Week 11: Lifecycle & Reproduction 1

Learning Objectives

Students will be able to explain the differences between endocrine and nervous system signaling and describe how hormones travel and act on target cells
Students will be able to identify the chemical classes of hormones and describe their mechanisms of action
Students will be able to describe the hypothalamic-pituitary axis and its role in regulating endocrine function
Students will be able to discuss hormones released from the anterior and posterior pituitary and explain their major target actions
Students will be able to describe thyroid hormone synthesis, explain why iodine is essential, and relate deficiency to goitre
Students will be able to explain fluid-balance and glucose-regulation hormones including ADH, ANP, aldosterone, insulin, and glucagon
Students will be able to explain the stress response including the roles of the sympathetic nervous system and adrenal hormones
Students will be able to describe male reproductive anatomy, spermatogenesis, and the hormonal control of sperm production
Students will be able to explain female reproductive anatomy, oogenesis, the menstrual cycle, and how reproductive capacity changes with age

Hormones and Reproduction: Chemical Messengers

Hormones are like chemical messengers that travel through your bloodstream to coordinate activities throughout your body. Think of them as text messages sent by different glands to tell organs what to do. Unlike the nervous system, which sends rapid electrical signals, hormones work more slowly but have longer-lasting effects. Your endocrine system includes classic glands like the pituitary, thyroid, and adrenal glands, but also includes cells in other organs that produce hormones.

When you experience stress, your body activates a fight-or-flight response. This begins in your brain's hypothalamus, which signals your adrenal glands to release adrenaline and noradrenaline. These hormones increase your heart rate, dilate your pupils, and prepare your muscles for action - just like preparing to run from danger. If stress continues, your body enters a resistance phase with different hormones like cortisol helping you adapt.

The male reproductive system produces sperm through a process called spermatogenesis. About 300 million sperm mature daily in the testes. Each sperm has a head containing genetic material and enzymes to penetrate an egg, a middle section packed with mitochondria for energy, and a tail for swimming. As men age, sperm production and quality gradually decline.

The female reproductive system is more complex. Women are born with all the egg cells they'll ever have - approximately 1-2 million. These eggs develop through oogenesis, which actually begins before birth. During each menstrual cycle, several follicles (egg-containing sacs) begin developing, but usually only one releases a mature egg during ovulation. The remaining eggs gradually decline in both quantity and quality with age, which is why female fertility decreases significantly after age 35-40. Hormones like estrogen and progesterone control these cycles, and their levels fluctuate throughout a woman's reproductive life.

Aging and Reproductive Capacity

Female Reproductive Aging: Women are born with a finite number of oocytes (1-2 million) that decline throughout life. By puberty, only 300,000-400,000 remain. This ovarian reserve depletes with age through follicular atresia (programmed cell death). Fecundability (probability of conception per cycle) decreases progressively: age 20-24 (~25%), age 30-34 (~20%), age 35-39 (~10%), age 40-44 (~5%). After menopause (average age 51), reproductive capacity ends completely.

The decline in both oocyte quantity and quality with age explains increased rates of aneuploidy (chromosomal abnormalities) and infertility. Miscarriage rates increase from 10% at age 20 to 50% by age 45. Assisted reproductive technologies have limited success in older women due to decreased ovarian response.

Male Reproductive Aging: Unlike females, men continue producing sperm throughout life via ongoing spermatogenesis. However, reproductive function still declines with age, though gradually. Testosterone levels decrease approximately 1% per year after age 30. By age 70, 20-30% of men have low testosterone.

Age-related changes in males include: decreased libido and erectile function, reduced sperm quality (motility and morphology decline, though sperm count may remain normal), increased time to conception, and higher rates of genetic mutations in sperm DNA. Unlike the abrupt menopause in females, male reproductive aging is gradual, and many men maintain some reproductive capacity into their 80s-90s.

Both sexes experience decreased fertility, changes in sexual function, and altered hormone profiles with advancing age, though the patterns differ significantly between males and females.

Endocrine System and Reproductive Physiology

The endocrine system comprises glands and specialized cells that synthesize and secrete hormones - chemical messengers that regulate physiological processes at extremely low concentrations. Hormones travel via blood or interstitial fluid to target cells bearing specific receptors, with the endocrine system generally producing slower, more prolonged effects compared to the rapid neural system. Hormones act in concert with the nervous system to coordinate and integrate cellular activity throughout the body, maintaining homeostasis.

Hormone Classification: Hormones are chemically classified as amino acid-derived (epinephrine, thyroid hormones), peptide/protein (insulin, growth hormone), or steroid (estrogen, testosterone, cortisol), each class exhibiting distinct mechanisms of action, transport characteristics, and receptor locations. Steroid hormones, being lipophilic, diffuse through cell membranes to bind intracellular receptors affecting gene transcription, while peptide hormones and catecholamines bind cell surface receptors activating second messenger systems.

Hypothalamic-Pituitary Axis: Represents a critical hierarchical control system. The hypothalamus produces releasing and inhibiting hormones that regulate the anterior pituitary, which in turn secretes tropic hormones (TSH, ACTH, FSH, LH) controlling peripheral endocrine glands. This cascade enables fine-tuned regulation and feedback control.

Stress Response: Stressors activate the hypothalamus, triggering sympathetic nervous system activation and adrenal medullary release of catecholamines (epinephrine, norepinephrine) producing immediate fight-or-flight effects: increased heart rate and contractility, bronchodilation, hepatic glycogenolysis, and vasoconstriction in non-essential tissues (skin, viscera) with vasodilation in essential organs (heart, brain, skeletal muscle). Concurrently, the hypothalamus-pituitary-adrenal axis activates, releasing ACTH stimulating cortisol production. Cortisol maintains blood glucose, suppresses inflammation, and enhances catecholamine sensitivity.

Male Reproduction: Spermatogenesis occurs in seminiferous tubules, producing approximately 300 million sperm daily. Spermatozoa are haploid cells with a head (nucleus and acrosome containing enzymes for egg penetration), midpiece (mitochondria for ATP production), and tail (flagellum for motility). Testosterone from Leydig cells regulates spermatogenesis and secondary sexual characteristics.

Female Reproduction: Women are born with 1-2 million primordial follicles. Oogenesis begins in fetal development, arresting in prophase I until puberty. Each cycle, FSH stimulates follicular development; the dominant follicle produces estrogen triggering the LH surge that induces ovulation. The corpus luteum forms from the ruptured follicle, producing progesterone to maintain the endometrium. Ovarian reserve declines with age, with accelerated loss after age 35-40.

Aging and Reproductive Function

Female Reproductive Aging: Women possess a finite oocyte endowment established during fetal development. Approximately 1-2 million oocytes are present at birth; by puberty, only 300,000-400,000 remain due to continuous follicular atresia. Throughout reproductive life, approximately 400-500 oocytes will ovulate, with the remainder lost through atresia.

Ovarian Reserve Decline: The rate of follicular depletion accelerates after age 35-37, with marked decline in both oocyte quantity and quality. Anti-Müllerian hormone (AMH) and antral follicle count (AFC) are clinical markers of ovarian reserve. Fecundability decreases progressively: age 20-24 (~25% per cycle), age 30-34 (~20%), age 35-39 (~10%), age 40-44 (~5%).

Oocyte Quality: Meiotic spindle abnormalities increase with maternal age, causing elevated rates of aneuploidy. Trisomy 21 (Down syndrome) risk increases from 1:1500 at age 20 to 1:30 at age 45. Miscarriage rates increase from ~10% at age 20 to ~50% by age 45, primarily due to chromosomal abnormalities.

Menopause: Defined as 12 consecutive months of amenorrhea due to ovarian failure. Average age 51 years (range 45-55). Depletion of ovarian follicles below threshold (~1000) results in estrogen deficiency and FSH elevation. Marks absolute end of natural reproductive capacity.

Male Reproductive Aging: Unlike the finite oocyte pool in females, spermatogenesis continues throughout male life. However, age-related changes occur:

Testosterone Decline: Total testosterone decreases ~1% per year after age 30; free testosterone (bioavailable) declines more rapidly due to increased SHBG. By age 70, 20-30% of men have testosterone below young adult reference range. In some men this contributes to late-onset hypogonadism, with decreased libido, erectile dysfunction, reduced muscle mass, fatigue, and mood changes.

Sperm Quality: While sperm count often remains within normal range, sperm motility and morphology decline with age. Increased sperm DNA fragmentation and mutations accumulate due to ongoing germ cell replication (spermatogonia divide continuously, accumulating mutations). Epigenetic changes in sperm may affect offspring.

Fertility Impact: Time to conception increases with paternal age. Offspring of older fathers have slightly increased risk of autism, schizophrenia, and certain genetic conditions due to de novo mutations.

Clinical Implications: Female age is the single most important factor in fertility treatment success. In vitro fertilization (IVF) pregnancy rates decline sharply after age 40 (10-15% vs 40-50% under age 35). Donor oocytes from young women achieve high success rates regardless of recipient age. For males, reproductive capacity is generally maintained longer, though quality declines.

Hormone Signaling Strategies and Interactions

Endocrine, paracrine, and autocrine signaling: Endocrine hormones travel in the bloodstream to distant targets. Paracrine signals act locally on nearby cells. Autocrine signals act back on the same cell that released them.

Humoral, neural, and hormonal control: Hormone secretion can be triggered by chemical changes in the blood (humoral stimuli), by nerves (neural stimuli), or by other hormones (hormonal stimuli).

Target-cell sensitivity: A hormone only affects a cell with the correct receptor. Cells can become more sensitive by making more receptors (up-regulation) or less sensitive by reducing receptor numbers (down-regulation).

Hormone interactions: Hormones may help each other (synergistic), oppose each other (antagonistic), or require one another for a full effect (permissive). Examples this week include insulin vs glucagon and thyroid hormone increasing responsiveness to catecholamines.

Week 11b: Major Hormones in the Body

This source lecture focuses on the major hormones that maintain homeostasis by linking each hormone to its gland, stimulus, and main action.

Anterior pituitary: Releases seven hormones, including growth hormone, TSH, ACTH, FSH, LH, prolactin, and MSH. Growth hormone supports growth and metabolism partly through insulin-like growth factors (IGFs).

Posterior pituitary: Does not synthesize its own hormones. It stores and releases ADH and oxytocin that were produced in the hypothalamus.

Thyroid, calcium, fluid, and glucose control: T3 and T4 require iodine; calcitonin, PTH, and calcitriol regulate calcium; ADH, ANP, and aldosterone regulate fluid balance; insulin, glucagon, and somatostatin regulate glucose homeostasis.

Pituitary, Thyroid, Fluid Balance, and Glucose Regulation

Pituitary gland: The pituitary sits below the hypothalamus and acts as a major control gland. The anterior pituitary releases hormones such as TSH, ACTH, FSH, LH, growth hormone, and prolactin. The posterior pituitary stores and releases ADH and oxytocin that were made in the hypothalamus.

Thyroid hormones and iodine: The thyroid produces T4 and T3, hormones that increase metabolic rate, support growth, and help regulate temperature. Iodine is essential for thyroid hormone synthesis. If iodine intake is too low, the thyroid enlarges in an attempt to keep up, producing a goitre.

Fluid balance hormones: ADH helps the kidneys retain water, so urine volume falls when the body needs to conserve fluid. Aldosterone helps retain sodium and water while promoting potassium loss. ANP works in the opposite direction by encouraging sodium and water excretion when blood volume is too high.

Glucose regulation: The endocrine pancreas helps keep blood glucose within a safe range. Insulin lowers blood glucose by promoting uptake and storage, while glucagon raises blood glucose by stimulating release from stored energy reserves. Somatostatin helps modulate both systems.

Calcium Homeostasis

Your body maintains blood calcium within a very narrow range because calcium is essential for muscle contraction, nerve function, and bone health. Three main hormones control this balance:

Parathyroid Hormone (PTH): When blood calcium drops too low, four tiny parathyroid glands on the back of your thyroid release PTH. Think of PTH as a "calcium-raising" hormone. It signals bones to release calcium, tells kidneys to hold onto calcium instead of letting it leave in urine, and activates vitamin D to help absorb more calcium from food.

Calcitonin: When blood calcium gets too high, the thyroid gland releases calcitonin. This hormone tells bones to store calcium and kidneys to excrete it. Calcitonin acts quickly but has a relatively minor role in day-to-day calcium control compared to PTH.

Calcitriol (Active Vitamin D): Your skin makes vitamin D when exposed to sunlight, and it is also found in some foods. The kidneys convert vitamin D into its active form, calcitriol. This hormone helps intestines absorb calcium from food and works with PTH to maintain bone health.

Clinical Relevance: Problems with calcium regulation are common. Osteoporosis affects millions, especially postmenopausal women, when bone breakdown outpaces formation. Hypocalcemia (low calcium) causes muscle spasms and tingling. Hypercalcemia (high calcium) can cause kidney stones, confusion, and weakness. Parathyroid disorders require careful monitoring and treatment.

Calcium homeostasis represents a tightly regulated system maintaining extracellular calcium concentration (2.2-2.6 mmol/L) through coordinated hormonal control. Calcium is essential for membrane stability, neuromuscular excitability, muscle contraction, blood clotting, and skeletal mineralization.

Parathyroid Hormone (PTH): Secreted by chief cells of the four parathyroid glands in response to hypocalcemia. PTH is the primary calcium-regulating hormone with three main target organs:

Bone: Stimulates osteoclast-mediated bone resorption, releasing calcium and phosphate
Kidney: Increases calcium reabsorption in distal tubules; inhibits phosphate reabsorption (phosphaturic effect)
Intestine (indirect): Stimulates renal 1α-hydroxylase to convert 25-hydroxyvitamin D to active 1,25-dihydroxyvitamin D (calcitriol)

Calcitonin: Produced by parafollicular C-cells of the thyroid gland in response to hypercalcemia. Acts via cAMP to inhibit osteoclast activity and promote renal calcium excretion. While pharmacologically significant, its physiological role in humans appears less critical than PTH.

Calcitriol (1,25-dihydroxyvitamin D): The hormonally active form of vitamin D synthesized in the kidney proximal tubule. Functions include:

Intestinal: Increases calcium and phosphate absorption through genomic and non-genomic mechanisms
Bone: Required for normal mineralization; at high doses stimulates bone resorption
Kidney: Increases calcium reabsorption

Integrated Control: Hypocalcemia stimulates PTH release → PTH increases bone resorption, renal calcium reabsorption, and calcitriol synthesis → calcitriol enhances intestinal calcium absorption → blood calcium rises, inhibiting further PTH release (negative feedback). Hypercalcemia suppresses PTH and stimulates calcitonin.

Clinical Relevance:

Primary Hyperparathyroidism: Excess PTH from parathyroid adenoma; causes hypercalcemia, kidney stones, bone loss
Hypoparathyroidism: Post-surgical or autoimmune; causes hypocalcemia, tetany, seizures
Osteoporosis: Reduced bone mass and microarchitectural deterioration; common in postmenopausal women (estrogen deficiency accelerates bone resorption)
Vitamin D Deficiency: Causes rickets (children) or osteomalacia (adults); impairs calcium absorption
Pseudohypoparathyroidism: Target organ resistance to PTH; hypocalcemia with elevated PTH

Additional Endocrine and Reproductive Clinical Links

Endocrine embryology: Endocrine organs come from different embryological tissues. For example, the pituitary has an oral ectoderm part and a neural ectoderm part, the thyroid begins near the tongue base, and the adrenal gland has both cortical and neural crest contributions.

Pineal gland and melatonin: The pineal gland releases melatonin, especially in darkness. Melatonin helps regulate the circadian rhythm, including normal sleep-wake timing.

Low calcium clinical cues: Hypocalcaemia can cause tingling, muscle cramps, and tetany. Clinically, signs such as carpopedal spasm, Chvostek sign, or Trousseau sign may appear, especially after thyroid or neck surgery if the parathyroid glands are affected.

Cryptorchidism: Cryptorchidism means an undescended testis. It matters because a testis that remains outside the scrotum is exposed to higher temperature, which increases later infertility and malignancy risk.

Male Reproductive System

The male reproductive system produces sperm and delivers them to the female reproductive tract. It includes both external and internal structures:

External Anatomy:

Scrotum: Skin sac holding testes outside the body. Keeps testes at 2-3°C below body temperature for optimal sperm production
Testes: Pair of oval organs that produce sperm and testosterone
Penis: Delivers sperm to female reproductive tract; contains erectile tissue

Internal Anatomy:

Epididymis: Coiled tube on testis where sperm mature and are stored (2-3 week maturation)
Vas Deferens: Muscular tube transporting sperm from epididymis to urethra during ejaculation
Seminal Vesicles: Glands producing 60% of semen fluid; provide fructose energy for sperm
Prostate Gland: Walnut-sized gland producing 25% of semen; alkaline fluid neutralizes vaginal acidity
Bulbourethral Glands: Pea-sized glands secreting pre-ejaculate fluid for lubrication

Spermatogenesis Process: Sperm production takes about 74 days and occurs in three stages:

1. Mitosis: Spermatogonia (stem cells) divide to maintain the cell line
2. Meiosis: Primary spermatocytes divide twice to reduce chromosomes from 46 to 23
3. Spermiogenesis: Round spermatids transform into streamlined sperm with head, midpiece, and tail

Erection Mechanics: Sexual stimulation triggers nitric oxide release, causing penile blood vessels to dilate. Blood fills spongy erectile tissue, compressing veins to trap blood and maintain rigidity. This involves parasympathetic nervous system activation.

Ejaculation Process: Sympathetic nervous system triggers emission (sperm and glandular fluids enter urethra) followed by expulsion (rhythmic muscle contractions propel semen out). Typically 2-5 mL of semen containing 40-250 million sperm.

Semen Composition: Only 5% sperm cells; 95% is seminal fluid providing nutrients, transport medium, and protection from acidic environments.

The male reproductive system comprises external genitalia (scrotum, penis) and internal structures (testes, epididymis, vas deferens, accessory glands) functioning to produce, mature, store, and deliver spermatozoa.

Scrotum and Testes: The scrotum maintains testicular temperature 2-3°C below core body temperature via dartos muscle (wrinkling for insulation) and cremaster muscle (elevation toward body when cold). The testes contain:

Seminiferous tubules: Site of spermatogenesis; contain Sertoli cells supporting germ cell development
Leydig cells (interstitial): Produce testosterone under LH stimulation
Blood-testis barrier: Tight junctions between Sertoli cells protect developing sperm from immune recognition

Complete Spermatogenesis: A 74-day process involving:

Proliferation: Type A spermatogonia (stem cells) divide by mitosis to maintain the pool; Type B spermatogonia differentiate
Meiosis I: Primary spermatocytes (2n DNA) undergo reduction division → secondary spermatocytes (1n DNA)
Meiosis II: Secondary spermatocytes divide equationally → spermatids (1n)
Spermiogenesis: Round spermatids differentiate: nuclear condensation, acrosome formation (Golgi), flagellum development (centriole), mitochondrial sheath formation, cytoplasmic shedding

Erection Physiology: Psychogenic or reflex stimulation activates parasympathetic fibers (pelvic splanchnic nerves, S2-S4) causing:

Nitric oxide (NO) release from endothelial cells and nNOS nerves
NO activates guanylate cyclase → cGMP ↑ → smooth muscle relaxation
Arterial dilation increases blood flow 20-40 fold
Corpus cavernosum engorgement compresses subtunical veins against tunica albuginea
Venous outflow obstruction maintains tumescence (trap mechanism)

Detumescence: PDE5 breaks down cGMP; sympathetic activation constricts arteries and relaxes trabecular smooth muscle.

Ejaculation Reflex: Two phases:

Emission: Sympathetic fibers (L1-L2) contract epididymis, vas deferens, seminal vesicles, prostate → sperm and secretions deposited in posterior urethra; internal urethral sphincter contracts to prevent retrograde flow
Expulsion: Somatic motor fibers (pudendal nerve S2-S4) stimulate rhythmic contractions of bulbospongiosus and ischiocavernosus muscles at 0.8-second intervals; external urethral sphincter relaxes

Semen Composition:

Spermatozoa: 40-250 million per ejaculate; 5% of volume
Seminal vesicle fluid (60%): Fructose (primary energy source), prostaglandins (uterine contraction), fibrinogen (clotting)
Prostatic fluid (25%): Citric acid, acid phosphatase, proteolytic enzymes (liquefaction), zinc
Bulbourethral gland fluid (<1%): Mucus for lubrication, buffering of urethral acidity
pH: 7.2-8.0 (alkaline to neutralize vaginal pH ~4.0)

Female Reproductive System

The female reproductive system produces eggs, supports fertilization and pregnancy, and nurtures offspring through lactation:

Anatomy:

Ovaries: Almond-sized organs producing eggs and hormones (estrogen, progesterone)
Fallopian Tubes: Thin tubes capturing eggs at ovulation and transporting them to the uterus; usual site of fertilization
Uterus: Pear-shaped muscular organ where embryo implants and develops
Cervix: Lower uterine neck connecting to vagina; produces mucus that changes consistency during cycle
Vagina: Muscular canal receiving sperm and serving as birth canal
Vulva: External genitalia including labia, clitoris, and vestibule

Ovarian Cycle: Each menstrual cycle, several follicles begin developing under FSH stimulation. Usually one becomes dominant and ovulates:

Follicular Development: Primordial → Primary → Secondary → Tertiary (antral) → Graafian (mature)
Ovulation: LH surge triggers egg release from dominant follicle (day 14 of 28-day cycle)
Corpus Luteum: Ruptured follicle transforms into hormone-producing structure for 14 days

Menstrual Cycle Phases:

Follicular Phase (Days 1-14): Menstruation occurs first 3-5 days; endometrium rebuilds under estrogen; follicle develops
Ovulation (Day 14): Egg released; cervical mucus becomes thin and stretchy to allow sperm passage
Luteal Phase (Days 15-28): Corpus luteum produces progesterone; endometrium becomes thick and secretory to support potential pregnancy

Mammary Glands and Lactation: Breasts contain glandular tissue producing milk after childbirth. Prolactin stimulates milk production; oxytocin triggers milk ejection. Estrogen and progesterone during pregnancy prepare breasts for lactation but inhibit milk production until after delivery.

The female reproductive system consists of internal organs (ovaries, uterine tubes, uterus, vagina) and external genitalia (vulva) functioning in gamete production, fertilization, pregnancy support, and lactation.

Ovaries: Paired almond-sized organs in the ovarian fossa with dual function:

Gametogenic: Oogenesis and ovulation
Endocrine: Synthesis of estrogen, progesterone, and inhibin

Uterine (Fallopian) Tubes: 10-12 cm muscular tubes with four regions (infundibulum with fimbriae, ampulla, isthmus, intramural part). Ciliated epithelium and smooth muscle contractions transport ova. Ampulla is the usual fertilization site. Tubal patency is essential for natural conception.

Uterus: Pear-shaped thick-walled muscular organ (7.5 x 5 x 2.5 cm) composed of:

Perimetrium: Serous outer layer
Myometrium: Thick smooth muscle layer responsible for contractions
Endometrium: Mucosal lining with functional layer (sheds monthly) and basal layer (regenerates)

Ovarian Cycle: The developmental sequence of follicles:

Primordial follicles: Oocyte arrested in prophase I surrounded by flattened follicular cells (dormant since fetal life)
Primary follicle: Oocyte enlarges; follicular cells become cuboidal and proliferate
Secondary follicle: Multiple granulosa cell layers; theca interna and externa form
Tertiary (antral) follicle: Fluid-filled cavity (antrum) appears; granulosa cells produce estrogen
Graafian (pre-ovulatory) follicle: Large antrum; cumulus oophorus surrounds oocyte
Ovulation: LH surge → prostaglandin release → follicular rupture → oocyte-cumulus complex extruded
Corpus luteum: Granulosa and theca cells luteinize → produce progesterone and estrogen for 14 days

Menstrual Cycle: Coordinated endometrial changes (28-day average):

Menses (Days 1-5): Spiral arteriole vasospasm → ischemia → functional layer necrosis and shedding; 30-80 mL blood loss
Proliferative phase (Days 6-14): Estrogen from developing follicles stimulates endometrial regeneration and proliferation; glands elongate; spiral arteries grow
Secretory phase (Days 15-28): Progesterone from corpus luteum converts endometrium to secretory state; glands become coiled and secretory (glycogen-rich); stroma becomes edematous; optimal for implantation
Ischemic phase: Corpus luteum regression → progesterone withdrawal → spiral artery vasospasm → premenstrual ischemia

Mammary Gland Structure and Lactation:

Structure: Modified apocrine sweat glands; 15-20 lobes with alveoli (milk-producing sacs); lactiferous ducts drain to nipple; suspensory ligaments (Cooper's) provide support
Development: Estrogen stimulates ductal growth; progesterone stimulates lobuloalveolar development; placental lactogen, prolactin, growth hormone contribute
Lactogenesis I: Mid-pregnancy; limited milk secretion (high progesterone inhibits full production)
Lactogenesis II: Postpartum (2-3 days); progesterone withdrawal triggers copious milk production
Milk ejection reflex: Infant suckling stimulates mechanoreceptors → afferent to hypothalamus → posterior pituitary releases oxytocin → myoepithelial cell contraction → milk expulsion
Prolactin: Anterior pituitary hormone stimulated by suckling; promotes milk synthesis; inhibits GnRH (lactational amenorrhea)

Aging and the Endocrine System

Hormone levels change throughout life, with significant shifts during aging affecting metabolism, reproduction, bone health, and overall well-being:

Hormonal Changes with Aging:

Growth Hormone: Declines steadily after puberty; by age 60, levels are about 1/3 of young adult levels; contributes to decreased muscle mass and increased body fat
Melatonin: Decreases with age, affecting sleep quality
Thyroid Function: TSH may increase slightly; T3 decreases; metabolic rate slows
Insulin: Sensitivity decreases; diabetes risk increases significantly after age 45
DHEA: Adrenal androgen peaks in 20s, declines to 10-20% by age 70

Menopause: Average age 51 (range 45-55). Ovarian follicles depleted, causing dramatic estrogen decline. Symptoms include hot flashes, night sweats, mood changes, vaginal dryness, and sleep disturbances. Bone loss accelerates (osteoporosis risk), and cardiovascular risk increases. Hormone therapy may help symptoms but has risks to consider.

Male Reproductive Aging / Late-Onset Hypogonadism: Testosterone declines gradually with age (~1% per year from age 30). By age 70, 20-30% of men have low testosterone. Symptoms may include decreased libido, erectile dysfunction, fatigue, reduced muscle mass, and mood changes. Unlike menopause, there is no universal abrupt pause in reproductive function, and many men maintain reproductive capacity later in life.

Osteoporosis and Bone Loss: Bone density peaks around age 30, then gradually declines. Accelerated loss occurs in women after menopause due to estrogen deficiency. Men lose bone more slowly but also face increased fracture risk with age. Weight-bearing exercise, calcium, and vitamin D help maintain bone health.

Diabetes Risk: Type 2 diabetes risk doubles every decade after age 35 due to decreased insulin sensitivity, reduced physical activity, and weight gain. Monitoring blood glucose and maintaining healthy weight become increasingly important.

The endocrine system undergoes significant age-related changes affecting virtually all hormonal axes. These changes contribute to the phenotype of aging including altered body composition, decreased metabolic rate, reduced reproductive function, and increased disease risk.

Somatotropic Axis: Growth hormone (GH) secretion follows a pulsatile pattern with highest amplitude during sleep. GH declines progressively after puberty (somatopause):

IGF-1 levels: Fall 50-70% between age 20 and 60
Consequences: Decreased muscle mass (sarcopenia), increased visceral adiposity, reduced bone mineral density, impaired lipolysis
Potential interventions: GH replacement controversial due to side effects; exercise and sleep optimization partially restore GH secretion

Hypothalamic-Pituitary-Thyroid Axis:

TSH: Slight increase with age (reduced thyroid hormone feedback sensitivity)
T3: Decreases due to reduced peripheral conversion (lower deiodinase activity)
Metabolic rate: Declines 2-3% per decade; contributes to age-related weight gain
Clinical: Subclinical hypothyroidism common in elderly; treatment decisions individualized

Male Reproductive Aging:

Testosterone: Total testosterone decreases ~1% per year from age 30; free testosterone declines more rapidly (SHBG increases with age)
Prevalence: 20-30% of men >70 have testosterone below young adult reference range
Symptoms: Decreased libido, erectile dysfunction, reduced muscle mass and strength, increased fat mass, decreased bone density, fatigue, depressed mood, cognitive changes
Late-onset hypogonadism: More accurate clinical term for symptomatic testosterone deficiency in some aging men; there is no universally accepted abrupt male equivalent of menopause
Testosterone replacement: Benefits muscle, bone, and sexual function; risks include erythrocytosis, sleep apnea, potential cardiovascular effects

Female Reproductive Aging:

Menopausal transition (perimenopause): Begins ~4 years before final menstrual period; characterized by menstrual irregularity, vasomotor symptoms, sleep disturbance
Menopause: 12 months of amenorrhea; median age 51; caused by follicular depletion below threshold
Hormonal changes: Estrogen and progesterone fall dramatically; FSH rises (no negative feedback); inhibin undetectable
Symptoms: Vasomotor (hot flashes, night sweats - affect 75%), urogenital atrophy, sleep disturbance, mood changes, cognitive changes
Long-term consequences: Osteoporosis (bone loss 3-5% per year first 5 years), cardiovascular disease risk increases, genitourinary syndrome of menopause
Hormone therapy: Most effective for vasomotor symptoms; benefits vs risks individualized based on age, time since menopause, cardiovascular risk

Glucose Homeostasis:

Insulin resistance: Increases with age due to reduced physical activity, increased adiposity, mitochondrial dysfunction, chronic low-grade inflammation
Beta cell function: Gradual decline in glucose-stimulated insulin secretion
Type 2 diabetes: Prevalence increases from 2% at age 20 to 25% at age 65
Complications: Cardiovascular disease, nephropathy, retinopathy, neuropathy risk all increase with age

Bone Metabolism:

Peak bone mass: Achieved in late 20s; genetic and environmental factors determine magnitude
Postmenopausal women: Estrogen deficiency increases osteoclast activity → rapid bone loss (cortical 2-3%/year, trabecular 5-10%/year)
Older men: Slower bone loss (0.5-1%/year) but increased fracture risk due to falls and age-related bone changes
Fracture risk: Doubles every 7-8 years for women, every 10-12 years for men after age 50

Stress and Hormonal Interactions

Your body responds to stress through two main hormonal pathways that prepare you to face challenges:

HPA Axis (Hypothalamus-Pituitary-Adrenal): This slower pathway kicks in within minutes and lasts hours:

Stress signals reach the hypothalamus in your brain
Hypothalamus releases CRH (corticotropin-releasing hormone)
CRH tells the pituitary gland to release ACTH into the blood
ACTH travels to adrenal glands and stimulates cortisol release
Cortisol helps maintain blood sugar, reduces inflammation, and helps you cope with prolonged stress

SAM Axis (Sympathetic-Adrenal-Medullary): This is the rapid fight-or-flight response:

Hypothalamus activates the sympathetic nervous system
Nerve signals reach the adrenal medulla (center of adrenal glands)
Adrenal medulla releases epinephrine (adrenaline) and norepinephrine
These hormones surge within seconds, increasing heart rate, opening airways, and diverting blood to muscles

Cortisol and Stress Response: Often called the "stress hormone," cortisol has wide-ranging effects:

Mobilizes energy by breaking down proteins and fats
Increases blood sugar for brain fuel
Suppresses immune function temporarily
Enhances memory formation of stressful events
Follows daily rhythm - highest in morning, lowest at night

Catecholamines: Epinephrine and norepinephrine work together:

Epinephrine: Mainly from adrenal medulla; affects heart, lungs, blood vessels throughout body
Norepinephrine: From adrenal medulla and nerve endings; causes vasoconstriction and increases alertness

Chronic Stress Effects: When stress is constant, these systems stay activated:

High cortisol leads to weight gain (especially belly fat), high blood pressure, and diabetes risk
Weakened immune system increases infection risk
Disrupted sleep and memory problems
Anxiety and depression
Digestive problems and ulcers

Stress management techniques like exercise, meditation, and social connection help regulate these hormonal responses.

Stress activates integrated neuroendocrine responses mediated by the HPA axis and SAM axis, which mobilize energy, enhance cardiovascular function, and modulate immune responses to maintain homeostasis during challenges.

HPA Axis (Hypothalamic-Pituitary-Adrenal): The primary neuroendocrine response to stressors:

Initiation: Stressors activate parvocellular neurosecretory neurons in the paraventricular nucleus (PVN) of the hypothalamus
CRH release: Corticotropin-releasing hormone secreted into hypophyseal portal system
ACTH release: CRH stimulates corticotrope cells in anterior pituitary to release adrenocorticotropic hormone (ACTH); vasopressin potentiates this response
Adrenal stimulation: ACTH binds melanocortin-2 receptors on zona fasciculata cells → cAMP/PKA activation → cortisol synthesis and release
Negative feedback: Cortisol inhibits CRH/ACTH at hypothalamus and pituitary (long loop); ACTH inhibits CRH (short loop)

SAM Axis (Sympathetic-Adrenal-Medullary): The rapid catecholamine response:

Sympathetic activation: Hypothalamus activates preganglionic sympathetic neurons
Adrenal medulla: Preganglionic fibers synapse on chromaffin cells (modified postganglionic neurons)
Catecholamine release: Chromaffin cells release epinephrine (~80%), norepinephrine (~20%), and dopamine into circulation
Direct neural effects: Sympathetic nerves release norepinephrine at target organs

Cortisol Actions: Glucocorticoid effects are pleiotropic and context-dependent:

Metabolic: Stimulates hepatic gluconeogenesis; promotes lipolysis; increases protein catabolism; maintains blood glucose during fasting
Cardiovascular: Enhances catecholamine sensitivity (permissive effect); maintains vascular tone
Immune/Inflammatory: Suppresses immune cell proliferation; inhibits cytokine production; anti-inflammatory; increases infection risk with chronic elevation
Neural: Enhances memory consolidation (hippocampus); excessive cortisol is neurotoxic to hippocampus (cognitive impairment)
Development: Essential for lung maturation (fetal cortisol); promotes surfactant production

Catecholamine Actions: Epinephrine and norepinephrine act via adrenergic receptors:

α1-adrenergic: Vasoconstriction, smooth muscle contraction
α2-adrenergic: Presynaptic inhibition, platelet aggregation
β1-adrenergic: Cardiac stimulation (increased rate, contractility, conduction velocity)
β2-adrenergic: Bronchodilation, uterine relaxation, hepatic glycogenolysis, skeletal muscle vasodilation
β3-adrenergic: Lipolysis, thermogenesis

Chronic Stress Pathology: Dysregulated stress responses contribute to disease:

Metabolic syndrome: Chronic cortisol promotes visceral obesity, insulin resistance, dyslipidemia, hypertension
Cardiovascular: Elevated catecholamines cause hypertension and promote atherosclerosis; cortisol impairs endothelial function
Immune dysfunction: Chronic cortisol immunosuppression increases infection risk; alters Th1/Th2 balance; may promote autoimmunity
Psychiatric: HPA axis hyperactivity in depression; cortisol neurotoxicity to hippocampus in PTSD; anxiety disorders
Gastrointestinal: Stress ulceration; altered gut motility (IBS); increased intestinal permeability
Reproductive: CRH and cortisol suppress GnRH → hypothalamic amenorrhea, infertility, decreased libido

Allostatic Load: The cumulative wear and tear on the body from chronic or repeated stress responses exceeding adaptive capacity. Markers include elevated basal cortisol or flattened diurnal rhythm, impaired negative feedback, increased inflammatory markers (CRP, IL-6), and accelerated cellular aging (telomere shortening).

Specific Hormones Reference

A comprehensive overview of major hormones organized by gland:

Anterior Pituitary Hormones:

TSH (Thyroid-Stimulating Hormone): Tells thyroid to make T3 and T4
ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to make cortisol
FSH (Follicle-Stimulating Hormone): Stimulates egg/sperm development
LH (Luteinizing Hormone): Triggers ovulation and testosterone
GH (Growth Hormone): Promotes growth and cell reproduction
Prolactin: Stimulates milk production after childbirth
MSH (Melanocyte-Stimulating Hormone): Controls skin pigmentation

Posterior Pituitary Hormones:

ADH/Antidiuretic Hormone (Vasopressin): Reduces urine output; raises blood pressure
Oxytocin: Triggers uterine contractions and milk ejection; promotes bonding

Thyroid Hormones:

T3 (Triiodothyronine): Active hormone; regulates metabolism
T4 (Thyroxine): Prohormone converted to T3 in tissues
Calcitonin: Lowers blood calcium

Adrenal Hormones:

Cortisol: Stress hormone; maintains blood sugar
Aldosterone: Regulates salt and water balance
DHEA: Weak androgen; precursor to sex hormones
Epinephrine: Fight-or-flight hormone
Norepinephrine: Alertness and blood pressure

Pancreatic Hormones:

Insulin: Lowers blood glucose
Glucagon: Raises blood glucose
Somatostatin: Regulates/inhibits both insulin and glucagon

Reproductive Hormones:

Testosterone: Male sex hormone; anabolic effects
Estrogen: Female sex hormone; regulates cycle
Progesterone: Prepares/maintains pregnancy
hCG: Pregnancy hormone
Inhibin: Inhibits FSH

Comprehensive reference of major hormones organized by gland of origin:

Anterior Pituitary (Adenohypophysis):

TSH (Thyrotropin): Glycoprotein; stimulates thyroid follicular cells to synthesize and release T3/T4; regulated by TRH and thyroid hormone negative feedback
ACTH (Corticotropin): Peptide (39 amino acids); stimulates adrenal cortex zona fasciculata to synthesize cortisol; regulated by CRH and cortisol feedback
FSH (Follicle-Stimulating Hormone): Glycoprotein; stimulates ovarian follicular development and spermatogenesis; regulated by GnRH and inhibin/sex steroids
LH (Luteinizing Hormone): Glycoprotein; triggers ovulation, corpus luteum formation, and testosterone synthesis; regulated by GnRH and sex steroids
GH (Growth Hormone/Somatotropin): 191 amino acid peptide; promotes growth via IGF-1; metabolic effects include lipolysis and decreased glucose uptake; regulated by GHRH (stimulatory) and somatostatin (inhibitory)
Prolactin: 199 amino acid peptide; stimulates mammary gland milk production; regulated by dopamine (prolactin-inhibiting factor) and TRH (stimulatory)
MSH (Melanocyte-Stimulating Hormone): Derived from POMC cleavage; regulates melanin synthesis; minimal role in humans

Posterior Pituitary (Neurohypophysis): Stores and releases hormones synthesized in hypothalamic nuclei:

ADH/Vasopressin: 9 amino acid peptide synthesized in supraoptic and paraventricular nuclei; V2 receptors in kidney collecting ducts → aquaporin-2 insertion → water reabsorption; V1 receptors on vascular smooth muscle → vasoconstriction; osmoreceptors and baroreceptors regulate release
Oxytocin: 9 amino acid peptide synthesized in paraventricular and supraoptic nuclei; stimulates uterine contraction (parturition) and myoepithelial cell contraction (milk ejection); promotes social bonding and maternal behavior; regulated by suckling reflex and cervical stimulation

Thyroid Gland:

T4 (Thyroxine): Prohormone with four iodine atoms; converted to T3 peripherally by deiodinases; binds thyroid receptors affecting gene transcription; regulates basal metabolic rate, thermogenesis, growth, and development
T3 (Triiodothyronine): Biologically active form (three iodine atoms); 10x more potent than T4; most intracellular T3 derived from T4 conversion; binds nuclear thyroid receptors with higher affinity
Calcitonin: 32 amino acid peptide from parafollicular C-cells; lowers blood calcium by inhibiting osteoclast activity; physiological role in humans minor compared to PTH
Thyroglobulin: Precursor protein synthesized in thyroid follicular cells; iodinated and stored in colloid; proteolysis releases T3/T4

Adrenal Gland:

Cortex - Zona Glomerulosa: Aldosterone (mineralocorticoid); regulated by RAAS and potassium; promotes sodium/water retention
Cortex - Zona Fasciculata: Cortisol (glucocorticoid); regulated by ACTH; metabolic, anti-inflammatory, cardiovascular effects
Cortex - Zona Reticularis: DHEA and androstenedione (weak androgens); regulated by ACTH; converted to testosterone and estrogen peripherally
Medulla: Epinephrine (80%), norepinephrine (20%), dopamine; regulated by preganglionic sympathetic fibers; catecholamine effects via adrenergic receptors

Pancreatic Islets:

Insulin (Beta cells): 51 amino acid peptide; lowers blood glucose by stimulating GLUT4 translocation, glycogen synthesis, lipogenesis, protein synthesis; deficiency causes diabetes mellitus
Glucagon (Alpha cells): 29 amino acid peptide; raises blood glucose by stimulating hepatic glycogenolysis and gluconeogenesis; reciprocal regulation with insulin
Somatostatin (Delta cells): 14/28 amino acid forms; paracrine inhibition of insulin and glucagon secretion; suppresses gastrointestinal function
Pancreatic Polypeptide (PP cells): Inhibits pancreatic enzyme secretion and gallbladder contraction

🎥 Video Lectures

Overview

Introduction to the endocrine system and reproduction.

Topic Title

Select a topic from the list to view detailed information.

📄 Lecture Notes

Key Terms

Hormone

Chemical messenger produced by endocrine glands that travels through blood to regulate target cell function at low concentrations

Endocrine System

System of glands and cells that secrete hormones directly into the bloodstream to regulate body functions

Target Cell

A cell that possesses specific receptors for a particular hormone and responds to its presence

Hypothalamus

Brain region that controls the pituitary gland and regulates body temperature, hunger, thirst, and hormone release

Pituitary Gland

Master endocrine gland located at the base of the brain; secretes hormones that regulate other endocrine glands

Adrenal Gland

Glands located atop kidneys consisting of cortex (producing corticosteroids) and medulla (producing catecholamines)

Cortisol

Glucocorticoid hormone produced by adrenal cortex; regulates metabolism, immune response, and stress adaptation

Epinephrine

Adrenaline; hormone and neurotransmitter produced by adrenal medulla that triggers fight-or-flight response

Fight or Flight

Physiological stress response preparing body for immediate action through sympathetic nervous system activation

Spermatogenesis

The process of sperm cell development in the testes; produces approximately 300 million sperm daily

Spermatozoa

Mature male gametes; haploid cells with head (nucleus and acrosome), midpiece (mitochondria), and tail (flagellum)

Acrosome

Cap-like vesicle on sperm head containing proteolytic enzymes necessary for penetrating the egg's outer layers

Oogenesis

The process of egg cell development; begins in fetal period and involves meiotic divisions to produce oocytes

Follicle

Ovarian structure containing a developing oocyte surrounded by granulosa and theca cells that produce hormones

Ovulation

The release of a mature secondary oocyte from the dominant follicle, typically occurring mid-cycle

Corpus Luteum

Endocrine structure formed from the ruptured follicle after ovulation; produces progesterone and estrogen

Estrogen

Primary female sex hormone produced by developing follicles; promotes endometrial proliferation and secondary sexual characteristics

Progesterone

Hormone produced by corpus luteum and placenta; maintains pregnancy by preparing and preserving the endometrium

FSH (Follicle-Stimulating Hormone)

Anterior pituitary hormone that stimulates follicular development in females and spermatogenesis in males

LH (Luteinizing Hormone)

Anterior pituitary hormone that triggers ovulation and corpus luteum formation in females; stimulates testosterone in males

Ovarian Reserve

The number and quality of remaining oocytes in the ovaries. Declines with age: 1-2 million at birth, 300,000-400,000 at puberty, rapid decline after age 35-37. Measured by AMH and antral follicle count.

Female Reproductive Aging

Finite oocyte pool with progressive decline in quantity and quality. Fecundability decreases from ~25% per cycle (age 20-24) to ~5% (age 40-44). Miscarriage rates increase from 10% to 50%. Menopause (average age 51) marks end of reproductive capacity.

Male Reproductive Aging

Testosterone declines ~1% per year from age 30. Spermatogenesis continues but sperm quality (motility, morphology, DNA integrity) declines. Unlike female menopause, reproductive capacity maintained longer though fertility decreases.

Age-Related Aneuploidy

Increased chromosomal abnormalities in oocytes with maternal age due to meiotic spindle dysfunction. Down syndrome risk: 1:1500 at age 20, 1:30 at age 45.

Late-Onset Hypogonadism

Clinical term for symptomatic testosterone deficiency in some aging men. More accurate than treating male aging as a direct equivalent of menopause.

TSH (Thyroid-Stimulating Hormone)

Anterior pituitary hormone that stimulates the thyroid gland to produce T3 and T4. Regulated by TRH from hypothalamus and negative feedback from thyroid hormones.

ACTH (Adrenocorticotropic Hormone)

Anterior pituitary hormone that stimulates the adrenal cortex to synthesize and release cortisol. Part of the HPA axis stress response system.

Growth Hormone (GH)

Anterior pituitary hormone that promotes growth, cell reproduction, and regeneration. Stimulates IGF-1 production. Declines significantly with age (somatopause).

Prolactin

Anterior pituitary hormone that stimulates milk production in mammary glands after childbirth. Regulated by dopamine (inhibitory) and TRH (stimulatory).

MSH (Melanocyte-Stimulating Hormone)

Anterior pituitary hormone derived from POMC cleavage. Regulates melanin synthesis and skin pigmentation. Minimal role in human physiology compared to other species.

ADH/Vasopressin

Posterior pituitary hormone that promotes water reabsorption in kidneys (V2 receptors) and causes vasoconstriction (V1 receptors). Regulated by osmoreceptors and baroreceptors.

Oxytocin

Posterior pituitary hormone that stimulates uterine contractions during labor and milk ejection during breastfeeding. Also promotes social bonding and maternal behavior.

T3 (Triiodothyronine)

Active thyroid hormone with three iodine atoms. 10x more potent than T4. Regulates metabolic rate, thermogenesis, growth, and development. Most intracellular T3 comes from T4 conversion.

T4 (Thyroxine)

Prohormone with four iodine atoms produced by thyroid follicular cells. Converted to active T3 in peripheral tissues. Binds nuclear receptors to regulate gene transcription.

Calcitonin

Thyroid hormone from parafollicular C-cells that lowers blood calcium by inhibiting osteoclast activity. Role in humans is relatively minor compared to PTH.

Thyroglobulin

Precursor protein synthesized in thyroid follicular cells, iodinated, and stored in colloid. Proteolysis releases T3 and T4 into circulation.

Goitre

Enlargement of the thyroid gland, often associated with iodine deficiency or chronic thyroid stimulation.

Parathyroid Hormone (PTH)

Primary calcium-regulating hormone from parathyroid glands. Raises blood calcium by stimulating bone resorption, renal calcium reabsorption, and calcitriol synthesis.

Calcitriol (1,25-dihydroxyvitamin D)

Active form of vitamin D synthesized in kidneys. Increases intestinal calcium absorption, promotes bone mineralization, and works with PTH to maintain calcium homeostasis.

Osteoclast

Bone cell that breaks down bone tissue, releasing calcium and phosphate into blood. Activated by PTH and inhibited by calcitonin. Essential for calcium homeostasis.

Aldosterone

Mineralocorticoid from adrenal cortex zona glomerulosa. Regulates sodium and potassium balance, promoting sodium/water retention. Regulated by renin-angiotensin system.

ANP (Atrial Natriuretic Peptide)

Hormone released by the atria of the heart that promotes sodium and water excretion, helping reduce blood volume and pressure.

Insulin

Pancreatic hormone from beta cells that lowers blood glucose by promoting cellular uptake and storage of nutrients.

Glucagon

Pancreatic hormone from alpha cells that raises blood glucose by stimulating glycogen breakdown and glucose release.

DHEA

Weak adrenal androgen from zona reticularis converted to testosterone and estrogen peripherally. Peaks in 20s, declines to 10-20% by age 70.

Norepinephrine

Catecholamine neurotransmitter and hormone from adrenal medulla and sympathetic nerves. Causes vasoconstriction and increases alertness through alpha-adrenergic receptors.

Zona Glomerulosa

Outer layer of adrenal cortex producing mineralocorticoids (aldosterone). Regulated by renin-angiotensin system and potassium levels.

Zona Fasciculata

Middle layer of adrenal cortex producing glucocorticoids (cortisol). Regulated by ACTH from anterior pituitary.

Zona Reticularis

Inner layer of adrenal cortex producing sex steroids (DHEA, androgens). Regulated by ACTH. Contributes to puberty onset (adrenarche).

Scrotum

External skin sac holding testes outside the body. Maintains testicular temperature 2-3°C below body temperature via dartos and cremaster muscles for optimal spermatogenesis.

Epididymis

Coiled tube on posterior testis where sperm mature and are stored. Site of sperm maturation taking 2-3 weeks. Sperm gain motility and fertilizing capacity here.

Vas Deferens

Muscular tube transporting mature sperm from epididymis to ejaculatory duct during ejaculation. Cut during vasectomy for male contraception.

Seminal Vesicles

Accessory glands producing 60% of semen fluid. Secrete fructose (sperm energy source), prostaglandins, and clotting proteins.

Prostate Gland

Walnut-sized gland producing 25% of semen volume. Secretes alkaline fluid containing citric acid, enzymes (PSA), and zinc to neutralize vaginal acidity.

Bulbourethral Glands

Pea-sized glands secreting pre-ejaculate fluid (Cowper's fluid) for lubrication and neutralizing urinary acidity in the urethra.

Spermatogonium

Diploid stem cells in seminiferous tubules that divide by mitosis to maintain the germ cell line and produce primary spermatocytes.

Primary Spermatocyte

Diploid cell (2n DNA) that undergoes meiosis I to form two secondary spermatocytes. First step in reducing chromosome number.

Secondary Spermatocyte

Haploid cell (1n DNA, 2c chromatids) that undergoes meiosis II to form two spermatids. Short-lived stage in spermatogenesis.

Spermatid

Haploid round cells (1n) formed from meiosis II. Undergo spermiogenesis to differentiate into mature spermatozoa with head, midpiece, and tail.

Sertoli Cells

Supportive cells in seminiferous tubules that nourish developing sperm, form blood-testis barrier, and produce inhibin. Essential for spermatogenesis.

Leydig Cells

Interstitial cells between seminiferous tubules that produce testosterone in response to LH stimulation. Essential for spermatogenesis and male secondary characteristics.

Ovaries

Almond-sized female gonads producing ova and sex hormones (estrogen, progesterone, inhibin). Contain 1-2 million primordial follicles at birth declining to menopause.

Fallopian Tubes

10-12 cm muscular tubes capturing ova at ovulation and transporting to uterus. Fimbriae sweep egg into tube. Ampulla is usual fertilization site.

Uterus

Pear-shaped muscular organ where embryo implants and develops. Composed of perimetrium (outer), myometrium (muscle), and endometrium (lining).

Endometrium

Uterine lining with functional layer (sheds during menstruation) and basal layer (regenerates). Prepares for implantation under estrogen and progesterone influence.

Cervix

Lower uterine neck connecting to vagina. Produces mucus that changes from thick (infertile) to thin and stretchy (fertile) during cycle.

Vagina

Muscular canal extending from cervix to exterior. Receives sperm, serves as birth canal, and routes menstrual flow. Maintains acidic pH (~4.0).

Vulva

External female genitalia including labia majora, labia minora, clitoris, and vestibule. Clitoris is highly innervated erectile tissue.

Primordial Follicle

Immature follicle with oocyte arrested in prophase I surrounded by flattened follicular cells. Present from birth; 1-2 million at birth, declining to menopause.

Primary Follicle

Developing follicle with growing oocyte and cuboidal granulosa cells. Early stage in follicular development under FSH stimulation.

Secondary Follicle

Follicle with multiple granulosa cell layers and developing theca interna/externa. Continues growth under FSH influence.

Graafian Follicle

Mature pre-ovulatory follicle with large antrum, cumulus oophorus surrounding oocyte. Dominant follicle that will ovulate under LH surge.

Menses

Menstrual phase (days 1-5) with shedding of endometrial functional layer. Caused by progesterone withdrawal when corpus luteum regresses. Blood loss 30-80 mL.

Proliferative Phase

Follicular phase endometrial regeneration (days 6-14) under estrogen stimulation. Endometrium rebuilds, glands elongate, spiral arteries grow.

Secretory Phase

Luteal phase endometrial preparation (days 15-28) under progesterone dominance. Glands become coiled and secretory; optimal for embryo implantation.

Mammary Glands

Modified apocrine sweat glands producing milk. 15-20 lobes with alveoli. Estrogen stimulates ductal growth; progesterone stimulates lobuloalveolar development.

Lactation

Milk production and ejection regulated by prolactin (synthesis) and oxytocin (ejection). Progesterone withdrawal postpartum triggers copious milk production.

Insulin

Pancreatic beta cell hormone lowering blood glucose by promoting GLUT4 translocation, glycogen synthesis, and lipogenesis. Deficiency causes diabetes mellitus.

Glucagon

Pancreatic alpha cell hormone raising blood glucose by stimulating hepatic glycogenolysis and gluconeogenesis. Reciprocal regulation with insulin.

Somatostatin

Pancreatic delta cell hormone inhibiting both insulin and glucagon secretion. Also suppresses gastrointestinal function. Paracrine regulator of glucose homeostasis.

CRH (Corticotropin-Releasing Hormone)

Hypothalamic hormone stimulating ACTH release from anterior pituitary. Initiates HPA axis stress response. Regulated by cortisol negative feedback.

GnRH (Gonadotropin-Releasing Hormone)

Hypothalamic hormone pulsatile release stimulating FSH and LH from anterior pituitary. Controls reproductive function. Inhibited by sex steroid negative feedback.

TRH (Thyrotropin-Releasing Hormone)

Hypothalamic hormone stimulating TSH and prolactin release. Regulates thyroid function. Inhibited by thyroid hormone negative feedback.

HPA Axis

Hypothalamic-Pituitary-Adrenal axis coordinating stress response. CRH → ACTH → cortisol cascade. Negative feedback maintains homeostasis.

SAM Axis

Sympathetic-Adrenal-Medullary axis providing rapid fight-or-flight response. Sympathetic activation → adrenal medulla catecholamine release (epinephrine, norepinephrine).

Catecholamines

Epinephrine, norepinephrine, and dopamine. Adrenal medulla releases 80% epinephrine and 20% norepinephrine. Act via adrenergic receptors for fight-or-flight.

Osteoporosis

Reduced bone mass and microarchitectural deterioration increasing fracture risk. Common in postmenopausal women due to estrogen deficiency accelerating bone resorption.

Adrenarche

Adrenal androgen secretion increase around age 6-8 leading to pubic and axillary hair growth. Occurs before gonadarche.

Gonadarche

Activation of gonadal function at puberty with increased GnRH pulsatility, rising sex steroids, and development of secondary sexual characteristics.

Pubarche

Appearance of pubic hair due to adrenal androgen secretion (adrenarche). Typically first visible sign of puberty in girls.

Menarche

First menstrual period marking onset of menstruation and potential fertility. Average age 12-13 in developed countries.

Inhibin

Gonadal hormone (Sertoli cells in males, granulosa cells in females) inhibiting FSH secretion. Provides negative feedback on gonadotropins.

hCG (Human Chorionic Gonadotropin)

Placental hormone maintaining corpus luteum in early pregnancy. Similar structure to LH. Used as pregnancy test marker.

Interactive Activity: Endocrine Flashcards

Review key endocrine system terms with these interactive flashcards. Learn about hormones, glands, reproductive processes, and the stress response.

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End of Week Test

Test your understanding of endocrine signalling, thyroid and calcium regulation, fluid-balance hormones, stress responses, and reproductive anatomy with the full end-of-week assessment.

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Clinical Case Study

Apply your knowledge of Endocrine System to a clinical scenario.

Open Case: The Thyroid Storm →

Week 11: Lifecycle & Reproduction 1

Learning Objectives

Hormones and Reproduction: Chemical Messengers

Aging and Reproductive Capacity

Endocrine System and Reproductive Physiology

Aging and Reproductive Function

Hormone Signaling Strategies and Interactions

Week 11b: Major Hormones in the Body

Pituitary, Thyroid, Fluid Balance, and Glucose Regulation

Calcium Homeostasis

Additional Endocrine and Reproductive Clinical Links

Male Reproductive System

Female Reproductive System

Aging and the Endocrine System

Stress and Hormonal Interactions

Specific Hormones Reference

🎥 Video Lectures

Overview

Topic Title

📄 Lecture Notes

Select a PDF to view

Key Terms

Hormone

Endocrine System

Target Cell

Hypothalamus

Pituitary Gland

Adrenal Gland

Cortisol

Epinephrine

Fight or Flight

Spermatogenesis

Spermatozoa

Acrosome

Oogenesis

Follicle

Ovulation

Corpus Luteum

Estrogen

Progesterone

FSH (Follicle-Stimulating Hormone)

LH (Luteinizing Hormone)

Ovarian Reserve

Female Reproductive Aging

Male Reproductive Aging

Age-Related Aneuploidy

Late-Onset Hypogonadism

TSH (Thyroid-Stimulating Hormone)

ACTH (Adrenocorticotropic Hormone)

Growth Hormone (GH)

Prolactin

MSH (Melanocyte-Stimulating Hormone)

ADH/Vasopressin

Oxytocin

T3 (Triiodothyronine)

T4 (Thyroxine)

Calcitonin

Thyroglobulin

Goitre

Parathyroid Hormone (PTH)

Calcitriol (1,25-dihydroxyvitamin D)

Osteoclast

Aldosterone

ANP (Atrial Natriuretic Peptide)

Insulin

Glucagon

DHEA

Norepinephrine

Zona Glomerulosa

Zona Fasciculata

Zona Reticularis

Scrotum

Epididymis

Vas Deferens

Seminal Vesicles

Prostate Gland

Bulbourethral Glands

Spermatogonium

Primary Spermatocyte

Secondary Spermatocyte