---

Short Notes - Reproductive Physiology

---

1. Indicators of Ovulation - Clinical Significance

• Progesterone, secreted in large quantities after ovulation from the corpus luteum, has a thermogenic effect on the hypothalamus.
• BBT rises by 0.2-0.5°C (about 0.5°F) after ovulation and remains elevated until menstruation.
• The temperature remains elevated throughout the luteal phase due to sustained progesterone secretion.
• Clinical significance: A chart showing a biphasic temperature pattern (low pre-ovulatory, high post-ovulatory) confirms that ovulation has occurred.

• The most reliable indicator. LH secretion rises 6-10 fold approximately 2 days before ovulation and peaks about 16 hours before follicle rupture.
• FSH also rises 2-3 fold simultaneously.
• Clinical significance: Urinary LH detection kits (ovulation predictor kits) detect this surge and are used to time intercourse for conception or to confirm ovulation.

• Some women experience lower abdominal pain, usually on one side, at mid-cycle due to irritation of the peritoneum by follicular fluid released at ovulation.
• Clinical significance: A useful clinical symptom to identify fertile window.

• Under estrogen influence just before ovulation, cervical mucus becomes profuse, clear, slippery (spinnbarkeit - can be stretched into a thread), and shows a fern pattern on drying (ferning/arborization).
• After ovulation, under progesterone influence, mucus becomes thick, scanty, and viscous.
• Clinical significance: Used in the Billings ovulation method and infertility evaluation.

• The preovulatory rise in estrogen leads to a proliferative endometrium. After ovulation, under progesterone, the endometrium becomes secretory.
• Clinical significance: Endometrial biopsy showing secretory changes confirms ovulation (now replaced by hormone assays).

• Serum progesterone measured on day 21 of the cycle (mid-luteal phase) >5 ng/mL (ideally >10 ng/mL) confirms ovulation.
• Serial estradiol and LH levels can track follicular development and predict ovulation.
• Clinical significance: Used in infertility workup, assisted reproductive technologies (ART), and monitoring ovulation induction.

• Follicle monitoring (folliculometry) tracks dominant follicle growth. Ovulation is confirmed by collapse of the follicle (from ~18-22 mm to absent/irregular) and appearance of free fluid in the pouch of Douglas.
• Clinical significance: Gold standard monitoring tool in IVF and ovulation induction protocols.

---

2. Methods of Contraception: Temporary, Physiological Basis of Oral Contraceptives, Safe Period

1. Suppression of ovulation: Estrogen and progesterone in the pill, when administered in appropriate quantities during the first half of the cycle, prevent the preovulatory surge of LH from the anterior pituitary. This is because the combined hormones block the positive feedback effect of rising estrogen on the pituitary that normally triggers the LH surge. Without this surge, ovulation cannot occur.
2. Alteration of cervical mucus: Progestins make cervical mucus thick and viscous, preventing sperm penetration.
3. Alteration of endometrium: The endometrium becomes atrophic and unreceptive to implantation.
4. Fallopian tube motility: Altered motility reduces the chances of fertilization.

• The pill contains synthetic estrogens (ethinyl estradiol or mestranol) and synthetic progestins (norethindrone, norethynodrel, ethynodiol, norgestrel).
• Synthetic forms are used because natural hormones are almost entirely destroyed by the liver (first-pass metabolism) when absorbed from the GI tract into the portal circulation. Synthetic analogues resist hepatic degradation.
• The pill is typically started early in the cycle and stopped after the expected ovulation date, allowing menstruation to occur.

• Based on the fact that the ovum can be fertilized for only 24-48 hours after ovulation, and sperm can survive up to 3-5 days in the female genital tract.
• Ovulation normally occurs 14 days before the next menstruation (not 14 days after the last - important distinction).
• Intercourse should be avoided for 4 days before and 3 days after the calculated day of ovulation.
• This method can only be used reliably in women with regular menstrual cycles.
• Failure rate: Up to 20-25% per year, making it the least reliable of contraceptive methods.
• The Billings method (monitoring cervical mucus changes) is a related physiological method.

---

3. Spermatogenesis: Stages and Factors Affecting It

• Primordial germ cells migrate into the testes during embryonic development and become spermatogonia, which lie in 2-3 layers on the inner surface of the seminiferous tubules.
• At puberty, spermatogonia begin to undergo mitotic division, continually proliferating.
• Spermatogonia migrate through the tight junctions of the Sertoli cell barrier (blood-testis barrier) toward the central lumen.
• Sertoli cells provide nutritional support and envelope the developing spermatogonia.

• As spermatogonia cross the blood-testis barrier, they are progressively modified and enlarged to form large primary spermatocytes (46 chromosomes, 2n).

• Each primary spermatocyte undergoes meiosis I (reductional division) to form two secondary spermatocytes (23 chromosomes, 1n).
• This is the point where genetic diversity is generated through crossing over.

• Secondary spermatocytes undergo meiosis II (equational division) to form spermatids (23 chromosomes, 1n).
• Each primary spermatocyte gives rise to 4 spermatids.

• Spermatids undergo morphological transformation into spermatozoa (no further cell division).
• Changes include: formation of acrosome (from Golgi apparatus), condensation of the nucleus, formation of the flagellum (tail), loss of most cytoplasm.

• Spermatozoa released into the lumen pass to the epididymis (6 m long coiled tube) where they undergo further maturation and gain motility over 10-14 days.

• Sperm carry either an X or Y chromosome. Fertilization of the ovum by an X-bearing sperm yields a female (XX); by a Y-bearing sperm yields a male (XY).

---

4. Physiological Changes in Pregnancy

• The uterus increases from ~50 g to ~1100 g due to hypertrophy and hyperplasia of myometrial cells.
• The vagina enlarges, breasts approximately double in size, and the introitus opens more widely.
• The placenta takes over progesterone and estrogen production from the corpus luteum by weeks 7-12.

• hCG: Rises from week 2, peaks at 10-12 weeks, maintains corpus luteum and prevents menstruation.
• Progesterone: Rises progressively; maintains uterine quiescence, thickens cervical mucus, prepares alveoli.
• Estrogens: Rise steadily; stimulate uterine growth, ductal breast development, and oxytocin receptor expression.
• Human Chorionic Somatomammotropin (hCS/hPL): Has growth hormone-like effects, promotes fat utilization, and is mildly lactogenic.
• Relaxin: Secreted by corpus luteum and placenta; relaxes pelvic ligaments and softens the cervix.
• Thyroid hormones: Basal metabolic rate rises ~15% in the latter half of pregnancy.
• Adrenocortical hormones: Increase; glucocorticoids help mature fetal lungs (surfactant).
• Aldosterone: Rises markedly, increasing sodium and water retention.

• Blood volume increases by ~30-40% (mainly plasma), causing dilutional anemia (physiological anemia of pregnancy).
• Cardiac output increases ~30-40%.
• Heart rate rises ~10-15 bpm.
• Blood pressure may fall slightly in the second trimester due to reduced peripheral resistance from progesterone.

• Tidal volume increases (due to progesterone stimulating the respiratory center).
• Respiratory rate may increase slightly.
• Total ventilation increases, leading to a mild respiratory alkalosis.
• The growing uterus elevates the diaphragm, reducing functional residual capacity.

• Glomerular filtration rate (GFR) and renal blood flow increase by ~50%.
• Glucose may spill into urine (physiological glycosuria) even with normal blood glucose.
• Urinary frequency is common due to pressure from the enlarged uterus.
• Mild dilation of renal pelvis and ureters (due to progesterone relaxing smooth muscle and mechanical compression).

• BMR increases ~15% in the latter half.
• Weight gain: Average 25-35 pounds total - fetus (~8 lb), placenta + amniotic fluid (~4 lb), uterus (~3 lb), breasts (~2 lb), blood and extracellular fluid (~5 lb), fat accumulation (~3-13 lb).
• Insulin resistance increases in late pregnancy (due to hCS, cortisol, progesterone) - diabetogenic state.
• Fat is stored in early pregnancy; in late pregnancy, fat is mobilized for maternal energy, sparing glucose for the fetus.
• Iron requirement increases (fetus needs ~375 mg; mother needs ~600 mg extra for her red cells).
• Calcium and phosphate needs increase for fetal bone development.

• Nausea and vomiting (morning sickness) in first trimester, likely due to hCG.
• Constipation due to reduced GI motility (progesterone effect).
• Gastroesophageal reflux increases.
• Increased appetite due to fetal demands.

---

5. Maternal Changes in the Last Trimester of Pregnancy

• The fetus gains the most weight in the last trimester - its weight nearly doubles in the last 2 months.
• The mother cannot absorb sufficient protein, calcium, phosphates, and iron from her diet alone during these months to supply fetal needs.
• In anticipation, the mother's body stores these substances earlier in pregnancy - some in the placenta, most in maternal storage depots.
• Calcium absorption requires vitamin D; deficiency causes fetal rickets.
• Vitamin K is often supplemented near term to ensure adequate prothrombin in the neonate, preventing hemorrhagic disease of the newborn (especially intracranial hemorrhage during birth).

• Blood volume and cardiac output are at their peak.
• The gravid uterus may compress the inferior vena cava when the mother lies supine (supine hypotension syndrome), reducing venous return and cardiac output.
• The mother is advised to lie in the left lateral decubitus position.
• Dependent edema of the ankles and lower legs is common due to venous compression.

• The enlarging uterus pushes the diaphragm upward, reducing lung volume.
• The mother may experience dyspnea on exertion.
• Total ventilation increases due to progesterone stimulation of the respiratory center.

• GFR remains elevated; urinary frequency is more pronounced due to fetal head engagement pressing on the bladder.
• Protein may appear in urine; significant proteinuria with hypertension raises concern for pre-eclampsia.

• The pubic symphysis and sacroiliac joints soften under relaxin, increasing pelvic diameter.
• Lumbar lordosis increases as the center of gravity shifts, commonly causing low back pain.

• Toward the end of the third trimester, the uterus becomes progressively more excitable (Braxton Hicks contractions become more frequent and stronger).
• The estrogen-to-progesterone ratio increases as progesterone stabilizes while estrogen continues to rise, increasing uterine sensitivity to oxytocin.
• The number of oxytocin receptors in uterine muscle increases markedly near term.
• Prostaglandins released by fetal membranes further increase uterine excitability.

• Under the continued influence of estrogen and progesterone, the ductal and alveolar system of the breasts reaches full development.
• Prolactin levels rise to 10-20 times non-pregnant values by term, but milk secretion is suppressed by the high estrogen and progesterone.
• Colostrum (protein-rich, immunoglobulin-rich pre-milk) begins to be secreted.

• Most of the 25-35 pound total weight gain has occurred by this stage.
• About 5 pounds represents extra fluid in blood and extracellular space; 3-13 pounds is fat accumulation.
• Peripheral edema is physiological and common.

---

6. Mechanism of Ovulation; Contraception - Rhythm Method

• From day 1-13, rising FSH stimulates follicular growth; one dominant follicle (1-1.5 cm) reaches maturity. Estrogen produced by the dominant follicle exerts positive feedback on the anterior pituitary.

• About 2 days before ovulation, LH secretion rises 6-10 fold, peaking about 16 hours before ovulation.
• FSH rises 2-3 fold simultaneously.
• LH acts on granulosa and theca cells, converting them to progesterone-secreting cells. Estrogen secretion begins to fall ~1 day before ovulation while progesterone begins to rise.
• Without the preovulatory LH surge, ovulation cannot occur.

• LH triggers secretion of follicular steroid hormones containing progesterone.
• Within hours, two events occur:
  • Proteolytic enzymes (from lysosomes in the theca externa) are released, causing dissolution of the follicular capsular wall, weakening it, and causing degenerative changes in the stigma.
  • Rapid angiogenesis (new blood vessel growth) occurs in the follicle wall, and prostaglandins (causing vasodilation) are secreted into follicular tissues.

• These two effects cause plasma transudation into the follicle, increasing intrafollicular pressure.
• The combination of follicle swelling and degeneration of the stigma leads to follicle rupture.
• The ovum, surrounded by the corona radiata (several thousand granulosa cells), is discharged into the peritoneal cavity.
• The fimbriae of the fallopian tube sweep the ovum into the tube.

• The ovum survives and is fertilizable for only about 24-48 hours after ovulation.
• Sperm survive in the female genital tract for up to 3-5 days.
• Ovulation occurs 14 days before the next expected menstruation.

• The fertile window extends from 4 days before ovulation to 3 days after ovulation (to account for sperm survival and ovum lifespan).
• Intercourse is avoided during this period.
• In a regular 28-day cycle, this means avoiding days 10-17.

• BBT charting (ovulation confirmed by post-ovulatory temperature rise)
• Cervical mucus monitoring (Billings method)
• Ovulation predictor kits (LH detection)

• Failure rate: 20-25% per year
• Only reliable in women with regular menstrual cycles.
• Illness, stress, travel, and medication can shift ovulation timing.
• Difficult to apply in women with irregular cycles.

---

7. Menstrual Cycle - Phases

• Ovarian cycle (changes in the ovary)
• Endometrial/Uterine cycle (changes in the uterus)

---

A. OVARIAN CYCLE

• Under the influence of FSH (secreted by anterior pituitary), primordial follicles are recruited and undergo maturation.
• Granulosa cells proliferate around the oocyte; the follicle enlarges and develops an antrum (antral/Graafian follicle).
• Rising estrogen from the growing follicle:
  • Causes proliferation of endometrial stroma and glands
  • Exerts negative feedback on FSH (suppresses further follicle recruitment)
  • Eventually exerts positive feedback on the pituitary, triggering the LH surge
• One dominant follicle (1-1.5 cm) is selected; the rest undergo atresia.

• Triggered by the LH surge (6-10 fold rise).
• The mature follicle ruptures and releases the ovum (see Mechanism of Ovulation above).
• The ovum is swept into the fallopian tube by the fimbriae.

• After ovulation, the ruptured follicle collapses; granulosa and theca interna cells are rapidly converted into lutein cells (luteinization).
• This forms the corpus luteum (yellow body), which secretes large amounts of progesterone and estrogens under LH influence.
• The corpus luteum reaches maximum size about 7-8 days after ovulation.
• Progesterone from the corpus luteum:
  • Converts the endometrium to secretory phase
  • Inhibits uterine contractility
  • Thickens cervical mucus
  • Raises BBT
• The corpus luteum secretes inhibin, which suppresses FSH and LH.
• If fertilization does not occur, the corpus luteum involutes (degenerates) about 12 days after ovulation due to withdrawal of LH.
• Falling progesterone and estrogen levels lead to menstruation.

---

B. ENDOMETRIAL (UTERINE) CYCLE

• The sudden withdrawal of estrogen and progesterone (corpus luteum involution) causes vasoconstriction of spiral arterioles, ischemia, and necrosis of the endometrium.
• The functional layer (stratum functionalis, ~5 mm thick) is shed along with ~40-80 mL of blood and tissue.
• The basal layer (stratum basalis) remains intact and serves as the source of regeneration.

• Rising estrogen from the growing follicle stimulates:
  • Regeneration of the endometrium from the basal layer
  • Proliferation of endometrial glands and stroma
  • Increase in endometrial thickness from ~0.5 mm to ~5 mm
  • Increased ciliated cells and activity in the fallopian tubes
• The endometrial glands become straight and narrow.

• After ovulation, progesterone from the corpus luteum converts the endometrium into a secretory organ.
• Endometrial glands become tortuous/coiled (corkscrew glands) and begin to secrete glycogen-rich fluid.
• Stroma becomes edematous and decidua-like.
• Blood supply via spiral arterioles increases.
• The endometrium is now maximally prepared for implantation of the blastocyst (occurs ~6-7 days after ovulation = day 20-21 of cycle).
• If implantation occurs, hCG from the trophoblast rescues the corpus luteum.
• If not, corpus luteum involutes → progesterone falls → menstruation begins.

---

8. Circulatory Changes at Birth

• Fetal lungs are non-functional (filled with fluid). The placenta serves as the organ of gas exchange.
• Oxygenated blood from the placenta reaches the fetus via the umbilical vein → enters the liver/ductus venosus → inferior vena cava → right atrium.
• Due to high pulmonary vascular resistance (hypoxic pulmonary vasoconstriction in unexpanded lungs), most blood bypasses the lungs via:
  • Foramen ovale: Opening between right and left atria allows blood to pass from right atrium directly to left atrium.
  • Ductus arteriosus: Connects pulmonary artery to the aorta, shunting blood away from pulmonary circulation to systemic circulation.
  • Ductus venosus: Bypasses hepatic circulation (closes after birth).
• Deoxygenated blood returns to the placenta via two umbilical arteries.

• The first breath inflates the lungs, dramatically reduces pulmonary vascular resistance, and increases pulmonary blood flow 5-fold.
• Pulmonary arterioles dilate due to: (a) mechanical expansion, (b) rise in PO₂, (c) fall in PCO₂, (d) endothelin and nitric oxide release.

• With the onset of breathing, pulmonary blood flow increases massively → more blood returns to the left atrium via pulmonary veins → left atrial pressure rises above right atrial pressure.
• This pressure reversal pushes the valve of the foramen ovale against the interatrial septum, causing functional closure within minutes to hours.
• Anatomical closure (fusion) occurs by age 1 year in most individuals.
• In ~25-30% of adults, the foramen ovale remains patent (PFO) but causes no clinical problem as long as left atrial pressure exceeds right atrial pressure.

• The rise in arterial PO₂ after birth causes intense vasoconstriction of the ductus arteriosus smooth muscle.
• Functional closure occurs within 1-8 hours after birth.
• Anatomical closure (fibrous obliteration) occurs within 1-4 months, converting it to the ligamentum arteriosum.
• Prostaglandins (PGE₂) maintain ductal patency in fetal life; non-steroidal anti-inflammatory drugs (indomethacin) can be used to close a patent ductus arteriosus (PDA) pharmacologically.
• If the ductus remains patent (PDA), a left-to-right shunt develops, increasing pulmonary blood flow.

• Clamping of the umbilical cord stops portal flow, and the ductus venosus closes within a few days.
• It becomes the ligamentum venosum.

• Umbilical arteries and vein constrict and thrombose after cord clamping.
• Umbilical arteries become medial umbilical ligaments; umbilical vein becomes the ligamentum teres hepatis.

---

9. Parturition: Role of Oxytocin

1. Progressive hormonal changes
2. Progressive mechanical changes

---

• Estrogens increase uterine contractility (partly by increasing gap junctions between uterine smooth muscle cells, facilitating electrical coupling).
• Progesterone inhibits uterine contractility throughout pregnancy.
• From the 7th month onward, estrogen secretion continues to rise while progesterone remains constant or even decreases slightly.
• This rising estrogen-to-progesterone ratio makes the uterus progressively more excitable and more sensitive to stimuli (including oxytocin).

1. Increased oxytocin receptors: The uterine muscle markedly increases its number of oxytocin receptors during the latter few months of pregnancy. This increased receptor density means the uterus becomes progressively more sensitive to any given dose of oxytocin. The 100-fold increase in oxytocin receptors near term is one of the critical events that initiates labor.
2. Increased oxytocin secretion: The rate of oxytocin secretion by the neurohypophysis is considerably increased at the time of labor.
3. Prolonged labor after hypophysectomy: Although hypophysectomized animals can still deliver their young at term, labor is significantly prolonged - indicating that while not absolutely essential, oxytocin makes labor more efficient and timely.
4. Ferguson's Reflex (Cervical reflex): Irritation or stretching of the uterine cervix during labor sends neurogenic signals through the paraventricular and supraoptic nuclei of the hypothalamus → increased oxytocin release from the posterior pituitary → stronger uterine contractions → more cervical stretch → more oxytocin. This positive feedback loop (Ferguson reflex) is critical for the progressive intensification of labor until delivery.

• The fetal pituitary secretes increasing oxytocin (contributes to uterine stimulation).
• Fetal adrenal glands secrete large quantities of cortisol (uterine stimulant and induces surfactant in fetal lungs).
• Fetal membranes release prostaglandins in high concentration at the time of labor, further intensifying uterine contractions.

---

• As the fetus grows, the uterine muscle is progressively stretched. Stretched smooth muscle tends to be more contractile.
• Intermittent fetal movements also stimulate uterine contractions.
• Evidence: Twins are born on average 19 days earlier than single babies, emphasizing the role of mechanical stretch.

• Stretching or irritating the uterine cervix greatly increases uterine contractility, partly through direct reflexes from the cervix to the body of the uterus.

---

• First stage: Progressive cervical dilation (0 → 10 cm) through uterine contractions; most prolonged stage.
• Second stage: Expulsion of the fetus (delivery).
• Third stage: Delivery of the placenta (within 30 minutes).

• Oxytocin (Syntocinon) is used clinically to induce or augment labor.
• It is also used to control postpartum hemorrhage (causes uterine involution).
• Misoprostol (prostaglandin E₁ analogue) is used similarly for cervical ripening and labor induction.

---

10. Lactation: Role of Prolactin and Oxytocin

---

• Estrogen: Stimulates growth of the ductal system and fat deposition in the stroma.
• Progesterone: Required for lobule-alveolar development; causes budding of alveoli and secretory differentiation of alveolar cells (acting synergistically with estrogen, growth hormone, prolactin, and glucocorticoids).
• Growth hormone, prolactin, glucocorticoids, insulin: All contribute to ductal and alveolar development.

---

• Prolactin levels rise steadily from the 5th week of pregnancy, reaching 10-20 times the normal non-pregnant level at term.
• Despite high prolactin levels, milk secretion during pregnancy is inhibited because estrogen and progesterone directly suppress milk secretion at the alveolar cell level (even though they are needed for breast development).
• The placenta also secretes human chorionic somatomammotropin (hCS/hPL) which has weak lactogenic properties and supports prolactin action.
• Only a few mL of fluid are secreted per day before birth; this is colostrum - protein-rich, contains maternal IgA immunoglobulins important for neonatal passive immunity.

• Delivery of the placenta causes a sudden dramatic fall in estrogen and progesterone.
• With the removal of the suppressive effect of estrogen and progesterone, prolactin can now exert its full lactogenic action.
• Milk secretion begins fully within 2-3 days postpartum.

• Binds to receptors on alveolar epithelial cells.
• Stimulates transcription and synthesis of milk proteins (casein, lactalbumin, lactoglobulin).
• Stimulates lactose synthesis.
• Stimulates milk fat synthesis.

• During nursing, mechanoreceptors in the nipple and areola send afferent signals via the spinal cord to the hypothalamus.
• These signals inhibit the release of dopamine (prolactin-inhibiting factor, PIF) from the hypothalamus.
• Reduced dopamine → increased prolactin secretion from the anterior pituitary.
• If the mother does not breastfeed, prolactin levels fall to normal within a week.
• Frequent suckling maintains high prolactin → sustained milk production.

• High prolactin inhibits GnRH pulsatility and gonadotropin release, suppressing ovulation.
• This provides natural, temporary contraception (lactational amenorrhea method).

---

1. Suckling or stimulation of the nipple → mechanoreceptor afferents → spinal cord → hypothalamus.
2. Hypothalamic nuclei (paraventricular and supraoptic) are activated → oxytocin is released from the posterior pituitary into the bloodstream.
3. Oxytocin acts on the myoepithelial cells surrounding the alveoli and small ducts of the breast.
4. Myoepithelial cells contract → milk is squeezed from the alveoli into the larger lactiferous ducts and sinuses → nipple.
5. This process occurs within 30-60 seconds of suckling (or even in anticipation of suckling - conditioned reflex).

• Emotional stress, pain, or anxiety can inhibit oxytocin release and block milk ejection.
• A calm, comfortable environment facilitates the reflex.
• Hearing the baby's cry or thinking about the baby can trigger milk let-down (conditioned reflex via the cerebral cortex).

• Human milk contains: lactose (principal carbohydrate), fat (main caloric source), proteins (casein, whey), calcium, phosphate, and vitamins.
• Milk secretion causes a major drain on the mother's nutritional reserves.
• Daily milk production can be 1500-2000 mL in well-nourished mothers.

---

Source: All notes are based on Guyton and Hall Textbook of Medical Physiology, Chapters 81, 82, 83, and 84.