Class 12 Biology · NCERT chapter 3 · 8 topics · free to read
Male Reproductive System
The male reproductive system consists of a pair of testes (housed in the scrotum), accessory ducts (rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct, urethra), accessory glands (seminal vesicles, prostate, bulbourethral glands), and external genitalia (penis). Testes produce sperm and testosterone.
Testes are located in the scrotum (2-2.5°C below body temperature) to maintain the lower temperature essential for spermatogenesis. Each testis has ~250 compartments called testicular lobules.
The testes are a pair of oval-shaped organs located outside the abdominal cavity in a pouch called the scrotum. The scrotal temperature is 2–2.5°C lower than body temperature — this is CRITICAL because spermatogenesis cannot occur at 37°C.
Each testis is 4–5 cm long, surrounded by a dense white fibrous capsule called the tunica albuginea
Each testis has about 250 testicular lobules
Each lobule contains 1–3 highly coiled seminiferous tubules — the site of sperm production
Total length of seminiferous tubules in both testes: ~250 metres (yes, metres!)
Testes serve a dual function: sperm production (exocrine) + testosterone production (endocrine)
Inside the Seminiferous Tubules
Seminiferous tubules are lined with two types of cells: germ cells (spermatogonia) that produce sperm, and Sertoli cells that nourish and support the developing sperm.
Each seminiferous tubule is lined internally by two types of cells: (1) Spermatogonia (male germ cells) that undergo spermatogenesis to produce sperm, and (2) Sertoli cells (also called sustentacular cells or nurse cells) that provide nutrition and support to developing sperm.
Spermatogonia are diploid (2n) male germ cells that line the inner wall of seminiferous tubules
Sertoli cells provide nourishment, protection, and structural support to developing sperm
Sertoli cells also secrete inhibin (negative feedback on FSH) and Androgen-Binding Protein (ABP)
Sertoli cells form the blood-testis barrier — protects developing sperm from immune attack
The region outside the seminiferous tubules is called interstitial space — contains Leydig cells
Leydig Cells
Leydig cells (interstitial cells) are present in the spaces between seminiferous tubules. They synthesize and secrete testosterone under the influence of LH from the anterior pituitary.
Leydig cells (also called interstitial cells) are present in the interstitial spaces between seminiferous tubules. Their primary function is to synthesize and secrete androgens, the most important being testosterone.
Leydig cells are stimulated by LH (Luteinizing Hormone) from the anterior pituitary
They secrete testosterone — the principal male sex hormone
Testosterone is essential for: spermatogenesis, development of secondary sexual characters, sex drive, muscle mass
Testosterone also acts in a feedback loop: high testosterone → inhibits GnRH and LH release
Accessory Ducts
Sperm travel through a series of ducts: rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra. Each segment has a specific function in sperm transport and maturation.
After production in seminiferous tubules, sperm travel through a series of ducts before ejaculation. The pathway is: Seminiferous tubules → Rete testis → Vasa efferentia → Epididymis → Vas deferens → Ejaculatory duct → Urethra.
Rete testis — network of tubules in the testis that collects sperm from seminiferous tubules
Vasa efferentia — ~12–20 small ducts that carry sperm from rete testis to epididymis
Epididymis — 6-metre coiled tube along the posterior surface of testis; site of sperm maturation and storage
Vas deferens (ductus deferens) — long muscular tube that ascends from epididymis, passes through inguinal canal into abdomen
Ejaculatory duct — formed by union of vas deferens + duct of seminal vesicle; opens into urethra
Urethra — shared passage for urine and semen (but never simultaneously); exits through penis
Accessory Glands
Three sets of glands contribute secretions to form semen: seminal vesicles (~60% of semen volume), prostate gland (~30%), and bulbourethral/Cowper's glands (pre-ejaculatory fluid).
The male accessory glands secrete fluids that together with sperm form semen (seminal plasma + sperm). These secretions provide nutrition, activate sperm, and create a suitable pH for sperm survival in the acidic female reproductive tract.
Seminal vesicles (paired) — contribute ~60% of semen volume; secrete fructose (energy for sperm), prostaglandins, and fibrinogen
Prostate gland (single) — contributes ~30% of semen volume; secretes slightly alkaline fluid with citric acid, lipids, enzymes, and zinc
Bulbourethral/Cowper's glands (paired) — secrete a clear, mucus-like pre-ejaculatory fluid that lubricates urethra and neutralizes urine acidity
Normal semen volume per ejaculation: 2–5 mL, containing 200–300 million sperm
The alkaline secretions of these glands neutralize the acidic pH of the female vaginal tract, protecting sperm
External Genitalia
The penis is the male copulatory organ made of erectile tissue. It has a root, body (shaft), and glans covered by foreskin. Erection occurs when erectile tissue fills with blood.
The penis is the male external genital organ that serves as both the copulatory organ and the passage for urine. It is made of special erectile tissue — spongy tissue with blood sinuses that can engorge with blood to cause erection.
Penis contains 3 columns of erectile tissue: 2 corpora cavernosa (dorsal) + 1 corpus spongiosum (ventral, surrounds urethra)
Glans penis — the enlarged, sensitive tip of the penis
Prepuce (foreskin) — loose skin covering the glans; sometimes removed by circumcision
Erection occurs when parasympathetic stimulation causes blood to fill the sinuses of erectile tissue
The urethra running through the corpus spongiosum serves as the common passage for both urine and semen
NCERT mentions the penis briefly but NEET occasionally tests the erectile tissue names. Know that corpora cavernosa (2, dorsal) and corpus spongiosum (1, ventral with urethra) exist.
Spermatogenesis
The process of sperm formation from spermatogonia in seminiferous tubules. Involves three phases: multiplicative (mitosis), growth, and maturation (meiosis). Spermatogonia (2n) → primary spermatocytes (2n) → secondary spermatocytes (n) → spermatids (n) → spermatozoa. Controlled by GnRH, FSH, LH, and testosterone.
Spermatogenesis begins at puberty and continues throughout life. It occurs in the seminiferous tubules under hormonal control and takes about 74 days for one complete cycle.
Spermatogenesis is the process by which spermatogonia (diploid male germ cells) are transformed into mature spermatozoa (haploid sperm). It begins at puberty under the influence of GnRH and continues virtually throughout life.
Occurs in seminiferous tubules of the testes
Begins at puberty and continues into old age
Involves mitosis (multiplication), meiosis (reduction division), and differentiation (spermiogenesis)
Each spermatogonium ultimately produces 4 functional spermatozoa
Entire process takes approximately 74 days (64 days spermatogenesis + 10 days spermiogenesis)
Unlike oogenesis (which starts before birth and has long arrest periods), spermatogenesis is a continuous, uninterrupted process from puberty onward.
Stages of Spermatogenesis
Three sequential phases: (1) Multiplicative phase — spermatogonia multiply by mitosis; (2) Growth phase — spermatogonia enlarge into primary spermatocytes; (3) Maturation phase — meiosis I and II produce spermatids.
Spermatogenesis proceeds through three phases, converting diploid spermatogonia into haploid spermatids. The spermatids then undergo morphological transformation (spermiogenesis) to become functional sperm.
Phase 1 — Multiplicative phase: Spermatogonia (2n = 46) divide repeatedly by mitosis to increase their number. Some cells remain as stem cells, others proceed to the next phase
Phase 2 — Growth phase: Some spermatogonia grow in size, accumulate nutrients → become primary spermatocytes (2n = 46). This is the largest cell in spermatogenesis
Phase 3 — Maturation phase (Meiosis):
Primary spermatocyte (2n) → Meiosis I → 2 secondary spermatocytes (n = 23, each with 2 chromatids)
Each secondary spermatocyte → Meiosis II → 2 spermatids (n = 23)
Net result: 1 primary spermatocyte → 4 spermatids (all are equal in size and functional)
Spermatogonium mitosis Primary spermatocyte (2n) meiosis I 2 Secondary spermatocytes (n) meiosis II 4 Spermatids (n)
Spermiogenesis & Spermiation
Spermiogenesis is the transformation of spermatids into spermatozoa (no cell division involved). Spermiation is the release of mature sperm from Sertoli cells into the lumen of seminiferous tubules.
Spermatids are round, non-motile cells — they look nothing like sperm. The transformation of spermatids into slender, motile spermatozoa is called spermiogenesis. This involves dramatic morphological changes but NO further cell division.
Spermiogenesis = transformation of spermatids → spermatozoa (morphological change, NOT cell division)
Changes during spermiogenesis: nucleus condenses, acrosome forms from Golgi, mitochondria spiral around middle piece, tail develops from centriole
Spermiation = release of mature spermatozoa from Sertoli cells into the lumen of seminiferous tubules
Released sperm are structurally mature but NOT yet capable of fertilization
Sperm gain motility in the epididymis and fertilizing capacity in the female tract (capacitation)
Hormonal Control
Spermatogenesis is controlled by the hypothalamo-hypophyseal-gonadal axis. GnRH from hypothalamus triggers FSH and LH release from anterior pituitary. LH acts on Leydig cells (testosterone), FSH acts on Sertoli cells.
Spermatogenesis is under tight hormonal regulation via the HPG axis (Hypothalamus → Pituitary → Gonads). At puberty, the hypothalamus begins pulsatile secretion of GnRH, which triggers the entire cascade.
Hypothalamus releases GnRH (Gonadotropin-Releasing Hormone) in pulses
GnRH stimulates anterior pituitary to secrete FSH + LH (gonadotropins)
Both FSH and testosterone are needed for spermatogenesis — neither alone is sufficient
Sperm Structure
A mature human spermatozoon is about 60 μm long with four distinct regions: head (contains nucleus + acrosome), neck (contains centrioles), middle piece (packed with mitochondria — the powerhouse), and tail (flagellum for motility). Each part has a specific function in reaching and fertilizing the ovum.
The head is the most important part of the sperm. It contains the highly condensed haploid nucleus (23 chromosomes) covered anteriorly by the acrosome — a cap-like structure filled with hydrolytic enzymes for egg penetration.
The sperm head is oval-shaped and flattened, roughly 4–5 μm long. It contains two critical structures: the nucleus with tightly packed haploid DNA (23 chromosomes), and the acrosome — a cap-like structure covering the anterior 2/3 of the head.
Nucleus — contains 23 chromosomes (haploid); DNA is extremely condensed with protamines (replacing histones)
Acrosome — derived from the Golgi apparatus during spermiogenesis
These enzymes digest the zona pellucida and corona radiata of the ovum during fertilization
Without a functional acrosome, sperm CANNOT fertilize the egg — this is the basis of the acrosomal reaction
Neck & Middle Piece
The neck contains the proximal and distal centrioles. The middle piece is packed with mitochondria arranged in a spiral (mitochondrial sheath) that generates ATP for tail movement.
The neck is a short connecting segment between head and middle piece. It contains a proximal centriole (which enters the egg and forms the first mitotic spindle of the zygote) and a distal centriole (which gives rise to the axial filament of the tail).
Proximal centriole — enters the ovum during fertilization; forms the first cleavage spindle of the zygote
Distal centriole — gives rise to the axoneme (core of the tail)
Middle piece (~5–7 μm) — contains tightly packed mitochondria arranged in a spiral called the mitochondrial sheath
Mitochondria generate ATP — the energy currency that powers tail movement
Middle piece is often called the 'powerhouse of the sperm'
Tail (Flagellum)
The tail is the longest part (~45 μm) and generates the whip-like movement that propels sperm through the female reproductive tract at about 1–3 mm per minute.
The tail (flagellum) is the longest part of the sperm (~45 μm) and provides the propulsive force for movement. It contains the axoneme — a core of microtubules arranged in the classic 9+2 pattern (9 doublet microtubules surrounding 2 central singlets).
Tail has the 9+2 axonemal arrangement of microtubules (same as cilia and flagella)
Tail movement is powered by ATP from middle piece mitochondria
Sperm swim at about 1–3 mm per minute in the female tract
The tail is divided into: principal piece (longest, with fibrous sheath) and end piece (short, only axoneme)
Sperm motility is essential for traversing the ~15–18 cm distance from vagina to ampulla of fallopian tube
Quick recall
What are the four parts of a sperm in order from front to back?
Head (acrosome + nucleus) → Neck (centrioles) → Middle piece (mitochondria) → Tail (flagellum). Total length ~60 μm.
Sperm Plasma Membrane
The entire sperm (head, neck, middle piece, and tail) is enclosed by a single continuous plasma membrane. This membrane plays a critical role in capacitation and the acrosomal reaction during fertilization.
An important NCERT-stated fact: the entire sperm is enveloped by a single, continuous plasma membrane. This membrane is crucial during fertilization — specific receptor proteins on the sperm membrane interact with glycoproteins (ZP3) on the egg's zona pellucida.
One continuous plasma membrane covers head + neck + middle piece + tail
Sperm membrane contains receptors that recognize ZP3 glycoprotein on the zona pellucida of the ovum
During capacitation in the female tract, the membrane undergoes changes that enable the acrosomal reaction
Human sperm can survive in the female tract for up to 72 hours (3 days)
After release, a sperm must undergo capacitation to become fertilization-competent
NCERT explicitly states the plasma membrane detail. While it sounds trivial, NEET has tested this — any assertion question on sperm structure could include this as a true/false option.
Female Reproductive System
The female reproductive system consists of a pair of ovaries (primary sex organs), fallopian tubes (oviducts), uterus, cervix, vagina, external genitalia (vulva), and mammary glands. Ovaries produce ova and secrete estrogen and progesterone.
Ovaries follicles Ova + theca/granulosa Estrogen, Progesterone
Ovaries
Ovaries are paired, almond-shaped organs located in the pelvic cavity. They are the female gonads with dual function: production of ova (oogenesis) and secretion of female sex hormones (estrogen, progesterone).
The ovaries are a pair of almond-shaped organs (~2–4 cm long) located one on each side of the lower abdomen, attached to the uterus by the ovarian ligament and to the pelvic wall by the mesovarium (part of the broad ligament).
Each ovary is covered by a thin layer of germinal epithelium (not truly germinal — name is historical)
Below the epithelium is a dense connective tissue layer called the tunica albuginea
Ovarian cortex — outer region containing ovarian follicles at various stages of development
Ovarian medulla — inner region with blood vessels, nerves, and connective tissue
Each ovary contains approximately 60,000–80,000 primary follicles at puberty (down from ~2 million at birth)
Only about 400–450 follicles will actually ovulate during a woman's reproductive lifetime
Fallopian Tubes (Oviducts)
The fallopian tubes are paired muscular tubes (~10–12 cm) connecting the ovary to the uterus. They have four parts: infundibulum (with fimbriae), ampulla, isthmus, and uterine part. Fertilization occurs in the ampulla.
The fallopian tubes (oviducts) are muscular tubes about 10–12 cm long. They don't physically connect to the ovaries — the infundibulum opens near the ovary and its finger-like projections (fimbriae) catch the released ovum.
Infundibulum — funnel-shaped opening near the ovary, with finger-like fimbriae that collect the ovum after ovulation
Ampulla — wider middle part; site of fertilization (this is where sperm meets egg)
Isthmus — narrow part connecting to the uterus; has thick muscular walls
Uterine (interstitial) part — passes through the uterine wall; narrowest segment
Inner lining has ciliated epithelium — cilia help move the ovum/embryo toward the uterus
Ectopic pregnancy can occur if the embryo implants in the fallopian tube instead of the uterus — a medical emergency
Uterus, Cervix & Vagina
The uterus (womb) is a pear-shaped muscular organ where the embryo implants and develops. Its wall has three layers: perimetrium, myometrium, and endometrium. The cervix connects the uterus to the vagina.
The uterus is an inverted pear-shaped muscular organ, about 7.5 cm long, located in the pelvic cavity between the bladder and rectum. It is supported by ligaments and is the site of implantation and fetal development.
Endometrium undergoes cyclical changes during the menstrual cycle; site of implantation
Endometrium has two layers: stratum basale (permanent) and stratum functionalis (shed during menstruation)
Cervix — narrow lower part of the uterus that opens into the vagina; cervical canal is lined with mucus-secreting cells
Vagina — fibromuscular canal (~7–10 cm); serves as the birth canal, receives penis during intercourse, and passage for menstrual flow
Mammary Glands
Mammary glands are modified sweat glands present in both sexes but functional only in females after pregnancy. Each breast has 15–20 mammary lobes with alveoli that produce milk under prolactin stimulation.
Mammary glands are paired structures (one in each breast) that are accessory reproductive organs — not directly involved in reproduction but essential for nourishing the newborn. They are present in both sexes but functional only in females after childbirth.
Each breast has 15–20 mammary lobes containing clusters of cells called alveoli that secrete milk
Alveoli open into mammary tubules → mammary ducts → mammary ampulla → lactiferous duct → nipple
Prolactin (from anterior pituitary) stimulates milk production
Mammary glands are actually modified sweat glands (apocrine type) — an amazing evolutionary adaptation
NCERT includes mammary glands under the female reproductive system. Know the basic structure (lobes → alveoli → ducts → nipple) and hormonal control (prolactin = production, oxytocin = ejection).
External Genitalia (Vulva)
The external genitalia of the female are collectively called the vulva. They include the mons pubis, labia majora, labia minora, clitoris, and hymen.
The female external genitalia are collectively called the vulva. These structures protect the openings of the vagina and urethra.
Mons pubis — cushion of fatty tissue over the pubic bone, covered with pubic hair after puberty
Labia majora — fleshy outer folds of skin extending from mons pubis; homologous to the male scrotum
Labia minora — smaller inner folds, devoid of fat and hair; enclose the vestibule
Clitoris — small, sensitive erectile structure at the anterior junction of labia minora; homologous to the male penis
Hymen — thin membrane that partially covers the vaginal opening; may be torn during physical activity, intercourse, or examination
Vestibule — space enclosed by labia minora, contains openings of vagina, urethra, and Bartholin's glands
Oogenesis
Formation of the ovum from oogonia in the ovary. Begins during fetal life, pauses at the primary oocyte stage (prophase I) until puberty, then resumes cyclically. 1 oogonium → 1 functional ovum + 3 polar bodies. Markedly different from spermatogenesis in timing, output, and cytoplasm distribution.
Unlike spermatogenesis which starts at puberty, oogenesis begins during fetal development. The process has two major arrest points and is only completed if the egg is fertilized.
Oogenesis is the formation of a mature ovum (egg) from an oogonium. The process is remarkably different from spermatogenesis — it begins before birth, has two arrest points, and from one oogonium, only one functional egg is produced (not four like sperm).
Oogenesis begins during fetal development (3rd month of gestation)
Oogonia multiply by mitosis → then enter meiosis I → arrested at prophase I as primary oocytes
At birth: ~2 million primary oocytes (all in prophase I arrest), each surrounded by a layer of granulosa cells → primary follicle
By puberty: only ~60,000–80,000 remain (rest degenerate by atresia)
From puberty: each menstrual cycle, one primary oocyte resumes meiosis I → secondary oocyte (arrested at metaphase II)
Meiosis II is completed ONLY if the secondary oocyte is fertilized by a sperm
Stages of Oogenesis
Oogonia (2n) multiply by mitosis during fetal life → primary oocytes (arrested at prophase I) → secondary oocyte + first polar body (arrested at metaphase II) → ovum + second polar body (only upon fertilization).
Oogenesis has three phases — but unlike spermatogenesis, the cytoplasm is unequally divided at each meiotic division, producing one large egg and tiny polar bodies.
Multiplicative phase (fetal life): Oogonia (2n) multiply by mitosis to form millions of cells
Growth phase (fetal life): Oogonia enlarge, accumulate nutrients → become primary oocytes (2n); enter meiosis I but arrest at prophase I (dictyotene stage)
Maturation phase (puberty onward, cyclically):
Primary oocyte (2n) completes meiosis I → 1 large secondary oocyte (n) + 1 tiny first polar body (n)
Secondary oocyte begins meiosis II but arrests at metaphase II — this is what is ovulated
Upon fertilization: Secondary oocyte completes meiosis II → 1 ovum (n) + 1 second polar body (n)
First polar body may or may not divide → total polar bodies = 2 or 3 (all degenerate)
Oogonium (2n) mitosis Primary oocyte (2n) meiosis I Secondary oocyte (n) _ large + 1st polar body (n) _ tiny [at fertilization] meiosis II Ovum (n) _ functional + 2nd polar body (n) _ degenerates
Oogenesis vs Spermatogenesis
A detailed comparison showing how the two gametogenic processes differ in timing, output, arrest points, and cytoplasm distribution.
Begins at puberty
Begins during fetal life
Continuous process (no arrest)
Two arrest points (prophase I, metaphase II)
1 spermatogonium → 4 functional sperm
1 oogonium → 1 functional ovum + 3 polar bodies
Equal cytoplasm division
Unequal cytoplasm division
Completed within the testis
Completed only upon fertilization
Millions of sperm per day
~1 ovum per month
Continues into old age
Stops at menopause (~50 years)
All 4 products are functional
Only 1 of 4 products is functional
Occurs in seminiferous tubules
Occurs in ovarian follicles
Takes ~74 days per cycle
One primary oocyte can wait decades in arrest
Follicle Development
As the oocyte develops, the surrounding cells also develop into follicles: primordial → primary → secondary → tertiary (antral) → Graafian follicle. The Graafian follicle ruptures to release the secondary oocyte (ovulation).
The developing oocyte is always surrounded by a follicle — layers of granulosa and theca cells that nourish and protect it. As oogenesis progresses, the follicle also develops through distinct stages.
Primary follicle — primary oocyte + one layer of granulosa cells
Secondary follicle — primary oocyte + multiple layers of granulosa cells + theca layer forms outside
Tertiary (antral) follicle — fluid-filled cavity (antrum) appears in the granulosa layer
Graafian follicle — mature follicle; large antrum, oocyte surrounded by zona pellucida and corona radiata
After ovulation, the remaining follicle transforms into the corpus luteum (secretes progesterone)
Menstrual Cycle
The ~28-day reproductive cycle in human females involving coordinated changes in the ovary (ovarian cycle) and uterus (uterine cycle). Three phases: menstrual (days 1–5), follicular/proliferative (days 6–13), and luteal/secretory (days 15–28), with ovulation on ~day 14. Regulated by the interplay of FSH, LH, estrogen, and progesterone.
The menstrual cycle is the cyclical preparation of the uterus for pregnancy. If pregnancy doesn't occur, the endometrium sheds (menstruation) and the cycle restarts. It begins at puberty (menarche) and ends at menopause.
The menstrual cycle is a repeating ~28-day cycle (range: 21–35 days) involving coordinated changes in the ovary (follicle development → ovulation → corpus luteum) and uterus (endometrium thickening → shedding). It is regulated by the hypothalamus-pituitary-ovarian axis.
Average cycle length: 28 days (day 1 = first day of menstrual bleeding)
Menarche — first menstruation, typically at age 11–13 years
Menopause — cessation of menstruation, typically at age ~50 years
The cycle involves simultaneous changes in: ovary (ovarian cycle) and uterus (uterine cycle)
Three phases: Menstrual → Follicular/Proliferative → Luteal/Secretory
Ovulation occurs approximately on day 14 in a 28-day cycle
Menstrual Phase (Days 1–5)
The cycle begins with menstruation — shedding of the stratum functionalis of the endometrium along with blood and unfertilized ovum. Triggered by a drop in progesterone and estrogen levels when the corpus luteum degenerates.
Day 1 of the menstrual cycle is defined as the first day of menstrual bleeding. The menstrual phase lasts 3–5 days. During this phase, the stratum functionalis of the endometrium breaks down and is shed along with blood, mucus, and the unfertilized ovum.
Duration: 3–5 days (average blood loss: 50–80 mL)
Cause: fall in progesterone and estrogen levels due to degeneration of the corpus luteum
What is shed: stratum functionalis of endometrium + blood + unfertilized ovum + mucus
The stratum basale remains intact — it regenerates the endometrium in the next phase
FSH levels begin to rise toward the end of this phase → initiating new follicle development
Menstruation does NOT occur if the woman is pregnant (progesterone stays high due to the corpus luteum/placenta)
Follicular/Proliferative Phase (Days 6–13)
FSH stimulates follicle growth in the ovary. The growing follicle secretes estrogen, which causes the endometrium to thicken (proliferate). One dominant follicle matures into a Graafian follicle. Rising estrogen triggers the LH surge.
During the follicular phase (ovarian) / proliferative phase (uterine), rising FSH stimulates several follicles to grow, but usually only one becomes dominant (the rest undergo atresia). The growing follicle secretes estrogen, which thickens the endometrium.
FSH from the anterior pituitary stimulates growth of ovarian follicles
Estrogen causes: endometrium to thicken (proliferate), new blood vessels to form, uterine glands to grow
Rising estrogen initially inhibits FSH/LH (negative feedback) — this is how only one dominant follicle survives
When estrogen reaches a peak level, it switches to positive feedback → triggers the LH surge (day 13–14)
Ovulation (Day 14)
A sharp spike in LH (the LH surge) triggers ovulation — the rupture of the mature Graafian follicle and release of the secondary oocyte (arrested at metaphase II) into the peritoneal cavity near the fimbriae.
Ovulation occurs approximately on day 14 of a 28-day cycle. The LH surge (a sharp, dramatic increase in LH levels induced by positive feedback from high estrogen) causes the mature Graafian follicle to rupture and release the secondary oocyte.
The LH surge (maximum LH level) is the direct trigger for ovulation
The Graafian follicle ruptures and releases the secondary oocyte (NOT a mature ovum)
The secondary oocyte is arrested at metaphase II of meiosis
It is released along with the corona radiata (layer of granulosa cells) and zona pellucida
The fimbriae of the fallopian tube collect the released oocyte
The oocyte is viable for only ~24 hours after ovulation — if not fertilized, it degenerates
Luteal/Secretory Phase (Days 15–28)
After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone. Progesterone maintains the thick, glandular endometrium for possible implantation. If no pregnancy occurs, the corpus luteum degenerates and the cycle restarts.
After ovulation, the remaining cells of the Graafian follicle collapse and fill with blood (corpus hemorrhagicum), then transform into the corpus luteum — a temporary endocrine gland. The corpus luteum secretes large amounts of progesterone and some estrogen.
Corpus luteum forms from the ruptured Graafian follicle under the influence of LH
Corpus luteum secretes progesterone (primary) and estrogen (secondary)
Progesterone: maintains thick endometrium, stimulates endometrial glands to secrete nutrients, raises body temperature slightly
High progesterone inhibits GnRH, FSH, and LH (negative feedback) — prevents new follicle development and ovulation
If pregnancy occurs: embryo secretes hCG → corpus luteum is maintained → continues progesterone production
If no pregnancy: corpus luteum degenerates (by day 24–26) → progesterone drops → endometrium sheds → menstruation
Hormonal Interplay Summary
A summary of how GnRH, FSH, LH, estrogen, and progesterone coordinate the menstrual cycle through feedback loops.
The menstrual cycle is orchestrated by four key hormones operating through feedback loops between the hypothalamus, anterior pituitary, and ovaries.
FSH — stimulates follicle growth + estrogen production (dominant in follicular phase)
LH — triggers ovulation (LH surge on day 14) + maintains corpus luteum + stimulates progesterone
Estrogen — thickens endometrium; at high levels triggers LH surge (positive feedback); at low levels inhibits FSH/LH (negative feedback)
Progesterone — maintains endometrium in secretory phase; inhibits GnRH/FSH/LH (negative feedback); its fall triggers menstruation
Quick recall
A woman's progesterone level suddenly drops on day 26 of her cycle. What happens next?
The endometrium begins to break down. Within 1–2 days, menstruation starts (day 1 of the next cycle). The drop in progesterone also releases the hypothalamus/pituitary from negative feedback, so FSH begins to rise → stimulating new follicle growth.
Fertilization & Implantation
Fertilization occurs in the ampulla of the fallopian tube. The sperm undergoes capacitation and acrosomal reaction to penetrate the zona pellucida. Fusion of nuclei (karyogamy) forms the diploid zygote. The zygote undergoes cleavage while moving toward the uterus, forming a blastocyst that implants in the endometrium ~7 days after fertilization.
Capacitation is the physiological change sperm undergo in the female reproductive tract that enables them to fertilize. The acrosomal reaction is the release of enzymes from the acrosome that allows sperm to penetrate the egg's protective layers.
Sperm deposited in the vagina are NOT immediately capable of fertilizing an egg. They must first undergo capacitation — a process that takes ~7 hours in the female reproductive tract. Capacitation involves changes in the sperm membrane that enable the acrosomal reaction.
Capacitation occurs in the female reproductive tract (uterus and fallopian tubes); takes ~7 hours
During capacitation: cholesterol is removed from sperm membrane, membrane fluidity increases, sperm become hyperactive
Of ~300 million sperm ejaculated, only ~100–200 reach the egg in the ampulla
When a capacitated sperm contacts the zona pellucida, the acrosomal reaction is triggered
Acrosomal enzymes (hyaluronidase, acrosin) digest the corona radiata and zona pellucida
Sperm membrane fuses with the oocyte membrane → sperm nucleus enters the oocyte cytoplasm
Cortical Reaction
The moment one sperm fuses with the oocyte, the cortical reaction is triggered — cortical granules release their contents, causing the zona pellucida to harden. This prevents additional sperm from entering (polyspermy block).
As soon as one sperm fuses with the secondary oocyte, the oocyte immediately triggers the cortical reaction — a wave of cortical granule exocytosis that modifies the zona pellucida (called the zona reaction), making it impenetrable to additional sperm.
Cortical reaction = release of cortical granules from the oocyte's cortex
Cortical granule contents modify ZP3 glycoproteins on the zona pellucida → zona hardens → polyspermy block
This ensures only ONE sperm fertilizes the egg (polyspermy would be lethal — triploidy)
Simultaneously, the secondary oocyte completes meiosis II → produces the ovum + second polar body
The sperm entry also triggers the activation of the oocyte — metabolic processes resume
Fertilization (Syngamy)
After sperm entry, the sperm and egg nuclei (pronuclei) swell and approach each other. Their nuclear membranes break down, and chromosomes fuse — this is karyogamy/syngamy, producing the diploid zygote (2n = 46).
After sperm entry, the sperm nucleus decondenses and swells to form the male pronucleus. The oocyte (now completing meiosis II) forms the female pronucleus. The two pronuclei approach each other, their membranes break down, and chromosomes intermingle — this is karyogamy (fusion of nuclei), forming the zygote (2n = 46).
Fertilization occurs in the ampulla of the fallopian tube
Sperm contributes: 23 chromosomes (including either X or Y sex chromosome) + a centriole (for first cleavage)
Ovum contributes: 23 chromosomes (always X) + almost ALL the cytoplasm and organelles
Sex determination: Sperm carries X → girl (XX); sperm carries Y → boy (XY). Father determines the sex!
The zygote is the first cell of the new individual — totipotent, capable of forming an entire organism
Fertilization restores the diploid number (2n = 46) from two haploid gametes
Cleavage, Morula & Blastocyst
The zygote undergoes rapid mitotic divisions (cleavage) while travelling down the fallopian tube. 2 → 4 → 8 → 16 cells (morula) → blastocyst (with inner cell mass and trophoblast). The blastocyst reaches the uterus by day 4–5.
The zygote doesn't wait — it begins dividing immediately. Cleavage is a series of rapid mitotic divisions with NO growth between divisions (cells get smaller with each division). The dividing embryo travels down the fallopian tube toward the uterus.
Cleavage = rapid mitotic divisions of the zygote (without cell growth → cells get progressively smaller)
Morula continues dividing and develops a fluid-filled cavity → becomes a blastocyst
Blastocyst has two parts: Trophoblast (outer layer, becomes part of placenta) + Inner Cell Mass (ICM) / embryoblast (becomes the embryo)
Blastocyst reaches the uterus by day 4–5 after fertilization
The fluid-filled cavity of the blastocyst is called the blastocoel (or blastocoele)
Implantation
The blastocyst attaches to the uterine endometrium and burrows into it. The trophoblast invades the endometrium, establishing connections with the maternal blood supply. Implantation is complete by about day 7 after fertilization.
Implantation occurs about 7 days (6–7 days) after fertilization. The blastocyst's trophoblast attaches to the endometrium of the uterus (usually the posterior wall of the upper body) and then invades it, burrowing into the thick, progesterone-primed endometrial tissue.
Implantation occurs ~day 7 after fertilization (day ~21 of the menstrual cycle)
The trophoblast cells of the blastocyst attach to and invade the endometrium
After implantation, the trophoblast starts secreting hCG (human Chorionic Gonadotropin)
hCG maintains the corpus luteum → progesterone production continues → no menstruation
This is why pregnancy tests detect hCG in urine — its presence = implantation has occurred = pregnancy
Pregnancy, Parturition & Lactation
Pregnancy (gestation) lasts ~9 months (266 days) in humans. The placenta is a temporary organ connecting mother and fetus for exchange of nutrients, gases, and waste. Key hormones: hCG, hPL, estrogen, progesterone, relaxin, oxytocin. Parturition (childbirth) is triggered by a positive feedback loop involving oxytocin. Lactation begins with colostrum.
The placenta is a disc-shaped temporary organ formed by interdigitation of fetal (chorionic villi) and maternal (uterine) tissue. It functions as the fetal lung, kidney, intestine, liver, and endocrine gland — all in one.
The placenta is a remarkable temporary organ that develops after implantation and is expelled after birth. It is formed by the interdigitation of chorionic villi (fetal tissue from the trophoblast) with the uterine endometrium (maternal tissue). It is fully functional by the end of the first trimester.
Structure: finger-like chorionic villi from fetus interlock with uterine tissue of mother
Connected to the fetus via the umbilical cord (2 arteries + 1 vein)
Transport functions: O₂ and nutrients from mother → fetus; CO₂ and waste from fetus → mother
Barrier function: prevents mixing of fetal and maternal blood (but allows diffusion of small molecules)
After the first trimester, the placenta replaces the corpus luteum as the main source of progesterone
Embryo Development Milestones
Key developmental milestones: inner cell mass differentiates into ectoderm, mesoderm, endoderm (gastrulation); organogenesis begins in the first trimester; heart beat starts by week 6; limbs and digits form; major organs complete by the end of the first trimester.
After implantation, the inner cell mass (ICM) differentiates into the embryo through a process called gastrulation — formation of three primary germ layers: ectoderm, mesoderm, and endoderm. These layers give rise to ALL tissues and organs of the body.
Ectoderm → nervous system, skin, hair, nails, lens of eye, enamel of teeth
First month: Heart formation begins; heart starts beating (~day 22–24)
Second month: Limb buds appear; digits form; embryo is called a fetus from 9th week onward
End of first trimester (12 weeks): Major organs formed; external genitalia visible; limbs and digits fully formed
Fifth month: First movements of the fetus felt by the mother (quickening); hair appears on head
End of 9 months (38 weeks): Fetus is fully developed and ready for birth
Hormones During Pregnancy
Pregnancy is maintained by a complex interplay of hormones. hCG maintains the corpus luteum initially; the placenta takes over hormone production by the second trimester. Progesterone prevents uterine contractions; estrogen stimulates uterine growth.
Pregnancy is a hormone-dependent state. Without adequate progesterone, the endometrium would shed and the pregnancy would terminate. Multiple hormones work together to maintain and support the developing embryo/fetus.
hCG (human Chorionic Gonadotropin) — first hormone of pregnancy; maintains corpus luteum; basis of pregnancy tests; peaks at ~8–10 weeks
Progesterone — initially from corpus luteum, later from placenta; maintains endometrium, prevents uterine contractions, suppresses immune response against fetus
Estrogen — stimulates growth of uterus and mammary glands; increases blood supply to uterus
hPL (human Placental Lactogen) — prepares mammary glands for lactation; mobilizes maternal glucose for fetal use
Relaxin — secreted by corpus luteum and placenta; relaxes pelvic ligaments and cervix before birth
All these hormones suppress the menstrual cycle — no menstruation during pregnancy
Parturition (Childbirth)
Parturition is the delivery of the fetus after ~9 months of gestation. It is triggered by a complex neuroendocrine mechanism involving a positive feedback loop: fetal signals → oxytocin → uterine contractions → more oxytocin → stronger contractions → delivery.
Parturition (childbirth) occurs approximately 266 days (~38 weeks / ~9 months) after fertilization. It is triggered by signals from the fully developed fetus and involves powerful rhythmic contractions of the myometrium (uterine muscle).
Signals from the fetus and placenta trigger the release of oxytocin from the posterior pituitary of the mother
Oxytocin causes strong contractions of the myometrium (uterine smooth muscle)
These contractions push the baby toward the cervix → cervix stretches → sends signals back → MORE oxytocin released
This is a positive feedback mechanism — contractions increase oxytocin, which increases contractions
The loop intensifies until the baby is delivered through the birth canal (cervix + vagina)
After the baby, the placenta is expelled as the afterbirth
Lactation
Lactation begins after parturition. The first milk (colostrum) is rich in antibodies (IgA). Prolactin stimulates milk production; oxytocin stimulates milk ejection (let-down reflex). Breastfeeding provides passive immunity to the newborn.
After delivery, the mammary glands begin producing milk — a process called lactation. The initial secretion from the mammary glands during the first few days is called colostrum — it is not regular milk but is immensely important for the newborn.
Colostrum — yellowish fluid produced in the first 2–3 days after birth; rich in antibodies (IgA) and immune cells
Colostrum provides passive immunity to the newborn (protects against infections)
Prolactin (from anterior pituitary) stimulates milk production by mammary alveoli