The testes are located outside the abdominal cavity within a pouch called Scrotum (with two scrotal sacs), except in prototherian mammals, elephants and whales. The scrotum maintains the temperature of testes at 2–2.5°C lower than normal body temperature — necessary for spermatogenesis.
Testes are formed inside intra-abdominal cavity and migrate to scrotum during the 7th month of pregnancy under influence of Testosterone and ABP.
Cryptorchidism
Failure of testis to descend into scrotum
Orchidopexy
Surgical descending of testis
Castration (Orchidectomy)
Crushing/removal of testis → animal becomes docile (↓ testosterone)
Provide structural support, nutrition and protection to developing sperm
Act as blood-sperm barrier
Phagocyte dead and immature sperms
Secrete: Androgen Binding Protein (ABP) — at 7th month of foetal life, at puberty for spermatogenesis; Inhibin → inhibits FSH; Anti-Mullerian Factor (AMF); Sperm maturation factor
💡 Sertoli Cell Effect: In males → AMF suppresses Mullerian duct → it becomes vestigial. Wolffian duct → converts to male reproductive structures (Epididymis, Vas deferens) under Testosterone. In females → Mullerian duct converts to female structures (Fallopian tube, Uterus, Vagina) since testosterone is absent.
Interstitial / Leydig Cells
Present in interstitial fluid of testis. Stimulated by LH (ICSH) at puberty via Hypothalamus → GnRH → Pituitary → LH → Leydig cells → release Androgen (Testosterone) → along with Sertoli cells (ABP) → stimulate spermatogenesis → Stop (feedback inhibition).
Highly coiled duct formed by union of 10–12 Vasa efferentia and opens into Vas deferens. Function: Storage, functional maturation of sperms and phagocytosis of dead sperms by Stereo Ciliated Pseudostratified Columnar epithelium.
Vas Deferens (Length — 40 cm)
Arises from Cauda epididymis and opens into ejaculatory duct. Function: Storage and transport of sperm.
Urethra
Male: 20 cm · Female: 4 cm (UTI more common in females due to short urethra)
In male — common passage for both Urine & Semen → called Urinogenital duct
Made up of special tissue for erection. Enlarged end = Glans penis covered by loose skin called Foreskin.
Corpus Spongiosum (Single): surrounds urethra, mid-ventral and anterior, produces Glans penis Corpus Cavernosum (One pair): surrounds blood vessels, position — dorsal
Accessory Glands
Gland
Number
% of Semen
Nature
Contains/Functions
Seminal Vesicles
One pair
60–70%
Alkaline (pH 7.4), Yellow viscous
Fructose, Prostaglandins (local hormone), Clotting proteins (fibrinogen) — help semen coagulate after ejaculation
Prostate Gland
Single, surrounds urethra
20–25%
Slightly acidic (pH 6.5)
Citric acid, Proteolytic enzymes (fibrinolysin), Seminal plasmin protein (antibiotic for bacteria), Ca²⁺ and phosphate
Cowper's / Bulbourethral Glands
One pair (size like peas)
Small amount
Alkaline
Secretes alkaline fluid that neutralizes acid from urine in urethra; also secretes mucus that lubricates penis
💡 Semen: Sperm + Seminal Plasma (fluid of accessory glands). Slightly alkaline (pH 7.2–7.7). Volume in typical ejaculation = 2.5–5 ml with 200–300 million sperms. For normal fertility: ≥60% sperms must have normal shape/size and ≥40% must show vigorous motility.
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Female Reproductive System
Section 04
The female reproductive system consists of:
Ovaries (one pair) — Primary Sex Organ
Ductus System → Fallopian tube (Oviduct), Uterus (Hystera/Womb), Cervix, Vagina
External Genitalia (Vulva) → Mons pubis, Labia minora, Hymen, Clitoris
Accessory Glands
Mammary Glands (one pair) → Secondary Sexual Character
Modified apocrine sweat (Sudoriferous) gland that produces milk. Attached with pectoralis major muscles through Cooper's Ligaments. Consists of 15–20 lobes; each lobe has several smaller compartments called Lobules composed of grape-like clusters of milk secreting glands called Alveoli. Each lactiferous duct carries milk from one lobe to the exterior.
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Ovary & Fallopian Tube
Section 05
Ovaries (One pair — Primary Sex Organ)
Position: One on each side of lower abdomen, connected to pelvic wall and uterus by ligaments
Size: Length 2–4 cm, Almond shape
Origin: Mesodermal
Structure of Ovary
Each ovary is covered by thin epithelium enclosing Ovarian Stroma, divided into two zones:
Outer Cortex → contains follicles
Inner Medulla → Artery, Vein, Lymph Vessels
Wall of Ovary: (1) Outer — Germinal epithelium (simple cuboidal, does NOT give rise to ova) (2) Inner — Tunica Albuginea (whitish capsule of dense irregular connective tissue)
Shape: Inverted pear shape · Size: 7.5 cm long, 5 cm wide, 2.5 cm thick
Three parts: Fundus (upper dome, superior to tubes), Body/Corpus (middle/main part), Cervix (inferior narrow — opens into vagina, formed by most powerful sphincter muscles)
Shows cyclic changes during reproductive cycle; implantation site
Vagina
Tubular 8–10 cm long canal lined with mucous membrane. Consists of Nonkeratinized Stratified Squamous epithelium. Mucosa contains large stores of glycogen; decomposition by Lactobacillus acidophillus produces organic acid → acidic environment retards microbial growth (but also harmful to sperm). Glands are ABSENT in vagina.
External opening = Vaginal Orifice, guarded by Hymen (thin fold of vascularized mucous membrane). The hymen is NOT a reliable indicator of virginity or sexual experience.
Vulva / Pudendum / External Genitalia
Mons Pubis — Adipose tissue + hair
Labia Majora — Adipose + hair (homologous to scrotum)
Labia Minora — Homologous to spongy (penile) urethra
Clitoris — Homologous to glans penis
Hymen — Perforated mucus layer (if still present)
Vestibule — Space within labia minora (homologous to membranous urethra of males)
Bartholine glands — Homologous to male Cowper's glands
Skene's glands / Para-urethral glands (one pair)
💊
Spermatogenesis
Section 07
Begins during puberty and continues till death. In humans, process takes 65–75 days.
Two Phases of Spermatogenesis
1. Spermatocytogenesis — Formation of spermatids from spermatogonia
2. Spermiogenesis / Spermateliosis (Longest phase) — Differentiation/metamorphosis of spermatids into sperms
Steps of Spermatocytogenesis
Spermatogonia (2n = 44+XY) — origin: Extra-gonadal, Embryonic life → enter gonades
In embryo (millions) → Oogonia (2n) by mitosis → Primary Oocyte (2n = 44+XX) → Meiosis I arrested at Prophase I → birth
At puberty (60,000–80,000 follicles) → Meiosis I completes just before ovulation → Secondary Oocyte (n) + 1st Polar body → Secondary oocyte enters Fallopian tube
Fertilization → Meiosis II completes → Ovum (n) + 2nd Polar body
Folliculogenesis
Follicle Stage
Key Features
Hormone
Primordial Follicle
2 million before birth; single layer of granulosa cells; primary oocyte (2n); Meiosis arrest in Prophase I
Degeneration of Corpus luteum; Breakdown & sloughing of Endometrium; Blood loss = 50–150 ml; Blood clotting in uterus → breakdown by Fibrinolysin released from uterine wall
Follicular / Proliferative Phase
6–13 (7–8 days)
FSH ↑ → Estrogen ↑, Inhibin ↑
Development of follicles; Dominant follicle → Estrogen & Inhibin (inhibit FSH & other follicle development); Proliferation/division/thickening of Endometrium
Rearrangement of glycoproteins in Plasma Membrane of Sperm. Activation of sperm after ejaculation in vaginal tract:
Removal of cholesterol, glycoproteins and proteins from plasma membrane around head of sperm
Dilution of decapacitation factors
Entry of Ca²⁺ into sperms → mobile sperms swim rapidly → pass through cervix → enter uterus → finally reach ampullary region of fallopian tube → fertilization takes place
Of 200–300 million sperms: 10,000 reach cervix → 250 reach ampulla → 250 sperms attach to surface of egg (Agglutination)
Acrosomal Reaction
250 sperms attach to surface of egg (agglutination) and acrosome starts releasing hydrolysing enzymes (Sperm lysins):
Zona Lysin (Acrosin) [Antifertilizin] — dissolves Zona Pellucida (ZP3 receptor → Fertilizin)
Fertilization Cone & Prevention of Polyspermy
Fertilization Cone — Contact of acrosome stimulates outgrowth by PM of oocyte (Cone of reception) to receive incoming sperm → Na channel opens → membrane depolarized
Zona Reaction (Fast block to polyspermy) — immediate depolarization
Cortical Reaction (Slow block to polyspermy) — Ca²⁺ released → Cortical granules exocytose → formation of new layer → Zona Pellucida hardens → prevents other sperms from entering
⚠️ Fertilizin-Antifertilizin Reaction: Zona pellucida → ZP3 receptor → contains Fertilizin. Sperm acrosome → contains Antifertilizin (Zona Lysin / Acrosin). This specific binding ensures species-specific fertilization.
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Cleavage, Implantation & Embryonic Development
Section 11
Cleavage
After fertilization — rapid mitotic cell divisions of zygote converting single-celled zygote into multicellular Morula.
During cleavage: total size remains constant (both growth phases G₁ & G₂ absent)
Nuclear:Cytoplasmic ratio increases (total cytoplasm same → DNA increases)
No. of Blastomeres increases → Size of blastomere decreases
Cleavage ends with formation of multicellular Morula
Hatching → Blastocyst Formation
Morula → Blastula → Blastocyst
Inner Cell Mass (Embryo proper / Cell of Rubber / Stem cells) → may give new embryo (Monozygotic Twins)
Trophoblast → releases lytic enzymes + hCG → for implantation
Blastocoel — fluid-filled cavity
Implantation (7–9 days after fertilization)
Attachment or embedding of developing blastocyst with wall of Uterus (Endometrium) with help of lytic enzymes released from trophoblast of embryo. During implantation, blastocyst completely sinks in wall of uterus and endometrium becomes more vascularized. After implantation, endometrium is known as Decidua.
Embryonic Development
After implantation → Trophoblast divides into Cytotrophoblast and Syncytiotrophoblast
Inner Cell Mass (cell of rubber = cell of trophoblast connected with inner cell mass) → Epiblast (Columnar) + Hypoblast (Cuboidal) → Germ Disc → Embryonic Disc
Gastrulation (First major event of 3rd week)
Occurs about 15 days after fertilization. The bilaminar embryonic disc (epiblast + hypoblast) transforms into Trilaminar embryonic disc with three primary germ layers:
Structural and functional unit between developing embryo and maternal body. Type in humans → Haemochorial placenta — maternal blood is in direct contact with chorion of foetus.
Foetal part: Trophoblast/Chorion + Conn. Tissue + Wall of Blood Vessels Maternal part: Blood of mother → Diffusion of CO₂/O₂/Nutrition
Functions of Placenta
1. Acts as endocrine gland — releases hormones only during pregnancy:
Estrogen (maintains Corpus luteum)
Progesterone (released from urine)
Human Chorionic Gonadotrophin (hCG)
Chorionic Thyrotrophin
Chorionic Corticotrophin
Human Placental Lactogen (hPL) / Chorionic Somatomammotrophin
Relaxin
2. Ultrafilter — allows passage of soluble inorganic & organic materials, nutrients, hormones, gases, antibody against diphtheria, smallpox and measles (from mother to foetus)
3. Virus infection (Rubella/German measles) and certain chemicals (teratogens) may cause malformation in developing embryo.
⚠️ Important: hCG is the basis of pregnancy detection tests (home pregnancy kits). hCG is detectable in urine 7–10 days after fertilization. Corpus luteum is maintained by hCG during first 14 weeks of pregnancy until placenta takes over progesterone production.
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MCQ Practice — Human Reproduction
Section 13
Q1. The site of fertilization in humans is:
Ampulla (widest portion of Fallopian tube) is the site of fertilization. The secondary oocyte released during ovulation is fertilized here.
Q2. Which of the following contributes the highest percentage to semen volume?
Seminal vesicles contribute 60–70% of the volume of semen. Prostate gland contributes 20–25%. Bulbourethral glands contribute only a small amount.
Q3. Corpus luteum is maintained during pregnancy by:
hCG (Human Chorionic Gonadotrophin) released from Trophoblast/Placenta maintains the Corpus Luteum for the first 14 weeks of pregnancy. After that, the placenta itself takes over hormone production.
Q4. The longest phase of spermatogenesis is:
Spermiogenesis (also called Spermateliosis) is the longest phase — it is the differentiation/metamorphosis of spermatids into spermatozoa. This is facilitated by Sperm Maturation Factor (SMF) from Sertoli cells.
Q5. At what day does ovulation occur if the total menstrual cycle is 28 days?
Day of Ovulation = Total days of M.C. − 14 = 28 − 14 = 14th day. This formula applies to any cycle length. For 30-day cycle: 30−14 = 16th day.
Q6. Which cells in testis secrete Androgen Binding Protein (ABP)?
Sertoli cells (Nurse cells) secrete ABP (Androgen Binding Protein), Inhibin, and Anti-Mullerian Factor (AMF). Leydig cells secrete Testosterone/Androgen when stimulated by LH (ICSH).
Fertilization in humans takes place in the Ampullary region (ampulla) of the Fallopian tube — the widest portion. The secondary oocyte released during ovulation is fertilized here by sperm.
Spermatogenesis: occurs in testis seminiferous tubules, begins at puberty, continuous lifelong, produces 4 functional sperms from one primary spermatocyte. Oogenesis: occurs in ovary, begins in fetal life, arrested at prophase I until puberty, produces only 1 functional ovum + 3 polar bodies from one primary oocyte.
Implantation is the attachment and embedding of the developing blastocyst into the uterine wall (endometrium). It occurs 7-9 days after fertilization. Trophoblast releases lytic enzymes to help penetrate endometrium. After implantation, the endometrium is called decidua.
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