1. Overview of the Reproductive System

The reproductive system of sexually reproducing animals is dedicated to the continuation of the species through the production, support, and delivery of gametes, as well as the secretion of sex hormones that regulate sexual maturation and physiological function.

Functional Classifications of Reproductive Organs

Figure: Organizational Hierarchy of Reproductive Organs. Primary organs (gonads) produce gametes and sex hormones directly. Secondary and accessory organs branch into ducts (transport), glands (secretion/nourishment), and structures (copulation/delivery) that support but do not themselves generate gametes.

Male vs. Female Secondary Sex Organs

2. Male Reproductive System

The male reproductive system consists of the testes (gonads), a network of excretory ducts, accessory sex glands, and supporting structures including the scrotum and penis.

Figure: The Male Reproductive Pathway. Sperm travel from the seminiferous tubules through the rete testis, efferent ducts, and epididymis (where they mature over ~14 days), then along the ductus deferens. Seminal vesicle fluid joins at the ejaculatory duct; prostatic and bulbourethral secretions are added further downstream before semen exits via the urethra.

2.1 Testis Structure and Histology

• Descent
  Development & Descent

  Testes develop high in the abdominal cavity near the kidneys during embryonic development, then descend through the inguinal canal into the scrotum during the last two months of fetal development or shortly after birth. Scrotal temperature is maintained ~3°C below core body temperature — essential for viable spermatogenesis. Cryptorchidism (failure of descent) causes sterility if uncorrected, due to thermal damage to spermatogenic cells.

• Framework
  Connective Tissue & Lobules

  Tunica albuginea: dense fibrous capsule wrapping each testis. Septa: internal extensions partitioning the testis into ~250 lobules. Each lobule contains 1–4 coiled seminiferous tubules, which converge into straight tubules feeding the rete testis.

Cell Types within the Seminiferous Tubules

• Cell 1
  Spermatogenic Cells

  Germ cells at various stages of meiotic/mitotic development: spermatogonia → primary spermatocytes → secondary spermatocytes → spermatids → spermatozoa.

• Cell 2
  Sertoli Cells (Sustentacular)

  Large supporting cells spanning basement membrane to lumen. Provide structural support and nourishment to developing sperm; phagocytose residual cytoplasm from spermiogenesis; produce fluid for sperm transport; secrete inhibin; mediate FSH activity.

• Cell 3
  Leydig Cells (Interstitial)

  Located in connective tissue between seminiferous tubules. Synthesize and secrete testosterone under the influence of Luteinizing Hormone (LH).

Figure: Seminiferous Tubule Cross-Section. Germ cells mature in stages as they move from the basement membrane toward the lumen — mitosis, Meiosis I, Meiosis II, and spermiogenesis. Sertoli cells span the full height of the epithelium, nourishing and regulating this process, while Leydig cells in the surrounding interstitial space secrete testosterone.

2.2 Spermatogenesis and Sperm Morphology

Spermatogenesis is the complete process of sperm production occurring within the seminiferous tubules, taking approximately 64–74 days, divided into three major phases: proliferative (mitosis), meiotic, and postmeiotic differentiation (spermiogenesis). Daily sperm production reaches approximately 300 million.

• Phase 1
  Proliferative Phase (Mitosis)

  Diploid spermatogonia (2n) line the inner periphery of the seminiferous tubule basement membrane and undergo mitotic division to maintain the stem cell pool and yield primary spermatocytes (2n).

• Phase 2
  Meiotic Phase

  The primary spermatocyte (2n) replicates its DNA and enters Meiosis I, dividing into two haploid secondary spermatocytes (n). Each secondary spermatocyte rapidly enters Meiosis II without further DNA replication, dividing to form four haploid spermatids (n) total.

• Phase 3
  Spermiogenesis (Postmeiotic Differentiation)

  Spherical, non-motile spermatids transform into motile spermatozoa: chromatin condenses, an acrosomal cap forms over the nucleus, a flagellum grows, and excess cytoplasm is shed as residual bodies (phagocytosed by Sertoli cells).

Figure: Spermatogenesis Flowchart. One spermatogonium (2n) yields, through mitosis and two meiotic divisions, four haploid spermatids — each of which differentiates via spermiogenesis into a mature spermatozoon.

Detailed Anatomy of a Mature Spermatozoon

A mature sperm is approximately 60 µm long and structurally adapted for swimming through the female reproductive tract to penetrate the secondary oocyte.

Figure: Mature Sperm Anatomy. The head carries the condensed haploid nucleus beneath an enzyme-filled acrosomal cap; the tail (neck, midpiece, principal piece, endpiece) provides the mitochondrial energy and flagellar apparatus for motility.

2.3 Male Reproductive Ducts and Accessory Sex Glands

System of Ducts

• Duct 1
  Seminiferous Tubules & Rete Testis

  Sertoli-cell fluid pressure pushes non-motile sperm along straight tubules into the rete testis and through the efferent ducts.

• Duct 2
  Epididymis

  A comma-shaped organ (~4 cm long; ~6 m uncoiled) along the posterior testis border. Site of sperm maturation (~14 days) — sperm acquire motility and fertilizing ability; also stores sperm, reabsorbing un-ejaculated sperm.

• Duct 3
  Ductus Deferens (Vas Deferens)

  Ascends along the epididymis, through the inguinal canal within the spermatic cord, into the pelvic cavity. Its dilated terminal portion is the ampulla. Peristaltic smooth-muscle contractions convey sperm toward the urethra during arousal.

• Duct 4
  Spermatic Cord

  Ascends out of the scrotum, containing the ductus deferens, testicular artery, pampiniform venous plexus (cools arterial blood), autonomic nerves, lymphatics, and the cremaster muscle.

• Duct 5
  Ejaculatory Ducts

  Short ducts (~2 cm) formed by the union of the seminal vesicle duct and the ampulla; pass through the prostate and terminate in the prostatic urethra.

• Duct 6
  Urethra

  Shared terminal duct (~20 cm) for the reproductive and urinary systems, divided into prostatic, membranous, and spongy (penile) urethra.

Accessory Sex Glands

Semen Composition

A mixture of spermatozoa and seminal fluid from the seminiferous tubules, seminal vesicles, prostate, and bulbourethral glands.

2.4 Hormonal Control of the Male Reproductive System

Male reproductive function is controlled by a neuroendocrine feedback cascade involving the hypothalamus, anterior pituitary gland, and testes.

Figure: Hypothalamic–Pituitary–Testicular Axis. GnRH drives pituitary release of FSH and LH. FSH acts on Sertoli cells to support spermatogenesis and stimulate inhibin release (which feeds back to suppress FSH); LH acts on Leydig cells to stimulate testosterone synthesis, which supports spermatogenesis directly and feeds back to suppress both GnRH and LH.

Endocrine Cascades

• Hormone
  GnRH

  Secreted by the hypothalamus in a pulsatile manner at puberty, stimulating the anterior pituitary.

• Hormone
  FSH

  Acts on Sertoli cells to promote spermatogenesis by stimulating secretion of Androgen-Binding Protein (ABP), which binds testosterone and maintains high local androgen concentrations. Sertoli cells also release inhibin, which selectively suppresses FSH via negative feedback when sperm counts are high.

• Hormone
  LH / ICSH

  Acts on Leydig cells between seminiferous tubules, stimulating synthesis and secretion of testosterone.

• Hormone
  Testosterone & DHT

  Mediates prenatal male development, secondary sex characteristics (deep voice, hair distribution, muscle mass), libido, and reproductive organ maintenance. Elevated blood testosterone inhibits GnRH release from the hypothalamus and LH release from the anterior pituitary.

3. Female Reproductive System

The female reproductive system produces ova (gametes), secretes sex hormones, transports gametes to the site of fertilization, provides a favorable anatomical environment for the developing fetus, moves the fetus externally during childbirth, and supports the newborn.

Figure: Female Reproductive Anatomy Overview. The secondary oocyte released at ovulation is swept into the uterine tube (site of fertilization), travels to the uterus (site of implantation and gestation), and at birth passes through the cervix, vagina, and vulva.

3.1 Ovary Anatomy and Oogenesis

Ovarian Structure

• Location
  Position

  Paired oval organs located in shallow depressions called ovarian fossae on the lateral walls of the pelvic cavity.

• Layers
  Histological Layers
  • Germinal Epithelium: simple cuboidal/squamous epithelium covering the outer surface.
  • Tunica Albuginea: dense connective tissue layer beneath the germinal epithelium.
  • Cortex: outer region containing ovarian follicles at various stages, surrounded by stromal cells.
  • Medulla: inner vascular core of loose connective tissue, blood vessels, lymphatics, and nerves.

Process of Oogenesis

Unlike spermatogenesis (which begins at puberty and continues continuously), oogenesis begins during embryonic development, halts until puberty, and ceases completely at menopause.

Figure: Oogenesis Timeline & Phases. Oogonia enter Meiosis I before birth but arrest at prophase I until puberty. Each reproductive cycle thereafter, one oocyte resumes meiosis, completing Meiosis I asymmetrically (secondary oocyte + degenerating polar body), then arresting again at Metaphase II until ovulation. Meiosis II only completes if a sperm penetrates the oocyte, yielding a mature ovum and a second polar body; otherwise the oocyte degenerates.

• Stage 1
  Prenatal Development (Before Birth)

  Primordial germ cells migrate from the yolk sac into the developing ovaries, differentiating into oogonia (2n). Oogonia divide mitotically to form millions of germ cells — most degenerate (atresia). Surviving oogonia become primary oocytes (2n), enter Meiosis I, and arrest at prophase I (diplotene stage) before birth, remaining inside primordial follicles until puberty.

• Stage 2
  Post-Pubertal Maturation

  Each month, a cohort of primary oocytes is recruited by FSH. Just prior to ovulation, the primary oocyte completes Meiosis I asymmetrically: the secondary oocyte (n) receives nearly all the cytoplasm, while the first polar body (n) (discarded chromosomes) eventually degenerates. The secondary oocyte begins Meiosis II but arrests at metaphase II.

• Stage 3
  Fertilization

  The secondary oocyte is released at ovulation. If sperm penetrates it, Meiosis II completes, producing a mature ovum (n) and a second polar body (n, degenerates); sperm and ovum nuclei fuse into a diploid zygote (2n). If fertilization does not occur, the secondary oocyte degenerates without completing Meiosis II.

3.2 Folliculogenesis and Follicular Histology

Folliculogenesis is the maturation process of the ovarian follicle surrounding the oocyte through distinct histological stages.

Figure: Stages of Folliculogenesis. A resting primordial follicle activates and grows through primary, secondary, and mature (Graafian) stages before ovulation. The ruptured follicle becomes a hormone-secreting corpus luteum, which degenerates into the corpus albicans if pregnancy does not occur.

Detailed Histology of Follicle Stages

Figure: Follicle Histology Comparison. The primordial follicle is a simple oocyte-plus-flat-cell-layer structure. The mature antral follicle nests several nourishing and protective layers — theca externa, theca interna, granulosa cells, and fluid-filled antrum — around the corona radiata, zona pellucida, and secondary oocyte at its core.

3.3 Female Reproductive Tract Anatomy

• Structure 1
  Uterine Tubes (Fallopian Tubes / Oviducts)

  Extend laterally from the uterus toward the ovaries (~10 cm long).

  • Infundibulum: funnel-shaped terminal portion open to the pelvic cavity, bearing fimbriae that sweep over the ovary to catch the released oocyte.
  • Ampulla: the widest, longest central portion (~2/3 of length) — primary site of fertilization.
  • Isthmus: short, thick-walled, narrow medial portion joining the uterine wall.
  • Histology: ciliated simple columnar epithelium and non-ciliated peg cells (secrete fluid); ciliary action and peristalsis move the oocyte/zygote toward the uterus.
• Structure 2
  Uterus (Womb)

  Pathway for sperm transport, site of blastocyst implantation, fetal development, and labor.

  • Fundus: dome-shaped superior portion above the uterine tube openings.
  • Body: central, major tapering portion containing the uterine cavity.
  • Isthmus: constricted 1-cm region between body and cervix.
  • Cervix: narrow inferior portion opening into the vagina via the cervical canal (bounded by internal os superiorly, external os inferiorly).
• Structure 3
  Vagina

  A tubular 10-cm fibromuscular canal lined with non-keratinized stratified squamous epithelium, extending from the exterior of the body to the cervix. The recess around the cervix is the fornix (contraceptive diaphragm placement site). The vaginal orifice is partially bordered by the hymen.

• Structure 4
  Vulva (External Genitalia)
  • Labia Majora: homologous to the male scrotum; outer hair-covered skin folds.
  • Labia Minora: homologous to spongy urethral skin; inner hairless folds bounding the vestibule.
  • Clitoris: homologous to the glans penis; small erectile mass at the anterior junction of the labia minora.
  • Vestibule: region containing the openings of the urethra, vagina, and vestibular glands.

Figure: Uterine Wall Layers. From outside in: the perimetrium (serosa), the thick muscular myometrium, and the endometrium — a permanent stratum basalis beneath a cyclically shed stratum functionalis — lining the uterine cavity.

4. Female Reproductive Cycle

The female reproductive cycle encompasses the ovarian cycle (changes in the ovary during and after oocyte maturation) and the concurrent uterine (menstrual) cycle (changes in the endometrium to prepare for embryo implantation), regulated by neuroendocrine feedback loops over an average 28-day duration.

Figure: The 28-Day Female Reproductive Cycle. The ovarian cycle (follicular → ovulation → luteal) runs in parallel with the uterine cycle (menstrual → proliferative → secretory), synchronized by the rise and fall of FSH, LH, estrogen, and progesterone.

4.1 Phases of the Ovarian Cycle

• Days 1–13
  Follicular Phase

  Driven by FSH secreted by the anterior pituitary. Primordial follicles develop into primary, secondary, and finally a single dominant mature (Graafian) follicle. Maturing granulosa cells secrete increasing levels of estrogen and inhibin.

• Day 14
  Ovulatory Phase

  High, sustained estrogen exerts positive feedback on the hypothalamus and anterior pituitary, triggering a massive LH surge. The LH surge causes the Graafian follicle to rupture and release the secondary oocyte into the pelvic cavity, caught by the uterine tube fimbriae.

• Days 15–28
  Luteal Phase

  Following ovulation, LH transforms the ruptured follicle into the corpus luteum, which secretes large amounts of progesterone and moderate estrogen. If no pregnancy occurs: high progesterone and estrogen exert negative feedback, suppressing GnRH, FSH, and LH. Lacking LH support, the corpus luteum degenerates into the corpus albicans by Day 26–28, and hormone levels plummet.

4.2 Phases of the Uterine (Menstrual) Cycle

• Days 1–5
  Menstrual Phase

  Triggered by the sharp drop in progesterone and estrogen at the end of the previous cycle. Constriction of uterine spiral arteries causes ischemia and necrosis of the stratum functionalis, which sloughs off along with ~50–150 mL of blood and fluid, exiting via the vagina (menses).

• Days 6–13
  Proliferative Phase

  Estrogen produced by growing ovarian follicles stimulates the stratum basalis to rebuild the stratum functionalis. Endometrial thickness increases from 1 mm to 2–3 mm; endometrial glands elongate and spiral arteries grow.

• Days 15–28
  Secretory Phase

  Progesterone produced by the corpus luteum drives maximum vascularization and glycogen secretion by enlarged, tortuous endometrial glands. Endometrial thickness reaches 4–6 mm, fully prepared to nourish an implanted embryo. If fertilization & implantation occur: the embryonic trophoblast secretes human Chorionic Gonadotropin (hCG), which maintains the corpus luteum and keeps progesterone elevated, preventing menses.

Figure: Ovarian & Uterine Hormone Curves. Estrogen rises through the follicular phase, triggering the LH surge at ovulation (Day 14). Progesterone then dominates the luteal phase, peaking around Day 21–22 before declining toward Day 28 if pregnancy does not occur.

4.3 Summary of Ovarian and Uterine Cycles

4.4 Menopause

• Definition
  What It Is

  The permanent cessation of monthly reproductive cycles and menstruation, typically occurring between ages 45 and 55.

• Cause
  Physiological Cause

  Aging ovaries become depleted of follicles and cease responding to anterior pituitary gonadotropins (FSH and LH).

• Hormones
  Hormonal Profile

  Plasma estrogen levels drop significantly, while blood levels of FSH and LH remain persistently elevated due to the loss of negative feedback inhibition by estrogen and inhibin.