The Reproductive System: Structure, Function, and Regulation

The Male Reproductive System

Overview and Main Structures

The male reproductive system is responsible for the production, maturation, and delivery of spermatozoa, as well as the secretion of male sex hormones. It consists of both internal and external organs, each with specialized functions.

• Internal organs: Testes, epididymis, vas deferens, seminal vesicles, ejaculatory ducts, prostate gland, urethra, bulbo-urethral glands

• External organs: Penis, scrotum

Testes

The testes are the primary male reproductive organs, located within the scrotum. They contain seminiferous tubules where spermatogenesis (sperm production) occurs. The testes require a temperature approximately 1°C below core body temperature for optimal sperm production.

• Spermatogenesis: The process of sperm cell development within the seminiferous tubules.

• Hormone production: The testes also produce testosterone, the main male sex hormone.

Epididymis

The epididymis is a long, coiled duct (about 6 meters) attached to the posterior of each testis. It serves as the site for sperm maturation and storage. Sperm can remain viable here for up to 42 days before being phagocytosed if not ejaculated.

Vas (Ductus) Deferens

The vas deferens is a muscular tube (about 45 cm long) that transports sperm from the epididymis to the ejaculatory duct. It runs anterior to the pubic bone and ends behind the urinary bladder.

Seminal Vesicles and Ejaculatory Ducts

The seminal vesicles are paired glands that secrete a fluid making up about 60% of semen volume. This fluid nourishes and protects sperm. The ejaculatory ducts are formed by the union of the vas deferens and the duct from the seminal vesicle, passing through the prostate to join the urethra.

Prostate Gland

The prostate gland surrounds the first part of the urethra and contributes about 30% of the seminal fluid. Its secretions enhance sperm motility and help neutralize vaginal acidity.

Bulbo-urethral (Cowper's) Glands and Urethra

The bulbo-urethral glands secrete a lubricating mucus into the urethra. The male urethra (about 20 cm long) serves as a common passage for urine and semen and is divided into prostatic, membranous, and penile sections.

Penis and Erectile Tissue

The penis is the external organ of copulation, consisting of a root, shaft, and glans. It contains three columns of erectile tissue: two corpora cavernosa and one corpus spongiosum (surrounding the urethra). Erection is achieved by blood filling these tissues.

Sex Cells and Chromosomes

Male and female gametes (sperm and ova) each contain half the normal number of chromosomes. Fertilization restores the diploid number, forming a zygote. Males have XY sex chromosomes, females have XX.

Hormonal Regulation of Male Reproduction

The hypothalamus and anterior pituitary regulate testicular function via two key hormones: Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). FSH stimulates spermatogenesis, while LH stimulates testosterone production by Leydig cells.

Testosterone

Testosterone is essential for sperm maturation, maintenance of accessory organs, development of secondary sexual characteristics, and stimulation of protein synthesis, muscle, and bone growth. It also influences brain development and sexual behavior.

Spermatozoa Structure

A mature sperm cell consists of a head (containing the nucleus and acrosome), midpiece (packed with mitochondria), and tail (flagellum for motility).

The Female Reproductive System

Overview and Main Structures

The female reproductive system is responsible for the production of ova, reception of sperm, fertilization, and support of fetal development. It includes both internal and external organs.

• Internal organs: Ovaries, fallopian tubes, uterus, vagina

• External organs (vulva): Clitoris, labia majora, labia minora, hymen, perineum

Mammary Glands (Breasts)

The mammary glands are accessory organs adapted for milk production (lactation). The areola surrounds the nipple, and Montgomery’s tubules secrete lubricating fluid to protect the skin during breastfeeding.

External Genitalia (Vulva)

The vulva includes the clitoris, labia majora, labia minora, hymen, and perineum. These structures protect the internal genital organs and play a role in sexual arousal.

Female Genital Mutilation (FGM)

FGM refers to procedures that intentionally alter or cause injury to the female genital organs for non-medical reasons. It is a violation of human rights and has significant health risks.

Internal Organs and Structures

The main internal organs are the uterus, ovaries, fallopian tubes, and vagina. The uterus is a hollow, muscular organ that supports fetal development. The cervix is the lower part of the uterus, connecting it to the vagina.

Vagina

The vagina is a muscular tube lined with mucous membrane, extending from the cervix to the external genitalia. It serves as the organ of copulation and the birth canal.

Fallopian Tubes

Each fallopian tube extends from the uterus to the ovary and ends in finger-like fimbriae that help capture the ovulated egg. Fertilization typically occurs in the distal third of the tube.

Uterus

The uterus consists of the body, fundus, and cervix. It maintains an environment suitable for implantation and development of a fertilized egg. If fertilization does not occur, the endometrial lining is shed during menstruation.

Ovaries and Oogenesis

The ovaries are oval organs that store and mature eggs (ova) through oogenesis. Each ovary contains thousands of follicles, each holding an immature oocyte. Ovaries also secrete estrogen and progesterone.

Ovarian Follicles and Ovulation

Females are born with all the follicles they will ever have. Each month, typically one follicle matures and releases an egg (ovulation). Only 400–500 eggs are ovulated in a woman’s lifetime.

The Ovarian Cycle

The ovarian cycle consists of three phases:

• Follicular Phase (Days 1–14): FSH stimulates follicle growth.

• Ovulation (Day 14): LH surge triggers release of the mature oocyte.

• Luteal Phase (Days 14–28): The ruptured follicle becomes the corpus luteum, which secretes progesterone to prepare the endometrium for implantation.

The Menstrual Cycle

The menstrual cycle is typically 28 days and consists of three phases:

• Menstrual Phase (Days 1–5): Shedding of the endometrial lining; average blood loss is 50–100 mL.

• Proliferative Phase (Days 6–14): Endometrial regeneration under the influence of estrogen.

• Secretory Phase (Days 15–28): Progesterone from the corpus luteum prepares the endometrium for implantation; if no pregnancy, the corpus luteum degenerates and menstruation begins.

Fertilization

Fertilization occurs when a sperm cell penetrates an ovum, usually in the fallopian tube. The resulting zygote contains genetic material from both parents and restores the diploid chromosome number.

Key Terms and Concepts

• Corpus luteum: The structure formed from the ruptured follicle after ovulation; secretes progesterone.

• Endometrium: The inner lining of the uterus that is shed during menstruation.

• FSH (Follicle Stimulating Hormone): Stimulates growth of ovarian follicles.

• LH (Luteinizing Hormone): Triggers ovulation and corpus luteum formation.

Summary Table: Male vs. Female Reproductive Structures

Male Structure	Function	Female Structure	Function
Testes	Produce sperm and testosterone	Ovaries	Produce ova, estrogen, progesterone
Epididymis	Sperm maturation/storage	Fallopian Tubes	Transport ova; site of fertilization
Vas Deferens	Transport sperm	Uterus	Supports fetal development
Penis	Copulation, semen delivery	Vagina	Copulation, birth canal