ch. 27 Reproductive System

Fundamental Objectives of the Reproductive System

• The reproductive system is unique in that it does not become functional until the period of puberty.

• The primary purpose of the system is to produce offspring through four critical tasks:

  • Formation of Gametes: Producing specialized cells for sexual reproduction ($\text{sperm}$ in males and $\text{ova}$ or eggs in females).

  • Copulation: Bringing male and female gametes together through sexual intercourse.

  • Fertilization: Combining the genetic information from gametes to form a $\text{zygote}$. The zygote is the first cell of a new organism from which all body cells arise.

  • Support of Ontogeny: Supporting the development of the fetus (gestation) and the birth of the baby (parturition).

Shared Features of Male and Female Systems

• Homologues: Many structures share a common origin and often shared functions.

  • Male testes are homologous to female ovaries.

  • The male penis is homologous to the female clitoris.

• Gonads (Primary Sex Organs): Testes and ovaries produce two essential products:

  • Gametes: Produced via a specialized cell division called $\text{meiosis}$.

  • Sex Hormones (Steroids): $\text{Testosterone}$ in males; $\text{estrogens}$ and $\text{progesterone}$ in females. These are vital for the development and function of reproductive and other organs, as well as sexual behavior and drives.

• Accessory Reproductive Organs: These include various ducts, glands, and external genitalia.

The Hypothalamic-Pituitary-Gonadal (HPG) Axis

• The HPG axis regulates gamete and hormone production through complex interactions between the hypothalamus, anterior pituitary, and gonads.

• Gonadotropin-releasing hormone (GnRH): Secreted by the hypothalamus, it reaches the anterior pituitary via the hypophyseal portal system.

• Gonadotropins: In response to GnRH, the anterior pituitary releases Follicle-stimulating hormone ($\text{FSH}$) and Luteinizing hormone ($\text{LH}$).

  • In both sexes, these act on the gonads to stimulate gamete production and hormone secretion.

• Feedback Loops:

  • Sex hormones from the gonads exert negative feedback at the hypothalamus and anterior pituitary.

  • Inhibin: Released by the gonads (stimulated by $\text{FSH}$), it exerts specific negative feedback on $\text{FSH}$ release from the anterior pituitary.

• Puberty Activation:

  • Before puberty, low levels of sex hormones are sufficient to suppress $\text{GnRH}$.

  • As puberty approaches, the hypothalamus becomes less sensitive to this inhibition and begins releasing $\text{GnRH}$ in a pulse-like manner.

  • The threshold for inhibition continues to rise until adult hormonal patterns are established.

The Process and Terminology of Meiosis

• Meiosis is a unique nuclear division occurring only in the gonads to reduce chromosome numbers by half.

• Key Terminology:

  • Sister Chromatids: Identical copies of a chromosome joined by a centromere, created during DNA replication in interphase.

  • Homologous Pairs: One paternal and one maternal chromosome that carry genes for the same characters (e.g., eye color). Different versions are called traits.

  • Nonsister Chromatids: Paternal and maternal chromatids within a pair that exchange DNA via crossing over.

  • Diploid ($2n$): Cells with two sets of chromosomes. In humans, $2n = 46$ ($23\text{ pairs}$).

  • Haploid ($n$): Gametes contain one set of chromosomes. In humans, $n = 23$.

• Sequence of Events:

  • One round of DNA replication is followed by two consecutive nuclear divisions: Meiosis I and Meiosis II, resulting in four haploid daughter cells.

  • Meiosis I (Reduction Division): Homologous pairs separate, reducing the number from $2n$ to $n$.

    • Synapsis: Homologous chromosomes pair up into tetrads (four chromatids).

    • Crossing Over: Exchange of genetic material at points called $\text{chiasmata}$.

    • Independent Assortment: Tetrads line up randomly at the metaphase plate.

  • Meiosis II (Equational Division): Sister chromatids separate, resulting in four genetically unique haploid cells. No DNA replication occurs between Meiosis I and II (interkinesis).

Anatomy of the Male Reproductive System

• Testes: The primary male gonads located in the scrotum.

  • Tunics: Surrounded by the outer $\text{tunica vaginalis}$ (derived from the peritoneum) and the inner fibrous $\text{tunica albuginea}$.

  • Lobules: Inner septa divide the testis into lobules containing $\text{1--4 seminiferous tubules}$, the sites of sperm production.

  • Sperm Path: Seminiferous tubules $\rightarrow$ straight tubule $\rightarrow$ rete testis $\rightarrow$ efferent ductules $\rightarrow$ epididymis.

• The Scrotum: A pouch of skin and superficial fascia suspended at the root of the penis.

  • Maintains testes at approximately $3^{\circ}\text{C}$ lower than core body temperature, which is necessary for viable sperm production.

  • Two muscle sets contract when cold and relax when warm to regulate temperature.

• The Spermatic Cord: Encloses nerve fibers, blood vessels, lymphatics, and the ductus deferens in a connective tissue sheath.

• Accessory Ducts:

  • Epididymis: Comma-shaped organ where sperm mature (taking about $20\text{ days}$) and gain motility. Sperm can be stored here for several months.

  • Ductus Deferens (Vas Deferens): Extends through the inguinal canal into the pelvic cavity. It joins the seminal gland duct to form the ejaculatory duct. Vasectomy involves cutting and ligating this duct.

  • Urethra: Conveys both urine and semen. It has three regions: prostatic, intermediate (membranous), and spongy (runs through the penis).

• The Penis: The copulatory organ consisting of a root, body, and glans penis.

  • Erectile Tissue: Includes the paired $\text{corpora cavernosa}$ and the single $\text{corpus spongiosum}$ (which surrounds the urethra).

  • Prepuce (Foreskin): A loose cuff of skin; surgical removal is called circumcision.

• Male Accessory Glands:

  • Seminal Glands: Produce a yellowish, viscous alkaline fluid containing fructose, citric acid, and prostaglandins. Accounts for \text{70%} of semen volume.

  • Prostate: Encircles the urethra; secretes a milky, slightly acidic fluid containing citrate and prostate-specific antigen ($\text{PSA}$). Accounts for \text{30%} of semen volume.

  • Bulbo-urethral Glands: Produce thick, clear mucus during arousal to lubricate the glans and neutralize acidic urine.

Male Sexual Response and Clinical Imbalances

• Erection: A parasympathetic reflex. Sexual excitement triggers the release of nitric oxide ($\text{NO}$), causing local vasodilation and engorgement of erectile bodies. Expansion of corpora cavernosa compresses veins, trapping blood.

• Ejaculation: A sympathetic spinal reflex.

  • Emission: Ductus deferens, prostate, and seminal glands contract, emptying contents into the prostatic urethra.

  • Expulsion: Sebum in the urethra triggers rhythmic contractions of the bulbospongiosus muscles, propelling semen at speeds up to $500\text{ cm/s}$ ($11\text{ mph}$).

• Resolution: A period of relaxation following ejaculation. The latent (refractory) period is the time during which a male cannot achieve another orgasm, which lengthens with age.

• Homeostatic Imbalances:

  • Testicular Cancer: Most common in males aged $15\text{--}35$. Cryptorchidism (undescended testes) is a major risk factor.

  • Benign Prostatic Hyperplasia (BPH): Affects 50\text{%} of men at age $50$ and 80\text{%} at age $70$. Constricts the urethra.

  • Prostate Cancer: Third leading cause of cancer death in males.

  • Erectile Dysfunction (ED): Affects 50\text{%} of American males over $40$. Often linked to diabetes or vascular issues.

Spermatogenesis: The Formation of Sperm

• Spermatogenesis occurs in the seminiferous tubules starting at puberty (approx. age $14$). Healthy adults produce $\text{400 million}$ sperm daily.

• Cell Types in Tubules:

  • Sustentocytes (Nurse Cells): Surround spermatogenic cells, provide nutrients, and form the blood-testis barrier via tight junctions to protect haploid cells from the immune system.

  • Spermatogenic Cells: Develop into sperm.

  • Myoid Cells: Contract to move fluid through tubules.

  • Interstitial Endocrine (Leydig) Cells: Produce androgens, primarily testosterone.

• Stages of Spermatogenesis:

  1. Mitosis of Spermatogonia: Spermatogonia divide into Type A (stem cell precursor) and Type B (becomes primary spermatocyte).

  2. Meiosis: Primary spermatocyte ($2n$) $\rightarrow$ Meiosis I $\rightarrow$ two secondary spermatocytes ($n$) $\rightarrow$ Meiosis II $\rightarrow$ four spermatids ($n$).

  3. Spermiogenesis: Transformation of spermatids into functional sperm by growing a tail and losing excess cytoplasm.

     • Head: Contains the nucleus and the enzyme-rich $\text{acrosome}$.

     • Midpiece: Contains mitochondria for energy.

     • Tail: Flagellum for locomotion.

Anatomy of the Female Reproductive System

• Ovaries: Female gonads that produce ova and hormones (estrogens and progesterone).

  • Held by the ovarian ligament (medial), suspensory ligament (lateral), and mesovarium.

• Female Duct System:

  • Uterine Tubes (Fallopian Tubes): Site of fertilization. Regions include the $\text{infundibulum}$ (with fimbriae), $\text{ampulla}$ (typical fertilization site), and $\text{isthmus}$.

  • Uterus (Womb): Hollow, thick-walled organ. Regions include the body, fundus, and cervix. The uterine wall has three layers: $\text{perimetrium}$ (outer), $\text{myometrium}$ (smooth muscle), and $\text{endometrium}$ (mucosal lining).

    • The endometrium has a stratum functionalis (shed during menses) and a stratum basalis (rebuilds the functional layer).

  • Vagina: Birth canal and organ of copulation. Mucosa is stratified squamous epithelium. The acidic pH protects from infection but is hostile to sperm.

• External Genitalia (Vulva):

  • Mons Pubis: Adipose over the pubic symphysis.

  • Labia Majora: Hair-covered skin folds (homologue of the scrotum).

  • Labia Minora: Skin folds enclosed by the majora (homologue of the spongy urethra).

  • Clitoris: Erectile tissue (homologue of the penis), rich in innervation.

• Mammary Glands: Modified sweat glands with $15\text{--}25$ lobes. Alveoli produce milk which flows into lactiferous ducts and sinuses to the nipple.

Oogenesis and the Ovarian Cycle

• Oogenesis:

  • Begins in the female fetus; oogonia produce primary oocytes arrested in Prophase I.

  • At birth, a female has approx. $\text{1 million}$ oocytes (reduced from $\text{7 million}$ in the fetus). By puberty, $\text{400,000}$ remain.

  • Each month, one primary oocyte completes Meiosis I to form a secondary oocyte and a first polar body.

  • The secondary oocyte arrests in Metaphase II and only completes Meiosis II if fertilized, yielding an ovum and a second polar body.

• The Ovarian Cycle:

  • Follicular Phase (Days 1\text{--}14): Dominant follicle is selected and secretes increasing estrogens.

  • Ovulation: High estrogen levels trigger an LH surge, causing the ovary wall to rupture and release the secondary oocyte.

  • Luteal Phase (Days 15\text{--}28): Ruptured follicle transforms into the corpus luteum, which secretes progesterone and estrogen. If no pregnancy occurs, it degenerates into the corpus albicans.

The Uterine (Menstrual) Cycle

• Menstrual Phase (Days 0\text{--}4): Functional layer of the endometrium is shed; ovarian hormones are at their lowest.

• Proliferative Phase (Days 5\text{--}14): Estrogens prompt the generation of a new functional layer and thin the cervical mucus.

• Secretory Phase (Days 15\text{--}28): Progesterone from the corpus luteum converts the functional layer into a secretory mucosa and thickens cervical mucus to form a plug.

• Homeostatic Imbalances:

  • Ectopic Pregnancy: Implantation outside the uterus, usually in the uterine tube.

  • Amenorrhea: Cessation of menstruation, often seen in female athletes with extremely low body fat, linked to low levels of the hormone $\text{leptin}$.

Sexually Transmitted Infections (STIs)

• Chlamydia: Most common bacterial STI in the U.S. Major cause of Pelvic Inflammatory Disease (PID) and sterility.

• Trichomoniasis: Parasitic infection; symptoms include yellow-green discharge with odor.

• Gonorrhea: Bacterial infection of reproductive/urinary mucosae; becoming increasingly antibiotic-resistant.

• Syphilis: Bacterial infection; features a painless $\text{chancre}$ in primary stage; secondary stage involves a rash; tertiary stage can destroy bones and the CNS. Treated with penicillin.

• Human Papillomavirus (HPV): Most common STI overall; linked to nearly all cervical cancers.

• Genital Herpes: Caused by herpes simplex virus; characterized by blister-like lesions and periods of remission.

Developmental and Late-Life Aspects

• Genetic Sex: Determined by the sperm at fertilization. $\text{XX}$ is female; $\text{XY}$ is male. The SRY gene on the Y chromosome initiates testes development.

• Nondisjunction:

  • Turner's Syndrome ($45,XO$): Females with one X; usually short and fail to develop ovaries.

  • Klinefelter's Syndrome ($47,XXY$): Males with extra X chromosomes; usually sterile.

• Menopause: Occurs when a year has passed without menstruation, usually between ages $46\text{--}54$. Estrogen decline leads to organ atrophy, bone loss, and cardiovascular risks.

• Andropause: The steady decline in testosterone secretion in aging males.