Gamete

Reproductive Strategies in Animals

  • Reproduction strategies evolved to support offspring survival in terrestrial and aquatic environments.
  • Oviparity (egg-laying)
    • Embryo develops outside the mother's body inside an egg, receiving nourishment from the yolk.
    • Occurs in:
    • Most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds.
    • Egg shell types (related to protection and water loss):
    • Reptiles and insects produce leathery eggs.
    • Birds and turtles produce hard shells with high calcium carbonate (CaCO₃) content; chicken eggs are an example of hard-shelled eggs.
  • Ovoviviparity
    • Fertilized eggs are retained in the female; embryo nourished by the yolk inside the egg.
    • Young are fully developed when hatched.
    • Occurs in: some bony fishes (e.g., Lebistes reticulatus, the guppy), some sharks, some lizards, some snakes (e.g., Thamnophis sirtalis), some vipers, and some invertebrates (e.g., Madagascar hissing cockroach, Gromphadorhina portentosa).
  • Viviparity
    • The young develop within the female and are nourished from the mother’s blood via a placenta; offspring are born alive.
    • Occurs in: most mammals, some cartilaginous fish, and a few reptiles.
  • Internal fertilization: advantages and implications
    • Protects fertilized eggs from dehydration on land.
    • Embryo is sheltered inside the female, reducing predation on the young.
    • Enhances fertilization by a specific male.
    • Fewer offspring are produced, but their survival rate is higher than with external fertilization.

The Evolution of Reproduction

  • After multicellularity and tissue/organ specialization, reproduction evolved with gonads producing gametes (sperm and eggs).
  • Early reproductive development observed in annelids:
    • Sperm and eggs produced from undifferentiated coelomic cells; stored in the coelom.
    • When the coelom fills, cells are released through an excretory opening or by body rupture.
  • Gonads and meiosis
    • Gonads (testes and ovaries) produce sperm and eggs.
    • Meiosis reduces chromosome number by half (diploid to haploid) and increases the number of cells via division.
  • Insects: complete reproductive systems with separate sexes
    • Sperm produced in testes; travel through coiled tubes to epididymis for storage.
    • Eggs mature in the ovaries; released eggs travel to uterine tubes for fertilization.
    • Spermatheca in some insects stores sperm for extended periods (sometimes up to a year).
    • Fertilization can be timed with environmental or food conditions to optimize offspring survival.
  • Vertebrates: similarities and differences
    • Non-mammals (birds/reptiles) commonly have a cloaca for digestive, excretory, and reproductive systems.
    • Sperm transfer in birds often involves cloacal contact (opposite openings).
    • Mammals have separate openings for different systems; many have a uterus to support developing offspring.
    • Uterine structure varies:
    • Some species have two-chambered uteri when producing large numbers of offspring.
    • Primates and other species that tend to produce fewer offspring may have a single uterus.
  • Sperm transfer modes across vertebrates
    • External fertilization in aquatic environments.
    • Cloacal coupling in some birds.
    • Direct sperm delivery via penis in many mammals.

43.3 | Human Reproductive Anatomy and Gametogenesis (Overview)

  • By the end of this section, you should be able to:
    • Describe human male and female reproductive anatomies.
    • Discuss the human sexual response.
    • Describe spermatogenesis and oogenesis and discuss their differences and similarities.
  • Human reproductive structures and development
    • In utero, tissues develop similarly in both sexes until a low level of testosterone from male gonads triggers male differentiation.
    • Testosterone → male sexual organs.
    • Absence of testosterone → female development.
    • Primitive gonads become: testes (males) or ovaries (females).
    • Tissues that become penis (males) or clitoris (females) are homologous structures.
    • The tissue that becomes the scrotum in males becomes the labia in females.

43.3 | Human Reproductive Anatomy: Male Reproductive Anatomy

  • Scrotum and testes
    • The scrotum houses the testes and provides protection and pathways for vessels, nerves, and muscles.
    • Testes produce sperm and male hormones.
    • Each testis: ≈ 2.5extcmimes3.8extcm2.5 ext{ cm} imes 3.8 ext{ cm} (≈ 1.5′′imes1′′1.5'' imes 1'').
    • The testes are divided into wedge-shaped lobules by septa; inside are seminiferous tubules that produce sperm.
    • Sperm are immobile at body temperature; therefore, the scrotum and penis are external to maintain cooler temperatures needed for motility.
    • In many land mammals, testes are descended outside the body by about 2°C cooler than body temperature; failure to descend can cause infertility.
  • Male reproductive ducts and glands
    • Epididymis: site of sperm maturation; lies along the top/posterior of the testes; has a C-shaped (comma) appearance.
    • Vas deferens (ductus deferens): carries sperm behind the bladder; forms the ejaculatory duct with the duct from the seminal vesicles.
    • Vasectomy: section of the vas deferens is removed to prevent passage of sperm during ejaculation.
  • Semen and components
    • Ejaculate volume: typically 2ext–5extmL2 ext{–}5 ext{ mL} of fluid.
    • Sperm concentration: 50ext–120extmillionsperm/mL50 ext{–}120 ext{ million sperm/mL}.
    • Sperm are haploid; basic sperm anatomy:
    • Tail (flagellum) for motility.
    • Neck containing mitochondria (energy production).
    • Head containing genetic material.
    • Acrosome on the head contains lysosomal enzymes to help penetrate the egg.
    • Semen consists of sperm plus secretions from accessory glands (~90% from glands; ~10% sperm).
  • Accessory glands and semen production
    • Seminal vesicles
    • Located along the posterior border of the urinary bladder.
    • Produce a thick, yellowish, alkaline solution containing mucus, fructose (sperm mitochondrial nutrient), a coagulating enzyme, ascorbic acid, and prostaglandins.
    • Account for about 60% of semen volume.
    • Prostate gland
    • Surrounds the urethra near the bladder.
    • Produces a milky fluid containing citrate, enzymes, and prostate-specific antigen (PSA) to liquefy semen after ejaculation.
    • Accounts for about 30% of semen volume.
    • Bulbourethral glands (Cowper’s glands)
    • Secrete mucus prior to the main semen discharge.
    • Neutralize residual acidity in the urethra from urine.
    • Contribute a small amount to the ejaculate and may contain some sperm.
    • Pre-ejaculate may contain sperm; withdrawal method may not prevent pregnancy.
  • The penis and erectile tissue
    • Penis contains three tubes of erectile tissue: two corpora cavernosa (dorsal) and one corpus spongiosum (ventral).
    • Engorgement with blood causes erection, enabling intercourse.
    • Urethra runs through the penis to the external opening.
    • Sphincters at the bladder entrance close during erection to prevent urine entry.
  • Orgasm and ejaculation
    • Orgasm is a two-stage process:
    • First, glands and accessory organs contract; semen is expelled through the urethra (ejaculation).
    • Second, erectile tissue drains and the penis becomes flaccid.
  • Anatomy summary (Table 43.1)
    • Scrotum: external; function — carry and support testes.
    • Penis: external; function — deliver urine; copulating organ.
    • Testes: internal; function — produce sperm and male hormones.
    • Seminal Vesicles: internal; function — contribute to semen production.
    • Prostate Gland: internal; function — contribute to semen production.
    • Bulbourethral Glands: internal; function — clean urethra at ejaculation.

43.4 | Human Reproductive Anatomy: Female Reproductive Anatomy

  • External female structures (the vulva and breasts as visible exterior features)
    • Breasts: mammary glands plus fat; size influenced by fat behind the gland.
    • Vulva includes: mons pubis, clitoris, labia majora, labia minora, vestibular glands.
  • Internal female structures
    • Ovaries: site of egg production; held in place by ligaments; medulla (nerves and blood vessels) and cortex (follicular tissue).
    • Oviducts (Fallopian tubes): extend from the uterus to the ovaries; not directly attached to ovaries; fimbriae at the distal ends capture the released egg.
    • Uterus: supports embryo/fetus during gestation; endometrium rich in blood vessels and mucus glands; thick smooth muscle wall; contractions aid childbirth; menstrual shedding occurs in part of the endometrium.
    • Cervix: lower portion of uterus that protrudes into the upper vagina; forms the birth canal.
    • Vagina: muscular tube; serves as menstrual flow passage, receptacle for the penis during intercourse, and birth canal.
  • Ovaries and oogenesis (brief overview)
    • Ovaries contain follicles; ovulation releases an egg which travels through the oviduct toward the uterus.
    • Tubal ligation (tubal occlusion) is a form of sterilization analogous to male vasectomy.
  • Follicular dynamics and ovulation
    • Ovary structure includes medulla and cortex; cortex houses follicles containing eggs.
    • At puberty, a cohort of follicles develops; usually one follicle ruptures each cycle to release an egg (ovulation).
    • The released egg is moved toward the uterus via the ciliated, muscular oviduct.
    • The oviduct environment facilitates fertilization typically within its length and time frame.
  • Uterus, endometrium, and labor
    • Endometrium is richly vascularized to support implantation and growth of the embryo.
    • The uterus contracts during labor to help push the baby through the cervix and vagina.
  • The female reproductive cycle and glands
    • The greater vestibular glands lubricate the vaginal opening.

The Breasts and Mammary Glands

  • The breasts consist of mammary glands and surrounding fat.
  • Gland structure: typically 15–25 lobes with ducts that drain at the nipple.
  • Milk production and immune protection example: milk provides nutrients and antibodies for the newborn.

The Oviducts, Uterus, and Menstrual Cycle (Key Concepts)

  • Oviducts (Fallopian tubes)
    • Capture released eggs via fimbriae; ciliated epithelium and smooth muscle propel the egg toward the uterus.
    • Fertilization usually occurs in the oviduct; embryo moves to the uterus within about a week.
  • Uterus
    • Endometrium supports embryo/fetus; cervix functions as part of the birth canal.
    • Menstrual shedding and rebuilding of the endometrium prepare for implantation.
  • Menstrual cycle overview (context for hormonal control)
    • Involves coordinated ovarian follicle development and uterine lining changes in response to hormones.

Sexual Response and Reproduction

  • Sexual response in humans involves both psychological and physiological components.
  • Four phases of the sexual response:
    • Phase 1 – Excitement: vasodilation and vasocongestion; erection; nipple, clitoris, labia, and penis engorge; vaginal lubrication.
    • Phase 2 – Plateau: continued stimulation; outer third of vaginal wall thickens; increased breathing and heart rate.
    • Phase 3 – Orgasm: rhythmic muscular contractions; in males, ejaculation; in females, uterine and vaginal contractions.
    • Phase 4 – Resolution: body returns to baseline; males experience a refractory period during which erection/ejaculation cannot be maintained (minutes to hours).

Gametogenesis: Spermatogenesis and Oogenesis

  • Meiosis produces haploid gametes for sexual reproduction.
  • Spermatogenesis (male)
    • Occurs in the walls of the seminiferous tubules.
    • Spermatogonia (diploid stem cells) reside at the periphery; mitosis yields cells that will differentiate into sperm.
    • Primary spermatocytes undergo the first meiotic division to form secondary spermatocytes (haploid).
    • Secondary spermatocytes undergo the second meiotic division to yield spermatids, which mature into spermatozoa with a flagellum.
    • From one primary spermatocyte, four sperm are produced.
    • Spermatogenesis begins at adolescence under gonadotropic hormone influence and continues throughout life.
  • Oogenesis (female)
    • Occurs in the outer layers of the ovaries; begins with oogonia (germ cells) that proliferate by mitosis during fetal development.
    • By birth, females have a finite pool of oogonia that become primary oocytes arrested in prophase I.
    • At puberty, hormonal signals stimulate follicle development; each cycle may resume meiosis for selected primary oocytes.
    • The primary oocyte completes the first meiotic division to yield a secondary oocyte and a polar body (unequal cytoplasmic division).
    • The secondary oocyte is arrested at metaphase II until fertilization.
    • If fertilized, meiosis II completes, producing a fertilized egg (with a full set of 46 chromosomes) and a second polar body.
    • Egg production is initiated before birth, arrested during fetal development, and resumes cyclically from puberty onward; typically one egg is produced per meiotic cycle, while polar bodies degenerate.

Hormonal Control of Human Reproduction (43.4)

  • The hypothalamus-pituitary-gonadal (HPG) axis

    • The hypothalamus releases gonadotropin-releasing hormone (GnRH).
    • GnRH stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
    • FSH and LH act on the gonads to regulate gametogenesis and sex hormone production.
    • Puberty is necessary for adrenals to release certain hormones required for GnRH production.
    • FSH and LH are produced in both sexes and regulate reproductive processes in both genders (though their roles are commonly described in female contexts).

  • Male hormones and feedback loops

    • FSH acts on Sertoli cells in the testes to promote spermatogenesis; this process is aided by testosterone from Leydig cells.
    • LH stimulates Leydig cells to produce testosterone, contributing to spermatogenesis and the development of male secondary sexual characteristics (e.g., deeper voice, facial/pub hair, sex drive).
    • Testosterone feeds back to the hypothalamus and anterior pituitary to inhibit GnRH, FSH, and LH production (negative feedback).
    • Sertoli cells secrete inhibin, which is released when sperm count is high and inhibits GnRH and FSH to regulate sperm production (negative feedback).

  • Key conceptual links

    • The hypothalamus–pituitary axis integrates signals from puberty and environmental cues to regulate reproductive function.
    • Hormones coordinate gamete production, sexual development, and reproductive cycles.

  • Quick study checks (conceptual recap)

    • Location and function of major male reproductive structures: testes, epididymis, vas deferens, seminal vesicles, prostate, bulbourethral glands, penis.
    • Key features of spermatogenesis vs. oogenesis: sites, timing, and products (4 sperm per primary spermatocyte; single egg per cycle with polar bodies).
    • Hormonal regulation basics: GnRH → FSH/LH; FSH/LH regulate Sertoli/Leydig cells; negative feedback via testosterone and inhibin.
    • Reproductive strategies and their evolutionary trade-offs: number of offspring vs. survival probability; parental investment.

  • Formulas and numerical references

    • Ejaculate volume and sperm count range:
    • Volume ≈ 2ext–5extmL2 ext{–}5 ext{ mL}
    • Sperm concentration ≈ 50ext–120imes106extsperm/mL50 ext{–}120 imes 10^{6} ext{ sperm/mL}
    • Testes temperature requirement for viability: approximately T<em>exttestesextis2extoextClowerthanT</em>extbodyT<em>{ ext{testes}} ext{ is } 2^ ext{o} ext{C lower than } T</em>{ ext{body}}
    • Primary spermatocytes yield four sperm via meiosis: "Four sperm result from each primary spermatocyte."

  • Connections to real-world relevance

    • Understanding placental nourishment and viviparity highlights differences between mammals and other vertebrates in reproductive strategies.
    • The temperature sensitivity of spermatogenesis explains why many mammals have external testes and why some species have seasonal breeding tied to environmental cues.
    • Contraceptive methods like vasectomy (blockage of the vas deferens) or tubal ligation (blockage of oviducts) illustrate practical applications of anatomy.