Sexual Reproduction and Development

Introduction to Sexual Reproduction

  • Germ cells (gametes) such as sperm and ova are formed in the gonads through meiosis, which halves the chromosome number.
  • Fertilization involves the fusion of ova and sperm, restoring the original chromosome number.
  • The new individual develops from a zygote to an embryo and then to a fetus.

Sex Determination

  • Each zygote receives 23 chromosomes from each parent, resulting in 23 pairs of homologous chromosomes, which is the diploid number.
  • 22 pairs are autosomal chromosomes, which contain the same genes (but not identical).
  • The remaining pair are the sex chromosomes.

Sex Chromosomes

  • Females have two X chromosomes and always pass on an X chromosome.
  • Males have one X and one Y chromosome and can pass on either.
  • The sperm determines the sex of the child.

Interaction Between the Hypothalamus, Anterior Pituitary, and Gonads

  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are produced in the anterior pituitary glands of both males and females, leading to:
    • Stimulation of spermatogenesis or oogenesis
    • Stimulation of gonadal hormone secretion
    • Maintenance of the structures of the gonads

Onset of Puberty

  • FSH and LH secretion is elevated at birth, remains high for about 6 months, and then declines until puberty.
  • Puberty begins with pulsatile release of LH due to decreased hypothalamic sensitivity to the negative feedback of gonadal hormones.
  • This results in increased secretion of testosterone or estradiol-17β, leading to the development of secondary sex characteristics.

Secondary Sex Characteristics

  • In girls: growth spurt, breast development, menarche (first menstrual flow).
  • In boys: later growth spurt; growth of body, muscle, penis, and testes.
  • In both sexes: body hair stimulated by adrenal androgens.

Female Sexual Development at Puberty

  • Appearance of breast buds: ages 8-13, stimulated by estrogen, progesterone, growth hormone, thyroxine, insulin, and cortisol.
  • Pubic hair: ages 8-14, stimulated by adrenal androgens.
  • Menarche: ages 10-16, stimulated by estrogen and progesterone.
  • Axillary hair: about 2 years after pubic hair, stimulated by adrenal androgens.
  • Eccrine sweat and sebaceous glands, acne: around the same time as axillary hair, stimulated by adrenal androgens.

Male Sexual Development at Puberty

  • Growth of testes: ages 10-14, stimulated by testosterone, FSH, and growth hormone.
  • Pubic and axillary hair: ages 10-15, stimulated by adrenal androgens.
  • Body growth: ages 10-15, stimulated by testosterone and growth hormone.
  • Growth of penis: ages 11-15, stimulated by testosterone.
  • Growth of larynx (voice lowers): same time as penis growth, stimulated by testosterone.
  • Facial hair: about 2 years after pubic hair, stimulated by testosterone.
  • Eccrine sweat and sebaceous glands, acne: around the same time as facial and axillary hair, stimulated by testosterone.

Factors Affecting Puberty

  • Onset of puberty depends on activity levels and body fat.
  • Leptin, secreted by adipose cells, is necessary for the onset of puberty.
  • Exercise may inhibit GnRH secretion, causing more active, slimmer girls to begin puberty later.

Human Sexual Response

  • Four phases:
    • Excitation: increased muscle tone and vasocongestion of sexual organs (arousal).
    • Plateau: continued vasocongestion.
    • Orgasm: contraction of uterus/vagina and male ejaculatory organs.
    • Resolution: body returns to pre-excitation condition.
  • Men experience a refractory period, during which they are unable to ejaculate.

Testes

  • Composed of two compartments:
    • Seminiferous tubules: site of spermatogenesis; Sertoli cells have FSH receptors, and FSH influences spermatogenesis.
    • Interstitial tissue: contains Leydig cells that produce testosterone; also contains blood and lymphatic capillaries. Leydig cells have LH receptors, and testosterone is secreted in response to LH.

Control of Gonadotropin Secretion

  • Testosterone is converted into derivatives in brain cells.
    • Converted by 5α-reductase to DHT, other androgens, or to estradiol by aromatase enzyme.
    • Estradiol inhibits LH secretion.
  • Male and female brains differ due to the effects of testosterone and estradiol.

Testosterone Derivatives Equations

  • Testosterone --(5α-reductase)--> 5α-DHT
  • Testosterone --(aromatase)--> Estradiol-17β

Testosterone Secretion and Age

  • Negative feedback effects of testosterone and inhibin maintain relatively constant gonadotropin secretion in males.
  • Androgen secretion declines slowly, leading to a hypogonadal state by age 70.
  • Physical inactivity, obesity, and drugs can also affect testosterone secretion.

Spermatogenesis

  • Germ cells migrate from the yolk sac to the testes early in embryonic development.
  • Diploid spermatogonia undergo mitosis to increase cell number; daughter cells either become primary spermatocytes or remain spermatogonial cells.
  • Primary spermatocytes undergo meiosis I, resulting in 2 secondary spermatocytes.
  • Secondary spermatocytes undergo meiosis II, resulting in 4 spermatids.
  • This process occurs as cells move toward the lumen of the seminiferous tubules.

Male Accessory Sex Organs

  • Spermatids move from the seminiferous tubules to the rete testis, then to the efferent ductules, and finally to the epididymis.
  • The epididymis is the site of sperm maturation and storage, where sperm become motile.
  • During ejaculation, spermatozoa move from the epididymis to the ductus deferens, then to the ejaculatory duct, and finally to the urethra.

Semen Composition

  • Seminal vesicles and prostate gland add fluid to the sperm to form semen.
    • Seminal fluid: contains fructose (energy for sperm).
    • Prostate fluid: contains citric acid, calcium, and coagulation proteins.

Erection, Emission, and Ejaculation

  • Erection: results from increased blood flow into the erectile tissues of the penis (corpora cavernosa and corpus spongiosum) due to parasympathetic nerve-induced vasodilation of arterioles.
  • Nitric oxide acts as a vasodilator.
  • Venous outflow of blood is partially blocked during erection.
  • Emission: movement of semen into the urethra.
  • Ejaculation: forceful expulsion of semen from the urethra.
  • Both are under sympathetic nervous system control and involve contraction of smooth muscles in tubules, seminal vesicle, prostate, and base of penis.

Male Fertility

  • A sperm count < 15 million/ml semen is called oligospermia, which may cause infertility.
  • Causes: heat, drugs, or anabolic steroids.

Male Contraception

  • Vasectomy: cutting and tying the vas deferens to prevent sperm transport.
  • Does not affect testosterone production or ejaculation.
  • About 70% of men develop antisperm antibodies after a vasectomy.

Female Reproductive Organs

  • Ovaries: female gonads; site of oocyte and sex steroid production.
  • Uterine (Fallopian) tubes: have fimbriae that partially wrap around the ovaries to "catch" the oocyte after ovulation; most common site of fertilization.
  • Uterus: site of embryonic development.
    • Endometrium: inner layer where embryo implants and develops; composed of stratum basale and stratum functionale.
    • Myometrium: middle muscle layer; contracts to expel baby at birth.
    • Perimetrium: outer connective tissue layer.
    • Cervix: narrow bottom region of uterus.
  • Vagina: organ of copulation opening between the labia minora.
  • Clitoris: erectile tissue.

Ovarian Cycle

  • Primary Oocytes: oogonia begin meiosis during gestation, become primary oocytes. A newborn girl has ~2 million, reduced to ~400,000 by puberty, with only ~400 ovulated in her lifetime.
  • Follicles: primary oocytes within primary follicles (one cell layer). FSH causes primary follicles to grow, forming multiple granulosa cell layers.
  • Some develop into secondary follicles with fluid-filled vesicles.

Follicle Development

  • Continued growth fuses vesicles into a single antrum, creating a mature Graafian follicle.
  • Cell layers (corona radiata and zona pellucida) form around the oocyte, acting as barriers to sperm entry.
  • FSH, estradiol, and paracrine signals stimulate Graafian follicle development.
  • Secondary Oocytes
    • As the Graafian follicle grows, the primary oocyte completes meiosis I to become a secondary oocyte (plus a polar body).
    • The secondary oocyte begins meiosis II but stops at metaphase II and will complete only if fertilization occurs.
Ovulation
  • Around the 10th-14th day after menstruation begins, one follicle matures into a Graafian follicle while others regress (atresia/apoptosis).
  • The mature follicle bulges from the ovary, and hormones stimulate it to burst, releasing the secondary oocyte.
  • If not fertilized, the oocyte degenerates after a few days.
Corpus Luteum
  • After ovulation, the remaining follicle becomes the corpus luteum, which secretes estradiol and progesterone.
  • These hormones play a crucial role in the menstrual cycle.

Menstrual Cycle

  • A 28-day cycle of endometrial buildup and sloughing in response to ovarian hormones.
  • Three phases: menstrual, proliferative, and secretory.
  • Changes in the endometrium correlate with changes in ovarian follicles.

Cyclic Changes in the Endometrium

  • Menstrual cycle begins with menstruation. Endometrial development regulated by ovarian estradiol and progesterone.
Proliferative Phase
  • Occurs during the ovarian follicular phase.
  • Rising estradiol levels stimulate growth of the stratum functionale.
  • Spiral arteries develop and the endometrium becomes more vascular and develops progesterone receptors.
Secretory Phase
  • Occurs during the ovarian luteal phase.
  • Progesterone stimulates development of uterine glands to store glycogen.
  • Endometrium thickens and becomes more vascular to nourish an embryo, if fertilized.
Menstrual Phase
  • Results from decreased estradiol and progesterone due to corpus luteum degeneration.
  • Endometrial arteries constrict, cells of the stratum functionale die, and the region is sloughed off.

Menopause

  • Characterized by cessation of ovarian activity and menses.
  • Caused by changes in the ovaries, not decreases in FSH and LH (levels are elevated due to lack of negative feedback).
  • Typically occurs after age 50.

Symptoms of Menopause

  • Due to loss of estradiol.
  • Hot flashes caused by vasomotor disturbances.
  • Atrophy of urethra and vagina walls; vaginal glands no longer produce lubrication.
  • Increased risk of atherosclerosis and osteoporosis.

Osteoporosis

  • Estradiol is needed for bone deposition.
  • Adipose tissue produces estrone (weak estradiol), reducing osteoporosis risk in heavier women.

Introduction to Fertilization, Pregnancy, and Parturition

  • At ejaculation, over 300 million sperm enter the female, but about 100 reach the fallopian tube.
  • Sperm must undergo capacitation to fertilize the ovum, which takes at least 7 hours after ejaculation. This involves:
    • pH increase.
    • Hyperactivation of the flagellum due to progesterone.
  • Capacitated sperm are guided to the oocyte by chemotaxis and thermotaxis.

Fertilization

  • Usually occurs in the uterine tubes.
  • The acrosome cap and zona pellucida cells stimulate the entry of Ca2+Ca^{2+} and the release of acrosomal enzymes. These enzymes digest the zona pellucida.
  • When the sperm enters, Ca2+Ca^{2+} is released from the endoplasmic reticulum, initiating a calcium wave to the opposite side of the entry. This:
    • Prevents polyspermy.
    • Activates the oocyte to complete meiosis and become a haploid ovum.

Fertilization Completion:

  • 12 hours after sperm entry, the ovum's nuclear envelope disappears, and chromosomes from sperm and egg join to form the diploid zygote.

Twins

  • Monozygotic (identical) twins result from a single ovum splitting.
  • Dizygotic (fraternal) twins result from 2 eggs being fertilized by 2 sperm.

Contributions to Zygote

  • Sperm contributes ½ chromosomes and the centrosome.
  • Egg contributes ½ chromosomes, cytoplasm, and all other organelles.

HCG and Pregnancy

  • HCG is secreted by the placenta during pregnancy.
  • It stimulates progesterone production and supports the corpus luteum.
  • Levels double every 2-3 days early in pregnancy.
  • Pregnancy tests detect HCG in urine or blood, indicating pregnancy after a fertilized egg attaches to the uterine lining.

Labor and Parturition

  • Powerful contractions leading to fetus expulsion are stimulated by oxytocin (from the mother’s hypothalamus) and prostaglandins (from the placenta).
  • Labor is initiated by rising levels of corticotropin-releasing hormone (CRH) from the placenta.

Lactation

  • Prolactin (from the pituitary gland) stimulates milk protein production (casein and lactalbumin).
  • Nursing stimulates oxytocin secretion, causing contraction of lactiferous ducts for milk flow (milk-ejection reflex/milk letdown).

Breast Feeding and Immunity

  • IgG antibodies are transferred from mother to child in utero.
  • IgA antibodies are transferred through breast milk.
  • This provides passive immunity for the first months, allowing development of the baby’s active immunity.