Chapter 24 - Reproduction and Development (Oct. 21, 1999)

Basic Patterns of Reproduction

Gametogenesis Begins in utero and Resumes during Puberty

Sexual Differentiation in the 2nd Month of Human Embryo

Germ cells of embryonic gonads undergo many mitotic divisions - this increases number of germ cells called spermatogonia or oogonia.

Germ cells then undergo meiosis, i.e., chromosomal reduction from 46 (diploid) to 23 (haploid) and lead to maturation. Involves 1st and 2nd meiotic division that respectively produce primary and secondary spermatocyte or oocyte.

Process similar, but timing of gametogenesis very different in male & female

Male: mitosis up to 2-3 months, then stops until puberty. Meiosis & maturation then occurs, but mitosis of germ spermatogonia continues throughout "life"

Female: mitotic division & 1st meiotic division completed by 5th month. Resumes in puberty on a periodic basis known as the reproductive cycle. Selected secondary oocytes mature and one is released as a mature egg at ovulation.

Hormonal Control of Reproduction is Directed by the Brain

Gonadotropin releasing hormone (GnRH) control secretion of FSH and LH

Feedback pathways: long loop (gonadal steroids) & short loop (GnRH, FSH, LH) responses

Male Reproduction

Anatomy

penis, scrotum, corpus spongiosum, corpus cavernosum, central artery, dorsal blood vessels, testes, seminiferous tubules, epididymis, ductus (vas) deferens, accessory glands (prostate gland, bulbourethral gland, seminal vesicles)

Sperm Production takes place in the Testes

Seminiferous tubules: 2 types of cells, developing spermatocytes and Sertoli cells.

Spermatogonia that undergo replicative divison (mitosis) are clustered at the periphery of the tubules. As they enter meiosis they develop into spermatocytes and then into haploid spermatids, all while migrating toward the lumen, surrounded and nursed by Sertoli cells. Spermatids undergo a morphological transformation into sperm (spermatozoa), and are released into lumen accompanied by Sertoli fluid secretion. In the epididymis they complete their maturation into motile cells.

Leydig cells, located in interstitium outside of semiferous tubules, secrete testosterone

Spermatogenesis requires gonadotropins and testosterone.

FSH works on Sertoli cells; LH works on Leydig cells to secrete testosterone, which in turn works on Sertoli cells. Sertoli cells then promote and modulate spermatogenesis.

Male Accessory Glands Contribute Secretions to the Semen

Androgens Influence Primary & Secondary Sex Characteristics

1° sex characteristics include the differentiaon of the internal and external sex organs
2° include body shape, body hair, muscular development, vocal chords, sex drive, etc.

Female Reproduction

Female Reproductive Anatomy

labia majora & minora, clitoris, vagina, hymen, cervix, uterus, endometrium, myometrium, Fallopian tube (oviduct), fimbriae, ovary, stroma, ovarian follicles, primary oocyte, primary follicle, granulosa cells, theca, corpus luteum

The Ovary, like the Testis, Produces both Gametes and Hormones

Human Female Reproductive Cycles last about 1 Month

Menstrual cycle: composed of 2 interrelated cycles

Ovarian cycle: 3 phases, follicular phase, ovulation, luteal phase

Uterine cycle: uterine lining changes; menses, proliferative phase, secretory phase

Ovarian & uterine cycles under control of GnRH and FSH & LH. Cells of the ovary produce steroid & peptide hormones in response to gonadotropins.

Early follicular phase: First day of menstruation is start of cycle. Rising FSH influences maturation of several primary follicles. As they grow FSH influences production of estrogen by granulosa cells. LH stimulates androgen production by thecal cells, which go to granulosa cells for conversion to estrogen. Estrogen has negative effect on release of GnRH, FSH & LH by hypothalamus and pituitary, but has positive effect on estrogen production by granulosa cells. Usually one follicle goes to full maturity. Menstruation is completed.

Late follicular phase: Estrogens promote growth of endometrial lining (i.e., proliferative phase of uterine cycle). Rapidly rising levels of estrogen now exert positive feedback on release of GnRH, FSH & LH. There is a surge of GnRH secretion which causes a surge in LH (also FSH) secretion. The primary oocyte undergoes meiotic division eliminating the first polar body, becoming an egg or secondary oocyte.

Ovulation: ~16-24 hr after LH peaks ovulation occurs. The egg, surrounded by some granulosa cells is swept into Fallopian tubes and carried away to be fertilized or die. Rise in LH also causes remaining follicular cells to transform into luteal cells (corpus luteum).

Early luteal Phase: Corpus luteum produces steadily increasing amounts of progesterone and some estrogen, which in turn exerts negative feedback on hypothalamic/ent. pituitary axis. This effect further promoted by luteal hormone inhibin. Progesterone causes maturation of endometrium, with endometrial glands coiling and becoming highly vasculaized, preparing for pregnancy, which in turn leads to ovulation. Progesterone also has thermogenic effect: during luteal phase the body temp. rises by ~0.3-0.5°C.

Late Luteal Phase: If there is no pregnancy, corpus luteum lives for about 12 days. Degeneration causes decrease in progesterone and estrogen, removing negative feedback on hypothalamic/ent. pituitary axis, thus FSH and LH secretion increase. Lack of progesterone causes degeneration of endometrial lining; blood vessels constrict and cells begin to die and slough its surface leading to menstruation and the beginning of the next cycle.

Estrogens and Androgens Contribute to Secondary Sex Characteristics

fat deposition & female shape, breast development (some female 2° sex characteristics are governed by androgens, e.g., pubic & axillary hair, sex drive)

Procreation

The Human Sexual Response

Coitus has 4 phases: 1) excitment, 2) plateau, 3) orgasm, and 4) resolution

Erection and Ejaculation

Penile erection is a state of vasocongestion, i.e., arterial inflow to spongy tissue exceds venous outflow. Tactile stimuli sensed by mechanoreceptors. Sensory neurons - spinal integration center - inhibition of sympathetic vasoconstriction of penile arterioles. Simultaneous parasympathetic vasodilation of penile arterioles via nitric oxide (NO). [NO helps to promote formation of cyclic GMP which promotes vasodilation and erection; breakdown of cyclic GMP reverses the effect; viagra slows down the breakdown of cyclicGMP]. Arterial blood inflow swells penis which also serves to passively compress veins, therefore trapping blood. In male climax coincides with emission (movement of sperm from vas deferens into urethra) and ejaculation (rapid muscular contractions propelling sperm and semen).

Contraception