Chap 46

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Hormonal control of the testes.

Gonadotropin–releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary to secrete two hormones.

Follicle-Stimulating Hormone (FSH) acts on Sertoli cells, which nourish developing sperm.

Luteinizing Hormone (LH) acts on Leydig cells, which produce androgens, chiefly testosterone.

Negative feedback by testosterone regulates production of these hormones.

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Earthworms are hermaphroditic; mature individuals (indicated by the presence of a swollen clitellum) posses both male and female gonads.

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Reproductive anatomy of the human female.

Eggs are produced in the ovary and released into the oviduct where fertilization can occur.

The fertilized zygote travels to the uterus, where it implants in the wall and develop into an embryo.

When development is complete, the child is born through the vagina .

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The trophoblast (outer layer of the blastocyst), together with the endometrium, forms the placenta.
The embryo secretes Human Chorionic Gonadotropin (HCG) to maintain secretion of progesterone and estrogens by the corpus luteum.

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Reproductive anatomy of the human male.

The testis (held in an extension of the abdomen called the scrotum) is where sperm is formed.

Sperm complete maturation in the epididymis, and is stored in the vas deferens (ductus deferens).

Seminal fluid produced in the seminal vesicle, prostate gland, and bulbourethral gland, mix with sperm to produce semen and is delivered (ejaculated) to the urethra, which is also the exit duct from the urinary bladder.

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  1. The hypothalamus releases GnRH (Gonadotropin-Releasing Hormone).
  2. The pituitary secretes FSH and LH.
  3. The FSH stimulates follicle growth, aided by LH.
  4. The follicles start to make estrogen. Several follicles begin to grow with each cycle, but usually only one matures; the others disintegrate.
  5. Around day 12 estrogen levels rise steeply.
  6. Rising estrogen stimulates a LH surge by positive feedback.
  7. A day after, the follicle ruptures, releasing the secondary oocyte by ovulation.
  8. The follicular tissue left behind in the ovary forms the corpus luteum, which secretes progesterone and estrogen.
  9. Rising progesterone and estrogen levels stimulate thickening of the uterine wall, or endometrium.
  10. If pregnancy had occurred, the corpus luteum continues secreting progesterone and estrogen to maintain the endometrium; otherwise it disintegrates and hormone levels drop, resulting in the loss of excess uterine tissue as menstrual flow.

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Oogenesis.

Germ cells differentiate into oogonia.

Oogonia profliferate by mitotsis, then start meiosis but the process stops at prophase I; these are primary oocytes, enclosed in small follicles.

Beginning at puberty, periodic secretion FSH induces a few primary oocytes to complete meiosis I, each producing a polar body and a haploid secondary oocyte.

One of the secondary oocytes completes growth in the ovary and begins meiosis II, whch is arrested at metaphase II.

The mature follicle ruptures and releases the oocyte from the ovary, leaving behind the corpus luteum; this is ovulation.

Completion of meiosis II does not occur until the oocyte is penetrated by a sperm, producing a second polar body and the ovum.


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After ovulation, the follicle left in the ovary becomes the corpus luteum, which secretes progesterone and estrogen.

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Parthenogenesis. The whiptail lizard (Cnemidophorus uniparens) is an all–female species, and reproduce by parthenogenesis, the development of an unfertilized egg. The lizard on top plays the role of a male; they switch sex roles every 2 or 3 weeks during the breeding season, mediated by sex hormones. The ovaries grow as estrogen levels rise, and the lizard behaves like a female. After ovulation, estrogen level drops and progesterone level rises; eliciting male behavior. Many aphid species reproduce by parthenogenesis without sexual behavior.

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The sperm head is tipped with an acrosome, which contains hydrolytic enzymes to help the sperm penetrate the egg.

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Spermatogenesis.

Germ cells near the outer wall of the seminiferous tubules differentiate into spermatogonia.

Each spermatogonium differntiates into a primary spermatocyte.

By meiosis, each primary spermatocyte produces haploid secondary spermatocytes, then 4 equal-sized spermatids, migrating towards the lumen (central opening).

Sertoli cells supply nutrients for the spermatids, which mature into motile sperm in the epididymis.


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