Topic 2 Human Reproductive Anatomy: Gametogenesis in Humans

Question 1

• Gametogenesis describes the meiotic cell divisions that produce eggs (oogenesis) and sperm (spermatogenesis). The egg contains most of the cytoplasm, RNA, organelles, and nutrients needed by the developing embryo, which explains why the egg is much larger in size than the sperm. The sperm contributes very little cytoplasm during fertilization. Let’s go through the processes of oogenesis and spermatogenesis:

Answer 1

Note

Question 2

• Begins during the female embryonic development (before birth). Oogonia (fetal cells) undergo mitosis and become primary oocytes. These primary oocytes begin meiosis, but remain in prophase I until puberty. During puberty, one primary oocyte is selected and stimulated via FSH to continue its development through meiosis I during the ~28 day menstrual cycle.
  a. This primary oocyte continues its development within a follicle, which is formed via encircling cells that protect and nourish the oocyte.
  b. Within the follicle, the oocyte completes meiosis I and becomes the secondary oocyte, which consists mostly of cytoplasm. The secondary oocyte also contains a polar body, which has much smaller cytoplasm content, and may or may not divide, but its products disintegrate.
  c. The secondary oocyte remains arrested in metaphase II until ovulation occurs.

Answer 2

1. Oogenesis

Question 3

1. Releases the secondary oocyte from a vesicular follicle, which is stimulated by an LH surge. If this oocyte is fertilized by a sperm, the oocyte completes meiosis II, and the resulting ovum/egg becomes diploid. The polar body degenerates.
   a. At puberty, FSH stimulates the growth of granulosa cells around the primary oocyte, which release a viscous substance that forms the zone pellucida, a jelly like layer around the egg. The structure at this stage is still a primary follicle
   b. Next, theca cells differentiate from the interstitial tissue and grow around the follicle to form a secondary follicle. Upon stimulation, by LH, theca cells secrete androgen, which is converted to estradiol (a type of estrogen) by the granulosa cells in the presence of FSH and is secreted into the blood.
   c. Typically, estradiol inhibits LH secretion by the anterior pituitary, but just before ovulation, estradiol levels rise rapidly, causing a dramatic increase in LH secretion.

Answer 3

2. Ovulation

Question 4

• A common misconception regarding a developing human ovum is that the route from ovary to uterus is completely enclosed. After ovulation, a secondary oocyte is briefly located in the body cavity prior to entering the oviduct.

Answer 4

Note

Question 5

• Spermatogenesis begins at puberty within the seminiferous tubules of testes. This differs from oogenesis, which begins during fetal development.

Answer 5

Spermatogenesis

Question 6

• Undergo mitosis and become primary spermatocytes. These primary spermatocytes undergo meiosis I and form two secondary spermatocytes, which undergo meiosis II to become four spermatids

Answer 6

1. Spermatogonia Cells

Question 7

• Are found in seminiferous tubules, provide nourishment, and “nurse” spermatids as they differentiate into mature spermatozoa (sperm). The sperm complete maturation (gain motility and are stored) in the epididymis.

Answer 7

2. Sertoli Cells

Question 8

• The combination of spermatozoa and fluids that leave the penis upon ejaculation.

Answer 8

3. Semen

Question 9

• The penultimate step in the maturation of the spermatozoa while in the vagina that allows for egg penetration. This is the final maturation of spermatozoa.
  a. During capacitation, physiological changes occur to the sperm, including changes in intracellular ion concentration, motility, and metabolism.

Answer 9

4. Capacitation

Question 10

• Eggs release progesterone which aids in sperm motility and increases the likelihood of fertilization. Also, spermatogenesis is a continuous process, while oogenesis is discontinuous.

Answer 10

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