Gametogenesis, the Menstrual Cycle and Fertilisation: Quick Revision

A brief note on Gametogenesis, the Menstrual Cycle and Fertilisation. Read it before a test. Try each question at the end before you open its answer.

Gametogenesis

• Spermatogenesis: spermatogonia (46) divide by mitosis. A primary spermatocyte (46) gives two secondary spermatocytes (23) and then four spermatids (23). Spermiogenesis makes the sperms, and spermiation sets them free.
• Hormones in the male: GnRH from the hypothalamus. LH acts on the Leydig cells (androgens). FSH acts on the Sertoli cells.
• Oogenesis: begins in the foetus. The primary oocyte stops in prophase I. A tertiary follicle completes meiosis I and gives a secondary oocyte and a first polar body. Meiosis II is completed only at fertilisation and gives the ovum and the second polar body.
• Results: one primary spermatocyte gives four sperms. One primary oocyte gives one ovum and up to three polar bodies.

The menstrual cycle

• Menarche at puberty and menopause at about 50 years. Average cycle 28 or 29 days. One ovum in each cycle.
• Phases: menstrual (3 to 5 days), follicular, ovulation (about day 14), luteal.
• Hormones: the surge of LH causes ovulation. The corpus luteum makes progesterone. Oestrogen comes from the growing follicles.
• No fertilisation: the corpus luteum degenerates, the progesterone falls, and menstruation begins. In pregnancy the cycle stops.
• Ovulation day is the cycle length minus 14.

Fertilisation to implantation

• Insemination, then fertilisation in the ampulla. One sperm enters, other sperms are blocked, meiosis II is completed, and the nuclei fuse to make a zygote (46).
• Sex: the ovum always carries X. The sperm carries X or Y.
• Cleavage: 2, 4, 8, 16 blastomeres. Morula: 8 to 16 cells. Blastocyst: trophoblast and inner cell mass.
• Implantation: the trophoblast attaches to the endometrium, and the blastocyst becomes embedded.

A worked example

A woman has cycles of 32 days. On which day does she ovulate?

1. Ovulation takes place 14 days before the next menstruation.
2. $32 - 14 = 18$. She ovulates on day 18.

Practice with new numbers

A program to try

The program finds the end of the follicular phase and the luteal days of a cycle.

Mistakes to avoid

• Saying that a primary oocyte gives four ova. It gives one ovum and polar bodies.
• Saying that ovulation is always on day 14. That is true only for a cycle of 28 days.
• Putting the trophoblast and the inner cell mass the wrong way. The trophoblast attaches. The inner cell mass becomes the embryo.
• Saying that the sex is decided by the ovum. The ovum always carries X.

Check yourself

1. Which hormone causes ovulation?

2. Which cells of the testis does FSH act on?

3. Where does fertilisation usually take place?

4. What does the zygote become after several cleavage divisions, before it is a blastocyst?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapters 27 and 28.