Oogenesis:
- Formation of a mature female gamete or ovum is called oogenesis.
- Oogenesis starts during embryonic stage, 25th week of the fetal age.
- Germinal epithelium of ovary divided mitotically to produce millions of gamete mother cell or oogonia.
- No oogonia formed or added after birth.
- Oogonia enters into meiosis-I and proceeds upto diakinesis of Prophase-I and get suspended, at this stage called primary Oocytes.
- Each primary oocyte surrounded by layers of granulose cells and then called primary follicle.
- At puberty only 60,000 to 80,000 primary oocytes are left in each ovary.
- After puberty primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
- The secondary follicle transformed into tertiary follicle, characterized by a fluid filled cavity called antrum.
- The theca layers organized into an inner theca interna and outer theca externa.
- During the growth of primary follicle into tertiary follicle during puberty, the primary oocyte restarts its first meiotic division and completes it within tertiary follicle resulting two unequal haploid cells.
- Large haploid cell is called secondary oocyte.
- A tiny cell called first polar body.
- The secondary oocyte retains bulk of the nutrient rich cytoplasm of primary oocyte.
- The tertiary follicle having secondary oocyte further changes into Graafian follicle.
- The secondary oocyte surrounded by a new membrane, zona pellucida.
- The secondary oocyte undergoes second meiotic division continued upto metaphase-II and get suspended until entry of sperm.
- At this stage Graafian follicle releases secondary oocyte from the ovary by the process called ovulation.
- On entry of a sperm into the secondary oocytes stimulates it to complete meiosis-II and there is formation of a haploid ovum and a second polar body (n).
Menstrual cycle:
- Reproductive cycle of female primates is called menstrual cycle.
- The first menstruation begins at puberty is called Menarche.
- Menstrual cycle repeated at an average interval of 28/29 days.
- One ovum is released in the middle of each menstrual cycle.
Menstrual cycle has following phases:
Menstrual phase:
- 1st phase of menstrual cycle.
- Menstrual flow occurs.
- Lasts for 3-5 days.
- Breakdown of endometrial lining and blood vessel.
- Mucus and blood comes out through vagina.
- It occurs only when ovum released but no fertilization.
- Lack of menstruation is the indication of pregnancy.
Follicular phase:
- Menstrual phase followed by follicular phase.
- Primary follicle becomes Graafian follicle.
- Regeneration and proliferation of uterine endometrium.
- LH and FSH level increases gradually in follicular phase.
- Level of estrogen increases as it is secreted from growing follicle.
- It lasts for 5-13 days.
Ovulatory phase:
- FSH and LH attain peak level in this period (14th day).
- This is called LH surge, which induces rupture of Graafian follicle and release of ovum from the ovary called ovulation.
Luteal phase:
- Remaining part of Graafian follicle transformed into corpus luteum.
- Coupus luteum produces large amount of progesterone.
- Progesterone maintains the uterine endometrium, and prepares it for implantation.
- Thickness of uterine endometrium increase in many folds, due to proliferation.
- If there is fertilization, corpus luteum grows further and pregnancy continued, menstrual cycle stopped.
- In the absence of fertilization corpus luteum degenerates.
- Disintegration of endometrium leading to menstruation.
- Menstrual cycle ceases around 50 years of age, called menopause.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
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