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1. OOGENESIS DR. BRANCH

2. LESSON OBJECTIVES To understand the steps involved in oogenesis The student after completing this module should know the stages of development of the oocyte The student should also be able to relate the previous class on meiosis to oogenesis

3. INTRODUCTION Oogenesis is the process whereby oogonia differentiate into mature oocytes. Maturation of oocytes begins before birth

4. RECAP OF MEIOSIS Meiosis I Events Synapsis : pairing of 46 homologous duplicated chromosomes. Crossing over : exchange of large segments of DNA. Alignment : alignment of 46 homologous duplicated chromosomes at the metaphase plate. Disjunction : separation of 46 homologous duplicated chromosomes from each other; centromeres do not split . Cell division: formation of two secondary gametocytes (23 duplicated chromosomes, 2N). Meiosis II Events Synapsis: absent. Crossing over: absent. Alignment: alignment of 23 duplicated chromosomes at the metaphase plate. Disjunction: separation of 23 duplicated chromosomes to form 23 single chromosomes; centromeres split. Cell division: formation of four gametes (23 single chromosomes, 1N).

5. MATURATION OF OOCYTES BEGINS BEFORE BIRTH Once Primordial Germ cells (46, 2N) have arrived in the gonad of a genetic female ( week 6 ), they differentiate into oogonia . These cells undergo a number of mitotic divisions , and by the end of the third month , they are arranged in clusters surrounded by a layer of flat epithelial cells Whereas all of the oogonia in one cluster are probably derived from a single cell, the flat epithelial cells, known as follicular cells, originate from surface epithelium covering the ovary.

6. MATURATION OF OOCYTES BEGINS BEFORE BIRTH Oogonia enter meiosis I and arrest in prophase I to form primary oocytes (46, 4N). All primary oocytes are formed by the 5 th month of life (7 million primary oocytes). No oogonia are present at birth. At this time (5 th month), cell death begins, and many oogonia as well as primary oocytes degenerate and become atretic. By the 7 th month, majority of oogonia have degenerated except for a few near the surface.

7. MATURATION OF OOCYTES BEGINS BEFORE BIRTH All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a layer of flat follicular epithelial cells. A primary oocyte, together with its surrounding flat epithelial cells, is known as a primordial follicle . Primary oocytes remain dormant in prophase (diplotene stage) of meiosis I from month 5 of fetal life until puberty.

8. MATURATION OF OOCYTES BEGINS BEFORE BIRTH Diplotene stage of prophase : a resting stage during prophase that is characterized by a lacy network of chromatin This arrested state is produced by oocyte maturation inhibitor (OMI), a small peptide secreted by follicular cells.

9. MATURATION OF OOCYTES BEGINS BEFORE BIRTH At puberty, a pool of growing follicles is established and continuously maintained from the supply of primordial follicles. Each month, 15 to 20 follicles selected from this pool begin to mature. As primordial follicles begin to grow, surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells, and the unit is called a primary follicle

10. MATURATION OF OOCYTES BEGINS BEFORE BIRTH Granulosa cells rest on a basement membrane separating them from surrounding ovarian connective tissue (stromal cells) that form the theca folliculi . Also, granulosa cells and the oocyte secrete a layer of glycoproteins on the surface of the oocyte, forming the zona pellucida As follicles continue to grow, cells of the theca folliculi organize into an inner layer of secretory cells, the theca interna, and an outer fibrous capsule, the theca externa .

11. MATURATION OF OOCYTES BEGINS BEFORE BIRTH Also, small, fingerlike processes of the follicular cells extend across the zona pellucida and interdigitate with microvilli of the plasma membrane of the oocyte. These processes are important for transport of materials from follicular cells to the oocyte.

12. MATURATION OF OOCYTES BEGINS BEFORE BIRTH Some of the follicles die, whereas others begin to have fluid filled spaces appearing between the granulosa cells which coalesce to form the antrum, thereby entering the antral or vesicular stage Fluid continues to accumulate in the antrum such that, immediately prior to ovulation, follicles are quite swollen and are called mature vesicular follicles or graafian follicles. The antral stage is the longest, whereas the mature vesicular stage encompasses approximately 37 hours prior to ovulation .

13. MATURATION OF OOCYTES BEGINS BEFORE BIRTH After puberty, 5 to 15 primary oocytes begin maturation with each ovarian cycle, with usually only 1 reaching full maturity in each cycle (the others degenerate). During the ovarian cycle and triggered by the luteinizing hormone (LH) surge, a primary oocyte completes meiosis I to form two daughter cells: the secondary oocyte (23, 2N) and the first polar body.

14. MATURATION OF OOCYTES BEGINS BEFORE BIRTH The secondary oocyte promptly begins meiosis II but is arrested in metaphase of meiosis II about 3 hours before ovulation. The secondary oocyte remains arrested in metaphase of meiosis II until fertilization occurs. Meiosis II is completed only if the oocyte is fertilized; otherwise, the cell degenerates approximately 24 hours after ovulation.

15. APPROXIMATE NUMBER OF OOCYTES Primary oocytes: At month 5 of fetal life, 7 million primary oocytes are present. At birth, 2 million are present (5 million have degenerated). At puberty, 40,000 are present (1.96 million more have degenerated

16. APPROXIMATE NUMBER OF OOCYTES Secondary oocytes: Twelve secondary oocytes are ovulated per year Up to 480 secondary oocytes are ovulated over the entire reproductive life of the woman (40 years × 12 secondary oocytes per year = 480). This number (480) is obviously overly simplified since it is reduced in women who take birth control pills (which prevent ovulation), in women who become pregnant (ovulation stops during pregnancy), and in women who may have anovulatory cycles.

17. Female Gametogenesis Summary PRIMORDIAL GERM CELLS (46,2N) from the wall of the yolk sac arrive in the ovary at week 6 of embryonic development and differentiate into oogonia (46,2N). Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (46,4N). All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase (dictyotene stage) of meiosis I until puberty. During a woman’s ovarian cycle, a primary oocyte completes meiosis I to form a secondary oocyte (23,2N) and a first polar body, which probably degenerates. The secondary oocyte enters meiosis II, and ovulation occurs when the chromosomes align at metaphase. The secondary oocyte remains arrested in metaphase of meiosis II until fertilization occurs. At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23,1N) and a second polar body

18. Question 1 Which of the following is a major characteristic of meiosis I? (A) Splitting of the centromere (B) Pairing of homologous chromosomes (C) Reducing the amount of DNA to 1N (D) Achieving the diploid number of chromosomes (E) Producing primordial germ cells

19. Question 2 All primary oocytes are formed by? (A) week 4 of embryonic life (B) month 5 of fetal life (C) birth (D) month 5 of infancy (E) puberty

20. Question 3 In the production of female gametes, which of the following cells can remain dormant for 12 to 40 years? (A) Primordial germ cell (B) First polar body (C) Secondary oocyte (D) Primary oocyte (E) Second polar body

21. Question 4 Concerning maturation of the female gamete (oogenesis), when do the oogonia enter meiosis I and undergo DNA replication to form primary oocytes? (A) During fetal life (B) At birth (C) At puberty (D) With each ovarian cycle (E) Following fertilization

22. References Sadler, T.W (2019). Langman’s Medical Embryology, 14 th Ed. 2:44-48 Dudek, R.W (2014). Board Review Series Embryology, 6 th Ed. 1:1-4 Google images