![• cumulus oophorus:
– follicular cells around primary oocyte form
cumulus oophorus
– Antrum progressively enlarges so that primary
oocyte becomes eccentric in position
• Under the influence of FSH and LH, the
secondary follicle matures further
– antrum increases, and the follicle becomes fully
mature; and is referred to as the Graafian follicle
(tertiary follicle or mature follicle) [20 mm across]
Postnatal Maturation of Oocytes](https://image.slidesharecdn.com/4oogenesis-240312203633-b4c1a6b9/85/4_Oogenesis-gametogenesis-lectureeeee-pdf-14-320.jpg)
![• About 12-24 hours prior to ovulation, there is an LH
surge, and this triggers the ejection of secondary
oocyte from a ruptured Graafian follicle
• This secondary oocyte is at the metaphase stage of
meiosis II when it is released into the peritoneal
cavity
• However, secondary oocyte is only able to complete
meiosis II if this cell is fertilized.
– Otherwise, it degenerates about 24 hours post-ovulation
• If fertilization occurs, secondary oocyte completes
meiosis II to form a mature ovum and 2nd polar body
(each normally with haploid [23] chromosomes)
Postnatal Maturation of Oocytes](https://image.slidesharecdn.com/4oogenesis-240312203633-b4c1a6b9/85/4_Oogenesis-gametogenesis-lectureeeee-pdf-17-320.jpg)
4_Oogenesis gametogenesis lectureeeee.pdf
- 1. OOGENESIS Dr. O. B. Akinola, Ph.D.
- 2. Study objectives • Students should be able to define gametogenesis, spermatogenesis and ovogenesis • Students should also be able to describe the formation of human gametes, including abnormalities of gametogenesis.
- 3. Oogenesis (Ovogenesis) • Definition: formation of mature oocytes (from primordial germ cells) • Primordial germ cells – derived from epiblast – appear in the wall of the yolk sac by 3rd week – are found in the developing gonads by 5th week • Oogenesis commences in utero and is completed after puberty
- 4. • Differentiation of primordial germ cells: – Following their arrival in the developing ovaries, germ cells differentiate into oogonia – by 4th month in utero: Oogonia undergo series of mitosis to form additional oogonia – Each cell is surrounded by a layer of flat cells (follicular cells) derived from coelomic epithelium
- 5. Prenatal maturation of oocyte • By 5th month in utero: – Mitotic division of oogonia continues – Some oogonia have differentiated into larger cells called primary oocytes – Each ovary at this point has about 7 million germ cells • By 7th month in utero: – Most oogonia have degenerated and majority of germ cells are primary oocytes – Each of the primary oocytes has replicated its DNA and has commenced the prophase of meiosis I – A primary oocyte and its surrounding squamous follicular cells constitute a primordial follicle
- 7. • At birth: –each ovary has about 1 million primary oocytes –Each primary oocyte has started prophase I –However, the cells suspend their meiotic division –instead of preceding to metaphase I, they enter a dictyotene stage (resting phase) until puberty Prenatal maturation of oocyte
- 8. Postnatal Maturation of Oocytes • Between birth and puberty: –most primary oocytes become atretic –only about 400,000 are surviving at puberty –Out of these, only about 400 will mature and be ovulated during the reproductive life of the female
- 9. • In childhood: – primary oocytes remain in the dictyotene stage until puberty when oocyte maturation resumes – The arrest of oocyte meiotic division during dictyotene phase is mediated by oocyte maturation inhibitor (OMI) – OMI is produced by follicular cells • Between puberty (11–13 years) and menopause (48–55 years), maturation of the primary oocytes resumes Postnatal Maturation of Oocytes
- 10. • About 5–15 follicles begin to mature during each ovarian cycle (in the ovary) – This cycle is under the control of gonadotropins (FSH & LH) • Under the influence of FSH, primary oocyte begins to increase in size, while the flattened follicular cells of the primordial follicle become cuboidal and stratified Postnatal Maturation of Oocytes
- 11. • A primary oocyte with its covering of stratified cuboidal (follicular) cells is called a primary follicle – A layer of glycoprotein (zona pellucida) is produced by the follicular cells and oocyte; – it surrounds primary oocyte
- 12. • theca folliculi: –As the primary follicle grows, certain cells of the ovarian stroma surround the follicular cells as theca folliculi –Soon, the theca is organized into an inner theca interna (containing secretory cells) and an outer theca externa, • Theca externa is made of connective tissue with fibroblast-like cells –Cells of the theca interna secrete oestrogens and progesterone Postnatal Maturation of Oocytes
- 13. • Secondary follicle: – forms as the primary follicle develops an antrum – Antrum contains follicular fluid Secondary & Graafian follicles
- 14. • cumulus oophorus: – follicular cells around primary oocyte form cumulus oophorus – Antrum progressively enlarges so that primary oocyte becomes eccentric in position • Under the influence of FSH and LH, the secondary follicle matures further – antrum increases, and the follicle becomes fully mature; and is referred to as the Graafian follicle (tertiary follicle or mature follicle) [20 mm across] Postnatal Maturation of Oocytes
- 15. Graafian follicle (20 mm across)
- 16. – Graafian follicle forms a local elevation on the surface of the ovary. • The tertiary follicle is formed around the middle of the menstrual cycle – As this follicle is formed, the primary oocyte completes meiosis I (just before ovulation; about 14th day of a 28-day cycle) – This forms a secondary oocyte and the 1st polar body (each with 23 (double-stranded) chromosomes) – Secondary oocyte retains virtually all the cytoplasm, while the 1st polar body has sparse cytoplasm Postnatal Maturation of Oocytes
- 17. • About 12-24 hours prior to ovulation, there is an LH surge, and this triggers the ejection of secondary oocyte from a ruptured Graafian follicle • This secondary oocyte is at the metaphase stage of meiosis II when it is released into the peritoneal cavity • However, secondary oocyte is only able to complete meiosis II if this cell is fertilized. – Otherwise, it degenerates about 24 hours post-ovulation • If fertilization occurs, secondary oocyte completes meiosis II to form a mature ovum and 2nd polar body (each normally with haploid [23] chromosomes) Postnatal Maturation of Oocytes
- 18. Oogenesis/fertilization Primary oocyte Secondary oocyte Secondary oocyte @ metaphase II fertilizatn Zygote
- 19. Clinical correlates • Incidence of chromosomal anomalies of the gametes increases with increasing maternal age Numerical or structural aberration of the chromosome may occur – Down’s syndrome (trisomy), achondroplasia (gene mutation), Klinefelter’s syndrome (trisomy), Turner’s syndrome (monosomy), etc • Only one mature graafian follicle is usually formed during each ovarian cycle • Several mature follicles may mature when ovulation is enhanced with gonadotropins or a drug such as clomiphene citrate
- 20. Study questions 1. Define gametogenesis 2. Give an account of the stages in spermatogenesis 3. Describe the events of the prenatal phase of oogenesis 4. Highlight the chromosomal and morphologic abnormalities associated with gametogenesis. 5. With the aid of diagrams, describe how a primary oocyte is transformed into a mature ovum.