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Oocyte
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- 2. ■ An oocyteis a female gametocyte or germ cell involved in reproduction ■ It is an immature ovum or an egg ■ An oocyte is produced in the ovary during female gametogenesis ■ The female germ cells produce a primordial germ cell pgc which undergoes mitosis to form an oogonium ■ During oogenesis the oogonium becomes a primary oocyte
- 3. ■ It isthe creation of an ovum ■ It is the female form of gametogenesis and involves the development of various stages of the immature ovum ■ In mammals the first part of oogensis starts in germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary ■ Oogenesis consists of several sub processes – oocytogenesis, ootidogenesis and the maturation to form an ovum ■ Folliculogenesis is a separate sub process and supports all three oogenetic sub process
- 5. ■ Oogenesis startswith the process of developing oogonia,which occurs via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis ■ Oocytogenesis is complete either before or shortly after birth
- 6. NUMBER OF PRIMARYOOCYTES ■ Oocytogenesis is complete ■ Primary oocytes reach their max. development at 20 weeks of gestational age,when approx 7 million primary oocytes have been created,this no. will be approx. reduced to 1-2 million
- 7. OOTIDOGENESIS ■ The succeedingphase of ootidogenesis occurs when the primary oocyte develops into an ootid. ■ This is achieved by the process of meiosis ■ Although this process begins at pre natal age, it stops at prophase I. ■ All oocytes ,still primary oocytes, have halted at this stage of development,called dictyate ■ After menarche these cells then continue to develop
- 8. MEIOSIS I ■ MeiosisI of ootidogenesis begins during embryonic development ,but halts in the diplotene stage of prophase I until puberty ■ For those primary oocytes that continue to develop in each menstrual cycle synapsis occurs and tetrads form enabling chromosomal cross over to occur ■ As a result of meiosis I the primary oocyte develops into the secondary oocyte and the first polar body
- 9. MEIOSIS II ■ Immediatelyafter meiosis I ,the haploid secondary oocyte initiates meiosisII. ■ This process is also halted at the mataphase II stage until fertilization ■ When meiosisII has completed ,an ootid and another polar body have now been created
- 10. MATURATION INTO OVUM ■Both polar bodies disintegrate at the end of meiosis II,leaving only the ootid,which then undergoes maturation into mature ovum ■ The function of forming polar bodies is to discard the extra haploid sets of chromosomes
- 11. FOLLICULOGENESIS ■ It isthe maturation of ovarian follicle,a densely packed shell of somatic cells that contains an immature oocyte ■ This describes the progression of number of small primordial follicles into large pre ovulatory follicles that enter the menstrual cycle ■ Folliculogenesis ends when the remaining follicles in the ovaries are incapable of responding to the hormones that had recruited some of the follicles to mature ■ This depletion of follicle supply signals the beginning of menopause
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- 13. HUMAN OOGENESIS ■ Atthe start of the menstrual cycle some 12-20 primary follicles begin to develop under the influence of elevated FSH to form secondary follicles. ■ By around day 9 of the cycle only one healthy secondary follicle remains with the rest having undergone ovarian follicle atrasia ■ The remaining follicle is called the dominant follicle and is responsible for producing large amount of estradiol during the late follicular phase.
- 14. ■ Estradiol productiondepends upon the co operation between theca and granulosa cells ■ On day 14 of the cycle an LH surge occurs which is triggered by the estradiol ■ This causes the secondary follicle to develop into tertiary follicle which then ovulates 24-36 hours later ■ When the development of the tertiary follicle occurs the primary oocyte completes the first meiotic division resulting in the formation of a polar body and a secondary oocyte ■ The empty follicle then forms a corpus luteum which releases progestrone hormone
- 15. CHARACTERISTICS ■ CYTOPLASM ■ Oocytesare rich in cytoplasm which contains yolk granules to nourish the cell in early development
- 16. NUCLEUS ■ During theprimary oocyte stage of oogenesis the nucleus is called a germinal vesicle ■ The human type secondary oocyte has 23rd sex chromosomes as 23,x female determining chromosome
- 17. NEST • The spacewhere an ovum or immature ovum is located is the cell nest
- 18. MITOCHONDRIA ■ The oocytereceives mitochondria from maternal cells,which will go on to control embryonic metabolism ■ The partitioning of mitochondria is carried out by a system of microtubules which will localize mitochondria throughout the oocyte
- 19. NUCLEOLUS ■ The nucleolusof the oocyte is derived solely from the maternal ceels ■ It is a structure found within the nucleus, is the location where the rRNA is transcribed and assembled into ribosomes ■ Nucleolus is dense and inactive in mature oocyte, it is reuired for proper development of the embryo
- 20. RIBOSOMES ■ Maternal cellsalso synthesise and contribute a store of ribosomes that are reuired for the translation of proteins before the zygotic genomes is activated ■ Maternally derived ribosomes and some mRNAs are stored in a structure called cytoplasmic lattices ■ They are the network of fibrils, protein and RNA s
- 21. PATERNAL CONTRIBUTIONS • Thespermatozoon which fertilizes an oocyte will contribute its pronucleus ,the other half of the zygotic genome • The sperm provides three essential parts • . An activating factor which causes the dormant oocyte to activate • The haploid paternal genome • The centrosome which is responsible for maintaining the microtubule system
- 22. ABNORMALITIES ■ Nondisjunction ■ Afailure of proper homolog separation in meiosisI or sister chromatid separation in meiosis II can lead to aneuploidy in which the oocyte has the wrong number of chromosomes -22x,or 24x ■ Downs syndrome and edwards syndrome ■ Advanced maternal age ■ Some oocytes have multiple nuclei ,although it is thought they never mature
- 23. Different Patterns ofCumulus cells
- 24. Oocytes under theStereo-microscope
- 25. Grading of Oocyte ■Zona Pellucida ■ Perivetiline Space ■ Polar Body ■ CytoplasmicTexture ■ Inclusions
- 26. Zona Pellucida ■ Thezona pellucida (ZP) may vary in thickness and rigidity. ■ Some patients have clearly thiner ZP than others. ■ It is usually a characteristics associated with the patient more than with the oocyte. ■ Although thickness may vary slightly around the oocyte, the following categories can be defined.
- 27. Parameters Parameters Morphological aspects Thinthe ZP is less than 15 µm Thick the ZP is greater or equal to 25 µm Normal the ZP is around 20 µm (15-25) Abnormal the ZP is quite irregular both in thickness and in shape
- 28. Periviteline Space ■ Theperiviteline space surrounds the ooplasm. It is generally clearly visible, at least in the vicinity of the polar bodies. It may contain fragments. Parameter Parameter Morphological aspect Small the ooplasma is adherent to the ZP and the periviteline space is almsot inexistent Large the ooplasma is clearly retraced away from the ZP leaving a clearly identifiable space all around the cytoplasm With fragments presence of clearly identifiable fragments Normal the ooplasm is slightly retracted away from the ZP at least in the vicinity of the polar body
- 29. Polar Body ■ Thepolar body appear when the first meiotic division has been completed. It appears as an small cell close to the oocyte and may undergo a second division. ■ It usually rapidly degenerates and appears as a fragmented structure Parameter MorphologicalAspects Unclear or absent the polar body may not always be clearly identifiable, depending on how the oocyte is oriented. It is absent in a metaphase I oocyte (before the first meiotic division) Intact clearly identifiable smooth round/oval cell lying close to the oocytes Fragmented the polar body material is irregular in shape and show clear signs of disorganization Giant usually intact, giant polar bodies a clearly recognizable, they may achive size of about half of that of an oocyte
- 30. CytoplasmicTexture ■ The cytoplasmof the oocyte exhibits usually a granular texture. ■ Depending on the oocyte, this texture may appear homogeneous or exhibit a gradient in the granulated texture. ■ In some cases, it may contain a dark cluster and exhibit sign of fragmentation.
- 31. Parameters Parameter MorphologicalAspect Fragmented thecytoplasm shows signs of fragmentation or appear as an undefined mass with absent or degraded oolema Homogeneous the cytoplasmic material appears homogeneously distributed with little signs of granulation Polarised zones more or less granulated are visible as two opposing hemispheres Dark cluster a dark zone is clearly identifiable inside the cytoplasm
- 32. Inclusions ■ The cytoplasmmay contain several remarkable structures. Parameter Morphological aspect Vacuole one large or several small vacuoles bounded with a clearly identifiable membrane SER cluster smooth endoplasmic reticulum appearing as clear spherical zones inside the cytoplasm.The boundaries of these structures may not be easily identified as in the case of vacuoles Refractile body Refractile bodies constitute one of the main morphological abnormalities in human oocytes and are made of auto fluorescing lipofucsin (appear yellowish) None absence of the above mentioned structures
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