Oogenesis Lecture

1/30/2019 1 Before birth 1/30/2019 2 Gametes are derived from Primordial germ cells Formed In A Epiblast 1/30/2019 3 During the second week of development B Cells move to the Wall Of Yolk Sac 1/30/2019 4 During Forth week till the end of Fifth week Cells begin to migrate toward C ( Developing Gonads) < Gonads of genetic females / ovaries > PGC’s differentiate into oogonia by mitotic divisions mitotic divisions Primordial germ cells Oogonia 1/30/2019 5 By the End of Third Month Oogonia arranged in clusters surrounded By Large Flat epithelial cells (Follicular Cells ) Why do oogonia need follicles?? Oogonia continue to divide by Mitosis But some of them arrest in their cell divisions in (Prophase Meiosis 1) And form 1/30/2019 7 Then In The Next few months Oogonia increase rapidly in numbers 1/30/2019 8 By the end of Fifth month Total number of germs cells in ovary reachs maximum Estimated 7 Millions At this time Many oogonia and primary oocyte become atretic 1/30/2019 9 By Seventh Month Majority of oogonia have degenerated except for few near surface All surviving primary oocyts WILL Enter prophase of meiosis 1 Individually surrounded by a layer of flat cells (Follicular epithelial cells ) This structure is named ( Primordial Follicle ) 1/30/2019 10 Near the time of birth all primary oocytes have started prophase of meiosis I but instead of proceeding into metaphase, they enter the (diplotene stage) 1/30/2019 11 Primary oocytes remain arrested in prophase of meiosis I diplotene stage and do not finish their first meiotic division before puberty is reached. This arrested state is produced by oocyte maturation inhibitor (OMI) a small peptide secreted by follicular cells 1/30/2019 12 The total number of primary oocytes at birth is estimated to vary from 600,000 to 800,000 Note 1: primary oocyte and flat epithelial cells known as (primordial follicle). Note 2: It’s still “primary oocyte” rested in (diplotene stage in meiosis 1) 1/30/2019 13 most oocytes become atretic; only approximately 40,000 are present by the beginning of puberty 1/30/2019 14 1/30/2019 15 Hypothalamus Stimulates Pituitary gland To secrete FSH (Follicle stimulating hormone) Stimulates 15 to 20 follicles selected to be mature and pass through 3 stages each month , and only one of these follicle reaches full maturity 1/30/2019 16 Primordial follicle 1. Primary (preantral) follicle 2.Secondary (antral) follicle 3.preovulatory (Graafian) follicle FSH Stimulate follicular (granulosa) cells around oocyte. Zona pellucida Nucleus Flat epithelial To Cuboidal epi To Stratified granulosa 1/30/2019 18 1/30/2019 19 Primary follicle Primordial follicle Growing follicle Zona pellucida 1/30/2019 Flat epithelial Cuboidal epithelial Stratified epi. Of granulosa 20 As the Primary follicle continue to grow Primary Follicle Granulosa cells rest on a basement membrane separating them from surrounding ovarian connective tissue (stromal cells) that form the( theca folliculi) theca interna (secretory cells) theca externa Fibrous capsule, Granulosa theca externa Fibrous capsule, theca interna (secretory cells) Granulosa 1/30/2019 22 Granulosa and primary oocyte Secrete a layer of glycoproteins on the surface of the oocyte, forming the ( zona pellucida ) Theca interna and granulosa Produce : Effect in uterine endometrium Thinning of cervical Mucus for sperm to pass Stimulate anterior Pituitary gland to secrete (LH) Note: primary oocyte still in diploten stage 1/30/2019 24 Primary follicle To Secondary Follicle fluid-filled spaces appear between granulosa cells (antrum) At that moment when antrum appeared follicle is termed (a vesicular or an antral or secondary follicle) Secondary Follicle Primary follicle antrum1/30/2019 25 Follicular antrum Theca externa Granulosa cells produce estrogens Theca interna cells produce estrogens Why do we need them? Primary oocyte arrested in diplotene stage of the prophase stage of meiosis Ι Zona pellucida 1/30/2019 26 Note: primary oocyte still in diploten stage When the secondary follicle is mature, a surge in luteinizing hormone (LH) promote the Preovulatory growth phase 1/30/2019 27 Secondary Follicle Preovulator y Follicle To Three things happen a) By influence of (LH) Meiosis I is completed, resulting in formation of two daughter cells the first polar the secondary oocyte (enters meiosis 2 And arrested in Metaphase) 3 hours Before ovulation c) Granulosa cells surrounding the oocyte remain intact and form the b) Antrum gets enlarged cumulus oophorus with time Antrum Cumulus oophorus Theca interna 1/30/2019 Secondary Follicle Theca externa Preovulatory Follicle 28 Preovulatory (Graafian follicle) ‫هام جدا‬ Granulosa cells Cumulus oophorus Consists of: 1-Granulosa cells 2-Zona pellucida 3-Secondry oocyte arrested in metaphase stage of meiosis ΙΙ 4-First polar body! Zona pellucida 1/30/2019 29 Preovulatory Follicle To Ovulation (LH) increase The surface of ovary bulges locally Digestion of collagen around follicle Local muscular contraction In ovarian wall All this will help in extrude the oocyte with surrounding granulosa cells that form (cumulus oophours ) BREAK FREE (ovulation) and float out of the ovary 1/30/2019 30 Some of (cumulus oophorus ) rearranged them selves around zona pellucida to form ( Corona Radiata ) Granulosa cells Oocyte in 2nd meiotic division Cumulus oophorus cells1/30/2019 31 After Ovulation To Fertilization Secondary oocyte and Corona radiata carried into the tube by these sweeping movements of the fimbriae and by motion of cilia on the epithelial lining 1/30/2019 32 After ovulation, granulosa cells remaining in the wall of the ruptured follicle, together with cells from the theca interna, are vascularized by surrounding vessels. Under the influence of LH, these cells develop a yellowish pigment and change into lutein cells, which form the corpus luteum and secrete estrogens and progesterone Progesterone, together with some estrogen, causes the uterine mucosa to enter the progestational or secretory stage in preparation for implantation of the embryo. 1/30/2019 33 Corpus luteum 1/30/2019 34 Fertilization oocyte : IF No sperm oocyte is NOT fertilized; the cell degenerates approximately 24 hours after ovulation. IF Sperm in tube oocyte is fertilized; Meiosis II is completed in the oocyte, And sperm will complete the work 1/30/2019 35 1/30/2019 36 1/30/2019 37 ➢In humans, one ovarian follicle occasionally contains two or three clearly distinguishable primary oocytes Although these oocytes may give rise to twins or triplets, they usually degenerate before reaching maturity. ➢In rare cases, one primary oocyte contains two or even three nuclei .Such binucleated or trinucleated oocytes die before reaching maturity. . Abnormal Gametes 1/30/2019 38