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by @CONVMEDBOT by @CONVMEDBOT Dr. Sumeya Oogenesis Slide 1  Oogensis 2N Primary Oocyte Meiosis Secondary...

by @CONVMEDBOT by @CONVMEDBOT Dr. Sumeya Oogenesis Slide 1  Oogensis 2N Primary Oocyte Meiosis Secondary 1N 1N Polar Body oocyte Polar 1N Polar Ootid 1N 1N 1N body bodies morphogenesis 1N Ovum Die Die Die Slide 2  Oogenesis Is the process whereby oogonia differentiate into mature oocytes. Maturation of Oocytes Begin Before Birth Once PGCs have arrived in the gonad of a genetic female, they differentiate into oogonia. By the end of the third month: they are arranged in clusters surrounded by a layer of flat epithelial cells(follicular cells) originate from surface epithelium covering the ovary. All the oogonia in one cluster are properly derived from a single cell. Oogonia: majority: continue to devide by mitosis. some: enter meiosis and arrest their cell division in prophase MI forming primary oocytes. Slide 3  5th month: total number of germ cells(oogonia + primary oocytes) reach maximum(7 millions). At this time: cell death begins leading to many oogonia and primary oocytes degenerate and become atretic. 7th month: the majority of oogonia ----- degenerate except few near a surface. All surviving primary oocytes ------ entered prophase of miosis 1 , 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 Slide 4 Slide 5 Slide 6  Maturation of Oocytes Continues at Puberty Near birth time: all primary oocytes ------- started prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage, a resting stage during prophase that is characterized by a lacy network of chromatin Primary oocytes remain arrested in prophase and do not 7 nish their 7 rst meiotic division before puberty is reached. This arrested state is produced by oocyte maturation inhibitor (OMI), a small peptide secreted by follicular cells. The total number of primary oocytes at birth is estimated to vary from 600,000 to 800,000. During childhood, most oocytes become atretic; only approximately 40,000 are present by the beginning of puberty, and fewer than 500 will be ovulated. Slide 7  Some oocytes that reach maturity late in life have been dormant in the diplotene stage of the C rst meiotic division for 40 years or more before ovulation------- vulnerable to damage with age and increase chromosomal abnormalities. Slide 8 Slide 9  At puberty, 15 to 20 primordial follicles monthly begin to mature passing in 3 stages:1-Primary or preantral 2-Secondary or antral or vesicular 3-Preovulatory(GraaCan follicle) Some of these die, while others begin to accumulate Ouid in a space called the antrum, thereby entering the antral or vesicular stage Fluid continues to accumulate such that, immediately prior to ovulation, follicles are quite swollen and are called mature vesicular follicles or GraC an follicles. The antral stage is the longest, whereas the mature vesicular stage encompasses approximately 37 hours prior to ovulation. Slide 10  As primordial follicles begin to grow, surrounding follicular cells change from Oat to cuboidal and proliferate to produce a stratiCed epithelium of granulosa cells, and the unit is called a primary follicle. Granulosa cells: rest on a basement membrane separating them from surrounding ovarian connective tissue (stromal cells) that form the theca folliculi. Also, granulosa cells + the oocyte secrete a layer of glycoproteins on the surface of the oocyte, forming the zona pellucida. Slide 11  As follicles continue to grow, cells of the theca folliculi organize into an inner layer of secretory cells, the theca interna, and an outer Cbrous capsule, the theca externa. Also, small, Inger-like 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. Slide 12  As development continues, Ouid-Clled spaces appear between granulosa cells. Coalescence of these spaces forms the antrum, and the follicle is termed a vesicular or an antral follicle. Initially, the antrum is crescent-shaped, but with time, it enlarges. Granulosa cells surrounding the oocyte remain intact and form the cumulus oophorus. At maturity, the mature vesicular (GraaIan) follicle may be 25 mm or more in diameter. It is surrounded by the theca interna, which is composed of cells having characteristics of steroid secretion, rich in blood vessels, and the theca externa, outer Cbrous capsule layer which gradually merges with the ovarian connective tissue Slide 13  With each ovarian cycle, a number of follicles begin to develop, but usually only one reaches full maturity. The others degenerate and become atretic. When the secondary follicle is mature, a surge in luteinizing hormone (LH) induces the preovulatory growth phase. Meiosis I is completed, resulting in formation of two daughter cells of unequal size, each with 23 double-structured chromosomes Slide 14  One cell, the secondary oocyte, receives most of the cytoplasm; the other, the Irst polar body, receives practically none. The C rst polar body lies between the zona pellucida and the cell membrane of the secondary oocyte in the perivitelline space The cell then enters meiosis II but arrests in metaphase approximately 3 hours before ovulation. Meiosis II is completed only if the oocyte is fertilized; otherwise, the cell degenerates approximately 24 hours after ovulation. The C rst polar body may undergo a second division Slide 15 Slide 16  Secondary oocyte, 1N Polar body, 1N http://www.ucalgary.ca/UofC/eduweb/virtualembryo/PageMill_Images/image145.gif Slide 17 Ovary releases Secondary Oocyte and Follicle Cells Slide 18 Release of secondary oocyte (1N) surrounded by follicle cells (2N) Slide 19  Follicle Cells = 2N nurse cells Secondary oocyte = 1N Slide 20 Slide 21 Sperm use acrosome Plasma enzymes to penetrate membrane zona pellucida Egg (oocyte) cytoplasm http://www.nature.com/ncb/journal/v3/n2/images/ncb0201_e59_F1.gif Slide 22