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Lec40 fig46 - Figure:3 Human Oogenesis Oogenesis begins in...

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Unformatted text preview: Figure :3;- Human Oogenesis Oogenesis begins in the female embryo with differentiation of primordial germ cells into 1/ oo'gonia, ovary—specific stem cells. An oogo— Ovary—v— a.- nium multiplies by mitosis and begins meiosis, but the process stops at prophase l. The cells at this stage, called primary oocyles, remain qui— escent within small follicles (cavities lined with protective cellsl until puberty, when hormones reactivate them. Beginning at puberty, iollicie- l Differentiation stimulating hormone (FSl-l) periodically stimu- lates a follicle to grow and induces its primary .oocyte to complete meiosis l and start meiosis ll. Meiosis then stops again. Arrested at meta- phase ll, the secondary oocyte is released at division -- .. - ovulation, when its follicle breaks open. Usually _ only one oocyte matures and is released each Primary ‘ Primary germ cell in embryo E3 Dogonlu .- in ovary Oogonium -—-—'———_i month. Meiosis does not continue until a sperm oocl’le .' penetrates the. oocyte; only then is oogenesis ac— tually completed, producing an ovum. (In other animal species, the sperm may enter the of meiosis I oocyte at the same stage, earlier, or later.) (present at birth) The meiotic divisions in oogenesis involve unequal cytokinesis. with the smaller cells Completion 0f meiosisl within , follicle. Primary oocyte, - arrested in prophase becoming polar. bodies (the first polar body and DHSEt 0i ”195095 11 may or may not divide again). After meiosis is completed, the haploid nuclei of the sperm and the now mature ovum fuse; this event is fertilization. The ruptured follicle left behind after ovu— lation develops into the. corpus luteum. If the released oocyte is. not fertilized, however. the corpus luteum degenerates. Second For many years, scientists have believed that women, like most female mammals, are born with all the primary oocytes they will ever liaveuthat no new ones develop after birth. In March of 2004, however, researchers reported that multiplying oogonja exist in the ovaries of adult mice and can develop into oocyles. The researchers are now looking for similar cells-in human ovaries, It is possible that the marked decline in fertility that occurs as women age results. from the gradual deple— tion of oogonia rather than solely from the degeneration of aging oocytes. Secondary oocyte. arrested at meta- phase of meiOsis II Ovulation , Entry of sperm triggers completion of meiosis Il Ovum ener ' ' co ous'luteu -. _ 974 UNIT seven Animal Form and Function ...
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