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The Living Body 5 - Ovulation to Fertilization.docx

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York University
Natural Science
NATS 1610
Barbara Czaban

NATS 1610 WINTER TERM L24 – Ovulation to Fertilization Human life has two main periods of development 1. Prenatal 2. Postnatal • The prenatal period begins at fertilization and ends at birth Summary of Early Events during the Pre-Embryonic Period • Step 1: Ovulation: A secondary oocyte is released from the ovary and swept into the oviduct • Step 2: Fertilization: A single sperm penetrates the secondary oocyte. Eventually, the sperm and egg nuclei will fuse, resulting in a fertilized egg, or zygote. • Step 3: Cleavage: The zygote soon undergoes rapid mitotic cell division as it moves along the oviduct toward the uterus, becoming a pre-embryo that consists of two cells, then four cells, then eight cells, and so on • Step 4: Morula: By day 4, successive divisions produce a morula, a solid ball of cells that enters the uterus • Step 5: Blastocyst: By day 6, the pre-embryo becomes a blastocyst, a hollow ball of cells with a fluid-filled cavity. The blastocyst has freed itself from the zona pellucida and can increase in size • Step 6: Implantation: The blastocyst attaches to the uterine lining (endometrium) and begins to digest its way inward In Human females, oogenesis & ovulation begins before birth • During early fetal development − Diploid oogenia numbers increase in ovaries by mitosis • During month 3 − Oogenia enlarge & store nutrients − Become primary oocytes o Enter meiosis I but do not complete it o “arrested” & remain diploid − Meiosis I is completed years later just prior to ovulation o Starting at puberty, one at a time In summary • During early fetal development – in ovary – Follicle Cells surround each developing egg cell − This group of “follicle cells plus egg cell” are called primary follicles − This starts occurring by 3 months post fertilization – i.e., in utero (before birth) − Remain in this state until puberty (after birth) • Primary follicles begin completing meiosis I at puberty − In cycles, throughout adulthood • Just PRIOR to Ovulation – one primary follicle will complete meiosis I − Forming one haploid Secondary Oocyte & one polar body − Polar bodies do not mature into egg cells • Ovulation occurs about 12 hours after secondary oocyte forms − Matured follicle (with secondary oocyte) is released from ovary − Corpus luteum – some follicle cells remain in ovary after ovulation o These cells secrete estrogen & progesterone o Degenerate if fertilization does not occur • Secondary Oocyte completes meiosis II when sperm inseminates it − Cytoplasmic division produces one ovum
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