Early Embryogenesis - fertilization, cleavage, implantation
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| Ovum descent | After ovulation the ovum is drawn into oviduct by sweeping motion of fimbriae (also epithelial cilia); ovum propelled towards uterus by rhythmic contraction of oviduct and cilia; ~3-4 days to reach uterus
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| Sperm ascent | Muscular contraction of uterus and oviducts propel sperm up in womb, towards oviduct; requires 0.5 hr-6 day for sperm to reach egg in oviduct
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| Capacitation | As spermatozoa are transported up womb, they are prepared for fertilization by removal of glycoprotein coat from their plasma membranes
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| Where does fertilization usually occur? | In the ampulla of the oviduct
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| Compare how many sperm are deposited with how many reach the ovum | 200-300 million deposited; 300-500 reach ovum
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| How many sperms are allowed to typically fertilize an ovum | 1
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| Polyspermy | When more than one sperm fertilizes an egg; must be prevented because it results in lethal triploidy
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| How does the ovum prevent polspermy? | 3 barriers - corona radiata, zona pellucida, zona reaction
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| First barrier in polyspermy - corona radiata | Capacitated sperm secrete proteolytic enzymes to penetrate corona radiata
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| Second barrier in polyspermy - zona pellucida | Contain glycoproteins that sperm must penetrate; done by acrosome reaction; acrosome contacts zona, releases digestive enzymes (ex: acrosin) that dissolve glycoproteins of zona; 1st sperm penetrates zona & contacts PM of ovum, PM fuses with egg's membrane
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| Corticol reaction | Fusion of membrane induces rupture of corticol granules lining ovum's PM; enzymes (ex: peroxidase) released from granules
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| Third barrier in polyspermy - zona reaction | Enzymes from corticol reaction inactivates receptors for spermatozoa binding that are on surface of zona, preventing additional sperm and penetrating
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| Male pronucleus | Swelling of sperm nucleus that entered cytoplasm of egg; egg is fertilized
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| Fertilized egg | Zygote
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| Female pronucleus | Nucleus that remains in zygote from secondary oocyte that was released from meiotic metaphase II releasing 2nd polar body
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| Do pronuclei fuse? | NO
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| Cleavage | Series of rapid mitotic divisions of early embryo that serve to multiply genomes required for embryogenesis
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| Blastomeres | Cells of early embryo; embryo does not grow during this period; subdivides into cluster of progressively smaller blastomeres (embryo remains same size in ovum)
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| What happens at the 3rd cleavage (8 cell stage)? | Tight junctions form; also known as compaction (pulls blastomeres closer together); segregates inner cells from outer cells
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| Inner cells | Determined to produce the fetus
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| Outer cells | Contribute to chorion and placenta
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| When does the 4th cleavage occur? | ~3 days after fertilization; cell embryo now called morula; inner cells = inner cell mass; outer cells = outer cell mass
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| What happens when the morula reaches the uterus? | Fluid begins to seep through zona pellucida, forming intercellular spaces between blastomeres
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| Blastocele | Intercellular spaces between blastomeres that coalesce; embryo = blastocyst
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| Outer cell mass in blastocyst | Organized into spherical sheet known as trophoblast
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| Inner cell mass in blastocyst | Localized to one pole where it is known as embryoblast
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| Blastocyst at day 5 | Prepares for implantation by hatching from zona pellucida
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| What happens at ~6 days after fertilization? | Blastocyst embryo attaches to region of uterine wall, normally in body of uterus (anterior / posterior)
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| What cells of the embryoblast penetrate the epithelial cells of the uterine mucosa? | Trophoblast cells
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| What enzymes does the trophoblast secrete? | Proteolytic enzymes; induced by adjacent mucosa
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| Ectopic pregnancies | Implantation in the wrong place; usually result in death of embryo during 2nd month (often producing severe hemorrhaging & abdominal pain in mother); usually terminated by methotrexate injection
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| Most common site for incorrect implantation (95%) | Ampulla of oviduct; can also occur at any point along oviduct = tubal pregnancies; pregnancy must be terminated; fetus cannot survive in confined space; tube will rupture as fetus grows
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| Placenta previa | Ectopic implantations at internal os of uterus, where placenta will over-bridge opening resulting in severe bleeding during later part of pregnancy and delivery
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| Most common abdominal implantation | Peritoneal lining of rectouterine cavity (Douglas' pouch); implantation also relatively common in intestinal peritoneum/omentum
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| Primary ovarian pregnancy | Blastocyst implants on surface of ovary
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| Intracytoplasmic sperm injection | Deals w/ male infertility; sperm harvested; male incapable of ejaculatiion, sperm harvested from any point along reproductive tract; single sperm isolated & microinjected into cytoplasm of oocyte
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| In vitro fertilization | Follicular growth induced by gonadotropin treatment; secondary oocytes laparoscopically/transvaginally harvested from ovary by aspiration; several oocytes harvested
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| One drawback of harvesting multiple oocytes? | Increases chances of one surviving, but can also result in multiple gestations
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| Simplistically, how are the harvested oocytes fertilized? | Mixing them with diluted sperm; resulting embryos monitored to eight cell stage, and then placed in uterus
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Created by:
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