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WVSOM - Embryology-1

Early Embryogenesis - fertilization, cleavage, implantation

QuestionAnswer
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
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
Capacitation As spermatozoa are transported up womb, they are prepared for fertilization by removal of glycoprotein coat from their plasma membranes
Where does fertilization usually occur? In the ampulla of the oviduct
Compare how many sperm are deposited with how many reach the ovum 200-300 million deposited; 300-500 reach ovum
How many sperms are allowed to typically fertilize an ovum 1
Polyspermy When more than one sperm fertilizes an egg; must be prevented because it results in lethal triploidy
How does the ovum prevent polspermy? 3 barriers - corona radiata, zona pellucida, zona reaction
First barrier in polyspermy - corona radiata Capacitated sperm secrete proteolytic enzymes to penetrate corona radiata
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
Corticol reaction Fusion of membrane induces rupture of corticol granules lining ovum's PM; enzymes (ex: peroxidase) released from granules
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
Male pronucleus Swelling of sperm nucleus that entered cytoplasm of egg; egg is fertilized
Fertilized egg Zygote
Female pronucleus Nucleus that remains in zygote from secondary oocyte that was released from meiotic metaphase II releasing 2nd polar body
Do pronuclei fuse? NO
Cleavage Series of rapid mitotic divisions of early embryo that serve to multiply genomes required for embryogenesis
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)
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
Inner cells Determined to produce the fetus
Outer cells Contribute to chorion and placenta
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
What happens when the morula reaches the uterus? Fluid begins to seep through zona pellucida, forming intercellular spaces between blastomeres
Blastocele Intercellular spaces between blastomeres that coalesce; embryo = blastocyst
Outer cell mass in blastocyst Organized into spherical sheet known as trophoblast
Inner cell mass in blastocyst Localized to one pole where it is known as embryoblast
Blastocyst at day 5 Prepares for implantation by hatching from zona pellucida
What happens at ~6 days after fertilization? Blastocyst embryo attaches to region of uterine wall, normally in body of uterus (anterior / posterior)
What cells of the embryoblast penetrate the epithelial cells of the uterine mucosa? Trophoblast cells
What enzymes does the trophoblast secrete? Proteolytic enzymes; induced by adjacent mucosa
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
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
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
Most common abdominal implantation Peritoneal lining of rectouterine cavity (Douglas' pouch); implantation also relatively common in intestinal peritoneum/omentum
Primary ovarian pregnancy Blastocyst implants on surface of ovary
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
In vitro fertilization Follicular growth induced by gonadotropin treatment; secondary oocytes laparoscopically/transvaginally harvested from ovary by aspiration; several oocytes harvested
One drawback of harvesting multiple oocytes? Increases chances of one surviving, but can also result in multiple gestations
Simplistically, how are the harvested oocytes fertilized? Mixing them with diluted sperm; resulting embryos monitored to eight cell stage, and then placed in uterus
Created by: JaneO