BIOL 001 Lecture Notes - Lecture 32: Trophoblast, Spina Bifida, Neural Crest

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End of cleavage > blastocyst has formed. Blastocyst is the mammalian version of blastula. Arrives at uterus approx 6 days after fertilization. Approx 100 cells around the central cavity. Trophoblast contracts uterine lining secrete enzymes. > enzymes will erode area of endometrium. > embryo penetrates into the wall of the endometrium. > thickens, extend projections into endometrium that reach out to blood vessel linings in order to make it nutrient rich and increase surface area. Takes approx 1 week and usually takes place in 2nd-3rd week of pregnancy. Inner cell mass forms flat disc 2 layers. From these two layers are where we get our embryonic germ layers. Epiblast cells mix with hypoblast > endoderm. Some remain on the surface > ectoderm. Yolk sac - blood cells for early embryo. Allantois - blood vessels and part of placenta attached to umbilical. Nervous system is the 1st to develop in most vertebrates.

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