Lecture notes for Wednesday, January 10, 2018
Embryology of mammals compared with sea urchins
Sea urchin eggs are about the same size as mammalian eggs (about 100 micrometers). Both have yolk, but not nearly as much as in other vertebrates such as fish, frogs, and birds.
Links to videos:
link to the video of sea urchin cleavage, on the web site of Rachel Fink of Mt. Holyoke College
These two figures are taken from a larger figure in which early development is compared in additional animals. This figure will be shown in some later lectures also.
Similarities and differences:
1* In mammals, cleavage (the sequence of cell divisions after fertilization) isn't synchronous, not 1-2-4-8-16-32 etc. as it is in sea urchins.
2* Cleavage in mammals isn't rapid (16-20 hours from one division to the next, instead of 30-60 minutes in sea urchins and other animals whose eggs develop in water).
3* Cell cycle "check points" are not turned off in mammals, unlike urchins, and also frogs, fish, and birds.
4* Early cleavages don't form any special pattern in mammals (just a pile of cells).
5* Mammal embryos become triploid when the sperm fuses with the oocyte.
No species of vertebrate finishes both meiotic divisions in oocytes before fertilization.
Sea urchin oocytes really do become haploid before fertilization. Also jelly-fish.
6* In both mammals and sea urchins, the cells cleave all the way through (holoblastic cleavage, versus meroblastic cleavage, which happens in fish and birds).
7* Cells rearrange into a hollow ball. "Blastula" in urchins and most vertebrates
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