Lecture notes for Monday, January 11, 2016


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:


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 that were discussed in the lecture:

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
(That is because the oocyte is still diploid at the time of fertilization, in most mammals.)
(In dogs and foxes, oocytes are tetraploid at fertilization, after which meiosis occurs)

No species of vertebrate finishes both meiotic divisions in oocytes before fertilization.
Sperm precursor cells do become haploid before they differentiate into sperm.

Sea urchin oocytes really do become haploid before fertilization. Also jelly-fish.
But all other phyla are like vertebrates. Many are like dogs in this respect.

6* In both mammals and sea urchins, the cells leave 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
"Blastocyst" in mammals (which really are different)




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