Second lecture   January 11, 2012

Differences " # " between sea urchin embryonic development versus mammal development

And also similarities: " * "

*a) Oocytes are about 80 to 100 micrometer spheres. (1000 > times the volume of average cells)

*b) Cytoplasm contains yolk, but not nearly so concentrated as in bird eggs.

*c) Oocytes undergo two meiotic divisions,

Reducing chromosome number from 4 to 2 and then to 1 {+ one sperm nucleus will equal two}

*d) Producing first one and then two tiny cells, called "polar bodies" which normally degenerate.

#e) One diference is that mammal polar bodies are much bigger than on urchin eggs.

    Mammal polar bodies can be ~ 5 micrometers in diameter, or more.
    You will be able to see them in some of the videos.
    (& sometimes they get moved around) (And sometimes get made into part of the body, even in people!)

(Polar bodies are too small to see (<1 micrometer) in sea urchins.

#f) Another difference is that sea urchin oocytes finish both first and second meiotic divisions before they fuse with a sperm.

(People tend to assume this is universal among animals: actually it's the exception. It is true of all echinoderms (urchins, starfish, brittle-stars etc.) and also jelly fish, corals & Hydra)

But no species of vertebrate has been discovered in which egg cells complete both meiotic divisions before fertilization. Dog and fox oocytes are fertilized before either meiosis, when they still have FOUR complete sets of chromosomes; therefore they become pentaploid temporarily.
(Many kinds of invertebrates also do that: Oocytes need those chromosomes to transcribe RNA!)

Humans, mice, and the great majority of vertebrates undergo the first meiosis (produce one polar body) THEN fuse with a sperm (making them temporarily triploid) [for an hour or more]
And only after that undergo the second meiotic division (reducing chromosome number to diploid).

#g) Cleavage divisions are simultaneous (= synchronous) and also very fast & frequent in early embryos of sea urchin, fish, frogs, chickens (and most invertebrates) But NOT in humans, mice or any mammal.

So our embryos don't simply go 1 - 2 - 4 - 8 -16 - 32 cells (you can see this in the videos)

#h) A related difference is that time between divisions is 15-20 hours in vertebrate embryos (which is in the normal range of cell cycle time for faster growing cells in the adult body)

IN CONTRAST to times of one hour, or half an hour, in sea urchins (or ~ 11 minutes in earliest stages of insect embryos)

Both these two differences (g and h) are because mammal embryos don't turn off cell cycle controls, whereas sea urchin embryos, frogs, fish, birds, and many invertebrates DO turn them off.

Mammal embryo cells divide when they are good and ready, and also pay no attention to when other cells of that same embryo divide.

*i) Both urchin & mammal embryos develop into hollow balls of (mostly) epithelial cells

#j) Called the blastula stage in sea urchins (also frogs)

But called the blastocyst stage (ONLY) in mammal embryos.


Only about 1 in 20 of the mammal embryo cells become part of the body.
(The other 95% form placenta "extraembryonic membranes" for protection, nutrition etc.)

This twentieth that form the body are the "inner cell mass"

The 19/20ths that form placenta are called "the trophoblast".

ALL the cells of a sea urchin embryo become some part of the body (& That's also true of frog embryos, but NOT true of fish or bird embryos (which form extraembryonic membranes)

# ? ) Identical twins ("monozygotic twins")... how do they form?

In humans only sometimes by separation at the 2 cell stage (& I would guess NEVER)

About half of pairs of identical twins result from aggregations of cells into two masses, so there are 2 trophoblasts, each with its own inner cell mass.

Another ~49% of human identical twins result from forming two inner cell masses inside one trophoblast;

And the remaining > 1? result from two body axes developing side by side.

Of this third category, a very small minority are conjoined. "Siamese Twins"


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