The Embryo — Cleavage Continues -

Philip Iannaccone
Sep 4, 2018 · 3 min read
The second and third cleavages of the rat embryo. Image courtesy of Greg Taborn, Lurie Children’s Research Center.

One cell becomes two, two cells become four, and four cells become eight. Then we have an embryo set for patterning into the many tissues of a fetus. Each of the divisions is symmetrical in that two identical cells are formed by division. Even this fairly simple observation is a source of great controversy among developmental biologists. These symmetrical divisions comprise the so-called cleavage stage where the egg “cleaves” and forms two “daughter cells.” But some scientists believe that in fact each of the cells that derive from the division are somehow different from each other and that the divisions of the second cleavage do not occur at the same time.

This issue is important because there are species that establish pattern by making two daughter cells after division which are different from one another. Whether or not the division occurs at the same time and whether or not it produces identical daughter cells is based on a very important fact of early development in us and in animals like us. If the cleaving embryo is split into its constituent cells (blastomeres) in the earliest days after fertilization then remarkably each blastomere can produce an entire and perfectly normal individual animal. This is a kind twinning where each twin is essentially an identical copy of the other. This would not happen if the cleavage of the egg was not symmetrical.

After several of these cleavages and the embryo has eight cells, a remarkable event occurs called compaction. The embryo progresses from a collection of rounded, loosely associated cells (the blastomeres) into a spherical embryo where individual blastomeres are hard to discern as they change shape and flatten against one another. Interestingly, this packing starts to maximize the amount of cell-to-cell contact.

How does this happen?

Compaction occurs in stages three stages: the outer surface of the cells recognize each other and rings of very small finger like structures called microvilli form and attach to similar structures on other adjacent blastomeres as if they were shaking hands. Then amazingly the microvilli shorten and move to one end of the cell. Now since the cells have a recognizable orientation (only one end has the microvilli now) the cell is said to be polarized. Compaction seems to be reversible and can be reinduced, but the polarization of microvilli can occur only once. Some of us think that compaction is the most important event in our very early lives!

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.


Originally published at lcresearchcenter.tumblr.com.

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