Micromeres of the sea urchin embryo

Sixth cleavage sea urchin embryo.

The micromeres are four small blastomers which at the fourth cleavage are segregated at the vegetal pole of the embryo due to the fact that in the macromers the spindle is strongly shifted towards the vegetal pole. The micromeres are committed to the formation of the primary mesenchyme and exert two important roles in morphogenesis:

1. They are responsible for the control of gastrulation.
2. They act as pacemakers of cell divisions during cleavage.

It was discovered by Driesch that the mesenchyme blastula of sphaerechinus there are about 30 primary mesenchyme cells and about 55 in Echinus. Embryos which develop from one of the first two blastomers isolated after the first cleavage contain half the number of the primary mesenchyme cells: about 14 in sphaerechinus and about 27 in Echinus.

Sixth cleavage sea urchin embryo.

It is interesting to note that the micromeres cleave at a lower rate and their division is out of phase with respect to the other cells of the embryo. The cleavage of the first four micromeres give rise to eight cells only four of which, namely the outer ones, continue to divide while the four inner ones appear to have lost the ability to divide any further (atleast through the next two division cycles; later the micromeres become undistinguishable from the other cells of the embryo). Thus within the micromere cell population two sub-populations are soon segregated, each of which again appear to be programmed as to whether of not the inner micromeres are the precursors of a cell line different from that of the other micromeres.