from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A very small blastomere.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. An unusually small blastomere formed by unequal cleavage of a fertilized ovum
from the GNU version of the Collaborative International Dictionary of English
- n. One of the smaller cells, or blastomeres, resulting from the complete segmentation of a telolecithal ovum.
from The Century Dictionary and Cyclopedia
- n. The smaller one of two masses or moieties into which the vitellus of a lamelli branch, as a fresh-water mussel, divides; the so-called “animal cell” of Rabl, which further subdivides into blastomeres. See macromere.
- n. One of the small blastomeres, or cells, which result from the early cleavage of the egg: opposed to macromere or megamere.
In this study, we resolve the regulatory connections between the Pmar1-activated PMC-GRN and the E-EM / En-GRN by testing the responsiveness of every zygotically activated core factor of the E-EM / En-GRN to the ectopic expression of the micromere determinant Pmar1.
This study unexpectedly uncovers the existence of an additional uncharacterized micromere signal to endomesoderm progenitors, significantly revising existing models.
Thus, micromere-dependent SoxB1 clearance occurs normally when at least two Pmar1-mediated veg2 specification signals (ActivinB and Delta) are blocked.
Therefore, although SoxB1 clearance depends on an early micromere signal (s), it is a gradual process that is not completed until the mesenchyme blastula stage and relies on an unknown pathway (s) distinct from the one that regulates
Despite several decades of efforts, however, neither the molecular identity (s) of these micromere-originated signals nor their specific regulatory contribution to early endomesoderm specification has been elucidated.
(A) The micromere determinant Pmar1 (circled in red) activates the PMC-GRN in micromere progeny and is sufficient for micromere-derived endomesoderm-inducing signals.
(B) Schematic depicting an experiment that reveals micromere-derived endomesoderm inductive signals, which are sufficient to induce ectopic endo16 expression and complete archenteron formation in animal blastomeres, and are also necessary for normal vegetal endo16 expression and timely gastrulation in the sea urchin embryo.
A third, unknown Pmar1-mediated signal clears SoxB1 (black line leading to factor circled with solid black line) from endomesoderm precursors and, contrary to previous models of pregastrular endomesoderm development, is independent of micromere-mediated specification of these blastomeres.
Minokawa T, Amemiya S (1999) Timing of the potential of micromere-descendants in echinoid embryos to induce endoderm differentiation of mesomere-descendants.
The zygotically expressed core factors Z13, Eve, Wnt8, Blimp1, FoxA, and Brachyury (Bra) (circled in black) accumulate in presumptive endomesoderm during early developmental stages and could potentially respond to early inductive inputs from micromere descendants.