The main target of chromosome manipulation in these studies was inhibition of polar body (PB) formation or first cleavage division.
The objectives of this work were to document early development starting with meiosis in newly fertilized eggs through the first and second cleavage divisions, including centrosome inheritance and duplication observed by confocal microscopy.
The second cleavage division, which begins shortly before the first is complete, also occurs in a distal-proximal direction and is perpendicular to the first one.
By the fourth to fifth cleavage division, a cavity begins to form in the interior of the cell mass [ILLUSTRATION FOR FIGURE 11 OMITTED], while the surface remains uneven [ILLUSTRATION FOR FIGURE 8 OMITTED].
After 2 h the addition of fresh sperm did not result in the initiation of cleavage divisions.
Quadrant identities, and the dorsoventral axis, are established relatively late through cell-cell interactions following the fifth cleavage division.
The first and second cleavage divisions normally correlation to the frontal plane and the plane of bilateral symmetry, generating left- and right-ventral and left- and right-dorsal quadrants.
Are quadrant identities and the dorsoventral axis in nemerteans established late by virtue of cell-cell interaction, as in other equal-cleavers, or are they set up precociously as a consequence of the early cleavage divisions, perhaps relative to some underlying axial properties?
The concentration of ACh exhibits distinct peaks during early cleavage divisions, but the major, sustained increases occur after the beginning of gastrulation, in tandem with transcription of zygotic genes and attendant rises in choline acetyltransferase, the enzyme that synthesizes ACh (Buznikov et al.
DMAE added to the embryos at the two-cell stage in concentrations of 100-400 [micro]M did not alter cleavage divisions or blastulation, but at very high concentrations (600-800 [micro]M), cleavages were inhibited (Figures 2 and 3).
Although cleavage divisions were affected only at very high DMAE concentrations, embryos treated with lower concentrations at the two-cell stage subsequently developed malformations (Figure 3): overabundance of mesenchyme-like cells that accumulated first in the vegetal half of the blastocoel, and later occupied all of the blastocoel, blocking or inhibiting gastrulation; absence of a second pair of arms in mid-pluteus, or sometimes shortening or absence of one arm of the first pair (Figure 3H).
Like those of a large majority of taxa, sea urchin embryos establish a spatial coordinate system for the initial body plan from one axis, the animal-vegetal (A-V), that is fixed during oogenesis by asymmetric deposition of maternal molecules (the embryologists' "determinants") and a second axis, dorsal-ventral (or, more descriptively, oral-aboral), that is specified sometime during the first few cleavage divisions
(reviewed by Davidson, 1989).