When the ectodermal micromeres
divide faster than the macromeres, then 3D induction and mesentoblast formation occur at a more advanced cell stage.
The first mesenchyme cells to ingress are called primary mesenchyme and are derived from the small micromeres
that arise at the vegetal pole of the embryo during an unequal fourth cleavage.
Three additional quartets of micromeres
are formed (2a-2d, 3a-3d, and 4a-4d; Fig.
Feeding veligers acquire a metatroch and food groove, which in Crepidula fornicata originate from progeny of a group of 2nd tier micromeres
(Hejnol et al., 2007).
However, unequal distribution of red cytoplasm within the four macromeres resulting from second cleavage is not consistent with the notion that the initial four macromeres of equally cleaving gastropod eggs are identical prior to later induction of the dorsoventral axis by micromeres
. This non-identity among macromeres may not affect which of the macromeres later becomes the D macromere, but it may result in some subtle but unknown phenotypic difference among the resulting larvae.
Spicules of the sea urchin embryo are synthesized by primary mesenchyme cells (PMCs), which are descendants of the micromeres
of the 16-cell-stage embryo (Gustafson and Wolpert, 1967; Okazaki, 1975; Ettensohn et al., 1997).
Development of a "primitive" sea urchin (Eucidaris tribuloides): irregularities in the hyaline layer, micromeres
, and primary mesenchyme.
In embryos of echinoids with typical feeding larvae, the symmetry of cleavage is interrupted at the fourth cell division: the animal pole quartet of cells divides meridionally and equally, whereas the vegetal quartet of cells divides equatorially and unequally to yield a quartet of small daughter cells (micromeres
) at the vegetal pole of the embryo.
4C), with 4 cells of macromeres at the vegetal pole (lower hemisphere) and 4 cells of micromeres
at the animal pole (upper hemisphere).
Injection of fluorescent lineage tracers (DiI) into identified blastomeres of Mnemiopsis embryos showed that lithocytes originate from oral micromeres
of the E lineage and continue to produce new lithocytes in the epithelial floor throughout adult life (Martindale and Henry, 1999).
At the third division, micromeres
and macromeres could be differentiated and egg development progressed to the gastrula with cell cleavage being total, unequal and spiral (stages 6-11) (Fig.
Embryonic development up to the trocophore stage occurs within the egg membrane and corresponds to the typical development described for gastropod molluscs, with elimination of the two polar bodies after fertilization; segmentation is of the spiral type with the formation of micromeres