Juliano CE, Voronina E, Stack C, Aldrich M, Cameron AR (2006) Germ line determinants are not localized early in sea urchin development, but do accumulate in the small micromere
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.
The first two cleavages were equal and the third unequal, producing four micromeres
and four macromeres at the animal and vegetal poles, respectively (Fig.
Third cleavage leads to the formation of the first quartet of micromeres
located at the animal pole (1a-1d).
2] cells, which themselves are the vegetal daughters of the first quartet of micromeres
formed at third cleavage).
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.
AmqNK2/3/4 transcripts are first detected during cleavage in a small number of micromeres
per embryo (Fig.
How the genome changed the identification of network genes is well illustrated by the recent identification of HesC as the Repressor of Micromeres
(Revilla-i-Domingo et al.
For example, patellogastropods (true limpets) and euthyneuran gastropods (pulmonates and opisthobranchs) with equally cleaving eggs depend on regional segregation of morphogenetic determinants to specify the animal-vegetal axis (4), but an inductive interaction between micromeres
and one of the macromeres later specifies the dorsoventral axis of the embryo (5, 6).
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.
Recently, MAP kinase (MAPK) signaling cascades in the gastropod Ilyanassa obsoleta have been linked to cell fate specification within the D macromere lineage and the micromeres
of the A, B, and C quadrants (Lambert and Nagy, 2001).