[24.] 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.
rosaceus follows the typical pattern for echinoids with planktotrophic larvae, beginning with three equal cleavages followed by the formation of mesomeres, macromeres, and micromeres
at the 16-cell stage (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
In early development there is an apparent lack of a hyaline layer, irregularly arranged blastomeres, variable numbers of micromeres
, and an absence of primary mesenchyme (Schroeder, 1981).
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).
Cleavage is dexiotropic and unequal at the 8-cell stage: the animal cells (micromeres
) are larger than the vegetal cells (macromeres) (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).
AmqNK2/3/4 transcripts are first detected during cleavage in a small number of micromeres
per embryo (Fig.3A, B).
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., 2007).
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).