Cell lineage in molluscan development

Abstract

Cell lineage specification in molluscs is brought about by two mechanism: the segregation of morphogenetic plasms and inductive cell interactions. The evidence for the existence of morphogenetic plasms is largely circumstantial, but in one species, Bithynia, such a plasm has been identified in the polar lobe that forms at first cleavage. Inductive cell interactions are thought to be a prerequisite for the development of a large number of tissues and organs. The most extensively studied example is the specification of the mesodermal stem cell in Lymnaea and Patella, which occurs between 5th and 6th cleavage through an interaction between one macromere and a large number of micromeres. Both segregation and induction are tuned to the animal-vegetal polarity of the egg, at least during early development. This polarity probably arises during oogenesis and is manifest in regional differentiations of the surface architecture of the egg, in the distribution of inner membrane particles in the plasma membrane, in membrane fluidity characteristics, in ionic conductance properties of the plasma membrane, etc. All these phenomena have in common that they represent properties of the egg surface, suggesting that the polarity of the egg is somehow imprinted into the plasma membrane and the cortex of the egg during oogenesis.

Spiral cleavage and early embryology of a loxosomatid entoproct and the usefulness of spiralian apical cross patterns for phylogenetic inferences

“…Although the formation of the first micromere quartet typically appears in a clockwise direction in most taxa, including the supposedly basal solenogaster Epimenia babai [35], the polyplacophoran Stenoplax heathiana (formerly Ischnochiton heathiana ; [32]), the scaphopod Antalis entalis [33], as well as the gastropods Limax [31], Crepidula [28,38], Patella [39], and Ilyanassa [40], a counter-clockwise formation is sometimes found, e.g., in the bivalve Dreissena polymorpha [41] or in gastropods with sinistrally coiled shells such as Planorbis trivolvis [42], Physa heterostropha [43], or Lymnaea stagnalis [44]. Accordingly, it appears that in shell-bearing gastropods, the chirality of cleavage is strictly correlated with the direction of shell coiling [43-45]. In contrast to the sinistrally coiled gastropod species mentioned above, fourth cleavage takes place in a counter-clockwise direction [28,29,31,32,35,39].…”

Some aspects of spiralian development

“…Similarly, it is generally supposed that the spiralians retain the apical-basal orientation of the developing oocyte on the ovarial epithelium as the apical-blastoporal axis (Raven 1966;Dohmen 1992), but direct observations are few.…”

Oyster vasa-like gene as a marker of the germline cell development in Crassostrea gigas

“…However, the origin of germline is unclear in most mollusc species. In bivalves, the existence of PGCs was reported only in few species, such as Cyclas cornea, Sphaerium japonicum and Lasea rubra (21). In the clam Sphaerium, the PGCs were thought to arise from the 4d mesentoblast formed at the sixth cleavage.…”