At about the onset of gastrulation, the embryo developed a slight hump at the animal pole, and trochoblast
cells began to produce patches of prototrocal cilia (Fig.
In all spiralians these trochal cells are derived from the comparable trochoblast cell lines.
After the 32-cell stage, the sixth cleavage cycle is initiated by the two tiers of primary trochoblasts, and completed by the division of the macromeres 3A-3D at the transition of the 60- to the 64-cell stage.
2] cells, the primary trochoblasts, resume mitosis and the embryo attains a 28-cell stage.
The first larval stage differs from the classic gastropod trochophore in lacking a girdle of multiciliated trochoblast
cells; instead, it is fully ciliated, with simple cilia extending through pores of an external cuticular layer (Fig.
For instance, the accessory trochoblast
cells that contribute to the formation of the prototroch in many spiralians (2q derivatives) have been co-opted to form the secondary opposed feeding band ("metatroch") in C.
It may very well be, however, that the ciliated cells in question are derived from the trochoblast cell lineage, as has been suggested for pericalymma larvae of protobranch bivalve and neomenioid aplacophoran molluscs (Thompson, 1960; Zardus and Morse, 1998), and shown to be the case in the palaeonemertean Carcinoma tremaphoros (Maslakova et ah, 2004).
Most annelid larval forms, feeding or not, can be readily related to the trochophore ground plan, which Nielsen (2001, 2004) defined as a larva bearing an apical tuft, a preoral ciliated band derived from trochoblasts (prototroch), an adoral ciliary zone, and postoral metatroch, gastrotroch, and telotroch, all of which bands consist of multiciliate cells with compound cilia.
2004a, b) discovered that the larva of a palaeonemertean, Carinoma tremctphoros, possesses a "hidden prototroch"--a transitory preoral ring of large cleavage-arrested cells derived from the trochoblast cell lineage.
In each of these, the cells presumed to be derived from the trochoblast lineage originally cover nearly the entire larval surface and disappear as a distinct preoral domain during the course of larval development.
In this paper, we discuss the molecular and developmental aspects of trochoblast and mesentoblast formation and their significance to the analysis of the phyletic relations between spiralian phyla.
Detailed knowledge of trochoblast specification, however, is limited to Patella vulgate, the common limpet.