Segment Development in Capitella teleta
Although the origin of segmental founders in the sedentary capitellid Capitella teleta is not yet described with cellular resolution, cell proliferation assays and injection of cell tracers have provided a very complete picture of proliferation dynamics and fate map in this species (Seaver, Thamm, and Hill 2005; Thamm and Seaver 2008; Meyer and Seaver 2010; Meyer et al. 2010). In contrast to many other annelids, cell 4d in Capitella has a similar size to other third and fourth quartet cells. Surprisingly, cell-tracing experiments have shown that 4d descendants generate the primordial germ cells but do not contribute to either larval or adult segmental mesoderm. Instead, left and right segmental mesoderm forms from descendants of the 3c and 3d blastomeres, respectively. Each mesodermal band is initially visible as a row of subsurface cells extending anteriorly from the telotroch. These bands thicken, becoming several cells wide and tall (except at the posterior end, which contains a single large cell), and expand initially dorsally and then ventrally following an anterior-posterior progression.
In Capitella, trunk ectoderm between the prototroch and the telotroch is formed by the progeny of 2d11. This cell divides into anterior daughter 2d111 and posterior 2d112. Interestingly, 2d112 does not contribute to the ectoderm located between the prototroch and the mouth; thus, this region’s ectoderm derives from 2d111 progeny exclusively. 2d112 divides bilaterally into a right 2d1121 and left 2d1122, which further expand ventrally and dorsally to form right and left segmental ectoderm bands, including chaetal sacs and the ventral nerve cord. While both bands meet symmetrically at the ventral midline, they show variable asymmetry dorsally, so that a band from one side might not reach the dorsal midline or extend past the midline and into the contralateral side.
The mesodermal and ectodermal segmental plates develop into ten segments; segments from each side develop independently and eventually fuse at the midline. Labeled descendants of 2d", 3c and 3d also show at a region located right in front of the telotroch. This region becomes the posterior growth zone, from which three additional segments are generated, to complete this species’ 13 larval segments (after about 5 days post fertilization). At this point, segment formation stops, and resumes only after metamorphosis, at a rate of about one segment every 3 days (Seaver, Thamm, and Hill 2005).