Figure 2

A single cell (row) may be a segment. (A) Lateral view of a Ciona savignyi late tailbud-stage embryo where the notochord is composed of a column of single cells (yellow). The planar-cell polarity proteins Prickle and Strabismus (orange) are located at the anterior and the nucleus (blue) is located at the posterior of each cell. (A after [35].) (B, C) Segmentation in the trunk of Parhyale hawaiensis. Left side of the embryo is depicted, right side is mirror image. Red arrows represent progression in time. (B) Once ectodermal cells condense into rows (PSPRs), each PSPR divides to produce one parasegment of ectoderm. After the first PSPRs division, Ph-hedgehog (Ph-hh, orange) is expressed in the anterior row (row a/b) [40]. After the second division, both En and Ph-hh (red) are expressed in the anterior row (row a) [36]. While the division of one PSPR produces one parasegment of ectoderm, in general, one segment’s worth of ectoderm (bracket) forms from the Engrailed (En) negative cells of one parasegment (rows b to d), and the En positive cells from another parasegment (row a) [36]. (C) One row of mesoblasts produces one segment’s worth of mesoderm (bracket). After the first mesoblasts division, Ph-twist (green) and Ph-even-skipped (purple) are expressed in a subset of the anterior daughters [27, 37, 40]. (D) Segmentation in the leech ectoderm. One side of the embryo is depicted, other side is mirror image. The ectoderm is formed from the progeny of four ectoteloblasts, N, O, P, and Q, [41]. Each progeny, or blast cell (green), of O and P gives rise to one segmental unit. However, two adjacently produced blasts cells from N and Q, ns (yellow) and nf (blue) and qs (yellow) and qf (blue), respectively, give rise to one segmental unit. a, anterior; p, posterior, PSPR, parasegment precursor row.