An Axial Posterior Growth Zone (PGZ) Originates from the D Quadrant in Helobdella and Other Clitellate Annelids
Embryos of annelids, mollusks, and most other lophotrochozoan taxa undergo a conserved pattern of early cell divisions called spiral cleavage, in which the third cleavage is obliquely oriented and unequal. This results in eight-cell embryos composed of quartets of animal micromeres and vegetal macromeres, respectively, that are offset from one another by roughly 45 degrees around the animal-vegetal axis. In many annelids (Dohle 1999), the first and second mitoses are unequal, so that the four-cell stage already contains a uniquely determined D quadrant blastomere, indicating a heterochronic shift of the D quadrant specification process relative to the presumed equal cleaving spiralian ancestor (Freeman and Lundelius 1992). The D quadrant ultimately gives rise to much of the mesodermal and ectodermal tissues. In Clitellata, the unequal first and second cleavages segregate yolk-deficient domains of cytoplasm to the D macromere. In leeches, the yolk-deficient cytoplasm is enriched in mitochondria, maternal mRNAs, and ribosomes (Fernandez and Olea 1982; Fernandez et al. 1990, 1998; Holton et al. 1989, 1994) and it is this material, rather than the difference in cell size that is critical for specifying the D quadrant in Helobdella (Astrow et al. 1987; Nelson and Weisblat 1991, 1992).
C.O. Whitman’s pioneering cell lineage studies in the 19th century accurately established the outline and many of the details of the early development of glos- siphoniid leeches (Whitman 1878; see also Sandig and Dohle 1988; Bissen and Weisblat 1989). In Helobdella and other clitellate embryos (Figure 7.3), the D macromere undergoes an arcane and yet remarkably well conserved series of mostly unequal divisions, generating a mix of smaller, yolk-free cells (micromeres) and five bilateral pairs of larger, yolk-rich cells (M, N, О/P, О/P, and Q teloblasts-, Figure 7.3). The D macromere generates a total of 16 micromeres, and the other three quadrants each contribute three more. These 25 micromeres gives rise to exclusively non-segmental tissues, chiefly the proboscis, the dorsal anterior ganglion, and the squamous epithelium of a temporary integument that spread to cover the embryo during gastrulation. In contrast, as described later, the teloblasts function as individually identifiable, lineage-restricted stem cells. They constitute a posterior growth zone (PGZ) from which segmental mesoderm and ectoderm arise in anteroposterior progression.
FIGURE 7.3 Formation of the Helobdella posterior growth zone (PGZ). Top: Selected stages of Helobdella development. Bottom left: Each teloblast undergoes repeated, highly unequal divisions, forming a column of segmental founder cells (blast cells) for that lineage. Bottom center: Detailed depiction of an early stage 8 embryo, showing that the five ipsilateral columns of teloblast-derived bandlets merge into a parallel array (germinal band) at the surface of the embryo in the vicinity of the animal pole, with the four ectodermal bandlets lying over the mesodermal bandlet (not visible here) on each side of the embryo. Teloblasts and their proximal blast cells constitute the PGZ. For clarity, the micromere-derived epithelium that covers the germinal bands and the region behind them is not illustrated for early stage 8, but is depicted by small irregular contours for mid and late stage 8. Bottom right: Schematic representation of stereotyped blast cell divisions and other events in the early germinal band (red boxes). Grandparental N and Q lineages are depicted by alternating dark and light cells in the blue (n) and green (q) bandlets. Fate specification in the О/P equivalence group is represented by the transition from orange/yellow (equipotent о/p) cells at the posterior end of the germinal band to distinct yellow (P lineage) and orange (O lineage) cells more anteriorly. Horizontal dotted lines indicate that individual segmental complements in the О and P lineages are not yet in register with those in the N and Q lineages. See text for details.