Segment Development in Tubifex tubifex
In the clitellate naidid Tubifex tubifex, cleavage yields a 25-cell embryo in which large 2dm, 4d, and 4D blastomeres lie along the future midline (Figure 4.7A-B). Each of these blastomeres then divides bilaterally to generate ectodermal, mesodermal, and endodermal precursors, respectively (Penners 1922; Penners 1924; Shimizu 1982; Goto, Kitamura, and Shimizu 1999; Nakamoto, Arai, and Shimizu 2004; Shimizu and Nakamoto 2014).
4d division yields two paired mesoteloblasts: Mr and Ml (Figure 4.7C). These mesoteloblasts divide at regular intervals (2.5 hours at 22°C) to generate small m" primary blast cells (Figure 4.7J). Although the initial rounds of division are highly unequal, M mesoteloblasts become smaller each round, to the point that after the 35th division, M35 and m35 are almost the same size. Their fate after this point is unknown, but it is likely that they divide a few more times to generate mesodermal stem cells at the posterior growth zone of the juveniles. The nth division of the M mesoteloblasts adds an m" blast cell posteriorly adjacent to the previous m" 1 blast
FIGURE 4.7 Development of segmental mesoderm in Tubifex tubifex. A-H) Origin of ectodermal, mesodermal and endodermal precursors. Endodermal El and Er cells and their progeny (lilac) derive from bilateral division of 4D; mesoteloblasts Ml and Mr and their progeny (red) derive from bilateral division of 4d. Ectoteloblast pairs N, О, P, and Q and their progeny (dark blue) derive from bilateral division of 2d111. A) Formation of 3d cell after fifth cleavage; 17-cell embryo, 24 hours post-fertilization (hpf). B) Formation of 4d cell after sixth cleavage; 22-cell embryo, 28.5 hpf. C) Formation of M mesoteloblast pair; 33 hpf. D) Formation of E cell pair; 34 hpf. E) Formation of NOPQ precursor pair; 36 hpf. F) Segregation of N teloblast pair; 48 hpf. G) Segregation of Q teloblast pair; 64 hpf. H) Elongation of germ bands (early gastrula); 96 hpf. I) Cell lineage tree of the progeny of the 2d"1 cell. Stem cells in bold and uppercase, blast cells in lower case. J) Cell lineage tree of the progeny of the 4d cell. Abbreviations as in Figure 4.6. A-G modified after Shimizu (1982); H modified after Shimizu (1982) and Anderson (1973).
cell; in this way, Mr and Ml generate bilaterally paired columns of cells. Within each column, m" undergoes a stereotyped series of divisions, generating a clonal cluster of cells that will generate the mesoderm of a single segment. These columns, known as mesodermal germ bands, are initially located at the dorsal side of the embryo, but as they extend, they migrate laterally and then ventrally until they converge at the ventral midline. At this point, the anterior region of the bands begins to show evident segmental organization. After ventral convergence, the germ bands begin expanding dorsally, migrating between the thin outer layer of yolk sac ectoderm and the yolky endoderm, until they meet along the dorsal midline. Blast cell proliferation, convergence, and morphological segmentation follow a clear anteroposterior gradient.
Segmental ectoderm follows a similar teloblastic pattern. 2d111 divides bilaterally to yield paired ectoteloblast precursors 2d"11 and 2d1112, which in clitellate literature are known as NOPQr and NOPQ1, respectively (Figure 4.7E). Each ectoteloblast precursor divides twice very unequally producing small nopq1 and nopq2 cells and then divides less asymmetrically into an N ectoteloblast and an OPQ precursor (Figure 4.7F). OPQ also generates two small cells (opq1 and opq2) and then divides into a smaller Q ectoteloblast and a larger OP precursor (Figure 4.7G). OP divides very unequally four times (yielding op1 through op4) and then divides equally to yield О and P ectoteloblasts. After their birth, each of the eight large ectoteloblasts (four on each side) divides very unequally with a constant frequency (every 2.5 hours at 22°C) giving rise to bilateral columns of small cells called primary blast cells that are arranged into a bilateral pair of bandlets (Figure 4.7H). These bandlets overlie the mesodermal germ bands and show a similar behavior of ventral convergence followed by dorsal extension until meeting at the dorsal midline to enclose the yolky endoderm. Descendants from those primary blast cells proliferate and follow a stereotyped developmental program, contributing to a segment’s worth of specific tissues (Figure 4.71).
In Tubifex, as in other clitellates including leeches (see Chapter 7), both trunk mesoderm and ectoderm show unambiguous lineage-driven segmentation. The origin of the cell lineage present at the posterior growth zone of juveniles and adults has not yet been determined; however, the behavior of Tubifex M mesot- eloblasts is highly reminiscent to that of Platynereis (see earlier), strongly suggesting that after giving rise to the last embryonic m blast cell, the now small M mesoteloblast gives rise to the founder mesodermal stem cells at the PGZ. In the same vein, it is likely that N, О, P, and Q ectoteloblasts eventually form the ring of ectodermal stem cells at the PGZ. Assuming this to be the case, another fascinating question is whether the stem cell progeny of the N, О, P, and Q progeny retain the lineage restrictions exhibited by their respective parent cells during embryonic segmentation.