Ho R K, Weisblat D A
Dev Biol. 1987 Apr;120(2):520-34. doi: 10.1016/0012-1606(87)90255-7.
Segmental tissues of glossiphoniid leeches arise from rostrocaudally arrayed columns (bandlets) of segmental founder cells (primary m, n, o, p, and q blast cells) which undergo stereotyped sublineages to generate identifiable subsets of definitive progeny. The bandlets lie at the surface of the embryo beneath the squamous epithelium of a transient embryonic covering called the provisional integument. This "provisional epithelium" derives from microsomes produced during the early cleavage divisions. Previous experiments have shown that the primary o and p blast cells constitute an equivalence group, i.e., are initially developmentally equipotent and undergo hierarchical interactions which cause them to assume distinct O and P fates. Here, we examine the role of the provisional epithelium in determining the fates of the underlying o and p blast cells. Experiments entailing the microinjection of individual micromeres with cell lineage tracers show that, at stages 7-8 of normal development, the epithelium comprises coherent and relatively stereotyped domains derived from particular micromeres. Upon photoablating domains of epithelium labeled with photosensitizing lineage tracer, the normal assignment of O fates is disturbed; o blast cells divide symmetrically (as p blast cells do) and some supernumerary definitive progeny expressing P fates arise within the O lineage. We therefore conclude that the epithelium is essential for generation and/or reception of signal(s) by which the o and p blast cells' normally determine their fates. Finally, a new tracer substance, biotinylated fixable dextran (BFD), is described which was essential for this study by virtue of its superior resistance to photobleaching and which offers several other advantages as well.
舌蛭科水蛭的节段组织起源于节段性奠基细胞（原初的m、n、o、p和q胚细胞）沿头尾方向排列的柱体（条带），这些细胞经历定型的亚谱系以产生可识别的终末子代亚群。这些条带位于胚胎表面，在一种称为临时体被的短暂胚胎覆盖物的鳞状上皮之下。这种“临时上皮”源自早期卵裂分裂过程中产生的微粒体。先前的实验表明，原初的o和p胚细胞构成一个等价组，即最初在发育上具有同等能力，并经历层级相互作用，使它们呈现出不同的O和P命运。在这里，我们研究临时上皮在决定其下方的o和p胚细胞命运中的作用。用细胞谱系追踪剂对单个小卵裂球进行显微注射的实验表明，在正常发育的第7 - 8阶段，上皮由源自特定小卵裂球的连贯且相对定型的区域组成。在用光敏谱系追踪剂标记的上皮区域进行光消融后，O命运的正常分配受到干扰；o胚细胞对称分裂（如同p胚细胞那样），并且在O谱系中出现了一些表达P命运的额外终末子代。因此，我们得出结论，上皮对于o和p胚细胞正常决定其命运所依据的信号的产生和/或接收至关重要。最后，描述了一种新的追踪物质，生物素化可固定葡聚糖（BFD），由于其对光漂白具有优异的抗性，对本研究至关重要，并且还具有其他几个优点。
