Animal and vegetal teloplasms mix in the early embryo of the leech, Helobdella triserialis

Abstract

In embryos of the glossiphoniid leech, Helobdella triserialis, as in many annelids, cytoplasmic reorganization prior to first cleavage generates distinct animal and vegetal domains of yolk-deficient cytoplasm, called teloplasm. Both domains are sequestered to the D' macromere, progenitor of the definitive segmental tissues, during the first three rounds of cell division. And it has been believed that during the fourth round of cell division, the obliquely equatorial cleavage of macromere D' cleanly segregates animal teloplasm into an ectodermal precursor, cell DNOPQ, and vegetal teloplasm into a mesodermal precursor, cell DM. But here we report a hitherto unobserved cytoplasmic rearrangement between the second and the fourth divisions that seems to mix the animal and vegetal domains of teloplasm. The newly observed rearrangement consists of the movement of vegetal teloplasm toward the animal pole of cell D' between the second and the fourth cell divisions. Animal and vegetal teloplasms form a single pool of teloplasm in cell D' which is then divided between DM and DNOPQ at the fourth division. The movement of teloplasm was inferred by examination of embryos fixed and sectioned between the second and the fourth rounds of cleavage and was confirmed in living embryos microinjected with rhodamine 123, a fluorescent mitochondrial stain.