Leech eggs5/30/2023 ![]() In contrast to the unequal first cleavage, the unequal second cleavage does not result from down-regulation of one centrosome, nor from an asymmetry within the spindle itself. Here we show that stereotyped cellular dynamics, including the formation of an intercellular blastocoel, culminate in a morphological left–right asymmetry in the 2-cell embryo, which precedes cytokinesis and predicts the chirality of the second cleavage. When CD divides, the larger D and smaller C blastomeres arise invariantly on the left and right sides of the embryo, respectively. The unequal division of the CD blastomere at second cleavage is critical in establishing the second embryonic axis in the leech Helobdella, as in other unequally cleaving spiralians. ![]() The latter feature, associated with cells em1 and em2 in Helobdella, is seen with the M teloblast lineage in a second clitellate species, the sludgeworm Tubifex tubifex and, on the basis of previously published work, in the initial progeny of the M teloblast homologs in molluscan species, suggesting that it may be an ancestral feature of lophotrochozoan development. We show that the first six progeny of the M teloblasts (em1–em6) contribute five different sets of progeny to non-segmental mesoderm, mainly in the head and in the lining of the digestive tract. Here we analyze cell fates for the early progeny of the bilateral daughters (M teloblasts) of micromere 4d in the leech Helobdella sp. Thus, distinguishing the conserved and diversified features of cell fates in the 4d lineage among modern spiralians is required to understand how lophotrochozoan diversity has evolved by changes in developmental processes. In particular, bilateral mesoderm in most lophotrochozoan species arises from the progeny of micromere 4d, which is assumed to be homologous with a similar cell in the embryo of the ancestral lophotrochozoan, more than 650 million years ago. Moreover, in contrast to Ecdysozoa and Deuterostomia, most lophotrochozoans exhibit a conserved pattern of stereotyped early divisions called spiral cleavage. The super-phylum Lophotrochozoa contains the plurality of extant animal phyla and exhibits a corresponding diversity of adult body plans. When this process is disturbed by colchicine or cytochalasin B treatment, abortive or ghost pole cells may be formed. A cytokinetic contractile ring forms by assembly of cortical actin filaments at the base of the AP evagination. Poleward displacement of this wave causes evagination of the AP and ooplasmic segregation. Emission of the first pole cell is preceded by a contraction wave that seems to arise by condensation of subcortical actin filaments at the equator of the egg. Redistribution of microvilli, as well as rearrangement of the ectoplasmic actin lattice, lead to remodeling of the egg surface. ![]() Meanwhile, the central spindle pole is less modified and is reused during the second meiotic division. The peripheral spindle pole is greatly modified during its anchorage to the AP and is dismantled upon emission of the first pole cell. Rotation of the meiotic spindle, presumably as a result of changes in the length and arrangement of astral fibers, allows one of its poles to approach the prospective animal pole (AP), which appears as a differentiated region of the ectoplasm. Reorganization of the fertilized egg during completion of the first meiotic division was studied in the glossiphoniid leech Theromyzon rude.
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