Early events leading to fate decisions during leech embryogenesis

This paper reviews leech development up to the 12-cell embryo. Oogenesis proceeds by a system of nurse cells that contribute to oocyte growth via continuous cytoplasmic connections. Development begins when fertilized eggs are deposited: formation of the polar bodies, and centration of the male and female pro-nuclei is accompanied by cytoskeletal contractions, and formation of teloplasm (yolk-free cytoplasm). The first cleavages are asymmetric: cell D', the largest macromere in the eight-cell embryo, contains most of the teloplasm. At fourth cleavage D' divides equally; its animal and vegetal daughters are precursors of segmental ectoderm and mesoderm, respectively. Teloplasm is a determinant of the D' cell fate. The expression pattern of Hro-nos, a leech homolog to the Drosophila gene nanos, suggests that it may be a determinant associated with the animal cortex and inducing the ectodermal fate in the animal daughter cell of the D' macromere.