Meroblastic Cleavage (Molecular Biology)

Meroblastic cleavage is the incomplete cellular division found in embryos with large amounts of yolk (1). In embryos with little yolk, the early cleavage furrows can completely separate the daughter cells, or blastomeres. In many vertebrate embryos, the large amounts of yolk prevent the cleavage furrows from completely separating the daughter blastomeres. In these embryos, the cytoplasm is usually segregated to one end of the embryo and sits on top of the yolk. The cleavage furrows begin in the cytoplasm, but only proceeds part of the way through the yolk. The nuclei and associated cytoplasm are separated on one surface, but remain connected to the yolk (and to each other) at the interior. Later divisions tangential to the original cleavage furrows completely separate the cells in the dorsal and middle portions of the cytoplasmic region, while the cells more ventral and lateral continue to be connected cytoplasmically to the yolk and each other. Eventually, a disc of blastomere cells is formed that sits on top of the yolk, with the nearest cells still connected to the yolk. Meroblastic cleavage is common among birds (eg, the domestic chicken embryos that have been used extensively for embryological studies), reptiles, bony fishes, and amphibians.