isolecithal

Isolecithal is a term used in embryology to describe eggs in which yolk is sparse and evenly distributed throughout the cytoplasm. The yolk content is minimal and not concentrated at any pole, a condition sometimes referred to as microlecithal, though some sources reserve microlecithal for particularly tiny yolk.

Cleavage in isolecithal eggs is holoblastic, meaning the whole egg participates in cell divisions. Because yolk is evenly dispersed, blastomeres tend to divide into roughly equal-sized cells. In different lineages, the cleavage pattern may be radial, as seen in many echinoderms, or rotational, as observed in mammals such as humans.

Development proceeds from cleavage to the formation of a blastula, with a blastocoel developing inside. Gastrulation follows according to species-specific mechanisms, including invagination or epiboly. The uniform distribution of yolk allows relatively uniform early cleavages and developmental potential across the embryo.

Occurrence of isolecithal eggs spans various taxa, including many mammals (humans, mice, and other species) and certain invertebrates such as sea urchins and starfish. This contrasts with other yolk distributions: mesolecithal eggs contain a moderate amount of yolk, centrolecithal eggs have yolk concentrated in the center, and telolecithal eggs hold yolk at one end, often leading to discoidal or superficial cleavage.

Etymology of the term derives from Greek isos, equal, and lēkith, yolk. The concept helps explain differences in cleavage and early development across species with varying yolk content.