Cytoplasmic Determinants

∞ generated and posted on 2016.12.23 ∞

Aspects of unfertilized eggs and subsequent zygotes that give rise to positional differences between cells during early development.

In a sense, early embryogenesis is mapped out within the zygote as well as the egg cell before it in terms of what material ends up in progeny cells during early cleavage. This material in turn serves to control what gene expression occurs, thereby effecting the cell fate determination of resulting cell lineages and, through cell-to-cell communication, those cells that are found around them.

The factors responsible for this early mapping out, within zygotes, of subsequent cell fate determination are termed cytoplasmic determinants. They consist, for example, of stored mRNAs as well as proteins and various regulatory factors that found within the cytoplasm of the egg cell.

The cytoplasm of these progenitor cells possess chemical gradients that, through cleavage (cell division), are translated into different cell lineages. Subsequent gradients of signaling molecules that develop between cells provide additional spatial information and it is from these extracellular gradients that the direction of further cell fate determination is gleaned, i.e., signal induction.

The transition from the intracellular chemical gradients, called cytoplasmic determinants, to extracellular chemical gradients, as occurs during development, can viewed as paralleling evolutionary transitions that must have occurred in the course of the evolution of multicellularity, particularly given indeterminate cleavage.

That is, complex cellular morphologies involve the development of intracellular chemical gradients and cell-to-cell signaling is known to occur among unicellular organisms as well as within relatively undifferentiated colonies of cells. But for complex morphogenesis within multicellular organisms to occur, the "seed" of intracellular gradients must have transitioned into extracellular gradients, i.e., signal induction as we see during development today.