Autonomous Specification

Table of Contents

Introduction

Autonomous specification is a major strategy of cell commitment in the development of early embryos.
It refers to the apportionment of critical determination factors in different regions of the egg cytoplasm.
The differentiation of cells occurs through the regulation of gene expression by transcription factors that are present in different parts of the egg cytoplasm.
Autonomous specification means that cells determine their own fate without interacting with other cells.

The early blastomeres of snail Patella are already specified and determined to their fate.
Even in the early cleavage stages, blastomeres that will eventually develop into trochoblast cells can be isolated and will continue to develop into the same cell types as they would in the embryo.

Tunicate (sea squirt) embryos provide some of the best examples of autonomous specification.
In 1905, Edwin Grant Conklin published a fate map of the tunicate Styela partita that showed a visible yellow coloration in the egg cytoplasm that ultimately segregated to muscle lineages.
Cell removal experiments confirmed that muscle-forming cells always derived from the B4.1 blastomeres, and if these cells were removed, the larva had no tail muscles.
Blastomeres that separated from the rest of the embryo continued to form the same cell types, and if the yellow cytoplasm of the B4.1 cells was placed into other cells, those cells developed into tail muscles.
These results suggest that critical factors that control cell fate are present and differentially segregated in the cytoplasm of early blastomeres.

J. R. Whittaker provided biochemical confirmation of cytoplasmic segregation in 1973.
When B4.1 blastomeres were removed and placed in isolation, they produced muscle tissue, but no other blastomere was able to form muscle when separated.
The yellow-pigmented cytoplasm contained mRNA for a muscle-specific transcription factor called Macho, and only blastomeres that acquired this region of cytoplasm formed muscle cells.
Macho is required for tail muscle development in Styela, and loss of Macho mRNA leads to a loss of muscle differentiation, while microinjection of Macho mRNA into other blastomeres promotes ectopic muscle differentiation.
The tail muscles of tunicates are formed autonomously by acquiring and retaining the Macho mRNA from the egg cytoplasm with each round of mitosis.