Introduction

Autonomous specification refers to the process of cell fate determination in early tunicate embryos, where cell fates are determined by intrinsic factors. However, the nervous system in tunicate embryos arises through conditional specification, where cell fates are determined by interactions with other cells.

Conditional Specification

• Conditional specification refers to the process of cell fate determination where cell fates are determined by interactions with other cells, which include cell-to-cell contacts (juxtacrine factors), secreted signals (paracrine factors), or the physical properties of the local environment (mechanical stress).
• Example: if cells from one region of a vertebrate blastula are transplanted into the presumptive ventral region of another embryo, the transplanted cells will change their fates and differentiate into ventral cell types.

The Concept of Conditional Specification was Demonstrated by Attempts to Disprove It

• August Weismann proposed the first testable model of cell specification, the germ plasm theory, where each cell of the embryo would develop autonomously.
• However, Wilhelm Roux and Hans Dreisch obtained opposite results in their experiments with frog and sea urchin embryos. Dreisch’s isolation experiments showed that cell fates are altered based on the conditions and interactions with other cells, rather than specified by some cytoplasmic factor.

Cell Position Matters: Conditional Specification in the Sea Urchin Embryo

• Hans Dreisch’s isolation experiments with sea urchin embryos showed that each blastomere developed into a complete larva and that cell fates were altered based on the conditions and interactions with other cells.
• Driesch confirmed conditional development in sea urchin embryos through an intricate recombination experiment, which showed that changing the partitioning of nuclei during cleavages does not result in deformed development.