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The Developmental Mechanics of Cell Specification

An embryo's environment may be a tide pool, a pond, or a uterus. As we saw above, the embryo interacts with its environment, and its developmental trajectory can be guided by information from its surroundings. On a smaller scale, the environment of an embryonic cell consists of the surrounding tissues within the embryo, and the fate of that cell (for instance, whether it becomes part of the skin or part of the lens) often depends upon its interactions with other components of its immediate “ecosystem.”

Thus, a second research program of experimental embryology studies how interactions between embryonic cells generate the embryo. The development of specialized cell types is called differentiation . These overt changes in cellular biochemistry and function are preceded by a process involving the commitment of the cell to a certain fate. At this point, even though the cell or tissue does not appear phenotypically different from its uncommitted state, its developmental fate has become restricted. The process of commitment can be divided into two stages .

The first stage is a labile phase called specification. The fate of a cell or a tissue is said to be specified when it is capable of differentiating autonomously when placed in a neutral environment such as a petri dish or test tube. (The environment is neutral with respect to the developmental pathway.) At this stage, the commitment is still capable of being reversed.

The second stage of commitment is determination. A cell or tissue is said to be determined when it is capable of differentiating autonomously even when placed into another region of the embryo. If it is able to differentiate according to its original fate even under these circumstances, it is assumed that the commitment is irreversible.*

Autonomous specification was first demonstrated in 1887 by a French medical student, Laurent Chabry. Chabry desired to know the causes of birth defects, and he reasoned that such malformations might be caused by the lack of certain cells. He decided to perform experiments on tunicate embryos, since they have relatively large cells and were abundant in a nearby bay. This was a fortunate choice, because tunicate embryos develop rapidly into larvae with relatively few cells and cell types . Chabry set out to produce specific malformations by isolating or lancing specific blastomeres of the cleaving tunicate embryo. He discovered that each blastomere was responsible for producing a particular set of larval tissues .

In the absence of particular blastomeres, the larva lacked just those structures normally formed by those cells. Moreover, he observed that when particular cells were isolated from the rest of the embryo, they formed their characteristic structure apart from the context of the other cells. Thus, each of the tunicate cells appeared to be developing autonomously.†

A second mode of commitment involves interactions with neighboring cells. In this type of specification, each cell originally has the ability to become many different cell types. However, the interactions of the cell with other cells restricts the fate of one or both of the participants.

This mode of commitment is sometimes called conditional specification, because the fate of a cell depends upon the conditions in which the cell finds itself. If a blastomere is removed from an early embryo that uses conditional specification.

Three basic modes of commitment have been described
The first is called autonomous specification. In this case, if a particular blastomere is removed from an embryo early in its development, that isolated blastomere will produce the same cells that it would have made if it were still part of the embryo .

Thus, conditional specification gives rise to a pattern of embryogenesis called regulative development.‡ Regulative development is seen in most vertebrate embryos, and it is obviously critical in the development of identical twins. In the formation of such twins, the cleavage-stage cells of a single embryo divide into two groups, and each group of cells produces a fully developed individual .

Invertebrate embryos, especially those of molluscs, annelids, and tunicates, often use autonomous specification to determine the fate of their cells. In these embryos, morphogenetic determinants (certain proteins or messenger are placed in different regions of the egg cytoplasm and are apportioned to the different cells as the embryo divides. These morphogenetic determinants specify the cell type.

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Moreover, the embryo from which that cell is taken will lack those cells (and only those cells) that would have been produced by the missing blastomere. Autonomous specification gives rise to a pattern of development referred to as mosaic development, since the embryo appears to be constructed like a tile mosaic of independent self-differentiating parts.

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The remaining embryonic cells alter their fates so that the roles of the missing cells can be taken over. This ability of the embryonic cells to change their fates to compensate for the missing parts is called regulation . The isolated blastomere can also give rise to a wide variety of cell types (and sometimes generates cell types that the cell would normally not have made if it were part of the embryo).

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