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 (Harrison 1933; Slack 1991). 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
Three basic modes of commitment have been described in Figure. 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 (Figure 3.7). 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. 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 RNAs) are placed in different regions of the egg cytoplasm and are apportioned to the different cells as the embryo divides.
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Autonomous specification Characteristic of most invertebrates. Specification by differential acquisition of certain cytoplasmic molecules present in the egg. Invariant cleavages produce the same lineages in each embryo of the species. Blasto- mere fates are generally invariant. Cell type specification precedes any large-scale embryonic cell migration. Produces "mosaic" ("determinative") development: cells cannot change fate if a blasto mere is lost.
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Conditional specification Characteristic of all vertebrates and few invertebrates. Specification by interactions between cells. Relative positions are important. Variable cleavages produce no invariant fate assignments to cells. Massive cell rearrangements and migrations precede or accompany specification. Capacity for "regulative" development: allows cells to acquire different functions.
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Syncytial specification
Characteristic of most insect classes.
Specification of body regions by interactions between cytoplasmic regions prior to cellularization of the blastoderm. Variable cleavage produces no rigid cell fates for particular nuclei. After cellularization, conditional specification is most often seen.
Conditional specification
The phenomenon of conditional specification
A second mode of commitment involves interactions with neighbouring 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, 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 (Figure 3.11). 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). 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
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