Specification and Determination

During the differentiation process, cells gradually become committed towards developing into a given cell type. Here, the state of commitment may be described as “specification” representing a reversible type of commitment or “determination” representing irreversible commitment.

Although the two represent differential gene activity, the properties of cells in this stage is not completely similar to that of fully differentiated cells. For instance, in the specification state, cells are not stable over a long period of time.

There are two mechanisms that bring about altered commitments in the different regions of the early embryo.

These include:

• Cytoplasmic localization

• Induction

Cytoplasmic Localization – This occurs during the earliest stage of embryo development. Here, the embryo divides without growth and undergoes cleavage divisions that produce blastomeres (separate cells). Each of these cells inherit a given region of the cytoplasm of the original cell that may contain cytoplasmic determinants (reuratory substances).

Once the embryo becomes a morula (solid mass of blastomeres) it is composed of two or more differently committed cell populations. The cytoplasmic determinants may contain mRNA or protein a given state of activation that influence specific development.

Induction – In induction, a substance secreted by one group of cells causes changes in the development of another group. During early development, induction tends to be instructive in that tissue assumes a given state of commitment in the presence of the signal.

In induction, inductive signals also evoke various responses at varying concentrations which results in the formation of a sequence of groups of cells, each being in a different state of specification.

During the final phase of cell differentiation, there is formation of several types of differentiated cells from one population of stem cells of the precursor. Here, terminal differentiation occurs both in embryonic development as well as in tissues during postnatal life.

Control of the process largely depends on a system of lateral inhibition. That is, cells differentiating along a given pathway send out signals which repress similar differentiation by the neighboring cells. A good example of this is with the developing CNS of vertebrates (central nervous system).

In this system, neurons cells from the tube of neuropithelium possess a surface receptor known as Notch and a cell surface molecule known as Delta that can bind to the Notch of adjacent cells and activate them.

This activation results in a cascade of intracellular events that ultimately result in the suppression of Delta production as well as the suppression of neuronal differentiation. As a result, the neuropithelium ends up only generating a few cells with high expression of Delta surrounded by a larger number of cells with low expression of Delta.