The regulation and integration of thyroid follicular differentiation and function.

As in a number of other endocrine tissues in which expression of differentiated function is dependent upon a stable cellular architecture, the control of differentiation of the thyroid follicle requires a coordination between the ultrastructural and morphological responses to a number of endocrine growth factors, which may ultimately involve autocrine or paracrine-mediated effects within the immediate follicular microenvironment. Through analogy with other cell types, expression of appropriate differentiation characteristics within the thyroid follicle may involve the activation of specific c-oncogenes within each of the component cells. With the recent development and application of oncogene transfection technology, application of such procedures to the thyroid follicular cell should prove to be a particularly fruitful area for future research within the thyroid gland, leading to elucidation of the mechanisms whereby the cellular responses to growth and tissue-differentiating factors are mediated. Clearly however, further consideration must also be made of the roles played in maintaining follicular stability by both physical and chemical interactions between the component cells and the immediate extracellular environment. The contributory roles played by basement membrane and cell-surface components in cellular recognition, together with the physical effects imposed upon the apical surfaces of the follicle by luminal thyroglobulin have already been identified as fundamental factors in this respect. It is also readily apparent that morphological differentiation of the follicle bears critically upon the chemical characteristics of the microenvironment through the ability of the latter to promote expression of specific apical/basal recognition characteristics of the component cells, and thus maintain the unidirectional polarity upon which the functional capacity of the thyroid follicle is so critically-dependent.