Thymic reticulo-epithelial cells: key cells of neuroendocrine regulation.

The reticulo-epithelial (RE) cellular network of the thymic stromal cellular microenvironment plays a vital role in neuroendocrine regulation and lymphoid cell homing and development. Transmission electronmicroscopic observations have confirmed that there are four functional subtypes of medullar RE cells: undifferentiated; squamous; villous; and cystic. Immunocytochemical observations have shown that the secreted thymic hormones, thymosin alpha1 and thymopoietin (and its short form, thymopentin or TP5), are both produced by RE cells. Thymic RE cells also produce numerous cytokines, including IL-1 and -6, G-CSF, macrophage-CSF and GM-CSF that likely are important during the various stages of thymocyte activation and differentiation. The coexistence of pituitary hormone and neuropeptide secretion, such as growth hormone, prolactin, adrenocorticotopic hormone and thyroid-stimulating hormone, among many others, and the production of a number of interleukins and growth factors, as well as the expression of receptors for all, by the same RE cell, is an unique molecular biological phenomenon. The thymic RE cell network represents an important cellular and humoral microenvironment in the neuroendocrine homeopathic regulatory mechanisms of the multicellular organism.