Endocrine system cell differentiation

Fig. 4. Schematic representation of the vitamin D-inicro-endocrine system (stem cell differentiation) showing the localization of l,25(OH)2D3 receptors as well as cells which have been shown to synthesize l,25(OH)2D.,.

Steroid hormones can increase and decrease the level and/or activity of a large number of proteins in eukaryotes. Steroid hormones were first discovered in humans, where they play essential roles in development, differentiation, homeostasis, and endocrine therapies. However, current interest in steroid hormones is increasing because they constitute excellent model systems for examining the control of gene expression. Many human pathologies result from the inappropriate expression of protein(s). Thus, to treat disease states, it is critical to understand the normal processes governing how, when, and how much of the information encoded in the DNA of cells is transcribed to mRNAs and eventually into proteins, which perform most of the functions of cells. Steroid hormones provide excellent model systems with which to address these clinically relevant questions. 

In general, the quantities of TNFP synthesized are much less than those of TNFa. Consequently, TNpp is not easily detected in the circulation and is usually considered a locally acting paracrine factor and not a mediator of systemic injury. Like TNFa, it has many biological functions, from stimulation of fibroblasts and killing of tumor cells to modulation of immunoinflammatory responses. However, TNFa is a potent paracrine and endocrine mediator of immunoinflammatory reactions. It is also known to regulate growth and differentiation of a wide variety of types of cells and to be cytotoxic (especially in combination with IFNy) for many transformed cells. In vivo it promotes necrosis of murine sarcomas. 

Nor are these endogenous microorganisms necessarily benevolent, for they may also initiate disease, simply by persistence (Drexler, 2002, p. 214ff). Whereas replicating viruses ordinarily destroy cells, those that produce persistent infections may merely replicate continuously in what are called differentiated or specialized cells, such as endocrine cells that make hormones, such as insulin. The cell is not destroyed per se, but its specialized function is. Possibilities include MS, juvenile diabetes, and assorted psychiatric illnesses as caused by viral infections and sustained by the immune system s response to stiU other infections. 

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