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Bacterial endospores are complex structures residing inside endospore-forming, mainly gram-positive bacteria. The process of sporulation is considered a simple example of cell differentiation. Endospores enable the organism to resist environmental stresses. Sporulation can be divided into several stages, from axial DNA filamentation to mother cell lysis. The structure and formation of an endospore is an attractive model for the assembly of complex macromolecular structures during development. The expression of genes involved in spomlation is compartmentalized and different sets of genes are expressed in the prespore and mother cell, this being associated with the subsequent activation of four sporulation-specific o factors. Their synthesis and activity are tightly regulated and the regulatory mechanisms have overlapping roles.
This review briefly summarizes recent findings on lactotrophs in the pituitary gland and extrapituitary tissues as a no homologous group cell types of different embryonic origin, morphology and biological function. They display a remarkable adaptation to altered physiological condition. Their functions are derived from structural polymorphism, local synthesis, divergent intracellular signaling pathways and target genes. Most of them are heterogenous with respect to basal hormone release, electrical activity and responsiveness to stimulatory/inhibitory factors, depending upon gender and physiological state of animal. The circulating prolactin (PRL) produced by many types of lactotrophs can act in edocrine/paracrine/autocrine manner, respectively as a hormone, growth factor, neurotransmitter or immunoregulator. At the cellular level PRL exerts mitogenic, morphogenic and secretory activity. Numerous factors of the central and peripheral origin are involved in the mechanism regulating PRL secretion, causing an increase or decrease of the hormone concentration in the circulation. The certain feedback mechanism keep the pituitary lactotrophs to be not overactive
Steroid hormones in plants and in animals are very important for physiological and developmental regulation. In animals steroid hormones are recognized by nuclear re­ceptors, which transcriptionally regulate specific target genes following binding of the ligand. In addition, numerous rapid effects generated by steroids appear to be me­diated by a mechanism not depending on the activation of nuclear receptors. Although the existence of separate membrane receptors was postulated many years ago and hundreds of reports supporting this hypothesis have been published, no animal mem­brane steroid receptor has been cloned to date. Meanwhile, a plant steroid receptor from Arabidopsis thaliana has been identified and cloned. It is a transmembrane pro­tein which specifically recognizes plant steroids (brassinosteroids) at the cell surface and has a serine/threonine protein kinase activity. It seems that plants have no intracellular steroid receptors, since there are no genes homologous to the family of animal nuclear steroid receptors in the genome of A. thaliana. Since the reason of the rapid responses to steroid hormones in animal cells still re­mains obscure we show in this article two possible explanations of this phenomenon. Using 1,25-dihydroxyvitamin D3 as an example of animal steroid hormone, we review results of our and of other groups concordant with the hypothesis of membrane steroid receptors. We also review the results of experiments performed with ovarian hormones, that led their authors to the hypothesis explaining rapid steroid actions without distinct membrane steroid receptors. Finally, examples of polypeptide growth factor that similarly to steroids exhibit a dual mode of action, activating not only cell surface receptors, but also intracellular targets, are discussed.
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