Hormonal control of Ca homeostasis in lower vertebrates: Considering the evolution

Abstract

Early vertebrates may have had genes that would later produce hormones concerning Ca homeostasis in advanced vertebrates. These genes may possibly have expressed the respective product for the local control of Ca in some tissues, which must be a paracrine control. When vertebrates acquired the jaw, however, delicate but systemic Ca control system may have been needed to cope with fluctuations in blood Ca levels resulting from the digestion of food. Furthermore, when vertebrates were transferred from freshwater to seawater or to land, new control systems must have been required. Therefore, such vertebrates might have coped with two ways in their evolution. One is the increase of the number of local production sites of hormones, for the delicate control of Ca in various tissues, which must be the paracrine control. The other is the creation of new glands for the systemic control of Ca homeostasis by making paracrine cells independent from tissues. This style must be an endocrine control. Therefore, in modern vertebrates, in various tissues, calcitonin, stanniocalcin, parathyroid hormone-related protein, and maybe also prolactin, or at least substances immunoreactive to antibodies against these hormones, are expressed locally as paracrine tissues. At the same time, these hormones or similar molecules are also produced in special endocrine glands such as the ultimobranchial glands, Stannius corpuscles, the parathyroid glands and the pituitary gland, respectively. This supposition is summarized in Fig. 1.