Abstract
The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.
Highlights
Teleost fish comprise the largest vertebrate group with close to 30,000 species [1], including well established model species such as zebrafish (Danio rerio) and Japanese medaka (Oryzias latipes), which provide valuable tools for basic research on vertebrate physiology [2, 3]
It is well established that sex steroids participate in the regulation of gonadotrope plasticity
The effects mainly occur at the cellular level by modulating the sensitivity of the endocrine cells to ligands through the regulation of the number of receptors, or by regulating endocrine cell activity
Summary
Teleost fish comprise the largest vertebrate group with close to 30,000 species [1], including well established model species such as zebrafish (Danio rerio) and Japanese medaka (Oryzias latipes), which provide valuable tools for basic research on vertebrate physiology [2, 3]. Gonadotrope population changes in the pituitary can be due to proliferation of gonadotropes (Figure 2A), differentiation of progenitor cells, (Figure 2B) transdifferentiation (Figure 2C), and cell death (Figure 2D) These mechanisms underlie many changes in the rates of synthesis and release of Fsh and Lh by the pituitary, and there is increasing evidence that the sex steroids may play a role in these processes. Experimental evidence of sex steroids regulating endocrine pituitary cell apoptosis is lacking in teleosts, there is ample evidence in mammals of both androgens and estrogens modulating apoptosis in such cells, including gonadotropes [170, 171]. Whether apoptosis regulates the number of endocrine cells in teleosts remains to be elucidated
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