Salinity Regulation of Prolactin Cell Proliferation and Apoptosis in the Euryhaline Teleost, the Tilapia (Oreochromis mossambicus).
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Date
2008-12-09
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Abstract
The euryhaline tilapia, Oreochromis mossambicus, has the ability to live in both freshwater (FW) and saltwater (SW) environments. Prolactin (PRL) is the most critical hormone to promoting life in FW, and without it tilapia loses the capacity to osmoregulate in hypotonic environments. Consistent with PRL’s actions in FW adaptation, pituitary PRL synthesis, content, secretion, and cell activity are all elevated in FW compared to SW acclimated tilapia We found that the PRL region of a FW tilapia pituitary has a 3-fold larger volume than the PRL region of a SW pituitary. It is unclear whether this increased tissue volume is due to larger cells (hypertrophy) or more cells (hyperplasia). Therefore, we evaluated if PRL cell proliferation and apoptosis might be sensitive to salinity and could account for the greater abundance of PRL in FW versus SW fish. Freshwater tilapia were transferred to either SW or sham transferred to FW and SW fish were moved to either FW or SW over a time course of 7 days. Pituitaries were sampled over the course of salinity challenge and triple stained for determination of lactotroph cell density (nuclei staining with Hoescht dye), proliferation (BrdU labeling) and apoptosis (TUNEL assay). Lactotrophs were identified by immunostaining using tilapia specific PRL antisera. Lactotroph cell densities were 40% lower in FW than SW fish and declined when fish were transferred from SW to FW (P < 0.001). The larger volume of the PRL cell region within the pituitary area combined with lower number of lactotrophs per unit area suggests that the cells are larger in FW than SW acclimated fish. There were negligible levels of apoptosis in lactotrophs and salinity was ineffective in regulating programmed cell death. By contrast, we found a dramatic effect of salinity on lactotroph cell proliferation. The pituitaries of FW tilapia show a higher rate of PRL cell proliferation than those of SW fish. During transfer from SW to FW proliferation increased by almost 20-fold compared with controls (P < 0.001). When fish were transferred from FW to SW, proliferation declined within 1 day to levels observed in sham transferred SW fish. The enhanced proliferation combined with increased volume of the pituitary PRL cell region, decline in lactotroph cell density, and presumed increase in cell volume, suggests that the tilapia lactotroph undergoes hypertrophy and hyperplasia in FW environments. Apoptosis appears to play little role in regulating lactotroph density under different salinities. Overall, these results suggest that the elevated production and content of PRL critical to life in FW is mediated, in part, through enhanced lactotroph proliferation and hypertrophy.
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tilapia, teleost, proliferation, apoptosis
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Degree
MS
Discipline
Zoology
