Comparative quantitative ultrastructural studies of the choroidal epithelium of hydrocephalic (hpy/hpy) and normal mice, and the effect of stress induced by water deprivation.

Animals homozygous for the recessive, pleiotropic, mutation hpy(hydrocephalic-polydactyl) develop hydrocephalus early in the postnatal period. The condition develops in the apparent absence of any overt indications of obstruction in the cerebrospinal fluid (CSF) drainage system suggesting a continued, inappropriate, secretion of CSF. Electron microscope investigations were undertaken to both characterize and quantify the cell types present in the choroidal epithelium of mutants and their wild-type littermates and to gauge their response to a prolonged (24 h) deprivation of water, which has severe adverse effects on the general body fluid balance. Collectively, the findings indicate that the cellular makeup of the choroidal epithelium of normal animals and the manner of its response to changing fluid conditions is more complex than formerly anticipated. Also that inferences derived from a simple extrapolation of findings from other fluid-transporting epithelia to choroidal cells are misleading and erroneous. In wild-type animals allowed free access to water light cells with clavate microvilli (secretory cells) predominated whereas, following water deprivation there was a preponderance of dark cells with filiform microvilli, abundant mitochondria, multivesicular bodies and osmiophilic droplets (resorptive cells). In hydrocephalic mutants, the makeup of the choroidal epithelium of non-water-deprived animals resembled that of water-deprived wild-type mice and showed little change following water deprivation. These findings suggest that while the choroidal cells of mutants are capable of mounting a response to conditions having adverse effects on water balance (i.e., hydrocephalus) their response falls short of the level needed to fully redress the imbalance and is not materially increased by imposition of further, stringent, conditions (e.g., water deprivation). Thus, the findings lend support to the view that the mutational event affects the regulation of solute transport rather than effecting abrogation of the membrane pump itself.