Developmental pattern of cell type-specific calretinin immunoreactivity in the postnatal gerbil pineal gland

The postnatal development of immunoreactivity for the neuronal calcium-binding protein calretinin in the pineal gland of the Mongolian gerbil was investigated using immunostaining of serial semithin sections. Calretinin-positive pineal cells could readily be visualized from the day of birth (P0) onwards and coexpressed the intermediate filament (IF) protein vimentin. During the first half of the first postnatal week, many of the calretinin-/vimentin-positive cells were also immunopositive for synaptophysin and neuron-specific enolase (NSE) and thus corresponded to pinealocytes. However, the expression of calretinin in pinealocytes was only transitory and declined towards the end of the first postnatal week. Thereafter, calretinin immunoreactivity became restricted to vimentin-positive interstitial glial cells. Therefore, in the gerbil pineal gland, calretinin obviously is not required in mature pinealocytes but instead serves as yet unknown functions in interstitial cells. The unusual calretinin expression pattern adds to the notion that pineal interstitial cells differ from glial cells of other brain regions. This conclusion is also underlined by our present detection of the neuronal marker protein PGP 9.5 in interstitial cells during postnatal development.