Ultrastructural and immunocytochemical studies of prolactin-secreting cells in adenohypophysis of the mink (Mustela vison)

Biochemical and morphological diversity among folliculo-stellate cells of the mink (Mustela vison) anterior pituitary

The folliculo-stellate (FS) cells are agranular cells of the anterior pituitary whose origin and function are still a matter of debate. This study examined the presence, topography, and morphological characteristics of FS cells in the mink anterior pituitary throughout the annual reproductive cycle. The S-100 protein was used as a FS cell marker. Immunoperoxidase labeling on tissue sections demonstrated the presence of two types of S-100 positive cells. Type 1 cells were stellate-shaped cells whose nuclei were localized near the center of pituitary follicles. In this type, S-100 labeling was strong in anterior pituitary sections obtained during spring, a period characterized by high prolactin pituitary content and low gonadotropin pituitary content. Type 2 cells were rounded cells occupying the periphery of the follicles. During periods of low prolactin pituitary content and high gonadotropin anterior content the type 2 S-100 positive cells formed aggregates of several cells. The total number of S-100 positive cells was constant during these two periods of the annual reproductive cycle, suggesting that type 1 and type 2 may reflect different morphological and physiological states of the same cell. Of the two subunits, alpha and beta, that, combined, form three different dimeric S-100 proteins, mink FS cells expressed mostly the beta subunit. FS cells also expressed the glial fibrillary acidic protein (GFAP). In culture, 8 +/- 3% of anterior pituitary cells were S-100 positive. Cultured S-100 cells were elongated, polygonal, or rounded. The S-100 labeling accumulated in the cytoplasm around and within the nucleus, whereas it was weak in pseudopods and large cytoplasmic vacuoles. The presence of pseudopods suggests that cultured FS cells could migrate. The vacuoles may be related to the phagocytic activity ascribed to these cells. Some FS cells presented membrane blebbing and peripheral vesicles that were immunopositive for S-100 and that may indicate a secretory activity. Cultured FS cells possessed actin filaments organized as a peripheral network; a few actin cables were also observed running across the cytoplasm. Pseudopods depicted a highly organized actin network. The microtubules of FS cells expanded throughout the cytoplasm. The intermediate filaments expressed by cultured FS cells were GFAP and vimentin. GFAP labeling was punctate and vimentin was organized as filaments. All cultured S-100 cells were positive for vimentin, suggesting a mesenchymal origin for the cells, and all cultured S-100 positive cells were positive for GFAP, suggesting a neuroectodermal origin. In conclusion, S-100 positive cells are heterogeneous with respect to cell shape and expression of S-100 subunits in the mink anterior pituitary. The presence of morphologically different S-100 positive cells is modified in accordance with the endocrine status of the animal, suggesting that FS cells may be involved in the modulation of the anterior pituitary endocrine activity in the mink.

Changes in estrogen receptor expression and cell activity of lactotropes in female mink (Mustela vison) pituitary in response to variations in the gonadal steroid environment

This study was undertaken to get new information on the role played by estrogen (E) on the activity of mink lactotropes. Immunocytochemistry for estrogen receptor (ER) and prolactin (PRL) was applied to assess modifications in the protein production that occur as a result of in vivo changes in the gonadal steroid environment. Variations in the functional activity of lactotropes were demonstrated from the ultrastructural characteristics and morphometric parameters (cellular area, numerical density, and secretory granular size). The present study documents the presence of ER in mink lactotropes revealing the ability of E to regulate the expression of ER in the mink pituitary. Furthermore, all morphological and morphometric parameters of lactotropes activity appeared significantly increased in intact females, killed during the mating period, compared with castrated females under the same photoperiodic conditions. Castration thus blocks the stimulatory effect of photoperiod on metabolic activity of mink lactotropes suggesting that E may participate in the photoperiodic regulation of PRL.