Co-localization pattern of growth hormone (GH) and prolactin (PRL) within the anterior pituitary cells in the female rat and female musk shrew

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Abstract
By a double immunocytochemical labeling procedure, using the protein A‐gold method combined with electron microscopy, the co‐localization pattern of growth hormone (GH) and prolactin (PRL) was detected in the anterior pituitary cells of female rats and female musk shrews. Two types of co‐localization of GH and PRL were demonstrated. First, GH‐ and PRL‐containing secretory granules were intermixed within closely aggregated and interdigitated cell clusters that were composed of GH and PRL cells. This phenomenon was characteristically seen in pregnant rats and pregnant musk shrews. Therefore, the occurrence of an inter‐mixture of GH and PRL granules might be related to an enhanced cellular function for PRL synthesis. In another pattern of co‐localization of GH and PRL, both hormones were co‐packaged in the same secretory granules within a single cell. Such cells were scarce, small, irregularly shaped, and observed only in pregnant rats. These mixed GH‐PRL cells contained not only mixed GH‐PRL granules but also granules containing only GH or PRL. This suggests that these bihormonal cells are able to synthesize, synchronously or asynchronously, GH and PRL. Furthermore, granule extrusion from the mixed cells was clearly shown in this study. It seems likely that the mixed GH‐PRL cells reveal active cellular function in the pituitary gland of the pregnant rat.

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