Subcellular distribution of laminin and prolactin in stimulated and blocked prolactin cells in the pituitary of lactating rats

Abstract

Laminin (LAM), a glycoprotein component of basement membranes, has been previously detected within several subcellular compartments of prolactin (PRL) cells in the pituitary gland. The present work was aimed at comparing the subcellular localization of PRL, a specific secretory product, with that of LAM, in relation to the secretory activity of PRL cells. LAM and PRL were located in parallel, by ultrastructural immunocytochemistry, in PRL cells of lactating female Wistar rats, either stimulated by suckling, or blocked by weaning, or reactivated by suckle following short-term weaning. Variations in physiological conditions were correlated with a redistribution of PRL immunoreactivity within morphologically modified compartments. The Golgi apparatus became hypertrophied, and PRL impressively accumulated within saccules of the Golgi stacks of blocked cells. On the contrary, no apparent changes occurred in LAM distribution, at least at the Golgi level. Only a slight increase of LAM immunoreactivity was observed in rough endoplasmic reticulum after a long weaning period. PRL could be detected in most of the secretory granules and particularly in forming elements, whereas LAM was observable at the peripheral edge of some mature granules. Such a labeling was not markedly influenced by the physiological state. The prominent structures, indicative of crinophagic activity, characteristic of blocked cells, contained masses of dense material, which were always immunopositive with antibodies to PRL, but never to LAM. These observations could suggest that, in PRL cells, intracellular transport and exportation of LAM are controlled by mechanisms independent from those involved in the regulation of PRL secretion.

Extracellular Matrix Proteins in the Anterior Pituitary Gland

“…Additionally laminin was demonstrated to be in the Golgi and in some mature granules in PRL-expressing cells in anterior pituitaries of lactating female rats [48]. Distinct differences in distribution under a variety of physiological circumstances, including suckling and weaning, suggested that laminin and PRL are controlled by independent pathways [48]. Receptors are present, providing support for the concept that pituitary cell-to-ECM interactions occur through transmembrane receptors, such as those of the integrin family [38].…”

“…Laminin was observed to be expressed in basement membranes and in endocrine cells and nonendocrine folliculo-stellate cells of the rat pituitary [38,40,47] in vivo and in primary cultures. The presence of laminin in the adenohypophysis of lactating female rats was also demonstrated [48]. Studies of mice detailed the distribution of laminin around vessels, in the parenchyma and cytoplasm of most of the cells [46].…”

“…Laminin has been detected intracellularly in lactotrophs [40,47,48]. In the normal pituitary, cells expressing PRL are sited close to laminin-containing cells [46].…”

“…In the normal pituitary, cells expressing PRL are sited close to laminin-containing cells [46]. Additionally laminin was demonstrated to be in the Golgi and in some mature granules in PRL-expressing cells in anterior pituitaries of lactating female rats [48]. Distinct differences in distribution under a variety of physiological circumstances, including suckling and weaning, suggested that laminin and PRL are controlled by independent pathways [48].…”

“…Laminin has been detected intracellularly in gonadotrophs of rats [40,47,48], and also in the adult mouse in which laminin immunoreactivity has been detected in gonadotrophs (but not other cell types). Laminin is also a constituent of the basal laminae in the adenohypophysis of the mouse pituitary gland [86].…”