Immunohistochemical localization of different laminin isoforms in human normal and adenomatous anterior pituitary

Laminin (LM) is an integral component of basement membranes (BM), with important roles in various aspects of cell biology. Several different isoforms of LM have been described, and each comprises a molecule consisting of three subunit polypeptides, the A, B1, B2, M or S chain.

The distribution of different LM subunits was studied in human nontumoral anterior pituitaries obtained postmortem and in pituitary adenomas by immunocytochemical methods using specific monoclonal antibodies.

In normal tissue, the A, B1, and B2 chains had a ubiquitous localization in both parenchymatous and vascular BMs and in the pericapillary connective tissue space also. The S chain seemed to have principally a vascular localization, in contrast to the M chain that was mostly localized in the parenchymatous BM. The same antigens were investigated in 23 human pituitary adenomas of different secretory type, grade and invasiveness. In all cases, the five monoclonal antibodies gave a positive staining. The presence or the localization of LM chains did not display a specific pattern in the adenomas according to their secretory type, grade or extent of local invasion. In the adenomas, the anti-A, -B1, and -B2 monoclonal antibodies stained the stroma and in particular the vascular BMs as well as the sparse elements of parenchymatous BMs when they were present. The staining with anti-S antibody was localized in the stroma and around all the blood vessels. In contrast, the immunoreactive material to anti-M antibody was associated to the sparse fragments of parenchymatous BMs and it delineated the boundary of adenoma cells and stroma. Within the stroma, the anti-M antibody immunoreactivity was regularly associated with the arterial walls but rarely with the venule walls.

The human normal and tumoral anterior pituitary express all five LM subunits and thus contain several LM isoforms with different patterns of localization. The most striking difference between the human normal and tumoral anterior pituitary concerns the peculiar expression of LM isoforms by adenomatous neovessels.