THE SUBENDOTHELIAL SPACE IN CERTAIN ENDOCRINE TISSUES

A subendothelial space between the parenchymal cell plasma membrane and the plasma membrane of the endothelial lining of the regional blood sinusoids has been reported in both adrenal cortex and medulla and in the corpus luteum (1, 2, 3). To date, such a space or interval has been seen to best advantage in the parathyroid (Fig. 1). As described in other endocrine tissues (vide supra) the subendothelial interval communicates with intercellular spaces between adjacent parenchymal cells. With regard only to the membranous boundaries of both subendothelial and intercellular intervals (and not to the outlines of any contained material), it can be said that these freely communicate in the rat parathyroid (Fig. 1). For the most part in this tissue adjacent endothelial and parenchymal plasma membranes follow gentle curves and the interval between them is correspondingly regular (Fig. 1). In contrast the interparenchymal cell spaces are tortuous and vary greatly in width along their extent. I t follows that in some of their course adjacent parenchymal plasma membranes are convoluted and interdigitafing (Fig. 2). The use of the term "subendothelial space" probably dates back to Disse's (4) description of the interval between the Kupffer-endothelial lining of the sinusoids and the parenchymal cells of the liver. In the same connection it was more recently used by Fawcett (5). Collagen has been noted in small amount in the subendothelial space of the anterior pituitary (6); the thyroid (7); the adrenal medulla (2); the adrenal cortex (1), particularly in the zona reticularis and in increased amount following hypophysectomy (8). The fact that endocrine secretory material must cross the subendothelial space in order to gain the blood stream might have a bearing on the interpretation of relevant electron micrographs. In Fig. 1 there is a grey amorphous material applied to, but distinct from, the plasma membrane of both parenchymal and endothelial cells of the rat parathyroid. The same appearance, described by Farquhar and Rinehart in the anterior pituitary (6), is also found in the thyroid (Fig. 4), in which gland Dempsey and Peterson (7) interpreted the amorphous lining material as basement membrane. Again it is observed in both adrenal cortex and medulla, though in the former it is presented as a single concentra293