The three-dimensional organization of the collagen fibrillar framework in the rat adrenal gland was studied using an alkali-water maceration method and scanning electron microscopy. The structure thus obtained was a continuum of collagen fibril plexuses extending through the adrenal capsule, cortex and medulla. The capsule consisted of finely meshed collagen fibril sheaths and layered, coarsely meshed plexuses of bundles of collagen fibrils. In the cortex, the channels of cortical capillaries were surrounded by thin sheaths of collagen fibrils. Many slender bundles of collagen fibrils extended from these sheaths into intercellular spaces and interconnected adjacent pericapillary sheaths. The collagen fibril sheath tubes precisely reflected the cortical blood vascular architecture; thus the three cortical zones-zona glomerulosa, zona fasciculata and zona reticularis-were also clearly remarked. In the medulla, collagen fibrils were interwoven into more tightly meshed sheaths around the peripheral radicles and venous tributaries of the central veins. Basket-like collagen fibril sheaths divided the spaces among these tree-like perivascular collagen fibril sheath tubes into round compartments which, in intact tissues, contained chromaffin cell nodules and nervous elements. Small tube-like spaces for housing the medullary capillaries were located in the interstices of contiguous collagen fibril baskets. Besides supporting the organization of the gland, the collagen fibrillar framework in the adrenal gland is believed to play important roles in providing three-dimensionally elaborated extravascular spaces for the diffusion of metabolites and physiological messengers, including hormones.
