STRUCTURE AND BLOOD-SUPPLY OF INTRINSIC LYMPH-NODES IN THE WALL OF THE RABBIT URINARY-BLADDER - STUDIES WITH LIGHT-MICROSCOPY, ELECTRON-MICROSCOPY, AND VASCULAR CORROSION CASTING

The urinary bladder is especially subject to infection by virtue of it s direct connection to the external urethral opening, and it is natura l to anticipate the presence of a well-developed immunological mechani sm to respond to this potential threat. The present study describes sm all, very highly vascular lymph nodes located in the wall of the rabbi t bladder, which may be involved in a local response to foreign antige ns. The vasculature and structure of these lymph nodes was described u sing a combination of vascular corrosion casting, ink injection, and l ight and electron microscopy. The distal abdominal aorta was cannulate d, and after clearing the bladder vasculature with buffered saline, on e of the following procedures was used: 1) the bladder was perfuse-fix ed in preparation for light and electron microscopy; 2) the bladder va sculature was filled with India ink for vessel tracing; or 3) vascular corrosion casts of the vasculature were prepared by infusing resin co mprised of a mixture of Mercer, methyl methacrylate monomer, and catal yst. The resulting casts were cleaned with KOH, formic acid, and water in preparation for scanning electron microscopy. Vascular casts and I ndia ink injections revealed the presence of a number of isolated capi llary tufts consisting of clusters of one to five ''glomeruli,'' close ly associated with the major vesicular vessels along the lateral walls of the bladder, and supplied by tertiary branches of these vessels. L ight and electron microscopy showed that the capillary tufts represent ed the blood supply to small, ovoid lymph nodes located near the seros al surface of the bladder wall and usually restricted to the basal hal f of the bladder. These nodes were encapsulated and exhibited subcapsu lar sinuses, numerous small blood vessels, a limited number of high en dothelial cells, and, occasionally, nerves and a follicular substructu re. The nodes contained abundant lymphocytes, stellate stromal cells, macrophages, and eosinophils, but lacked the obvious cortical and medu llary organization and germinal centers often seen in larger lymph nod es. Vascular corrosion casts, vascular ink injections, and microscopic examination confirmed the presence of small, highly vascular lymph no des closely associated with the main vesicular vessels along the later al walls of the rabbit bladder. A follicular substructure of the nodes appears to correspond with the ''glomerular'' capillary arrangement w ithin the nodes as seen with corrosion casts. The rich blood supply ma y be indicative of the high metabolic demand of lymphatic tissue, and may be altered in response to the level of activity of the node. The c lose association between the lymphatic tissue and the rich blood suppl y to the nodes may allow a rapid mobilization of lymphocytes during a local immune response to foreign agents. Anat. Rec. 252:477-484, 1998. (C) 1998 Wiley-Liss, Inc.