Microvasculature of Normal and Involuted Mouse Thymus

Abstract

The spatial arrangement of blood vessels in the thymus of normal and hydrocortisone-injected mice was studied by light and electron microscopy. The thymus is supplied by one thymic artery which branches into arterioles as it enters the parenchyma.These, in turn, feed capillary networks in the cortex and in the medulla. Cortical networks at the periphery of the lobule form loops which return blood to postcapillary venules at the corticomedullary junction and in the medulla. There is no subcapsular venous drainage. The overall distribution of blood vessels in the involuted thymus is essentially the same as in the normal thymus but the pattern becomes irregular and the vessels are tortuous. The endothelium of the postcapillary venules is flat and surrounded by a wide perivascular space containing many lymphocytes. This space is delimited by basal laminae, on the one side by that of the abluminal surface of the venular endothelium, on the other side by that of a thin, sheet-like epithelial layer formed by cytoplasmic processes of reticular cells. The perivascular space is in continuity with the surrounding interstitial space via gaps in its epithelial sheet. It does not form continuous longitudinal channels along the venules, but is interrupted by epithelial trabeculae.There is no obvious difference in structure between the postcapillary venules and their perivascular spaces of normal and those of involuted thymuses. Lymphocytes are intercalated in the venular endothelium, particularly in involuted glands. They are present in the perivascular space and in gaps of the outer epithelial sheet.These findings suggest that the postcapillary venules and the perivascular spaces may function as pathways for the migration of thymic lymphocytes into or out of the blood circulation.

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“…The current study showed that in the normal human thymus, an L-selectin ligand, PNAd, is localized in some of the vessels surrounded by PVS, which, based on earlier morphologic studies, seems to be postcapillary venules (9,10). The current study showed that in the normal human thymus, an L-selectin ligand, PNAd, is localized in some of the vessels surrounded by PVS, which, based on earlier morphologic studies, seems to be postcapillary venules (9,10).…”

“…The spaces around vessels where loosely arranged lymphocytes were lined by polygonal epithelial cells in hematoxylin and eosin-stained sections or by cytokeratin-positive cells in immunostained sections were thought to be PVSs (9).…”

“…Postcapillary venules in normal mouse thymus are localized at the corticomedullary region and in the adjacent medulla, where they are surrounded by a perivascular space (PVS) (9). The postcapillary venule-PVS regions may serve as functional units for the selective migration of lymphocytes into the thymus from the circulation.…”

Analysis of the human thymic perivascular space during aging

“…Recent studies have focused attention on a third anatomic region of the thymus, known as the perivascular space (PVS) (1, 2) for its location adjacent to the blood vessels. The PVS is located within the thymic capsule, but is separated from the TES by a basement membrane and does not contain developing thymocytes (3,4,5). Early in life, the thymus is composed primarily of TES that supports thymopoiesis and generation of the peripheral T-cell repertoire.…”

Thymic microvascular system

“…While these reports provide general information on the fine structure and distribution of PCVs and PVSs, they do not address the structural organization. The three-dimensional relationship of the blood vessels, the reticular framework, and the cells forming the thymic parenchyma have been examined in the rat (Hwang et al, 1974;Ushiki, 1986), guinea pig (Kato and Schoefl, 1987), and mouse (Kato and Schoefl, 1989) by combined scanning and transmission electron microscopy (SEM, TEM).…”