A scanning electron microscopic study on the lymphatic microcirculation of the rabbit mesenteric lymph node. A corrosion cast study

Abstract

A cast of the rabbit mesenteric lymph node was made by injection of Mercox resin through the afferent lymphatic vessel and was observed with a scanning electron microscope. In order to confirm the exact nature of the spaces which were filled with Mercox and formed the casts, additional comparative light-microscopic, transmission-electron-microscopic and scanning-electron-microscopic observations were made on specimens of the washed-out lymph node by perfusion with Ringer’s solution, or of the lymph node infused with gelatin or Mercox, and the precise lymphatic microcirculation of the lymph node was clarified. The marginal sinus appeared to be a dome-like space beneath the capsule of the lymph node. From the marginal sinus, the lymph flowed into the trabecular sinus, then into the tubular sinus with about 50 µm in diameter in the deep cortex, and finally into the medullary sinus. This tubular sinus has never been described in the literature and was tentatively named the tubular cortical sinus. In addition, the lymph entered the diffuse lymphoid tissue from the marginal sinus or from the trabecular sinus, run through the channel-like space through the meshwork of the reticulum cells or fibers of the diffuse lymphoid tissue, and entered the medullary sinus. This channel-like space was named the diffuse lymphoid tissue channel. About one half of the area in the outer cortex, except for the cortical nodule, was occupied by the diffuse lymphoid tissue; the other half was a different type lymphoid tissue, in which reticulum cells and lymphoid cells were closely packed and lymphatic spaces poorly developed. This tissue was tentatively named the ‘dense lymphoid tissue’ or ‘densely packed lymphoid tissue’. The cortical nodule was rather cave-like structure, its ceiling consisting of the marginal sinus and its wall of the trabecular sinus. The germinal center usually had saccular lymphatic space communicating with the trabecular sinus by a few narrow routes. Two different types of medullary sinuses were distinguished based on their differences in structure and size. The one situated in the distal part of the medulla had a tubular structure completely surrounded by medullary cords. Reticulum cell network could not be seen in them. This type of medullary sinus was named the ‘distal medullary sinus’. The distal medullary sinus received all lymphatic pathways of the cortex. Adjacent to the hilum, the distal medullary sinus gradually transformed into a second type, the ‘proximal medullary sinus’, which was small in size and had rather irregular spaces, partly surrounded by medullary cords. The reticulum cell network was well developed in its space. This type of sinus finally joined the efferent lymphatic vessel at the hilum.