Tridimensional study of the deep cortex of the rat lymph node. V: Postnatal development of the deep cortex units

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Abstract
Recently, the deep cortex of the adult rat lymph node was shown to be made up of semirounded lymphocytic “units.” Each unit is contiguous to the peripheral cortex, bulges into the medulla, and is centered on the opening(s) of an afferent lymphatic vessel of a node. Furthermore, each unit comprises a “center” and a “periphery,” bearing distinct morphological features. The present study investigated the postnatal development of the units in rats of various ages. One minute after birth, no lymphocytic structures were detected in the nodes. One day after birth, tiny rounded lymphocytic areas were detected in the developing cortex. These areas were topographically related to the openings of afferent lymphatic vessels. One week after birth, small semirounded lymphocytic areas with some morphological features of the adult deep cortex units were observed. Two weeks after birth, typical units were present in the nodes. The observations indicated that the rounded lymphocytic areas observed in nodes of rats aged 1 week or less were actually developing deep cortex units. The overall findings further provided information on the morphological processes involved during the postnatal development of the deep cortex units. Key words: lymph node, deep cortex, development of deep cortex, rat.