Ontogeny of non-lymphoid and lymphoid cells in the rat gut with special reference to large mononuclear Ia-positive dendritic cells.

The ontogeny of rat Peyer's patches was studied with special reference to Ia-positive dendritic cells. The 16-day-old foetus contained large Ia-positive dendritic cells randomly distributed throughout the gut wall and mesentery. In cell suspensions prepared from the foetal gut, these cells showed the typical movement pattern of veiled cells, and they shared ultrastructural features with these and other antigen-presenting cells. The gut was found to be the first organ in which large numbers of these Ia-positive dendritic cells were found. At 16 days gestation, the Ia-positive dendritic cells were randomly distributed throughout the primitive gut wall, but at later stages accumulated in structures which were clearly recognizable as Peyer's patches from 20 days gestation onwards. T and B lymphocytes could be seen on the day of birth at the earliest, at first randomly distributed throughout the Peyer's patch, but afterwards concentrating in separate T- and B-cell regions. Two weeks after birth Peyer's patches consisted of densely populated B-cell nodules, mainly comprising surface IgM (sIgM) positive cells, and internodular T-cell regions. Ia-positive dendritic cells were situated between the epithelial cells of the dome area, just underneath this epithelium in a loose reticular area and in the internodular T-cell regions. These Ia-positive dendritic cells were acid phosphatase negative or only weakly positive. Some Ia-positive dendritic cells were also present in small intestinal villi at all ages studied. However, these cells started to express an increasing acid phosphatase activity in the 2 weeks following birth. Thus they resemble macrophages, while Peyer's patch dendritic cells show characteristics of antigen-presenting cells.