Development and functional anatomy of the human tonsilla palatina.

The tonsilla palatina, belonging to the gut-associated lymphoepithelial organs (GALT), shows close morphological and functional correlation between the lymphatic tissue of mesenchymal origin and the endodermal epithelium of the second pharyngeal pouch. During the 4th fetal month epithelial crypts grow down into the connective tissue and are infiltrated by non-epithelial cells. In the underlying lymphoid tissue the first primary follicles with precursors of follicular dendritic reticulum cells can be localized already in the 14th fetal week post conceptionem (p.c.). The parafollicular areas develop into T-cell regions. Interdigitating cells in these regions as well as within the crypt epithelium express the HLA-DR antigen. They were frequently found in close contact with T-helper cells. This fact is discussed as an indication of the tonsillar crypt epithelium partly serving as T-cell region. But also B-cells and macrophages invade the crypt epithelium in great numbers with a distinctly different distribution. Natural killer (NK) cells (Leu-7-positive) are localized within the crypt epithelium. Small Leu-7-positive lymphocytes (probably precursors) have been found to be numerous in the germinal centers. The interaction of specific stationary cells in the T-cell and B-cell regions is obviously an important factor for the maturation of different subpopulations of immunocompetent lymphoid cells.