The normal human appendix: a light and electron microscopic study.

Human appendices from 3 to 12 years old children were studied by light and electron microscopy. Three morphological zones were determined: sub-epithelial (or lymphoid lamina propria), parafollicular, and follicular. The fine structure of these regions has been studied and discussed with regard to the thymus-dependent and thymus-independent regions of other lymphoid organs. Two types of lymphocytes, 'light' and 'dark', and intermediate forms, were also found. The light ones are the more abundant in the epithelium and within the parafollicular post-capillary venules; they form groups or clusters between epithelial cells, becoming like blast cells and possibly maturing into plasma cells in the sub-epithelial region. Whether light lymphocytes are T or B or both is discussed. The general conclusion is that the human appendix, at least in children, has the characteristics of a well-developed lymphoid organ, suggesting that it has important immunological functions.