Morphometry of normal human lymphoid tissues. Nuclear invaginations and clefts.

We performed morphometric analysis on samples of human lymphoid tissue involved by reactive hyperplasia to assess the basis for the concept of cleaved lymphocyte nuclei in current non-Hodgkin's lymphoma classification. Histologic sections from all regions of lymph node showed considerable variation (usually 9% to 16%) in the proportion of nuclear profiles, with invaginations of at least 0.4 micron. Invaginations sufficiently deep to label nuclear profiles as "cleaved" occurred in 2% or less of lymphocytes of all regions seen on 1-micron plastic-embedded sections. The majority of nuclear invaginations were less than 1.6 micron deep. Analysis of the morphometric data suggests that the nuclei of germinal center lymphocytes undergoing antigen-induced transformation do not progress through a specific phase in which they become increasingly cleft. With the increased angularity and irregularity of lymphocyte nuclear profiles in germinal centers, the "cleaved" appearance of these nuclei in paraffin-embedded sections seems to be due to linear creases and overlapping segments of the nucleus.