Membrana shrapnelli of maturing rats. The occurrence of mast cells, and histamine content in relation to otitis media with effusion.

The membrana shrapnelli or pars flaccida of 12 day old rats is thick (84 micron) and almost completely fills the lateral attic compartment. Histologically it is characterized by a thickened layer of stratified squamous epithelium and a lamina propria whereas an inner epithelial lining is difficult to discern. Mast cells are numerous and distributed throughout the lamina propria. At day 21 and day 30 the thickness of the pars flaccida is reduced and histologically it resembles that of the adult. Mast cells are mainly located in the vicinity of blood vessels and in a layer just beneath the squamous epithelium. The number of mast cells per unit area of pars flaccida is considerably higher in young rats as compared to adults. At day 12 their number is roughly 2.1 times that of the adult and at day 21 and 30 about 1.5 times. Except for a slightly raised level of histamine at day 30 the concentration of histamine in the pars flaccida of young rats did not differ from values obtained in the adult. Previous experimental studies have indicated that mast cells in the pars flaccida may play an important role in the pathogenesis of OME. It is interesting to note that the pars flaccida of young rats contains more mast cells than that of the adult. Should this also be the case in the pars flaccida of young humans it is tempting to suggest that mast cells may play a role in the pathogenesis of OME in children too.