Organisation and ultrastructure of the regenerative crypts in the midgut of the adult worker honeybee (L. Apis mellifera)

Abstract

The midgut epithelium of the adult honeybee consists of columnar and endocrine cells, both originating from regenerative crypt cells. The regenerative crypt is composed of stem cells and differentiating cells. The stem cells generate two forms of endocrine cells along with differentiating enterocytes of two distinct stages. At first they can be seen as light crypt cells which are not secretory active; they then develop into more electron dense, active secretory crypt cells. The developing enterocytes are arranged as tetrads, each composed of cells with the same degree of differentiation. This can be explained by the occurrence of cytoplasmic bridges between the cells of each tetrad. These fusomes are probably responsible for the apparent intercellular coordination. This is a new example of intercellular bridges between somatic cells and, as far as we know the first description of fusomes in the insect midgut epithelium.The microvilli on top of the crypt cells develop within a spherical extracellular space where glycosaminoglycans are secreted. This occurs by the coordinated activity of the four surrounding electron dense crypt cells. Microvilli formation therefore seems to be the first in a series of successive functions of honeybee enterocytes during their ontogeny.