Characterization of the apical cell in the testicular follicle of the silkworm Bombyx mori L. from the embryonic to adult stage

Abstract

In lepidopteran species, the apical cell is located in the testicular follicle, which is similar to that of locusts and the hub in Drosophila. The origin and functions of the apical cell remain unknown in lepidopteran species. In this study, the behavior of the apical cell in Bombyx mori from the embryonic to adult stage was studied by immunofluorescence staining of microtubules (MTs). In the embryonic stage, apical cells in contact with the testicular membrane were distinguished from primordial germ cells by MT-rich cytoplasm. Four cell masses, including a single apical cell and primordial germ cells were distinguished before the testis was separated into four follicles. In the second instar, the apical cell grew larger, and the cytoplasm with fewer MTs appeared in the peripheral region. The apical cell was surrounded by the germinal stem cells, spermatogonia appeared outside the stem cells, and cysts occurred in multiplying spermatogonia. Thus, the germinal proliferation centers (GPCs) formed. During the stage from the third to the fourth instar, the apical cell moved inwards, and separated from the testicular epithelium. In the last instar, the apical cell, the GPCs as well, enlarged further. After the spinning stage, the MT-poor region of the apical cell diminished. Characteristics of GPCs remained unchanged during pupal stages. The apical cell persisted until adult emergence, but shrinkage of the nucleus appeared in surrounding germ cells. These findings indicate that the apical cell continues to function as niche for the germinal stem cells from the larval to the pupal stages.