Intercellular bridges and the fusome in the germ cells of the Cecropia moth

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Abstract
Intercellular bridge development was compared in Ceropia by transmission electron microscopy in the germ cells of males and females. Bridge formation begins in the fourth instar, and the spindle remnants in newly formed bridges are replaced at this time by an amorphous material known as the fusome. In the four- and eight-celled cystocyte clusters of the female, the newly formed intercellular bridges migrate centripetally, forming a central complex of bridges surrounded by a rosette of germ cells. The fusomes become continuous and occupy all of the bridges in the complex. At each division each mitotic spindle orients itself with one pole toward the continuous fusome. In the female, mitosis stops at the eight-cell stage, and the cystocytes all enter first meiotic prophase. At the end of the fifth instar, when nurse cell differentiation commences in seven of the cells, the continuous fusome is replaced by a continuous mass of mitochondria and microtubules. In the male, bridge migration and rosette formation are abandoned during the later mitotic and meiotic divisions; during these stages the fusome is no longer continuous. The fusome of males disappears during spermatid differentiation and is not replaced by mitochondria and microtubules. The ability of the centrally located continuous fusome to orient the mototic spindles of succeeding mitoses could account for the earlier observations that all pre-existing bridges remain in only one of the daughter cells at each successive cystocyte division in the female.