Ultrastructural study of the prothoracic glands of silkworms (Antheraea pernyi and Bombyx mori) at the last two larval stages has shown that the essential modifications which take place during each intermolt affect the chondriome of secretory cells. A description is given of the differentiation of macromitochondria from typical mitochondria by a general swelling, a clearing of the matrix, and the formation of a complex tubular network. The hypothesis of fixation or anaesthesia artifacts has been dismissed because of the persistence of these transformations after different fixations and because of the existence of numerous intermediary stages between these two types of chondriosomes which imply the progressiveness of differentiation. The cytochemical demonstration of mitochondrial DNA fibers suggests that the genetic information, probably present in this type of nucleic acid, controls the differentiation and the specific metabolic activity of these organelles. The frequent relationships observed in Antheraea between the tubules of agranular reticulum and the macromitochondria which are reminiscent of the vacuoles-mitochondria associations of the adrenal cortex, may be related to the transfer of cholesterol and other precursors of steroidogenesis. In the last stages, the macromitochondria become transformed into vacuoles by a disappearance of the tubular network. The correlation revealed between mitochondrial transformations and the cyclical release of ecdysone (65) leads to the conclusion that a prominent fraction of chondriosomes is involved, in relation to the agranular reticulum, in the elaboration of steroid hormones such as ecdysone.

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