AN ELECTRON MICROSCOPE STUDY OF CHANGES IN MITOCHONDRIA OF FLIGHT MUSCLE OF AGEING HOUSEFLIES (MUSCA DOMESTICA)

The fine structure of flight muscle of adult houseflies (Musca domestica L.) has been investigated in order to study the changes in mitochondria and muscle cell structure with ageing. In all the specimens examined, two kinds of mitochondria are recognized: type A with simple folded cristae and a light matrix, and type B with complex multicristae surrounded by a dense matrix. However, the relative proportions of A and B, the extent of complexity of the cristae, the arrangement within the muscle fibres, and the degree to which they are surrounded with glycogen varies with age. In newly emerged flies A and B types are equally represented and are loosely arranged in between the muscle fibres and surrounded with large quantities of glycogen. About 12 days after emergence, the population of type B mitochondria increases; in some individuals the multicristae develop fenestrations and in others they become arranged as whorls resembling myelin figures. The amount of glycogen is reduced and the mitochondria are more closely packed between the muscle fibres. After 20 days the structural complexity of the mitochondria begins to decrease and degeneration starts. Type A degenerate by swelling due to non-uniform increase of their intracristal spaces, whereas type B degenerate by developing structureless areas in place of a dense matrix. The outer membrane in both types of mitochondria loosens up and acquires a fuzzy appearance. Along with a decline in mitochondrial number, the muscle substance also decreases in old flies. The relationship between the fine structure of mitochondria and biochemical changes recorded during the senescence of houseflies is discussed. © 1969.