Fungus invasion of human hair tissue in tinea capitis caused by Microsporum canis: light and electron microscopic study.

Previously, we reported a morphological change of Trichophyton violaceum in hair tissue in black dot ringworm. To investigate the morphology of Microsporum canis in human hais tissue, three cases of tinea capitis by M. canis were examined by both light and electron microscopy. The fungal elements, which were located deeplyin the keratogenous zone, showed nonseptate hyphae in the outer part of the hair cortex. With the upward development of hair tissues, some hyphae invaded the keratinized inner root sheath and were there transformed into arthrospores, which then occupied the large volume of the inner root sheath; each spore was surrounded by an electron-lucent halo. In some affected hair follicles, at the follicular isthmus level, a microabscess composed of polymorphonuclear leukocytes was often formed in the outer root sheath adjacent to the arthrospores in the keratinized inner root sheath. On the other hand, the remaining hyphae in the cortex became degenerated. Fungi did not invade the hair-germinative cells. There is a distinct relationship between the morphological change of fungi and the differentiation of hair cells in tinea capitis by M. canis as well as in that by T. violaceum, although the direction of invasion and pathological roles of fungal elements within hair tissue are significantly different between the two species of fungi.