Observations on spermiogenesis in the fungus gnat Sciara coprophila.

Although 9-membered centrioles are found in somatic tissues of Sciara, the centriole which lies at the spindle pole of the second meiotic division in male Sciara is composed of a row of approximately 70 short tubules in an oval array. Shortly after telophase of this unequal division, in the daughter cell destined to undergo spermiogenesis, microtubules become confluent with the tubules of the centriole. These tubules have the same density as other cytoplasmic microtubules after glutaraldehyde-OsO(4) fixation and, like them, are not preserved with OsO(4) fixation. As the centriole, now with approximately 70 attached, posteriorly directed, doublet tubules, migrates from the polar to the apolar end of the nucleus to take a final position in an oval groove which forms in the nuclear envelope, the tubules lengthen and become demonstrable after OsO(4) fixation and more electron opaque than other cytoplasmic microtubules following glutaraldehyde-OsO(4) fixation. Later, a singlet tubule appears peripherally to each doublet of the oval and 4 "arms" develop at specific sites on the tubules. Posteriorly, where the oval of tubules becomes discontinuous and forms a spiral, the arrangement of arms is different and the singlet tubules are lacking. Dense solid bodies develop inside this odd flagellum and become enclosed by a smooth double membrane. A single mitochondrial derivative has three components: a central area of homogeneous, moderately electron-opaque, proteinaceous material; a peripheral ring of cristae; and a crystalloid which is specifically oriented with respect to the flagellar tubules.