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The distribution of microtubules and mitochondria in central axons of an insect (Periplaneta americana) is assessed by comparison between counts on micrographs and computed axon random ‘models’. These studies show that the observed multiple association of microtubules with individual mitochondria is statistically highly significant. Electron micrographs of thin sections show that linkage is effected by physical cross-bridges, possibly comprising components from the microtubule and mitochondrion. Linear particle arrays are described on the outer mitochondrial membrane in freeze-fracture replicas, and tentatively related to the bridges seen in thin sections. The results are discussed in terms of proposed roles of microtubules in neurons and other cells.

Gross ultrastructural and histochemical details of the integumental milk glands of the tsetse fly Glossina morsitans have been examined during the pregnancy cycle. Structural evidence for protein secretion is found between Days 3–8 of the nine-day cycle: termination of activity is completed on the day of parturition. Onset of lactation is synchronized with the eclosion of the first instar larva. The changes in cell volume (notably in the extracellular reservoir) occurring throughout the pregnancy cycle are illustrated in electron micrographs, and a one hundred-fold volume increase in the reservoir volume between the inactive phase and the active period is illustrated and discussed in terms of membrane modulation of the limiting membrane of the reservoir. Intracellular membrane changes during the cycle, particularly the development of extensive ER arrays in the actively secreting cell, are illustrated and discussed. It is suggested that cytoplasmic microtubules play a part in maintaining the form of the distended secretory cell, at the height of secretory release and storage. Histochemical observations on the milk secretion, and the contents of the larval gut are presented.

Pure preparations of Trypanosoma brucei , free from plasma and cellular components were isolated from rat blood, and concentrated into loose pellets by low-speed centrifugation. Pellets were either processed for thin sectioning as a control for general morphology, or glycerol-treated after glutaraldehyde fixation for preparation of freeze-fracture replicas. Concentration of cells of 50,000–100,000/mm 2 of sectioned or fractured surface facilitated identification of fracture faces of the cell body, invaginated flagellar pocket and flagellum. Particle distribution and A and B faces of these regions of the cell are described. A collar of B face particles occurs around the neck of the flagellar pocket, possibly associated with a junction controlling ingress of ingested materials to coated vesicles formed along the membrane defining the pocket. A and B faces of the flagellum and adjoining surface of the cell body have shown that the only intra-membrane specialization corresponding to the miniature ‘maculae adherentes’ described previously in thin sections is probably an uninterrupted series of small clusters (3–6) of 80 A particles on the A face of the flagellar membrane. It is proposed that these arrays represent attachment points for strands linking the axoneme and paraxial rod to the flagellar surface, and are not directly concerned with the physical adhesion of the flagellum to the cell body surface—a linkage that appears to be established within the extracellular gap between these apposed surfaces of the cell. The potential use of freeze-etching in further study of the external antigens of the infective cell is discussed.

The fine structure of synapses in the central nervous system of lamprey (Petromyzon marinus) ammocoetes has been investigated. Both synapses within the neuropil and synaptic links between giant fibers (including Müller cells) and small postsynaptic units are described. The distribution of neurofilaments and microtubules in nerve profiles over a wide diameter range is described, and the possible role of these structures in intracellular transport is discussed. Electron micrographs indicate that small lucent "synaptic vesicles" occur sparsely throughout the axoplasm and in regular arrays in association with microtubules in the vicinity of synapses. Within a synaptic focus, immediately adjoining the presynaptic membrane, vesicles are randomly arranged and are not associated with microtubules. Neurofilaments are present, generally in large numbers, but these are not associated with vesicles or other particulates. The structural findings are considered in terms of current concepts of fast and slow transport in neurons and the mechanochemical control of intracellular movement of materials.