Organization of organelles within hyphae of Ashbya gossypii revealed by electron tomography.

Ashbya gossypii grows as multinucleated and constantly elongating hyphae. Nuclei are in continuous forward and backward motion, also move during mitosis, and frequently bypass each other. Whereas these nuclear movements are well documented, comparatively little is known about the density and morphology of organelles which very likely influence these movements. To understand the three-dimensional subcellular organization of hyphae at high resolution, we performed large-scale electron tomography of the tip regions in A. gossypii. Here, we present a comprehensive space-filling model in which most membrane-limited organelles including nuclei, mitochondria, endosomes, multivesicular bodies, vacuoles, autophagosomes, peroxisomes, and vesicles are modeled. Nuclei revealed different morphologies and protrusions filled by the nucleolus. Mitochondria are very abundant and form a tubular network with a polarized spherical fraction. The organelles of the degradative pathways show a clustered organization. By analyzing vesicle-like bodies, we identified three size classes of electron-dense vesicles (∼200, ∼150, and ∼100 nm) homogeneously distributed in the cytoplasm which most likely represent peroxisomes. Finally, coated and uncoated vesicles with approximately 40-nm diameters show a polarized distribution toward the hyphal tip with the coated vesicles preferentially localizing at the hyphal periphery.

Reciprocal innervation of outer hair cells in a human infant.

Reciprocal synapses are characterized by the presence of both afferent and efferent types of synaptic specializations between two cells. They have been described at the neural poles of outer hair cells (OHCs) in humans with advanced age and two monkey species. Our objective was to study the innervation of the OHCs and determine if reciprocal synapses were present in a young (8-month-old infant) human subject. We studied the synaptic and cytoplasmic morphology of 162 nerve terminals innervating 29 OHCs using serial section transmission electron microscopy. Seventy-six percent of all OHCs were innervated by terminals with reciprocal synapses. This prevalence increased from the first toward the third row (p < 0.001), and 100% of OHCs in the third row demonstrated at least one reciprocal synapse. The prevalence of terminals with reciprocal synapses was higher in the human infant than in older human subjects and was very similar to what has been reported for the chimpanzee. Reciprocal synapses occur in sufficient numbers to be physiologically significant in primates. The nerve terminals were found to segregate into two groups on the basis of their cytoplasmic morphological characteristics: (1) vesicle-rich/neurofilament-poor (VR/NP) and (2) vesicle-poor/neurofilament-rich (VP/NR). All afferent and reciprocal terminals were of the VP/NR variety. The majority of the efferent terminals originated from VR/NP nerve fibers (classical olivocochlear morphology), but 23.5% of the efferent terminals were VP/NR. The hypothesis that peripheral processes of type II spiral ganglion cells form classical afferent, reciprocal, and a number of purely presynaptic terminals on OHCs is discussed. The presence of different types of synaptic specializations on OHCs formed by nerve fibers of the same type (VP/NR) suggests the existence of reciprocal neuronal circuits between OHCs sharing the dendritic arborization of a type II spiral ganglion cell.