Search PubMed⌕ Search

Biomedical subjects

M C Whitehead

Publications and source records attributed to M C Whitehead.

25 records · Page 2Linked to original sources
PubMed1985

The growth of cochlear fibers and the formation of their synaptic endings in the avian inner ear: a study with the electron microscope.

The developmental sequence of nerve-epithelial cell contacts, leading up to the formation of the mature receptoneuronal synapse, has been studied in the basilar papilla of chick embryos with electron microscopy. The receptor epithelium before innervation, on embryonic days 3-4, consists of a homogeneous population of primitive cells; hair cells and supporting cells cannot be distinguished. During innervation of the epithelium (embryonic days 5-7), the invading peripheral fibers of cochlear ganglion cells penetrate the basal lamina and form nerve-epithelial attachments with the epithelial cell bases. Once within the epithelium some fibers turn and spread in the transverse dimension across the basilar papilla through channels formed between the basal epithelial processes. Subsequently, nerve-epithelial attachments are observed more superficially within the epithelium. Hair cells and supporting cells differentiate during early synaptogenesis (embryonic days 8-9). Receptoneural synapses, possibly derived from the nerve-epithelial attachments formed during the innervation stage, are first seen during this period. They are characterized by symmetrical or asymmetrical membrane densities, separated by a cleft containing a dense material. At many of these junctions synaptic bodies, as well as dense-cored and coated vesicles, gather in the hair cells. During mid-synaptogenesis (embryonic days 11-13) the hair cells proliferate synaptic bodies, many of which are not located at receptoneural junctions. The preterminal portions of the sensory endings form large swellings, containing flocculent material, endoplasmic reticulum and vesicles. Late in synaptogenesis (embryonic days 15-17) the swellings disappear, while synaptic endings are transformed to foot-shaped terminals. In the hair cells, synaptic bodies not associated with junctions disappear. Efferent synapses are first seen during this period. This sequence of ultrastructural changes, which the developing sensory nerve endings and their target cells undergo in parallel, can be correlated with observations of Golgi preparations from a companion study. These correlations suggest that the innervation of the cochlea involves the following developmental processes. Initially the peripheral fibers of the ganglion cells grow directly toward the otocyst in fascicles. Having reached the base of the primitive receptor epithelium, the axonal endings, including some with growth cones, encounter a barrier in the basal lamina. When they enter some of the fibers attach to the basal end-feet of the primitive epithelial cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1983

Anatomy of the gustatory system in the hamster: central projections of the chorda tympani and the lingual nerve.

The sensory modalities of taste and touch, for the anterior tongue, are relegated to separate cranial nerves. The lingual branch of the trigeminal nerve mediates touch: the chorda tympani branch of the facial nerve mediates taste. The chorda tympani also contains efferent axons which originate in the superior salivatory nucleus. The central projections of these two nerves have been visualized in the hamster by anterograde labelling with horseradish peroxidase (HRP). Afferent fibers of the chorda tympani distribute to all rostral-caudal levels of the solitary nucleus. They synapse heavily in the dorsal half of the nucleus at its rostral extreme; synaptic endings are sparser and located laterally in caudal regions. These taste afferents travel caudally in the solitary tract and reach different levels by a series of collateral branches which extend medially in the the solitary nucleus, where they exhibit preterminal and terminal swellings. Taste afferent axons range in diameter from 0.2 micrometer to 1.5 micrometers. The thickest axons project exclusively to the rostral and intermediate subdivisions of the solitary nucleus; the find ones may distribute predominantly to the caudal subdivision. Afferent fibers of the lingual nerve terminate heavily in the dorsal one-third of the spinal nucleus of the trigeminal nerve and also as a dense patch in the lateral solitary nucleus at the midpoint between its rostral and caudal poles. This latter projection overlaps that of the chorda tympani. Thus the two sensory nerves which subserve taste and touch from coincident peripheral fields on the tongue converge centrally on the intermediate subdivision of the solitary nucleus. Efferent neurons of the superior salivatory nucleus were labelled retrogradely following application of HRP to the chorda tympani. These cells are located ipsilaterally in the medullary reticular formation ventral to the rostral pole of the solitary nucleus; their dendrites are oriented dorsoventrally. The efferent axons course dorsally, form a genu lateral to the facial somatomotor genu, and course ventrolaterally through the spinal nucleus of the trigeminal nerve to exit the brain ventral to the entering facial afferents.

Afferent Pathways↗
PubMed1982

Development of neuron-specific enolase immunoreactivity in avian nervous tissue in vivo and in vitro.

Neuron-specific enolase (NSE) is a glycolytic isoenzyme that is primarily located in neurons and neuroendocrine cells. The development of NSE immunoreactivity in th avian nervous system at the level of the hind limb has been examined using immunocytochemical methods. NSE immunoreactivity is first detected in ventral horn motor neurons and dorsal root ganglion neurons at embryonic day 9-10. This is at least 2-3 days after some neurons in both these populations are capable of electrical activity. The glycogen body, a non-neuronal structure, also exhibits NSE (+) staining, but the onset of this immunoreactivity is earlier, at 8 days of embryonic development. NSE immunoreactivity was absent from the cell bodies of paravertebral sympathetic ganglia throughout development, but was present in cellular processes and terminals in the adult ganglia. NSE immunoreactivity also develops in tissue cultures containing cells of neural tube and neural crest origin.

Animals↗
PubMed1982

Synapse formation is related to the onset of neuron-specific enolase immunoreactivity in the avian auditory and vestibular systems.

Neuron-specific enolase (NSE) is a unique form of the glycolytic enzyme, enolase, which, in the brain, is strictly localized in differentiated neurons and neuroendocrine cells. In this report evidence is presented that NSE is first seen in the cell bodies of developing chick auditory and vestibular neurons shortly after the time when they first form immature synapses. Subsequently, immunoreactive NSE increases and spreads into transiently appearing structures which participate in the morphogenesis of mature synaptic arrangements. These data correlate synaptogenesis with the differentiation of a specific neuronal enzyme.

Animals↗
PubMed1981

Spatial sampling by dendritic trees in visual cortex.

Kittens were reared in vertically or horizontally striped cylinders. After rearing exposures of 400-500 h, responses of single neurons were determined as a function of orientation of a square wave grating stimulus. These data suggest that the rearing environment did alter orientation preference in some of the kittens. The visual cortices of the stripe-reared kittens and of control kittens were impregnated according to a Golgi-Cox method. Dendrites of layer IV stellate cells were tracked and analyzed in three dimensions by a computer-microscope. Four methods of analyzing the spatial distribution of dendrites are described and discussed. Two methods previously described in the literature were not sufficiently sensitive to detect any differences among kittens exposed to vertical or horizontal stripes or to a control environment. Two newly developed methods were able to provide initial evidence for rearing effects on dendritic trees in visual cortex. The more detailed of these new methods describes the angular location of dendritic segments, with respect to standard brain axes, as a function of distance from the cell body. Data obtained by means of this method of dendritic angular distribution (DAD) plots suggests a number of conclusions. Rearing animals in a striped environment may influence the way in which dendrites of layer IV stellate cells of visual cortex distribute themselves in the neuropil. The effect of selective rearing on dendritic distribution does not appear to extend back to those portions of the dendritic tree closest to the cell body. This influence of rearing in a selective environment may be explained by hypothesizing that during development dendrites distribute themselves in ways that tend to maximize the effects of spatiotemporal summation for the postsynaptic neurons.

Animals↗