Muriel D. Ross

Observations on Normal and Degenerating Human Otoconia

Specimens of human otoconia obtained from autopsy material and representing various stages from fetal to advanced old age, were studied by microdissection, scanning electron microscopy, electron microprobe analysis, and x-ray powder diffraction. The typical adult otoconial configuration is a cylindrical, finely serrated body with pointed ends; crystallographically, it corresponds to a single crystal of calcite. Other, less numerous types include joined otoconia, pure rhombohedrons and multifaceted, presumably immature forms. Many otoconia achieve the adult configuration during fetal development. The multifaceted otoconia are most numerous, and the rhombohedrons proliferate, during childhood in the utricle. Crystals from both end organs are virtually identical in composition in the young adult, but saccular otoconia are the larger. In middle and advanced age the otoconia decrease in number, especially in the saccule. Saccular otoconia degenerate progressively in a posteroanterior direction across the macula; they assume a specific, fibrous, hollowed-out appearance, which is not duplicated by either chemical etching or autolysis. Neogenesis and growth of otoconia appear to occur postnatally, with different characteristic growth potentials for those of the saccule and the utricle. Age-related saccular otoconial degeneration appears to involve the organic material, which disappears either before or simultaneously with the mineral substance.

Otoconial formation in the fetal rat

The development of otoconia in the fetal rat was investigated by scanning and transmission electron microscopy and by x-ray elemental analysis. The transmission electron microscopical results indicate that primitive otoconia are highly organic appearing but are trigonal in cross section, indicating that they already possess a three-fold axis of symmetry and a complement of calcite. These otoconia develop into spindle-shaped units which accrue fibrous, organic material at an angle to their surfaces. Dumbbell-shaped otoconia, with distinct central cores and peripheral zones, result. These otoconia then mature to the adult crystal configuration having a more cylindrical body and pointed ends. The existence of trigonal, spindle- and dumbbell-shaped otoconia was verified by scanning electron microscopy of fresh-frozen material. Tissues prepared for transmission electron microscopy proved (by elemental analysis) to have been decalcified inadvertently, fortuitously revealing the arrangement of the organic material. Subsequent transmission electron microscopy of dumbbell-shaped otoconia not exposed to fluids during embedment showed that calcite deposits mimicked the arrangement of the organic material. X-ray elemental analysis demonstrated that calcium was present in lower quantities in the central core than peripherally. Findings are interpreted to indicate that organic material is essential to otoconial seeding and directs otoconial growth.

The Nature and Crystal Growth of Otoconia in the Rat

Several types of otoconia are present in the macular regions of young rats. These include multifaceted, transitional and rounded body forms, some variant otoconia and a few rhombohedrons. The adult form has typically rounded but nonsmooth body surfaces and pointed ends with three planar faces. The multifaceted and transitional otoconia fracture and etch more readily than do the adult type. The differences in properties of the otoconia are considered in the light of known facts concerning inorganic crystal nucleation and growth. This integrated approach indicates that many otoconia originate by seeding of multiple subunits on an organic substrate and develop by the mechanism of parallel growth. The basic structural unit is the rhombohedron. By analogy to inorganic crystals of calcite, it would seem that the typical otoconium grows on the end faces but growth on the side faces is suppressed by some unknown chemical factor. Some otoconia are exceptions, evidently seeding and growing in the pure rhombohedral form. Decalcification of cleaved otoconia shows that organic material is incorporated during growth. The observations are interpreted to indicate that organic substance influences growth and achievement of the adult otoconial form.

Multipolar Neurons In The Spiral Ganglion Of The Rat

Light- and electronmicroscopical studies of the rat spiral ganglion demonstrate that small neurons which are filamentous, thinly encapsulated and contain sparse rough endoplasmic reticulum are consistently present at all ages from weanling to sexually mature. These cells are further characterized by eccentricity of the nucleus, and by the presence of dense-cored vesicles (1200-1400 A in diameter) scattered in the vicinity of the Golgi complex, in the cytoplasm, or in the cell processes. These neurons comprise about 74% of the ganglion cells and correspond to the unmyelinated neurons described for other species. By use of serial sections, the cells have been determined to be multipolar. Ultrastructurally, they resemble known parasympathetic neurons. Thus, it is suggested that the small, unmyelinated spiral ganglion cells are autonomic (parasympathetic) neurons.

Innervation Patterns in Rat Saccular Macula A Structural Basis for Complex Sensory Processing

Serial sections through the anterior part of rat saccular macula were reconstructed as montages. Findings are that type II hair cells are integrated into the neural circuitry of type I cells, chiefly by synapses with neighboring calyces and their collaterals; and that complex interactions between afferent- and efferent-type nerve elements take place. Three basic types of nerve/calyx pattern are present: U-type nerves lose their myelin before they enter the macula and have complex calyces with several collaterals; M-type nerves are myelinated up to the calyx, which lacks collaterals; and M/U-type nerves have short, unmyelinated segments proximal to their calyces, which have few collaterals. Both afferent- and efferent-type collaterals spring from calyces, chiefly from those of U-type nerves. Type II cells are presynaptic both to electron-lucent and to vesiculated terminals; some synapses are reciprocal. Electron-lucent boutons sometimes are presynaptic to calyces and to type II hair cells; and morphologically afferent-to-afferent kinds of synapses occur in the neuroepithelium. The anatomical findings indicate that complex information processing must occur in mammalian gravity receptors.

The suprastructure of the saccular macula.

Inclusion of tannic acid in the primary fixative fortuitously preserved the macular suprastructure, from the macular surface to the otoconia, in rat. Results demonstrate that fluid-filled channels around the stereociliary tufts and a slit-like space immediately above the macula are artefacts of tissue preparation. Filaments fill the area between, and show linkage to, the macular surface and the more peripheral, closely knit layer identified here as the true otoconial membrane. The material also passes through the otoconial membrane at various sites to surround the otoconia and extends well beyond the maculas in all directions. Although it may prove later to be endolymph, it is referred to here as supramacular substance. Kinocilia and the tallest stereocilia as well as the otoconia are tethered to the otoconial membrane. Findings were confirmed in decalcified material. Results are discussed in terms of functional and biochemical implications, and by analogy to man-made acceleration sensing devices.

Horseradish peroxidase acute ototoxicity and the uptake and movement of the peroxidase in the auditory system of the Guinea pig

When guinea pig cochlea was perfused in vivo with a solution of 1% horseradish peroxidase (HRP) in artificial perilymph, the enzyme was found in the basilar membrane, spiral limbus, some outer and inner hair cells and some supporting cells and it was gradually cleared away with time. Acute signs of cell damage included swelling, vacuolization and diffuse labeling of some hair cells, but stereocilia remained normal in configuration. Albino melanocytes of the spiral ligament were also damaged, and vacuolization of Reissners membrane occurred after 10% HRP. Both concentrations caused a gradual decline in CM, showing that HRP is acutely ototoxic but its mode of action is unknown. No retrograde transport of HRP to spiral ganglion cells or to brain stem neurons occurred, but some brain stem neurons took up HRP from the neuropil following diffusion from the cochlea.

Morphological Evidence of Vestibular Pathology in Long-term Experimental Diabetes Mellitus: II. Connective Tissue and Neuroepithelial Pathology

The influence of long-term experimental diabetes on the saccule and utricle was investigated with light and electron microscopy. Pathological changes involving the connective tissue cells of the subneuroepithelial connective tissue suggest that these cells may play a potentially important role in diabetic pathology of the inner ear. In diabetic animals there was an increased incidence of secondary lysosomes within the connective tissue cells as well as an accumulation of intracellular lipid droplets that increased with the level of hyperglycemia. Five animals with relatively more severe diabetes also had an extensive accumulation of extracellular matrix. Two of these rats with longer diabetes duration had a small number of degenerating type I hair cells scattered through the saccular neuroepithelium, possibly due to chronic stresses from impaired diffusion of oxygen, nutrients and waste material through the dense extracellular matrix. Utricles of these same animals did not have any degenerating hair cells.

THE INFLUENCE OF GRAVITY ON STRUCTURE AND FUNCTION OF ANIMALS

Gravity is the only environmental parameter that has remained constant during the period of evolution of living matter on Earth. Thus, it must have been a major force in shaping living things. The influence of gravitational loading in evolution of the vertebrate skeleton is well recognized, and scale effects have been studied. This paper, however, considers in addition four pivotal events in early evolution that would seem to have been significant for the later success and diversification of animal life. These are evolution of the cytoskeleton, cell motility (flagellae and cilia), gravity detecting devices (accelerometers), and biomineralization. All are functionally calcium dependent in eukaryotes and all occurred or were foreshadowed in prokaryotes. A major question is why calcium was selected as an ion of great importance to the structure and function of living matter; another is whether gravity played a role in its selection.

Distribution and significance of norepinephrine in the lateral cochlear wall of pigmented and albino rats.

Picogram quantities of norepinephrine were found in cochlear regions of pigmented and non-pigmented rats. These regions of the cochlea were the modiolus, organ of Corti--osseous spiral lamina and the lateral cochlear wall. The content of norepinephrine in the modiolus and lateral cochlear wall of the pigmented rat was significantly greater than that in areas of the non-pigmented rat. In contrast, there was no statistical difference between the norepinephrine content of the organ of Corti--osseous spiral lamina region of the pigmented rat and that of the albino rat. Since a major difference between the pigmented and albino rats is the presence of melanin-containing melanocytes in the modiolus and lateral cochlear wall region of the pigmented animals, it is possible that norepinephrine is stored in cochlear melanocytes.