Adaptation of skeletal muscle to immobilization in a shortened position.

This study determined the morphological changes and adaptations that occur following immobilization of rat soleus and gastrocnemius muscles when the ankle joint is placed in complete plantar flexion for 2, 5, 7, 14, 21, and 28 days by means of plaster casts. Previous studies of such shortened muscles have shown that the number of sarcomeres in series is reduced, but how the sarcomeres are reduced has not been determined. We observed that the fibers in the mid-belly region of the muscles demonstrated a progressive degenerative process over the first few weeks. Myofibrils across the entire width of the affected fibers underwent dissolution. However, by 4 weeks new myofibrils were being formed, and sarcomere lengths appeared normal. Portions of the fibers near the tendon underwent segmental necrosis. These findings are similar to the response of the soleus and gastrocnemius muscles to tenotomy and are clinically relevant to orthopedic procedures that maintain muscles in shortened conditions for prolonged periods.

Degeneration and alteration of axons and intrafusal muscle fibers in spindles following tenotomy.

The effect of tenotomy (cutting of distal and proximal tendons) on the spindles in soleus muscle of adult rats was examined by light and electron microscopy in tissue obtained 1, 2, 3, 4, 5, 6, 7, and 14 days after surgery. Degenerative changes in the spindles progressed with time; also, the degree of alteration observed in a spindle varied with its position in the muscle, i.e., the earliest and most extensive changes occurred in spindles situated near the tendons. The disorganization and breakdown of myofibrils in the intrafusal muscle fibers were observed 3 days after tenotomy and continued with longer periods after surgery. The progression of morphological changes included alteration in fiber outline and the external lamina of the intrafusal muscle fibers and changes in the extracellular environment within the spindle capsule. The intrafusal muscle fibers of some spindles were shortened, apparently due to the loss of their polar ends, and degenerated axons which apparently supplied the intrafusal muscle fibers were also observed. Completely necrotic intrafusal fibers were absent. A possible relationship between the observed morphologic alterations and an adjustment of the spindle to muscle shortening is discussed.

Sex behavior and the sexually dimorphic hypothalamic nucleus in male Zucker rats.

To determine whether or not impaired male sex behavior in obese male Zucker rats is accompanied by any anatomical alterations in a hypothalamic area implicated in the control of sex behavior, 6 lean and 5 obese male Zucker rats were studied behaviorally and anatomically at 14 months of age. Obese males showed markedly decreased male sex behavior relative to lean males, in spite of serum levels of testosterone and testicular weights comparable to those of lean rats. Obese rats had significant decreases in brain weight and volumes of sexually dimorphic nuclei per g of brain, relative to lean rats; volumes per g brain of other structures (paraventricular and suprachiasmatic nuclei) were not different between groups. It is suggested that an incomplete expression of sexually dimorphic features of the preoptic area-anterior hypothalamus, due perhaps to an impaired process of perinatal brain androgenization, may contribute to decreased male sex behavior in adult obese rats.

The morphology of growth cones of regenerating optic nerve axons.

The morphology of growth cones of regenerating optic nerve axons was examined by light and electron microscopy in adult frogs (Rana pipiens), using a horseradish peroxidase (HRP) fiber-filling method, during early and later phases of regeneration. Optic nerve regeneration was initiated unilaterally by crushing the optic nerve in mid-orbit. Fiber filling was accomplished by severing the affected nerve closer to the eye 24-48 hrs. prior to sacrifice and applying HRP to the central stump. Regenerating axons and their growth cones were observed in the optic nerves, chiasma, tract, pretectal neuropil, and optic tectum. Growth cones of normal-appearing axons varied in shape and size. Flattened, foliate growth cones similar to those commonly described in vitro were observed in the pretectal neuropil and optic tectum. Other growth cones having vermiform, lanceolate, spatulate, and bulbous forms were observed throughout the optic pathway at all stages examined, although the longer (up to 70 micrograms) wormlike structures appeared only in the optic tract during the early period of outgrowth. Nearly complete serial-section reconstructions were obtained for two growth cones in the contralateral optic tectum at 8 wks. regeneration time. One was thinly flattened (to 30-50 nm in places) and extended broadly (8 micrograms in diameter) in contact with a neuronal perikaryon. The other formed a hood over the blind end of a severed, nonregenerating myelinated axon, which was normal-appearing except at its end within the confines of the growth cone. Morphological variation among the growth cones is discussed in relation to other descriptive in vivo studies and views concerning growth cone motility.

Long-term survival of centrally projecting axons in the optic nerve of the frog following destruction of the retina.

A significant number of unmyelinated axons and their synaptic endings in the frog, Rana pipiens, were found to retain a normal morphology long after separation from their cell bodies. At the end of various survival periods following unilateral removal of the retina, horseradish peroxidase (HRP) was administered to the optic nerve stump by a fiber-filling method. In frogs maintained at 20 degrees C, unmyelinated optic nerve axons conducted HRP from the site of application in the orbit to layers A, C, and E of the contralateral optic tectum, even though their retinas had been removed up to 69 days earlier. Such fiber-filling was absent beyond 19 days in other frogs surviving at 35 degrees C. No labeled fibers were continuous with any intracerebral neurons. The HRP was always localized intraaxonally, and the marked axons and terminals were ultrastructurally normal. Counts of surviving axons from electron micrographs of the optic nerves showed that, at 20 degrees C, more than half of the normal complement of unmyelinated axons disappeared in the first 10 days. All the myelinated axons degenerated during the first 6 weeks survival. However, approximately 55,000 normal-appearing unmyelinated axons (12% of the unmyelinated fiber population) persisted in the optic nerve at 10 weeks following removal of the retina. The survival rate was lower at 35 degrees C. In other frogs, one eye was injected with 3H-leucine to initiate axonal transport into the retinal ganglion cell axons. That eye was removed 48 hours later. Autoradiographic analysis of brain sections of frog surviving an additional 31 to 61 days at 20 degrees C showed strong labeling of the optic tract and layers A, C, and E of the contralateral optic tectum. The absence of displaced ganglion cells that might exist within the optic nerve was verified by other observations. It is hypothesized that the potential shown by frog optic axons for long-term survival in the absence of the cell-body expresses a general property of vertebrate (and invertebrate) axons, rather than a special property of the frog optic nerve.

Comparison of the ultrastructure of stimulated and unstimulated mechanoreceptors in the taste hairs of the blowfly Phaenicia serricata.

The structure of mechanoreceptors at the base of labellar taste hairs of the blowfly Phaenicia serricata were examined in stimulated and unstimulated conditions (i.e. with the hair bent or unbent). Physiological recordings from the mechano-receptor showed that the receptors responded when the hair is bent dorsally or ventrally and when the hair is bent at extreme angles. These conditions are the same as those placed on hairs in the anatomical studies. Bending the hair toward the ventral labellar surface caused the hair base to compress and indent the tubular body and its surrounding membrane and sheath at the distal end of the mechanoreceptor dendrite. In compressed tubular bodies, microtubules oriented longitudinally were bent and separated a greater distance from each other. Separation as much as 70 nm was observed in compressed tubular bodies as compared with a maximum of 26 nm between micro-tubules in tubular bodies of unbent hairs. The dense amorphous material between microtubules of compressed tubular bodies formed prominent bridges 18 nm thick connecting the microtubules at intervals of 48-74 nm. Thin 10 nm filaments were also evident in the spaces between microtubules. When the hair was bent toward the proximal end of the proboscis, the tip of the tubular body was bent about 15 degrees. The tubular body appears to function as a firm but resilient structure over which the dendritic membrane can be stretched during mechanostimulation. Comparison of morphology of bent and unbent hairs suggests a means by which mechanical force from the movement of the hair is transferred to the receptors by structures in the hair socket region. No differences were found in ciliary structures of stimulated and unstimulated receptors.