![]() Such protrusions of myelin lamellae into dendrites appear to have been first described by Antal and Szekely (1987) in frog spinal cord. Such protrusions may also be the source of profiles of dendrites that contain membranous components in their cytoplasm, the assumption being that such profiles arise when the protrusion from the myelin sheath is not in the plane of section. ![]() Some profiles of dendrites that show dendrites being invaginated by protrusions from myelin sheaths have also been encountered ( Fig. Since the cytoplasm is electron dense it is assumed that these are profiles of dendrites that are degenerating. Such profiles are identified as belonging to dendrites because they are synapsing with axon terminals. Some profiles of dendrites are electron dense and shrunken ( Fig. Profiles of some dendrites in the aging cerebral cortex have membrane bound vacuoles in their cytoplasm (Figs. That these profiles belong to dendrites is shown by the fact that their bounding plasma membrane can be involved in the formation of synapses with axon terminals.ģ. 2.2 may be an early stage in degeneration, leading to the formation of profiles of dendrites that are almost completely devoid of organelles ( Fig. The profiles of dendrites such as shown in Fig. Profiles such as these are common in layer 1 of the aging cortex, a layer which becomes thinner with age as the apical tufts of pyramidal neurons retract and degenerate.Ģ. The profiles of some dendrites in the cortices of old monkeys show a cytoplasm devoid of organelles beyond a few microtubules beneath the plasma membrane ( Fig. Nevertheless, unusual images of dendrites that probably reflect the effects of age are sometimes encountered and some of these images are as follows:ġ. Although no systematic studies have been made of the effects of age on the fine structure of dendrites, most profiles of dendrites in electron micrographs show no or few obvious age changes. It is known that with age dendrites, especially those in the apical tufts of pyramidal cells, retract and degenerate. Consequently profiles of their dendrites have more regular shapes and these dendrites receive most of their synapses on their shafts. In contrast to the dendrites of pyramidal neurons, the dendrites of nonpyramidal, inhibitory neurons have smoother outlines, with few or no dendritic spines. The dendritic spines receive the majority of axon terminals synapsing with dendrites of pyramidal and spiny stellate neurons, but electron microscopic images that show the entire lengths of dendritic spines are not commonly encountered and have to be sought. ![]() The dendrites of the excitatory pyramidal cells are also profusely decorated with dendritic spines so that profiles of their dendrites tend to have somewhat irregular outlines. They travel singly through the neuropil and have a cytoplasm that contains regularly spaced microtubules, ribosomes and rough endoplasmic reticulum, mitochondria, and long cisternae of smooth endoplasmic reticulum ( Figs. Dendrites are processes that extend from the cell bodies of cortical neurons. ![]()
0 Comments
Leave a Reply. |