摘要:

AbstractThe normal development of the mesencephalic nucleus of the trigeminal nerve has been studied in a series of chicks from the fourth day of incubation until the eighteenth day post‐hatching. In addition, the time of origin of the cells in the nucleus has been determined by3H‐thymidine autoradiography, using a variant of the cumulative labeling method.The fully‐formed nucleus consists of two distinct groups of unipolar cells, a numerically larger medial division within the tectal commissure and a smaller, lateral division within thestratum griseum periventriculareof the optic tectum. The cells in the lateral group are, on the average, larger than those in the medial part of the nucleus at all stages of development. The entire population of neurons is generated between the third and fourth days of incubation (stages 14 to 24) with the cells in the lateral division being formed slightly earlier (stages 14 to 20) than the medially located cells (stages 16 to 24). When the nucleus is numerically complete, at the eighth or ninth day of incubation, it contains between 4,000 and 4,500 cells. Between days 11 and 13 there is a dramatic loss of cells, especially from the caudal part of the nucleus, to a final population of about 1,100 neurons. This normally occurring cell loss seems to occur at the time muscle spindles are being formed, and appears to be due to the inability of the outgrowing processes of the cells to form appropriate peripheral contacts since it is accentuated by the early extirpation of the mesoderm of the mandibular lobe. These experiments also suggest that in the chick the jaw musculature is, in part, innervated bilaterally by the mesencephalic nu

ISSN:0092-7317    

DOI:10.1002/cne.901470302

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1973

数据来源: WILEY

摘要:

AbstractAn investigation of synaptic plasticity in the lateral geniculate nucleus has been carried out in rabbits surviving one day to one year following ipsilateral visual cortex removal. There is extensive retrograde degeneration of LGN neurons, and the altered nucleus thus provides a model for examining reorganization of optic tract axons which have been deprived of their normal postsynaptic membrane.The distribution of synaptic contacts in normal LGN was quantitatively determined. In the animals surviving 1–14 days after cortex ablation, there was extensive cell death and loss of the axodendritic synapses of cortical origin, which constitute the most numerous synaptic type in normal LGN.In animals surviving 4–12 months, there were few remaining neurons and dendrites. The fine structure of the nucleus was characterized by a shift from axodendritic synaptic contacts, as in the normal, to a ten–fold increase in axoaxonal synapses, which became the dominant population. The contribution of optic axons to this synaptic population was determined by contralateral retinal removal. It was found that the synaptic axoaxonal organization in the altered LGN follows the rules of normal organization, in that optic afferents are invariably the presynaptic component in round to flat vesicle axoaxonal synapses.The LGN's undergoing chronic retrograde degeneration shrank to about twothirds the size of the normal nucleus. There was no evidence of reduction in the length of optic tract axons within the nucleus.The evidence indicates that there are new synaptic contacts formed between surviving axons in the degenerating LGN, as a consequence of the loss of postsynaptic dendritic membrane. The new synapse formation is guided by the normal rules of axoaxonal organization, indicating a maintenance of recognition of appropriate membrane for synaptic contact in the reordered LGN. It cannot be stated whether sprouting of axons accompanies the formation of new synapses or whether the new synaptic contacts are exclusively between preexisting synaptic knobs i

ISSN:0092-7317    

DOI:10.1002/cne.901470303

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1973

数据来源: WILEY

摘要:

AbstractAs part of a larger ultrastructural study on the photoreceptors of the squid,Loligo pealei, this paper reports findings on non‐synaptic regions. The cross‐sectional profiles of individual distal segments coincided in horizontal sections as far as 75 μ apart, but progressively less with greater separations. Areas of cross‐section showed a unimodal distribution skewed towards 6 μ2, the smallest areas. The distribution of ratios of villous to smooth border was unimodal and skewed with 1.0 being the most numerous and minimal ratio. This ratio tended to increase linearly with area. Cells of small cross‐section were completely surrounded by villous border of other cells and about 60° of the perimeter of any cell had microvilli on both sides of the border. Cells participated in odd or even numbers of rhabdomes, up to eight. Pentalaminar membrane junctions between microvilli of the same or neighboring cells were observed. Cytoplasmic microtubules were concentrated near villous borders. In light adaptation some ommin granules entered the photoreceptor tips, and the rhabdomes which formerly began near the surface, now began somewhat below this level. A terminal bar zone existed proximal to the rhabdomes. Here glial cytoplasm separated photoreceptors and both cells contained ommin granules. Photoreceptor somas lacked glial separation and were highly interplicated. Mitochondria were longitudinally arranged along the somal membrane. The planes of the lamellate membrane bodies, seen in the somal cytoplasm of the cells, did not exhibit a rectilinear pattern such as that displayed by the basically rectangular distal segment cross‐sections and microvillar

ISSN:0092-7317    

DOI:10.1002/cne.901470304

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1973

数据来源: WILEY

摘要:

AbstractThe plexus level of the retina of the common squid,Loligo pealeiwas studied by electron microscopy and was found to be layered in the sclerad direction by imprecise zones respectively characterized by receptor collaterals, synaptic profiles, horizontal fibers, and receptor axons and other neurites ensheathed in glia. Receptor axons characteristically contained microtubules and dense mitochondria, passed vertically through the layer of horizontal fibers and then turned horizontally as they became collectively ensheathed. Receptor collaterals emerged at the level of the most proximal receptor nuclei and at intermittent regions across the retina entered into gap junctions with similar appearing processes. Horizontal fibers may represent efferents which originate in similar fibers sheathed in retinal glia and also seen in the optic nerve and lobe. They occasionally formeden passantsynapses on receptors which were characterized by clusters of small clear vesicles and increased membrane density. They also gave rise to branches whose terminals have not been directly traced. Some of these branches showed both clear and dense‐cored vesicles. Synapses on receptors had a cluster of small clear vesicles and one to three large dense‐cored vesicles which were not involved in the cluster. Increased density of the synaptic membrane was best seen after primary osmium fixation. Aldehydeosmium and direct osmium fixed material are presented for comparison. Fibers somewhat similar in appearance to horizontal fibers but which were smaller and more dense had regions filled with smaller dense‐cored vesicles which never showed clustering on a membrane or a consistent relationship with a putative post‐synaptic cell. The possible origins of the neurites of the retinal plexus are di

ISSN:0092-7317    

DOI:10.1002/cne.901470305

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1973

数据来源: WILEY

摘要:

AbstractAn ultrastructural investigation of the optic lobe of the squid,Loligo pealei, revealed that the optic fibers, like those ofOctopus, terminated in the outer plexiform layer in long carrot‐shaped bags containing small clear vesicles. The outer 4–5 μ of a terminal was devoid of conventional synapses but came into direct contiguity and specialized contacts with adjacent terminals. These specializations often included protrusions of finger‐like processes from one cell into another. At contact sites the intercellular space narrowed to a minimum of about 25 Å and immediately adjacent, in the cytoplasm of each contacting cell, was a sub‐membranal cistern. These contacts could be electrotonic junctions. The remainder of the receptor terminal received dendritic endings of post‐synaptic cells in deep invaginations. A pre‐synaptic accumulation of vesicles and increased membrane density was evident at these junctions. Neurons of the outer nuclear layer are described, as well as vertical fascicles of large pale neurites which pass through the outer nuclear and plexiform layers and which may include efferents to the retina. Specialized glial cells and a dense, intercellular material located in pools or clefts in certain regions of the cortex of the lobe a

ISSN:0092-7317    

DOI:10.1002/cne.901470306

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1973

数据来源: WILEY

ISSN:0092-7317    

DOI:10.1002/cne.901470301

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1973

数据来源: WILEY