摘要:

AbstractThe effect of complete and partial limb bud removal on motoneuron survival was studied inXenopus laevistadpoles. Amputation of both hind limb buds at stage 49 before limb innervation begins caused the subsequent death of all motoneurons. This result confirms that there are no exceptions to the rule that developing motoneurons must contact the limb to survive. Partial removals of the limb at early stages of innervation caused the subsequent death of motoneurons that normally project to the deleted segments while motoneurons for the remaining segments survived. Before dying, the motoneurons deprived of their normal targets invaded the remaining limb segments thereby forming erroneous projections. Since the motoneurons died despite contacting limb tissue, it is concluded that they must be dependent on contact with specific limb regions and that errors of projection lead to death. Since large‐scale errors of the type induced by this experiment affect only a minority of developing motoneurons in normal embryos, the possibility of small‐scale errors is discussed in an attempt to equate all naturally occurring motoneuron death with error correct

ISSN:0092-7317    

DOI:10.1002/cne.902030202

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe ontogeny of the somatostatin (SRIF) neuron system in the lower brainstem of the rat was analyzed using the indirect immunofluorescence technique of Coons.SRIF‐positive structures first appeared in the primordium of the ventral nucleus of the lemniscus lateralis of the rat fetus corresponding to gestational day 15 (12–14‐mm embryos). On and after gestational day 15, these structures appeared in progressively more diverse areas of the lower brainstem and continued to show an increase in number and intensity up till birth. The maximum SRIF‐positive structures were histochemically identified in the lower brainstem at the perinatal stage. However, after birth, the numbers of SRIF‐positive structures in the lower brainstem began to decrease as the rat grew and none or only a few SRIF‐positive structures were detected in the adult rats. This ontogenetical study of the SRIF neuron system in the lower brainstem of the rat strongly suggests that SRIF might play an important role in the development of the lower brainstem other than in its neurotransmitter or neuromodulat

ISSN:0092-7317    

DOI:10.1002/cne.902030203

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe anterograde transport of3H proline and of horseradish peroxidase has been used to study the retinogeniculate pathway in normal adult ferrets and in young ferrets during postnatal development. The lateral geniculate nucleus in adults shows a characteristic “carnivore” pattern, with layers A, A1, C, C1, C2, and C3, and a medial interlaminar nucleus recognizable either cytoarchitectonically or on the basis of the retinogeniculate innervation. In addition, there is a well‐defined, rather large perigeniculate nucleus.At birth the lateral geniculate nucleus is unlaminated and essentially all parts are reached by afferents from both eyes. The crossed component is by far the larger. It extends from the optic tract medially well into the perigeniculate field, in contrast to the uncrossed component which barely reaches the perigeniculate field. During the first 3 postnatal days the uncrossed fibers restrict their arbors to a small posterior and medial region, the precursor of the binocular segment of the nucleus. The crossed fibers gradually retreat from the region within which the uncrossed fibers have concentrated. Between the fourth and eighth postnatal days the field occupied by the ipsilateral component expands again to form a major focus that will define lamina A1 and a minor focus that will define C1. At this stage the crossed and the uncrossed fibers overlap at the borders of lamina A1 and the whole region of lamina C1 is also occupied by arbors of the crossed component. The perigeniculate field becomes clearly distinguishable from the lateral geniculate nucleus and the medial interlaminar nucleus is becoming clearly recognizable between days 3 and 8.Between days 8 and 15 the cytoarchitectonic borders between layers A and A1 become clearly defined, but the retinogeniculate axons from each eye still extend across this border. These axons retreat into their appropriate lamina after the 15th postnatal day and the nucleus reaches its essentially adult structure by about the fourth postnatal week. Segregation of retinofugal axons in the C layers occurs after segregation in the A layers, but many of the cells within the C layers show signs of cytological maturity earlier than those of the A layers. The nucleus undergoes a series of migrations and changes of shape as the ipsilateral and contralateral components become segregated. Whereas in the newborn the nucleus is roughly comma‐shaped and on the lateral aspect of the dorsal thalamus, in the adult it is “L”‐shaped and mainly on the posterior aspect of the do

ISSN:0092-7317    

DOI:10.1002/cne.902030204

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

年代:1981

数据来源: WILEY

摘要:

AbstractA paralemniscal zone in the lateral midbrain tegmentum of the cat has been identified in a possible pathway from the superior colliculus to the facial nucleus that may control pinna movements (Henkel and Edwards, '78). Other brainstem afferent projections to this paralemniscal zone have been mapped in the present study using the retrograde horseradish peroxidase tracing method and are discussed in three groups. First, potential sources of auditory afferents were limited mainly to the external nucleus of the inferior colliculus, the nucleus sagulum, and the dorsomedial periolivary cell group. Labeled cells in other superior olivary regions and the dorsal cochlear nucleus were apparently related to uptake of horseradish peroxidase from the axons of the lateral lemniscus. Second, afferents from several premotor regions involved in aspects of gaze control were identified. These were mainly from the nucleus prepositus hypoglossi and adjacent pontine reticular formation, butalso included projections from the medial vestibular and abducens nuclei and possibly subthalamic regions such as the zona incerta and fields of Forel. Third, a relatively large group of midbrain afferents was closely related to the origin of the collicular projection to the paralemniscal zone. This group consisted of labeled cells in the periaqueductal gray matter, nucleus cuneiformis, and pretectum. The relatively sparse labeling in auditory regions that projected to the paralemniscal zone seems to indicate that the sensorimotor integration necessary to guide pinna movements does not take place primarily in the lateral midbrain tegmentum. The interaction of gaze‐related sources with the pinna‐related pathway is also discus

ISSN:0092-7317    

DOI:10.1002/cne.902030205

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

年代:1981

数据来源: WILEY

摘要:

AbstractExperiments were performed to determine (1) under what conditions early brain surgery can cause sensory afferents to the thalamus to form connections at abnormal thalamic sites and (2) the extent to which such ectopic projections are receptotopically organized.In newborn Syrian hamsters, two of the retina's principal synaptic targets, the superior colliculus and dorsal lateral geniculate nucleus, were destroyed, respectively, by a direct lesion and by retrograde degeneration following a lesion of the occipital cortex. In the same brains, alternative terminal space for the retinofugal axons was made available in auditory (medial geniculate) or somatosensory (ventrobasal) thalamic nuclei by lesions of ascending auditory or somatosensory pathways, respectively; additional terminal space was made in the lateral posterior nucleus by degeneration of afferents from the superior colliculus. The projections of the contralateral retina were traced in neonatally operated adults by making one or two small peripheral retinal lesionsandintraocular injections of3H‐proline 5 days and 1 day, respectively, prior to sacrifice.The neonatal surgery reliably produced anomalous crossed retinal projections to the partially deafferented structures. These projections terminate preferentially at the nuclear surfaces.Computer reconstructions from serial sections demonstrated several signs of spatial order suggestive of receptotopic organization in the anomalous retinothalamic projections. In order of increasing stringency, these signs (which are not mutually exclusive) are: (1) In each nucleus, a restricted retinal sector gives rise to a limited part of the abnormal projection. (2) In each nucleus, different parts of the retina give rise to different parts of the anomalous projection. (3) In each nucleus, there is a more or less consistent polarity of the anomalous connection.Each small retinal sector appears to be represented along a “line of projection” in each of its abnormal thalamic targets, as it normally is in the dorsal and ventral lateral geniculate nuclei and in the superior colliculus.In some brains, some of the abnormal projections produce only a partial representation of the retina. However, in a single animal, a retinal sector not represented in the anomalous projections to one nucleus can contribute to the abnormal connections with another nucleus.In additional experiments, an attempt was made to direct developing auditory and somatosensory fibers normally terminating in the medial geniculate and ventrobasal nuclei, respectively, to anomalous thalamic targets. The axons were deprived of some of their normal thalamic sites of termination and alternative terminal space was made available in another thalamic sensory nucleus. These experiments failed to produce reliable evidence of ectopic auditory or somatosensory thalamic projections.The anomalous retinal projections to nuclei that normally recieve little (lateral posterior) or no (medial geniculate, ventrobasal) optic tract input, show that the preference of retinal axons for their normal targets is relative, not absolute. The orderliness of the ectopic projections opposes the hypothesis that the formation of retinotopic connections depends upon the matching of a set of signals distributed among the retinofugal fibers and a corresponding set of cues unique to the normal terminal fields of optic axons. The results are consistent with the formation of receptotopic connections by interactions among developing axons and suggest the action of additional factors that determine the terminal sites and organization of central neuronal connec

ISSN:0092-7317    

DOI:10.1002/cne.902030206

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

年代:1981

数据来源: WILEY

摘要:

AbstractContrary to previous assumptions, we have found that a single dose of3H‐thymidine (25 μCi), injected into the yolk sac of White Leghorn chick eggs on 2 days of incubation (d.i.) only remains available for DNA‐synthesizing (proliferating) cells for 48 hours following the time of injection. This finding now makes it possible to date the time of neuronal origin in the avian embryo using a single injection of isotope and a long survival time (30 days posthatch) as in mammalian studies where3H‐thymidine is only available as a short “pulse.” Using this method, we have determined that neurons in the chick locus coeruleus (LC) cease proliferation on 2–6 d.i. with a peak of neuronal genesis on 3–5 d.i. In addition, neuronal genesis is not homogeneous throughout the LC cell population, but occurs in a predominantly caudorostral gradient. Conversely, the cerebellar Purkinje cells cease division on 3–8 d.i. with a peak of heavy labeling on 4–6 d.i., 1 day later than that

ISSN:0092-7317    

DOI:10.1002/cne.902030207

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

年代:1981

数据来源: WILEY

摘要:

AbstractAn attempt has been made to characterize the photoreceptors and neurons of the frontal organ ofRana pipiensand the synaptic contacts of these cells, in order to provide morphological correlates for previous physiological findings. Neurons, glial cells, and photoreceptors with two different appearances can be seen in normal anatomical preparations. The photoreceptors make ribbon synaptic contacts onto both neurons and other photoreceptors. Retrograde transport of either cobalt or horseradish peroxidase through the frontal nerve results in labeling of neurons which are often located near the exit point of the frontal nerve from the frontal organ. Some of the dendritic processes of these neurons ramify among the neurons themselves, while others ramify among the photoreceptors. In addition, the frontal organ contains two photoreceptor types which synaptically contact both neurons and other photoreceptors. In some frontal organs labeled with cobalt (but never in peroxidase‐labeled preparations) certain photoreceptors were also labeled. The labeled photoreceptors were probably the result of transcellular cobalt movement from labeled neurons, and not the result of some photoreceptors having axons in the frontal nerve. These results are discussed in conjunction with physiological evidence that chromatic interactions occur within single photoreceptors, each of which contains a single visual pigment with two physiologically active state

ISSN:0092-7317    

DOI:10.1002/cne.902030208

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

年代:1981

数据来源: WILEY

摘要:

AbstractThe failure of axonal regeneration in the transected spinal cord of mammals has been attributed to many factors, including an intrinsic lack of regenerative capacity of mature CNS neurons, mechanical obstruction of axonal elongation by glial‐connective tissue scars, necrosis of spinal tissue resulting in cavitation, lack of trophic influences sufficient to sustain outgrowth, and contact inhibition resulting from the formation of aberrant synapses. Assessment of the relative importance of each of these factors requires animal models in which one or more of these pathological processes can be eliminated. We therefore examined the effects of spinal transection in the hibernating animal because, during hibernation, collagen formation is depressed while nerve regeneration and slow axonal transport are maintained. Midthoracic spinal transections were performed in hibernating ground squirrels and the spinal cords were examined histologically 1–6 months later. The lesion site was composed primarily of a loose accumulation of macrophages and showed minimal glial and collagenous scarring, or cavitation. There was extensive regeneration of intrinsic spinal cord and dorsal root fibers. These axons grew to the margin of the lesion where they turned abruptly and continued growing along the interface between the lesion and the spinal cord. We conclude (1) that mammalian spinal‐cord neurons have considerable regenerative potential; (2) that such mechanical impediments as collagenous and glial scarring, cyst formation, and cavitation cannot provide the sole explanation of why regeneration in the mammalian CNS is abortive; and (3) that specific physical and chemical properties of the cells in the environment of the growth cone regulate the extent and orientation of regenerative axonal outg

ISSN:0092-7317    

DOI:10.1002/cne.902030209

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

年代:1981

数据来源: WILEY

ISSN:0092-7317    

DOI:10.1002/cne.902030210

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

年代:1981

数据来源: WILEY