Facundo Valverde

Apical dendritic spines of the visual cortex and light deprivation in the mouse

SummaryThe effects of light deprivation on the number of apical dendritic spines have been studied in the visual cortex of the mouse. In the portion of the apical dendrites of layer V-pyramidal cells traversing layer IV, dendritic segments of 50 μ in length from different cells were selected. The number of spines on each of 50 different segments per animal was counted. The countings were done in the areae striata and temporalis prima from mice raised in complete darkness since birth up to 22–25 days old. The observations were compared with the countings obtained in the areae striata and temporalis prima from mice raised under normal conditions. The results indicated that mice raised in darkness had a significant reduction in the number of spines per dendritic segment at the level of layer IV in area striata when compared with control animals. No significant difference was found in the number of spines per dendritic segment in layer IV between both groups of normal and dark raised mice in the area temporalis prima. The mean number of dendritic spines per consecutive segments along complete apical dendrites of layer V-pyramidal cells in area striata has been found to increase exponentially with the distance from the cell body. The same exponential relation, but with somewhat lower values, was obtained in the apical dendrites in area striata in mice raised in darkness. The significance of these findings were discussed. It was concluded that: First, visual sensory deprivation affect the fine structure of the central nervous system. Second, the results observed support the assumption that structural changes in the nerve cells occur as the result of experience.

Structural changes in the area striata of the mouse after enucleation

SummaryThe optic pathways of the mouse have been studied by tracing of degenerating fibers after enucleation and coagulation of the lateral geniculate nucleus. The effects of unilateral enucleation at birth in the contralateral area striata of the mouse have been studied with the Golgi method. The number of spines on three different portions of the apical dendrites of layer V pyramidal cells have been counted in the affected area striata of mice 24 and 48 days old enucleated at birth. The results were compared with the countings obtained in the area striata homolateral to the enucleated side and with controls of the same ages. The results indicated that enucleation produces, through a series of transneuronal changes, significant diminution of the number of spines in the apical dendritic segments located in layer IV. The diminution of dendritic spines is more pronounced in younger animals. Specific variations in the orientation of dendrites of stellate cells with ascending axons have been observed in enucleated animals. The significance of these findings has been discussed suggesting the existence of compensatory mechanisms which affect significantly the intrinsic organization of the area striata.

Formation of an olfactory glomerulus: morphological aspects of development and organization.

We have studied the development of olfactory nerves in the rat from their first contact with the telencephalic vesicle until the formation of glomerular structures in the olfactory bulb at early postnatal period. The study is based on serial semithin and ultrathin sections of material prepared for electron microscopy and antibodies to label radial glial cells, glial fibrillary acidic protein and Rat-401. Beginning on embryonic day 12, developing olfactory axons from the olfactory placode are accompanied by migratory cells, also derived from the olfactory placode, that reach the prospective olfactory bulb by embryonic day 13. The mass of migratory cells accumulate superficial to the telencephalic vesicle. The cells increase in number by mitotic divisions. The majority of these cells represent precursor elements that will later develop into the ensheathing cells of the olfactory nerves and olfactory nerve layer of the adult. Some migratory cells penetrate into the prospective olfactory bulb early during development. The first synaptic contacts of olfactory axons with dendritic processes in the olfactory bulb were observed at embryonic day 18. Glomerular formation is initiated by penetration of cells from the migratory mass into the prospective glomerular layer by embryonic day 20 to postnatal day 0. These cells form walls surrounding zones of high synaptic density forming protoglomeruli. Postnatally, the peripheral processes of radial glial cells branch profusely delimiting glomerular formations and transform into periglomerular astrocytes. Rat-401 stains radial glial cells from embryonic day 14. Immunoreactivity becomes restricted to the olfactory glomeruli during the first postnatal weeks and it virtually disappears by the end of the first postnatal month. We conclude that the early penetration of cells from the migratory mass into the prospective olfactory bulb, observed immediately after the first synaptic contacts were established, initiates the formation of olfactory glomeruli which becomes completed by the transformation of radial glial cells into periglomerular astrocytes.

Rate and extent of recovery from dark rearing in the visual cortex of the mouse

Abstract (1) A number of dendritic spines (those found before opening of eyes) in the apical dendrites of pyramidal cells of the visual cortex develop through the induction of morphogenetic agencies. The growing of these spines is not dependent on the presence or absence of visual stimuli. (2) A second group of dendritic spines depends on normal arrival of visual impulses after the spontaneous opening of the eyes. (3) If the animals are kept in darkness during the time of eye-lid opening the the second group of spines does not develop, and/or possibly a third group of spines grows in response to non-visual stimuli. The total number of spines will then never reach normal values. (4) If the animals are removedfrom darkness after lid opening and are allowed to live under normal conditions, a permanent numerical loss in spines persists in some of the apical dendrites, while in others the number of spines reaches normal average values. (5) The recovery of dendritic spines for some apical dendrites is noticeable two days after removal from darkness.

Transitory population of cells in the temporal cortex of kittens

A correlated study, using conventional electron microscopy and the Golgi method in the subplate layer and lower part of layer VI, in the developing temporal cortex of kittens has been made. Neuronal death was identified in semithin sections and with the electron microscope. The existence of presumptive degenerating cells in Golgi preparations has been suggested. This transitory population of early-generated subplate cells might be involved in the organization of cortical afferents before the final targets have completed their migrations.

Transient pattern of exuberant projections of olfactory axons during development in the rat

The purpose of our study was twofold: (1) to trace the development of the olfactory axons from early embryonic stages until the mature pattern of connectivity and (2) to determine whether a transient penetration of them exists beyond the olfactory glomeruli. Two techniques were employed: DiI applied in the olfactory epithelium after aldehyde fixation, and olfactory marker protein (OMP) immunostaining. At E13 and E14 olfactory axons were observed spreading over the telencephalic vesicle and entering deeply into the prospective olfactory bulb, extending near the ventricular zone. Growth cones were seen at the end of these axons. At E15, the bundles of olfactory axons form a network, in which axons, growth cones and cells were seen. Some of these axons entered the olfactory bulb. Using OMP immunostaining olfactory axons were observed along the external plexiform layer, the mitral cell layer and in the granular layer from E19 to P6. At P9 some OMP immunoreactive axons were observed in the external plexiform layer. No OMP immunostained axons could be observed outside the glomeruli at P10. Our conclusions are that a transient immature pattern of early invasion over the telencephalic vesicle and of the olfactory bulb by olfactory axons occurs in the olfactory system. By the second postnatal week the glomerular layer reaches its mature configuration, and no olfactory fibers are seen outside the glomerular layer.

Evidence for intrinsic development of olfactory structures in Pax-6 mutant mice.

It has been reported that the arrival of primary olfactory axons is required to induce the development of the olfactory bulb (OB). On the other hand, the SeyNeu/SeyNeu mutant mouse (Small eye) has been previously described as a model for the absence of olfactory bulbs, owing to the lack of olfactory epithelium (OE). In the present report, we take advantage of this mutant and study a neural structure in the rostral pole of the telencephalon that phenotypically resembles the prospective OB. We named this formation olfactory bulb‐like structure (OBLS). We also report the occurrence, in the mutants, of small epithelial vesicles in the malformed craneofacial pits, resembling an atrophic OE, although a mature olfactory nerve was not identified. Axonal tracing, birthdating, immunohistochemistry, and in situ hybridization using antibodies and probes expressed in the olfactory system, indicated that two distinct structures observed in the OBLS correspond to the main and accessory olfactory bulbs of the control mouse. We propose that the OBLS has developed independently of the external influences exerted by the olfactory nerve. The presence of a prospective OB in the mutants, without intervening olfactory fibers, suggests that intrinsic factors could define brain territories even in absence of the proper afferent innervation. The intrinsic mechanisms and environmental cues in the telencephalon could be sufficient to promote axonogenesis in the projection neurons of the OB and guide their axons in a lateral prospective tract, in the absence of olfactory axons. J. Comp. Neurol. 428:511–526, 2000.

Dynamic architecture of the visual cortex

Abstract Several aspects of the organization of the visual cortex in the mouse have been examined in Golgi preparations. Special attention has been devoted to the afferent systems innervating the basal dendrites of pyramidal cells of layers III and V. A computer analysis using matrix algebra of the projected dendritic field of the basal and apical-collateral dendrites of pyramidal cells of layers III and V of the visual cortex has been carried out. The data were obtained from Golgi preparations from three age groups of normal and neonatally enucleated mice. Statistical comparisons of the dendritic fields showed, in enucleated mice, the existence of decreased dendrite density in an area of the dendritic field below the cell body, bounded by two lines forming an angle of 90° with vertex in the cell body and bisector perpendicular to the surface of the brain. The dendrite diminution is more pronounced in younger enucleated subjects. It is also more intense in pyramidal cells of layer III. The possible dependence of these differences on different innervations of basal dendrites has been discussed.

THE TELENCEPHALIC VESICLES ARE INNERVATED BY OLFACTORY PLACODE-DERIVED CELLS: A POSSIBLE MECHANISM TO INDUCE NEOCORTICAL DEVELOPMENT

During early embryonic development, the olfactory placode is the source of different cell types migrating toward the telencephalic vesicle. Among these cell types are the ensheathing cells, the luteinizing hormone-releasing hormone-producing cells and the olfactory marker protein-immunoreactive cells. We have identified a novel group of olfactory placode-derived migratory cells using an antibody against beta-tubulin to label neurons and acetylcholinesterase histochemistry to label posmitotic cells. In this paper we describe the morphology, migration and fate of this novel group of cells. The first neurons detected in the rostral prosencephalon with acetylcholinesterase and anti-beta-tubulin antibody are localized in the olfactory placodes at embryonic day 11 in the rate. At embryonic day 12, anti-beta-tubulin antibody-positive cells were observed in the mesenchymal tissue between the olfactory pit and the rostral pole of the telencephalic vesicle. Anti-beta-tubulin antibody-positive cells were seen running superficially over the pial (dorsal) side of the telencephalic vesicle at embryonic day 13. The majority of these cells have a bipolar profile with short leading and trailing processes, suggesting that they are migratory elements. However, some of these cells showed elaborate processes extending for quite long distances, overlying the pial surface of the telencephalic vesicle. A mass of cells extending over the telencephalic vesicle from the developing olfactory epithelium were observed at embryonic day 13 using acetylcholinesterase histochemistry. Some of these acetylcholinesterase-positive cells were identified as neurons with the specific neuronal marker anti-beta-tubulin antibody. On embryonic day 12, neurons from the olfactory epithelium send axonal fibers toward the telencephalic vesicles. Most of these fibers spread over the anteroventral pole of the vesicles but others entered deep into the telencephalon, reaching the germinal ventricular zone. We also show that fibers run rostrocaudally over the surface of the telencephalic vesicles. We suggest that these cells and fibers, apparently originating in the olfactory placode and migrating through non-conventional routes, might play a significant role in the earliest stages of telencephalic vesicle development.

Dendritic spines in the visual cortex of the mouse: Introduction to a mathematical model

SummaryThe spines of apical dendrites of the layer V pyramidal cells of the area striata in the mouse represent a sequence of post-synaptic structures receiving a variety of contacts from terminal fibers derived fundamentally from short axon cells and superficial pyramidal cells. The study of Golgi preparations of mice 180 days old shows the existence of the most complicated terminal structures over portions of apical dendrites at the levels of layers III and IV. Observations on young mice reveals the terminations of the specific afferent fibers on the dendrites of short axon cells. A mathematical model which defines the distribution of spines along the apical dendrites is introduced. The principal equation of the model has been adjusted from the data processing of microscope countings through a series of programs written for an IBM 7070. The equation defines satisfactorily the different distributions of dendritic spines in mice 10–180 days old raised in normal conditions and in complete darkness. The equation defines also the distribution of dendritic spines in the visual cortex of mice enucleated at birth on one side, and the distribution along the apical dendrites of various cortical areas of the hamster, cat and man. The number of dendritic spines increases with the age of the subject and their distribution varies significantly according to the values of the parameters of the model.