The median ventricular formation. A distinct structure at the mesencephalic apex

A role for tectal midline glia in the unilateral containment of retinocollicular axons

Retinal fibers approach close to the tectal midline but do not encroach on the other side. Just before the entry of retinal axons into the superior colliculus (SC), a group of radial glia differentiates at the tectal midline; the spatiotemporal deployment of these cells points to their involvement in the unilateral containment of retinotectal axons. To test for such a barrier function of the tectal midline cells, we used two lesion paradigms for disrupting their radial processes in the neonatal hamster: (1) a heat lesion was used to destroy the superficial layers of the right SC, including the midline region, and (2) a horizontally oriented hooked wire was inserted from the lateral edge of the left SC toward the midline and was used to undercut the midline cells, leaving intact the retinorecipient layers in the right SC. In both cases, the left SC was denervated by removing its contralateral retinal input. Animals were killed 12 hr to 2 weeks later, after intraocular injections of anterograde tracers to label the axons from the remaining eye. Both lesions resulted in degeneration of the distal processes of the tectal raphe glia and in an abnormal crossing of the tectal midline by retinal axons, leading to an innervation of the opposite ("wrong") tectum. The crossover occurred only where glial cell attachments were disrupted. These results document that during normal development, the integrity of the midline septum is critical in compartmentalizing retinal axons and in retaining the laterality of the retinotectal projection.

Glial environment in the developing superior colliculus of hamsters in relation to the timing of retinal axon ingrowth

We have examined the developmental changes of glial cell organization in the superior colliculus of embryonic and neonatal hamsters in reference to the known sequence of retinal axon ingrowth and arborization in the midbrain. Immunolocalization of vimentin, a marker for neuronal and glial cell precursors, reveals a uniform distribution of radially oriented cells, with perikarya located at the ventricular surface and thin, elongated processes fanning out toward the pia. These vimentin-positive cells, referred to as the lateral radial cells, are present in the tectum from embryonic day (E) 10 (earliest day examined) until approximately postnatal day (P) 5. Vimentin expression in the lateral radial cells decreases markedly during the second week of postnatal life: application of DiI to the ventricular surface reveals that the pial attachment of the lateral radial cells is withdrawn and that the radial processes are gradually pulled back toward the ventricular zone. By P14, virtually no vimentin-positive radial cells are detectable in the superior colliculus. At no time during development are the lateral radial cells immunopositive for the glial fibrillary acidic protein (GFAP); however, shorter, vimentin-positive astrocytic profiles can be seen in the tectum around the time the radial fibers have been withdrawn, suggesting that at least some radial cells are transformed into astrocytes that will colonize the mature colliculus. At approximately E12, a second group of cells, referred to as the midline radial glia, is detected at the tectal midline. These cells are tightly bundled, forming a raphe in the tectum. They are intensely vimentin positive from E13 until at least P14. From the time of birth, the midline radial cells also exhibit intense immunoreactivity for GFAP. The lateral radial cells are present in the superior colliculus prior to and during the period of neurogenesis but remain well past the time when collicular neuronal migration is completed. Pial processes of the lateral radial cells are present within the superficial tectal layers during the time retinal axons are entering this target; they may be involved in directing the growth and initial collateralization of retinotectal axons. Their withdrawal from retinorecipient collicular zones begins at about the time arbors are being elaborated on retinal axons. In contrast, the midline glia become distinct just prior to the time retinal axons enter the superior colliculus and persist during the time retinotectal projections are being fully established. These raphe glia may be involved in maintaining the laterality of the retinotectal projection.

Postnatal development of vimentin-positive cells in the rabbit superior colliculus

The present study examined the postnatal development of the radial glia in the rabbit superior colliculus during the first 40 postnatal days. An antivimentin monoclonal antibody and the carbocyanine fluorescent tracer DiI were used in order to investigate the development of laminar connectivity in the superior colliculus. We focused our study on the superficial gray layer, the intermediate layer, and the deep layers of the superior colliculus, the periaqueductal gray matter (PAGM), and the medial intercollicular region. Vimentin-positive structures of glial lineage consisted of 1) the main radial system, which in the newborn rabbit was made up of wavy fibers that ran from the aqueduct to the pial surface, where they terminated in end-feet. At postnatal day 15, these fibers diminished to 100-200 microns long wavy tracts, which emanated from the aqueduct, and to a few straight or arched fragments in the superficial gray layer; 2) the median ventricular formation, which extends from mesencephalic aqueduct to the intercollicular sulcus, was characterized by a series of ascending, vimentin-positive fibers, some of large caliber, which persisted until postnatal day 40; 3) the tangential fiber system, which was made up of fibers that diverged from the median ventricular formation and of a number of short tracts running perpendicular to the periaqueductal radial fibers; these structures may provide support for migrating subpopulations of neurons; 4) immature and mature-like protoplasmic and fibrous astrocytes, which appeared during the second postnatal week. Thereafter, the number of vimentin-positive astrocytes decreased sharply. Our findings generally support earlier descriptions of the radial glia, except for the persistence, in superficial layers of the superior colliculus, of straight and curved fragments of fibers, which may participate in the organization of visual afferents at this level.

Dopaminergic cells align along radial glia in the developing mesencephalon of the rat

Studies were performed to examine the relation of dopaminergic cells and radial glia in the developing mesencephalon of the rat at ages E12-E20. Dopaminergic cells were immunolabelled with an antiserum which recognizes tyrosine hydroxylase, and radial glia were immunolabelled with a monoclonal antibody which recognizes vimentin. The vimentin-immunoreactive fibres of radial glia were noted at E12. At E12, and more clearly at later time points, the radial glia extended from the aqueduct to the pial surface, and this pattern persisted throughout the prenatal period. Tyrosine hydroxylase-immunoreactive cells were located along the ventral surface of the mesencephalon at age E13. At age E15, E16, and E18 the tyrosine hydroxylase-immunoreactive cells were present from the aqueduct to the ventral pial surface of the mesencephalon and were aligned along radial glia. Our study suggests that radial glia provide paths for migration of dopaminergic cells in the mantle layer from E15 to E18 of the developing mesencephalon. It also suggests that some dopaminergic cells between E15 and E18 may express tyrosine hydroxylase during their migration through the mantle layer and prior to reaching the location they occupy in the adult brain.

Organization of radial glia and related cells in the developing murine CNS. An analysis based upon a new monoclonal antibody marker

A monoclonal antibody, RC1, has been generated which provides a selective and sensitive immunohistochemical marker of radial glial cells and related cell forms during development of the mouse CNS. Beginning on embryonic day E10, immunocytochemistry performed on cryostat sections stains throughout the CNS a subpopulation of cells in the ventricular zone with radial processes that terminate with endfeet at the pial surface. These processes become fasciculated and attain maximal densities by E12-14 in the spinal cord and lower brainstem and by E14-16 in the midbrain, cerebellum and forebrain. Fasciculation is especially prominent for a subclass of these cells at the midline of the brainstem and spinal cord. As nuclear and cortical structures develop, the trajectories of the radial fiber fascicles undergo systematic and region-specific distortions in their initially simple linear configuration, in the process maintaining a consistent spatial registration of germinal ventricular zones with distal sites of assembly of post-migratory neurons. In the late fetal period, radial glial progressively disappear and scattered immature astrocytes bearing multiple fine processes appear in most regions of the CNS. In the spinal cord, a transitional unipolar radial form is identified in the emerging ventral and lateral funiculi between E13 and E17. In the cerebellum, precursors to the unipolar Bergmann glial cell are identified by E15, and in the retina, precursors of the bipolar Müller cell are identified by E16. Postnatally, RC1-stained radial glia become sparse, and after one week, immunoreactive cells include only ependymal cells, hypothalamic tanycytes, Bergmann glia, Müller cells, a unipolar radial form in the dentate gyrus, and a subpopulation of white matter astrocytes. These results suggest that radial cells of astroglial lineage comprise a diverse set of cell classes which subserve multiple functions in the developing and adult brain.

Analysis of differentiation and transformation of cells by lectins

During differentiation cells are known to change their biological behavior according to their genotype. This is thought to be accompanied by a modulation of cell surface determinants expressed on the outer cell membrane. Vice versa, cell surface molecules are suggested to mediate extracellular signals to the genome. Most of these molecules integrated in the cell membrane have been proven to be glycoconjugates. The carbohydrate moieties of these molecules can be detected by means of lectins that are characterized by their ability to react specifically with distinct terminal sugar sequences. Thus, lectins have been used as appropriate tools for studying the modulation of functionally important membrane-associated molecules during the differentiation of cells, in particular of B- and T-lymphocytes. Moreover, lectins have been proven to distinguish between differentiated cells and malignant cell clones, according to the hypothesis that transformed cells possess a glycoconjugate profile that corresponds to the stage of differentiation at which they are arrested. Since lectins, like monoclonal antibodies, make it possible to study functionally important molecules that are associated with differentiation and malignancy, they might be of value for diagnostic purposes and, moreover, for analyzing malignant transformation.

The first appearance of the future cerebral hemispheres in the human embryo at stage 14

Thirty-five embryos of stage 14 (32 days) were studied in detail and graphic reconstructions of four of them were prepared. Characteristic features of this stage include the beginning formation of the future cerebral hemispheres and the cerebellar plates. The ventral boundary between telencephalon medium and diencephalon is the preoptic recess. Although a velum transversum is not yet distinguishable as a dorsal boundary, its site is indicated by a change in the thickness of the roof of the forebrain. As the cerebral vesicles (future hemispheres) begin to evaginate, a di-telencephalic sulcus and a corresponding lateral ventricle and ventricular ridge (torus hemisphericus) develop. The telencephalic wall is mainly ventricular layer but three areas show advanced differentiation: olfactory area, future amygdaloid body (which lies at first mainly in the diencephalon), and primordium of the hippocampus. The telencephalon is growing in length, and the forebrain now occupies almost one quarter of the total length of the brain. The two neuromeres of the diencephalon are no longer as clearly delineated. The floor of D1 presents a thickened chiasmatic plate; that of D2 includes the infundibulum, which is closely related to the adenohypophysial pouch. The ventricular surface of D1 presents elevations for the dorsal and ventral thalami, separated by the sulcus medius. Other features of the diencephalon include the ventricular eminence (medial ventricular ridge) of the basal nuclei and the hypothalamic cell cord, from which the preopticohypothalamotegmental tract arises. The roof of D2 contains the evaginating part of the synencephalon. The mesencephalic angle continues to diminish. Two neuromeres, M1 and M2, are still distinguishable. The oculomotor nucleus emits nerve fibres, as does also the trochlear nucleus, which lies in the isthmic segment. Some extracerebral oculomotor fibres are present, but decussating and extracerebral trochlear fibres have not yet appeared. In the region of the tectum, two nuclei are discernible, and will form the medial tectobulbar tract and the mesencephalic root of the trigeminal nerve, respectively. The medial longitudinal fasciculus is present. A "median ventricular formation" is sometimes found in the mesencephalic roof. The cerebellum is the widest part of the brain. Two neuromeres (isthmic segment and Rh1) are involved in its formation. Most of the cerebellar plate has differentiated an intermediate layer, and the future rhombic lip is discernible. Indications of an efferent fibre system are present. In addition to the cerebellum, the rhombencephalon includes Rh1 to Rh7, and RhD.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer ranking of the sequence of appearance of 73 features of the brain and related structures in staged human embryos during the sixth week of development

The sequence of events in the development of the brain in human embryos, already published for stages 8-15, is here continued for stages 16 and 17. With the aid of a computerized bubble-sort algorithm, 71 individual embryos were ranked in ascending order of the features present. Whereas these numbered 100 in the previous study, the increasing structural complexity gave 27 new features in the two stages now under investigation. The chief characteristics of stage 16 (approximately 37 postovulatory days) are protruding basal nuclei, the caudal olfactory elevation (olfactory tubercle), the tectobulbar tracts, and ascending fibers to the cerebellum. The main features of stage 17 (approximately 41 postovulatory days) are the cortical nucleus of the amygdaloid body, an intermediate layer in the tectum mesencephali, the posterior commissure, and the habenulo-interpeduncular tract. In addition, a typical feature at stage 17 is the crescentic shape of the lens cavity.

Development of cell and fiber lamination in the mouse superior colliculus

The emergence of laminar organization in the superior colliculus was investigated in the mouse with several anatomical methods, including tritiated-thymidine autoradiography, Golgi impregnation, and general stains for cell bodies and for fibers. The sequence of neurogenesis, cell migration, and early morphological differentiation of neurons was shown to exhibit a discontinuity between the lower and upper divisions (i.e., between the deep and intermediate "gray" and "white" layers and the superficial "gray" and "white" layers). These events proceed in an inside-out order within the lower division, but the same events within the upper division commence in advance of the completion of this progression. Thus, peak generation times for layers of the lower division proceed from (embryonic day) E11 to E13 and for the upper division from E12 to E13. Cell migration, as monitored with tritiated-thymidine labelling, reflects closely the pattern of cytogenesis. This is most clearly evident on E15 when a population of E11-labelled cells is divided into superficial and deep layers (the strata superficiale and profundum--SS and SP) by the interposition of E13-labelled cells at an intermediate level (stratum intermedium--SI). A contingent of the latter cells continue their migration and join their predecessors within the SS on E17, a time point when cell migrations are largely complete. Paralleling this sequence of arrival of neurons and the formation of three primary layers, both the time course of accumulation of fiber fascicles and the early morphological differentiation of neurons in the interval from E13 to E17 tends to proceed from SP to SS and from SS to SI. Thus, the transverse fiber system and large multipolar neurons of SP develop in advance of the longitudinal fiber system and vertically oriented neurons of SS, which in turn develop precociously with respect to the longitudinal fibers and medium-sized multipolar neurons of SI. In contrast, later events of differentiation that underly a major radial growth and an architectonic sublamination of the primary strata proceed in a simpler inside-out sequence from E17 to (postnatal day) P6. The major morphogenetic events underlying the establishment of statification in the colliculus appear to involve the operation of relatively independent programs of assembly for the two basic subdivisions. It is probable that selective cell-cell interactions contribute to the delivery of concurrently generated neurons to different laminae as well as to the deployment of axons in a manner that respects laminar boundaries.