Interhemispheric connections between primary visual areas: beyond the midline rule

In the last five years, a number of detailed anatomical, electrophysiological, optical imaging and simulation studies performed in a variety of non-human species have revealed that the functional organization of callosal connections between primary visual areas is more elaborate than previously thought. Callosal cell bodies and terminals are clustered in columns whose correspondence to features mapped in the visual cortex, such as orientation and ocularity, are starting to be understood. Callosal connections are not restricted to the vertical midline representation nor do they establish merely point-to-point retinotopic correspondences across the hemispheres, as traditionally believed. In addition, anatomical studies have revealed the existence of an ipsilateral component of callosal axons. The aim of this short review is to propose how these new data can be integrated into an updated scheme of the circuits responsible for assembling the primary visual field map

Extracellular matrix molecules play diverse roles in the growth and guidance of central nervous system axons

Axon growth and guidance represent complex biological processes in which probably intervene diverse sets of molecular cues that allow for the appropriate wiring of the central nervous system (CNS). The extracellular matrix (ECM) represents a major contributor of molecular signals either diffusible or membrane-bound that may regulate different stages of neural development. Some of the brain ECM molecules form tridimensional structures (tunnels and boundaries) that appear during time- and space-regulated events, possibly playing relevant roles in the control of axon elongation and pathfinding. This short review focuses mainly on the recognized roles played by proteoglycans, laminin, fibronectin and tenascin in axonal development during ontogenesis

Topographic organization in the anterior commissure of developing hamsters

The present study has examined the existence of a topographic organization in the anterior commissure (AC) of developing hamsters. Fluorescent carbocyanine crystals (DiI and/or DiA) were implanted into different rostrocaudal and dorsoventral sectors of the paleocortex of hamsters ranging in age from E15 to P10 (E16 = P1 = date of birth). The cerebral hemispheres of each brain were cut horizontally and sagittally, respectively, and the sections were observed under a fluorescence microscope coupled to a computerized reconstruction system. A distinct topographic organization of AC fibers was observed along the rostrocaudal axis starting at E15, and continued unchanged thereafter. These results support the hypothesis that the orderly pattern of AC fibers is achieved by active positioning during the first days after crossing the midplane rather than by a regressive sculpting from an initially disorganized pattern.

Pioneer axons in the anterior commissure of hamster embryos

The existence and the morphological characteristics of pioneer axons in the anterior commissure (AC) were investigated in hamster embryos using fluorescent carbocyanine crystals implanted into the paleocortex. On embryonic days 12-13(E12-E13), a group of tortuous fibers was seen close to the midline. After this group of fibers reached the midline on E13.5-E14, pioneer axons were seen in the AC, and from E14.5 to E15.5 both pioneers and followers approached the targets, the former ahead of the latter by about 500 µm. Arborization took place as soon followers approached the targets, the former ahead of the latter by about 500 µm. Aarborization took place as soon as the followers reached the targets (E15.5-E16). The pioneer cells were labeled retrogradely and were seen in the most superficial layears of the paleocortex, along the rostrocaudal extent of the hemisphere. The fact that pionners appear only during or after crossing suggests the existence of factors either in the midline region and/or in the opposite hemisphere, which selectively accelerate the growth of the pioneers or decelerate the growth of the followers

The effects of total and partial callosal agenesis on the rotatory behavior of Balb/cCF mice

We report a study on rotational behavior (free swin test) of 30 adult male mice of an inbred strain in which about 20% of the animals are born with callosal defects (the BALB/cCF strain). The experiment was designied to study the relationship between the development of the corpus callosum and the degree of lateralization in rotatory behavior. Most of the BALB/cCF mice (77%) were lateralized rotators. Although most animals with a normal callosal area presented individual asymetry, there was no signficant tendency for a populational asymetry toward one side in this group. In contrast, 75% of the animals with abnormally callosal areas were left rotators. These animals, therefore, presented not only individual asymetry, but also a tendency for a populational asymetry. It was concluded that disturbances in the development of the corpus callosum are related to the appearence of directional populational asymetry in rotatory behavior

Development of interhemispheric connections through the anterior commissure in hamsters

The development of interhemispheric connections through the anterior commissure was studied in hamsters by use of the postmortem fluorescent tracer diI. Labelled commissural axons were seen to aaproach midline and cross it on E14 (E1, day of conception), extend into the opposite hemisphere and rech the terminal targets on E15, and start to arborize on E16. OnP1(day of birth), a few labelled neurons could already be seen in the anterior olfactory nucleous, olfactoru tubercle, piriform and insular cortices, nucleus of the lateral olfactory tract, bed nucleous of the stria terminalis, amygdaloid complex, temporal, perihinal and entorhinal cortices. In all these regions, labelled neurons became progressively more numerous on P3, P5 and P10, but their distribution did not change. Since no evidence of topographical exuberance of connections could be found, it is hypothesized that the development of anterior commissure connections is entirely progressive, lacking the regressive events that characterize callosal ontogenesis