Morphological and electrophysiological studies were performed on neocortices of adult Connexin 32 (Cx32)-deficient mice and wild-type mice to investigate the consequences of a lack of the gap junction subunit Cx32 on neocortical structure and function. Morphometrical analysis revealed a reduced volume fraction of myelin within the neuropil and a decreased thickness of the axonal myelin sheaths in the neocortex of Cx32-deficient mice. Intracellular recordings from neurons in neocortical slice preparations provided evidence for an increased membrane input resistance in neurons of Cx32-null mutant mice as compared to neurons of wild-type mice. Consequently, neurons of Cx32-deficient mice displayed an enhanced intrinsic excitability. In addition, approximately 50% of the neurons investigated in slices of Cx32-deficient mice responded to afferent stimulation with delayed and large glutamatergic excitatory postsynaptic potentials resembling paroxysmal depolarizations. GABAergic inhibition sufficient to efficiently control synaptic excitability was virtually absent in these cells. The changes in intrinsic membrane properties observed in neurons of Cx32-null mutant mice were independent of the alterations in synaptic function, since increased membrane resistances were observed also in neurons with normal synaptic response pattern. Thus, in the neocortex, lack of Cx32 correlates with myelination defects, alterations in intrinsic membrane properties and dysfunction of inhibitory synaptic transmission.

This paper presents morphological (light- and electron-microscopical) evidence for the role of the mesonephros in contributing cells to the differentiating indifferent gonad and, after sexual differentiation, to the testis. A continuous process is revealed during which segregation of cells occurs from the developing and regressing mesonephros. Additionally, the complementary role of the coelomic epithelium in gonadal ridge and testis formation is demonstrated. The differentiation of testicular cords, their remodelling from a primary reticulum, and the composition and further change of the cellular content during the period after sexual differentiation is described using a computer-aided three-dimensional reconstruction system. Apart from these morphogenetic events, cytodifferentiation in the somatic cells of the indifferent gonad and of the early differentiated testis is demonstrated using indirect immunofluorescence in combination with monoclonal antibodies to the intermediate filament proteins keratin 8 and 18 and vimentin. The immunohistochemical results show that different forms of cytodifferentiation coexist among the somatic cells present in the indifferent gonad and in the testis early after sexual differentiation.

Time schedule and mechanisms of the appearance of dendrite bundles in laminae IV and II/III of the visual cortex have been investigated in the rabbit from the first appearance of the cortical plate during fetal development up to adult stages. Sections cut either perpendicularly or tangentially to the cortical surface were used for light- and electron-microscopic analysis. Dendrite bundles appear during the late fetal period and the first postnatal days in a biphasic process. The first step takes place during late fetal development. Due to migration of neuroblasts along radial processes of glial cells, column-like compartments of neuropil are formed in laminae II-IV which contain the apical processes of the nerve cells situated in deeper layers. They represent the units of origin for the later dendrite bundles. The second step is initiated immediately after birth when axons arrive and small oblique and horizontal dendrites start to sprout from perikarya as well as from the apical dendrites. These new cell processes proliferate and grow rapidly. However, during this growth process groups of apical dendrites of pyramidal cells remain together, and they later form the bundles. In the beginning of this process many apical dendrites are connected with each other by punctae adhaerentiae. The basic pattern of dendrite bundles is present before the eyes open and before the majority of spines and synaptic contacts are formed.

In semithin sections cut tangentially with respect to the surface of the cerebral cortex and with a side length of at most a few mm, it is extremely difficult to be certain which lamina has been sectioned, for instance lamina III or upper lamina IV. In order to provide criteria to overcome this difficulty, a systematic study was carried out in which 1 micron semithin sections through the visual cortex of the rabbit were cut in both the frontal and tangential planes and compared with 8-12 micron paraffin sections stained for cells and/or myelin. It was found that the arrangement of thick dendrites, of myelinated fibres and of unmyelinated profiles which may be either dendrites or neurites, permits subdivision of the visual cortex into three characteristic horizontal zones which are termed zones A, B and C. The relation between these three zones and the cytoarchitectonic pattern is as follows: Zone A corresponds to the cytoarchitectonic lamina I; zone B comprises lamina II/III, lamina IV and the upper half of lamina V; zone C corresponds to the lower half of lamina V and all of lamina VI. Zones A and B can further be subdivided: Zone A consists of two layers, whereas zone B can be divided into three tiers, each of which is characterized by a particular arrangement of the myelinated and unmyelinated profiles.(ABSTRACT TRUNCATED AT 250 WORDS)

Due to the bundling of apical dendrites throughout laminae IV-II of the cerebral cortex, two compartments of neuropil are distinguished: (1) the neuropil of the dendrite bundles consisting of closely associated, vertically oriented apical dendrites of pyramidal cells and the fine cell processes between them and (2) the neuropil separating the dendrite bundles. The aim of the present study is to obtain more information concerning differences in composition and arrangement of fine cell processes in the two compartments and to quantify differences in the configuration and size of the extracellular space. To this end, a comparative stereological study of the two compartments has been performed in the upper half of lamina II/III in the visual cortex of 8 adult rabbits. The various constituent elements of the neuropil have been evaluated in electron micrographs with respect (a) to their volume fractions and (b) to their surface fractions, i.e. the surface area of their membranes comprised in a given volume of tissue. It was found that the volume fractions of thin axons, glial processes and spines are significantly higher between than within dendrite bundles, whereas the volume fraction of dendrites is about 50% higher in the compartment of the bundles. However, when excluding the dendrites in both compartments from calculations, the composition of the remaining neuropil within and between dendrite bundles was found to be identical. As far as surface fractions are concerned, significant differences were shown to exist for all kinds of cell processes except dendrites and myelinated axons. Surface fractions of small axons, glial processes and spines are higher between than within dendrite bundles.(ABSTRACT TRUNCATED AT 250 WORDS)