Bibliothek

Notizen: Summary Synapse formation and maturation were examined in the visual cortex of albino rats from birth to maturity. During the first few days of postnatal life, synapses were sparsely scattered in the subplate zone and in layer I. They appeared immature as judged by the irregular shapes of the presynaptic and postsynaptic profiles, the relatively poorly defined membrane specializations and the presence of only a few synaptic vesicles in the presynaptic structures. As the neuropil matured, synapses were observed throughout the cortex, showing increased thickening of the membrane specializations and more vesicles. However, it was not until the end of the fourth postnatal week that they appeared qualitatively indistinguishable from synapses identified in the adult material. A feature characteristic of the developing visual cortex was the presence of vacant membrane specializations that resembled type I postsynaptic densities. These specializations, which were located either opposite extracellular space or opposite another neuronal process, were only evident during the initial stages of synaptogenesis and their frequency decreased as the number of synapses increased. In addition, transitional forms between these densities and true type I synapses were identified during the first two postnatal weeks. Structures that resembled vacant postsynaptic densities typical of type II synapses were not observed. The earliest identified forms of type II synaptic contacts identified consisted of two profiles that exhibited symmetrical membrane specializations and cleft material. Based on these observations, a scheme has been proposed for the formation of type I and type II synapses in the visual cortex of the rat.

Notizen: Summary We have examined quantitatively the formation and maturation of synapses in the visual cortex of the rat. The density of the total number of synapses (synapses per 100 μm2 neuropil) as well as the densities of Gray's type I and type II contacts were estimated from photographic montages of coronal strips of visual cortex from rats of various postnatal ages. Histograms of synaptic density as a function of depth were prepared, and the mean values of the postsynaptic density length and vesicle number per terminal were estimated for the two synapse types at each age examined. During the first few days of life, synapses were concentrated in the subplate region. By the latter part of the second postnatal week they were present throughout the cortex and an adult-like distribution, in which the highest densities were present in the superficial layers, was achieved by day 14. The postsynaptic density length of the type I synapses remained relatively unchanged during development but that of the type II synapses was more variable. Specifically, it was significantly longer during the second and third postnatal weeks compared to earlier ages and to adult values. The mean number of vesicles per terminal for the two synapse types increased with age until day 28. Subsequently, it only increased slightly between days 28 and 90 for the type I synapses but decreased significantly for the type II synaptic contacts. At all ages examined, type I synapses formed the majority of synaptic contacts. The developmental pattern appeared to differ for the two synapse types. The density of type I synapses increased continuously during the first three weeks and achieved a mean value close to that of adult animals by day 20. In contrast, the density of type II synapses did not increase significantly until day 6, increased dramatically in the second and third postnatal weeks, and then declined markedly between days 20 and 90. The observed decrease in the density of type II synaptic contacts is a clear example of synapse elimination in the visual cortex.

Notizen: Summary Electron microscopy and tritiated thymidine autoradiographic techniques were used to study the life history of Retzius-Cajal cells in the developing visual cortex of the rat, a subject which has long been debated by investigators. The findings show unequivocally that at least some of these characteristic cells of the immature animals remain in the adult cortex in the form of typical nonpyramidal neurons.

Notizen: Summary The ontogenesis of Retzius-Cajal cells, a unique feature of developing cortical layer I in a variety of mammalian species, was examined with the electron microscope in coronal or tangential sections of the visual cortex of rats whose ages were closely spaced in time between day 17 of gestation and adulthood. At 17 days of gestation, Retzius-Cajal cells already display a characteristic appearance and some of the cytoplasmic organelles by which they are identified in the perinatal period. At birth they are recognized by their large size, horizontally oriented long processes, dark cytoplasmic ground substance and abundance of tightly packed organelles. One feature which is most typical of these cells at this, and later stages of development, is the presence in the cytoplasm of numerous wide cisterns of granular endoplasmic reticulum filled with electron-opaque material. Synapses are rarely seen on the perikarya and processes during the first week of postnatal life but become more frequent later in development. A pattern of modifications becomes noticeable in the morphology of these cells during the first postnatal week with the appearance of growth cone-like differentiations and new processes of varying sizes. Furthermore, their cytoplasm slowly acquires a lighter appearance, and the thickness of the characteristically long processes diminishes. The frequency of Retzius-Cajal cells decreases with age and at the end of the third postnatal week only very few can be recognized with certainty. Careful examination of a large series of sections during subsequent days revealed that the morphological characteristics of Retzius-Cajal cells continue to change until these cells can no longer be distinguished from classical layer I nonpyramidal neurons.

Notizen: Summary Using conventional immunocytochemical techniques, we have examined the morphology and distribution of somatostatin-like immunoreactive neurons in the visual cortex of albino rats between the first postnatal day and maturity. In the adult, somatostatin-immunoreactive neurons were observed in layers II to VI but were concentrated in layers II and III. These cells displayed morphological features characteristic of the multipolar and bitufted varieties of cortical non-pyramidal neurons as described in Golgi preparations of rat visual cortex. On the first postnatal day and in the subsequent few days, immunoreactivity was confined to immature bipolar and multipolar neurons concentrated in layers V and VI. Labelled cells first appeared in the more superficial layers at the beginning of the second postnatal week and attained a distribution similar to that observed in adult animals at the end of this week. At this time they closely resembled their adult counterparts from which they appeared indistinguishable by the end of the third postnatal week. The late appearance of labelled cells in the superficial layers, where they are predominantly located in adult animals, suggests that the somatostatin immunoreactivity exhibited by most of these neurons develops several days after they have completed their migration and assumed their positions in the visual cortex.

Notizen: Summary The peroxidase-antiperoxidase immunocytochemical technique has been used to examine the development of the ultrastructural features of somatostatin (SRIF)-immunoreactive neurons in the visual cortex of the rat between embryonic day 17 and postnatal day 32. In the adult, stained neurons are distributed in layers II through VI and characterized by an abundance of cytoplasm containing a plethora of organelles, most conspicuous of which are cisternae of granular endoplasmic reticulum organized in parallel arrays. In embryonic tissue, SRIF-positive cells are present in the subplate and in the border between the cortical plate and marginal zone. These cells possess scanty cytoplasm containing a few organelles; synapses onto immunoreactive perikarya and dendrites are evident at this stage. At birth and in early postnatal life, labelled cells are confined to the subplate region. Already at this age a number of cells display signs of ultrastructural features which characterize them in adult life. At the end of the first postnatal week, SRIF-immunoreactive neurons span a considerable spectrum of maturity. At one extreme are a few cells with little cytoplasm surrounding a large nucleus and at the other are the majority of labelled neurons showing abundant cytoplasm including prominent arrays of granular endoplasmic reticulum. Labelled cells first appear in the more superficial layers at the beginning of the second postnatal week and attain a distribution similar to that observed in adult animals at the end of this week. At this time their ultrastructural features closely resemble those of their adult counterparts, and differences in cytoplasmic maturity between superficial and deep labelled cells are not evident. This suggests that the SRIF-producing neurons in the superficial layers begin to express this peptide after they complete their migration and have acquired their morphological features. Maturation proceeds during the third postnatal week; at this stage most cells acquire their mature nuclear and cytoplasmic features and an adult complement of synapses. However, a number of SRIF-immunoreactive cells contain a particularly prominent accumulation of cytoplasmic organelles and appear hypertrophic.