W. Maxwell Cowan

The development of the chick optic tectum. II. Autoradiographic studies.

Abstract The time of origin and the pattern of migration of the cells in each of the principal layers of the chick optic tectum have been studied autoradiographically using a variant of the ‘cumulative labeling’ technique. The earliest unlabeled cells ( i.e. , the first cells to be formed) are the large, multipolar neurons of the stratum griseum centrale; these appear in the rostroventral portion of the tectum between the third and forth days of incubation. The last cells to arise on the ninth day form the deepest laminae of the stratum griseum et fibrosum superficiale in the caudo-dorso-medial pole of the tectum. Viewed as a whole, the layers of the tectum are formed as the result of 3 spatially distinct, but chronologically overlapping, phases of cell proliferation and migration. The first of these occurs between days 4 and 6 and gives rise to the cells in the 4 deepest strata (the stratum griseum centrale, stratum album centrale, stratum griseum periventriculare and stratum fibrosum periventriculare). The second phase results in the formation of the outer laminae of the stratum griseum et fibrosum superficiale in which the retinal afferents terminate; this phase extends from the fourth to the eighth days of incubation. In the final phase (from days 5 to 9) the deeper laminae (h-j) of the stratum griseum et fibrosum superficiale are formed. In all 3 developmental zones there is a general rostro-ventro-lateral to caudo-dorso-medial gradient, and, in addition, the cells in each zone arise in a characteristics sequence. In the inner and intermediate zones the more superficial cells are formed before the deeper ( i.e. , they follow an ‘outside-in’ gradient); in the outer zone this sequence is reversed with the cells in the deeper laminae being formed between one and two days earlier than those in the outer layers (an ‘inside-out’ gradient). The cells in the 3 developmental zones migrate at apparent rates of about 1.5–1.8 μm/h, 3.5–5 μm/h, and 3.5–4μm/h, respectively.

Studies on the development of the chick optic tectum. III. Effects of early eye removal

The effects of early deafferentation upon the development of the optic tectum have been studied in a series of chicks from which the right eye had been removed during the second or third day of incubation. No change could be observed in the tectum contralateral to the eye removal (apart from the absence of thestratum opticum in which the retinal fibers are normally found) until the twelfth day of incubation. From this it is concluded that deafferentation has no effect upon cell proliferation, neuronal differentiation or the initial migration of neurons in the tectum, since these morphogenetic processes are largely completed by day 12. Beyond the fourteenth day of incubation there was a progressive atrophy of the tectum, so that by the time of hatching the affected side was markedly reduced in volume and had undergone a considerable degree of cell loss. In addition, the cytoarchitectonic differentiation of the layers in which the retinal afferents terminate was disrupted, and in certain of the deeper layers there was a conspicuous shrinkage of the surviving neurons. These findings have been compared to the effects of eye removal some time after retino-tectal connections were established. Deafferentation at this time results in a qualitatively similar, but quantitatively less severe, atrophy of the tectum. A consideration of these observations, and the findings of similar studies in the literature, leads to the general conclusion that in the avian nervous system the principal effect of early deafferentation is an atrophy, or degeneration, of differentiated neurons; this may, in time, lead to a disruption of the normal pattern of growth and cytoarchitectonic structure.

An On-Line Digital-Computer System for the Semiautomatic Analysis of Golgi-Impregnated Neurons

Rapid and accurate measurements of neuronal processes in Golgi preparations are possible with the aid of a small computer (Glaser and Van der Loos, 1965). This paper describes a system for doing this, using a small digital computer which controls stepping motors (0.5-?m steps) attached to the stage (x, y axes) and fine focus (z axis) of a microscope. The observer tracks the processes, topological information, such as the location of the soma, dendritic origins, branch points, and ends of processes, is signaled to the computer by special controls, and the x, y, and z coordinates of each are stored in digital format. The computer printout yields 1) individual x, y, and z coordinates with associated topological identifiers; 2) quantitative data for each dendritic segment in order (i.e., primary and secondary branches, etc.); and 3) computes the actual linear dimensions of each segment in microns. An associated oscilloscope display can 1) display the whole neuron, or individual processes, by connecting recorded points by vectors; 2) identify each dendrite; 3) rotate (continuously or by a specified angle) the whole cell, or individual dendrites, around any selected point or axis; and 4) indicate spatial relationships by dynamic rotation, intensity modulation, or stereo pairs. The performance of this system has been evaluated for accuracy and the repeatability of measurements.

On the Distribution of Axonal Terminals Containing Spheroidal and Flattened Synaptic Vesicles in the Hippocampus and Dentate Gyrus of the Rat and Cat

SummaryA quantitative analysis has been made of the distribution of presynaptic profiles containing round (or spheroidal) and flattened (or ellipsoidal) synaptic vesicles in the apical and basal dendritic zones and in the layer of pyramidal cell somata of fields CA1 and CA3 of the hippocampus, and in the molecular and granular layers of the dentate gyrus of the rat and cat.In the apical and basal dendritic zones of fields CA1 and CA3 the overwhelming majority of the synapses are of the asymmetrical variety, the axon terminals ending principally upon dendritic spines, and to a lesser extent upon the shafts and secondary or tertiary branches of the dendrites. Between 1 and 8% of the axon terminals in these zones contained flattened vesicles: all of these formed symmetrical contacts upon medium-sized or large dendritic shafts. In the molecular layer of the dentate gyrus a slightly higher percentage of flattened vesicle containing profiles was observed (∼10%); again these formed symmetrical contacts upon dendritic shafts. In the stratum pyramidale of the hippocampal fields and the stratum granulosum of the dentate gyrus of the rat, flattened vesicle containing synapses are two or three times more numerous than those with spheroidal vesicles. In the cat hippocampus the axosomatic synapses are about equally distributed between those containing round, and those with flattened vesicles.The finding that at the focus of post-synaptic inhibition, at the level of the pyramidal cell somata, the majority of the axon terminals contains flattened synaptic vesicles, whereas in the region of termination of the extrinsic, commissural and long association pathways (all of which are excitatory) virtually all the synapses contain round vesicles, strongly supports the view that endings containing flattened vesicles mediate post-synaptic inhibition in the hippocampal formation.

Evidence for the sprouting of entorhinal afferents into the “hippocampal zone” of the molecular layer of the dentate gyrus

SummaryFragments of the dentate gyrus containing portions of the stratum granulosum and the overlying stratum moleculare were isolated from their commissural, associational and hypothalamic inputs in 11 and 28-day-old rats. Six weeks later the distribution of the entorhinal afferents to the isolated portion of the stratum moleculare was determined autoradiographically by the injection of 3H-proline into the medial entorhinal area, and the appearance of the molecular layer was examined in Timms sulfide silver preparations. It is evident from our material that under these circumstances the entorhinal afferents to the dentate gyrus can extend right up to the stratum granulosum and that the normal staining pattern that characterizes the “hippocampal zone” of the molecular layer in Timms preparations may disappear and be replaced, in part, by a narrow zone of dense, mossy fiber-like staining.The simplest interpretation of these findings is that fibers from the medial entorhinal area are capable of sprouting into the deafferented “hippocampal zone” of the dentate gyrus. However, the observation that entorhinal afferents may occupy the entire thickness of the molecular layer does not, by itself, establish this point because in every case examined the whole molecular layer was shrunken. While this calls for some caution in the interpretation of the findings, our results suggest that there is no special barrier to the growth of entorhinal fibers and that the inner part of molecular layer of the dentate gyrus is capable of receiving afferents other than those normally present.