The effect of visual deprivation upon the basal dendrites of Meynert cells in the striate cortex of the monkey

The basal dendrites of Meynert cells in the striate cortex have been studied with the Golgi method in the brains of monkeys that had been reared for varying periods with the eyelids closed over one eye. The lengths and arrangement of the dendrites were compared with those in normal brains. In the visually deprived brain almost half of the cells had basal dendrites that were apparently normal with the dendritic fields in the form of an ellipse and the long axes parallel to the direction of the ocular dominance bands. The other cells had dendritic fields that have rarely been seen in normal material and two distinct types could be recognized. The 'lop-sided' cell had an ellipsoidal dendritic field with the major axis parallel to the ocular dominance bands, but the extents of the dendrites along the minor axis were very asymmetric; the ratio of the means of the long and short arms of the minor axis of the 'lop-sided' cell is 2.3:1 compared with 1.1:1 in normal brains. The 'perpendicular' type of cell also had an ellipsoidal dendritic field but the relation of the major and minor axes to the direction of the ocular dominance bands was the reverse of the normal cell, with the long axis of the ellipse being aligned perpendicular to the bands. 'Lop-sided' cells formed approximately 18% of the total of Meynert cells studied and the 'perpendicular' 32%. The proportion of the cells with abnormal basal dendritic fields, and particularly the 'perpendicular', increased with longer durations of eyelid closure. It is suggested that the alterations in the dendritic fields of the 'lop-sided' and 'perpendicular' cells may be correlated with the changes in width of the ocular dominance bands that are known to occur after monocular eyelid suture.     

Some additional morphological and metrical observations on Pan brain casts and their relevance to the Taung endocast

Using an increased sample of chimpanzee brains and brain casts, 32 hemispheres were measured to determine the variability of OP-FP (occipital-pole) and OP-LS (occipital pole-lunate sulcus) arc dimensions and their ratios. The Taung endocast was remeasured to test Falk's assertions that the lunate sulcus is in a pongid position. The average ratio for the chimpanzee brains was 0.218, a value more than 2+ S.D.'s posterior to Falk's placement of the lunate sulcus on the Taung specimen. It is suggested that the chimpanzee and Taung occipital poles have a different morphometric pattern, the former being coincident with the caudal end of the LC (lateral calcarine) fissure. The Taung OP-LS arc consistently measured at least 42 mm, and not 40 mm as claimed by Falk.     

Ape-like endocast of “ape-man” Taung

I have identified and illustrated a spherical “dimple” or “depression” on the Taung endocast as indicating the most likely position of the medial end of the lunate sulcus but have not drawn an actual lunate sulcus on Taung because one is not visible. In a recent paper, R.L. Holloway (Am. J. Phys. Anthropol. 77:27–33, 1988) drew a lunate sulcus on his copy of the Taung endocast, incorrectly attributed this sulcus to me, and used it to obtain a ratio of 0.254 to describe “Falk's” position of the lunate sulcus. My published ratio of 0.242 for Taung (Falk: Am. J. Phys. Anthropol. 67:313–315, 1985a) was not considered, although the focus of Holloway's paper was my assessment of the position of the lunate sulcus. Holloway also excluded published ratios for a chimpanzee in my collection from his statistical analysis but, even so, my published ratio for Taung is still only 1.5 standard deviations from his chimpanzee mean. If my chimpanzee brain is included in the sample, the ratio for Taung is 1.2 standard deviations from the mean. Furthermore, one of Holloway's own chimpanzees (B60–7) has a ratio of 0.241, just 0.001 below my ratio for Taung. There is no sulcus where Holloway has drawn one on Taung, his “F(LS)” is not mine, his 2 mm error is not mine, and the correct ratio for my measurement of Tuang is the one that I published, not the one that Holloway attributes to me. Assessment of Holloway's chimpanzee data supports my claim that the dimple on the Taung endocast is within the chimpanzee range for the medial end of the lunate sulcus.     

An autism-associated variant of Epac2 reveals a role for Ras/Epac2 signaling in controlling basal dendrite maintenance in mice

The architecture of dendritic arbors determines circuit connectivity, receptive fields, and computational properties of neurons, and dendritic structure is impaired in several psychiatric disorders. While apical and basal dendritic compartments of pyramidal neurons are functionally specialized and differentially regulated, little is known about mechanisms that selectively maintain basal dendrites. Here we identified a role for the Ras/Epac2 pathway in maintaining basal dendrite complexity of cortical neurons. Epac2 is a guanine nucleotide exchange factor (GEF) for the Ras-like small GTPase Rap, and it is highly enriched in the adult mouse brain. We found that in vivo Epac2 knockdown in layer 2/3 cortical neurons via in utero electroporation reduced basal dendritic architecture, and that Epac2 knockdown in mature cortical neurons in vitro mimicked this effect. Overexpression of an Epac2 rare coding variant, found in human subjects diagnosed with autism, also impaired basal dendritic morphology. This mutation disrupted Epac2's interaction with Ras, and inhibition of Ras selectively interfered with basal dendrite maintenance. Finally, we observed that components of the Ras/Epac2/Rap pathway exhibited differential abundance in the basal versus apical dendritic compartments. These findings define a role for Epac2 in enabling crosstalk between Ras and Rap signaling in maintaining basal dendrite complexity, and exemplify how rare coding variants, in addition to their disease relevance, can provide insight into cellular mechanisms relevant for brain connectivity.     

Wulst Neurons in Nestlings of the Pied Flycatcher Ficedula hypoleuca at the Pre-Visual Stage of Development

Neurons of the Wulst region, an analog of the mammalian visual cortex, were studied in Golgi-impregnated preparations of brain of non-precocial 1-day old nestlings of the pied flycatcher Ficedula hypoleuca. At this age, vision does not function in nestlings, their behavior is provided by an acoustic analyzer. Two populations of nerve cells, large and small juvenile neurons, were differentiated at visual examination. The comparative morphometry has shown these groups to differ significantly from each other by most studied parameters: the area of the profile field of their soma, the total length of dendrites, branching of the cell, the number of the maximal branching foci, the character of distribution of dendrite free endings in three concentric zones of the cell dendritic field. The distribution of dendrites in the dendritic neuron field was similar in the both groups of neurons. An increased density of dendrites was observed from the side of the afferent input. At the same time, study of orientation of the longest dendrite has shown it to be located parallel to the plane of the afferent input practically in all cells of the both groups. It is suggested that such orientation of the longest dendrite broadens the area of cell contacts, which is necessary for search for the maximal number of afferents. The predominant orientation of dendrites in the direction to the afferent input forms foundation for establishing a more extended zone of contacts with growing visual afferents.     

Apples, oranges, and the lunate sulcus

Holloway (1984) used a method of direct tape-arc measurements on chimpanzee brain casts to reject the hypothesis that the lunate sulcus is located in an anterior position in the Taung endocast. However, Holloway neglected to measure the occipital pole-lunate sulcus (OP-LS) arc directly on the Taung endocast as he did on chimpanzee brain casts (a crucial part of his methodology); instead, he determined the relative position of Taung's lunate sulcus on the basis of a calculation that confounds direct measurements and measurements from photographs. When arc OP-LS is measured directly on Taung according to Holloway's methods, the feature that has been identified as the medial end of the lunate sulcus is shown to be located within the range that Holloway determined for chimpanzees. Thus Holloway's methodology and data support rather than refute the claim that the lunate sulcus is located in a pongid-like position in australopithecines.     

Differentiation of Apical and Basal Dendrites in Pyramidal Cells and Granule Cells in Dissociated Hippocampal Cultures

Hippocampal pyramidal cells and dentate granule cells develop morphologically distinct dendritic arbors, yet also share some common features. Both cell types form a long apical dendrite which extends from the apex of the cell soma, while short basal dendrites are developed only in pyramidal cells. Using quantitative morphometric analyses of mouse hippocampal cultures, we evaluated the differences in dendritic arborization patterns between pyramidal and granule cells. Furthermore, we observed and described the final apical dendrite determination during dendritic polarization by time-lapse imaging. Pyramidal and granule cells in culture exhibited similar dendritic patterns with a single principal dendrite and several minor dendrites so that the cell types were not readily distinguished by appearance. While basal dendrites in granule cells are normally degraded by adulthood in vivo, cultured granule cells retained their minor dendrites. Asymmetric growth of a single principal dendrite harboring the Golgi was observed in both cell types soon after the onset of dendritic growth. Time-lapse imaging revealed that up until the second week in culture, final principal dendrite designation was not stabilized, but was frequently replaced by other minor dendrites. Before dendritic polarity was stabilized, the Golgi moved dynamically within the soma and was repeatedly repositioned at newly emerging principal dendrites. Our results suggest that polarized growth of the apical dendrite is regulated by cell intrinsic programs, while regression of basal dendrites requires cue(s) from the extracellular environment in the dentate gyrus. The apical dendrite designation is determined from among multiple growing dendrites of young developing neurons.     

Observations on the ultrastructure of a cephalic sense organ of the nematode Mermis nigrescens

This cephalic sense organ of Mermis nigrescens is unusual in that it is innervated by a dendrite which ends in two dendritic processes. These dendritic processes contain numerous neurotubules. There is a basal plate at the base of the dendritic process, and this is associated with a well-developed system of striated fibres which extend into the cytoplasm of the dendrite.     

Polarized secretory trafficking directs cargo for asymmetric dendrite growth and morphogenesis

Proper growth of dendrites is critical to the formation of neuronal circuits, but the cellular machinery that directs the addition of membrane components to generate dendritic architecture remains obscure. Here, we demonstrate that post-Golgi membrane trafficking is polarized toward longer dendrites of hippocampal pyramidal neurons in vitro and toward apical dendrites in vivo. Small Golgi outposts partition selectively into longer dendrites and are excluded from axons. In dendrites, Golgi outposts concentrate at branchpoints where they engage in post-Golgi trafficking. Within the cell body, the Golgi apparatus orients toward the longest dendrite, and this Golgi polarity precedes asymmetric dendrite growth. Manipulations that selectively block post-Golgi trafficking halt dendrite growth in developing neurons and cause a shrinkage of dendrites in mature pyramidal neurons. Further, disruption of Golgi polarity produces neurons with symmetric dendritic arbors lacking a single longest principal dendrite. These results define a novel polarized organization of neuronal secretory trafficking and demonstrate a mechanistic link between directed membrane trafficking and asymmetric dendrite growth.     

Structure and differentiation of the sensilla of the ventral sensory field on the maxillary palps ofPeriplaneta americana (Insecta,Blattodea), paying special attention to the ciliogenesis of the sensory cells

Summary The embryonic development of palpal contact chemosensitive sensilla was studied from 42% of development up to the hatching of the larvae. Ciliogenesis of the sensory cells can be observed at the earliest stages investigated. A complex consisting of two basal bodies and a cap-like ciliary vesicle is localized in the dendritic inner segment. It migrates apically and fuses with the cytoplasmic membrane. At the same time, microtubule doublets of the distal basal body elongate, thus generating the dendritic outer segment. Furthermore, the typical accessory structures of a motile cilium are formed. Although the central pair of microtubules is lacking, the dendritic outer segment can be considered as a modified motile cilium. At about 84% of development the hair structure starts to be formed. Whereas the socket is generated by the tormogen cell, the trichogen cell produces the hair shaft and terminal porus. The dendrite sheath, which rises above the newly formed hair, is attached apically to the embryonic cuticle forming an irregular pore. In larvae and imagines, the inner surface of the dendrite sheath is highly differentiated. A range of circular ledges and filamentous structures wrapping around the dendritic outer segments can be distinguished. These may have a stabilizing function. Furthermore, in cryofixed specimens, the dendritic outer segments possess regularly spaced swellings which are about 1 m in length and about 0.5 m in diameter. Their functional significance is still unclear.     

An electron microscopy study of exsolved phases in natural black Australian sapphire

Natural Australian sapphire (α-Al₂O₃ with around 1% Fe and up to 0.03% Ti) occurs in many colours, generally with an abundance of exsolved phases. Such ‘silky’ stones, cut en cabochon, exhibit the well-known phenomenon of asterism. For stones with a well-defined ‘star’ needle-like precipitate phases can be seen under an optical microscope. The needle axes are perpendicular to the star axes and generally parallel to the prism a-axes. (For artificial star sapphire the precipitate needle axes are perpendicular to the prism axes, e.g. Phillips et al., 1980.) Many authors have assumed that these precipitate phases in natural sapphire were needles of rutile (TiO₂) only, as is the case for artificial sapphire (Phillips et al., 1980). Our studies on natural Australian black sapphire, however, confirm the existence of a variety of iron/titanium rich phases, as well as rutile.Sample specimens were prepared by illuminating a polished basal plane with a He-Ne laser to obtain the star directions and then cutting slices of the stone parallel to the star axes. The polished sections were then ion-beam thinned and examined in a JEOL 100CX (S)TEM, equipped with an energy dispersive X-ray spectrometer. Precipitate phases were identified by electron diffraction and from their X-ray spectrum.     

Cell type-specific dendritic polarity in the absence of spatially organized external cues

Pyramidal neurons of the hippocampus and cortex have polarized dendritic arbors, but little is known about the cellular mechanisms distinguishing apical and basal dendrites. We used morphometric analysis and time lapse imaging of cultured hippocampal neurons to show that glutamatergic neurons develop progressive dendritic asymmetry in the absence of polarized extrinsic cues. Thus, pyramidal neurons have a cellular program for polarized dendrite growth independent of tissue microenvironment.     

Lunate morphology

Fifty healthy volunteers were subjected to the CT examination of the wrist joint to provide normal database of the shape and size of the lunate. The various parameters of the lunate were measured taking help of the reformatted images in sagittal, coronal and axial planes. The mean maximum antero-dorsal diameter of the lunate measured on axial section was 16.96mm (SD 1.60) with the range of 13-19mm while the mean medio-lateral diameter of the lunate was 12.80mm (SD 1.37) with the range of 10-15mm. The mean axes of the scaphoid and the triquetral articular surfaces of the lunate were 11.83 degrees (SD 9.33) and 1.54 degrees (SD 9.70), respectively, while the mean axial index was 2.04 (SD 1.33).Lunate is reported to have shapes of three different types on plain radiographs. The CT measurements of most lunates failed to classify them into the described three shapes since many lunates showed dissimilar typing on the various chosen sagittal sections of the same lunate. The classical wedged lunate with its apex towards the dorsum has been described to have a tendency to extend under the capitate compressive force. However, in a study on plain radiography no correlation was reported between the radio lunate angle and the shape of the lunate measured in the direction of the lunate's axis. Our study confirmed the same on plain radiographs and on the CT also. We measured lunate's shape in the direction of the capitate's axis too, which demonstrated significant correlation with the RLA (p<0.001).     

Atlas of olfactory organs of Drosophila melanogaster 2. Internal organization and cellular architecture of olfactory sensilla

Antennae and maxillary palps of Drosophila melanogaster were studied with the electron microscope on serial sections of cryofixed specimens. The number of epidermal cells roughly equals the number of sensilla, except for regions where the latter are scarce or absent. Each epidermal cell forms about two non-innervated spinules, a prominent subcuticular space and a conspicuous basal labyrinth, suggesting a high rate of fluid transport through the sensory epithelium. The internal organization and fine structure of trichoid, intermediate and basiconic sensilla is very similar. Receptor cell somata are invested by thin glial sheaths extending distad to the inner dendritic segments. Further distally, the thecogen cell forms a sleeve around the dendrites, but an extracellular dendrite sheath is absent. At the base of the cuticular apparatus, the inner sensillum-lymph space around the ciliary and outer dendritic segments is confluent with the large outer sensillum-lymph space formed by the trichogen and tormogen cells. All three auxiliary cells exhibit many features of secretory and transport cells but extend only thin basal processes towards the haemolymph sinus. The bauplan and fine structure of coeloconic sensilla differs in the following aspects: (1) the ciliary segment of the dendrites is located deeper below the base of the cuticular apparatus than in the other sensillum types; (2) a prominent dendrite sheath is always present, separating inner and outer sensillum-lymph spaces completely; (3) the apical microlamellae of the auxiliary cells are more elaborate, but free sensillum-lymph spaces are almost absent; (4) there are always four not three auxiliary cells. Morphometric data are presented on the diameter of inner and outer dendritic segments and on the size of receptor cells, as well as of the receptor and auxiliary cell nuclei. The special fine structural features of Drosophila olfactory sensilla are discussed under the aspects of sensillar function and the localization of proteins relevant for stimulus transduction.     

The taung endocast and the lunate sulcus: A rejection of the hypothesis of its anterior position

Using an independent method of direct tape-arc measurements on six chimpanzee brain casts, it is shown that Falk's (1980, 1983) claims regarding an anterior pongidlike placement of a lunate sulcus on the Taung specimen remain unconfirmed. Thus Holloway's (1981) stereoplotting method of testing Falk's hypothesis is independently confirmed, using the actual specimens rather than photographs of them. Falk's (1980) placement of a lunate sulcus falls at least 2.5 standard deviations anterior to a position expected on the basis of a Pan location.     

Local dendritic activity sets release probability at hippocampal synapses

The arrival of an action potential at a synapse triggers neurotransmitter release with a limited probability, p(r). Although p(r) is a fundamental parameter in defining synaptic efficacy, it is not uniform across all synapses, and the mechanisms by which a given synapse sets its basal release probability are unknown. By measuring p(r) at single presynaptic terminals in connected pairs of hippocampal neurons, we show that neighboring synapses on the same dendritic branch have very similar release probabilities, and p(r) is negatively correlated with the number of synapses on the branch. Increasing dendritic depolarization elicits a homeostatic decrease in p(r), and equalizing activity in the dendrite significantly reduces its variability. Our results indicate that local dendritic activity is the major determinant of basal release probability, and we suggest that this feedback regulation might be required to maintain synapses in their operational range.     

Integrins establish dendrite-substrate relationships that promote dendritic self-avoidance and patterning in drosophila sensory neurons

Dendrites achieve characteristic spacing patterns during development to ensure appropriate coverage of territories. Mechanisms of dendrite positioning via?repulsive dendrite-dendrite interactions are beginning to be elucidated, but the control, and importance, of dendrite positioning relative to their substrate is poorly understood. We found that dendritic branches of Drosophila dendritic arborization sensory neurons can be positioned either at the basal surface of epidermal cells, or enclosed within epidermal invaginations. We show that integrins control dendrite positioning on or within the epidermis in a cell autonomous manner by promoting dendritic retention on the basal surface. Loss of integrin function in neurons resulted in excessive self-crossing and dendrite maintenance defects, the former indicating a role for substrate interactions in self-avoidance. In contrast to a contact-mediated mechanism, we find that integrins prevent crossings that are noncontacting between dendrites in different three-dimensional positions, revealing a requirement?for combined dendrite-dendrite and dendrite-substrate interactions in self-avoidance.     

An Elliptometer: A Simple Method of Measuring the Areas of Structures in Microscopic Sections

A simple method of measuring the cross sectional areas of objects (such as cells) in microscopic sections is described. A beam of light is passed through an adjustable diaphragm and focussed by a lens on a screen of mm. ruled graph paper. The screen may be rotated about a horizontal axis. Adjustment of the aperture of the diaphragm and the plane of the screen yields illuminated areas of variable size and degree of ellipticity. As close a “fit” as possible is made between the illuminated area and the camera lucida tracing of the object to be measured. The lengths of the major and minor axes of the appropriate ellipse are read from the screen. The application of the formula for the area of an ellipse to mean major and minor axes for a group of cells gives the mean cross sectional area for the cell population under investigation.     

The mechanism of sensory transduction in the sensilla of the trochanteral hair plate of the cockroach,Periplaneta americana

Summary The trochanteral hair plate of the cockroach leg contains approximately 60 hair sensilla that are deflected by a joint membrane during flexion of the leg. Previous work has shown that the organ is a mechanoreceptor which limits leg flexion during walking by reflex connections to flexor and extensor motoneurons. Functional analysis of the largest sensilla has shown that their behaviour may be well approximated by a velocity detector followed by a unidirectional rectifier.We report here the results of an examination of the largest sensilla by scanning and transmission electron microscopy in an attempt to correlate the structure with the known functional elements. Each hair is innervated by a single sensory dendrite which is surrounded by an electron dense dendritic sheath. The dendrite terminates below the hair shaft in a tubular body containing a parallel array of microtubules embedded in an electron dense matrix, while the dendritic sheath extends beyond the tubular body to form the walls of the ecdysial canal. At the proximal end of the tubular body the dendritic sheath and sensory dendrite are anchored to the cuticular socket by a fibrous dome which seems to form a fulcrum around which the tubular body can be deflected by movements of the hair. We suggest that the basis for the detection of velocity may be mechanical differentiation by a fluid space between the dendritic sheath and the tubular body. The structure is also discussed with relation to the mechanism of sensory transduction and the possible causes of the unidirectional sensitivity.Supported by the Canadian Medical Research Council. The authors gratefully acknowledge the expert technical assistance of Sita Prasad     

The pyramidal cells of Betz within the cingulate and precentral gigantopyramidal field in the human brain

Summary It can be demonstrated with the aid of Golgi-, Nissl-, and pigment preparations that the Betz cells represent a homogeneous class of giant cells within the human brain, which can readily be distinguished from other large pyramids by their densely aggregated lipofuscin deposits. In addition to the primary motor field (4, Brodmann), there exists only a small area on the medial surface of the hemisphere in front of the central sulcus which also contains large Betz pyramids in layer Vb. This recently discovered gigantopyramidal field is almost totally buried in the depth of the cingulate sulcus (Braak, 1976b). Compared with the Betz cells of the primary motor field (4, Brodmann), those of the cingulate area display numerous primitive traits. A small number of short basal dendrites springs off from the cell body. The apical dendrite forks in a short distance from the perikaryon repeatedly but issues only few side branches. A spine-free proximal dendritic segment is poorly developed or lacking. Moreover, numerous spines are encountered along the surface of the soma. In view of their primitive features the large pyramids of the cingulate gigantopyramidal area are interpreted as the forerunners of the precentral Betz pyramids.Supported by the Deutsche Forschungsgemeinschaft (Br 317/7). Dedicated to Prof. Dr. med. Drs. h.c. W. Bargmann in honour of this 70th birthday