Development of geniculocortical axon arbors in a primate

Patchy distributions of myelin and vesicular glutamate transporter 2 align with cytochrome oxidase blobs and interblobs in the superficial layers of the primary visual cortex

Blobs are a modular component of the primary visual cortex (area 17) of all primates, but not of other mammals closely related to primates. They are characterized as an even distribution of patches, puffs, or blobs of dense cytochrome oxidase (CO) expression in layer III of area 17, and are now known to differ from surrounding, nonblob cortex in thalamic, intrinsic, and extrastriate connections. Previous studies have also recognized a blob-like pattern of myelin-dense patches in layer III of area 17 of primates, and more recently the vesicular glutamate transporter (VGLUT)-2 isoform of the VGLUT family has been found to selectively distribute to layer III patches in a similar blob-like pattern. Here, we sought to determine if the blob-like patterns all identify the same modular structures in area 17 of primates by staining alternate brain sections cut parallel to the surface of area 17 of a prosimian primate (Otolemur garnettii) for CO, myelin, and VGLUT2. By aligning the sections from the three preparations, we provide clear evidence that the three preparations all identify the same modular blob structures. The results provide a further understanding of the functional nature of the blobs by demonstrating that their higher level of CO activity is related to thalamic inputs from the lateral geniculate nucleus that use VGLUT2 as their main glutamate transporter, and via myelinated axons.

Delayed luminance and chromatic contrast sensitivity in infants with spontaneously regressed retinopathy of prematurity

BACKGROUND

The current study assessed whether contrast sensitivity is affected in preterm infants with a history of spontaneously regressed retinopathy of prematurity (ROP, Stages 1-3). Specifically, we employed luminance (light/dark) and chromatic (red/green) stimuli, which are mediated by the magnocellular (M) and parvocellular (P) subcortical pathways, respectively.

METHODS

Contrast sensitivity (CS) was measured using forced-choice preferential looking testing in 21 infants with a history of ROP and 41 control preterm infants who were born prematurely but did not develop ROP, tested between 8 and 47 weeks (2-11 months) postterm age. Infants were presented with chromatic and luminance drifting sinusoidal gratings, which appeared randomly on the left or right side of the monitor in each trial. The contrast of the stimuli varied across trials and was defined in terms of root mean squared cone contrast for long- and medium-wavelength cones.

RESULTS

Between 8 and 25 weeks postterm, ROP infants had significantly worse CS, and there was a trend for greater impairment for luminance than chromatic CS. This delay was not seen at older ages between 26 and 47 weeks postterm.

CONCLUSIONS

These findings are consistent with the concept that early maturation of the M pathway is vulnerable to biological insult, as in the case of ROP, to a greater extent than in the P pathway.

Chromatic and luminance contrast sensitivity in fullterm and preterm infants

In order to investigate the contributions of visual experience vs. preprogrammed mechanisms on visual development, the current study compared contrast sensitivity in preterm vs. fullterm infants. If development is tied to time since conception, preterm infants should match the developmental trajectories of fullterm infants when plotted in postterm age. By contrast, if development is influenced by visual experience, preterm and fullterm infants should match when plotted in postnatal age. Luminance (light/dark) and chromatic (red/green) contrast sensitivities (CS) were measured in 25 preterm (born, on average, 6.6 weeks early) and 77 fullterm infants, between 1 and 6 months postterm. In the first few months, luminance CS was found to be predicted by postterm age, suggesting that preprogrammed development is sufficient to account for luminance CS. By contrast, chromatic CS exceeded that predicted by postterm age, which suggests that time since birth confers a benefit on chromatic CS. The preterms' 6.6 weeks of additional time since birth is roughly equivalent to 3.7 weeks of development in chromatic CS. In sum, these results suggest that chromatic CS is more influenced by early postnatal visual experience than luminance CS, which may have implications for development of parvocellular and magnocellular pathways.

Does visual modularity increase over the course of development?

Early in postnatal development, the brain produces exuberant connections, some of which are later retracted, a process that is thought to play a role in the formation of functionally segregated modules in the brain. In the case of visual development, retraction between visual areas might underlie the known psychophysical and neural segregation of processing for different aspects of vision (e.g., color, motion, form, depth) known to exist in adults. This review covers the psychophysical evidence for increasing dissociation between visual modules over the course of development, and provides insight into the possible functions of this developmental alteration.

Functional organization of visual cortex in the prosimian bush baby revealed by optical imaging of intrinsic signals

Cells in primary visual cortex (V1) of primates and carnivores respond most strongly to a visual stimulus presented to one eye, in a particular visual field location, and at a particular orientation. Each of these stimulus attributes is mapped across the cortical surface, and, in macaque monkeys and cats, strong geometrical relationships exist between these feature maps. In macaque V1 and V2, correlations between feature maps and cytochrome oxidase (CO)-rich modules have also been observed. To see if such relationships reflect a conserved principle of V1 functional architecture among primate species, we examined these maps in the prosimian bush baby, a species that has been proposed to represent the ancestral primate organization. We found that the layout of individual feature maps in bush baby V1 is similar to that of other primates, but we found an entirely different organization of orientation preference in bush baby V2 compared with that reported in simian primates. Another striking distinction between bush baby and simian species is that we observed no strong relationships among maps of orientation, ocular dominance, and CO blobs in V1. Thus our findings suggest that precise relationships between feature maps are not a common element of the functional organization in all primates and that such relationships are not necessary for achieving basic coverage of stimulus feature combinations. In addition, our results suggest that specific relationships between feature maps in V1, and the subdivision of V2 into functional compartments, may have arisen comparatively late in the evolution of primates.

Visual cortical organization at the single axon level: a beginning

Single axon analysis of visual cortical connections is an important extension of previous anterograde studies using 3H-amino acids or wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). The higher resolution tracers-Phaseolus vulgaris-leucoagglutinin (PHA-L), biocytin, biotinylated dextran amine (BDA) and dextran-conjugates-have already produced new results, simply by providing improved visualization, concerning laminar definition and possible subtypes of connections, as well as the beginning of a database of morphometrics and microstructure. The comparative approach, comparing geniculocortical terminations and cortical connections across several areas, has suggested both specific structural-functional correlations (for example, in extrastriate area MT/V5) and more subtle, possibly gradient-wise variations. Likely future directions for this line of research include more direct correlations of axon geometry with functional architectures, investigations of microcircuitry at the level of electron or confocal microscopy, anatomical and functional investigations of connectional convergence and interactions, and, not least, a more comprehensive database.

Development of individual axon arbors in a thalamocortical circuit necessary for song learning in zebra finches

Individual axon arbors within developing neural circuits are remodeled during restricted sensitive periods, leading to the emergence of precise patterns of connectivity and specialized adaptive behaviors. In male zebra finches, the circuit connecting the medial dorsolateral nucleus of the thalamus (DLM) and its cortical target, the lateral magnocellular nucleus of the anterior neostriatum (lMAN), is crucial for the acquisition of a normal vocal pattern during the sensitive period for song learning. The shell subregion of lMAN as well as the entire terminal field of DLM axons within lMAN undergo a striking increase in overall volume during early stages of vocal learning followed by an equally substantial decrease by adulthood, by which time birds have acquired stable song patterns. Because the total number of DLM neurons remains stable throughout this period, the dramatic changes within the overall DLM-->lMAN circuit are presumably attributable to dynamic rearrangements at the level of individual DLM axon arbors over the course of vocal learning. To study such rearrangements directly, we reconstructed individual DLM axon arbors in three dimensions at different stages during vocal learning. Unlike axon arbors in other model systems, in which the number of branches increases during development, DLM arbors are unusual in that they have the greatest number of branches at the onset of vocal learning and undergo large-scale retraction during the sensitive period for song learning. Decreases in the degree of overlap between DLM arbors apparently contribute to the increased overall volume of the DLM-->lMAN circuit during vocal learning. These developmental changes in DLM axon arbors occur at the height of the sensitive period for vocal learning, and hence may represent either a morphological correlate of song learning or a necessary prerequisite for acquisition of song.

Development of psychophysically-derived detection contours in L- and M-cone contrast space

In order to investigate the development of color mechanisms in infants we fitted elliptical detection contours to psychophysically-derived contrast thresholds plotted in L- and M-cone contrast space. Detection ellipses were obtained for 47 infants (ages 2-5 months of age), and were compared to those of six adults tested under nearly identical conditions. The parameters of the fitted ellipses allowed us to address several aspects of color development. First, the lengths and widths were used to assess the relative development of chromatic, with respect to luminance, sensitivity. The results of these analyses revealed a sharp increase in chromatic sensitivity between 3 and 4 months of age, suggesting an accelerated development of chromatic mechanisms around this time. Second, the angles of the ellipses provided estimates of individual red/green isoluminance points. In line with previous reports, we found that isoluminance points do not vary significantly with age. Finally, our ellipse-fitting procedures were used to assess whether color sensitivity is best described by a model that assumes independence between post-receptoral chromatic and luminance mechanisms. Similar to previous results of Kelly and Chang [Kelly, J. P. & Chang, S. (2000). Vision Research 40, 1887-1906] obtained using steady-state visually evoked potentials, only a proportion (approximately half) of our infants exhibited detection contours that were consistent with independent mechanisms, a finding that most likely results from statistical noise in the infant data sets.

Infant temporal contrast sensitivity functions (tCSFs) mature earlier for luminance than for chromatic stimuli: evidence for precocious magnocellular development?

In order to investigate the development of luminance and chromatic temporal contrast sensitivity functions (tCSFs), we obtained chromatic and luminance contrast thresholds from individual 3- and 4-month old infants, and compared them to previously obtained functions in adults. Stimuli were moving sinusoidal gratings of 0.27 cyc/deg, presented at one of five temporal frequencies: 1.0, 2.1, 4.2, 9.4 or 19 Hz (corresponding speeds: 3.8, 7.7, 15, 34, 69 deg/s). Previous studies, including our own, have shown that adult tCSFs are bandpass for luminance stimuli (peaking at 5-10 Hz), yet lowpass for chromatic stimuli (sensitivity falling at > 2 Hz), and that the two functions cross one another near 4-5 Hz when plotted in terms of cone contrast. In the present study, we find that the shapes and peaks of the luminance tCSF in both 3- and 4-months-olds appear quite similar to those of adults. By contrast, chromatic tCSFs in infants are markedly different from those of adults. In agreement with our earlier report (Dobkins, K. R., Lia, B., & Teller, D. Y. (1997). Vision Research, 37(19), 2699-2716), the chromatic function in 3-month-olds is rather flat, lacking the sharp high temporal frequency fall-off characteristic of the adult function. In addition, the luminance tCSF in 3-month-olds is elevated above the chromatic tCSF, and the two functions do not exhibit an adult-like cross-over within the range of temporal frequencies tested. By 4 months of age, substantial development of chromatic contrast sensitivity takes place at the lowest temporal frequencies. Although still immature, the 4-month-old chromatic tCSF has begun to adopt a more adult-like shape. In addition, similar to adults, luminance and chromatic tCSFs in 4-month-olds cross one another near 5 Hz. In adults, magnocellular (M) and parvocellular (P) pathways are thought to underlie the bandpass luminance and lowpass chromatic tCSF, respectively (e.g. Lee, B. B., Pokorny, J., Smith, V. C., Martin, P. R., & Valberg, A. (1990). Journal of the Optical Society of America (a), 7(12), 2223-2236). Based on this correspondence between psychophysical and neural responses in adults, our results suggest that the relatively slow development of the chromatic tCSF in infants may reflect immature chromatic responses in the P pathway and/or reliance on chromatic responses originating in the M pathway.

Morphology of M-cell axon arbors in striate cortex of monkeys reared with monocular aphakia

Although the effects of visual deprivation on the development of ocular dominance columns have been well described in primates, nothing is known in primates about the impact of the deprivation on the axonal profiles that make up the ocular dominance columns. We now show that the effects of monocular deprivation on the morphology of geniculostriate axons involve not only shifts in terminal arbor sizes, much as would be expected from the ocular dominance data, but also changes in the proliferation of terminal arbor branches. In macaque monkeys reared from birth with unilateral lens removal (aphakia), terminal arbors of geniculostriate axons were bulk-filled with horseradish peroxidase (HRP) in brain-slice preparations and reconstructed from serial sections through striate cortex (area 17). Our focus was on the arbors that terminate in the upper tier of layer IV, the target of cells in the magnocellular (M) layers of the LGN. Of the 26 M-cell arbors reconstructed from three aphakic monkeys, eight were unique in having few very simple terminal arbor branches. These also tended to be smaller in total extent than the average M-cell axons reconstructed from 1 normal monkey. In contrast, eight arbors had very rich terminal branching patterns, and seven of these were larger than any of those from the normal monkey. We propose that the small, sparse axon arbors are related to the deprived eye, and the large, dense arbors are related to the non-deprived eye. These morphological changes reflect abnormalities in the growth patterns of geniculostriate inputs that undoubtedly have important persisting consequences for visual performance.

Infant color vision: temporal contrast sensitivity functions for chromatic (red/green) stimuli in 3-month-olds

In order to investigate the development of temporal contrast sensitivity functions (tCSFs) for chromatic (red/green) stimuli, we obtained chromatic contrast thresholds from 3-month-old infants and adults using behavioral techniques. Stimuli were moving or counterphase-reversing sinusoidal gratings of 0.25 c/deg. Five temporal frequencies were used: 0.7, 2.1, 5.6, 11 and 17 Hz (corresponding speeds = 2.8, 8.4, 22, 44 and 67 deg/sec). In order to compare chromatic results with those obtained under luminance-defined conditions, luminance tCSFs were also obtained from adults, and previously obtained infant luminance tCSFs were used (from Dobkins & Teller, 1996a). In accordance with previous studies, adults exhibited bandpass luminance tCSFs with peaks near 5 Hz and lowpass chromatic tCSFs that declined rapidly at temporal frequencies greater than 2 Hz, and the two curves crossed one another near 4 Hz. By contrast, infants exhibited bandpass rather than lowpass chromatic tCSFs with peaks near 5 Hz. These chromatic curves were quite similar in peak frequency and general shape to previously obtained infant tCSFs for luminance stimuli. Moreover, both chromatic and luminance tCSFs in infants were found to be quite similar in peak and shape to luminance tCSFs observed in adults. These findings point to the possibility that, for 3-month-old infants, both chromatic and luminance stimuli are detected by the same underlying mechanism under these conditions. We propose that such a mechanism is probably a physiological pathway dominated by magnocellular input. Earlier studies of infant color vision are discussed in this context.

Parvalbumin, calbindin, and calretinin mark distinct pathways during development of monkey dorsal lateral geniculate nucleus

Immunocyochemical labeling was applied to follow the developmental changes in the calcium-binding proteins parvalbumin (PV), calbindin D28k (CaB), and calretinin (CaR) during fetal and infant development of Macaca monkey dorsal lateral geniculate nucleus (LGN). For all three proteins, LGN cell body and retinal ganglion cell (RGC) axon labeling patterns changed temporally and spatially over development, and many of these were LGN laminar specific. CaR+ and CaB+ cells were present at the youngest age studied, fetal day 55 (F55). After lamination of the LGN occurred between F90 and F115, CaR+ and CaB+ neurons were specific markers for the S, intercalated, and interlaminar layers. Double label immunocytochemistry showed that all CaR+ cells contained CaB, and none contained GABA. CaR+ cell bodies decreased in number soon after birth so that adult LGN contained only a very small number of CaR+ cells. These patterns and cell counts indicated that a downregulation of CaR had occurred in the CaB+ population. Although CaB+ cell density in S and interlaminar zones declined in the adult, cell counts indicated that this is due to dilution of a stable population into a much larger nucleus during development. PV+ cells appeared at F85 only within the putative magnocellular (M) and parvocellular (P) layers, and PV remained a marker for these layers throughout development. Fetal PV cells also contained GABA, indicating that they were LGN interneurons. After birth, GABA-/PV+ cell numbers increased dramatically throughout the whole nucleus so that by the end of the first year, P and M layers were filled with PV+ cells. Their number and size indicated that these were the LGN projection neurons. Beginning at F66, bundles of PV+ axons occupied the anterior-middle LGN and filled the optic tract. Up to F101, PV+ synaptic terminals were restricted to Players, but after F132 labeling in M layers was heavier than in P layers. Axonal labeling for CaR began at F125. Prenatally CaR+ terminals were present mainly in P layers, whereas by postnatal 9 weeks labeling in M layers much exceeded P layers. Axonal labeling for CaB was present at F132, but CaB+ terminals were observed only after birth with labeling always heavier in M than P layers. By postnatal 9 weeks, PV, CaR, and CaB were colocalized in the same axons and terminals. These experiments indicated that during development and in the adult LGN, both CaR and CaB were markers for the LGN neurons in the S and intercalated pathway. CaR was present transiently while CaB persisted into adulthood. PV was a M and P layer marker first for interneurons and later for projection cells. The complex temporal developmental patterns found in this study suggested that viewing PV, CaB, and CaR simply as calcium-buffering proteins severely underestimates their functional roles during visual system maturation.

Infant motion: detection (M:D) ratios for chromatically defined and luminance-defined moving stimuli

In order to assess the relative contributions of chromatic vs luminance information to motion processing in infants, we employed a motion:detection (M:D) paradigm. Stimuli consisted of 27 deg by 40 deg, 0.25 c/deg sinusoidal gratings moving at 22 deg/sec (5.6 Hz), and were either chromatically defined or luminance-defined. Contrast thresholds for direction-of-motion (M) were obtained using a directional eye movement technique. Contrast thresholds for detection (D) were obtained using forced-choice preferential looking. M:D threshold ratios were obtained for individual infant subjects, and results were compared to those of adults. As expected, adult M:D threshold ratios were near 1:1 for luminance-defined stimuli, but greater than 1:1 for chromatically defined stimuli. This suggests that, for adults, luminance-defined, but not chromatically defined, stimuli are detected by mechanisms labeled for direction of motion. By contrast, infant M:D ratios for chromatically and luminance-defined stimuli were approximately equal and close to 1:1, suggesting that, for infants, luminance- as well as chromatically defined stimuli are detected by mechanisms that are labeled for direction of motion.

Myelinated axons and the pyramidal cell modules in monkey primary visual cortex

In addition to the horizontal bands of myelinated axons that produce the line of Gennari and the inner band of Baillarger, the macaque primary visual cortex contains prominent vertical bundles of myelinated axons. In tangential sections through layer IVC, these axon bundles are regularly arranged. They have a mean center-to-center spacing of about 23 microns, and each one contains an average of 34 (S.D. +/- 13) myelinated axons. These bundles seem to be largely composed of efferent fibers, because in material in which pyramidal cells have been labelled in layer II/III and in layers IVA and IVB the axons of these neurons descend towards the white matter in bundles. However, it is doubtful whether all of the descending myelinated axons from the superficial layers emerge from the cortex, since counts show that the bundles contain maximum numbers of myelinated axons at the level of layer IVC, and that in layers V and VI their number is reduced by about 30%. Perhaps some of the axons enter the line of Baillarger, in layer V. When the bundles of myelinated axons and the clusters of apical dendrites of the layer V pyramidal cells are visualized simultaneously within layer IVC in electron microscopic preparations, it is apparent that their center-to-center spacing is similar, namely, about 23 microns and that a bundle of axons has a cluster of apical dendrites lying adjacent to it. Because of this association, and because axons from layer III pyramidal cells have been shown to enter the bundles, it is suggested that the myelinated axon bundles contain the efferent axons from the projection neurons in the individual pyramidal cell modules. However, in addition to the myelinated axons, the bundles contain unmyelinated axons, so that they also probably serve as the conduits for axons forming connections between layers. It is proposed that the pyramidal cell modules are the basic, functional neuronal units of the visual cortex, and since the neurons within a particular module can be expected to have slightly different inputs and response properties from those in neighboring modules, the individual axon bundles that emerge from each module would be expected to carry a unique set of efferent information.

Qualitative and quantitative features of axons projecting from caudal to rostral inferior temporal cortex of squirrel monkeys

On the basis of cortical and subcortical connections and architectonics, inferior temporal (IT) cortex of squirrel monkeys consists of a caudal region, ITC, with dorsal (ITCd) and ventral (ITCv) subdivisions; a rostral region, ITR; and possibly a third region intermediate to ITC and ITR, ITI (Weller & Steele, 1992; Steele & Weller, 1993). The present study qualitatively and quantitatively examined the terminal arborizations of 26 axons in ITR and ITI labeled by injections of biocytin or, in one case, horseradish peroxidase, in ITCv. The majority of axons gave rise to a single terminal arbor, with a small number branching into two overlapping or nearby arbors. Presumptive terminal specializations consisted of rounded, bead-like swellings, most often located en passant. All axons terminated in layer 4 of cortex, and most had additional terminations in layers 3 and 5. The total extent of each axon's terminal arbor was 125-750 microns dorsoventrally (mean = 360.6 microns) and 150-725 microns anteroposteriorly (mean = 328.1 microns; all values uncorrected for shrinkage). In most axons, especially those with larger terminal fields, boutons were not uniformly distributed, but formed 2-4 clumps (mean = 2.2), with a mean width of 149 microns, separated by narrower regions of fewer boutons. Based on a cluster analysis of characteristics of the 26 axons, axons projecting from caudal (ITCv) to rostral (ITR or ITI) IT cortex of squirrel monkeys comprised three groups that we called Type I, Type II, and Type III. Type I axons, the smallest in area extent of terminal arbor, terminated predominantly in dorsal ITR. Type III axons, largest in areal extent, and Type II axons, intermediate in areal extent, terminated in ventral ITR and throughout ITI. The three classes of axons may correspond to different types of visual information entering rostral IT cortex. The clumping of boutons suggests that individual axons terminate in limited patches within their terminal fields.

Interhemispheric connections in neonatal owl monkeys (Aotus trivirgatus) and galagos (Galago crassicaudatus)

Interhemispheric connections were studied by injecting a mixture of horseradish peroxidase (HRP) and wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) into multiple sites in dorsolateral occipital and parietal cortex of one cerebral hemisphere of three galagos (Galago crassicaudatus) and two owl monkeys (Aotus trivirgatus) within seven days of birth. Cortex was either separated from the rest of the brain, flattened and cut parallel to the surface to aid reconstructing surface-view patterns of labeled neurons and processes, or cut in standard coronal or parasagittal planes to better reveal laminar patterns of connections. In both primate species, the surface-view pattern of callosal connections in infants was remarkably adult-like. In infant owl monkeys, callosal connections were concentrated along the margin of area 18 with area 17, and only a few labeled cells were found within area 17. Other visual areas including the second visual area, V-II, and the middle temporal visual area, MT, had patchy distributions of labeled neurons that extended over large parts of the visual field representations. Primary motor, auditory, and somatosensory fields also had patchy distributions of labeled neurons, with regions of areas 3b and adjoining somatosensory fields having few callosal connections in portions that appeared to correspond with representations of the hand and foot. Results were very similar in galagos, except that newborn galagos, as in adults, had a patchy distribution of callosally projecting neurons that extended well within area 17. Furthermore, the labeled neurons were concentrated in patches that aligned with the cytochrome oxidase blobs of area 17. Finally, callosal connections were concentrated in cytochrome oxidase poor regions of area 3b.

The postnatal development of geniculocortical axon arbors in owl monkeys

To characterize the postnatal development of geniculocortical axon arbor morphology in owl monkeys at a series of ages from birth to adulthood, individual arbors were bulk-filled with HRP in brain slice preparations and were reconstructed from serial sections. At all ages, cortical layers and sublayers were obvious. Presumed M or magnocellular arbors were largely confined to layer IV alpha, but they also extended into layer IIIc (IVB of Brodmann, 1909); presumed P or parvocellular arbors were almost exclusively confined to layer IV beta. Other axons that may reflect feedback projections from MT terminated in layer IIIc. Overall, M axon arbors increased in size and complexity from birth to adulthood with mean surface-view arbor areas ranging from 0.08 +/- 0.01 mm2 in newborns to 0.24 +/- 0.02 mm2 in adults. The developing P arbor areas were, on average, as large or larger than adult (newborn = 0.07 +/- 0.01 mm2, adult = 0.047 +/- 0.01 mm2; n.s.) but the arbors were somewhat less complex. Since the brain and area 17 increase in size postnatally, the proportion of area 17 subserved by each P arbor would decrease in postnatal development. Terminal boutons with immature features were evident in both M and P populations at all developmental ages. The results indicate that, while both LGN axon types in monkeys undergo morphological changes postnatally, M arbors appear to mature by increasing arbor size and terminal branching complexity, whereas P arbors increase in complexity but not in size. These distinct programs of axon arbor development suggest that the periods of susceptibility of geniculocortical axon arbors to postnatal influences of the environment, and the types of plastic responses they potentially exhibit, are class-specific.

Morphological details of primate axons and dendrites revealed by extracellular injection of biocytin: an economic and reliable alternative to PHA-L

The objective of this study was to determine if biocytin would reliably label details of distant axons and dendrites when injected extracellularly in primates. Biocytin (2.5-5%) was injected iontophoretically or by pressure into several areas of the visual and somatosensory systems of macaque monkeys, squirrel monkeys, tree shrews and galagos. After survival times that ranged from 9 h to 2 weeks, fine details of anterogradely filled axons and/or retrogradely filled dendrites were reliably revealed with an avidin-biotin-HRP complex (ABC solution) that was enhanced with heavy metals. Biocytin labeling was successfully combined with choline acetyltransferase (ChAT) or cytochrome oxidase (CO) histochemistry to reveal double-labeled cells. Our results show that biocytin is a versatile, easy-to-use label that completely fills cell processes both anterogradely and retrogradely in several primate species.

Development of the calcium-binding protein parvalbumin and calbindin in monkey striate cortex

The development of immunoreactivity for the calcium-binding proteins parvalbumin (PV) and calbindin-D28K (Cal) was studied in Macaca nemestrina striate cortex from fetal (F) 60 days to postnatal (P) 5 + years. We correlated changes in PV and Cal staining patterns with the well-documented developmental sequence for primate striate cortex neuron generation and maturation, synaptogenesis, and thalamocortical axon interactions in an attempt to deduce a functional role for these proteins. Our major findings is that Cal and PV have diametrically opposed developmental patterns except in layer 1. At F60 days both are present only in neurons of layer 1 and the number of labeled cell bodies and processes increases up to F125 days. Almost all Cal+ and PV+ cells in layer 1 disappear by P12 weeks. Cal is present by F113 days in pyramidal and stellate neurons, particularly layers 4-6. The numbers and staining density of cells in layers 2-6 increases up to birth and then both decline by P9-12 weeks. Supragranular layers show a second increase in Cal labeling from P20-36 weeks, and then there is a slow decline to the adult pattern which is reached by P1-2 years. Cell bodies in layers 4A, 4C alpha, and deep 4C beta are heavily Cal+ during pre- and early post-natal periods, but upper 4C beta remains unlabeled. PV is not seen until F155-162 days in layers 2-6. Large stellate and a few pyramidal cells appear first in layers 5/6 and 4C alpha, but PV+ stellate neurons are found in all layers except 4C beta by P6 weeks. Layer 4C beta contains a few PV+ cell bodies at P3 weeks, and light neuropile staining at P6 weeks, but then PV labeling rapidly increases so that by P12 weeks the density of 4C beta exceeds that of 4C alpha. Striate cortex has an adult pattern of cell number and neuropile density by P20 weeks. These developmental patterns suggest that the highest density of Cal cell body staining does not correlate with synaptogenesis, or the postnatal critical period of visually driven, binocular interactions. Rather Cal appears when lateral geniculate axons arrive in cortex, persists over the entire span of thalamocortical interactions, and disappears during the decline of cortical plasticity. The appearance of PV is highly correlated with the onset of complex visually driven activity at birth, while both the number of PV+ cell bodies and the density of PV+ neuropile reach adult levels coincident with the completion of thalamocortical connections.