Selective early expression of the orphan nuclear receptor Nr4a2 identifies the claustrum homolog in the avian mesopallium: Impact on sauropsidian/mammalian pallium comparisons.

The transcription factor Nr4a2 was recently revealed as a very early developmental marker of the claustrum (CL) proper in the mouse. The earliest claustral primordium was identified superficially, dorsal to the olfactory cortex, and was subsequently covered by the Nr4a2-negative cells of the insular cortex. Some tangentially migrating claustral derivatives (subplate cells and some endopiriform elements) also expressed this marker. The present study employs the same genetic marker to explore the presence of a comparable pallial division in chicken in which, in principle, the same pallial sectors exist as in mammals. We were indeed able to delineate an early-developing Nr4a2-positive mantle domain at the expected topologic position within the developing chicken lateral pallium. In the chicken as well as in the turtle (from data in the literature), the earliest postmitotic lateropallial cells likewise express Nr4a2 and occupy a corticoid superficial stratum of the mesopallium, which is clearly comparable in spatial and chronological profile to the mouse CL. Other cells produced in this pallial sector include various tangentially migrating Nr4a2-labeled derivatives as well as Nr4a2-negative and Nr4a2-positive local deeper subpopulations that partially interdigitate, forming mesopallial core and shell populations. We hold that the deep avian and reptilian mesopallial formation developing under the superficial corticoid CL homolog represents a field homolog of the insula, although additional studies are required to underpin this hypothesis.

Radial and tangential migration of telencephalic somatostatin neurons originated from the mouse diagonal area.

The telencephalic subpallium is the source of various GABAergic interneuron cohorts that invade the pallium via tangential migration. Based on genoarchitectonic studies, the subpallium has been subdivided into four major domains: striatum, pallidum, diagonal area and preoptic area (Puelles et al. 2013; Allen Developing Mouse Brain Atlas), and a larger set of molecularly distinct progenitor areas (Flames et al. 2007). Fate mapping, genetic lineage-tracing studies, and other approaches have suggested that each subpallial subdivision produces specific sorts of inhibitory interneurons, distinguished by differential peptidic content, which are distributed tangentially to pallial and subpallial target territories (e.g., olfactory bulb, isocortex, hippocampus, pallial and subpallial amygdala, striatum, pallidum, septum). In this report, we map descriptively the early differentiation and apparent migratory dispersion of mouse subpallial somatostatin-expressing (Sst) cells from E10.5 onward, comparing their topography with the expression patterns of the genes Dlx5, Gbx2, Lhx7-8, Nkx2.1, Nkx5.1 (Hmx3), and Shh, which variously label parts of the subpallium. Whereas some experimental results suggest that Sst cells are pallidal, our data reveal that many, if not most, telencephalic Sst cells derive from de diagonal area (Dg). Sst-positive cells initially only present at the embryonic Dg selectively populate radially the medial part of the bed nucleus striae terminalis (from paraseptal to amygdaloid regions) and part of the central amygdala; they also invade tangentially the striatum, while eschewing the globus pallidum and the preoptic area, and integrate within most cortical and nuclear pallial areas between E10.5 and E16.5.

Distal-less-like protein distribution in the larval lamprey forebrain.

A polyclonal antibody against the Drosophila distal-less (DLL) protein, cross-reactive with cognate vertebrate proteins, was employed to map DLL-like expression in the midlarval lamprey forebrain. This work aimed to characterize in detail the separate diencephalic and telencephalic DLL expression domains, in order to test our previous modified definition of the lamprey prethalamus [Pombal and Puelles (1999) J Comp Neurol 414:391-422], adapt our earlier schema of prosomeric subdivisions in the lamprey forebrain to more recent versions of this model [Pombal et al. (2009) Brain Behav Evol 74:7-19] and reexamine the pallio-subpallial regionalization of the lamprey telencephalon. We observed a large-scale conservation of the topologic distribution of the DLL protein, in consonance with patterns of Dlx expression present in other vertebrates studied. Moreover, evidence was obtained of substantial numbers of DLL-positive neurons in the olfactory bulb and the cerebral hemispheres, in a pattern consistent with possible tangential migration out of the subpallium into the overlying pallium, as occurs in mammals, birds, frogs and teleost fishes.

Genoarchitectonic profile of developing nuclear groups in the chicken pretectum.

Earlier results on molecularly coded progenitor domains in the chicken pretectum revealed an anteroposterior subdivision of the pretectum in precommissural (PcP), juxtacommissural (JcP), and commissural (CoP) histogenetic areas, each specified differentially (Ferran et al. [2007] J Comp Neurol 505:379-403). Here we examined the nuclei derived from these areas with regard to characteristic gene expression patterns and gradual histogenesis (eventually, migration patterns). We sought a genoarchitectonic schema of the avian pretectum within the prosomeric model of the vertebrate forebrain (Puelles and Rubenstein [2003] Trends Neurosci 26:469-476; Puelles et al. [2007] San Diego: Academic Press). Transcription-factor gene markers were used to selectively map derivatives of the three pretectal histogenetic domains: Pax7 and Pax6 (CoP); FoxP1 and Six3 (JcP); and FoxP2, Ebf1, and Bhlhb4 (PcP). The combination of this genoarchitectonic information with additional data on Lim1, Tal2, and Nbea mRNA expression and other chemoarchitectonic results allowed unambiguous characterization of some 30 pretectal nuclei. Apart from grouping them as derivatives of the three early anteroposterior domains, we also assigned them to postulated dorsoventral subdomains (Ferran et al. [2007]). Several previously unknown neuronal populations were detected, thus expanding the list of pretectal structures, and we corrected some apparently confused concepts in the earlier literature. The composite gene expression map represents a substantial advance in anatomical and embryological knowledge of the avian pretectum. Many nuclear primordia can be recognized long before the mature differentiated state of the pretectum is achieved. This study provides fundamental notions for ultimate scientific study of the specification and regionalization processes building up this brain area, both in birds and other vertebrates.

Chicken lateral septal organ and other circumventricular organs form in a striatal subdomain abutting the molecular striatopallidal border.

The avian lateral septal organ (LSO) is a telencephalic circumventricular specialization with liquor-contacting neurons (Kuenzel and van Tienhoven [1982] J. Comp. Neurol. 206:293-313). We studied the topological position of the chicken LSO relative to molecular borders defined previously within the telencephalic subpallium (Puelles et al. [2000] J. Comp. Neurol. 424:409-438). Differential expression of Dlx5 and Nkx2.1 homeobox genes, or the Shh gene encoding a secreted morphogen, allows distinction of striatal, pallidal, and preoptic subpallial sectors. The chicken LSO complex was characterized chemoarchitectonically from embryonic to posthatching stages, by using immunohistochemistry for calbindin, tyrosine hydroxylase, NKX2.1, and BEN proteins and in situ hybridization for Nkx2.1, Nkx2.2, Nkx6.1, Shh, and Dlx5 mRNA. Medial and lateral parts of LSO appear, respectively, at the striatal part of the septum and adjacent bottom of the lateral ventricle (accumbens), in lateral continuity with another circumventricular organ that forms along a thin subregion of the entire striatum, abutting the molecular striatopallidal boundary; we called this the "striatopallidal organ" (SPO). The SPO displays associated distal periventricular cells, which are lacking in the LSO. Moreover, the SPO is continuous caudomedially with a thin, linear ependymal specialization found around the extended amygdala and preoptic areas. This differs from SPO and LSO in some molecular aspects. We tentatively identified this structure as being composed of an "extended amygdala organ" (EAO) and a "preoptohypothalamic organ" (PHO). The position of LSO, SPO, EAO, and PHO within a linear Dlx5-expressing ventricular domain that surrounds the Nkx2.1-expressing pallidopreoptic domain provides an unexpected insight into possible common and differential causal mechanisms underlying their formation.

Anatomical and gene expression mapping of the ventral pallium in a three-dimensional model of developing human brain.

Combining gene expression data with morphological information has revolutionized developmental neuroanatomy in the last decade. Visualization and interpretation of complex images have been crucial to these advances in our understanding of mechanisms underlying early brain development, as most developmental processes are spatially oriented, in topologically invariant patterns that become overtly distorted during brain morphogenesis. It has also become clear that more powerful methodologies are needed to accommodate the increasing volume of data available and the increasingly sophisticated analyses that are required, for example analyzing anatomy and multiple gene expression patterns at individual developmental stages, or identifying and analyzing homologous structures through time and/or between species. Three-dimensional models have long been recognized as a valuable way of providing a visual interpretation and overview of complex morphological data. We have used a recently developed method, optical projection tomography, to generate digital three-dimensional models of early human brain development. These models can be used both as frameworks, onto which normal or experimental gene expression data can be mapped, and as objects, within which topological morphological relationships can be investigated in silico. Gene expression patterns and selected morphological structures or boundaries can then be visualized individually or in different combinations in order to study their respective morphogenetic significance. Here, we review briefly the optical projection tomography method, placing it in the context of other methods used to generate developmental three dimensional models, and show the definition of some CNS anatomical domains within a Carnegie stage 19 human model. We also map the telencephalic EMX1 and PAX6 gene expression patterns to this model, corroborating for the first time the existence of a ventral pallium primordium in the telencephalon of human embryos, a distinct claustroamygdaloid histogenetic area comparable to the recently defined mouse primordium given that name [Puelles L, Kuwana E, Puelles E, Bulfone A, Shimamura K, Keleher J, Smiga S, Rubenstein JLR (2000) Pallial and subpallial derivatives in the embryonic chick and mouse telencephalon, traced by the expression of the genes Dlx-2, Emx-1, Nkx-2.1, Pax-6, and Tbr-1. J Comp Neurol 424:409-438; Puelles L, Martínez S, Martínez-de-la-Torre M, Rubenstein JLR (2004) Gene maps and related histogenetic domains in the forebrain and midbrain. In: The rat nervous system, 3rd ed (Paxinos G, ed), pp 3-25. San Diego: Academic Press].

Morphologic fate of diencephalic prosomeres and their subdivisions revealed by mapping cadherin expression.

The expression of four cadherins (cadherin-6B, cadherin-7, R-cadherin, and N-cadherin) was mapped in the diencephalon of chicken embryos at 11 days and 15 days of incubation and was compared with Nissl stains and radial glial topology. Results showed that each cadherin is expressed in a restricted manner by a different set of embryonic divisions, brain nuclei, and their subregions. An analysis of the segmental organization based on the prosomeric model indicated that, in the mature diencephalon, each prosomere persists and forms a coherent domain of gray matter extending across the entire transverse dimension of the neural tube, from the ventricular surface to the pial surface. Moreover, the results suggest the presence of a novel set of secondary subdivisions for the dorsal thalamus (dorsal, intermediate, and ventral tiers and anteroventral subregion). They also confirm the presence of secondary subdivisions in the pretectum (commissural, juxtacommissural, and precommissural). At most of the borders between the prosomeres and their secondary subdivisions, changes in radial glial fiber density were observed. The diencephalic brain nuclei that derive from each of the subdivisions were determined. In addition, a number of previously less well-characterized gray matter regions of the diencephalon were defined in more detail based on the mapping of cadherin expression. The results demonstrate in detail how the divisions of the early embryonic diencephalon persist and transform into mature gray matter architecture during brain morphogenesis, and they support the hypothesis that cadherins play a role in this process by providing a framework of potentially adhesive specificities.

Diencephalic neuronal populations projecting axons into the basal plate in a lizard (Gallotia galloti).

Lizard diencephalic populations sending axons into the basal plate were studied by the in vitro HRP technique in the lizard Gallotia. Retrograde labeled cells were concentrated in distinct neuronal groups within alar plates of prosomeres p1 and p3, whereas the alar plate of p2 was poorly labeled. Efferent fibers from alar p1 and p3 populations entered the basal plate of the diencephalon along topologically dorsoventral courses, bifurcating thereafter into longitudinal ascending (rostral) and descending (caudal) trajectories. Thus, diencephalic segments p1 and p3 have alar cell populations contributing to the longitudinal premotor connectivity of the neural axis , whereas the alar p2 segment projects via the fasciculus retroflexus, the efferent tract of the epithalamus. However, the axons from the habenular complex bifurcate within or adjacent to the floor plate and not within the basal plate.

Pharmacological characterization, anatomical distribution and sex differences of the non-NMDA excitatory amino acid receptors in the quail brain as identified by CNQX binding.

The distribution of non-N-methyl-D-aspartate binding sites was studied in coronal and sagittal sections through the brain of adult Japanese quail by quantitative autoradiography, using tritiated 6-cyano-7-nitroquinoxaline-2,3-dione as a radioligand. Saturation binding experiments were, in addition, carried out in areas showing high levels of binding (cerebellar molecular layer, nucleus anterior medialis and nucleus infundibularis) and demonstrated that the binding of tritiated ligand was specific and saturable. Competition studies with alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid and kainic acid indicated that kainic acid strongly inhibited ligand binding in all brain areas. alpha-Amino-3-hydroxy-methyl-4-isoxazole propionic acid was only a weak inhibitor in the hypothalamic nuclei whereas in the cerebellar molecular layer both high and low affinity inhibitions were detected. The highest binding levels of tritiated ligand were observed in the molecular layer of the cerebellum. Very high levels of binding were detected in various preoptic/hypothalamic sites including the nucleus suprachiasmaticus pars medialis, nucleus anterior medialis hypothalami, nucleus infundibularis, nucleus mammillaris medialis, nucleus posteromediale hypothalami and nucleus hypothalami ventromedialis. High levels of binding were also detected in the bulbus olfactorius, bed nucleus commissuralis anterior, bed nucleus commissuralis pallii, nucleus accumbens, bed nucleus striae terminalis and nucleus interpeduncularis. In the preoptic area/hypothalamus, high levels of binding were clearly present in all areas that contain gonadotropin releasing hormone cells or fibers. In the pons and mesencephalon, moderate levels of binding were associated with catecholaminergic areas such as the area ventralis tegmentalis (area ventralis of Tsai) and the locus coeruleus. Saturation analysis demonstrated the presence of a higher number of binding sites in females than in males in the cerebellar molecular layer, nucleus infundibularis and nucleus anterior medialis. This latter difference was confirmed in the one point assays that also identified higher levels of specific binding in the nucleus suprachiasmaticus pars medialis of males as compared with females. These anatomical data suggest a possible implication of non-N-methyl-D-aspartate receptors in the synthesis and/or release of both gonadotropin releasing hormone and catecholaminergic neurotransmitters that should now be tested by pharmacological experiments.

Intertectal commissural projection in the lizard Gallotia stehlini: origin and midline topography.

The retinotectal projection of reptiles is largely crossed. The intertectal commissure is an important pathway that interconnects directly the two sides of the optic tectum. The rostrocaudal topography of intertectal commissural fibers at the dorsal midplane was examined by means of the in vitro horseradish peroxidase (HRP) labelling technique in the lizard Gallotia stehlini. Unilateral large deposits of tracer in the optic tectum as well as smaller deposits restricted to one quadrant were used to map the intertectal fibers anterogradely. Most commissural axons reached the contralateral side grouped into a dense bundle at the transition between two structurally distinct parts of the midbrain dorsal midline. The smaller rostral zone relates laterally to the griseum tectale, whereas the larger caudal zone relates to the tectum. The intertectal fibers seem to converge on the rostralmost part of the latter midline region, even though they originate throughout the optic tectum. A rough rostrocaudal tectotopic order was detected at the midline. Retrogradely labelled neurons were best obtained by depositing HRP directly within the compact commissure at the midline. These belong to pyriform cells in the periventricular layers 3 and 5. Axons labelled from the tectum did not enter the posterior commissure nor the intervening commissural region related to the griseum tectale.

A segmental map of architectonic subdivisions in the diencephalon of the frog Rana perezi: acetylcholinesterase-histochemical observations.

The work examines frog diencephalic subdivisions from a segmental viewpoint and adds a number of details to the atlas of the bullfrog diencephalon reported by Neary and Northcutt [1983]. Acetylcholinesterase histochemistry was performed on brains of Rana perezi, sectioned either sagittally or in a plane roughly parallel to the optic tract to optimize detection of segmented landmarks. This material provided a very detailed picture of individual neurons, neuropils and some fiber tracts expressing the enzyme in diverse patterns characteristic for each diencephalic region. The main diencephalic areas previously recognized in the bullfrog appeared subdivided into smaller AChE-chemoarchitectonic units. Modified subdivisions are proposed for several entities: preoptic, suprachiasmatic, entopeduncular, ventral thalamic, anterior thalamic, pretectal, hypothalamic and tuberculum posterior regions. A number of cell groups are described for the first time in frogs. This mapping is expected to be useful for the interpretation of immunocytochemical and experimental hodologic results in the diencephalon of frogs and opens new possibilities for comparative analysis.

New subdivision schema for the avian torus semicircularis: neurochemical maps in the chick.

Chemoarchitectonic subdivisions in the chicken torus semicircularis were mapped by means of acetylcholinesterase histochemistry and immunocytochemical labeling of leucine-enkephalin, choline acetyltransferase, neuropeptide Y, and calbindin/calretinin in adjacent sections. The torus semicircularis was found to consist of three main divisions: intercollicular area, toral nucleus, and preisthmic superficial area. All three appear variously subdivided. The intercollicular area is a mid-mesencephalic ventral periventricular region and appears subdivided into core and shell intercollicular regions. The toral nucleus is formed by a large caudal periventricular cytoarchitectonic complex, consisting of a periventricular lamina subdivided into core and shell regions, a pericentral, diffuse external nucleus, a central nucleus subdivided into core and shell regions, a caudomedial shell nucleus, a paracentral nucleus, and a posterior hiliar nucleus, apart from other minor parcellations. The preisthmic superficial area extends superficially at the caudomedial end of the toral nucleus, reaching the paramedian dorsal brain surface just rostral to the isthmo-optic nucleus. It is subdivided into core and shell regions. This previously unnoticed area is distinguished here from the intercollicular area and from the caudomedial shell and paracentral nuclei, all of which are frequently mixed in the literature under the concept "intercollicular nucleus." The revised terminology and subdivision for the avian torus clarifies many chemoarchitectonic and hodological mappings reported in the literature. It also suggests new research subjects and eliminates some causes of confusion.

Observations on the fate of nucleus superficialis magnocellularis of Rendahl in the avian diencephalon, bearing on the organization and nomenclature of neighboring retinorecipient nuclei.

The cytoarchitectonic development of an. superficialis magnocellularis (dorsal thalamus, posterior parencephalon) was studied from 3 days of incubation up to the mature state after hatching in the chick. A hypothesis of Kuhlenbeck (1937) on a partial transformation or contribution of SM cells into a different neighbouring griseum was tested, in the wider context of divergent interpretations of diencephalic development either within Herrick's (1910) longitudinal columnar theory, or within a modified neuromeric conception (Puelles et al. 1987 a). Nucleus SM develops early within the alar region of the posterior parencephalon, forming an outer mantle stratum over the main telencephalopetal thalamic inner cell mass. Thymidine-labeling data pinpoint its generation period mainly between 3 and 4.5 days of incubation. Throughout its subsequent development, SM remains within the primary interneuromeric limits that separate it from ventral thalamus and pretectum. After 8 days of incubation, SM subdivides into superficial (compact) and deep (disperse) sublaminae. The superficial one becomes much compressed between n. geniculatus ventralis and n. synencephali superficialis. Some of its cells migrate interstitially into the optic tract (12-16 days in ovo) and later disappear. The corresponding mature remnant was called n. interstitialis tractus opticus (ITO). The deep sublamina of SM forms a cap around n. rotundus. It becomes increasingly dispersed due to many passing fibers, and may be recognized in the mature brain as an area perirotundica (ApR). Clarification of the fate of embryonic SM bears on the confused terminology for various visual diencephalic nuclei. It is argued that the terms n. geniculatus dorsalis p. principalis and p. intercalaris, n. superficialis magnocellularis (in its wrong usage), n. lamminaris precommissuralis, n. lentiformis mesencephali p. medialis, p. parvocellularis and p. magnocellularis should be considered obsolete, on various embryological and hodologic grounds. An embryologically consistent terminology is proposed.

Retinal and tectal connections of embryonic nucleus superficialis magnocellularis and its mature derivatives in the chick.

In a companion paper (Puelles et al, this issue), the cytoarchitectonic development of the thalamic primordium called nucleus superficialis magnocellularis (SM) and its adult configuration in the chick were studied, correcting the misinterpretations that have impeded proper study of this neuronal group. Given its superficial position in the diencephalon, in contact with the optic tract and neighbouring retinorecipient grisea (SS, GV), as well as with the tecto-recipient n. rotundus, SM was suspected to have connections with centers of the visual pathway. In this paper we report the existence of a non-topographic retinal projection over the superficial adult derivate of SM (n. interstitialis tractus opticus, ITO) and a non-topographic, diffuse projection of the whole SM-derived population (area perirotundica, ApR, and ITO) onto the optic tectum. The latter was demonstrated throughout the late embryonic period in which SM loses its embryonic unitary character and becomes dispersed into its ill-defined, definitive adult portions (ITO, ApR). Golgi-like HRP- or DiI-labeling of SM cells showed a protracted immature appearance of their dendrites, expressed coincidently with a capacity to translocate superficially into the optic tract.

Acetylcholinesterase-histochemical differential staining of subdivisions within the nucleus rotundus in the chick.

Histochemical mapping of AChE activity in the chick diencephalon shows differential staining of several subregions within the nucleus rotundus. The topography and extent of these subdivisions were studied in transverse, horizontal and sagittal sections. A correlation with rotundic hodologic subdivisions reported in the literature is feasible, whereas several other chemoarchitectonic or functional markers show a homogeneous distribution throughout the n. rotundus. Moreover, cholinergic markers do not detect cholinergic afferents within the rotundic neuropile. Late embryonic appearance of the AChE heterogeneity suggests a modulation of neuropile AChE levels subsequent to synaptogenetic adjustment of differential hodology.

The locus of optic nerve head representation in the retinotopic projection over nucleus geniculatus lateralis ventralis and nucleus griseum tectalis in the chick also lacks a retinal projection.

Recently we have demonstrated the presence of a gap in the avian retinotectal projection, that corresponds with the locus of retinotopic representation of the elongated optic nerve head. The present report describes an equivalent gap in the optic neuropiles of the avian ventral geniculate nucleus and griseum tectalis formation, detected after filling optic terminals anterogradely from the contralateral eye with peroxidase. A projection-less strip appears at the expected retinotopic position in both grisea intersecting radially all the strata of the corresponding neuropiles. It is speculated that a common synaptogenetic mechanism must account for these gaps.

Segment-related, mosaic neurogenetic pattern in the forebrain and mesencephalon of early chick embryos: I. Topography of AChE-positive neuroblasts up to stage HH18.

Histochemical mapping of AChE-positive neuroblasts in sectioned and whole-mounted preparations of the chick embryo mesencephalon and prosencephalon allows a correlation of early neural tube morphogenesis (segmentation, longitudinal compartmentation) with the heterochronic pattern of neurogenesis. One significant finding is that the initial appearance of neuroblasts in the forebrain does not follow neuromeric segmentation, but evolves in parallel with it. Early neuroblasts appear as separate, distinct groups within specific matrix territories at the center of the transverse neuromeric segments. Neighbouring segments display different spatiotemporal patterns of neurogenesis. Overall gradients of differentiation in the rostrocaudal and ventrodorsal directions are absent, whereas a clear-cut segment-related, mosaic pattern becomes evident. Notwithstanding this, gross regularities of heterochrony in the neurogenetic behavior of the different segments lead to a definition of elemental longitudinal compartments of the forebrain and mesencephalon (floor, paramedian, basal, and alar regions) on the basis of precocious differentiation of the basal region and retarded differentiation of the paramedian and alar regions.

Location of the rostral end of the longitudinal brain axis: review of an old topic in the light of marking experiments on the closing rostral neuropore.

The rostral end of the forebrain was classically defined on the basis of descriptive data. Different assumptions on the mode of closure of the rostral neuropore caused three different theories of the rostral end of the forebrain to be formulated (His 1893a; von Kupffer, '06; Johnston, '09). Some recent descriptive and experimental data have put these theories into question. A piece of black nylon thread was inserted through the rostral neuropore of chick embryos and was fixed to its ventral lip. These operations were done at all intermediate stages during the process of closure of the rostral neuropore. The embryos were sacrificed at a later stage, by which time the neuropore had disappeared. In the cleared specimens the threads always lay at the same site, namely the upper border of lamina terminalis, irrespective of the stage at which the marker was inserted. These results stand against His's conception (1893a,b) of a sutura terminalis and support the single mechanism of sutura dorsalis during closure of the rostral neuropore. The marking data therefore imply that the topologic rostral end of the forebrain lies at the upper limit of lamina terminalis, as proposed by von Kupffer, '06).

The locus of optic nerve head representation in the chick retinotectal map lacks a retinal projection.

Retinotopic representation of the optic nerve head of the contralateral eye lies at the rostrodorsal face of the avian tectum. Since the homologous mammalian retino-collicular map shows an optic disc gap, the present anterograde HRP transport experiments were designed to detect an equivalent gap in the avian tectal retinorecipient strata. Sections tangential to the tectum at the locus of pecten representation displayed a thin, elongated, projection-less strip, crossing radially all retinorecipient laminae. Histochemical demonstration of NADH-diaphorase on similar sections showed low activity of this enzyme along an equivalent, elongated strip.

Solitary magnocellular neurons in the avian optic tectum: cytoarchitectonic, histochemical and [3H]thymidine autoradiographic characterization.

Solitary magnocellular neurons are described in the adult chick optic tectum on the basis of their large size, polygonal shape, intensely basophilic perikarya, characteristic position at the border between the stratum griseum centrale and the stratum album centrale, decreasing density along a rostrocaudal gradient, and intense activity of NADH-diaphorase. These characteristics distinguish this population from the adjacent ganglion cells of the stratum griseum centrale, which are more numerous, smaller, paler staining and have background levels of NADH-diaphorase. Moreover, the solitary magnocellular neurons appear unlabeled after tritiated-thymidine administration after stage 17+, and are thus born before the stratum griseum centrale neurons, which are generated after stage 19. These large cells may correspond to a class of stellate ganglion cells with thick spiny dendrites described by Ramón (1943).