Miriam González-Gómez

Neurons in the white matter of the adult human neocortex

The white matter (WM) of the adult human neocortex contains the so-called “interstitial neurons”. They are most numerous in the superficial WM underlying the cortical gyri, and decrease in density toward the deep WM. They are morphologically heterogeneous. A subgroup of interstitial neurons display pyramidal-cell like morphologies, characterized by a polarized dendritic tree with a dominant apical dendrite, and covered with a variable number of dendritic spines. In addition, a large contingent of interstitial neurons can be classified as interneurons based on their neurochemical profile as well as on morphological criteria. WM- interneurons have multipolar or bipolar shapes and express GABA and a variety of other neuronal markers, such as calbindin and calretinin, the extracellular matrix protein reelin, or neuropeptide Y, somatostatin, and nitric oxide synthase. The heterogeneity of interstitial neurons may be relevant for the pathogenesis of Alzheimer disease and schizophrenia. Interstitial neurons are most prominent in human brain, and only rudimentary in the brain of non-primate mammals. These evolutionary differences have precluded adequate experimental work on this cell population, which is usually considered as a relict of the subplate, a transient compartment proper of development and without a known function in the adult brain. The primate-specific prominence of the subplate in late fetal stages points to an important role in the establishment of interstitial neurons. Neurons in the adult WM may be actively involved in coordinating inter-areal connectivity and regulation of blood flow. Further studies in primates will be needed to elucidate the developmental history, adult components and activities of this large neuronal system.

Comparative aspects of p73 and Reelin expression in Cajal-Retzius cells and the cortical hem in lizard, mouse and human

Cajal-Retzius (CR) cells of the mammalian neocortex co-express the extracellular matrix protein Reelin and p73, a transcription factor involved in cell death and survival. Most neocortical CR cells derive from the cortical hem, with minor additional sources. We analyzed the distribution of Reelin and p73 immunoreactive (ir) neurons in the telencephalon of Lacerta galloti from early embryonic stages to hatching. Numerous Reelin-ir cells appeared in the pallial MZ from the preplate stage onward. Conversely, p73-ir cells were rare in the pallial preplate and not observed in the cortical plate. Subpallial p73-ir cells spread from the septum and the telencephalic-diencephalic boundary to the pial surface of the basal forebrain and amygdala, respectively, where they co-expressed Reelin and p73. A small group of Reelin/p73-ir CR cells appeared in a rudimentary cortical hem at the interface of the medial cortex and choroid plexus. Comparison with early embryonic stages of mice and humans showed similar foci of p73-ir cells in the septum and at the telencephalic-diencephalic boundary and revealed an increasing prominence of the cortical hem, in parallel with increasing numbers of neocortical Reelin/p73 positive CR cells, which attain highest differentiation in the human brain. Our data show that Reelin-expression in the pallium is evolutionarily conserved and independent of a cortical hem, and suggest that p73 in the cortical hem may be involved in the evolutionary increase in number and complexity of the mammalian neocortical CR cells.

Lhx2 Regulates the Development of the Forebrain Hem System

Early brain development is regulated by the coordinated actions of multiple signaling centers at key boundaries between compartments. Three telencephalic midline structures are in a position to play such roles in forebrain patterning: The cortical hem, the septum, and the thalamic eminence at the diencephalic–telencephalic boundary. These structures express unique complements of signaling molecules, and they also produce distinct populations of Cajal–Retzius cells, which are thought to act as “mobile patterning units,” migrating tangentially to cover the telencephalic surface. We show that these 3 structures require the transcription factor Lhx2 to delimit their extent. In the absence of Lhx2 function, all 3 structures are greatly expanded, and the Cajal–Retzius cell population is dramatically increased. We propose that the hem, septum, and thalamic eminence together form a “forebrain hem system” that defines and regulates the formation of the telencephalic midline. Disruptions in the forebrain hem system may be implicated in severe brain malformations such as holoprosencephaly. Lhx2 functions as a central regulator of this systems development. Since all components of the forebrain hem system have been identified across several vertebrate species, the mechanisms that regulate them may have played a fundamental role in driving key aspects of forebrain evolution.

Dynamic expression of the p53 family members p63 and p73 in the mouse and human telencephalon during development and in adulthood

p63 and p73, family members of the tumor suppressor p53, are critically involved in the life and death of mammalian cells. They display high homology and may act in concert. The p73 gene is relevant for brain development, and p73-deficient mice display important malformations of the telencephalon. In turn, p63 is essential for the development of stratified epithelia and may also play a part in neuronal survival and aging. We show here that p63 and p73 are dynamically expressed in the embryonic and adult mouse and human telencephalon. During embryonic stages, Cajal-Retzius cells derived from the cortical hem co-express p73 and p63. Comparison of the brain phenotypes of p63- and p73- deficient mice shows that only the loss of p73 function leads to the loss of Cajal-Retzius cells, whereas p63 is apparently not essential for brain development and Cajal-Retzius cell formation. In postnatal mice, p53, p63, and p73 are present in cells of the subventricular zone (SVZ) of the lateral ventricle, a site of continued neurogenesis. The neurogenetic niche is reduced in size in p73-deficient mice, and the numbers of young neurons near the ventricular wall, marked with doublecortin, Tbr1 and calretinin, are dramatically decreased, suggesting that p73 is important for SVZ proliferation. In contrast to their restricted expression during brain development, p73 and p63 are widely detected in pyramidal neurons of the adult human cortex and hippocampus at protein and mRNA levels, pointing to a role of both genes in neuronal maintenance in adulthood.

Dynamic expression of calretinin in embryonic and early fetal human cortex.

Calretinin (CR) is one of the earliest neurochemical markers in human corticogenesis. In embryos from Carnegie stages (CS) 17 to 23, calbindin (CB) and CR stain opposite poles of the incipient cortex suggesting early regionalization: CB marks the neuroepithelium of the medial boundary of the cortex with the choroid plexus (cortical hem). By contrast, CR is confined to the subventricular zone (SVZ) of the lateral and caudal ganglionic eminences at the pallial-subpallial boundary (PSB, or antihem), from where CR+/Tbr1- neurons migrate toward piriform cortex and amygdala as a component of the lateral cortical stream. At CS 19, columns of CR+ cells arise in the rostral cortex, and contribute at CS 20 to the “monolayer” of horizontal Tbr1+/CR+ and GAD+ cells in the preplate. At CS 21, the “pioneer cortical plate” appears as a radial aggregation of CR+/Tbr1+ neurons, which cover the entire future neocortex and extend the first corticofugal axons. CR expression in early human corticogenesis is thus not restricted to interneurons, but is also present in the first excitatory projection neurons of the cortex. At CS 21/22, the cortical plate is established following a lateral to medial gradient, when Tbr1+/CR- neurons settle within the pioneer cortical plate, and thus separate superficial and deep pioneer neurons. CR+ pioneer neurons disappear shortly after the formation of the cortical plate. Reelin+ Cajal-Retzius cells begin to express CR around CS21 (7/8 PCW). At CS 21–23, the CR+ SVZ at the PSB is the source of CR+ interneurons migrating into the cortical SVZ. In turn, CB+ interneurons migrate from the subpallium into the intermediate zone following the fibers of the internal capsule. Early CR+ and CB+ interneurons thus have different origins and migratory routes. CR+ cell populations in the embryonic telencephalon take part in a complex sequence of events not analyzed so far in other mammalian species, which may represent a distinctive trait of the initial steps of human corticogenesis.

Cortical hypoplasia and ventriculomegaly of p73-deficient mice: Developmental and adult analysis

Trp73, a member of the p53 gene family, plays a crucial role in neural development. We describe two main phenotypic variants of p73 deficiency in the brain, a severe one characterized by massive apoptosis in the cortex leading to early postnatal death and a milder, non‐/low‐apoptosis one in which 50% of pups may reach adulthood using an intensive‐care breeding protocol. Both variants display the core triad of p73 deficiency: cortical hypoplasia, hippocampal malformations, and ventriculomegaly. We studied the development of the neocortex in p73 KO mice from early embryonic life into advanced age (25 months). Already at E14.5, the incipient cortical plate of the p73 KO brains showed a reduced width. Examination of adult neocortex revealed a generalized, nonprogressive reduction by 10–20%. Area‐specific architectonic landmarks and lamination were preserved in all cortical areas. The surviving adult animals had moderate ventricular distension, whereas pups of the early lethal phenotypic variant showed severe ventriculomegaly. Ependymal cells of wild‐type ventricles strongly express p73 and are particularly vulnerable to p73 deficiency. Ependymal denudation by apoptosis and reduction of ependymal cilia were already evident in young mice, with complete absence of cilia in older animals. Loss of p73 function in the ependyma may thus be one determining factor for chronic hydrocephalus, which leads to atrophy of subcortical structures (striatum, septum, amygdala). p73 Is thus involved in a variety of CNS activities ranging from embryonic regulation of brain size to the control of cerebrospinal fluid homeostasis in the adult brain via maintenance of the ependyma. J. Comp. Neurol. 522:2663–2679, 2014.

Telocyte Behaviour During Inflammation, Repair and Tumour Stroma Formation

In this chapter, we outline the role of human CD34+ stromal cells/telocytes (CD34+ SC/TCs) as progenitor cells during repair. The in vivo activation phenomena of CD34+ SC/TCs in this process include increased size; separation from the neighbouring structures (mainly of the vascular walls); association with inflammatory cells, predominantly macrophages; development of the organelles of synthesis (rough endoplasmic reticulum and Golgi apparatus); cell proliferation with presence of mitosis and high proliferative index (transit-amplifying cells); and fibroblastic and myofibroblastic differentiation. A procedure to study these tissue-resident cells, comparison of their behaviour in vivo and in vitro and different behaviour depending on location, time, type of injury (including tumour stroma) and greater or lesser proximity to the injury are also considered.

The Subpial Granular Layer and Transient Versus Persisting Cajal-Retzius Neurons of the Fetal Human Cortex

Neurons of the subpial granular layer (SGL) in the human marginal zone (MZ) migrate tangentially from the periolfactory subventricular zone all over the neocortex. After an immature stage, from 14 to 18 gestational weeks (GW), the SGL attains maximum prominence around midgestation. At 20-25 GW, a transient miniature cell type in the MZ expresses glutamate decarboxylase (GAD) and calretinin, and extends a varicose plexus surrounding somata of large transient Cajal-Retzius neurons (tCRN), potentially modulating their activity. The compact Reelin+ horizontal axon plexus of tCRN forms a transient interface between cortical plate and MZ; it may serve as a migration substrate for cortical interneurons, and attracting NPY+ fibers from the subplate. Around 30 GW, after the disappearance of SGL and tCRN, a population of persisting Cajal-Retzius neurons (pCRN) appears and remains into adult life. pCRNs express Reelin, Tbr1, calretinin, nitric oxide synthase, and the cytokine receptor CXCR4. They are characterized by subpial location, closeness to blood vessels, and aggregation in the walls of developing sulci. Unlike tCRNs, pCRNs do not develop a compact axon plexus in the lower MZ. Occasional mitoses in the midgestation SGL suggest that CRN progenitor cells may give rise to late-appearing pCRNs populating the definitive molecular layer.

The heterogeneity of human Cajal-Retzius neurons

The definition of a Cajal-Retzius neuron (CRN) is still controversial, in part possibly due to species differences. We review the developmental history of CRN in human neocortex and focus on two main CRN family members, transient (t) and persisting (p) CRN. They share the expression of Reelin andTbr1, complemented by p73, calretinin, CXCR4 and NOS, but differ in their moment of appearance, fate and morphology. The distinctive feature of tCRN is the axon plexus in the lower third of the marginal zone, which innervates the apical dendritic tufts of pyramidal cells and may serve as a migration substrate and waiting compartment for interneurons descending from the subpial granular layer (SGL) into the cortical plate. Around midgestation, the SGL also gives rise to a transient interneuron type, the miniature neuron, that provides the GABAergic innervation of tCRN, which eventually, through diverse signaling pathways involving p73, contribute to the demise of tCRN and the breakdown of their plexus. The pCRN appear in the last trimester of gestation and may derive from committed CRN progenitors which migrate with the SGL from the periolfactory forebrain. They lack the horizontal CR plexus, and may be implicated in cortical folding, distribution of blood vessels, and plasticity of microcircuits in the molecular layer.

Sinusoidal hemangioma and intravascular papillary endothelial hyperplasia: Interrelated processes that share a histogenetic piecemeal angiogenic mechanism

Sinusoidal hemangioma, characterized by interconnecting thin-walled vascular spaces, may present papillae/pseudo-papillae and zones that resemble intravascular papillary endothelial hyperplasia (IPEH). Our objectives are to explore the existence of zones in IPEH with sinusoidal hemangioma characteristics, the mechanism of papillary and septa formation in sinusoidal hemangioma and the comparison of this mechanism with that in IPEH. For these purposes, specimens of 4 cases of each entity were selected and studied by serial histologic sections and by immunochemistry and immunofluorescence procedures. The results showed a) zones with characteristics of sinusoidal hemangioma in IPEH cases, b) presence in both entities of papillae with a cover formed by a monolayer of CD34+ and CD31+ endothelial cells (ECs) and a core formed by either type I collagen and αSMA+ cells (presenting a pericyte/smooth muscle cell aspect) or thrombotic components, and c) a similar piecemeal angiogenic mechanism in papillary formation, including sprouting of intimal ECs toward the vessel wall itself or intravascular thrombi, formation of vascular loops that encircle and separate vessel wall or thrombus components, and parietal or thrombotic papillae development. The major differences between both entities were the number, arrangement and substrate of papillae: myriad, densely grouped, parietal and thrombotic papillae in IPEH, and a linear arrangement of predominant parietal papillae in sinusoidal hemangioma, originating septa (segmentation). In conclusion, sinusoidal hemangioma and IPEH are interrelated processes, which share morphologic findings and a piecemeal angiogenic mechanism, combining sprouting and intussusceptive angiogenesis, and leading to papillary formation and vessel segmentation.