The distribution of doublecortin-immunopositive cells in the brains of four afrotherian mammals: the Hottentot golden mole (Amblysomus hottentotus), the rock hyrax (Procavia capensis), the eastern rock sengi (Elephantulus myurus) and the four-toed sengi (Petrodromus tetradactylus)

Methods to study adult hippocampal neurogenesis in humans and across the phylogeny

The hippocampus hosts the continuous addition of new neurons throughout life-a phenomenon named adult hippocampal neurogenesis (AHN). Here we revisit the occurrence of AHN in more than 110 mammalian species, including humans, and discuss the further validation of these data by single-cell RNAseq and other alternative techniques. In this regard, our recent studies have addressed the long-standing controversy in the field, namely whether cells positive for AHN markers are present in the adult human dentate gyrus (DG). Here we review how we developed a tightly controlled methodology, based on the use of high-quality brain samples (characterized by short postmortem delays and ≤24 h of fixation in freshly prepared 4% paraformaldehyde), to address human AHN. We review that the detection of AHN markers in samples fixed for 24 h required mild antigen retrieval and chemical elimination of autofluorescence. However, these steps were not necessary for samples subjected to shorter fixation periods. Moreover, the detection of labile epitopes (such as Nestin) in the human hippocampus required the use of mild detergents. The application of this strictly controlled methodology allowed reconstruction of the entire AHN process, thus revealing the presence of neural stem cells, proliferative progenitors, neuroblasts, and immature neurons at distinct stages of differentiation in the human DG. The data reviewed here demonstrate that methodology is of utmost importance when studying AHN by means of distinct techniques across the phylogenetic scale. In this regard, we summarize the major findings made by our group that emphasize that overlooking fundamental technical principles might have consequences for any given research field.

Doublecortin-Expressing Neurons in Human Cerebral Cortex Layer II and Amygdala from Infancy to 100 Years Old

A cohort of morphologically heterogenous doublecortin immunoreactive (DCX +) "immature neurons" has been identified in the cerebral cortex largely around layer II and the amygdala largely in the paralaminar nucleus (PLN) among various mammals. To gain a wide spatiotemporal view on these neurons in humans, we examined layer II and amygdalar DCX + neurons in the brains of infants to 100-year-old individuals. Layer II DCX + neurons occurred throughout the cerebrum in the infants/toddlers, mainly in the temporal lobe in the adolescents and adults, and only in the temporal cortex surrounding the amygdala in the elderly. Amygdalar DCX + neurons occurred in all age groups, localized primarily to the PLN, and reduced in number with age. The small-sized DCX + neurons were unipolar or bipolar, and formed migratory chains extending tangentially, obliquely, and inwardly in layers I-III in the cortex, and from the PLN to other nuclei in the amygdala. Morphologically mature-looking neurons had a relatively larger soma and weaker DCX reactivity. In contrast to the above, DCX + neurons in the hippocampal dentate gyrus were only detected in the infant cases in parallelly processed cerebral sections. The present study reveals a broader regional distribution of the cortical layer II DCX + neurons than previously documented in human cerebrum, especially during childhood and adolescence, while both layer II and amygdalar DCX + neurons persist in the temporal lobe lifelong. Layer II and amygdalar DCX + neurons may serve as an essential immature neuronal system to support functional network plasticity in human cerebrum in an age/region-dependent manner.

Neural stem cell research in Africa: current realities and future prospects

Neural stem cells (NSCs) are immature progenitor cells that are found in developing and adult brains that have the potential of dividing actively and renewing themselves, with a complex form of gene expression. The generation of new brain cells in adult individuals was initially considered impossible, however, the landmark discovery of human neural stem cells in the hippocampus has been followed by further discoveries in other discreet regions of the brain. Investigation into the current state in Africa of the research and use of NSCs shows relatively limited activities on the continent. Information on the African application of NSCs for modelling disease mechanisms, drug discovery, and therapeutics is still limited. The International Brain Research Organization (IBRO)-African Regional Committee (ARC), with support from the Company of Biologists, and the Movement Disorder Society, sponsored the first African Basic School on NSC in Ibadan, Nigeria, with the vision of bringing together young neuroscientists and physicians across different fields in neuroscience to learn from leaders who have applied NSCs in stem cell research, the pathophysiology of neurodegenerative diseases, neuroanatomy, and neurotherapeutics. Twenty early-career researchers in academic institutions at junior and senior faculty cadres were selected from South Africa, Uganda and Nigeria. The students and organizer of the school, who wrote this review on the state of NSCs research in Africa, recommended the following: (1) other African countries can take a cue from South Africa and Nigeria in probing the phenomena of adult neurogenesis in unique animal species on the continent; (2) Africa should leverage the expertise and facilities of South African scientists and international collaborators in scaling up NSC research into these unique species and (3) Centers of Excellence should be established on the continent to serve as research hubs for training postgraduate students, and facilities for African scientists who trained overseas on NSCs.

Postnatal and Adult Neurogenesis in Mammals, Including Marsupials

In mammals, neurogenesis occurs during both embryonic and postnatal development. In eutherians, most brain structures develop embryonically; conversely, in marsupials, a number of brain structures develop after birth. The exception is the generation of granule cells in the dentate gyrus, olfactory bulb, and cerebellum of eutherian species. The formation of these structures starts during embryogenesis and continues postnatally. In both eutherians and marsupials, neurogenesis continues in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation throughout life. The majority of proliferated cells from the SVZ migrate to the olfactory bulb, whereas, in the dentate gyrus, cells reside within this structure after division and differentiation into neurons. A key aim of this review is to evaluate advances in understanding developmental neurogenesis that occurs postnatally in both marsupials and eutherians, with a particular emphasis on the generation of granule cells during the formation of the olfactory bulb, dentate gyrus, and cerebellum. We debate the significance of immature neurons in the piriform cortex of young mammals. We also synthesize the knowledge of adult neurogenesis in the olfactory bulb and the dentate gyrus of marsupials by considering whether adult-born neurons are essential for the functioning of a given area.

Doublecortin-Expressing Neurons in Chinese Tree Shrew Forebrain Exhibit Mixed Rodent and Primate-Like Topographic Characteristics

Doublecortin (DCX) is transiently expressed in new-born neurons in the subventricular zone (SVZ) and subgranular zone (SGZ) related to adult neurogenesis in the olfactory bulb (OB) and hippocampal formation. DCX immunoreactive (DCX+) immature neurons also occur in the cerebral cortex primarily over layer II and the amygdala around the paralaminar nucleus (PLN) in various mammals, with interspecies differences pointing to phylogenic variation. The tree shrews (Tupaia belangeri) are phylogenetically closer to primates than to rodents. Little is known about DCX+ neurons in the brain of this species. In the present study, we characterized DCX immunoreactivity (IR) in the forebrain of Chinese tree shrews aged from 2 months- to 6 years-old (n = 18). DCX+ cells were present in the OB, SVZ, SGZ, the piriform cortex over layer II, and the amygdala around the PLN. The numerical densities of DCX+ neurons were reduced in all above neuroanatomical regions with age, particularly dramatic in the DG in the 5–6 years-old animals. Thus, DCX+ neurons are present in the two established neurogenic sites (SVZ and SGZ) in the Chinese tree shrew as seen in other mammals. DCX+ cortical neurons in this animal exhibit a topographic pattern comparable to that in mice and rats, while these immature neurons are also present in the amygdala, concentrating around the PLN as seen in primates and some nonprimate mammals.

Widespread Doublecortin Expression in the Cerebral Cortex of the Octodon degus

It has been demonstrated that in adulthood rodents show newly born neurons in the subgranular layer (SGL) of the dentate gyrus (DG), and in the subventricular zone (SVZ). The neurons generated in the SVZ migrate through the rostral migratory stream (RMS) to the olfactory bulb. One of the markers of newly generated neurons is doublecortin (DCX). The degu similarly shows significant numbers of DCX-labeled neurons in the SGL, SVZ, and RMS. Further, most of the nuclei of these DCX-expressing neurons are also labeled by proliferating nuclear antigen (PCNA) and Ki67. Finally, whereas in rats and mice DCX-labeled neurons are predominantly present in the SGL and SVZ, with only a few DCX neurons present in piriform cortex, the degu also shows significant numbers of DCX expressing neurons in areas outside of SVZ, DG, and PC. Many areas of neocortex in degu demonstrate DCX-labeled neurons in layer II, and most of these neurons are found in the limbic cortices. The DCX-labeled cells do not stain with NeuN, indicating they are immature neurons.

The hypercholinergic brain of the Cape golden mole (Chrysochloris asiatica)

Studies detailing the anatomy of the brain of the golden moles are few. A recent study indicated that in the Hottentot golden mole (a member of the Amblysominae clade), there was a broad, atypical, distribution of cholinergic interneurons in the olfactory bulb, cerebral cortex, hippocampus and amygdala. To determine whether this broad distribution of cholinergic neurons is shared by other species of golden mole, we here examine the brain of the Cape golden mole (a member of the Chrysochlorinae clade, representing the second major clade within the family Chrysochloridae). Our analyses indicates the presence of a similar widespread distribution of cholinergic interneurons in the Cape golden mole. Thus, we conclude that these features are derived morphological traits in the brains of golden moles. In addition, we describe the nuclei generally considered to be part of the typical cholinergic system in mammals. Whereas the vast majority of these generally reported cholinergic nuclei were the same as recorded in other Eutherian mammals, it was noted that the cholinergic nuclei involved in oculomotion were substantially reduced in size, or absent in the case of the abducens nucleus. In addition, there was an absence of the cholinergic medial septal nucleus, but the presence of a cholinergic lateral septal nucleus. The laterodorsal and pedunculopontine tegmental nuclei evince regions where the cholinergic neurons are densely packed. These are atypical features of the mammalian cholinergic system, which when combined with the widespread atypical distribution of cholinergic interneurons, reveals a family-specific complement of cholinergic nuclei in the Chrysochloridae.

Cellular Plasticity in the Adult Murine Piriform Cortex: Continuous Maturation of Dormant Precursors Into Excitatory Neurons

Neurogenesis in the healthy adult murine brain is based on proliferation and integration of stem/progenitor cells and is thought to be restricted to 2 neurogenic niches: the subventricular zone and the dentate gyrus. Intriguingly, cells expressing the immature neuronal marker doublecortin (DCX) and the polysialylated-neural cell adhesion molecule reside in layer II of the piriform cortex. Apparently, these cells progressively disappear along the course of ageing, while their fate and function remain unclear. Using DCX-CreER^T2/Flox-EGFP transgenic mice, we demonstrate that these immature neurons located in the murine piriform cortex do not vanish in the course of aging, but progressively resume their maturation into glutamatergic (TBR1⁺, CaMKII⁺) neurons. We provide evidence for a putative functional integration of these newly differentiated neurons as indicated by the increase in perisomatic puncta expressing synaptic markers, the development of complex apical dendrites decorated with numerous spines and the appearance of an axonal initial segment. Since immature neurons found in layer II of the piriform cortex are generated prenatally and devoid of proliferative capacity in the postnatal cortex, the gradual maturation and integration of these cells outside of the canonical neurogenic niches implies that they represent a valuable, but nonrenewable reservoir for cortical plasticity.

The brain of the tree pangolin (Manis tricuspis). IV. The hippocampal formation

Employing a range of standard and immunohistochemical stains we provide a description of the hippocampal formation in the brain of the tree pangolin. For the most part, the architecture, chemical neuroanatomy, and topological relationships of the component parts of the hippocampal formation of the tree pangolin were consistent with that observed in other mammalian species. Within the hippocampus proper fields CA1, 3, and 4 could be identified with certainty, while CA2 was tentatively identified as a small transitional zone between the CA1 and CA3 fields. Within the dentate gyrus evidence for adult hippocampal neurogenesis at a rate comparable to other mammals was observed. The subicular complex and entorhinal cortex also exhibited divisions typically observed in other mammalian species. In contrast to many other mammals, an architecturally and neurochemically distinct CA4 field was observed, supporting Lorente de Nó's proposed CA4 field, at least in some mammalian species. In addition, up to seven laminae were evident in the dentate gyrus. Calretinin immunostaining revealed the three sublamina of the molecular layer, while immunostaining for vesicular glutamate transporter 2 and neurofilament H indicate that the granule cell layer was composed of two sublamina. The similarities and differences observed in the tree pangolin indicate that the hippocampal formation is an anatomically and neurochemically conserved neural unit in mammalian evolution, but minor changes may relate to specific life history features and habits of species.

Doublecortin-expressing cell types in temporal lobe epilepsy

Doublecortin (DCX) is widely regarded as a marker of immature and migrating neurons during development. While DCX expression persists in adults, particularly in the temporal lobe and neurogenic regions, it is unknown how seizures influence its expression. The aim of the present study was to explore the distribution and characteristics of DCX-expressing cells in surgical and postmortem samples from 40 adult and paediatric patients, with epilepsy and with or without hippocampal sclerosis (HS), compared to post mortem controls. The hippocampus (pes and body), parahippocampal gyrus, amygdala, temporal pole and temporal cortex were examined with DCX immunohistochemistry using four commercially-available DCX antibodies, labelled cells were quantified in different regions of interest as well as their co-expression with cell type specific markers (CD68, Iba1, GFAP, GFAP∂, nestin, SOX2, CD34, OLIG2, PDGFRβ, NeuN) and cell cycle marker (MCM2). Histological findings were compared with clinical data, as well as gene expression data obtained from the temporal cortex of 83 temporal lobe epilepsy cases with HS. DCX immunohistochemistry identified immature (Nestin⁻/NeuN⁻) neurons in layer II of the temporal neocortex in patients with and without epilepsy. Their number declined significantly with age but was not associated with the presence of hippocampal sclerosis, seizure semiology or memory dysfunction. DCX⁺ cells were prominent in the paralaminar nuclei and periamygdalar cortex and these declined with age but were not significantly associated with epilepsy history. DCX expressing cells with ramified processes were prominent in all regions, particularly in the hippocampal subgranular zone, where significantly increased numbers were observed in epilepsy samples compared to controls. DCX ramified cells co-expressed Iba1, CD68 and PDGFRβ, and less frequently MCM2, OLIG2 and SOX2, but no co-localization was observed with CD34, nestin or GFAP/GFAP ∂. Gene expression data from neocortical samples in patients with TLE and HS supported ongoing DCX expression in adults. We conclude that DCX identifies a range of morphological cell types in temporal lobe epilepsy, including immature populations, glial and microglial cell types. Their clinical relevance and biological function requires further study but we show some evidence for alteration with age and in epilepsy.

Nuclear organization of the African elephant (Loxodonta africana) amygdaloid complex: an unusual mammalian amygdala

Here we describe the nuclear organization of the African elephant amygdaloid complex using Nissl, myelin, and a range of immunohistochemical stains. The African elephant is thought to exhibit many affect-laden and social-empathic behaviours; however, to date the amygdaloid complex, which is the generator of emotional states of the brain is yet to be fully explored in the elephants. For the most part, the amygdaloid complex of the African elephant is similar to that observed in other mammals in terms of the presence of nuclei and their topological relationships; however, we did observe several specific differences in amygdaloid organization. The elephant amygdala has undergone rotation in both the coronal and sagittal planes, seemingly associated with the expansion of the temporal lobe. Numerous scalloped cell clusters, termed glomeruli, forming the intermediate nuclei of the basal, accessory basal and central nuclear groups, were occupied by structures immunopositive to doublecortin. The nuclei typically associated with the accessory olfactory system (posterior cortical nucleus and medial nuclear complex) were absent from the elephant amygdala. The anterior cortical nucleus is very large and appears to be comprised of two subdivisions. The lateral nuclear complex is expanded and has two novel subdivisions. The amygdalohippocampal area appears relatively enlarged. The numerous shared and derived characters make the elephant amygdaloid complex very unusual and unique amongst mammals, but the derived characters appear to relate to observed elephant affect-laden behaviours.

The Distribution of Ki-67 and Doublecortin Immunopositive Cells in the Brains of Three Microchiropteran Species, Hipposideros fuliginosus, Triaenops persicus, and Asellia tridens

This study uses Ki-67 and doublecortin (DCX) immunohistochemistry to delineate potential neurogenic zones, migratory pathways, and terminal fields associated with adult neurogenesis in the brains of three microchiropterans. As with most mammals studied to date, the canonical subgranular and subventricular neurogenic zones were observed. Distinct labeling of newly born cells and immature neurons within the dentate gyrus of the hippocampus was observed in all species. A distinct rostral migratory stream (RMS) that appears to split around the medial aspect of the caudate nucleus was observed. These two rostral stream divisions appear to merge at the rostroventral corner of the caudate nucleus to turn and enter the olfactory bulb, where a large terminal field of immature neurons was observed. DCX immunolabeled neurons were observed mostly in the rostral neocortex, but a potential migratory stream to the neocortex was not identified. A broad swathe of newly born cells and immature neurons was found between the caudoventral division of the RMS and the piriform cortex. In addition, occasional immature neurons were observed in the amygdala and DCX-immunopositive axons were observed in the anterior commissure. While the majority of these features have been found in several mammal species, the large number of DCX immunolabeled cells found between the RMS and the piriform cortex and the presence of DCX immunostained axons in the anterior commissure are features only observed in microchiropterans and insectivores to date. In the diphyletic scenario of chiropteran evolution, these observations align the microchiropterans with the insectivores. Anat Rec, 299:1548-1560, 2016. © 2016 Wiley Periodicals, Inc.

Sp8 expression in putative neural progenitor cells in guinea pig and human cerebrum

Neural stem/progenitor cells have been characterized at neurogenic sites in adult mammalian brain with various molecular markers. Here it has been demonstrated that Sp8, a transcription factor typically expressed among mature GABAergic interneurons, also labels putative neural precursors in adult guinea pig and human cerebrum. In guinea pigs, Sp8 immunoreactive (Sp8+) cells were localized largely in the superficial layers of the cortex including layer I, as well as the subventricular zone (SVZ) and subgranular zone (SGZ). Sp8+ cells at the SGZ showed little colocalization with mature and immature neuronal markers, but co-expressed neural stem cell markers including Sox2. Some layer I Sp8+ cells also co-expressed Sox2. The amount of Sp8+ cells in the dentate gyrus was maintained 2 weeks after X-ray irradiation, while that of doublecortin (DCX+) cells was greatly reduced. Mild ischemic insult caused a transient increase of Sp8+ cells in the SGZ and layer I, with the subgranular Sp8+ cells exhibited an increased colabeling for the mitotic marker Ki67 and pulse-chased bromodeoxyuridine (BrdU). Sp8+ cells in the dentate gyrus showed an age-related decline in guinea pigs, in parallel with the loss of DCX+ cells in the same region. In adult humans, Sp8+ cells exhibited comparable morphological features as seen in guinea pigs, with those at the SGZ and some in cortical layer I co-expressed Sox2. Together, these results suggested that Sp8 may label putative neural progenitors in guinea pig and human cerebrum, with the labeled cells in the SGZ appeared largely not mitotically active under normal conditions. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 939-955, 2016.

Brain size and limits to adult neurogenesis

The walls of the cerebral ventricles in the developing embryo harbor the primary neural stem cells from which most neurons and glia derive. In many vertebrates, neurogenesis continues postnatally and into adulthood in this region. Adult neurogenesis at the ventricle has been most extensively studied in organisms with small brains, such as reptiles, birds, and rodents. In reptiles and birds, these progenitor cells give rise to young neurons that migrate into many regions of the forebrain. Neurogenesis in adult rodents is also relatively widespread along the lateral ventricles, but migration is largely restricted to the rostral migratory stream into the olfactory bulb. Recent work indicates that the wall of the lateral ventricle is highly regionalized, with progenitor cells giving rise to different types of neurons depending on their location. In species with larger brains, young neurons born in these spatially specified domains become dramatically separated from potential final destinations. Here we hypothesize that the increase in size and topographical complexity (e.g., intervening white matter tracts) in larger brains may severely limit the long-term contribution of new neurons born close to, or in, the ventricular wall. We compare the process of adult neuronal birth, migration, and integration across species with different brain sizes, and discuss how early regional specification of progenitor cells may interact with brain size and affect where and when new neurons are added.

Prenatal genesis of layer II doublecortin expressing neurons in neonatal and young adult guinea pig cerebral cortex

Cells expressing doublecortin (DCX+) occur at cortical layer II, predominantly over the paleocortex in mice/rats, but also across the neocortex among larger mammals. Here, we explored the time of origin of these cells in neonatal and 2-month-old guinea pigs following prenatal BrdU pulse-chasing. In the neocortex, BrdU+ cells birth-dated at embryonic day 21 (E21), E28, and E35 laminated over the cortical plate with an inside-out order. In the piriform cortex, cells generated at E21 and E28 occurred with a greater density in layer II than III. Many cells were generated at later time points until birth, occurring in the cortex without a laminar preference. DCX+ cells in the neocortex and piriform cortex partially co-colocalized with BrdU (up to 7.5%) in the newborns after pulse-chasing from E21 to E49 and in the 2 month-old animals after pulse-chasing from E28 to E60/61, with higher rates seen among the E21-E35 groups. Together, layer II DCX+ cells in neonatal and young adult guinea pigs may be produced over a wide prenatal time window, but mainly during the early phases of corticogenesis. Our data also show an earlier establishment of the basic lamination in the piriform relative to neocortical areas in guinea pigs.