Adult neurogenesis in the telencephalon of a lizard: a [3H]thymidine autoradiographic and bromodeoxyuridine immunocytochemical study

A qualitative and quantitative evaluation of adult neurogenesis in the telencephalon of a lizard was conducted using [3H]thymidine autoradiography and immunocytochemical detection of 5'-bromodeoxyuridine (BrdU). The sites of cell proliferation, the fate, and the phenotype of cells born in adulthood were determined by short and long survival experiments (7 and 30 days). Adult specimens of the Moorish gecko, Tarentola mauritanica (Sauria, Gekkonidae), received subcutaneous injections of either BrdU or [3H]thymidine. Seven days after BrdU administration most labelled cells were in the walls (ventricular zone; VZ) of the lateral ventricles, particularly close to the sulcal zones. Labelled cells in the VZ of other ventricles were practically absent. Following 30 days, labelled cells outside the VZ were seen in most telencephalic regions: olfactory bulbs (OB), medial, dorsal, and lateral cortices (MC, DC, LC), anterior dorsal ventricular ridge (ADVR), nucleus sphericus (NS), and striatum (St). Electron microscopic examination of [3H]thymidine labelled cells found in these areas revealed their neuronal identity. No labelled neurons were detected in the dorsomedial cerebral cortex (DMC), septum (Sp), or elsewhere in the telencephalon. We conclude that neurogenesis occur in most regions of the adult lizard telencephalon. The most intense neuronal production occurs in the MC. Other telencephalic regions, such as the OB, ADVR, and NS, also show a considerable rate of adult neurogenesis. We infer from our results that the new neurons are born in the walls of the lateral ventricles and then migrate to their destinations. Furthermore, we report some evidence suggesting that neurons formed in adulthood in the olfactory bulbs are generated in the distant proliferative VZ. The functional significance of neurogenesis in the telencephalon of adult lizards is unclear, but it seems to be restricted to areas that have been implicated in olfactory plasticity, learning, and memory.

Chain migration of neuronal precursors

In the brain of adult mice, cells that divide in the subventricular zone of the lateral ventricle migrate up to 5 millimeters to the olfactory bulb where they differentiate into neurons. These migrating cells were found to move as chains through a well-defined pathway, the rostral migratory stream. Electron microscopic analysis of serial sections showed that these chains contained only closely apposed, elongated neuroblasts connected by membrane specializations. A second cell type, which contained glial fibrillary acidic protein, ensheathed the chains of migrating neuroblasts. Thus, during chain migration, neural precursors moved associated with each other and were not guided by radial glial or axonal fibers.

Lesion and regeneration in the medial cerebral cortex of lizards

The cerebral cortex of Squamate reptiles (lizards and snakes) may be regarded as an archicortex or "reptilian hippocampus". In lizards, one cortical area, the medial cortex, may be considered as a true "fascia dentata" on grounds of its anatomy, connectivity and cyto- chemo-architectonics of its main zinc-rich axonal projection. Moreover, its late ontogenesis and postnatal development support this view. In normal conditions, it shows delayed postnatal neurogenesis and growth during the lizard's life span. Remnant neuroblasts in the medial cortical ependyma of adult lizards seasonally proliferate. The late-produced immature neurocytes migrate to the medial cortex cell layer where they differentiate and give off zinc-containing axons directed to the rest of cortical areas. This results in a continuous growth of the medial cortex and its zinc-rich axonal projection. Perhaps the most important characteristic of the lizard medial cortex is that it can regenerate after having been almost completely destroyed. Recent experiments in our laboratory have shown that chemical lesion of its neurons (up to 95%) results in a cascade of events; first, those related with massive neuronal death and axonal-dendritic retraction and, secondly, those related with a triggered neuroblast proliferation and subsequent neo-histogenesis, and the regeneration of an almost new medial cortex that shows itself undistinguishable from a normal undamaged one. This is the only report to our knowledge that an amniote central nervous centre may regenerate by new neuron production and neo-histogenesis. Perhaps the medial cortex of lizards may be used as a model for neuronal regeneration and/or transplant experiments in mammals or even in primates.

Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards

Systemic administration of the neurotoxin 3-acetylpyridine to adult lizards results in extensive loss of neurons in the medial cerebral cortex, other brain areas remaining largely unaffected. After the neurotoxic trauma, new cells are produced by mitotic division of cells in the ventricular wall. The new cells migrate along radial glial fibers and replace lost neurons in the medial cortex. Electron microscopic examination of cells labeled with [3H]thymidine confirms that the newly generated cells are neurons. Thus, neuron regeneration can occur in the cerebral cortex of adult lizards.

Morphological aspects of the ectopic granule-like cellular populations in the albino rat hippocampal formation: a Golgi study

Rat hippocampal formation was examined by the Golgi impregnation method. Three different ectopic granule-like populations of cells were differentiated: (1) Ectopic granule-like cells of the regio inferior, located in the stratum radiatum; one or two dendrites arose from the cell body and ran towards either the molecular layer of the fascia dentata or the stratum lacunosum-moleculare of the hippocampus, where they branched into secondary and tertiary dendrites. (2) Ectopic granule-like cells of the hilar region; this cell population showed bipolar and monopolar types of dendritic tree. Unipolar cells had dendrites oriented towards the granular layer where they branched profusely. Dendritic arborisation of bipolar cells was confined to the hilus. (3) Ectopic granule-like cells of the molecular layer; they showed several structural appearances depending on their location within the layer. Axonal tracts of ectopic granule-like cells gave rise to numerous collaterals; the main branch ran to the CA4 and CA3 hippocampal subregions. Several 'en passant' and mossy-like boutons were shown along this path.