Calbindin-D28k and calretinin expression in the forebrain of anuran and urodele amphibians: Further support for newly identified subdivisions

Immunohistochemical localization of calbindin-D28k and calretinin in the spinal cord of lungfishes

A common pattern of distribution of neurons and fibers containing the calcium-binding proteins calbindin-D28k (CB) and calretinin (CR) in the spinal cord of terrestrial vertebrates has been recently demonstrated. Lungfishes are considered the closest living relatives of tetrapods, but practically no experimental data exist on the organization of their spinal cord. By means of immunohistochemical techniques, the localization of CB and CR was investigated in the spinal cord of the African (Protopterus dolloi) and Australian (Neoceratodus forsteri) lungfishes. Abundant cell bodies and fibers immunoreactive for either CB or CR were widely distributed throughout the spinal cord. A large population of immunoreactive cells was found in the dorsal column of the gray matter in both species, and abundant cells were distributed in the lateral and ventral columns. Ventrolateral motoneurons and multipolar cells were only intensely CB and CR immunoreactive in Neoceratodus. For the most part, separate cell populations contained either CB or CR, but a small subset of dorsally located neurons contained both in the two lungfishes. Colocalization was found in motoneurons and in ventrolaterally located cells only in Neoceratodus. Fiber labeling showed a predominance of CR-containing axons in the lateral and ventral funiculi of presumed supraspinal origin. These results show that lung-fishes and tetrapods have many features in common, suggesting that primitive anatomical, and likely functional, organization of the spinal cord of tetrapods is present in lungfishes.

Lungfishes, like tetrapods, possess a vomeronasal system

The vomeronasal system (VNS) is an accessory olfactory system that in tetrapod vertebrates is composed of specific receptor neurons in the nasal organ and a set of centers in the forebrain that receive and relay the information consecutively towards the hypothalamus. Thus, only in tetrapods the VNS comprises a discrete vomeronasal (Jacobson's) organ, which contains receptor cells that are morphologically distinct from those of the olfactory epithelium and use different transduction mechanisms. The axons of the vomeronasal receptors in tetrapods project to the accessory olfactory bulb (AOB) in the rostral telencephalon. Secondary vomeronasal connections exist through the medial amygdala to the hypothalamus. Currently, the lungfishes are considered the closest living relatives of tetrapods. Here we show that the African lungfish, Protopterus dolloi, has epithelial crypts at the base of the lamellae of the olfactory epithelium that express markers of the vomeronasal receptors in tetrapods. The projections of these crypts allow us to identify an AOB on the lateral margin of the main olfactory bulb. The projections of this AOB reach a region that is topologically, hodologically, and immunohistochemically identical to the medial amygdala and could represent its homolog. Neurons of this putative medial amygdala were demonstrated to project to the lateral hypothalamus, as they do in tetrapods. All these features that lungfishes share with tetrapods indicate that lungfishes have the complete set of brain centers and connections involved in processing vomeronasal information and that these features were already present in the last common ancestor of lungfishes and tetrapods.

Sonic hedgehog expression during Xenopus laevis forebrain development

We have analyzed the developing expression pattern of x-Shh in the Xenopus forebrain, interpreting the results within the framework of the neuromeric model to assess evolutionary trends and clues. To achieve this goal, we have characterized phenotypically the developing x-Shh expressing forebrain subdivisions and neurons by means of the combination of in situ hybridization for x-Shh and immunohistochemistry for the detection of forebrain essential regulators and markers, such as the homeodomain transcription factors Islet 1, Orthopedia, NKX2.1 and NKX2.2 and tyrosine hydroxylase. Substantial evidence was found for x-Shh expression in the telencephalic commissural preoptic area and this is strongly correlated with the presence of a pallidum and/or a basal telencephalic cholinergic system. In the diencephalon, x-Shh was demonstrated in the zona limitans intrathalamica and the x-Shh expressing cells were extended into the prethalamus. Throughout development and in the adult hypothalamic x-Shh expression was strong in basal regions but, in addition, in the alar suprachiasmatic region. The findings obtained in the forebrain of Xenopus revealed a largely conserved pattern of Shh expression among tetrapods. However, interesting differences were also noted that could be related to evolutive changes in forebrain organization.