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Comparative studies on the forebrain across different model organisms are necessary to investigate the origin and degree of evolutionary conservation of this brain region and its derivatives. The catshark Scyliorhinus canicula has become a reliable model representative of cartilaginous fishes (Chondrichthyans), the oldest divergent lineage of extant gnathostomes (jawed vertebrates). Previous studies on the chemoarchitecture, connectivity, and development of the subpallium of S. canicula revealed the existence of subdivisions with an embryological origin and genetic specification similar to those of tetrapods, supporting homology with their basal ganglia and certain amygdaloid components. To better understand the evolutionary origin of these subpallial components, we present here a summary of the main neuroanatomical, chemoarchitectural, and developmental features of the area superficialis basalis of S. canicula , a nuclear aggrupation of the basal forebrain of all Chondrichthyans that has been related to the basal ganglia and the amygdala. Particular emphasis has been placed on characterizing the tyrosine hydroxylase-positive components of the subpallium to discern their involvement in the structural organization of putative basal ganglia homologs in the catshark. We propose a new interpretation that considers the area superficialis basalis as the central part of a subpallial complex formed also by its neighboring territories, where the basic components of the basal ganglia and the amygdala of gnathostomes would be integrated.

Neurogenesis is a multistep process by which progenitor cells become terminally differentiated neurons. Adult neurogenesis has gathered increasing interest with the aim of developing new cell-based treatments for neurodegenerative diseases in humans. Active sites of adult neurogenesis exist from fish to mammals, although in the adult mammalian brain the number and extension of neurogenic areas is considerably reduced in comparison to non-mammalian vertebrates and they become mostly reduced to the telencephalon. Much of our understanding in this field is based in studies on mammals and zebrafish, a modern bony fish. The use of the cartilaginous fish Scyliorhinus canicula (representative of basal gnathostomes) as a model expands the comparative framework to a species that shows highly neurogenic activity in the adult brain. In this work, we studied the proliferation pattern in the telencephalon of juvenile and adult specimens of S. canicula using antibodies against the proliferation marker proliferating cell nuclear antigen (PCNA). We have characterized proliferating niches using stem cell markers (Sex determining region Y-box 2), glial markers (glial fibrillary acidic protein, brain lipid binding protein and glutamine synthase), intermediate progenitor cell markers (Dlx2 and Tbr2) and markers for migrating neuroblasts (Doublecortin). Based in the expression pattern of these markers, we demonstrate the existence of different cell subtypes within the PCNA immunoreactive zones including non-glial stem cells, glial progenitors, intermediate progenitor-like cells and migratory neuroblasts, which were widely distributed in the ventricular zone of the pallium, suggesting that the main progenitor types that constitute the neurogenic niche in mammals are already present in cartilaginous fishes.

The cerebellum is present in all extant gnathostomes or jawed vertebrates, of which cartilaginous fishes represent the most ancient radiation. Since the isthmic organizer induces the formation of the cerebellum, comparative genoarchitectonic analysis on the meso-isthmo-cerebellar region of cartilaginous fishes with respect to that of jawless vertebrates could reveal why the isthmic organizer acquires the ability to induce the formation of the cerebellum in gnathostomes. In the present work we analyzed the expression pattern of a variety of genes related to the cerebellar formation and patterning ( ScOtx2 , ScGbx2 , ScFgf8 , ScLmx1b , ScIrx1 , ScIrx3, ScEn2 , ScPax6 and ScLhx9 ) by in situ hybridization, and the distribution of Pax6 protein in the developing hindbrain of the shark Scyliorhinus canicula . The genoarchitectonic code in this species revealed high degree of conservation with respect to that of other gnathostomes. This resemblance may reveal the features of the ancestral condition of the gene network operating for specification of the rostral hindbrain patterning. Accordingly, the main subdivisions of the rostral hindbrain of S. canicula could be recognized. Our results support the existence of a rhombomere 0, identified as the ScFgf8 / ScGbx2 / ScEn2 -positive and mainly negative ScIrx3 domain just caudal to the midbrain ScIrx1 / ScOtx2 / ScLmx1b -positive domain. The differential ScEn2 and Pax6 expression in the rhombomere 1 revealed anterior and posterior subdivisions. Interestingly, dissimilarities between S. canicula and lampreys (jawless vertebrates) were noted in the expression of Irx, Lhx and Pax genes, which could be part of significant gene network changes through evolution that caused the emergence of the cerebellum.

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.

Because the cerebellum emerged at the agnathan-gnathostome transition and cartilaginous fishes are at the base of the gnathostome lineage, this group is crucial to determine the basic developmental pattern of the cerebellum and to gain insights into its origin. We have systematically analyzed key events in the development of cerebellum and cerebellum-related structures of the shark Scyliorhinus canicula . Three developmental periods are distinguished based on anatomical observations combined with molecular analysis. We present neurochemical and genoarchitectonic evidence on the onset of cerebellar development, the rostral and caudal cerebellar boundaries, the compartmentalization of the cerebellum, and correspondence of cerebellar domains to rhombomeric segmentation of the rostral hindbrain. Our observations, mainly based on the expression pattern of ScHoxA2, support the origin of both the upper and lower auricular leaves from r1 and exclude any cerebellar origin from r2. Correlation between subrhombomeres r1a/r1b and cerebellar domains is proposed based on the ScEn2 expression. The ScEn2 and ScOtx2 expression patterns revealed an antero-posterior cerebellar compartmentalization similar to that of mammals, and supported certain fissures (commonly used to define cerebellar domains) as reliable anatomical landmarks. At difference from mammals, the expression of Sc En2 along the cerebellar median-lateral axis does not reveal a multiple-banded pattern. The present study provides an atlas of cerebellar development in one of the most basal extant gnathostome lineages and emphasizes the importance of combining classic descriptive with modern molecular studies to gain knowledge on the ancestral condition of cerebellar developmental processes and the origins and evolution of the cerebellum.

The hypothalamus is a key vertebrate brain region involved in survival and physiological functions. Understanding hypothalamic organization and evolution is important to decipher many aspects of vertebrate biology. Recent comparative studies based on gene expression patterns have proposed the existence of hypothalamic histogenetic domains (paraventricular, TPa/PPa; subparaventricular, TSPa/PSPa; tuberal, Tu/RTu; perimamillary, PM/PRM; mamillary, MM/RM), revealing conserved evolutionary trends. To shed light about the functional relevance of these histogenetic domains, the present work aims to interpret the location of developed cell groups according to the prosomeric model in the hypothalamus of the catshark Scyliorhinus canicula, a representative of Chondrichthyans (the sister group of Osteichthyes, at the base of the gnathostome lineage). To this end, we review in detail the expression patterns of ScOtp, ScDlx2 and ScPitx2 as well as Pax6-immunoreactivity in embryos at stage 32, when the morphology of the adult catshark hypothalamus is already organized. We also propose homologies with mammals when possible. The present study provides a comprehensive tool to better understand previous and novel data on hypothalamic development and evolution.

The retina, whose basic cellular structure is highly conserved across vertebrates, constitutes an accessible system for studying the central nervous system. In recent years, single-cell RNA sequencing studies have uncovered cellular diversity in the retina of a variety of species, providing new insights on retinal evolution and development. However, similar data in cartilaginous fishes, the sister group to all other extant jawed vertebrates, are still lacking. Here, we present a single-nucleus RNA sequencing atlas of the postnatal retina of the catshark Scyliorhinus canicula , consisting of the expression profiles for 17,438 individual cells from three female, juvenile catshark specimens. Unsupervised clustering revealed 22 distinct cell types comprising all major retinal cell classes, as well as retinal progenitor cells (whose presence reflects the persistence of proliferative activity in postnatal stages in sharks) and oligodendrocytes. Thus, our dataset serves as a foundation for further studies on the development and function of the catshark retina. Moreover, integration of our atlas with data from other species will allow for a better understanding of vertebrate retinal evolution.

The hypothalamus is an important physiologic center of the vertebrate brain involved in the elaboration of individual and species survival responses. To better understand the ancestral organization of the alar hypothalamus we revisit previous data on , , , 1 expression and Pax6 immunoreactivity jointly with new data on , , , and expression, in addition to immunoreactivity to serotonin (5-HT) and doublecortin (DCX) in the catshark , a key species for this purpose since cartilaginous fishes are basal representatives of gnathostomes (jawed vertebrates). Our study revealed a complex genoarchitecture for the chondrichthyan alar hypothalamus. We identified terminal (rostral) and peduncular (caudal) subdivisions in the prosomeric paraventricular and subparaventricular areas (TPa/PPa and TSPa/PSPa, respectively) evidenced by the expression pattern of developmental genes like (TPa) and immunoreactivity against Pax6 (PSPa) and 5-HT (PPa and PSPa). Dorso-ventral subdivisions were only evidenced in the SPa (SPaD, SPaV; respectively) by means of Pax6 and (respectively). Interestingly, expression overlaps over the alar-basal boundary, as does in other vertebrates. Our results reveal evidences for the existence of different groups of tangentially migrated cells expressing , , and . The genoarchitectonic comparative analysis suggests alternative interpretations of the rostral-most alar plate in prosomeric terms and reveals a conserved molecular background for the vertebrate alar hypothalamus likely acquired before/during the agnathan-gnathostome transition, on which , , , and are expressed in the Pa while and 8 are expressed in the SPa.

The nervus terminalis (or terminal nerve) system was discovered in an elasmobranch species more than a century ago. Over the past century, it has also been recognized in other vertebrate groups, from agnathans to mammals. However, its origin, functions or relationship with the olfactory system are still under debate. Despite the abundant literature about the nervus terminalis system in adult elasmobranchs, its development has been overlooked. Studies in other vertebrates have reported newly differentiated neurons of the terminal nerve system migrating from the olfactory epithelium to the telencephalon as part of a ‘migratory mass' of cells associated with the olfactory nerve. Whether the same occurs in developing elasmobranchs (adults showing anatomically separated nervus terminalis and olfactory systems) has not yet been determined. In this work we characterized for the first time the development of the terminal nerve and ganglia in an elasmobranch, the lesser spotted dogfish (Scyliorhinus canicula), by means of tract-tracing techniques combined with immunohistochemical markers for the terminal nerve (such as FMRF-amide peptide), for the developing components of the olfactory system (Gα0 protein, GFAP, Pax6), and markers for early postmitotic neurons (HuC/D) and migrating immature neurons (DCX). We discriminated between embryonic olfactory and terminal nerve systems and determined that both components may share a common origin in the migratory mass. We also localized the exact point where they split off near the olfactory nerve-olfactory bulb junction. The study of the development of the terminal nerve system in a basal gnathostome contributes to the knowledge of the ancestral features of this system in vertebrates, shedding light on its evolution and highlighting the importance of elasmobranchs for developmental and evolutionary studies.

Neurogenesis is the process by which progenitor cells generate new neurons. As development progresses neurogenesis becomes restricted to discrete neurogenic niches, where it persists during postnatal life. The retina of teleost fishes is thought to proliferate and produce new cells throughout life. Whether this capacity may be an ancestral characteristic of gnathostome vertebrates is completely unknown. Cartilaginous fishes occupy a key phylogenetic position to infer ancestral states fixed prior to the gnathostome radiation. Previous work from our group revealed that the juvenile retina of the catshark Scyliorhinus canicula , a cartilaginous fish, shows active proliferation and neurogenesis. Here, we compared the morphology and proliferative status of the retina in catshark juveniles and adults. Histological and immunohistochemical analyses revealed an important reduction in the size of the peripheral retina (where progenitor cells are mainly located), a decrease in the thickness of the inner nuclear layer (INL), an increase in the thickness of the inner plexiform layer and a decrease in the cell density in the INL and in the ganglion cell layer in adults. Contrary to what has been reported in teleost fish, mitotic activity in the catshark retina was virtually absent after sexual maturation. Based on these results, we carried out RNA-Sequencing (RNA-Seq) analyses comparing the retinal transcriptome of juveniles and adults, which revealed a statistically significant decrease in the expression of many genes involved in cell proliferation and neurogenesis in adult catsharks. Our RNA-Seq data provides an excellent resource to identify new signaling pathways controlling neurogenesis in the vertebrate retina.