AUTHOR=Santos-Durán Gabriel N. , Ferreiro-Galve Susana , Menuet Arnaud , Quintana-Urzainqui Idoia , Mazan Sylvie , Rodríguez-Moldes Isabel , Candal Eva 

TITLE=The Shark Alar Hypothalamus: Molecular Characterization of Prosomeric Subdivisions and Evolutionary Trends

JOURNAL=Frontiers in Neuroanatomy

VOLUME=10

YEAR=2016

URL=https://www.frontiersin.org/journals/neuroanatomy/articles/10.3389/fnana.2016.00113

DOI=10.3389/fnana.2016.00113

ISSN=1662-5129

ABSTRACT=<p>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 <italic>ScOtp</italic>, <italic>ScDlx2/5</italic>, <italic>ScTbr1</italic>, <italic>ScNkx2.</italic>1 expression and Pax6 immunoreactivity jointly with new data on <italic>ScNeurog2</italic>, <italic>ScLhx9</italic>, <italic>ScLhx5</italic>, and <italic>ScNkx2.8</italic> expression, in addition to immunoreactivity to serotonin (5-HT) and doublecortin (DCX) in the catshark <italic>Scyliorhinus canicula</italic>, 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 <italic>ScLhx5</italic> (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 <italic>ScNkx2.8</italic> (respectively). Interestingly, <italic>ScNkx2.8</italic> expression overlaps over the alar-basal boundary, as <italic>Nkx2.2</italic> does in other vertebrates. Our results reveal evidences for the existence of different groups of tangentially migrated cells expressing <italic>ScOtp</italic>, <italic>Pax6</italic>, and <italic>ScDlx2</italic>. 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 <italic>Otp</italic>, <italic>Pax6</italic>, <italic>Lhx5</italic>, and <italic>Neurog2</italic> are expressed in the Pa while <italic>Dlx</italic> and <italic>Nkx2.2/Nkx2.</italic>8 are expressed in the SPa.</p>