Abstract
The hypothalamus is a key integrative center of the vertebrate brain. To better understand its ancestral morphological organization and evolution, we previously analyzed the segmental organization of alar subdivisions in the catshark Scyliorhinus canicula, a cartilaginous fish and thus a basal representative of gnathostomes (jawed vertebrates). With the same aim, we deepen here in the segmental organization of the catshark basal hypothalamus by revisiting previous data on ScOtp, ScDlx2/5, ScNkx2.1, ScShh expression and Shh immunoreactivity jointly with new data on ScLhx5, ScEmx2, ScLmx1b, ScPitx2, ScPitx3a, ScFoxa1, ScFoxa2 and ScNeurog2 expression and proliferating cell nuclear antigen (PCNA) immunoreactivity. Our study reveals a complex genoarchitecture for chondrichthyan basal hypothalamus on which a total of 21 microdomains were identified. Six belong to the basal acroterminal region, the rostral-most point of the basal neural tube; seven are described in the tuberal region (Tu/RTu); four in the perimamillar region (PM/PRM) and four in the mamillar one (MM/RM). Interestingly, the same set of genes does not necessarily describe the same microdomains in mice, which in part contributes to explain how forebrain diversity is achieved. This study stresses the importance of analyzing data from basal vertebrates to better understand forebrain diversity and hypothalamic evolution.
Highlights
The hypothalamus is an important physiologic center of the brain
ScShh-Expression/Shh Immunoreactivity Since ScShh detection by means of in situ hybridization (ISH) at early developmental stages yields similar results to those obtained by IHC against anti-Shh (Santos-Durán et al, 2015), here we have used the antibody to analyze additional developmental stages and to ease pattern comparisons by means of double ISH-IHC staining
As described in Santos-Durán et al (2015), from stage 29 onwards, Shh immunoreactivity is observed in part of the rostral and dorsal tuberal area (Tu) domain and broadly detected within the retromamillary area (RM) domain, extending from here along the diencephalic basal plate
Summary
The hypothalamus is an important physiologic center of the brain. It integrates information from limbic, endocrine and autonomic sources to elaborate different kinds of homeostatic and behavioral responses such as feeding or reproduction. The prosomeric model, a modern paradigm of vertebrate brain development and organization (Puelles and Rubenstein, 2003, 2015; Puelles et al, 2012; Puelles, 2017), understands the hypothalamus to be located ventral to the telencephalon, being both located rostral to the diencephalon (or primary prosencephalon). Telencephalon and hypothalamus (known together as secondary prosencephalon) are subdivided into two true segments: hp, rostral or terminal; hp, caudal or peduncular (see Puelles et al, 2012). The intrahypothalamic border (IHB) separates hp from hp while the hypothalamodiencephalic border (HDB) separates hp from p3, the rostral-most unit of the tripartite segmental diencephalon (Puelles et al, 2012)
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