Crocodylians evolved scattered multi-sensory micro-organs

Abstract

BackgroundDuring their evolution towards a complete life cycle on land, stem reptiles developed both an impermeable multi-layered keratinized epidermis and skin appendages (scales) providing mechanical, thermal, and chemical protection. Previous studies have demonstrated that, despite the presence of a particularly armored skin, crocodylians have exquisite mechanosensory abilities thanks to the presence of small integumentary sensory organs (ISOs) distributed on postcranial and/or cranial scales.ResultsHere, we analyze and compare the structure, innervation, embryonic morphogenesis and sensory functions of postcranial, cranial, and lingual sensory organs of the Nile crocodile (Crocodylus niloticus) and the spectacled caiman (Caiman crocodilus). Our molecular analyses indicate that sensory neurons of crocodylian ISOs express a large repertoire of transduction channels involved in mechano-, thermo-, and chemosensory functions, and our electrophysiological analyses confirm that each ISO exhibits a combined sensitivity to mechanical, thermal and pH stimuli (but not hyper-osmotic salinity), making them remarkable multi-sensorial micro-organs with no equivalent in the sensory systems of other vertebrate lineages. We also show that ISOs all exhibit similar morphologies and modes of development, despite forming at different stages of scale morphogenesis across the body.ConclusionsThe ancestral vertebrate diffused sensory system of the skin was transformed in the crocodylian lineages into an array of discrete multi-sensory micro-organs innervated by multiple pools of sensory neurons. This discretization of skin sensory expression sites is unique among vertebrates and allowed crocodylians to develop a highly-armored, but very sensitive, skin.

Variations in the proliferative activity of the peripheral retina correlate with postnatal ocular growth in squamate reptiles

“…Finally, slides were mounted using Dako Faramount aqueous medium (Agilent). IHC fluorescent staining was performed as described previously (Di‐Poï & Milinkovitch, ), using heat‐induced epitope retrieval (HIER) and overnight incubation at 4°C with primary antibodies known to recognize reptile and/or chicken epitopes: BrdU (1:300, rat monoclonal, Abcam, cat# ab6326, ), calretinin (CR; 1:100, rabbit polyclonal, Abcam, cat# ab702, ), doublecortin (DCX; 1:200, rabbit polyclonal, Cell Signaling Technology, cat# 4604, ), glial fibrillary acidic protein (GFAP; 1:300, mouse monoclonal, LifeSpan, cat# LS‐C88015‐100, ), neuronal‐specific RNA‐binding proteins HuC/D (HuC/D; 1:200, mouse monoclonal, Thermo Fisher Scientific, cat# A‐21271, ), proliferating cell nuclear antigen (PCNA; 1:200, mouse monoclonal, BioLegend, cat# 307901, ), SRY (sex determining region Y)‐box 2 (SOX2; 1:400, rabbit polyclonal, Abcam, cat# ab97959, ), and SRY‐box 9 (SOX9; 1:400, rabbit polyclonal, Millipore, cat# AB5535, ). Last, incubation with Alexa Fluor‐conjugated secondary antibodies (Alexa Fluor‐488: 1:500, goat anti‐rabbit IgG, Thermo Fisher Scientific, cat# A‐11008, ; Alexa Fluor‐568: 1:500, goat anti‐rabbit IgG, Thermo Fisher Scientific, cat# A‐11011, ) was performed for 1 hr at RT, and slides were mounted with Fluoroshield mounting medium (Sigma‐Aldrich) containing 4′,6′‐diamidino‐2‐phenylindole (DAPI).…”

“…Finally, slides were mounted using Dako Faramount aqueous medium (Agilent). IHC fluorescent staining was performed as described previously (Di-Poï & Milinkovitch, 2013), using heat-induced epitope retrieval (HIER) and overnight incubation at 4 C with primary antibodies known to recognize reptile and/or chicken epitopes: BrdU (1:300, rat monoclonal, Abcam, cat# ab6326, RRID: AB_305426), calretinin (CR; 1:100, rabbit polyclonal, Abcam, cat# ab702, RRID: AB_305702), doublecortin (DCX; 1:200, rabbit polyclonal, Cell Signaling Technology, cat# 4604, RRID: AB_561007), glial fibrillary acidic protein (GFAP; 1:300, mouse monoclonal, LifeSpan, cat# LS-C88015-100, RRID: AB_1795608), neuronal- Fisher Scientific, cat# A-11011, RRID: AB_143157) was performed for 1 hr at RT, and slides were mounted with Fluoroshield mounting medium (Sigma-Aldrich) containing 4 0 ,6 0 -diamidino-2-phenylindole (DAPI). For ISH-IHC double labelings, ISH was performed first followed by IHC omitting the HIER step.…”

Short Note: Tubercles: What Sense Is There?

“…For instance, small domes distributed along the jaws of alligators (Alligator mississippiensis) and crocodiles (Crocodylus niloticus) are known to respond to water surface disturbances (Soares, 2002;Leitch & Catania, 2012). Interestingly, the domes also respond to thermal and chemical changes (Di-Poï & Milinkovitch, 2013), suggesting that they function as multimodal sensory organs. It seems unlikely that similar stimuli would be ecologically relevant for humpback whales given the variability of surface conditions in ocean environments, but it remains possible that tubercles might facilitate the detection of water movements related to current flow or turbulence patterns.…”

Anatomy and Ontogeny of the Mandibular Symphysis in Alligator mississippiensis

“…These biomechanical modifications evolved with an equally highly derived somatosensory system requiring rather porous bone to accommodate the dendritic assemblage of alveolar neurovascular bundles (Leitch and Catania, 2012;Di-Poï and Milinkovitch, 2013). These branches of the mandibular division of the trigeminal nerve conduct sensory impulses from the heavily innervated skin and are accompanied by branches from the mandibular and oromandibular arteries and veins (Leitch and Catania, 2012;George and Holliday, 2013;Porter et al, 2016).…”