Ultrastructural evidence of a mechanosensory function of scale organs (sensilla) in sea snakes (Hydrophiinae)

Abstract

The evolution of epidermal scales was a major innovation in lepidosaurs, providing a barrier to dehydration and physical stress, while functioning as a sensitive interface for detecting mechanical stimuli in the environment. In snakes, mechanoreception involves tiny scale organs (sensilla) that are concentrated on the surface of the head. The fully marine sea snakes (Hydrophiinae) are closely related to terrestrial hydrophiine snakes but have substantially more protruding (dome-shaped) scale organs that often cover a larger portion of the scale surface. Various divergent selection pressures in the marine environment could account for this morphological variation relating to detection of mechanical stimuli from direct contact with stimuli and/or indirect contact via water motion (i.e. 'hydrodynamic reception'), or co-option for alternate sensory or non-sensory functions. We addressed these hypotheses using immunohistochemistry, and light and electron microscopy, to describe the cells and nerve connections underlying scale organs in two sea snakes, Aipysurus laevis and Hydrophis stokesii. Our results show ultrastructural features in the cephalic scale organs of both marine species that closely resemble the mechanosensitive Meissner-like corpuscles that underlie terrestrial snake scale organs. We conclude that the scale organs of marine hydrophiines have retained a mechanosensory function, but future studies are needed to examine whether they are sensitive to hydrodynamic stimuli.

Keywords: cutaneous; mechanoreceptor; scale organs; sea snake; sensilla; skin.

Figure 1.

Gross morphology of the skin of sea snakes illustrating small, unpigmented scale organs (sensilla). Line drawing of sea snake indicates regions of skin sampled for this study: nasal scales from the head of Aipysurus laevis and Hydrophis stokesii, and supralabial scales from the head and caudal scales from the tail in A. laevis only. (a) Gross morphology of scale organs on the nasal scale of A. laevis. (b) Gross morphology of the caudal scales of A. laevis illustrating sparse scale organs. (c) Gross morphology of scale organs on the nasal scale of H. stokesii. Stereomicroscope images were taken from museum specimens: (a,b) WAMR174260 and (c) FMNH202826. Scale bars represent 1 mm. Line drawing based on the image of A. laevis from [40] and modified with permission.

Figure 2.

Light micrographs of a transverse section of cephalic skin (supralabial scale) from Aipysurus laevis. (a) Transverse section shows that scale organs (*) are skin elevations (bumps) created by dermal papillae; other features of the dermis are clearly visible including nerve bundles, blood vessels and collagen; note that the beta layer has artificially separated from the alpha layer. (b,c) Higher magnification of transverse section of scale organs (*) that show central cells within the dermal papilla, which displace the stratum germinativum of the epidermis; dermal papillae are vascularized by blood vessels; note the red blood cells (rbc), lamellar corpuscles (lc) and melanophores (m) within the dermis. Slides were stained with haematoxylin-eosin and magnified at (a) ×5.5, (b) ×20 and (c) ×30.

Figure 3.

Light micrographs of a transverse section of cephalic skin (nasal scale) from Hydrophis stokesii. (a) Transverse section shows that scale organs (*) are skin elevations (bumps) created by dermal papillae; other features of the dermis are clearly visible including nerve bundles, blood vessels and collagen fibres, and hinge region of the scale. (b) Higher magnification of transverse section of scale organ (*); the central cells within the dermal papilla displace the stratum germinativum of the epidermis. (c) Transverse section of edge of scale organ shows a small bundle of collagen fibres surrounded by central cells. Note the lamellar corpuscles (lc) within the dermis. Slides were stained with Gomori's one-step trichrome and magnified at (a) ×6.2, (b) ×22.8 and (c) ×23.

Figure 4.

Light micrographs of a transverse section of cephalic skin (nasal scale) from Hydrophis stokesii showing that dermal papillae are not associated with external skin elevations (bumps). (a,b) Central cells of a dermal papillae (*) displace surrounding stratum germinativum of the epidermis, but do not result in skin elevations. The nerve bundle is closely associated with base of the dermal papilla. Slide was stained with Gomori's one-step and magnified at (a) ×20 and (b) ×40.7.

Figure 5.

Immunoreactivity of a neuron-specific protein (PGP9.5) on cephalic skin (nasal scale) of Hydrophis stokesii; reactive protein appears dark pink. (a) Transverse section of scale organ (*) with neuronal-positive stain within the dermal papillae, as well as within the epidermis and alpha layer above the dermal papillae. Several neuronal-positive, discoid endings (arrows) are present within the stratum germinativum and alpha layers of the epidermis. Lamellar corpuscles (lc) within the dermis are also immuno-positive and can be distinguished from melanocytes (me) and dispersed melanophores (m), which have a dark brown coloration. (b) Transverse section of a scale organ showing neuronal-positive discoid endings (arrow). (c) A trail of neuronal-positive stain (arrow heads) leading to a forming scale organ (*). Negative control was conducted by omitting primary antibody. Slides were counterstained with Harris haematoxylin and magnified at (a) ×30, (b) ×50 and (c) ×50.

Figure 6.

Immunoreactivity of a neuron-specific protein (PGP9.5) of lamellar corpuscles (lc) in the cephalic dermis (nasal scale) of Hydrophis stokesii. The location within the dermis and co-localization of immuno-staining with lamellar structures suggests that they are Pacinian-like corpuscles. (a) Immunoreactivity of PGP9.5; reactive protein appears dark pink, showing immuno-positive stain localized to lamellar corpuscles (lc) in the dermis and discoid endings (arrows) in the epidermis. These structures can be distinguished from melanocytes (me) and dispersed melanophores (m), which have a dark brown coloration. (b) Transverse section of the skin showing structure of lamellar corpuscles and an associated blood vessel (bv) and nerve bundle (n). Slides were stained and magnified: (a) Harris haematoxylin, ×30, and (b) Gomori's one-step trichrome, ×50.

Figure 7.

Light micrograph and transmission electron micrographs (TEM) of sections of cephalic scale organs in sea snakes. (a) Transverse section of scale organs (*) in Aipysurus laevis showing the dermal papilla within the epidermis. (b) Higher magnification of dermal papilla (*) in A. laevis. First inset shows (a1)nuclei of central cells (c) and epidermal cells (keratinocytes; k), and collagen fibres (coll), a structure typically found within the dermis, in the intercellular domain of the dermal papilla. (a2) a putative myelinated axon (arrow heads) is present in the intercellular domain of the central cells; small phospholipid (p) inclusions are also present. Inset two (b1 and b2) shows intercellular junctions (desmosomes; d) at the membrane of central cells (c) and the keratinocytes (k). Note the fine keratin-like tonofibrils (tb) associated with the desmosomes and large aggregations of keratin-like filaments (tonofilaments; t) in the intracellular domain of the keratinocytes. Light micrograph slide was stained with hemotoxylin-eosin and magnified at (a) ×34.1; TEM: (b) ×1900, (a1) ×4800, (b1) ×6800; (a2) ×9300 and (b2) ×18 500.

Figure 8.

Light micrographs of transverse sections of tail skin (posterior caudal scales) of Aipysurus laevis. (a) Scale organs (*) in the tail are skin elevations created by a thickening of underlying epidermis. (b) Unknown dermal papillae (*) consisting of central cells that displaces surrounding stratum germinativum of the epidermis but does not result in skin elevations. Note that the dermis immediately underlying dermal papillae consists of loosely arranged collagen fibres devoid of melanophores (m). Slides were stained with haematoxylin-eosin and magnified at (a) ×16 and (b) ×17.2.