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. 2014 Oct 29:8:121.
doi: 10.3389/fnana.2014.00121. eCollection 2014.

Innervation patterns of sea otter (Enhydra lutris) mystacial follicle-sinus complexes

Christopher D Marshall  1 , Kelly Rozas  2 , Brian Kot  2 , Verena A Gill  3
Affiliations

Innervation patterns of sea otter (Enhydra lutris) mystacial follicle-sinus complexes

Christopher D Marshall et al. Front Neuroanat. .

Abstract

Sea otters (Enhydra lutris) are the most recent group of mammals to return to the sea, and may exemplify divergent somatosensory tactile systems among mammals. Therefore, we quantified the mystacial vibrissal array of sea otters and histologically processed follicle-sinus complexes (F - SCs) to test the hypotheses that the number of myelinated axons per F - SC is greater than that found for terrestrial mammalian vibrissae and that their organization and microstructure converge with those of pinniped vibrissae. A mean of 120.5 vibrissae were arranged rostrally on a broad, blunt muzzle in 7-8 rows and 9-13 columns. The F-SCs of sea otters are tripartite in their organization and similar in microstructure to pinnipeds rather than terrestrial species. Each F-SC was innervated by a mean 1339 ± 408.3 axons. Innervation to the entire mystacial vibrissal array was estimated at 161,313 axons. Our data support the hypothesis that the disproportionate expansion of the coronal gyrus in somatosensory cortex of sea otters is related to the high innervation investment of the mystacial vibrissal array, and that quantifying innervation investment is a good proxy for tactile sensitivity. We predict that the tactile performance of sea otter mystacial vibrissae is comparable to that of harbor seals, sea lions and walruses.

Keywords: F-SCs; axon investment; comparative neurobiology; marine mammals; otters; peripheral nervous system; somatosensory system; vibrissae.

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Figures

Figure 1
Figure 1
(A) Sea otter mystacial vibrissae in vivo with overlain representative map of columns and rows of individual vibrissa. (B) Isolated mystacial vibrissae from rows G-H, columns 9–11.
Figure 2
Figure 2
Longitudinal sections through the center of large mystacial F-SCs stained with trichrome (left) and a modified Bodian silver stain (right) for axons. UCS: upper cavernous sinus; RS: ring sinus; LCS: lower cavernous sinus; SG: sebaceous gland; HS: hair shaft; DC: dermal capsule of follicle: ICB: inner conical body; RW: ringwulst; MS: mesenchymal sheath; ORS: outer root sheath; DVN: deep vibrissal nerve. Note the penetration of the DVN at the base of the F - SC and its route through the LCS to the RW.
Figure 3
Figure 3
Sebaceous glands. Detailed micrograph of SG near the apical region of the UCS and surrounded by the apical DC.
Figure 4
Figure 4
Silver staining of sea otter F-SCs. (A) Longitudinal center section depicting the DVN ascending from the base of the LCS through the trabeculae and the MS to the RW within the RS. (B) Cross-section midway through the LCS showing the location and relative concentric organization of the DC, axon bundles within the trabeculae, MS, GM (glassy membrane), ORS and HS. (C) Silver stained axon bundles distributed throughout the trabeculae of the LCS.
Figure 5
Figure 5
Mechanoreceptors of the F - SC. (A) Longitudinal branch of the DVN coursing apically within the MS, but adjacent to the GM. White arrows show the location of presumptive Merkel-Neurite Complexes (MNCs) within the ORS adjacent to the GM (Bodian silver, 20X). (B) Longitudinal branch of the DVN coursing apically in the MS with numerous sub-branches crossing the GM to innervate presumptive lanceolate mechanoreceptors (black arrows) in the ORS (Bodian silver stain 40X). (C) Location of presumptive mechanoreceptors (asterisks) arranged in the ORS at the border of the GM (trichrome, 100X).
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