Afferent neurons and their mechanoreceptors provide critical sensory feedback for gait. The anatomical distribution and density of afferents and mechanoreceptors influence sensory feedback, as does mechanoreceptor function. Electrophysiological studies of hind paw skin reveal the different types of afferent responses and their receptive fields, however, the anatomical distribution of mechanoreceptor endings is unknown. Also, the role of integrin α1β1 in mechanoreceptor function is unclear, though it is expressed by keratinocytes in the stratum basale where it is likely involved in a variety of mechanotransduction pathways and ion channel functionalities. For example, it has been shown that integrin α1β1 is necessary for the function of TRPV4 that is highly expressed by afferent units. The purpose of this study, therefore, was to determine and compare the distribution of mechanoreceptors across the hind paw skin and the footfall patterns of itga1-null and wild type mice. The itga1-null mouse is lacking the integrin α1 subunit, which binds exclusively to the β1 subunit, thus rendering integrin α1β1 nonfunctional while leaving the numerous other pairings of the β1 subunit undisturbed. Intact hind paws were processed, serially sectioned, and stained to visualize mechanoreceptors. Footfall patterns were analyzed as a first step in correlating mechanoreceptor distribution and functionality. Merkel cells and Meissner-like corpuscles were present, however, Ruffini endings and Pacinian corpuscles were not observed. Meissner-like corpuscles were located exclusively in the glabrous skin of the footpads and digit tips, however, Merkel cells were found throughout hairy and glabrous skin. The increased density of Merkel cells and Meissner-like corpuscles in footpads 1 and 3 and Meissner-like corpuscles in footpad 4 suggests their role in anteroposterior balance, while Meissner-like corpuscle concentrations in digits 2 and 5 support their role in mediolateral balance. Finally, a larger density of Meissner-like corpuscles in footpads 3 and 4 in male itga1-null mice compared to wild type controls paves the way for future site-specific single fiber in vivo recordings to provide insight into the role of integrin α1β1 in tactile mechanotransduction.

Background: Mechanoreceptors at the terminal ends of afferent nerves in the foot sole are critical for gait and balance. While previous studies have mapped afferent receptive fields and innervation densities across the foot sole using microneurography, the location and distribution of mechanoreceptors in the foot sole skin is unknown. In humans, histological investigations are limited to specimens obtained during amputation or embalmed tissues. These can be difficult to obtain and the latter poorly preserved at the extremity of the foot sole. Mice serve as an appropriate animal model to study mechanoreceptors whose function is conserved across mammalian species, however serial sectioning of intact mouse hind paws is challenging due to the dense bones and variety of soft tissues present. Consequently, digits and footpads are typically isolated for analysis and inferences made regarding mechanoreceptor density across the whole hind paw. There are no published protocols for serial histological sectioning of mouse hind paws and attempts at serially sectioning intact rat hind paws have been unsuccessful. Methods: We conducted eight experiments optimizing tissue preparation, chemical processing and sectioning techniques to achieve serial sectioning and staining of intact mouse hind paws. Two additional approaches (isolated hind paw skin and footpad biopsies) were compared to the intact hind paw approach to determine the optimal method for mechanoreceptor visualization and localization. Results: The optimized protocol for serial sectioning of intact hind paws included five days of fixation in 4% paraformaldehyde, ten days of decalcification in Cal-Ex™️ II, a 9-hour automated tissue processing protocol, embedding in a special formulated paraffin, and sectioning with specific techniques. Of the three approaches, intact hind paws provided the most context to structures visualized in the hind paw skin and thus, was the recommended protocol for future studies. Meissner-like corpuscles were located in the footpads and most abundantly in the footpads adjacent to digits II to V of the hind paw. Conclusion: The method for serial sectioning of the intact hind paws outlined in this study will allow future analysis of mechanoreceptor distribution and density in transgenic or disease mouse models.