Ultrastructural insights into mammalian cutaneous mechanoreceptors

Abstract

Cutaneous mechanoreceptors are a morphologically and functionally diverse class of primary sensory neurons in mammals that transduce mechanical stimuli acting on the skin into electrical impulses. These neurons reside in trigeminal ganglia and dorsal root ganglia and form pseudounipolar axons, of which one branch innervates the skin and the other innervates the spinal cord dorsal horn and brainstem, where it forms synapses with second-order neurons. Although substantial progress has been made in characterizing the anatomical and physiological properties of mechanoreceptors, the mechanotransduction mechanisms within endings in the skin and the organizational logic of synapses in the spinal cord remain poorly understood. To gain insight into these two questions, I used electron microscopy (EM) in conjunction with mouse genetic tools to investigate the ultrastructural properties of mechanoreceptors and their associated glia in the skin as well as primary afferent synapses in the spinal cord. I generated an optimized peroxidase, dAPEX2, and created a collection of genetically encoded EM reporters that are robust, versatile, and suitable for multiplexed labeling and volume EM reconstructions. Using these EM reporters, I first investigated the ultrastructural properties of sensory axon central branches in the spinal cord dorsal horn, identifying distinct ultrastructural features of previously inaccessible C-fiber subtypes. In addition, I have initiated a large-scale EM reconstruction of the spinal cord dorsal horn, which is enabling visualization and characterization of spinal cord circuit motifs. In the skin, genetic EM labeling has revealed insights into the ultrastructural basis of different physiological response properties of two mechanoreceptor subtypes that innervate Meissner corpuscles, a mechanosensory end organ in the glabrous skin. Moreover, three-dimensional EM reconstructions of hair follicle afferents and Meissner corpuscle afferents using focused ion beam-scanning EM have revealed common ultrastructural features of these distinct end organs, leading to new models to explain mechanisms of mechanotransduction in the skin.