Yuxiang Wang, Kara Marshall, Rachel Clary, Hyojung Kang, Rachel Orlowsky, Ellen Lumpkin, Gregory J. Gerling
We sought to define the mechanical properties of mouse skin over a range of normal physiological states. We evaluated the skin's mechanics over the mouse hair cycle – loss and replacement of hair from head to tail in two separate waves between about 4 to10 weeks, before entering maturity – to study natural, systematic changes to the skin and neuron. Uniaxial compression measurements surveyed the skin’s mechanics over the hair cycle.
The skin serves a pivotal role in the transfer function from sensory stimuli to neuronal signaling
In two separate cycles, the hair of the mouse is replaced progressing through stages of Telogen, Anagen, and Catagen. These stages happen in waves over two separate points of time, and thereafter the skin and hairs enter a mature stage. At that point, the hair and skin are renewed in a mosaic fashion, i.e., where hairs are replaced not in a wave but in staggered points in time, on regular intervals.
Skin thickness, stiffness and modulus were quantitatively surveyed in adult, female mice. These measures were analyzed under uniaxial compression, which is relevant for touch reception and compression injuries, rather than tension, which is typically used to analyze skin mechanics. Compression tests were performed with 105 full-thickness, freshly isolated specimens from the hairy skin of the hind limb. Physiological variables included body weight, hair-cycle stage, maturity level, skin site and individual animal differences.
The results indicate that skin thickness and stiffness were dominated by hair-cycle stage at young (6–10 weeks) and intermediate (13–19 weeks) adult ages but by body weight in mature mice (26–34 weeks). Interestingly, stiffness varied inversely with thickness so that hyperelastic modulus was consistent across hair-cycle stages and body weights. By contrast, the mechanics of hairy skin differs markedly with anatomical location. In particular, skin containing fascial structures such as nerves and blood vessels showed significantly greater modulus than adjacent sites. Collectively, this systematic survey indicates that, although its structure changes dramatically throughout adult life, mouse skin at a given location maintains a constant elastic modulus to compression throughout normal physiological stages.
Although the skin’s structure, in particular its thickness and stiffness changed dramatically, we found that skin at a given location maintains a constant modulus throughout normal physiological stages
Over the population of animals, we observed the skin’s viscoelasticity to be quite variable, yet found systematic correlation of residual stress ratio with skin thickness and strain, and of relaxation time constants with strain rates. These findings indicate that the natural range of specimen thickness, as well as experimental controls of compression level and rate, significantly influence measurements of skin viscoelasticity. In particular, we find that there is faster relaxation with a higher strain rate (B, below), that thinner skin relaxes to a greater degree (C, below), and that there is more relaxation with a greater level of strain.
Skin viscoelasticity is dependent on specimen thickness and experimental controls of compression level and rate
From the neuron’s perspective, the branched Merkel cell neurite complex end organs innervated by slowly-adapting type I afferents renew dramatically as well, and our computational simulations (using Discrete Event Simulation) indicate that the observed remodeling follows simple rules (Marshall,2016; Kang, 2017).
To identify principles that specify how arbors change over the hair cycle, we built a computational model to evaluate policies of Merkel-cell and heminode dynamics
Thirty end organs were auto-generated by clustering Merkel cells at heminodes, whose numbers were probabilistically drawn from first-telogen distributions. Each end organ then went through a complete hair cycle, with iterations of remodeling following four policies: (1) the probability of losing a Merkel cell from a cluster is proportional to cluster size, (2) the probability of adding a Merkel cell is random across heminodes, (3) myelinated branches refine to achieve a range of three to four heminodes, and (4) heminodes that lack Merkel cells are most likely to be pruned (A, below). These simple rules produced distributions of Merkel cells (B, below) and heminodes (C, below) in anagen, catagen, and telogen that agreed with experimental observations. Thus, we propose that Merkel-cell loss and addition are probabilistic among heminodes but that the stability of heminodes, and thus myelinated branches, depends on Merkel-cell inputs.
Although important to skin renewal, such remodeling processes could be detrimental to touch sensitivity. However, our models predict that perceptual sensitivity is robust to changes in skin mechanics.
Finite element models were employed that were fitted to six representative mouse skin samples that varied across the spectrum identified above over the hair cycle. In particular, their thickness varied from 268-525 microns, and their hyper and viscoelastic properties were varied as well. The stress, strain and other mechanical measures were sampled and used as input into a neural model to predict the elicitation of action potentials. We moved the indenter tip using both force control and displacement control for comparison to one another.
We find that when the six models with different skin properties are indented with a displacement controlled stimulus that they generate quite variable prediction, in particular in the image, the firing bands are large and as the arrow indicates, any given spike firing, such as 35 spikes/sec might be the result of skin property variance. But when force control is used, these bands of prediction accuracy reduce significantly. Therefore, even amidst naturalistic changes in skin mechanical properties, the skin can reliably convey indentation magnitude (as shown below) as well as other properties such as rate and spatial geometry.
Amidst skin renewal, the modeled skin and afferent sensitivity to stimulus pressure, as opposed to displacement and other cues, affords low variability in predicted firing rates, and may be more naturalistic and optimal for active exploration of the tactile environment