The sympathetic nervous system innervates virtually every organ and maintains tissue homeostasis throughout life. With age, many tissues, including skin, bone marrow, heart and thymus, show a loss of sympathetic innervation, which has been linked to impaired regeneration. However, it remained unclear whether this denervation reflects the death of the neurons themselves or a deterioration of the target tissue that supports peripheral axons. In addition, sex differences in the aging of the sympathetic system, despite the known sexual dimorphism in autonomic regulation, have hardly been studied. The work sits off to the side of neighboring studies: nearby lie papers on brain age maps, quantum effects and cellular senescence, but there are no direct neighbors on sympathetic innervation of the skin.
The authors used C57BL/6JNia mice aged 2 to 20 months. Sympathetic nerves in the skin were visualized by immunostaining for tyrosine hydroxylase (TH) in thick sections and whole-mount preparations. For quantification they used machine learning and three-dimensional reconstruction: they measured the distance from the nerve to the follicle and the nerve volume within 5 µm of the follicle. Neurons projecting to the skin were labeled with a retrograde AAV-retro virus injected intradermally. To test the role of NGF, they used an AAV with the Myh11 promoter, specific for arrector pili smooth muscle, to selectively boost NGF expression in this niche. Sex effects were separated using the Four Core Genotypes (FCG) model, which uncouples gonadal sex from the set of sex chromosomes, as well as gonadectomy at 1 month of age and chemical ovariectomy with VCD.
With age, sympathetic innervation of hair follicles declined in both sexes, but in males it was significantly stronger: by 20 months, about 35% of follicles had completely lost their nerve, whereas in females most follicles remained innervated. The surviving terminals in old males were smaller in volume and located farther from the follicles. Retrograde labeling showed that the number of cutaneous sympathetic neurons in the ganglia did not change with age in either males or females, and apoptosis did not increase. This points to retraction of distal axons rather than neuron death. The arrector pili smooth muscle degenerated more in males, but many denervated follicles retained the muscle, meaning structural degeneration alone is not enough to explain the loss of nerve. NGF levels in the skin declined more in old males. Analysis of published single-cell RNA-seq data from human skin showed that smooth muscle cells in women retain neurotrophic support programs with age, while in men inflammatory and remodeling programs predominate. Selective expression of NGF in arrector pili in old males led to reinnervation: the proportion of follicles with a normal nerve increased, and the proportion of denervated ones decreased; in females there was no effect. Restoration of innervation was accompanied by improved hair regeneration in males. In the FCG model, old mice with testes had significantly lower innervation than mice with ovaries, regardless of chromosome set. Gonadectomy at 1 month smoothed out the differences between groups, while removal of ovaries in adults (VCD) did not affect innervation.
The work directly shows for the first time that age-related loss of sympathetic innervation in the skin is a reversible distal axonopathy, not neurodegeneration. Unlike neighboring studies that describe the aging of individual cell populations or systems, this one brings together neuroanatomy, molecular biology of the niche and the genetics of sex. The discovery that the fate of axons depends on the state of the target tissue and is established by gonadal hormones before puberty shifts the focus from neurons to the microenvironment. It also supports a model in which aging of stem cells, stroma and innervation are interconnected processes within a single regenerative unit.
The topic could move toward uncovering the molecular mechanisms by which early gonadal hormones program the niche. Experiments will be needed with gonadectomy at different stages, timed hormone replacement and tissue-specific knockout of androgen and estrogen receptors. It is also important to separate the direct and nerve-mediated effects of NGF on hair regeneration. In parallel, one could study how aging of different components of the neuroeffector unit influences one another over time.
The results could influence research on age-related denervation in the heart, bone marrow and thymus, where a link to impaired tissue function has already been shown. For regenerative medicine, an opportunity opens up to restore innervation by acting on the niche rather than on neurons.
Perform gonadectomy at different age windows and hormone replacement therapy to determine the critical period. Compare the effects of NGF on hair regeneration in mice with selective removal of sympathetic nerves.
Connection to unsolved problems in the neuroscience of aging: it remains unknown exactly how the target tissue supports axons and which signals trigger retraction. It is also unclear whether the loss of sympathetic innervation is a cause or a consequence of stem cell aging. The work shows that these processes are reversible and sex-dependent, which raises the question of the mechanisms of sexual dimorphism in the peripheral nervous system.
In old male mice, about 35% of hair follicles are completely devoid of sympathetic nerves, yet the neurons themselves in the ganglia are intact, as if the cords were unplugged from the socket while the socket itself stays live.
The idea of reversible denervation and restoring the connection between nerve and tissue echoes a motif from William Gibson's novel Neuromancer, where damaged neural connections can be reprogrammed and restored by acting on the environment.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.