The little bumps that appear on our skin when we’re cold, scared, excited or sexually stimulated are the body’s way of regulating hair growth, scientists say.
Goose pimples regulate the stem cells that regenerate hair follicles – the structure of cells and connective tissue that surrounds a hair, Harvard researchers reveal.
In response to the cold, the muscle in the hair follicle contracts, resulting in tiny bumps appearing all over the skin.
Goose pimples protect animals with thick fur from the cold, but researchers wanted to understand why this biological reaction has been preserved in humans, despite us having comparatively sparse amounts of hair around the body.
They conclude that our nervous system releases neurotransmitters that target hair follicle stem cells, causing them to activate and grow new hair.
‘We have always been interested in understanding how stem cell behaviours are regulated by external stimuli,’ said Ya-Chieh Hsu at the Harvard Stem Cell Institute, Harvard University.
‘The skin is a fascinating system – it has multiple stem cells surrounded by diverse cell types, and is located at the interface between our body and the outside world.
‘Therefore, its stem cells could potentially respond to a diverse array of stimuli – from the niche, the whole body, or even the outside environment.
‘In this study, we identify an interesting dual-component niche that not only regulates the stem cells under steady state, but also modulates stem cell behaviours according to temperature changes outside.’
In animals, goose pimples protect animals against the cold by creating an expanded layer of air that bridges the top of each hair and acts as insulation.
The erection of each individual pimple and hair attached in response to cold is more effective for animals with thick fur, however, than in humans, who have relatively thinner hairs sparsely covering the body.
But the biological reaction still exists in modern humans even though there is less need for it, following nearly 200,000 years of using textiles to protect ourselves from the cold, scientists estimate.
Goose pimples were of interest to the legendary British naturalist Charles Darwin, who mused about them in his writings on evolution
In his 1872 work The Expression of the Emotions in Man and Animals, Darwin pointed out the analogous reaction in the human scalp and skin associated with fear.
But now, Harvard researchers claim a new theory as to why this evolutionary trait is still important for the modern-day human.
Many organs are made of three types of tissue – epithelium, mesenchyme, and nerve – and in the skin, these three lineages are organised in a special arrangement.
Key to the process in humans is the sympathetic nerve, part of our nervous system that controls body homeostasis and our responses to external stimuli.
In the skin under out hair, the sympathetic nerve connects with a tiny smooth muscle in the mesenchyme.
This smooth muscle in turn connects to hair follicle stem cells, a type of epithelial stem cell critical for regenerating the hair follicle and repairing wounds.
The connection between the sympathetic nerve and the muscle has been well known, since they are the cellular basis behind goose bumps – the cold triggers sympathetic neurons to send a nerve signal, and the muscle reacts by contracting and causing the hair to stand on end.
To learn more, Harvard researchers examined the skin using electron microscopy, which involves beams of accelerated electrons as a source of illumination to produce extremely high resolution images.
The researchers found that the sympathetic nerve not only associated with the muscle, but also formed a direct connection to the hair follicle stem cells.
They said the nerve fibres wrapped around the hair follicle stem cells ‘like a ribbon’.
‘We could really see at an ultrastructure level how the nerve and the stem cell interact,’ Hsu said
‘Neurons tend to regulate excitable cells, like other neurons or muscle with synapses.
‘But we were surprised to find that they form similar synapse-like structures with an epithelial stem cell, which is not a very typical target for neurons.’
The sympathetic nervous system is normally activated at a constant low level to maintain what’s called body homeostasis – maintaining a stable, relatively constant internal physical and chemical conditions in the body.
During this low level of nerve activity, the stem cells were maintained in a poised state ready for regeneration, the team found.
Under prolonged cold, the nerve was activated at a much higher level and more neurotransmitters were released, causing the stem cells to activate quickly, regenerate the hair follicle and grow new hair.
The sympathetic nerve reacts to cold by contracting the muscle and causing goose bumps in the short term, and by driving hair follicle stem cell activation and new hair growth over the long term.
‘When the cold lasts, this becomes a nice mechanism for the stem cells to know it’s maybe time to regenerate new hair coat,’ said Yulia Shwartz, a postdoctoral fellow at Harvard and co-first author of the study.
In the future, the researchers will further explore how the external environment might influence the stem cells in the skin.
‘We live in a constantly changing environment, Hsu said.
‘Since the skin is always in contact with the outside world, it gives us a chance to study what mechanisms stem cells in our body use to integrate tissue production with changing demands, which is essential for organisms to thrive in this dynamic world.
The study has been published in the journal Cell.