Why do males with baldness have shiny skin on their head, whereas we don't experience such "shiny skin" in other parts of body without hair.
Think of what happens when you don't wash your hair for days. It gets oily and greasy. This oil-like substance on your hair is called sebum and mainly works as a lubricant . The Sebaceous glands responsible for excreting sebum don't stop working when the hair follicles attached to them do. The result is a "shiny" head.
It’s due to fibrosis. Men with androgenic allopecia suffer from chronic inflammation of the scalp. Male pattern baldness is a poorly understood disease, but what most people don’t know is that it’s an auto immune skin condition. The hormone DHT appears to trigger a cascade of events leading to inflammation and fibrosis This results in a build up of scar tissue, I.e. fibrosis that gives a swollen and shiny look to the scalp. Scalp fibrosis results in reduced blood flow and hair follicles shrinkage and eventual death. https://perfecthairhealth.com/the-ultimate-hair-loss-flowchart-why-we-lose-our-hair/ You may not realize it, but claims that bald men’s heads are shiny due to sweat and grease is quite insulting to people suffering from an embarassing and irritating auto immune disease. It’s just an uneducated guess in the truest sense. An attempt to explain something without having done any research into the subject. Armpits are usually sweaty and produce oils but are never shiny.
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Skin is naturally shiny thanys the waxes and oils on its surface, provided it is smooth. The skin over the top of the head is very smooth and if fairly taunt because the skin does not undergo much flexion. Note how people in with uncommonly smooth and hairless skin any large smooth taunt patch (like the thigh or cheek) will also be shiny unless they use makeup.