Sebum is oil produced by the sebaceous gland which covers all of the skin except for the palms of hands and soles of feet. Sebum contains triglycerides, diglycerides, fatty acids, wax esters, squalene and cholesterol [1] and sebaceous glands lie just below the dermis layer of the skin and sebum is secreted from them to the stratum corneum (the outermost layer) to maintain a hydrophobic barrier [3]- sebum makes skin waterproof. Sebum is not a product of fats consumed by the individual diet from rather manufactured specially to be stored by sebaceous glands.

Squalenes, lipids I had never heard of before, are hydrophobic chains of carbon atoms that form naturally occurring steroids and signal molecules.

Sebum is important because it lubricates the skin and hair [2] by creating an acidic environment of 4.5-5.5 pH which forces the epidermal and cuticle cells to lie flat so that cells don’t pull apart which then leads to dehydration and irritation of the skin and brittle hair.

However, too much sebum, produces oily skin and can block pores, leading to acne. Clogged pores leads to acne because it creates a low oxygen environment supporting the growth of acne-causing bacteria. Also, sebum is a great food source for acne-causing bacteria and therefore bacterial growth. When sebum is broken down, it may also release molecules that lead to inflammation and cause inflammatory acne [3] and it seems like people with oily skin have different type of sebum than those with normal skin like decreased fatty acids and increased levels of squalene and glycerides [3]; there must be a genetic factor.

Another factor is having an excess of the hormone dihydrotestosterone (DHT) [2] which triggers the sebaceous gland s to produce sebum so having higher levels of this hormone or having glands that may be more sensitive to the hormone leads to an excess of sebum production. DHT is also the reason why acne starts in the teenage years- it becomes more active in puberty and also why some females get more acne prone before and during menstruation; DHT rises at this time mirroring the rise of progesterone after ovulation.

Other factors like environmental factors, diet, genetics and metabolic conditions [3] could increase the production of sebum also because they are part of a balance that controls hormonal signals. Another hormone said to be involved in the acne party is testosterone and other androgen hormones, as males with high levels of them also have higher levels of sebum production and women with higher levels of those hormones have higher acne levels. Oestrogens are supposed to suppress anhydrous hormones [3] which links to how acne in women changes over menstruation

Sebaceous glands lie just below the dermis layer of the skin and sebum travels up through the dermis layer out of pores and hair follicles. The T-zone of the face has the most sebaceous glands- the forehead, nose and chin.

P.S. Shout out to praise for telling me to right about this!

Sources:

[1]https://askanesthetician.wordpress.com/2011/04/21/what-is-sebum-its-more-interesting-than-you-think/

[2]http://www.dermadoctor.com/blog/oily-skin/

[3] http://thescienceofacne.com/sebum/

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