Skin - Sebocytes
Sebocytes are the major cell components in the sebaceous glands present in the dermis of the skin, next to the hair follicles. They are specialised epithelial cells that accumulate lipids and cell debris over time, forming sebum. They eventually disrupt and dissolve, releasing sebum through the hair canal where it reaches the surface of the skin, in a process known as holocrine secretion.
The face, scalp, and chest have the highest concentrations of sebaceous glands, with up to 900 of these glands per square centimetre.
Functions of sebum
Sebum forms a protective layer over the skin and hair shafts. Its synthesis is regulated by hormones, in particular – androgens like testosterones, a type of male hormone. Sebum production peaks shortly after birth and decreases within the first week of life. During puberty, testosterone floods the body, triggering another spike in sebum production. Testosterone and sebum levels naturally decline with age.
Sebum is made up of a group of complex oils, including lipids like triglycerides and cholesterol, making it sticky or oily in consistency. While the main function of sebum is to lubricate the skin, protect it from friction and make it impervious to moisture, it also serves a variety of purposes.
Thermoregulatory: The sebaceous glands and the sweat glands work together to regulate body temperature in changing climates. When exposed to heat, sebum emulsifies sweat to form a “sweat sheet” which discourages the loss of water from the surface of the skin, thereby preventing dehydration. In colder conditions sebum changes its nature and repels rain from skin and hair, thereby maintaining body heat.
Antimicrobial: The pH. of sebum is between 4.5 and 6, making it slightly acidic. This prevents harmful pathogens from invading the skin. They actively participate in inflammatory processes by communicating with immune mediated cells.
Antioxidant : Sebum transports fat soluble vitamins like Vitamin E to the skin’s surface which helps with skin regeneration and prevents oxidative damage to the skin cells, by counteracting the effects of free radicals present in the environment.
Conditions due to altered sebum secretion
Disruptions in sebum secretion along with structural changes in sebaceous glands could lead to various skin diseases.
a. Excessive Production
Overactive sebaceous glands can affect both males and females and gives rise to excessively oily skin, which feels unpleasant, appears shiny and seems to get dirty quickly. This is known as seborrhoea, which in worsening conditions leads to red, greasy and scaly patches on the skin known as seborrheic dermatitis.
Excessive sebum production also gives rise to acne , as it may block the skin’s pores leading to accumulation of dirt and bacteria, with the formation of a hardened sebum plug in the hair follicle. When a sebum plug forms, commensal bacteria that usually live harmlessly on the surface of the skin, start growing within the follicle, causing inflammation that leads to a breakout. So, sebum plugs end up being precursors for blackheads and white heads.
The secretion of sebum many need to be controlled with topical retinols (vitamin A derivatives), topical steroids or hormonal medication.
b. Decreased Production
On the other hand , reduced sebum secretion due to premature degeneration of sebaceous glands, results in psoriatic alopecia . This causes hair loss with psoriatic plaques. Low levels of sebum may also give rise to dry skin that tends to crack, especially when exposed to cold.
References
1.Schneider, MR, Zouboulis, CC. Primary sebocytes and sebaceous gland cell lines for studying sebaceous lipogenesis and sebaceous gland diseases. Exp Dermatol. 2018; 27: 484– 488. https://doi.org/10.1111/exd.13513
2.Boer M, Duchnik E, Maleszka R, Marchlewicz M. Structural and biophysical characteristics of human skin in maintaining proper epidermal barrier function. Postepy Dermatol Alergol. 2016;33(1):1-5. doi:10.5114/pdia.2015.48037
3.Porter AM. Why do we have apocrine and sebaceous glands?. J R Soc Med. 2001;94(5):236-237. doi:10.1177/014107680109400509