Oil gland
The sebaceous glands are located in the mid-dermis and are made up of lipid-filled cells that develop embryologically in the fourth month of gestation as an epithelial budding of the hair follicle.
This gland is characterized by synthesizing sebum, a lipid substance whose function is to "lubricate" and protect the surface of the skin.
This glandular secretion is continuous, with a certain predominance during the anagen cycle of the hair follicle. The secretion of each lobe is of a holocrine nature, that is, with rupture of the individual cells, draining from the acini into the main sebaceous duct that ends in the hair canal.
These glands are found throughout the skin, varying in size and number depending on their location: on the face and scalp they are large and numerous (400 to 900 per cm²), on the trunk they are small and less abundant, increasing in the anterior part of the thorax and midline of the back.
Under an electron microscope, peripheral glandular cells are observed to contain tonofilaments, reflecting their epidermal origin, and scant lipids. As lipids are formed, glycogen is consumed, tonofilaments are displaced, and the cytoplasm is filled with vacuoles. In the cell, the vacuoles merge with each other, causing an increase in size up to one hundred times normal, acquiring the appearance of a foreign body cell. In a later stage, the membrane is disorganized and the cell ruptures, eliminating its contents into the sebaceous canal.
Several medical conditions involve sebum, including acne, sebaceous cysts, hyperplasia, and sebaceous adenoma. These are normally attributed to overactive sebaceous glands that produce excess sebum.
Structure
Location
Sebaceous glands are found throughout all areas of the skin except the palms of the hands and the soles of the feet. There are two types of sebaceous gland, those that are connected to the hair follicles, in pilosebaceous units and those that exist independently...
One or more glands may surround each hair follicle, and the glands are surrounded by erector pili muscles. The glands have an acinar structure, in which several glands exit from a central duct. The glands deposit sebum in the capillaries and bring it to the surface of the skin together with the hair shaft. The consistent structure of hair, hair follicle, erector pili muscles, and sebaceous gland is an epidermal invagination known as the pilosebaceous unit.
Sebaceous glands are also found in hairless areas of the eyelids, nose, penis, labia minora, the inner mucous membrane of the cheek, and nipples. Some sebaceous glands have unique names.
Development
Sebaceous glands are first visible between weeks 13 and 16 of fetal development. Sebaceous glands develop from the same tissue that gives rise to the epidermis of the skin. Overexpression of the signaling factors Wnt, Myc and SHH increase the probability of the presence of sebaceous glands.
The sebaceous glands of a human fetus secrete a substance called vernix caseosa, a translucent white, waxy substance that coats the skin of newborns. After birth the activity of the sebaceous glands decreases to the point of almost no presence. null during the ages of 2 to 6 years, then increases to its peak of activity during puberty due to high levels of androgens.
Function
The sebaceous glands secrete the oily, waxy substance called sebum which is made of triglycerides, wax esters, squalene and metabolites of fat-producing cells. Sebum is hydrophobic and lubricates the skin and mammalian hair. Sebaceous secretions in conjunction with apocrine glands play an important role in thermoregulation. In warm conditions, the secretions emulsify the sweat produced by the eccrine glands and this produces a sweat film that is not easily lost by sweat droplets. This is of great importance to delay dehydration. In colder conditions, the sebum composition becomes more lipidic, covering hair and skin, rain is repelled efficiently..
Sebum is produced in a holocrine process, in which cells within the sebaceous glands disintegrate while releasing sebum, and the rest of the cell is secreted along with the sebum. Cells are constantly replaced by mitosis in the base of the duct.
Sebum
Sebum, secreted by the sebaceous gland in humans, is composed primarily of triglycerides (~41%), wax esters (~26%), squalene (~12%), and free fatty acids (~16%). The composition of sebum varies between species. Wax esters and squalene are unique to sebum and are not produced elsewhere in the body. Sapienic acid is a fatty acid in sebum that is unique to humans and is implicated in the development of acne. Sebum is odorless, but its breakdown by bacteria can produce strong odors.
Sex steroids are known to affect the rate of sebum secretion: androgens such as testosterone have been shown to stimulate secretion and estrogens have been shown to inhibit secretion. Dihydrotestosterone acts as the primary androgen in the prostate and hair follicles.
Immune Function and Nutrition
The sebaceous glands are part of the body's integumentary system and serve to protect the body from germs. The sebaceous glands secrete acids that form the acid mantle. This is a very thin layer of acid on the skin's surface and acts as a barrier against bacteria, viruses, and other potential contaminants that can penetrate the skin. The skin's pH is between 4.5 and 6.2, and this acidity helps to neutralize the alkaline nature of contaminants.
Sebaceous lipids make an important contribution in maintaining the integrity of the skin barrier and express both pro-inflammatory and anti-inflammatory properties. Sebum can act as a delivery system for antioxidants, antimicrobial lipids, pheromones and hydration of the stratum corneum. The fatty acids contained within the sebum have extensive microbial activity. In addition, the secretion of sebaceous glands provides vitamin E to the upper layers of facial skin.
Single sebaceous glands
During the last three months of fetal development, the sebaceous glands of the fetus produce vernix caseosa, a white, waxy substance that forms a layer to protect the skin from amniotic fluid..
The areolar glands are in the areola that surrounds the nipple of a woman's breast. These glands secrete an oily fluid that lubricates the nipple and also secretes volatile compounds that are thought to serve as an olfactory stimulus to the newborn. During pregnancy and lactation these glands, also called Montgomery glands, become larger.
Meibomian glands in the eyelids secrete a form of sebum called meibum into the eye, which slows the evaporation of tears. It also serves to create a tight seal when the eyes are closed and its lipid quality also prevents tears eyelids stick together. Meibomian glands are also known as tarsal glands, Zeis glands, and palpebral glands. They attach directly to the eyelid follicles that are arranged vertically between the tartal plates and the eyelids.
Fordyce pimples, or Fordyce granules, are ectopic sebaceous glands found on the genitals and oral mucosa. They show up as yellowish white milia.
Ear wax is made up in part of sebum produced by glands in the ear canal. These secretions are viscous and have a high lipid content that provides good lubrication.
Clinical significance
Sebaceous glands are linked to skin problems such as acne and keratosis pilaris. In the pores of the skin, sebum and keratin can create a hyperkeratotic plug called a comedone.
Acne
Acne is a very common problem, particularly during puberty in adolescents, and is thought to be related to an increase in sebum production due to hormonal factors. Increased sebum production can lead to a blockage of the sebaceous gland duct. This can cause a comedone (blackhead), which can cause an infection, particularly with the bacterium Cutibacterium acnes. This can inflame comedones, which then change into characteristic acne lesions. Comedones generally appear in areas with more sebaceous glands, mainly the face, shoulders, upper chest, and back. Comedones can be white or black depending on the complete pilosebaceous unit or if the sebaceous duct is blocked.
There are many treatments available for acne, from reducing sugars in the diet to medications including antibiotics, benzoyl peroxide, retinoids, and hormonal treatments. Retinoids reduce the amount of sebum produced by the sebaceous glands. If the treatments fail, you can look for the presence of Demodex which could be a probable cause.
Others
Other conditions involving the sebaceous glands include:
- Abortion, refers to hyperactive sebaceous glands, which cause skin or oily hair.
- Sebaceous hyperplasia, refers to the excessive proliferation of cells within the glands, are macroscopically visible as small ampulas in the skin, particularly in the forehead, nose and cheeks.
- Seborreic dermatitis, a chronic form of dermatitis caused by changes in the sebaceous glands.
- Psoriasis as-seborreica ("Sebopsoriasis", and "Seborriasis") is a skin condition characterized by psoriasis with a seborreic dermatitis.
- Sebaceous adenoma, a slow tumor of benign growth, which, however, may in some cases be precursor to a cancer syndrome known as Muir-Torre syndrome.
- Sebaceous carcinoma, an aggressive and uncommon skin tumor.
- Sebaceous cyst it is a term used to refer both to an epidermoid cyst and a pillar cyst. Although none contains sebum, only keratin and does not originate in the sebaceous gland and therefore are not really sebaceous cysts. A true sebaceous cyst is relatively rare and is known as stool.
- Fimatous rosacea is a skin condition characterized by the excessive growth of the sebaceous glands.
History
The word sebaceous means "consisting of sebum", it was first used in 1728 and comes from the Latin adipem meaning sebum. Sebaceous glands have been documented since at least 1746 by Jean Astruc, who defined them as "...the glands that separate fat." He describes them in the oral cavities, head, eyelids and ears, as &# 34;universally" known. Astruc describes them as being blocked by "small animals" that they are "implanted" in the secretory ducts, and attributes its presence in the oral cavities to aphthous ulcers, noting "these glands naturally [secrete] a viscous humor, which puts on various colors and consistencies... in its natural state it is very smooth, balsamic, and intended to moisten and lubricate the mouth". In The Principles of Physiology 1834, Andrew Combe noted that the glands were not present on the palms of the hands or the soles of the feet.
Other animals
The foreskin glands of mice and rats are large modified sebaceous glands that produce pheromones used to mark territory. These and the sensory glands on the flanks of hamsters are similar in composition to human sebaceous glands, are sensitive to androgens and have been used as a basis for study.
Sebaceous adenitis is an autoimmune inflammatory disease that affects the sebaceous glands. It is known to occur primarily in dogs, particularly poodles and akitas, where it is generally believed to be of autosomal recessive inheritance. It has also been described in cats, and one report describes the condition in a rabbit. In these animals it causes hair loss, although the nature and distribution of hair loss differs greatly.
Additional images
Pylosebaceae unit base
Sebaceous gland insertion in a hair stem
Sagital cut through the upper eyelid
A hair follicle with associated structures
