Relative to keratinocytes that make up the hair follicle, sebaceous glands are composed of huge cells (sebocytes) with many large vesicles that contain the sebum. These cells express Na+ and Cl− ion channels,
ENaC and
CFTR (see Fig. 6 and Fig. 7 in reference Sebaceous secretions in conjunction with
apocrine glands also play an important
thermoregulatory role. In hot conditions, the secretions emulsify the sweat produced by the
eccrine sweat glands and this produces a sheet of sweat that is not readily lost in drops of sweat. This is of importance in delaying dehydration. In colder conditions, the nature of sebum becomes more lipid, and in coating the hair and skin, rain is effectively repelled. Sebum is produced in a
holocrine process, in which sebocyte cells within the sebaceous gland rupture and disintegrate as they release the sebum and the cell remnants are secreted together with the sebum. The cells are constantly replaced by
mitosis at the base of the duct. The composition of sebum varies across species.
Sapienic acid is a sebum fatty acid 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 hormones 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 in hair follicles.
Immune function and nutrition Sebaceous glands are part of the body's
integumentary system and serve to protect the body against microorganisms. Sebaceous glands secrete acids that form the
acid mantle. This is a thin, slightly
acidic film on the surface of the
skin that acts as a barrier to
microbes that might penetrate the skin. The
pH of the skin is between 4.5 and 6.2, an acidity that helps to neutralize the
alkaline nature of contaminants. Sebaceous lipids help maintain the integrity of the skin barrier and supply
vitamin E to the skin. Sebaceous glands actively participate in innate immunity by releasing
cytokines like IL-1α, IL-6, and TNF-α.
Unique sebaceous glands During the last three months of
fetal development, the sebaceous glands of the fetus produce
vernix caseosa, a waxy white substance that coats the skin to protect it from
amniotic fluid. The
areolar glands are in the
areola that surrounds the nipple in the female breast. These glands secrete an oily fluid that lubricates the nipple, and also secrete volatile compounds that are thought to serve as an olfactory stimulus for the newborn. During pregnancy and lactation these glands, also called Montgomery's glands, become enlarged.
Meibomian glands, in the
eyelids, secrete a form of sebum called
meibum onto the
eye, that slows the evaporation of
tears. They also serve to create an airtight seal when the eyes are closed, and their lipid quality also prevents the eyelids from sticking together. They attach directly to the follicles of the eyelashes, which are arranged vertically within the
tarsal plates of the eyelids.
Fordyce spots, or Fordyce granules, are ectopic sebaceous glands found on the genitals and
oral mucosa. They show themselves as yellowish-white
milia (milk spots).
Earwax is partly composed of sebum produced by glands in the ear canal. These secretions are
viscous and have a high
lipid content, which provides good lubrication. ==Clinical significance==