Sebaceous Glands at a Glance
- Sebaceous glands are unilobular or multilobular structures that consist of acini connected to a common excretory duct and are usually associated with a hair follicle.
- Sebaceous glands vary considerably in size, even in the same individual and in the same anatomic area.
- The sebaceous glands exude lipids by disintegration of entire cells, a process known as holocrine secretion.
- Human sebum, as it leaves the sebaceous gland, contains squalene, cholesterol, cholesterol esters, wax esters, and triglycerides.
- Sebaceous glands are regulated by androgens and retinoids.
- Other factors, such as melanocortins, and peroxisome proliferator-activated receptors (PPARs), may regulate sebaceous gland activity as well.
Sebaceous glands are unilobular or multilobular structures that consist of acini connected to a common excretory duct, which is composed of stratified squamous epithelium. Sebaceous glands are composed of lipid-producing sebocytes and of keratinocytes that line the sebaceous ducts and are usually associated with a hair follicle. The periphery of the sebaceous gland is a basal cell layer composed of small, cuboidal, nucleated, highly mitotic sebocytes. Cells progress toward the middle of the gland and accumulate lipid droplets as they terminally differentiate. These fully differentiated sebocytes are full of lipid and lack all other cellular organelles (Fig. 79-1). Surrounding the glands are connective tissue capsules composed of collagen fibers that provide physical support.1
Hematoxylin and eosin-stained section of the human sebaceous gland showing its multilobular structure.
Sebaceous glands are associated with hair follicles all over the body. A sebaceous gland associated with a hair follicle is termed a pilosebaceous unit. The glands may also be found in certain nonhairy sites, including the eyelids (Meibomian glands), the nipples (Montgomery glands), and around the genitals (Tyson glands). Only the palms and soles, which have no hair follicles, are totally devoid of sebaceous glands. Sebaceous glands vary considerably in size, even within the same individual and within the same anatomic area. On the external body surface, most glands are only a fraction of a millimeter in size. The largest glands and greatest density of glands (up to 400–900 glands/cm2) are located on the face and scalp.1–3 The hairs associated with these large glands are often tiny, and it has been suggested that the total structures be more properly termed sebaceous follicles rather than hair follicles.
In the oral epithelium, sebaceous glands known as Fordyce spots are sometimes present. Fordyce spots are visible to the unaided eye because of their large size (up to 2–3 mm) and the transparency of the oral epithelium. In this location, the sebaceous ducts open directly to the surface.
Embryogenesis and Morphogenesis
In the human fetus, sebaceous glands develop in the 13th to 16th week ...