Sebaceous Glands

1. Overview

Sebaceous glands are holocrine exocrine glands associated with the skin that secrete an oily substance known as sebum. These glands are a key component of the integumentary system and are involved in lubricating and waterproofing the skin and hair. Sebaceous glands play essential roles in maintaining skin barrier integrity, antimicrobial defense, and thermoregulation. Their activity is hormonally regulated and closely linked to conditions such as acne and seborrheic dermatitis.

2. Location

Sebaceous glands are distributed throughout most of the body’s skin, with varying densities depending on the region:

• High density: Face, scalp, chest, and upper back—areas prone to oiliness and acne.

• Moderate density: Trunk and limbs.

• Absent: Palms of the hands and soles of the feet.

• Association with hair follicles: Most sebaceous glands open into hair follicles and are considered part of the pilosebaceous unit.

• Free sebaceous glands: Found in hairless regions such as the eyelids (Meibomian glands), lips (Fordyce spots), and genital mucosa.

3. Structure

Sebaceous glands are composed of lobular clusters of specialized epithelial cells:

• Acini (lobules):

  • Grape-like clusters of sebocytes that synthesize and store sebum in cytoplasmic lipid droplets.

  • Located in the dermis, near the upper portion of hair follicles.

• Duct system:

  • Short duct connects the acini to the hair follicle or directly to the skin surface in hairless regions.

• Holocrine secretion:

  • Mature sebocytes rupture, releasing their entire contents (sebum) into the duct.

Sebaceous glands are surrounded by a basal layer of progenitor cells that continually replenish the sebocytes.

4. Function

Sebaceous glands perform several critical functions essential to skin health:

• Sebum secretion: Oily, lipid-rich secretion that coats the skin and hair shaft.

• Lubrication: Prevents drying and cracking of the epidermis and hair.

• Waterproofing: Helps reduce transepidermal water loss by forming a hydrophobic barrier.

• Antimicrobial action: Sebum contains free fatty acids and lipids with antibacterial and antifungal properties.

5. Physiological role(s)

Sebaceous glands contribute to several broader physiological processes:

• Skin barrier maintenance: Supports stratum corneum integrity and normal desquamation (shedding).

• Hormonal response: Androgens (e.g., testosterone) stimulate sebaceous gland growth and sebum production, especially during puberty.

• Innate immunity: Sebum forms part of the skin’s first-line defense against pathogens.

• Thermoregulation support: Although not directly involved in sweating, sebum influences surface heat and moisture retention.

6. Clinical Significance

Sebaceous glands are involved in a range of dermatological and systemic conditions:

• Acne vulgaris:

  • Caused by excess sebum, follicular hyperkeratinization, bacterial overgrowth (Cutibacterium acnes), and inflammation.

  • Most common during adolescence due to androgenic stimulation.

• Seborrheic dermatitis:

  • Inflammatory condition associated with overactive sebaceous glands and yeast colonization (Malassezia).

• Sebaceous cysts (epidermoid cysts):

  • Blocked sebaceous ducts may form fluid-filled sacs that can become infected or inflamed.

• Sebaceous hyperplasia:

  • Benign enlargement of sebaceous glands, typically seen as yellowish papules on the face in older adults.

• Sebaceous gland carcinoma:

  • Rare but aggressive malignant tumor, often arising in Meibomian glands of the eyelid; requires early diagnosis and treatment.

• Hormonal disorders:

  • Conditions such as polycystic ovary syndrome (PCOS) and androgen-secreting tumors can cause sebaceous overactivity and oily skin.