Sebaceous Gland: Anatomy and Physiology (MD Dermatology Notes)

1. Introduction

2. Embryology (brief)

• Develop from the epithelial placode (bulge) of the developing hair follicle at 13-16 weeks of fetal life, arising in a cephalocaudal sequence.
• The hair follicle bulge stem cell reservoir can regenerate sebaceous glands, though some evidence suggests glands may also be maintained independently of the bulge.
• Key signalling: Sonic hedgehog (Shh) and Myc signalling promote sebocyte differentiation; Wnt/β-catenin signalling promotes hair follicle differentiation instead (reciprocal switch) - a favorite viva question.

3. Anatomy

• Multilobular, holocrine glands composed of acini (lobules) draining into a common sebaceous duct lined by undifferentiated keratinocytes (stratified squamous epithelium).
• Gland periphery = single layer of small, cuboidal, mitotically active undifferentiated sebocytes (basal layer) → cells migrate centrally, accumulate lipid droplets, terminally differentiate, lose all organelles, then rupture and die, discharging contents into the duct.
• Usually opens into the hair follicle infundibulum, forming the pilosebaceous unit (gland + hair follicle + arrector pili muscle). When the associated hair is vestigial/tiny with a disproportionately large gland, this is called a sebaceous follicle (the site of acne).

• Present wherever hair follicles exist (whole body).
• Highest density: nose (~1600 glands/cm²) > face and scalp (400-900 glands/cm²) - "T-zone" seborrhea correlates directly with this density.
• Absent on palms and soles (no hair follicles); sparse on dorsa of hands/feet.
• Free (ectopic) sebaceous glands - not associated with hair, open directly onto epithelial surface:
  • Meibomian glands (tarsal glands) - eyelids
  • Montgomery glands (areolar glands) - nipple/areola
  • Tyson glands - prepuce/genital skin
  • Fordyce spots - lips, buccal mucosa, genital mucosa (visible 1-3 mm papules due to oral epithelial transparency)

Diagram

4. Physiology

A. Holocrine Secretion

• Not simple cell swelling; it is a programmed, lysosomal DNase2-mediated cell death pathway distinct from apoptosis, necroptosis, and cornification.
• Sebocytes accumulate lipid droplets as they move centrally → disintegrate near the duct → release neutral lipids (proteins/nucleic acids/membrane phospholipids are recycled, not secreted).
• Transit time: ~7.4 days total (4-7 days undifferentiated stage, 14-25 days differentiated/lipid-producing stage) - a commonly asked numeric fact.

B. Lipid Composition of Sebum

• Wax esters and squalene are unique to sebum (absent from internal organ lipids) - key differentiating feature in exams.
• Ductal bacterial lipases hydrolyze triglycerides → free fatty acids (implicated in acne/comedogenesis).
• Unique fatty acids: sapienic acid (16:1Δ6, major fatty acid of adult human sebum, from Δ6 desaturation of palmitic acid) and sebaleic acid (18:2Δ5,8) - both essentially unique to human sebum.

C. Regulation of Sebaceous Gland Size and Sebum Production

• Gland size increases with age (mean area rises from ~0.2 to ~0.4 mm²); gland number stays roughly constant through life, but turnover slows in older adults.
• Regulated by ligand-receptor systems on sebocytes:
  • Androgen and estrogen receptors (androgens are the principal stimulus for sebum production - clinically central to acne pathogenesis)
  • PPAR (peroxisome proliferator-activated receptor) and LXR (liver-X receptor)
  • Neuropeptide receptors (e.g., CRH receptors - corticotropin-releasing hormone increases lipogenesis and IL-6/IL-8 release, linking stress to acne flares)
  • Retinoid receptors (basis of isotretinoin's mechanism - sebosuppressive)
  • Vitamin D receptor
• Sebocytes retain stem-like, bipotential characteristics despite being programmed for terminal differentiation.

D. Functions of the Sebaceous Gland (Fitzpatrick's functional classification - good for a "functions of sebaceous gland" essay)

1. Protection
   • Natural UVB photoprotection
   • Thermoregulation and water-repelling properties
   • Possible role in wound healing
   • Production of vernix caseosa (neonatal skin protection)
2. Transportation
   • Vehicle for delivering antioxidants to/from the skin surface
   • Vehicle for pheromone/fragrance-related compounds
3. Inflammation and Immunity
   • Produces both pro- and anti-inflammatory lipids
   • TLR2 activation upregulates lipogenesis
   • Lipid-induced innate antimicrobial activity
   • Secretes antimicrobial peptides: cathelicidin, psoriasin, β-defensin-1, β-defensin-2 (direct antimicrobial activity, relevant to acne and rosacea pathogenesis)
   • Promotes Th17 differentiation (contributes to inflammatory skin disease)
4. Endocrine Properties
   • Sebaceous gland is a peripheral steroidogenic organ - expresses all steroidogenic enzymes and synthesizes androgens locally
   • Expresses vitamin D receptor and vitamin D-metabolizing enzymes
   • Expresses retinoid-metabolizing cytochrome P450 enzymes
   • Contributes to hormonally-mediated skin aging

5. Clinical Correlation (high-yield linkage for exams)

6. Quick Exam Recall Points

• Sebaceous glands = holocrine, sweat glands = merocrine
• Density highest at nose (1600/cm²); absent on palms/soles
• Transit time ~7.4 days
• Unique lipids: wax esters, squalene, sapienic acid, sebaleic acid
• Chief hormonal driver: androgens (basis for antiandrogen/isotretinoin therapy in acne)
• Free sebaceous glands mnemonic: "MFTM" - Meibomian (eyelid), Fordyce (mucosa), Tyson (prepuce), Montgomery (areola)