M04.01.015 The skin

The skin, the largest organ in the human body, comprises about 8% of total body mass. It is a versatile structure with varying compositions across different regions. This article covers the skin’s function, gross structure, and ultrastructure.

Functions of Skin

The skin serves as a vital barrier against mechanical, chemical, osmotic, thermal, UV damage, and microbial invasion. Additional functions include:

• Vitamin D synthesis
• Body temperature regulation
• Psychosexual communication
• Sensory reception for touch, temperature, and pain

Gross Structure

Skin composition varies; it can be thin, hairy, hirsute, or glabrous (hairless and thick, found on palms and soles). Skin consists of three layers: the epidermis, dermis, and hypodermis.

Ultrastructure

Epidermis:

• Keratinocytes: Undergo terminal maturation, producing keratin and migrating to the surface (cornification).
• Non-Keratinocytes: Include melanocytes (produce melanin), Langerhans cells (antigen-presenting), and Merkel cells (sensory mechanoreceptors).

Layers of the Epidermis (deep to superficial):

• Stratum basale: Keratinocyte mitosis
• Stratum spinosum: Desmosomes connect keratinocytes
• Stratum granulosum: Secretion of lipids and waterproofing molecules
• Stratum lucidum: Increased keratin production, loss of nuclei
• Stratum corneum: Keratin production continues, loss of organelles

Dermis:

• Layers: Superficial papillary layer and thicker reticular layer with collagen fibers
• Cell Types and Structures:
  • Fibroblasts: Synthesize extracellular matrix (collagen, elastin)
  • Mast cells: Contain histamine granules
  • Blood vessels and sensory nerves
  • Skin appendages: Hair follicles, nails, sebaceous and sweat glands

Hair Follicles and Sebaceous Glands:

• Form pilosebaceous units in hirsute skin
• Sebaceous glands release secretions into hair follicles
• Arrector pili muscles cause hair to stand upright

Sweat Glands:

• Eccrine glands: Major sweat glands, that release clear, odorless sweat for thermoregulation
• Apocrine glands: Larger glands in axillary and genital regions produce body odor when broken down by microbes

Hypodermis:

• Subcutaneous tissue, stores adipose tissue, and varies in size depending on fat content

Clinical Relevance – Disorders of Skin

• Alopecia Areata: Autoimmune destruction of hair follicles causing hair loss
• Vitiligo: Autoimmune destruction of melanocytes, leading to depigmentation
• Psoriasis: Increased keratinocyte mitosis in stratum basale, leading to thickened stratum spinosum and scaly skin

By understanding the skin’s structure and functions, as well as the clinical implications of its disorders, medical professionals can better diagnose and treat related conditions.