Skin: Structure, Functions, and Layers
The Integumentary System
The integument as an organ: The integument, also known as skin, is the body’s largest organ, accounting for 15% of body weight. The integumentary system includes the skin and its derivatives: hair, nails, and glands.
Derivatives of the Integument
- Hair: Functions include protection and sensing light touch. Hair is composed of columns of dead, keratinized cells bound together by extracellular proteins.
- Nails: Participate in grasping and handling small objects. Nails are plates of tightly packed, hard, keratinized epidermal cells.
- Glands: Participate in regulating body temperature. There are three main types of glands associated with the integument:
- Sebaceous: Oil glands located in the dermis that secrete sebum.
- Sudoriferous: Sweat glands, divided into two main types:
- Eccrine: Most common; their main function is regulating body temperature by evaporation.
- Apocrine: Responsible for the “cold sweat” associated with stress.
- Ceruminous: Lie in subcutaneous tissue below the dermis and secrete cerumen (ear wax) into the ear canal or sebaceous glands.
Functions of the Skin
- Thermoregulation: Evaporation of sweat and regulation of blood flow to the dermis.
- Cutaneous Sensation: Sensations like touch, pressure, vibration, pain, warmth, or coolness.
- Vitamin D Production: UV sunlight and a precursor molecule in the skin produce vitamin D.
- Protection: The skin acts as a physical barrier.
- Absorption and Secretion: The skin is involved in the absorption of water-soluble molecules and the excretion of water and sweat.
- Wound Healing:
- Minor Wounds: When a minor burn or abrasion occurs, basal cells of the epidermis break away from the basement membrane and migrate across the wound. They migrate as a sheet; when the sides meet, growth stops—this is called ‘contact inhibition.’
- Deep Wounds: A clot forms in the wound, blood flow increases, and many cells move to the wound. The clot becomes a scab; granulation tissue fills the wound, and intense growth of epithelial cells occurs beneath the scab. The scab falls off, and the skin returns to normal thickness.
The Two Layers of Skin
Epidermis
The epidermis is the thinner, more superficial layer of the skin. It is made up of four cell types:
- Keratinocytes: Produce keratin, a fibrous protein that helps protect the epidermis.
- Melanocytes: Produce the brown pigment melanin.
- Langerhans Cells: Participate in the immune response.
- Merkel Cells: Participate in the sense of touch.
Five Distinct Sub-layers of the Epidermis
- Stratum Corneum: The outermost layer, made of 25-30 layers of dead, flat keratinocytes. Lamellar granules provide water-repellent action and are continuously shed and replaced.
- Stratum Lucidum: Found only in the fingertips, palms of hands, and soles of feet. This layer is made up of 3-5 layers of flat, dead keratinocytes.
- Stratum Granulosum: Made up of 3-5 layers of keratinocytes; the site of keratin formation. Keratohyalin gives the granular appearance.
- Stratum Spinosum: Appears covered in thorn-like spikes; provides strength and flexibility to the skin.
- Stratum Basale: The deepest layer, made up of a single layer of cuboidal or columnar cells. Cells produced here constantly divide and move up to the apical surface.
Dermis
The dermis is the deeper, thicker layer composed of connective tissue, blood vessels, nerves, glands, and hair follicles. The dermis contains three cell types:
- Adipocytes
- Macrophages
- Fibroblasts
There are two main divisions of the dermal layer:
- Papillary Region: The superficial layer of the dermis, made up of loose areolar connective tissue with elastic fibers.
- Dermal Papillae: Finger-like structures that invade the epidermis and contain capillaries or Meissner corpuscles, which respond to touch.
- Reticular Region: Made up of dense, irregular connective and adipose tissue; contains sweat glands, sebaceous (oil) glands, and blood vessels.