Skin, Hair, and Nail Structure and Function
The Skin: Structure and Function
The skin is an organ with cutaneous annexes, such as hair and nails, and exocrine glands, including sebaceous and sweat glands.
Structure of the Skin
Epidermis
The epidermis is composed of stratified epithelial tissue. Its cells, called keratinocytes, divide by mitosis. Keratinocytes generate cell layers and produce keratin.
- Melanocytes: These cells have extensions that lie between nearby cells.
Layers of the Epidermis:
- Stratum Basale (Basal Layer): The deepest layer, consisting of a single row of dividing keratinocytes.
- Stratum Spinosum (Spinous Layer): Several rows of keratinocytes located above the basal layer.
- Stratum Granulosum (Granular Layer): Composed of 2-3 rows of flattened cells containing keratohyalin.
- Stratum Lucidum (Clear Layer): Found in the palms of the hands and soles of the feet. It consists of dead cells containing proteinase K, which provides elasticity.
- Stratum Corneum (Corneal Layer): Consists of several rows of dehydrated, flattened corneocytes (dead cells) that lack a nucleus and organelles, but contain keratin.
Dermis
Layers of the Dermis:
- Papillary Dermis: Separated from the epidermis by the basement membrane.
- Reticular Dermis: Connects to the hypodermis.
Cells of the Dermis:
- Fibroblasts: Dispersed in the dermis, these cells have extensions and synthesize macromolecules of fibers:
- Collagen: Composed of collagen.
- Elastic Fibers: Composed of elastin.
- Reticular Fibers: Composed of reticulin.
- Histiocytes: Star-shaped cells that trap foreign substances.
- Mast Cells: Involved in inflammation and allergies, these cells have a globular appearance.
Sensory Nerves
The skin contains various sensory nerves:
- Krause Corpuscles: Detect cold.
- Ruffini Corpuscles: Detect heat.
- Meissner’s Corpuscles: Detect contact.
- Pacinian Corpuscles: Detect pressure.
Hypodermis
The hypodermis is a layer of adipose tissue located below the dermis. It separates the skin from the underlying muscles. It is divided into lobes, and its cells, called adipocytes, store fat. This fat serves as a reserve and contributes to the body’s cellular processes.
Hair and Nails
Hair
Hair is a thin, flexible, keratinized epidermal filament that emerges from the skin. It is divided into the hair shaft and the root (located within the hair follicle).
- Hair Follicle: Allows hair growth. It is an epidermal structure with a germinal layer that surrounds the root.
- Dermal Papilla: A small indentation at the base of the follicle containing blood vessels that provide nutrients for the matrix cells.
- Matrix: Surrounds the dermal papilla. Its germ cells are constantly dividing. Melanocytes within the matrix produce the pigment that gives hair its color.
Follicular Sheath:
- Internal Root Sheath: Originates from the matrix cells.
- External Root Sheath: Formed by several cell layers at the level of the hair follicle opening, but its thickness decreases towards the bulb.
- Connective Tissue Sheath: Surrounds the follicle and is composed of dermal fibers.
- Sebaceous Glands: Discharge lubricating secretions into the hair follicle and onto the skin.
- Arrector Pili Muscle (Piloerector Muscle): Composed of smooth muscle, it is situated perpendicularly to the hair follicle. When it contracts, it causes the hair to stand on end (goosebumps).
Hair Shaft:
- Cuticle: Formed by 5-7 layers of cells without melanin.
- Cortex: Makes up 85% of the hair’s volume.
- Medulla: Occupies the center of the hair shaft.
Hair Cycle:
- Anagen Phase: Growth phase, lasting 3-4 years.
- Catagen Phase: Transition phase, lasting 3-4 weeks.
- Telogen Phase: Resting phase, lasting 3-4 months, which provokes the fall of the old hair and the expulsion by the new hair.
Nails
Nails are convex, hard, flexible, and translucent plates that line the dorsal surface of the distal phalanges of the fingers and toes. They originate from the keratinization process of epidermal cells.
Nail Growth:
Keratinization occurs at a rate of approximately 0.1 mm per day. Growth can be influenced by:
- Age (e.g., 0.15 mm/day at 10 years old, 0.10 mm/day in adulthood, 0.06 mm/day in old age).
- Temperature (nails grow faster in summer than in winter due to increased cell activity).