Pigmentation of the Skin

Structure of the Skin

• The skin is the largest organ in the body and contains glands, hair, and nails.
• It consists of two main layers: the epidermis and the dermis.
• The epidermis is the most superficial layer, composed of stratified keratinized squamous epithelium.
• The dermis is a tough and elastic connective tissue containing collagen fibers and elastic fibers.

The Epidermis

• The epidermis is thickest on the palms of the hands and soles of the feet.
• It has no blood vessels or nerve endings, but is bathed in interstitial fluid from the dermis.
• The epidermis has several layers of cells, extending from the deepest germinative layer to the most superficial stratum corneum.

Layers of the Epidermis

• Basal Layer/Germinal Layer: The deepest layer, made up of a single layer of columnar cells that rest on a basal lamina.
• Stratum Spinosum: Several layers of polygonal keratinocytes that constitute the Malpighian layer.
• Stratum Granulosum: Few layers of flattened cells characterized by the presence of deeply staining granules in their cytoplasm.
• Stratum Lucidum: A homogeneous layer superficial to the stratum granulosum.
• Stratum Corneum: An acellular layer made up of flattened scale-like elements containing keratin filaments embedded in protein.

Pigmentation of the Skin

• The number of melanocytes is constant, and differences in skin color depend on the amount of melanin secreted.
• Melanin protects the skin from harmful sunlight, and exposure to sunlight promotes melanin synthesis.
• The epidermis is translucent, allowing the color of blood to show, especially in light-skinned individuals.

Melanocytes

• Melanocytes are responsible for producing the pigment melanin.
• They have processes that transfer melanin granules to surrounding non-melanin-producing cells.
• Each melanocyte has dendritic processes.

The Dermis

• The dermis is a tough and elastic connective tissue containing collagen fibers and elastic fibers.
• It rests on the superficial fascia, which attaches it to deeper structures.
• The dermis has two main layers: the papillary layer and the reticular layer.
• Fibroblasts, macrophages, and mast cells are the main cells found in the dermis.

Blood and Lymph Supply

• Blood vessels do not penetrate the epidermis.
• Arterioles form a fine network with capillary branches supplying sweat glands, sebaceous glands, hair follicles, and the dermis.
• Veins from the dermal papillae drain into a venous plexus lying on deep fascia.

Nerve Supply

• Sensory receptors for touch, temperature, pressure, and pain are widely distributed in the dermis.
• Different types of sensory receptors are activated by various stimuli:
  • Meissner's corpuscle: light pressure
  • Pacinian corpuscle: deep pressure
  • Free nerve endings: pain

Appendages of the Skin

• Hair and Hair Follicle: Each hair consists of a visible part (shaft) and an embedded part (root).
• The root has an expanded lower end (bulb) and is surrounded by a tubular sheath (hair follicle).
• The surface of the hair is covered by a thin membrane (cuticle).
• The cortex is acellular and made up of keratin.

Sweat Glands

• The wall of the tube making up the gland consists of an inner epithelial lining, basal lamina, and supporting layers of connective tissue.
• In large sweat glands, flattened contractile myoepithelial cells are present between the epithelial cells and their basal lamina.
• Sweat glands are innervated by cholinergic nerves.

Nails

• Nails are present on fingers and toes.
• The main part of the nail is the body, which has a free distal edge.
• The proximal part of the nail is implanted into a groove on the skin and is known as the root or radix.
• The nail represents a modified part of the zone of keratinization of the epidermis.
• The nail substance consists of several layers of dead, cornified cells filled with keratin.