Dermis and Its Functions

Dermis

• The dermis is a connective tissue layer that resides beneath the epidermis, making up the majority of the skin.
• Its thickness ranges from 0.2 mm (eyelids) to 4 mm (palms, soles).
• Contains blood vessels, sweat glands, sebaceous glands, nerve endings, hair follicles, and nail roots.
• It's also known as corium.
• The dermis has a wavy boundary with the epidermis.
  • Dermal papillae are finger-like extensions of the dermis that project upward into the epidermis.
  • Epidermal ridges are downward waves of the epidermis.
  • Friction ridges are prominent waves found on fingers, creating fingerprints.
• The cells of the dermis are mesodermal in origin, except for nerves and melanocytes, which derive from the neural crest.

Functions of the Dermis

• Skin support: Provides pliability, elasticity, and tensile strength through collagen, hyaluronic acid, oxytalan fibers, and elastic tissue.
• Protection: Protects the body from mechanical injury.
• Thermoregulation: Vasoactive dermal vessels regulate body temperature, and glomus bodies contribute through AV shunt formation.
  • Glomus bodies are found in digits, palms, and soles, consisting of glomus cells, vessels, and smooth muscle.
• Sensation: Nerve endings in the dermis surround hair follicles, acting as mechanoreceptors for hair movement. Deep pressure receptors are also present.
  • Pacinian corpuscles: Deep pressure and vibration sensation.
  • Meissner's corpuscles: Low-frequency stimuli.

Layers of the Dermis

• Papillary Layer: Superficial zone that projects into the epidermal ridges.
  • Composed of loose connective tissue.
  • Thin zone of areolar tissue with capillaries, sensory nerve endings, and cytoplasm.
  • Allows for mobility of leukocytes and defense cells.
• Reticular Layer: Deeper and thicker layer, made of dense, irregular connective tissue.
  • Connects with the subcutaneous fatty tissue.
  • Contains blood vessels and nerves.

Interstitial Components

• Collagen:
  • Primary component of the dermis, making up 70% of its weight.
  • Predominantly Type I collagen with some Type III.
  • Found in tendons, ligaments, bone lining, and the dermis.
  • Provides stress resistance.
  • Degraded by collagenases and replaced by new fibers.
  • Collagen fibers are aggregations of fibrils.
    • Thin fibers in the papillary layer.
    • Thick fibers in the reticular layer.
• Elastic Fibers:
  • Composed of elastin and fibrillin microfibrils.
    • Elastin allows for gliding, stretching, and recoiling.
  • Thinner, less tough than collagen fibers.
  • Abundant in the dermis of the scalp, face, and extensible organs.
  • Deeper fibers are thicker.
  • In the reticular layer, they run parallel to the skin surface.
  • Connected to the lamina densa of glands, sweat ducts, smooth muscle, nerves, and blood vessels.
  • Subtypes:
    • Elaunin Fibers: Found near the papillary-reticular junction.
    • Oxytalan Fibers: Found in the papillary dermis, perpendicularly arranged.
• Reticular Fibers:
  • Composed of Type III collagen.
  • Thin reticular network compared to collagenous fibers.
  • Found in various tissues, forming supporting frameworks.

Cells of the Dermis

• Fibroblasts:
  • Primary cell type in the dermis.
  • Produce and secrete procollagen and elastic fibers.
  • Procollagen is catalyzed to form collagen, which is then cross-linked into strong layers.
• Macrophages (Histiocytes):
  • Tissue macrophages that intermingle with fibroblasts and help the immune system.
  • Assist in organ repair.
• Mast Cells:
  • Found in the perivascular areas.
  • Secrete vasoactive and proinflammatory mediators.
  • Contain granules with histamine and other chemicals.
• Schwann Cells: Specialized cutaneous Schwann cells initiate pain sensation.
• Dermal Adipocytes:
  • Distinct from subcutaneous adipocytes.
  • Involved in insulation, energy storage, hair follicle regeneration, and wound healing.

Vasculature

• Subcutaneous/Deep Dermal:
  • Arteries and veins branch superficially into the middermis, forming the cutaneous plexus.

### Nerve Endings

• Free-Nerve Endings: Sensitive to cold, found throughout the body, lips, tongue, and conjunctiva.
• Brushes of Ruffini: Detect tension deep in the skin, sensitive to pressure, stretching, and distortion.

Diseases

• Collagen-Related:
  • Ehlers-Danlos Syndrome: Genetic connective tissue disorders.
    • Defective collagen synthesis.
    • Symptoms: skin hyperextensibility, joint mobility, tissue fragility.
  • Osteogenesis Imperfecta: Genetic disorder of type I collagen.
    • Decreased dermal collagen, impaired skin elasticity, brittle bones.
  • Striae Distensae: Stretch marks caused by rapid stretching or shrinkage of the skin.
    • Rupture of collagen and elastin.
• Fibroblast-Related:
  • Hypertrophic Scars: Raised, erythematous, pruritic lesions confined to the wound boundaries, caused by myofibroblasts.
  • Keloids: Benign growths of fibrous tissue extending beyond the wound boundaries, also related to myofibroblasts.
  • Lichen Sclerosus: Inflammatory disorder with abnormal fibroblast function, causing dermal fibrosis and epidermal changes.
  • Acanthosis Nigricans: Increased growth factor receptor signaling, leading to epidermal and dermal proliferation.
• Mast Cell-Related:
  • Mastocytosis: Increase in mast cells in the skin and other organs.
    • Urticaria Pigmentosa: Reddish-brown lesions that form hives or blisters when stroked.

Hypodermis

• Also called the subcutaneous layer or superficial fascia.
• Connects the skin to underlying fascia of bones and muscles.
• Composed of well-vascularized loose areolar connective tissue and adipose tissue.
• Functions: fat storage, insulation, and integument cushioning.
• The panniculus (subcutaneous layer) varies in thickness based on the skin site.
• Considered an endocrine organ due to hormone conversions:
  • Androstenedione to estrone by aromatase.
  • Lipocytes produce leptin, which regulates body weight.