The epidermis is populated by specialized cells, each with a unique role:
- Keratinocytes: The most abundant cells, which produce keratin, a hard, waterproof protein. As they divide in the basale, they are pushed toward the surface, eventually dying and filling with keratin—a process known as cornification.
- Melanocytes: Located in the stratum basale, these cells produce the pigment melanin. Keratinocytes take up melanin-filled vesicles (melanosomes) via endocytosis.
- Tactile cells: Sensory receptors for fine touch, located in the stratum basale and associated with dermal nerve cells.
- Dendritic cells: Immune system cells found in the stratum spinosum and granulosum that alert the body to invading pathogens.
The Dermis
Often called the "true skin," the dermis is a deep layer containing loose/areolar connective tissue over dense irregular connective tissue. It houses several vital structures:
- Papillae: Cone-like projections that fit into the epidermis, forming fingerprints and containing blood vessels and touch receptors.
- Fibers: Produced by fibroblasts, these include collagen for strength and elastic fibers for extensibility.
- Nerve Endings: These include lamellated and tactile corpuscles for pressure/touch, and free nerve endings for pain.
- Hair Follicles: Structures that contain the hair root and are formed by the stratum basale.
- Cutaneous Glands: Specialized glands that secrete various substances.
The Hypodermis
While not technically part of the skin, the hypodermis is a layer of adipose connective tissue that attaches the skin to the underlying body structures.
Cutaneous Glands, Hair, and Nails
Cutaneous Glands
The dermis contains two primary types of glands:
- Sebaceous Glands: These produce sebum (oil), which is emptied into hair follicles to moisturize the skin and hair. Hormones like estrogen and testosterone increase sebum production; excess sebum can lead to acne.
- Sweat Glands:
- Merocrine glands: Found all over the body, they secrete clear, watery perspiration to cool the body via evaporation.
- Apocrine glands: Located in the axillary and genital regions, they secrete a milky sweat containing proteins and fats. While odorless initially, bacteria decompose these secretions to create body odor.
- Ceruminous glands: Produce earwax (cerumen) in the external auditory canal.
- Mammary glands: Located in breast tissue to produce milk.
Hair
Human hair can be categorized into three types: lanugo (fine, unpigmented hair on a fetus), vellus (fine, unpigmented hair on children/most women), and terminal (thick, pigmented hair found on the scalp, eyelashes, and pubic regions).
A hair consists of three sections: the bulb (at the follicle end), the root (extending to the surface), and the shaft (the part extending outside the skin). It also has three layers: the medulla, cortex, and cuticle.
Nails
Nails protect the ends of fingers and toes and aid in grasping. The nail matrix is the growth center composed of active keratinocytes. The visible part is the nail plate, which sits on the nail bed. The lunula is the visible white crescent where the matrix is thick enough to hide underlying blood vessels.
Exam Tip: Remember that the epidermis is epithelial tissue (protection/barrier), while the dermis is connective tissue (strength/nutrition/sensation).
Injury, Aging, and Disease
Skin Injury and Healing
When the skin is damaged, it heals through one of two processes:
- Regeneration: The restoration of normal function where stratum basale cells replace lost tissue.
- Fibrosis: The replacement of functional tissue with non-functional scar tissue produced by fibroblasts.
Burns are classified by the depth of tissue involvement:
- First-degree: Affects only the epidermis (redness, pain).
- Second-degree: Affects the epidermis and dermis (blisters, swelling).
- Third-degree: Involves the epidermis, dermis, and hypodermis (charring, often no pain at the site).
Effects of Aging
As the body ages, the integumentary system undergoes several changes:
- Loss of sebaceous and sweat glands.
- Reduction in melanocytes (leading to age spots and gray hair).
- Thinning of the dermis due to decreased collagen and elastic fibers, resulting in wrinkles and increased bruising.
Common Disorders
The skin is susceptible to various conditions, including:
- Skin Cancer: Includes basal cell carcinoma (most common), squamous cell carcinoma, and the highly dangerous malignant melanoma.
- Infections: Can be bacterial (impetigo, cellulitis), viral (warts, herpes), fungal (tinea/ringworm), or parasitic (scabies).
- Autoimmune/Other: Conditions like psoriasis (overgrowth of skin cells), atopic dermatitis (eczema), and scleroderma (hardening of skin due to excess collagen).
