Anatomy of skin

 
The skin (integumentary system) is the body's largest organ, covering a surface area of 1.5 to 2 m^2 in adults. More than just a simple covering, the skin is a complex, multi-layered organ that includes glands, hair, and nails. It is vital for protection, sensation, and temperature regulation.

The skin is primarily composed of two main layers: the outer Epidermis and the underlying Dermis. Below the dermis lies the Subcutaneous Layer (Hypodermis), which is made of adipose (fat) and areolar tissue that separates the skin from underlying structures.

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A. The Epidermis: The Protective Outer Shield

The epidermis is the thinner, upper layer of the skin. It is composed of stratified keratinized squamous epithelium and varies in thickness across the body (thickest on the palms and soles).

Epithelial cells in the epidermis, known as keratinocytes, lack capillaries and receive oxygen and nutrients from the dermis below.

Layers and Life Cycle

The epidermis consists of multiple layers (strata), continuously being replaced. The process takes approximately one month for a cell to move from the deep germinative layer to the surface.

• Stratum Germinativum (Stratum Basale):
• This is the deepest layer, resting on the dermis.
• Mitosis (cell division) occurs here continuously, producing new keratinocytes that are pushed toward the surface.
• Functions: Produces new surface cells, produces antimicrobial defensins, and synthesizes Vitamin D when cholesterol is exposed to UV rays.

• Stratum Corneum (Keratin):
• This is the outermost layer, consisting of ultra-thin, flat, dead cells that are constantly being rubbed away (shed).
• These dead cells are non-nucleated and contain the tough, fibrous, waterproof protein keratin.
• Functions: Provides a waterproof barrier (preventing water from entering or leaving) and protects against pathogens and many chemicals (if unbroken).

Specialized Cells in the Epidermis

The lower layers of the epidermis also contain specialized cells crucial for defense and pigmentation:

• Langerhans Cells (Dendritic Cells):
• Originating from red bone marrow, these cells are mobile and act as part of the immune system.
• Functions: They destroy invading bacteria via phagocytosis and present ingested pathogens to lymphocytes in lymph nodes, stimulating an immune response (antibody production).
• Melanocytes:
• Produce the dark pigment melanin.
• Functions: Produce melanin upon $\text{UV}$ ray exposure; melanin then helps to prevent the skin from absorbing excessive $\text{UV}$ rays. Skin color is determined by the level of melanocyte activity, not the number of cells.
• Merkel Cells:
• Functions: These are receptors that respond to touch.

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B. The Dermis: Strength, Elasticity, and Sensation

The dermis is the thicker layer beneath the epidermis, composed of irregularly arranged fibrous connective tissue. This structure provides the skin with both strength and elasticity.

Key Components of the Dermis:

• Fibers and Matrix:
• Fibroblasts produce collagen (provides tensile strength and absorbs water) and elastin (allows for stretch and recoil).
• The loss of collagen's tensile strength with age contributes to wrinkles. Stretch marks (striae) are caused by the tearing of elastic fibers during rapid weight gain or pregnancy.
• Cells: Most common cells include fibroblasts, macrophages, and mast cells.

• Papillary Layer:

• The uneven connection layer between the dermis and the epidermis.
• Function: Contains an abundance of capillaries that nourish the deeper stratum germinativum layer of the epidermis.

Structures Embedded in the Dermis

The dermis houses essential structures responsible for various sensory, protective, and regulatory functions:

• Hair (Follicles):
• Hair follicles are made of epidermal tissue. The arrector pili muscle (a small smooth muscle) attaches to the follicle and pulls the hair upright when stimulated by cold or fear (creating "goosebumps").
• Functions: Insulation against cold (especially head hair); hair in the nasal cavities and eyes helps keep dust out.

• Nails (Follicles):
• Produced by nail follicles at the ends of fingers and toes. New, strong keratin cells are formed by mitosis at the base of the nail and die as they move forward.
• Functions: Prevents mechanical injury to the ends of fingers and toes.

• Receptors:
• The dermis contains most of the sensory receptors for cutaneous senses (touch, pressure, heat, cold, and pain).
• Specific small structures (often encapsulated nerve endings) detect specific changes.
• Functions: Perception of changes in the environment as cutaneous sensations.

• Sebaceous Glands:
• Release the lipid substance sebum (oil), usually opening into a hair follicle.
• Functions: Produces sebum, which prevents hair and skin from drying out and inhibits bacterial growth.

• Ceruminous Glands:
• Located in the ear canal.
• Functions: Produces cerumen (ear wax), which lubricates and protects the eardrum.

• Eccrine Sweat Glands:
• Coiled secretory tubes found across the body (prominent on the forehead, palms, and soles).
• Functions: Produces sweat, which evaporates from the skin surface to cool the body during excessive heat.

• Arterioles:
• Small arteries in the dermis with smooth muscle walls that allow them to be constricted (closed) or dilated (opened).

• Functions:

• Warmth: Dilate to bring more blood (heat) to the surface for loss, cooling the body.
• Cold: Constrict to conserve body heat by diverting blood away from the surface.
• Stress: Constrict to divert blood to other major organs.