Integumentary System and Tissue Membranes: Structure, Function, and Clinical Relevance

Organs Within Body Cavities

Epithelial Tissue

Epithelial tissue lines body cavities and surfaces, providing protection and facilitating exchange with the environment. There are three main types of epithelial tissue membranes found in the body:

• Cutaneous: The skin, which covers the body surface.

• Serous: Lines body cavities closed to the outside environment (e.g., pericardial and peritoneal cavities). Composed of simple squamous epithelium called mesothelium.

• Mucous: Lines body cavities open to the outside environment (e.g., digestive tract, respiratory tract, and urogenital tracts).

Connective Tissue Membranes

Connective tissue membranes provide support and protection for organs and joints:

• Synovial linings: Found in synovial joints, tendon sheaths, and bursae. Secrete synovial fluid, which lubricates and nourishes the joint.

• Meningeal linings: Protect the brain and spinal cord. Consist of three layers: dura mater, pia mater, and arachnoid mater.

The Integumentary System

Overview

The integumentary system consists of the skin, hair, nails, and glands. It serves multiple functions essential for homeostasis and protection.

• Functions: Protection, sensation, temperature regulation, vitamin D production, and excretion.

• Fun fact: Hypodermis is referred to as subcutaneous tissue (not technically part of the skin); composed mainly of adipose tissue.

Epidermis: Structure and Layers

The epidermis is the outermost layer of the skin, composed of stratified squamous epithelium. It is organized into distinct layers from deep to superficial:

1. Stratum basale: Generates new cells via mitosis; cells are separated from the dermis by the basement membrane.

2. Stratum spinosum: Cells are alive but not dividing; contains a web-like system of prekeratin fibers, tonofilaments, and dendritic cells.

3. Stratum granulosum: Cells begin to die; several layers of keratinizing cells.

4. Stratum lucidum: Translucent layer, found only in thick skin (palms and soles of feet); cells are dead and filled with keratin.

5. Stratum corneum: Outermost layer, consisting of 20-30 layers of dead cells, fully keratinized; composed of stratified squamous epithelium.

Cells of the Epidermis

Several specialized cell types are found in the epidermis, each with distinct functions:

1. Keratinocytes: 90% of epidermal cells; primary cells of skin (found in stratum spinosum). Produce keratin, a protein that provides strength and waterproofing.

2. Melanocytes: Produce melanin, which protects against UV radiation. Melanin is transferred to keratinocytes in the stratum spinosum. Melanocytes increase melanin production in response to UV radiation (found in stratum basale).

3. Dendritic cells (Langerhans cells): Ingest foreign substances and are key activators of the immune system.

4. Merkel (tactile) cells: Connect to nerve endings in skin for the sensation of touch (found in the stratum basale).

Accessory Structures of the Skin

Glands

• Sudoriferous (sweat) glands: Two types:

  • Eccrine glands: Abundant on palms, soles, and forehead; function in thermoregulation.

  • Apocrine glands: Ducts connect to hair follicles; functional at puberty; found in axillary and anogenital regions.

• Oil glands (sebaceous glands): Secrete sebum; become active at puberty; secrete into hair follicles.

Hair and Nails

• Hair: Composed of dead, hard keratinized cells.

• Nails: Composed of dead, hard keratinized cells; protect the distal phalanges.

Clinical Considerations

Skin Cancer

Skin cancer arises from uncontrolled cell growth in the skin. The main types include:

• Basal cell carcinoma: Develops in basal cells in the stratum basale. Most common form; does not metastasize.

• Squamous cell carcinoma: Forms in the stratum spinosum. Can metastasize, but not common; treated with surgery or chemotherapy.

• Malignant melanoma: Develops from melanocytes in the stratum basale. Most malignant and serious; metastasizes rapidly.

Burns

Burns damage the skin and underlying tissues. The main threat is dehydration and electrolyte imbalance. After 24 hours, infection becomes the primary concern.

Vitamin D Synthesis

Vitamin D functions as a hormone and is essential for calcium incorporation into bone. The skin converts vitamin D into vitamin D3, which is then modified in the kidneys and liver as vitamin D.

• Equation:

$\text{7-dehydrocholesterol (in skin)} \xrightarrow{UV~light} \text{Vitamin D3 (cholecalciferol)}$

$\text{Vitamin D3} \xrightarrow{liver, kidney} \text{Calcitriol (active vitamin D)}$

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Mitotic activity, melanocytes, Merkel cells
Stratum spinosum	Above basale	Keratinocytes, dendritic cells
Stratum granulosum	Middle	Keratinization begins, cells start to die
Stratum lucidum	Thick skin only	Translucent, dead cells
Stratum corneum	Superficial	20-30 layers, dead, fully keratinized cells

Additional info:

• Melanin production increases with UV exposure, providing protection against DNA damage.

• Keratin provides mechanical strength and waterproofing to the skin.

• Vitamin D deficiency can lead to bone disorders such as rickets and osteomalacia.