Skin Structure and Function

Epidermis Layers

• Granular layer (stratum granulosum): Keratinocytes secrete waterproofing molecules for a protective barrier and moisture retention.
• Prickle cell layer (stratum spinosum): Keratinocytes are tightly joined by desmosomes, providing structural support.
• Basal layer (stratum basale/germinativum): Responsible for keratinocyte mitosis, the process of cell division.
• Horny layer (stratum corneum): Keratinocytes lose organelles and produce keratin.

Sweat Glands

• Apocrine gland: Large sweat gland leading to body odor, localized in axilla and groin.
• Eccrine gland: Secretes odourless, clear substance primarily composed of water and sodium chloride, vital for thermoregulation.

Keratinocytes

• Produce keratin, a fibrous protein for skin strength, found in hair and nails.
• Undergo mitosis in the basal layer.
• Move up through the prickle cell layer, losing the ability to divide and their nuclei, forming the horny layer.
• The horny layer provides a hydrophobic skin barrier due to keratin.

Melanocytes

• Located in the basal layer of the epidermis.
• Synthesize melanin, a pigment that darkens skin and offers UV protection.
• Darker-skinned individuals have melanocytes that produce more melanin, not more melanocytes.

Langerhans Cells

• Antigen presenting cells.
• Circulate between the epidermis and local lymph nodes.
• Mediate immune responses.

Merkel Cells

• Connected to keratinocytes and sensory nerves.
• Receive the sensation of touch.

Cell Junctions in Epidermis

• Epidermis structure: Avascular and multilayered with a basement membrane at the bottom.
• Cell junctions in epidermis: Tight junctions, desmosomes, gap junctions, and hemidesmosomes.

Skin Appendages

• Hair: Used for thermal regulation and display.
• Arrector pili: Muscle that erects hair, trapping air for heat retention.
• Sebaceous glands: Associated with hair follicles, secrete sebum for waterproofing.
• Sweat glands: Eccrine and apocrine.
• Nails: Provide physical protection, composed of hard layers of keratin.

Burns

• First and second degree burns are also known as partial thickness because not all of the skin layers are destroyed.
• Third degree burns are full thickness, affecting all layers and some underlying tissues.

Skin Function

• The skin is the largest organ of the body.
• Acts as a physical barrier, protecting against infection, physical damage, and chemical damage.
• Provides touch, pressure, pain, and temperature regulation.

Skin Layers

• Epidermis: Derived from ectoderm, keratinized stratified squamous epithelium, contains keratinocytes and Langerhans cells.
• Dermis: Dense connective tissue, derived from mesoderm, contains fibroblasts, collagen, blood, mast cells, receptors, and nerves.
• Hypodermis (subcutaneous tissue): Main blood supply for the rest of the skin, provides insulation.

Airway Structure

• The smooth muscle layer controls airway diameter and resistance to airflow, and becomes more prominent in smaller airways.
• The submucosa layer is located under the smooth muscle layer and contains seromucous glands.
• Cartilage prevents collapsing and becomes less prominent as tubes get smaller.

Variations in Airway Structure

• Trachea: C-shaped cartilage, mucous glands
• Bronchi: discontinuous cartilage, more smooth muscle, mucous glands
• Bronchioles: no cartilage, no submucosal glands, no goblet cells
• Alveolar Duct: no cilia, no glands
• Alveoli: contain Type 1 & 2 Pneumocytes

Vein Structure

• Tunica Intima: thin layer with endothelial lining
• Tunica Media: smooth muscle + elastic fibers
• Tunica Adventitia/Externa: thickest layer with longitudinally arranged thick collagen fibers

Artery Structure

• Elastic Arteries: innermost to outermost structure
• Dermis: where fibers aren't organized

Cartilage Structure

• Chondrocytes embedded in an amphorous hydrated ground substance
• Chondrocytes: cells that secrete cartilage matrix and embed with the matrix
• Provides structural support, resistance to compression, poorly vascularized

Bone Structure

• Osteocytes: bone cells formed from an osteoblast
• Osteocytes embedded in a mineralized matrix, not hydrated

Interstitial Compartment

• A type of loose connective tissue that supports the bronchial tree, arterio-venous tree & continuous tubes in the digestive system

Airway Layers

• Respiratory Epithelium: ciliated cells, includes a basement membrane
• Lamina Propria: contains connective tissue, blood + lymph, fibroelastic tissue
• Tunica Intima: thin layer with an endothelial lining, contains little collagenous connective tissue
• Tunica Media: smooth muscle + elastic fibers
• Tunica Adventitia/Externa: almost as thick as the media, merges with surrounding tissue, composed of collagen

Capillary Structure

• Composed of a single layer of endothelium/endothelial cells
• Very small diameter, forcing red blood cells to fold to pass through
• Clefts or slits between endothelial cells allow for effective material exchange

Gut Tube Structure

• Mucosa: divided into three layers: epithelial lining, supporting connective tissue (lamina propria), and a thin layer of smooth muscle (muscularis mucosae)
• Submucosa: a layer of connective tissue that supports the mucosa, contains larger blood vessels, lymphatics, and nerves
• Muscularis Externa/Propria: consists of smooth muscle
• Adventitia/Serosa: conducts major blood vessels and nerves, part of the visceral peritoneum when GI tube is below the diaphragm

Oesophagus

• Conducts swallowed substances from pharynx to stomach (about 25 cm)