Chapters 5, 6, and 7: Integumentary and Skeletal Systems – Study Notes

Integumentary System

Overview and Functions

The integumentary system is composed of the skin and its associated structures. It serves as the body's first line of defense and performs several vital functions.

• Protection: Acts as a barrier against mechanical injury, pathogens, and harmful substances.

• Sensation: Contains sensory receptors for touch, pain, temperature, and pressure.

• Thermoregulation: Regulates body temperature through sweat production and blood flow.

• Excretion: Eliminates wastes and toxins via sweat.

• Synthesis of Vitamin D: Skin synthesizes vitamin D when exposed to UV light, essential for calcium absorption.

Structure of the Skin

• Epidermis: The superficial layer of the skin, consisting of stratified squamous keratinized epithelium. It is avascular and relies on diffusion from the dermis for nutrients and oxygen.

• Dermis: The deeper layer, made of loose and dense irregular collagenous connective tissue. Contains blood vessels, nerves, and accessory structures.

• Hypodermis (Subcutaneous Tissue): The layer of adipose tissue beneath the dermis, providing insulation and energy storage.

Layers of the Epidermis (from deep to superficial)

• Stratum basale: Deepest layer; site of most mitosis.

• Stratum spinosum: Thickest layer; provides strength and flexibility.

• Stratum granulosum: Cells fill with keratin here.

• Stratum lucidum: Present only in thick skin (palms, soles).

• Stratum corneum: Outermost layer; dead, keratinized cells. Takes 40–50 days for cells to migrate here.

Mnemonic: "Come, Let's Get Sun Burned" (Corneum, Lucidum, Granulosum, Spinosum, Basale)

Thick skin (palms, soles) has all five layers and no hair follicles. Thin skin has four layers (lacks stratum lucidum).

Cells of the Epidermis

• Keratinocytes: Make up ~95% of the epidermis; produce keratin for protection and water resistance.

• Dendritic (Langerhans) cells: Immune cells that phagocytose pathogens.

• Merkel cells: Sensory cells in the stratum basale; detect light touch and texture.

• Melanocytes: Produce melanin, the pigment that protects against UV radiation.

Accessory Structures

• Hair (Pili): Filamentous structures for protection and sensation. Vellus hair is thin and non-pigmented; terminal hair is thick and pigmented. Arrector pili muscles cause hair to stand (goosebumps).

• Nails: Protective coverings on fingers and toes. Nail plate is the visible part; lunula is the crescent-shaped region; eponychium is the cuticle.

• Sweat Glands:

  • Eccrine glands: Most numerous; produce watery sweat for thermoregulation.

  • Apocrine glands: Found in axillae, anal, and areolar regions; secrete protein-rich sweat, active at puberty, can produce odor.

• Sebaceous Glands: Secrete sebum (oil) to lubricate skin and hair.

Pigments and Skin Color

• Melanin: Brown-black to orange-red pigment; protects from UV.

• Carotene: Orange pigment from diet (carrots, sweet potatoes).

• Hemoglobin: Red pigment in blood; gives pinkish hue to skin.

Clinical Terms: Cyanosis (blue skin, low oxygen), erythema (redness, increased blood flow), pallor (pale, decreased blood flow).

Thermoregulation and Feedback Loops

Body temperature is regulated by negative feedback mechanisms involving the hypothalamus.

• If temperature rises: Thermoreceptors signal the hypothalamus, which increases sweating and vasodilation. When normal, signals reduce sweating and vasodilation.

• If temperature drops: Thermoreceptors signal the hypothalamus, which triggers vasoconstriction and shivering to conserve heat.

Skin Disorders and Injuries

• Burns:

  • First-degree: Red, painful, no permanent damage.

  • Second-degree: Partial thickness, blisters, may require grafts.

  • Rule of nines: Body divided into 11 sections (9% each) plus 1% perineal area to estimate burn severity.

• Skin Cancer:

  • Basal cell carcinoma: Least likely to metastasize; from stratum basale.

  • Squamous cell carcinoma: From keratinocytes of stratum spinosum; more likely to metastasize than basal cell.

  • Malignant melanoma: From melanocytes; most likely to metastasize.

  • ABCDE rule: Asymmetry, Border, Color, Diameter, Evolving (used to identify melanoma).

Skeletal System

Functions of the Skeletal System

The skeletal system provides structure, protection, and support for the body, and is essential for movement and mineral storage.

• Protection: Shields vital organs (e.g., skull protects brain).

• Mineral storage: Reservoir for calcium and phosphate.

• Acid-base homeostasis: Buffers blood pH.

• Blood cell formation: Occurs in red bone marrow (hematopoiesis).

• Fat storage: Yellow bone marrow stores triglycerides.

• Movement and support: Provides attachment for muscles and supports body weight.

Classification of Bones

• Long bones: Longer than wide; have diaphysis and two epiphyses (e.g., femur).

• Short bones: Cube-shaped (e.g., carpals).

• Flat bones: Thin, often curved; two layers of compact bone with spongy bone in between (e.g., cranial bones).

• Irregular bones: Complex shapes (e.g., vertebrae).

• Sesamoid bones: Embedded in tendons (e.g., patella).

Structure of Long Bones

• Periosteum: Dense connective tissue covering the diaphysis.

• Diaphysis: Shaft of the bone.

• Epiphyses: Ends of the bone; contain spongy bone and red marrow.

• Medullary cavity: Central cavity containing yellow marrow.

• Epiphyseal line/plate: Remnant of growth plate; site of longitudinal growth.

Bone Tissue and Cells

• Compact bone: Dense, forms outer layer; contains osteons (cylindrical structures).

• Spongy bone: Honeycomb-like, resists forces from many directions; contains trabeculae.

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells, maintain bone matrix.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

• Matrix: Has inorganic (hydroxyapatite, for strength) and organic (collagen, for flexibility) components.

Bone Growth and Remodeling

• Intramembranous ossification: Forms flat bones (e.g., skull) from mesenchyme.

• Endochondral ossification: Forms most bones below the head; bone develops from hyaline cartilage model.

• Longitudinal growth: Occurs at epiphyseal plates; increases length.

• Appositional growth: Increases bone width.

• Bone remodeling: Continuous process of bone deposition and resorption.

Calcium Homeostasis

Blood calcium levels are regulated by negative feedback involving the parathyroid hormone (PTH) and calcitonin.

• If blood calcium is low: Parathyroid glands release PTH, stimulating osteoclasts to resorb bone and release calcium.

• If blood calcium is high: Calcitonin (from thyroid) promotes calcium deposition in bone.

Equation:

$\text{PTH} \uparrow \Rightarrow \text{Osteoclast activity} \uparrow \Rightarrow \text{Blood Ca}^{2+} \uparrow$

$\text{Calcitonin} \uparrow \Rightarrow \text{Osteoblast activity} \uparrow \Rightarrow \text{Blood Ca}^{2+} \downarrow$

Bone Repair

1. Hematoma forms.

2. Fibroblasts and chondroblasts form a soft callus.

3. Osteoblasts build a bone callus.

4. Bone is remodeled; primary bone replaced by secondary bone.

Bone repair generally takes 6–8 weeks.

Axial and Appendicular Skeleton

• Axial skeleton: Skull, vertebral column, thoracic cage.

• Appendicular skeleton: Limbs and girdles (pectoral and pelvic).

Major Bones and Landmarks

• Skull: 8 cranial bones (frontal, parietal, temporal, occipital, sphenoid, ethmoid), 14 facial bones (mandible, vomer, nasal, palatine, lacrimal, zygomatic, maxilla, inferior nasal conchae).

• Sutures: Coronal, sagittal, lambdoid, squamous.

• Vertebral column: 7 cervical, 12 thoracic, 5 lumbar vertebrae; sacrum and coccyx.

• Thoracic cage: Sternum (manubrium, body, xiphoid), ribs.

• Pectoral girdle: Clavicle (collarbone), scapula (shoulder blade).

• Upper limb: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic girdle: Ilium, ischium, pubis; acetabulum is the hip socket.

• Lower limb: Femur, patella, tibia, fibula, tarsals (including calcaneus and talus), metatarsals, phalanges.

Bone Markings and Features

• Foramen: Hole for passage of vessels/nerves.

• Fossa: Depression or indentation.

• Condyle: Rounded articular projection.

• Epicondyle: Small projection above a condyle.

• Tubercle: Small rounded projection.

• Trochanter: Large projection (femur only).

• Crest: Ridge-like projection.

• Spine: Sharp, slender projection.

Joints and Cartilage

• Intervertebral discs: Pads of fibrocartilage between vertebrae for shock absorption.

• Glenoid cavity: Socket in scapula for humerus.

• Acetabulum: Hip socket for femur.

Table: Types of Bones and Examples

Type	Description	Example
Long	Longer than wide, shaft and two ends	Femur, humerus
Short	Cube-shaped	Carpals, tarsals
Flat	Thin, often curved	Skull bones, sternum
Irregular	Complex shapes	Vertebrae
Sesamoid	Within tendons	Patella

Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Mitotic activity, melanocytes, Merkel cells
Stratum spinosum	Above basale	Thickest layer, keratinocytes, Langerhans cells
Stratum granulosum	Middle	Keratinization begins
Stratum lucidum	Thick skin only	Clear, dead cells
Stratum corneum	Superficial	Dead, keratinized cells

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard Anatomy & Physiology content.