Module 2.1: Support & Movement – Skin and Skeletal Tissues

Support & Movement: Overview

This module covers the anatomy and physiology of the skin (integumentary system) and skeletal tissues, focusing on their structure, function, and roles in homeostasis and movement.

Skin (Integumentary System)

Definition and Components

• Integument: Refers to the skin and its associated connective tissues.

• Integumentary system: The system comprising the skin, hair, nails, and skin glands. It is the body's largest organ.

Structure of the Skin

• The skin is classified as the cutaneous membrane.

• It consists of two primary layers:

  • Epidermis: The superficial, thinner layer composed mainly of epithelial tissue.

  • Dermis: The deeper, thicker layer made of connective tissue, providing strength and flexibility.

• Hypodermis (subcutaneous layer): Lies beneath the dermis, connecting the skin to underlying structures. Not technically part of the skin but important for fat storage and insulation.

Types of Skin

• Thick skin: Found on palms and soles; has five layers (strata) in the epidermis.

• Thin skin: Covers most of the body; has four layers in the epidermis.

Cells of the Epidermis

• Keratinoctyes: Make up about 90% of epidermal cells; produce keratin for protection and waterproofing.

• Melanocytes: Produce melanin, contributing to skin color and UV protection.

• Epidermal dendritic (Langerhans) cells: Immune defense cells.

• Merkel (tactile) cells: Associated with sensory nerve endings for light touch sensation.

Epidermal Growth and Regeneration

• Cells form in the stratum basale and migrate to the surface, a process taking about 28 days.

• Growth is regulated by factors such as epidermal growth factor, insulin-like growth factor 1, and hormones.

• Increased friction or pressure can thicken the stratum corneum, forming calluses.

Dermoepidermal Junction

• A basement membrane zone connecting the epidermis and dermis.

• Contains fibrous elements and a polysaccharide gel, serving as a partial barrier and providing structural integrity.

Dermis

• Also called the corium or "true skin"; much thicker than the epidermis.

• Composed of connective tissue, rich in collagen and elastin fibers.

• Functions:

  • Provides strength and elasticity.

  • Reservoir for water and electrolytes.

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

• Contains structures such as hair follicles, sweat glands, sebaceous glands, and blood vessels.

Skin Color

• Determined by the quantity, type, and distribution of melanin produced by melanocytes.

• Types of melanin:

  • Eumelanin: Brown-black pigment.

  • Pheomelanin: Red-yellow pigment.

• Factors influencing melanin production:

  • Genetics (heredity)

  • Sunlight exposure

  • Hormones

  • Presence of the amino acid tyrosine

• Other pigments affecting skin color:

  • Beta-carotene: Yellow-orange pigment from diet.

  • Hemoglobin: Red pigment in blood; low oxygen causes cyanosis (bluish color).

  • Lipofuscin: Yellow-brown pigment from cell breakdown.

Functions of the Skin

• Physical barrier to microorganisms and chemical hazards.

• Protection against mechanical trauma and UV radiation.

• Prevents dehydration by reducing water loss.

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

• Excretion of waste products through sweat.

• Vitamin D synthesis when exposed to sunlight.

• Regulation of body temperature (thermoregulation).

Homeostasis of Body Temperature

• Maintained by balancing heat production (metabolism, muscle activity) and heat loss (radiation, conduction, convection, evaporation).

• Regulated by the hypothalamus, which adjusts blood flow to the skin and sweat gland activity.

Accessory Structures: Hair, Nails, and Glands

• Hair:

  • Produced by hair follicles; growth occurs in cycles of growth and rest.

  • Color and texture determined by genetics and melanin content.

  • Arrector pili muscles cause hair to stand (goosebumps).

• Nails:

  • Composed of keratinized cells; growth occurs from the nail matrix.

  • Protects fingertips and aids in manipulation.

• Glands:

  • Sudoriferous (sweat) glands: Eccrine (widespread, regulate temperature) and apocrine (armpits, groin, active at puberty).

  • Sebaceous (oil) glands: Secrete sebum for lubrication and antibacterial activity.

Clinical Considerations

• Burns: Severity depends on depth and total body surface area affected.

• Skin color changes: Jaundice (yellow), cyanosis (blue), bruising (ecchymosis).

Skeletal Tissues

Functions of Bone

• Support and protection of organs.

• Leverage for movement.

• Mineral storage (especially calcium and phosphate).

• Hematopoiesis (blood cell formation in red marrow).

Types of Bones

• Long bones: e.g., femur, humerus.

• Short bones: e.g., carpals, tarsals.

• Flat bones: e.g., skull, ribs.

• Irregular bones: e.g., vertebrae.

Bone Tissue Structure

• Compact bone: Dense, strong outer layer providing strength.

• Cancellous (spongy) bone: Lighter, porous inner layer containing red marrow.

• Extracellular matrix is hard and calcified, providing rigidity and support.

Cells of Bone Tissue

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells maintaining bone tissue.

• Osteoclasts: Bone-resorbing cells, important for remodeling and calcium release.

Bone Marrow

• Red marrow: Site of blood cell production; found in spongy bone.

• Yellow marrow: Fat storage; can convert to red marrow if needed.

Cartilage Types and Functions

• Hyaline cartilage: Most common; found in joints, respiratory tract, and epiphyseal plates.

• Elastic cartilage: Flexible; found in external ear and eustachian tube.

• Fibrocartilage: Strong and rigid; found in intervertebral discs and pubic symphysis.

Type	Flexibility	Location
Elastic cartilage	Most flexible	External ear, nose, respiratory tract
Hyaline cartilage	Moderately flexible	Joints, ribs, epiphyseal plates
Fibrocartilage	Least flexible	Intervertebral discs, pubic symphysis

Calcium Regulation and Bone Remodeling

• Bones store the majority of the body's calcium.

• Calcium homeostasis is regulated by:

  • Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing renal reabsorption, and activating vitamin D synthesis.

  • Osteoblasts: Build bone, lowering blood calcium.

  • Osteoclasts: Break down bone, raising blood calcium.

Key Equation:

$\text{Bone Remodeling:} \quad \text{Osteoblast Activity} \leftrightarrow \text{Osteoclast Activity}$

Life Cycle of Bone

• The skeleton is fully ossified by the mid-20s.

• Bone density and strength decrease with age, especially in advanced adulthood.

• Exercise can help maintain bone mass and strength.

Summary Table: Skin and Skeletal Tissues

System	Main Components	Primary Functions
Integumentary	Skin, hair, nails, glands	Protection, sensation, temperature regulation, vitamin D synthesis
Skeletal	Bones, cartilage, ligaments	Support, movement, mineral storage, blood cell formation

Additional info: Some details, such as the exact time for epidermal turnover and the full list of hormones affecting skin and bone, were inferred from standard Anatomy & Physiology knowledge to ensure completeness.