Epithelial Tissues: Structure and Function

• Tissues are groups of similar cells carrying out related functions
• Tissues combine to form organs, which have specific, specialized functions
• Organs are organized into organ systems to perform broad functions

Epithelial Tissues

• Cover the outside of organs
• Line the lumens of organs
• Can be a single layer or multiple layers of cells
• Classified by the shapes of cells and number of layers
• Simple epithelia: single layer of cells
• Stratified epithelia: multiple layers of cells

Types of Epithelial Tissues

Squamous

• Flat, irregular round shape
• Simple squamous epithelium is found in lung alveoli and capillaries
• Stratified squamous epithelium is found in skin, mouth, and vagina

Cuboidal

• Cube-shaped with a central nucleus
• Found in glands and renal tubules, usually in a single layer

Columnar

• Tall, narrow shape with nucleus toward the base or along cell
• Simple columnar epithelium is found in the digestive tract
• Pseudostratified columnar epithelium is found in the respiratory tract

Transitional

• Round, simple but appear stratified
• Found in the urinary bladder

Squamous Epithelia

• Generally round, flat cells with a centrally located nucleus
• Cells fit together to form a covering or lining
• Simple squamous epithelium facilitates diffusion in tissues
• Stratified squamous epithelium provides protection in areas of abrasion and damage

Cuboidal Epithelia

• Cube-shaped cells with a single, central nucleus
• Commonly found in a single layer in glandular tissues
• Involved in preparing and secreting glandular material
• Found in walls of tubules and ducts of the kidney and liver

Columnar Epithelia

• Taller than they are wide, resembling a stack of columns
• Nuclei are lined up at the base of the cells in the digestive tract
• Absorb material from digestive tract lumen for entry into the body

Connective Tissues

• Includes loose/areolar, dense fibrous connective tissue, cartilage, bone, adipose, and blood

Loose/Areolar

• Contains fibroblasts, macrophages, lymphocytes, neutrophils
• Has few collagen, elastic, and reticular fibers
• Anchors around blood vessels

Dense Fibrous Connective Tissue

• Contains fibroblasts and macrophages
• Mostly collagen
• Irregular arrangement in skin; regular arrangement in tendons and ligaments

Cartilage

• Contains chondrocytes and chondroblasts
• Hyaline cartilage has few collagen
• Fibrocartilage has a large amount of collagen

Bone

• Contains osteoblasts, osteocytes, and osteoclasts
• Some collagen and elastic fibers
• Found in the vertebrate skeleton

Adipose

• Contains adipocytes
• Has few fibers
• Located in adipose (fat) tissue

Blood

• Contains red and white blood cells
• No fibers

Dense Regular Connective Tissue

• Found in tendons (connect muscle to bone) and ligaments (connect bone to bone)

Cartilage

• Matrix is made by cells called chondrocytes
• Chondrocytes are found in spaces called lacunae
• Hyaline cartilage has few collagen and elastic fibers
• Hyaline cartilage has a milky appearance
• Sharks have cartilaginous skeletons
• Hyaline cartilage is found at the ends of long bones

Elastic Cartilage

• Contains a large amount of elastic fibers providing flexibility
• Found in ears and portions of the larynx

Fibrocartilage

• Contains a large amount of collagen fibers
• Found in intervertebral discs
• Hyaline cartilage in movable joints can be damaged, replaced by fibrocartilage

Bone

Functions

• Supportive framework
• Supports and transmits weight
• Provides levers for locomotion
• Muscle and ligament attachment site
• Provides mechanical protection
• Stores calcium
• Forms blood in marrow

Types

• Compact (lamellar)
• Spongy (cancellous)
• Diploic

Compact Bone

• Outer portion of long bones
• Consists of cylindrical units called osteons

Spongy Bone

• Found on ends of long bones
• Contains bony bars and plates called trabeculae separated by spaces
• Red marrow here produces blood cells

Osteon Structure

• Includes osteocytes, lacunae, lamellae, central canal, canaliculi

Blood

• Connective tissue of cells suspended in plasma
• Red blood cells (erythrocytes) carry oxygen
• White blood cells (leukocytes) fight infection
• Platelets are fragments of giant cells found in bone marrow

Muscular Tissues

Types

• Smooth
• Cardiac
• Skeletal

Smooth Muscle

• Long, spindle-shaped cells
• Surround blood vessels, digestive tract, organs (e.g., stomach, bladder, uterus)
• Myofilaments are not parallel

Cardiac Muscle

• Long cardiac muscle cells (cardiocytes) with a single nucleus
• Myofibrils are similar to skeletal muscle
• Contractile cells do mechanical work
• Autorhythmic cells initiate and conduct action potentials

Skeletal Muscle

• Muscle fibers (cells) are elongated, unbranched, and voluntary
• Nuclei are at the periphery
• Striations (dark and light bands) are present
• Myofibrils are made of thick (myosin) and thin (actin) myofilaments
• Cross striations due to overlapping myosin and actin

Muscle Tissue Structure and Function

• Skeletal muscle moves or stabilizes skeleton, guards entrances/exits, generates heat, protects organs
• Cardiac muscle moves blood and maintains blood pressure
• Smooth muscle moves food, urine, and secretions; controls vessel diameter

Nervous Tissues

• Receive and transmit electrical impulses
• Neuron is the main cell, with a cell body containing a nucleus
• Dendrites receive input
• Axon transmits impulses
• Glial cells regulate the environment, insulate axons, and remove debris
• A nerve consists of neurons and glial cells