Anatomy: Epithelial Tissue

• Anatomy is the study of the body's structure.
• Tissues are groups of similar cells performing specific functions.
• Histology is the study of tissues.
• The four primary tissue types include: epithelial, connective, muscle, and nervous.

Epithelial Tissue

• Covers body surfaces, lines body cavities, and forms glands.
• Functions include: protection, absorption, filtration, excretion, secretion, and sensory reception.
• Characterized by closely packed cells with tight junctions and desmosomes.
• Supported by a basement membrane.
• Avascular but innervated.
• High regenerative capacity.

Classification of Epithelia

• Classified by the number of cell layers: simple (single layer) or stratified (multiple layers).
• Classified by cell shape: squamous (flattened), cuboidal (cube-shaped), or columnar (column-shaped).

Simple Epithelia

• Simple squamous epithelium: single layer of flattened cells; allows passage of materials by diffusion and filtration in sites where protection is not important.
• Location: air sacs of lungs, lining of heart, blood vessels, and lymphatic vessels.
• Simple cuboidal epithelium: single layer of cube-like cells with large, spherical central nuclei; secretion and absorption.
• Location: kidney tubules, ducts and secretory portions of small glands, ovary surface.
• Simple columnar epithelium: single layer of tall cells with round to oval nuclei; absorption and secretion of mucus, enzymes, and other substances.
• Location: lines most of the digestive tract (stomach to rectum), gallbladder, and excretory ducts of some glands.
• Pseudostratified columnar epithelium: single layer of cells of differing heights, some not reaching the surface; secretion, particularly of mucus; propulsion of mucus by ciliary action.
• Location: trachea, most of the upper respiratory tract.

Stratified Epithelia

• Stratified squamous epithelium: thick membrane composed of several cell layers; protects underlying tissues in areas subjected to abrasion.
• Location: lining of the esophagus, mouth, and vagina; epidermis of the skin (keratinized).
• Stratified cuboidal epithelium: typically two layers of cuboidal cells; protection.
• Location: largest ducts of sweat glands, mammary glands, and salivary glands.
• Stratified columnar epithelium: several cell layers; protection and secretion.
• Location: rare in the body; small amounts in the male urethra and in large ducts of some glands.
• Transitional epithelium: resembles both stratified squamous and stratified cuboidal; stretches readily and permits distension of urinary organ by contained urine.
• Location: lines the ureters, urinary bladder, and part of the urethra.

Glandular Epithelia

• Glandular epithelium: Composed of cells specialized to produce and secrete substances.
• Endocrine glands: ductless glands that secrete hormones into the bloodstream.
• Exocrine glands: secrete products onto body surfaces or into body cavities, such as sweat and salivary glands.

Connective Tissue

• Supports, protects, and binds other tissues together.
• Includes bone, cartilage, adipose tissue, and blood.
• Characterized by cells scattered within an extracellular matrix.
• Common origin is mesenchyme.
• Varying degrees of vascularity.

Components of Connective Tissue

• Ground substance: unstructured material that fills the space between cells and contains the fibers.
• Fibers: collagen fibers (strongest and most abundant), elastic fibers (provide flexibility), and reticular fibers (form delicate networks).
• Cells: fibroblasts (produce fibers and ground substance), chondrocytes (cartilage cells), osteocytes (bone cells), and blood cells.

Types of Connective Tissue

• Connective tissue proper: loose connective tissues (areolar, adipose, reticular) and dense connective tissues (dense regular, dense irregular, elastic).
• Cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.
• Bone: compact bone and spongy bone.
• Blood: red blood cells, white blood cells, and platelets in a plasma matrix.

Connective Tissue Proper

• Loose connective tissue:
  • Areolar connective tissue: gel-like matrix with all three fiber types; cells include fibroblasts, macrophages, mast cells, and some white blood cells; wraps and cushions organs.
  • Location: widely distributed under epithelia of body, e.g., forms lamina propria of mucous membranes; packages organs; surrounds capillaries.
  • Adipose connective tissue: matrix as in areolar, but very sparse; closely packed adipocytes, or fat cells, have nucleus pushed to the side by large fat droplet; provides reserve food fuel; insulates against heat loss; supports and protects organs.
  • Location: under skin in subcutaneous tissue; around kidneys and eyeballs; within abdomen; in breasts.
  • Reticular connective tissue: network of reticular fibers in a typical loose ground substance; reticular cells lie on the network; fibers form a soft internal skeleton (stroma) that supports other cell types including white blood cells, mast cells, and macrophages.
  • Location: lymphoid organs (lymph nodes, bone marrow, and spleen).
• Dense connective tissue:
  • Dense regular connective tissue: primarily parallel collagen fibers; a few elastic fibers; major cell type is the fibroblast; attaches muscles to bones or to muscles; attaches bones to bones; withstands great tensile stress when pulling force is applied in one direction.
  • Location: tendons, most ligaments.
  • Dense irregular connective tissue: primarily irregularly arranged collagen fibers; some elastic fibers; major cell type is the fibroblast; withstands tension exerted in many directions; provides structural strength.
  • Location: fibrous capsules of organs and of joints; dermis of the skin; submucosa of digestive tract.
  • Elastic connective tissue: dense regular connective tissue containing a high proportion of elastic fibers; allows tissue to recoil after stretching; maintains pulsatile flow of blood through arteries; aids passive recoil of lungs following inspiration.
  • Location: walls of large arteries; certain ligaments associated with the vertebral column; within the walls of the bronchial tubes.

Cartilage

• Stands up to tension and compression.
• Avascular and lacks nerve fibers.
• Hyaline cartilage: amorphous but firm matrix; collagen fibers form an imperceptible network; chondroblasts produce the matrix and when mature (chondrocytes) lie in lacunae; supports and reinforces; has resilient cushioning properties; resists compressive stress.
  • Location: forms most of the embryonic skeleton; covers the ends of long bones in joint cavities; forms costal cartilages of the ribs; cartilages of the nose, trachea, and larynx.
• Elastic cartilage: similar to hyaline cartilage, but more elastic fibers in matrix; maintains the shape of a structure while allowing great flexibility.
  • Location: supports the external ear (auricle); epiglottis.
• Fibrocartilage: matrix similar to but less firm than that in hyaline cartilage; thick collagen fibers predominate; tensile strength allows it to absorb compressive shock.
  • Location: intervertebral discs; pubic symphysis; discs of knee joint.

Bone

• Supports and protects body structures.
• Stores calcium and other minerals.
• The matrix contains collagen fibers and calcium salts.
• Osteocytes reside in lacunae.
• Compact bone: dense outer layer.
• Spongy bone: internal network of trabeculae.

Blood

• Contains red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.
• The matrix is plasma.
• Transports oxygen, carbon dioxide, nutrients, and wastes.

Muscle Tissue

• Highly vascularized tissues responsible for movement.
• Composed of cells called muscle fibers.
• Three types: skeletal, cardiac, and smooth.

Skeletal Muscle

• Attached to bones.
• Voluntary movement.
• Cells are long, cylindrical, multinucleate, and striated.

Cardiac Muscle

• Forms the walls of the heart.
• Involuntary movement.
• Cells are branched, uninucleate, striated, and connected by intercalated discs.

Smooth Muscle

• Walls of hollow organs.
• Involuntary movement.
• Cells are spindle-shaped, uninucleate, and lack striations.

Nervous Tissue

• Main component of the nervous system (brain, spinal cord, and nerves).
• Regulates and controls body functions.
• Composed of neurons and supporting cells (neuroglia).

Neurons

• Generate and conduct nerve impulses.
• Composed of a cell body, dendrites, and an axon.

Neuroglia

• Support, insulate, and protect neurons.

This lesson explores epithelial tissue, one of the four primary tissue types in the human body. It covers its functions, including protection, absorption, and secretion. The lesson also classifies epithelia by cell layers and shapes, such as simple squamous and stratified types.