Tissues: Structure, Function, and Classification in Human Anatomy

Topic #2, Part 1: Tissues

Introduction to Tissues

Tissues are groups of structurally similar cells that perform a common or related function. The organization of cells into tissues is fundamental to the structure and function of organs, contributing to the overall homeostasis of the human body. Understanding tissue types and their properties is essential for studying anatomy and physiology.

• Histology: The study of tissues and their cellular organization.

• Why different tissues? Individual body cells are specialized for division of labor. At the organ level, different tissues cooperate for the organ to function as a whole.

The Four Primary Tissue Types

• Epithelial tissue: Covers body surfaces and lines cavities. Example: skin, lining of GI tract

• Connective tissue: Supports, binds, and protects other tissues. Example: bone, blood, cartilage

• Muscle tissue: Responsible for movement. Example: skeletal muscle, cardiac muscle

• Nervous tissue: Controls and regulates body functions. Example: brain, spinal cord, nerves

Epithelial Tissue

Definition and Functions

Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity, creating boundaries between different environments. It is classified as either covering and lining epithelium or glandular epithelium.

• Protection: Mechanical, chemical, and infectious protection (e.g., skin)

• Absorption: Uptake of substances (e.g., GI tract)

• Filtration: Selective movement of substances (e.g., kidney)

• Excretion: Removal of waste (e.g., kidney)

• Secretion: Production and release of substances (e.g., glands, GI tract)

• Sensory reception: Detection of stimuli (e.g., taste buds, olfactory membranes)

Special Structural Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have an apical (top) surface and a basal (bottom) surface, each with distinct structures and functions.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes, which help maintain tissue integrity and control permeability.

• Supported by connective tissue: The basement membrane (composed of basal lamina and reticular lamina) anchors the epithelium to underlying connective tissue and acts as a selective barrier.

• Avascular but innervated: Epithelial tissues lack blood vessels but are supplied by nerve fibers. They receive nutrients by diffusion from underlying connective tissues.

• High regenerative capacity: Epithelial cells rapidly divide to replace lost or damaged cells, which is essential due to their protective and absorptive roles.

Additional info: Cancerous epithelial cells often lose the ability to respect boundaries imposed by the basement membrane, contributing to metastasis.

Classification of Epithelial Tissue

• Criteria: Epithelia are classified by cell shape and number of layers.

• Cell shapes: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall and column-like)

• Number of layers: Simple (one layer), Stratified (multiple layers)

Types of Simple Epithelia

• Simple squamous epithelium: Thin and permeable; functions in filtration and diffusion. Locations: endothelium (lining of blood vessels), kidney glomeruli, lung alveoli

• Simple cuboidal epithelium: Secretion and absorption. Locations: kidney tubules, small glands

• Simple columnar epithelium: Secretion and absorption. Locations: digestive tract, gallbladder

• Pseudostratified (ciliated) columnar epithelium: Appears stratified but is a single layer; often ciliated and involved in secretion and movement of mucus. Location: respiratory tract

Types of Stratified Epithelia

• Stratified squamous epithelium: Multiple layers; protects underlying tissues. Basal cells are cuboidal and undergo mitosis; apical cells are squamous and may be keratinized (skin) or non-keratinized (mouth, esophagus).

• Transitional epithelium: Stratified; basal cells are cuboidal, apical cells flatten as the organ fills. Location: urinary bladder

Glandular Epithelia

• Gland: One or more cells that make and secrete a particular product.

• Endocrine glands: Ductless; secrete hormones directly into the bloodstream.iiExocrine glands: Secrete products into ducts. Examples: mucus, sweat, oil, salivary glands, liver, pancreas

• Unicellular exocrine glands: Single cells (e.g., goblet cells) that secrete mucus.

• Multicellular exocrine glands: Composed of a duct and secretory unit, supported by connective tissue.

Modes of Secretion

• Merocrine: Secrete products by exocytosis (e.g., salivary glands, most sweat glands)

• Holocrine: Entire cell ruptures to release product (e.g., sebaceous glands)

• Apocrine: Apex of cell pinches off with product (e.g., mammary glands; debated if truly apocrine in humans)

Matching Epithelial Types to Locations

Location	Epithelial Type
Lines stomach, most of intestines	Simple columnar
Lines inside of the mouth	Stratified squamous
Lines much of the respiratory tract	Pseudostratified ciliated columnar
Endothelium (lining of blood vessels)	Simple squamous
Composes lung alveoli	Simple squamous

Connective Tissue (CT)

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type in the body. It supports, binds, and protects other tissues and organs.

• Main types: Mesenchyme, CT proper, cartilage, bone, blood

• Main functions:

  • Binding and supporting (e.g., tendons, ligaments)

  • Protecting (e.g., bone, cartilage)

  • Insulating (e.g., adipose tissue)

  • Storing reserve fuel (e.g., adipose tissue)

  • Transporting substances (e.g., blood)

Clinical Note: Rheumatoid arthritis is an autoimmune disease where the immune system attacks the synovial membranes of joints, leading to inflammation and joint damage.

Structural Organization of Connective Tissue

• Ground substance (matrix): Composed of interstitial fluid, cell adhesion proteins (e.g., fibronectin, laminin), and proteoglycans (which act as molecular sieves and help retain water).

• Fibers:

  • Collagen fibers: Provide high tensile strength.

  • Elastic fibers: Contain elastin; allow stretch and recoil.

  • Reticular fibers: Thin collagen fibers forming supportive networks.

• Cells: Immature cells ("-blast" cells) are active in growth and repair; mature cells ("-cyte" cells) maintain tissue.

Tissue Type	"Blast" Cell	"Cyte" Cell
Connective tissue proper	Fibroblast	Fibrocyte
Cartilage	Chondroblast	Chondrocyte
Bone	Osteoblast	Osteocyte
Blood	Hematopoietic stem cell	Blood cells (RBCs, WBCs)

Types of Connective Tissue Proper

Loose Connective Tissue

• Areolar CT: Gel-like matrix with all three fiber types; contains fibroblasts, macrophages, mast cells, and some WBCs. Functions: Cushions organs, holds tissue fluid, involved in inflammation. Location: Widely distributed under epithelia.

• Adipose CT: Modified areolar CT for nutrient storage; consists of fat-filled adipocytes. Functions: Energy storage, insulation, supports and protects organs. Location: Under skin, around kidneys and eyeballs, within abdomen and breasts.

• Reticular CT: Like areolar CT but only reticular fibers. Functions: Forms soft internal skeleton supporting other cell types. Location: Lymphoid organs (lymph nodes, bone marrow, spleen).

Dense Connective Tissue

• Dense regular CT: Parallel collagen fibers; provides great tensile strength. Location: Tendons, ligaments, aponeuroses.

• Dense irregular CT: Thicker, irregularly arranged collagen fibers; withstands tension in many directions. Location: Dermis, submucosa of GI tract, fibrous capsules of organs.

• Elastic CT: High content of elastic fibers; allows recoil. Location: Some ligaments, large arteries.

Key Questions and Applications

• Which fibrous element gives CT high tensile strength? Collagen fibers

• What type of CT can increase in volume without making more cells? Areolar CT

• Which kind of CT acts as a sponge, soaking up fluid when edema occurs? Areolar CT

Additional info: Other types of connective tissue, such as cartilage, bone, and blood, will be covered in subsequent lessons.