Tissues: Structure, Function, and Classification in Anatomy & Physiology

Tissues in the Human Body

Introduction to Tissues

Tissues are groups of structurally similar cells that perform a common or related function. The specialization of individual body cells allows for division of labor, enabling organs and systems to work efficiently as a whole.

• Definition: A tissue is a group of cells with similar structure and function.

• Importance: Specialized cells allow for complex functions and cooperation at the organ level.

Primary Types of Tissue

There are four primary tissue types in the human body, each with distinct roles:

• Epithelial tissue: Covers surfaces and lines cavities.

• Connective tissue: Provides support and structure.

• Muscle tissue: Responsible for movement.

• Nervous tissue: Controls and coordinates body activities.

Epithelial Tissue

General Characteristics

Epithelial tissue forms sheets that cover body surfaces or line body cavities. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Covering and lining epithelium: Skin, lining of organs.

• Glandular epithelium: Forms glands (e.g., salivary glands).

Functions of Epithelial Tissue

• Protection: Shields underlying tissues from mechanical and chemical injury, pathogens, and dehydration (e.g., skin).

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

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

• Excretion: Elimination of waste products.

• Secretion: Production of substances (e.g., glands).

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

Polarity and Specialization

• Apical surface: Faces the body surface or cavity; may have specializations like microvilli or cilia.

• Basal surface: Attached to underlying connective tissue via the basement membrane.

Note: Cancerous epithelial cells often fail to respect the boundary imposed by the basement membrane.

Classifying Epithelial Cells

Epithelial cells are classified by cell shape and number of layers:

• Cell shapes: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

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

Types of Simple Epithelia

• Simple squamous: Thin, allows for filtration and diffusion (e.g., kidney, lungs).

• Simple cuboidal: Secretion and absorption (e.g., kidney tubules).

• Simple columnar: Absorption and secretion (e.g., digestive tract).

• Pseudostratified columnar: Appears layered but is not; often ciliated (e.g., trachea).

Types of Stratified Epithelia

• Stratified squamous: Multiple layers, protects against abrasion (e.g., skin, mouth).

• Transitional epithelium: Stretches and recoils (e.g., bladder).

Glandular Epithelium

• Endocrine glands: Ductless, secrete hormones directly into the bloodstream.

• Exocrine glands: Secrete products through ducts (e.g., sweat, salivary glands).

Types of Exocrine Glands

• Unicellular: Single cells (e.g., goblet cells).

• Multicellular: Composed of many cells, may have simple or compound duct structures.

Multicellular Exocrine Glands Table

Simple Duct Structure	Compound Duct Structure
Simple tubular (intestinal glands)	Compound tubular (duodenal glands)
Simple branched tubular (stomach glands)	Compound alveolar (mammary glands)
Simple alveolar (no important examples in humans)	Compound tubuloalveolar (salivary glands)

Modes of Secretion

• Merocrine: Secrete by exocytosis (e.g., pancreas, sweat glands).

• Apocrine: Apex of cell pinches off (e.g., mammary glands).

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

Connective Tissue

General Characteristics

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

• Main functions: Binding/support, protection, insulation, transportation (blood).

Types of Connective Tissue

• Mesenchyme: Embryonic connective tissue.

• Connective tissue proper: Loose and dense types.

• Cartilage

• Bone

• Blood

Structural Organization/Elements

• Ground substance: Matrix of interstitial fluid, cell adhesion proteins, and proteoglycans.

• Fibers: Collagen (strength), elastic (stretch), reticular (support).

• Cells: "Blast" cells (actively dividing, matrix-secreting), "Cyte" cells (maintenance).

Connective Tissue Cell Types Table

Tissue Type	"Blast"	"Cyte"
CT proper	fibroblast	fibrocyte
Cartilage	chondroblast	chondrocyte
Bone	osteoblast	osteocyte
Blood	hemocytoblast	RBCs, WBCs, platelets

Types of Connective Tissue Proper

Loose Connective Tissue

Type	Description	Location	Function	Example
Areolar CT	Gel-like matrix with all fiber types; cells include fibroblasts, macrophages, mast cells, and some WBCs	Widely distributed under epithelia of body	Cushioning of organs, immunity, inflammation; fluid reservoir	The dermis of the skin
Adipose Tissue	Areolar CT modified to store nutrients in adipocytes; matrix is sparse	Under skin, around kidneys & eyeballs, within abdomen, in breasts	Fuel reserve, insulation, supports & protects organs	Subcutaneous fat, fat in bone marrow
Reticular CT	Like areolar CT but only reticular fibers	Lymphoid organs (lymph nodes, bone marrow, spleen)	Fibers form soft internal skeleton that supports other cell types	The kidneys contain reticular tissue; framework for developing blood cells

Dense Connective Tissue

Type	Description	Function	Example
Dense regular CT	Bundles of collagen fibers running parallel to direction of pull; resists tension	Attachment with strength	Tendons (muscle to bone), ligaments (bone to bone), aponeuroses
Dense irregular CT	Same as regular but collagen bundles thicker and arranged irregularly	Withstand tension exerted in many directions; strength	The submucosa of the digestive tract; fibrous capsules of organs & joints
Elastic CT	Like dense regular CT but with more elastic fibers	Allows for stretch; found in some very elastic ligaments	Elastic ligaments

Summary

• Tissues are fundamental units of structure and function in the body.

• Epithelial tissue covers surfaces, lines cavities, and forms glands.

• Connective tissue supports, binds, and protects organs and other tissues.

• Classification is based on cell type, arrangement, and function.