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

Chapter 4: Tissues

Levels of Biological Organization

Tissues represent a key level in the biological hierarchy, situated between cells and organs. Understanding tissues is essential for grasping how the body’s structures and functions are organized.

• Definition: A tissue is a group of similar cells and their extracellular matrix that perform a specific function.

• Hierarchy: Cells → Tissues → Organs → Organ Systems → Organism

• Example: Muscle tissue is made of muscle cells (fibers) and forms organs such as the heart or skeletal muscles.

Main Tissue Types

The human body contains four primary tissue types, each with distinct structures and functions.

• Epithelial tissue: Covers body surfaces, lines cavities, and forms glands.

• Connective tissue: Supports, binds, and protects other tissues and organs.

• Muscle tissue: Responsible for movement and force generation.

• Nervous tissue: Initiates and transmits electrical impulses for communication.

Epithelial Tissue

Definition and Locations

Epithelial tissue consists of closely packed cells that form continuous sheets, covering surfaces and lining cavities throughout the body.

• Locations: Skin (epidermis), lining of digestive tract, respiratory tract, blood vessels, and glands.

Characteristics and Functions

• Cellularity: Composed almost entirely of tightly packed cells with minimal extracellular matrix.

• Polarity: Has an apical surface (exposed to exterior or cavity) and a basal surface (attached to underlying tissue).

• Attachment: The basement membrane anchors epithelium to connective tissue below.

• Avascularity: Lacks blood vessels; nutrients diffuse from underlying tissues.

• Regeneration: High capacity for cell division and repair.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

Specializations of the Apical Surface

• Microvilli: Increase surface area for absorption (e.g., small intestine).

• Cilia: Move substances across the surface (e.g., respiratory tract).

Basement Membrane

• Definition: A thin, fibrous layer that anchors epithelial cells to underlying connective tissue.

• Function: Provides support and regulates exchange of materials.

Intercellular Junctions

• Tight junctions: Seal adjacent cells to prevent leakage.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap junctions: Allow communication and passage of ions between cells.

Criteria for Identifying Epithelial Tissue

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

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

Comparison Table: Types of Epithelia

Type	Shape	Layers	Main Function	Location
Simple squamous	Flat	1	Diffusion, filtration	Alveoli, blood vessels
Simple cuboidal	Cube	1	Secretion, absorption	Kidney tubules
Simple columnar	Tall	1	Absorption, secretion	Digestive tract
Stratified squamous	Flat	Multiple	Protection	Skin, mouth
Stratified cuboidal	Cube	Multiple	Protection	Gland ducts
Stratified columnar	Tall	Multiple	Protection, secretion	Male urethra

Glands: Exocrine vs. Endocrine

• Glands: Structures that produce and secrete substances.

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

• Endocrine glands: Release hormones directly into the bloodstream (e.g., thyroid, pituitary).

Connective Tissue

General Characteristics and Functions

• Most abundant tissue type in the body.

• Functions: Support, protection, insulation, storage, and transport.

• Components: Cells, fibers (collagen, elastic, reticular), and ground substance (matrix).

Table: General Components of Connective Tissue

Component	Description	Function
Cells	Fibroblasts, adipocytes, immune cells	Produce fibers, store energy, defense
Fibers	Collagen, elastic, reticular	Strength, elasticity, support
Ground substance	Gel-like material	Fills space, supports cells

Types of Connective Tissue

• Areolar tissue: Loose, supports and binds other tissues; found under epithelia.

• Adipose tissue: Stores fat, insulates, cushions; found under skin, around organs.

• Reticular tissue: Forms framework of lymphoid organs.

• Dense regular connective tissue: Parallel collagen fibers; found in tendons, ligaments.

• Dense irregular connective tissue: Irregular collagen fibers; found in dermis of skin.

• Cartilage: Hyaline (smooth, ends of bones), fibrocartilage (strong, intervertebral discs).

• Bone: Rigid, supports and protects; stores minerals.

• Blood: Fluid tissue; transports gases, nutrients, wastes.

Types of Membranes

Body membranes are sheets of tissue that cover surfaces, line cavities, and divide spaces or organs.

• Mucous membranes: Line body cavities open to the exterior (e.g., digestive, respiratory tracts); secrete mucus.

• Serous membranes: Line closed cavities (e.g., pleura, pericardium); secrete serous fluid.

• Cutaneous membrane: The skin; protects body surface.

• Synovial membranes: Line joint cavities; secrete synovial fluid.

Muscle Tissue

General Function

• Function: Produces movement by contracting; maintains posture and generates heat.

Types of Muscle Tissue

Type	Striated?	Voluntary?	Nuclei per cell	Main Location
Skeletal	Yes	Yes	Multinucleate	Attached to bones
Cardiac	Yes	No	Uninucleate	Heart
Smooth	No	No	Uninucleate	Walls of hollow organs

Nervous Tissue

General Function

• Function: Receives, processes, and transmits electrical signals throughout the body.

Cell Types

• Neurons: Conduct electrical impulses; main functional unit.

• Neuroglial cells: Support, protect, and nourish neurons.

Neuron Structure and Function

• Cell body: Contains nucleus and organelles.

• Dendrites: Receive signals.

• Axon: Transmits impulses away from cell body.

Inflammation and Tissue Repair

• Inflammation: The body’s response to injury; characterized by redness, heat, swelling, pain, and loss of function.

• Purpose: Isolates damaged area, removes pathogens, and initiates repair.

• Repair: Involves regeneration (replacement with same tissue) or fibrosis (replacement with scar tissue).

Aging and Tissue Structure

• Aging: Tissues lose elasticity, repair more slowly, and may accumulate damage over time.

• Examples: Thinner skin, decreased bone density, slower wound healing.

Additional info: Some details, such as the specific types of neuroglial cells or the molecular composition of basement membranes, were inferred for completeness and academic context.