Chapter 4: Tissue—The Living Fabric (Anatomy & Physiology Study Notes)

Tissues: The Living Fabric

Introduction to Tissues

Tissues are groups of cells that are similar in structure and perform a common function. The human body is composed of four primary types of tissues, each with distinct roles in maintaining homeostasis and supporting life.

• Epithelial tissue: Covers body surfaces and lines cavities.

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

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Enables internal communication via electrical impulses.

Example: The skin contains epithelial tissue (epidermis), connective tissue (dermis), muscle tissue (arrector pili muscles), and nervous tissue (sensory receptors).

Primary Tissues Overview

Functions and Locations

• Nervous tissue: Found in the brain, spinal cord, and nerves; responsible for transmitting electrical signals.

• Muscle tissue: Found attached to bones (skeletal), in the heart (cardiac), and in the walls of hollow organs (smooth).

• Epithelial tissue: Forms boundaries between environments, lines digestive tract organs, covers skin surface, and forms glands.

• Connective tissue: Found in tendons, fat, bone, and other soft padding tissues; provides support and protection.

Epithelial Tissue (Epithelium)

Main Types by Location

Epithelial tissue is classified based on its location and function:

• Covering and lining epithelia: Found on external and internal surfaces (e.g., skin, lining of organs).

• Glandular epithelia: Forms the secretory tissue in glands.

Characteristics of Epithelial Tissue

• Polarity: Cells have an apical (free) surface and a basal (attached) surface.

• Specialized surfaces:

  • Apical surfaces may have microvilli (brush border of intestinal lining) or cilia (lining of trachea).

  • Basal surface rests on a basement membrane composed of glycoproteins and collagen fibers.

• Cellularity: Composed of closely packed cells forming continuous sheets held together by tight junctions and desmosomes.

• Supported by connective tissue: The reticular lamina underlies the basement membrane.

• Avascular but innervated: Contains no blood vessels but is supplied by nerve fibers.

• High regenerative capacity: Rapidly replaces lost cells by cell division.

Classification of Epithelia

Epithelia are classified by two criteria:

• Number of cell layers:

  • Simple epithelium: Single cell layer thick.

  • Stratified epithelium: Two or more cell layers.

• Cell shape:

  • Squamous: Flattened cells.

  • Cuboidal: Cube-shaped cells.

  • Columnar: Column-shaped cells.

Example: The lining of the alveoli in the lungs is simple squamous epithelium, allowing efficient gas exchange.

Visual Classification of Epithelia

Epithelial tissues can be visually distinguished based on the number of layers and the shape of the cells:

• Simple: One layer of cells.

• Stratified: Multiple layers of cells.

• Squamous: Flat, scale-like cells.

• Cuboidal: Cube-like cells.

• Columnar: Tall, column-shaped cells.

Types of Epithelial Tissue

Simple Squamous Epithelium

Simple squamous epithelium consists of a single layer of flattened cells. It is specialized for rapid diffusion and filtration, and provides a slick, friction-reducing lining in lymphatic and cardiovascular systems.

• Functions:

  • Diffusion and filtration

  • Friction reduction

• Locations: Kidney glomeruli, lining of heart, blood vessels, lymphatic vessels, and serosae.

Simple Cuboidal Epithelium

Simple cuboidal epithelium is a single layer of cube-shaped cells, primarily involved in secretion and absorption.

• Functions: Secretion and absorption

• Locations: Kidney tubules, ducts and secretory portions of small glands, ovary surface.

Simple Columnar Epithelium

Simple columnar epithelium consists of a single layer of tall, column-shaped cells. Goblet cells, which secrete mucus, are often found in this layer. Many columnar cells contain microvilli or cilia.

• Functions: Secretion and absorption

• Special features:

  • Microvilli increase surface area for absorption (e.g., digestive tract lining).

  • Cilia help move substances through internal passageways (e.g., small bronchi, uterine tubes).

• Locations: Digestive tract, small bronchi, uterine tubes, regions of the uterus.

Example: Ciliated simple columnar epithelium in the uterine tubes helps move the ovum toward the uterus.