I. Introduction

- tissue: group of cells that are closely associated, similar in structure and function, and perform a common or related function.

- four primary tissues: epithelial tissue, connective tissue, muscle tissue, nervous tissue.

II. Epithelial tissue: cell layers are arranged in sheets.

A. General comments: epithelial tissues form the membranes and the glands of the body.

1. Membranes: covering and lining epithelium; sheets of cells that cover external surfaces or line internal surfaces; functions include protection, absorption, filtration, excretion, secretion.

2. Glands: composed of cells mostly specialized for secretion; develop as down growths of epithelial membranes into underlying connective tissue

B. Classification of epithelium: by number of cell layers present and by shape of the superficial cell layers.

1. Number of cell layers:

a. Simple epithelium: single layer of cells; important in diffusion, filtration, secretion, and absorption.

b. Stratified epithelium: two or more cell layers; important for protection in high abrasion areas.

c. Pseudostratified epithelium: simple epithelium in disguise; all cell touch basement membrane on which they grow

• important in secretion and protection.

2. Shape:

a. Squamous : flattened cells with flattened nuclei

b. Cuboidal: cube-like cells with spherical nuclei

c. Columnar: column-shaped, rectangular cells with oval nuclei

C. Types of epithelium:

1. Simple Epithelium:

a. Simple squamous epithelium: 

• single layer of flattened cells; located in air sacs of lungs
• found lining inside of blood vessels, air pocket in lungs
• function involves permitting easy passage of materials by simple diffusion where protection not critical

b. Simple cuboidal epithelium: 

• single layer of cube-like cell
• found in secretory portions of small glands, linings of ducts
• functions in secretion and absorption

c. Simple columnar epithelium: 

• single layer of tall cells
• found lining most of the digestive tract (stomach to anus), and regions of uterus
• functions in secretion of mucous and enzymes, and absorption

2. Stratified epithelium

• Stratified squamous epithelium: 
  • thick membrane composed of several layers of cells where basal cells are cuboidal or columnar and metabolically active, and surface cells are squamous and often dead
  • forms top layer of skin, the epidermis, lining of mouth, esophagus
  • functions in protection

3.  Pseudostratified epithelium

• important in secretion and protection
• found lining of parts of respiratory tract

III. Connective tissue - found everywhere in the body

• most abundant and widely distributed of primary tissues
• amount of connective tissue varies greatly from organ to organ 

A. Structural elements of connective tissue

- in any type of connective tissue there are two basic types of elements:

• cells -- produce extracellular matrix (ECM)
• ECM -- produced by cells
  • fibers
  • ground substance -- watery soup that surrounds fibers and cells

- the type of connective tissue depends on the type of cell that predominates, and the types, density, and arrangement of fibers 

C. Types of connective tissue -- soft connective tissues and specialized connective tissues

1. Soft connective tissues

a. Loose connective tissue

• cell is fibroblast
• few collagen fibers, a lot of ground substance
• found under skin, cushioning organs
• functions are cushioning, protection

b. Dense regular connective tissue

• cells are fibroblasts
• collagen fibers predominate over cells, little ground substance
• fibers arranged in parallel
• found in tendons ligaments
• functions are strength, support along one plane

c. Dense irregular connective tissue

• cells are fibroblasts
• collagen fibers predominate over cells, little ground substance
• fibers irregularly arranged in many planes
• present in lower layers of skin (dermis)
• function is withstanding tension from many directions

d.  Cartilage

• cells are chondrocytes
• collagen fibers present but invisible in preparation
• lot of ground substance -- high in water
• found in ends of long bones, nose, ear, parts of airways
• functions are support and flexibility where resiliency needed

2.  Specialized connective tissues

a.  Bone

• cells are osteocytes
• collagen fibers present
• ground substance is calcified
• present in bones of skeleton
• functions are support and movement

b.  Adipose tissue

• cells are adipocytes -- modified fibroblasts, specialize in storing triglycerides
• very few collagen fibers and ground substance -- most of space taken by cells
• found under skin, surrounding some organs
• functions are energy storage, padding, protection

c.  Blood

• cells are RBC, WBC
• matrix and fibers comprise plasma
• functions are transport of gases, nutrients and protection

IV. Muscle Tissue

- made up of cells specialized for movement, contractile cells

- types of muscle

A. Skeletal muscle

• found attached to bones
• long, cylindrical cells with alternating light and dark bands (striations)
• voluntary -- bone movement

B. Smooth muscle: 

• found in walls hollow tubes of body (digestive tract, uterus, blood vessels, parts of respiratory tract)
• spindly shaped cells
• involuntary -- movement of internal organs

C. Cardiac muscle: 

• found in walls of heart
• branched cells 
• junctions between cells visible as dark areas -- intercalated disks
• involuntary -- pump blood

V. Nervous tissue

• makes up the nervous system which controls body functions
• cell types present in neural tissue are predominantly supporting (glial) cells and neurons
• neurons are highly variable in shape -- have central area with many processes