The Tissue Level of Organization

Types of Tissues & Their Origins

• Tissue: A group of similar cells that usually have a similar embryological origin and are specialized for a particular function.

• The nature of the extracellular material that surrounds the tissue cells and connections between the cells that compose the tissue influence the structure and properties of a specific tissue.

Tissue Terms

• Histology: The science that deals with the study of tissues.

• Pathologists: Physicians who specialize in laboratory studies of cells and tissues, aid other physicians in making diagnoses, and perform autopsies. Pathologists also analyze biopsies (samples of living tissue removed from patients for microscopic examination).

Types of Tissues

• Depending on their function and structure, the various tissues of the body are classified into four principal types:

1. Epithelial: Covers the body surfaces, lines hollow organs, body cavities and ducts and forms glands.

2. Connective: Protects and supports the body and its organs, binds organs together, stores energy reserves as fat and provides immunity.

Types of Tissues

• Muscle Tissue: Responsible for movement and generation of force.

• Nervous Tissue: Initiates and transmits action potentials (nerve impulses) that help coordinate body activities.

Extracellular Materials

• Extracellular Fluid or ECF: Is EXTERNAL to ALL body cells, providing a medium for dissolving and mixing solutes, transporting substances and carrying out chemical reactions.

Types of Extracellular Fluid:

• Interstitial (intercellular) Fluid: A subdivision of of ECF, it fills the microscopic spaces (interstitial spaces) between cells in tissues.

• Plasma: A second subdivision of ECF, this is the liquid portion of the blood and is found in all blood vessels.

Extracellular Fluids Continued

• The extracellular substance of connective tissue consists of a matrix which contains protein fibers embedded in a fluid, gel or solid ground substance.

• Ground substance contains a number of large molecules including adhesion proteins.

• Normally, most cells within a tissue remain in place, anchored to other cells, basement membranes and connective tissues. Exceptions include: phagocytes and embryonic cells involved in differentiation and growth.

Cell Junctions

• Cell Junctions: Points of contact between adjacent plasma membranes.

Three Types of Cell Junctions:1. Tight Junctions: Form fluid-tight

seals between cells and are common among epithelial cells that line the stomach, intestines and urinary bladder.

Cell Junctions

2. Anchoring Junctions: Fasten cells to one another or to the extracellular material. They are common in tissues subjected to friction and stretching (e.g. outer layer of skin, muscle of the heart, neck of the uterus, lining of the gastrointestinal tract.) They include desmosomes, hemidesmosomes and adherens junctions.

Communicating Junctions

Communicating Junctions: Allow the rapid spread of action potentials from one cell to the next in some parts of the nervous system and in muscle of the heart and gastrointestinal tract.

Communicating Junctions:

• In a developing embryo, chemical and electrical signals that regulate growth and differentiation may travel via these junctions.

• The microscopic structure that functions as a communicating junction is the gap junction.

Epithelial Tissues

• General Features of Epithelia:

The subtypes of epithelium (epithelial

tissue) include covering and lining

epithelium and glandular

epithelium.

General Features of Epithelial Tissue:

1. Epithelium consists mostly of closely packed cells with little extra-cellular material.

2. Epithelial cells are arranged in sheets, in either single or multiple layers.

3. An epithelial cell has an apical surface and a basal surface attached to a basement membrane.

4. Many cells junctions are present, providing secure attachments among the cells.

General Features of Epithelial Tissue:

5. Epithelia are avascular; Exchange of materials between epithelium and adjacent connective tissue is by diffusion.

6. Epithelia adhere firmly to nearby connective tissue via a thin extracellular layer, the basement membrane.

General Features, continued

5. Epithelia have a nerve supply.

6. Epithelia have a high capacity for renewal (high mitotic rate.)

7. Epithelia are derived from all three primary germ layers. (Endoderm, Mesoderm, Ectoderm)

Functions of Epithelia

1. Protection2. Filtration 3. Lubrication4. Secretion5. Digestion6. Absorption7. Transportation8. Excretion9. Sensory Reception10. Reproduction

Types of Stratified Epithelium

Simple Squamous Epithelium

• Consists of a single layer of flat, scale-like cells.

• It is adapted for diffusion and filtration and is found in the lungs and kidneys; in serous membranes. It functions in osmosis and secretion.

• It is found in parts of the body that are subject to very little wear and tear.

• Endothelium lines the heart and blood vessels. Mesothelium lines the thoracic and abdominopelvic cavities and covers the organs that lie within them.

Simple Cuboidal Epithelium

• Consists of a single layer of cube-shaped cells

• It is adapted for secretion of mucus, perspiration or enzymes and absorption (the intake of fluids or other substances by cells.)

• It is found covering ovaries, in kidneys and eyes and lining some glandular ducts.

Simple Columnar Nonciliated Epithelium

• Consists of a single layer of nonciliated rectangular cells.

• It functions in secretion of mucus and in absorption.

• It lines most of the gastrointestinal tract, where specialized cells containing microvilli that perform absorption and goblet cells that secrete mucus.

Simple Columnar Ciliated Epithelium

• Consists of a single layer of ciliated rectangular cells.

• It moves fluids or particles along a passageway by ciliary action.

• It is found in a few portions of the upper respiratory tract, uterine tubes, uterus, some paranasal sinuses and the central canal of the spinal cord.

Stratified Squamous Epithelium

• Consists of several layers of cells in which the top layer is flat and performs a protective function.

• A keratinized variety forms the outer layer of skin; keratin is a protein that is waterproof, resistant to friction and helps repel bacteria.

• A nonkeratinized variety lines the mouth, esophagus, part of the epiglottis and vagina and covers the tongue.

Stratified Cuboidal Epithelium

• Consists of several layers of cells in which the top layer is cube-shaped.

• It functions in protection.

• It is found in ducts of adult sweat glands and parts of the male urethra.

Stratified Columnar Epithelium

• Consists of several layers of cells in which the top layer is rectangular.

• It protects and secretes.

• It is found in portions of the male urethra and large excretory ducts of some glands.

Transitional Epithelium

• Consists of several layers of cells whose appearance is variable.

• It is capable of stretching many times its size and thus permits distention of an organ.

• It lines the urinary bladder and portions of the ureters and the urethra.

Pseudostratified Columnar Epithelium

• Has only one layer but gives the appearance of many.

• It functions in secretion and movement of mucus by ciliary action.

• It lines larger excretory ducts of many large glands, part of the male urethra and the auditory (Eustachian) tubes. The ciliated variety with goblet cells lines most of the upper respiratory tract.