Can be separated into two basic overall types: a) covering and lining epithelia and b) glandular epithelia.
Special Characteristics of Epithelium include cellularity (made of close packed cells leaving very little extracellular material), specialized contacts (i.e. tight juctions and desmosomes), polarity (free apical surface and attached basal surface), supported by connective tissue (all epithelial sheets rest upon connective tissue), avascularity (contains no blood vessels) and regeneration (has a high capacity to regenerate).
Classification of Epithelial Tissue
A) Covering and Lining Epithelia
1. Simple Squamous Epithelium
-Squashed cells in a single thin, permeable layer with very little cytoplasm.
-Nuclei are centered and disc-shaped.
-Its function is to filter or rapidly diffuse substances.
Ex: lining of the heart
Simple Squamous Epithelium (www.technion.ac.il)
2. Simple Cuboidal Epithelium
-Cube shaped cells in a single layer.
-Nuclei are centered and spherical.
-Its function is to absorb and secrete.
Ex: kidney tubules
Simple Cuboidal Epithelium (www.technion.ac.il)
3.Simple Columnar Epithelium
-Single row of tall, column-like cells.
-Nuclei are round.
-Cilia may be present.
-Its function is to absorb and secrete.
-Secretes mucus.
-Cilia movement propels substances and cells.
Ex: lining of digestive tract
Simple Columnar Epithelium (www.technion.ac.il)
4. Pseudostratified Columnar Epithelium
-Column shaped cells in a single layer, but all vary in height.
-Cilia may be present.
-Its function is to secrete and absorb.
Ex: lining of respiratory tract
Pseudostratified Epithelium (http://erl.pathology.iupui.edu/HISTO/GENER60.HTM)
5. Stratified Squamous Epithelium
-Several layers of cells thick.
-The top layer is squamous cells and the deeper layers are cuboidal or columnar cells.
-Its function is to protect underlying tissues in areas of the body that are subjected to abrasion.
Ex: Epidermis
Stratified Squamous Epithelium (www.visualsunlimited.com)
6. Stratified Cuboidal Epithelium
-Two layers of cuboidal cells
Ex: mammary glands
Stratified Cuboidal Epithelium (www.keele.ac.uk)
7. Stratified Columnar Epithelium
-Apical layer is columnar cells.
Ex: pharynx
Stratified Columnar Epithelium (www.anatomy.dal.ca)
8. Transitional Epithelium
-Basal layer is cuboidal or columnar.
-Apical layer varies, depending upon how distended the area is.
-Its function is to stretch to allow distension.
Ex: lining of the urinary bladder
Transitional Epithelium (www.faculty.une.edu)
B) Glandular Epithelia
1. Endocrine Glands
-Ductless glands.
-Produce hormones, which secrete into extracellular space, then into blood and then travel to specific
organs to increase response.
-Secretions vary from amino acids to steroids.
Ex: adrenal gland
2. Exocrine Glands
-Have ducts.
-Secrete products onto skin or into body cavities
Ex: mucous glands
a. Unicellular Exocrine Glands
-one celled
Ex: Goblet cell in intestinal and respiratory tracts
b. Multicellular Exocrine Glands
-many celled
-have an epithelium-based duct and a secretory unit
1. Simple Multicellular Exocrine Glands
-unbranched duct
2. Compound Multicellular Exocrine Glands
-branched duct
3. Tubular Multicellular Exocrine Glands
-Secretory cell forms tubes
Ex: Intestinal glands
4. Alveolar Multicellular Exocrine Glands
-Secretory cells form small sacs
Ex: Mammary glands
5. Tubuloalveolar Multicellular Exocrine Glands
-both types of secretory units
Ex: Salivary glands
Connective Tissue: Most abundant tissue and it is found everywhere in the body.
It functions to bind and support, protect, insulate and transport (in the case of blood).
Special Characteristics of Connective Tissue include that it originates from mesenchyme, has a full range of vascularity, it is mainly composed of extracellular matrix (which gives it the ability to endure weight, tension, trauma and abrasion). The extracellular matrix is made up of ground substance and fibers (collagen, elastic or reticular).
Types of Connective Tissue
1. Mesenchyme (Embryonic Connective Tissue)
-First tissue formed from mesoderm
-Made up of mesenchymal cells and fluid ground substance with fibrils
-specializes into all other types of connective tissue.
2. Connective Tissue Proper
A. Loose Connective Tissue
1. Areolar Connective Tissue
-Most abundant connective tissue
-Binds body parts together, but allows for movement of these parts.
-It is wrapped around small blood vessels,nerves and glands.
-Makes up the subcutaneous tissue that cushions and attaches the skin to structures.
-Defends against infection.
-Stores nutrients as fat.
-Ex: lamina propria of mucous membranes
Areolar Connective Tissue (www.science.tjc.edu)
2. Adipose (Fat) Tissue
-Makes up 18% of average person’s body weight.
-Very vascular, indicating high metabolic activity.
-Functions as a shock absorber and insulation. Also prevents heat loss.
Ex: Abdomen
Adipose Tissue (www.science.tjc.edu)
3. Reticular Connective Tissue
-Reticular fibers are the only fibers in the matrix
-Forms a stroma that supports free blood cells (mainly lymphocytes)
Ex: lymph nodes
Reticular Connective Tissue (www.udel.edu)
B. Dense Connective Tissue
1. Dense Regular Connective Tissue
-Made up of dense collagen fibers running parallel to the direction of pull.
-Its function is to attach muscles or bones to bones.
-Has high tensile strength in one direction.
Ex: Tendons
Dense Regular Connective Tissue (www.pathology.mc.duke.edu)
2. Dense Irregular Connective Tissue
-Collagen fibers run in more than one plane.
-Has high tension strength in more than one direction.
Ex: Dermis of the skin
Dense Irregular Connective Tissue (www.sru.edu)
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