groups of cells that are similar in structure and perform a common or related function.

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Epithelial: cover body structures in order to absorb, secrete, protect, of filter materials in between layers of tissues.
Connective: support body structures.
Muscle: move body structures by contracting.
Nerve: control body structures by conducting electrical impulses throughout the body.

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Covering epithelium covers body surfaces and lines body cavities in order to form boundaries between different environments. (Forms the outer layer of skin, lines the thoracic and gastrointestinal cavity and forms the outer and inner coverings of organs).
Glandular epithelium forms the glands that secrete vital substances for the body.

 Absorption
 Secretion
 Protection
 Filtration

Special Characteristics of Epithelium:
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Cells are closely packed together with tiny amounts of extracellular material in the narrow spaces between them
Cells fit close together to form continues sheets. The cells are held together using tight junctions and desmosomes (proteins projected from the cell membrane that connect the two cells)
Epithelial cells have a top and bottom or an apical surface and a basal surface.
The apical surface is the upper surface exposed to the exterior or the cavity and the basal surface is attached to the next layer of cells (connective cells).
The apical surface may have microvilli to increase surface area in cells that absorb or secrete substances or they may have motile cilia on cells that propel substances across the surface.

Special Characteristics of Epithelium:
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All epithelial cells are supported by a layer of connective tissue. The two tissues create a thin sheet called a lamina or membrane that acts as a division between the two layers. This membrane acts as a selective filter and helps the epithelium resist stretching and tearing (made largely of collagen)
Epithelial tissues do not contain blood vessels (avascular) so they are supplied for by the connective tissue below diffusing substances.
Epithelial cells are extremely regenerative. This allows them to cope with hostel environments and constant abrasion.

Classification of Epithelium
 All epithelium have two names. The first indicates if the cells are one layer thick (simple) or several layers
(stratified). The second indicates the shape of the cells. All epithelium have six (somewhat irregular) sides, an apical surface and a basal surface.
However the height of the cells varies. Squamous cells are flat and scale like, cuboidal cells are boxlike and columnar cells are tall column shaped cells. The shape of the nucleus will match the shape of the cell (squamous- flat disc, cuboidalsphere, and columnar- elongated egg shape and near the basal surface).

 Simple epithelium tissues are most concerned with absorption, secretion, and filtration. Being only one cell thick they do not offer much protection. Stratified epithelium tissues are normally used for protective coverings.
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Simple squamous: allows for easy passage of materials by diffusion through the cell layer. These cells are found in blood vessels and lungs where oxygen, CO2, nutrients and waste need to diffuse quickly into and out of the surrounding tissues.
Simple cuboidal: mostly used for secretion and absorption. Found in the kidneys and in the ducts and pores of glands.

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Simple columnar: these cells are also involved in absorption and secretion.
They can have single celled glands
(goblet cells) that secrete mucus or enzymes. Many have microvilli to increase surface area and some have cilia to propel the secreted substances. These cells make up the lining of the digestive tract.
Pseudostratified columnar: all cells are attached to the basement membrane but some do not reach the free surface. These cells secrete mucus in the upper respiratory tract and have cilia to move the dust trapping mucus away from the lungs.

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Stratified squamous: protect body surfaces. This includes all of the skin and extends a small distance into every opening. It is composed of many layers of squamous cells with cuboidal and columnar cells underneath. Since epithelium cells depend on diffusion from connective tissues at the basal surface, the cells at the apical surface are often not viable or atrophied.
Stratified cuboidal and columnar: both a quit rare. Most occurrences are ducts of larger glands.
Transitional epithelium: forms the lining of the urinary system. The cells that make up the basal layer are cuboidal or columnar. The cells of the apical layers change shape as the tissues are stretched by an increase in urine.

 Glandular epithelium
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Glands: one or more cells that are responsible for secreting an aqueous solution (water based) containing proteins, lipids, or steroids.
Endocrine glands: produce hormones that are secreted through exocytosis directly into the space surrounding the cells. The hormones enter the blood stream or lymphatic fluid to travel to their destination.

 Exocrine glands: all secrete products onto body covering or into cavities through an epithelial lined duct. These include mucus, sweat, oil, and salivary glands, the liver
(which secretes bile) and the pancreas (which creates digestive enzymes). Glands are surrounded by connective tissues which supply the gland with blood and nerve fibers. The cells can secrete the products either through exocytosis or there can be multiple layers of cells in which case the top layer of cells rupture releasing the products and the lower cells are used to replace them by constant cell division.

 Connective tissue proper
 Cartilage
 Bone
 Blood

 Binding and support
 Protection
 Insulation
 Transport of substances

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Common origin: all connective tissue arise from the same embryonic tissue called the mesenchyme
Varying degrees of vascularity from avascular
(cartilage) to highly vascular.
Extracellular matrix: unlike the other tissue types the living cells of connective tissue are separated by a matrix of nonliving structures that allow the tissue to bear weight, withstand tension, and tolerate physical trauma.

Structural Elements of Connective Tissue
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Ground substance: part of the extracellular matrix that contain the fibers. This is composed of tissue fluid and adhesion proteins. This substance holds large amounts of water and works as a sieve through which nutrients and other substances can diffuse between blood vessels and cells.
Fibers: provide support for connective tissues. Three types:
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Collagen: most abundant and strongest fiber. Provide the matrix with high tensile strength.
Elastic: long thin fibers that are able to stretch and recoil like rubber bands.
 Reticular fibers: similar to collagen fibers but are thinner and allow more give. They support the soft tissue of organs and blood vessels.

Structural Elements of Connective Tissue
 Cells:
 Each type of connective tissue has two cell states. The first is indicated by the suffix “blast” and is a period of active mitosis and secretion of matrix components. Once the matrix is created the cells change into a less active state indicated by the suffix “cyte”. Each type of connective tissue has a different name for the cells found in it.
 Connective proper: fibroblast or fibrocyte
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Cartilage: chondroblast or chondrocyte
Bone: osteoblast or osteocyte
Blood: hemopoietic stem cells (always remain in the state of active division)

Structural Elements of Connective Tissue
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The cells of connective tissue are capable of converting from “blast” cells to “cyte” cells and back if the matrix is injured or in need of repair.
Connective tissue is also composed of accessory cells including nutrient-storing fat cells, white blood cells, plasma cells that produce antibodies, mast cells, and macrophages.

Types of Connective Tissue
 Connective tissue proper:
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Areolar connective tissue: support and bind other tissues, hold body fluids, defend against infection, and store nutrients. The most obvious structural feature is the loose arrangement of fibers.
When an area of the body becomes inflamed this tissue and matrix soaks up excess fluid causing swelling called edema.
Adipose tissue: this tissue has a greater capacity for storing nutrients. An almost pure oil droplet occupies most of the cell forcing the cytoplasm and nucleus to the edge creating a thin rim. Adipose tissue is highly vascularized and can accumulate in subcutaneous tissues to act as shock absorbers and insulation. It can also accumulate in areas of high activity such as around the heart to supply nutrients.

Types of Connective Tissue
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Reticular connective tissue: creates a supportive network for other cells in the lymph nodes, bone marrow, and spleen.
Dense regular connective tissue: have densely packed collagen fibers that are all parallel creating a very strong resistance to a pulling force. This creates the tendons that attach muscle to bone and the ligaments that connect bone to bone.
Some ligaments are very elastic (ligaments between vertebrae) while others are not.
Dense irregular connective tissue: similar to the regular dense tissue but the collagen fibers extend in many directions so that they can withstand tension from different directions. Forms a fibrous covering for some organs and the dermis of the skin.

Types of Connective Tissue
 Cartilage: stands up to both tension and compression giving it characteristics of bone and dense connective tissue (tough but flexible).
Has no nerve endings or blood vessels, instead cartilage gets its nutrients from diffusion through the matrix to the blood vessels in the connective tissue around it. Due to lack of blood flow cartilage is slow to heal when damages. There are three types:
 Hyaline cartilage: makes up most if the embryonic skeleton, trachea, larynx, nose, and attaches the ribs to the sternum.
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Elastic cartilage: the ear and epiglottis (flap that prevents food from entering the lungs).
This type provides strength and stretchability.
Fibrocartilage: in the knee and between the vertebrae where strong support and resistance of heavy pressure is required.

Types of Connective Tissue
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Bone: supports the body and provides cavities for the storage of fat and the creation of blood cells.
Unlike cartilage bone has blood vessels that supply it with nutrients.
Blood: made of many types of cells that are responsible for carrying nutrients, waste, respiratory gases and other substances throughout the body.

Nervous tissue is the main component of the nervous system that which will control body functions. The nervous tissue makes up the brain, spinal cord, and nerves.

Types of nervous tissue
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Neurons generate and conduct nerve impulses.
The cells are branched with cytoplasmic extensions (processes) that allow them to transmit these impulses over a longer distance.
Supporting cells support, insulate, and protect the fragile neurons

Muscle tissue is highly cellular and greatly vascularized.
They are responsible for all types of movement. Muscle cells have elaborate networks of actin and myosin called myofilaments that slide along each other to create contractions.

 Skeletal muscle: this muscle tissue is surrounded by connective tissues and attached the bones of the skeleton. As these muscles contract they pull the bones of the skeleton causing body movement. The cells that make up skeletal muscles contain many nuclei and appear striated because of the alignment of the myofilaments. The muscles are also called voluntary muscles as they are under our conscious control.

 Cardiac muscle: this type of muscle cell makes up the heart. The heart contracts in order to force blood around the body. These cells are also striated with myofilaments but they only have one nucleus and they branch apart and come back together at intercalated discs.
These muscles are involuntary because we are not conscious of their actions

 Smooth muscle: these cells have no visible striations and contain only one nucleus.
These muscle cells make up hollow organs of the digestive and urinary tract, the uterus, and certain blood vessels. Smooth muscles alternate contracting and relaxing in order to force materials through the hollow organ. These muscles are also involuntary.