Recall that tissues are groups of cells that have similar structures and functions. The following are the 4 major tissue types in the body: 1)epithelial tissues 2) connective tissues 3) muscle tissues and 4) nervous tissue.
Click here to see a summary table of the major characteristics of the four major tissue types
When you think of epithelial tissue you should think it is a covering, lining or glandular. Epithelial tissue covers the body and it also covers most organs. Epithelial tissue lines tubes of the body (like blood vessels, esophagus, etc.) and it also lines hollow organs of the body. Epithelial tissue also lines body cavities (thoracic cavity, abdominopelvic cavity, etc). Glandular tissue is also classified as a type of epithelial tissue.
One side of an epithelial tissue rests on a basement membrane. The basement membrane separates the epithelium from underlying connective tissue. The other side of an epithelium is "free". In other words, it touches the outside world (like the epithelium covering of skin) or it touches an open space (like the epithelium that lines the stomach).
Epithelial tissue is said to be avascular (does not have blood vessels). However it is innervated (has a nerve supply) and very mitotic (divides constantly). Cells of epithelial tissues are tightly packed together.
The following are different types of epithelial tissues.
This epithelium is called simple because it is composed of one layer of cells. It is called squamous because the cells are flat. This epithelium type is very thin and allows substances to move easily across it. This type of epithelium is found lining the air sacs of lungs where oxygen and gas move across it. It also forms the walls of capillaries (another site where substances move in and out of blood) and lining the walls of blood vessels in general.
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This type of epithelium consists of one layer of cube-shaped cells. This epithelium covers the ovaries and lines tubules of the kidneys. It also lines the ducts of many glands.
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This type of epithelium consists of one layer of tall cells. This epithelium also contains Goblet cells which are mucus secreting cells. It is most commonly found lining many organs of the digestive tract. The cells of the epithelium often possess microvilli which are finger-like projections of a cell membrane. These microvilli increase the surface area of these cells so they can secrete and absorb a large amount of substances.
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This type of epithelium is termed "falsely layered". Pseudo is the root for false and stratified means layered. This epithelium looks like it is many layers of tall cells but it really is just one layer of tall cells. The layering effect result from the nuclei of the cells lining up at different levels in the cells. Many organs of the respiratory tract (trachea, bronchi) are lined with this type of epithelium. This type of epithelium also contains Goblet cells. The cells of this epithelium often contain cilia. The cilia constantly move mucus with trapped dust out of the respiratory tract to the throat where it is swallowed.
Click here to see pseudostratified columnar epithelium
This type of epithelium is made up of many layers of cells (stratified). The cells on the free surface of this epithelium are flat hence the name squamous. This type of epithelium forms the outer layer of skin. As part of skin, a protein called keratin accumulates in the cells of this epithelium. Keratin makes the tissue water-proof and impermeable to most bacteria and viruses. This type of epithelium also lines the mouth, throat, anus, and vagina: however in these organs the epithelium does not accumulate keratin.
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This epithelium consists of layers of cube-shaped cells. This type of epithelium lines the larger ducts of glands. It also lines the seminiferous tubules of the testes.
This epithelium is made of several layers of tall cells. This type of epithelium is found lining the urethra of the male, the vas deferens (part of male reproductive tract) and in parts of the pharynx (throat).
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This epithelium is stratified (made of several layers of cells). It is called transitional because the cells closest to the free surface change their shapes regularly. This type of epithelium is found most commonly lining the urinary bladder. When the bladder is full and distended (stretched) the cells of this epithelium get stretched and tend to flatten. However, when the bladder is empty the cells are not stretched and tend to have a cube-like shape.
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Click here to see a summary table of the different types of epithelial tissue
Glandular epithelium is composed of cells that make a secrete substances. Glandular epithelium makes up glands. If a gland secretes is product into a gland, it is called an exocrine gland. If a gland secretes its product directly into tissue fluids or blood, it is celled an endocrine gland. Endocrine glands are ductless so they have to secrete their products directly into body fluids such as blood.
Exocrine glands can be classified as merocrine glands, apocrine glands, or holocrine glands. Merocrine glands secrete fluids. These fluids contain water, enzymes and sometimes mucus. Sweat glands, the pancreas, and salivary glands are types of merocrine glands. Apocrine glands secrete fluids and cell parts. Mammary glands and ceruminous glands (wax glands lining the ear canal) are examples of apocrine glands. Holocrine glands secrete whole cells and fluids. Sebaceous glands (oil glands of the skin) are examples of holocrine glands. These oil glands secrete whole cells that contain an oily substance. Before the cell reaches the surface of the skin, it lyses (bursts) to release the oily substance.
Connective tissue is the most abundant major tissue type. Cells of connective tissues generally do not touch each other like you see in epithelial tissue. The cells of connective tissues are separated by a matrix. Matrix can very simply be thought of as the "stuff" between the cells of connective tissue. The matrix can contain fibers, water, proteins, inorganic salts, and other substances. The components of the matrix varies depending on the connective tissue type. Connective tissues have rich blood supplies except for cartilage and dense connective tissues.
There are many different cell types that are found in connective tissues. Fibroblasts (blast means to make) are common cell types that make fibers. Mast cells are also common and they secrete substances such as heparin ( a blood thinner) and histamine ( promotes inflammation). Macrophages are another cell type also found in most connective tissues. They are phagocytic cells that destroy unwanted material such as bacteria or toxins.
Remember that the matrix of connective tissues often contain fibers. These fibers are made by fibroblasts. There are three types of fibers. Collagenous fibers are the strongest and most abundant fiber type. They cannot be stretched and are therefore found in tissues that tightly bind structures together. Elastic fibers can be stretched and are not as strong as collagenous fibers. This fiber type is found in organs that commonly are stretched such as the vocal cords. The last fiber type is called the reticular fiber. They really are just very thin collagenous fibers. They often form the framework of some organs such as the spleen.
Click here to see a summary table of the different components commonly found in connective tissues
The following lists and describes the different types of connective tissues:
Loose connective tissue loosely binds structures together. For example, it binds skin to underlying structures and binds muscles together. It also binds epithelial tissue to underlying connective tissues.
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Adipose tissue is more commonly called fat. Most adipose tissue in the body lies beneath the skin to act as an insulator. Adipose tissue also sits around the kidneys, on the surface of the heart, and behind the eyeballs to cushion these organs. Adipose tissue can also be used to make energy by cells.
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Dense connective tissue is called dense because it is packed with collagenous fibers. Therefore, this is a very strong tissue type that cannot be stretched. Tendons and ligaments are made of this connective tissue type. Tendons tightly bind muscles to bones and ligaments tightly bind bones to other bones. This tissue type is also found in the dermis of skin and the sclera (the white part) of the eyeball.
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The matrix of cartilage contains fibers and proteins that make the matrix rigid. The cells of cartilage are called chondrocytes and they sit in small holes called lacunae in the matrix of cartilage. Cartilage is an avascular tissue and must receive its nutrients from a membrane that surrounds it called the perichondrium. Cartilage often functions to support soft tissues.
There are three cartilage types: hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline cartilage is the most abundant type. It has a rigid but smooth matrix. It covers the ends of many bones, is found in the tip of the nose and in many respiratory tubes such as the trachea. Elastic cartilage contains elastic fibers in its matrix but is also rigid. It is more flexible than hyaline and forms the framework of the outer ear. Fibrocartilage contains many collagenous fibers in its matrix and is therefore the toughest cartilage type. It often exists as discs that can act as shock absorbers to reduce pressure on structures. For example, the discs between vertebrae are made of fibrocartilage.
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Click here to see elastic cartilage and fibrocartilage
Bone is the most rigid connective tissue type because the matrix of bone contains very hard mineral salts. Bone supports soft tissues of the body, protects organs and makes new blood cells. Osteocytes are cells of bone. They also sit in lacunae (holes) in the matrix of bone. We will look at the unique arrangement of bone tissue in a later chapter.
Blood is the most fluid type of connective tissue as the matrix of blood is plasma. Blood is the tissue that transports substances throughout the body. Erythrocytes (red blood cells) and leukocytes (white blood cells) are cells types of blood.
Click here to see a summary table of connective tissues
Muscle tissue is specialized to shorten and elongate; in other words, it contracts and relaxes! The following are the three types of muscle tissues:
Most skeletal muscle tissue is attached to the skeleton ( to bones), hence its name. This muscle tissue is called voluntary because we can consciously control its movement. For example, we can consciously decide to contract the skeletal muscles attached to our arm bones and make them move. It is also called striated because the cells of this muscle tissue type have striations or stripes in their cytoplasm. More about these striations in a later chapter.
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Smooth muscle tissue is located in the walls of hollow organs (except the heart), walls of blood vessels, and the dermis of the skin. It is not voluntary because we cannot consciously control its movement. For example, you do not get to decide when the smooth muscle of your stomach contacts or not. It is called smooth because its cells do not possess striations.
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Cardiac muscle tissue is located in the wall of the heart, hence its name. Like skeletal muscle, it is striated. Like smooth muscle it is not under voluntary control.
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Nervous tissue is found in the brain, spinal cord, and peripheral nerves. This tissue type is specialized to send impulses (electrical messages) to muscles and glands in the body. Neurons are the largest cell type of this tissue. Other cells called neuroglial cells are smaller and they support neurons.
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Click here to see a summary table of muscle and nervous tissue
