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Saturday, January 5, 2013

Tissue Types: Animal Structure and Function

The human body is a remarkably complex organization of substances, from cells to organs.  Essential to the makeup of the body are tissues.  There are four different main types of tissues in the body, each with several sub-types.

The first tissue type is the epithelial tissue, also commonly called the epithelia.  This tissue is made up of many layers of cells.  It makes up the skin, which covers the outside of the body, and covers organs and cavities within the body to protect them from damage.  This tissue type has five common sub-types:

1. The stratified squamous epithelium.

The stratified squamous epithelium, shown to the left, is a multi-layered tissue.  The most notable aspect of this tissue is the speed at which its cells are capable of replicating themselves.  New cells are formed at the base of the tissue and these cells rise to the top as old, dead cells are worn off.  This epithelial tissue is present on areas of the body that are frequently exposed to wear and the outside world, such as the skin.

2.  The pseudostratified columnar epithelium.

This tissue type is composed of one layer of cells that are a variety of heights, but whose nuclei look similar to those of the stratified epithelia.  This tissue type is most easily recognizable by the cilia on its surface, though pseudo stratified columnar epithelium can lack cilia in some cases.  This tissue often makes up a membrane of mucous (moved by the cilia) in the respiratory tract.

3.  The simple squamous epithelium.

The simple squamous epithelium is a single layer of large, flat cells.  It is the thinnest of any epithelial tissue.  This tissue type is notable for the incredibly fast rate at which substances can diffuse through it (owing to the fact that it is a single layer of very thin cells).  This tissue type is found in areas where diffusion of substances is essential, such as the lungs or the Bowman's capsule of the kidney.

4.  The simple columnar epithelium

The simple columnar epithelial tissue is made up of very large, long, rectangular cells, as shown in the image on the left.  These cells are key in absorption, particular in areas that are subject to abrasion and wear (i.e. the intestines and the digestive tract).  They also protect the rest of the body from any harmful bacteria that might have found there way into these areas by secreting a mucous coat, which is kept fluid by many microvilae.

5.  The cuboidal epithelium
The cuboidal epithelium is named for the cube-like cells that make up the tissue (shown on the left).  These cells are also key to secretion.  This tissue surrounds various structures within the body to protect from pathogens and wear and tear.  They are found in the kidneys and many glands within the body, where they assist with absorption.

The next tissue type is connective tissue.  Quite simply, this is the tissue that connects very many parts of the body.  This tissue makes up a matrix that holds the internal organs in place.  The matrix contains many cells, including fibroblasts and macrophages.  There are three different varieties of fiber that compose the connective tissue: collagenous fibers, reticular fibers, and elastic fibers.

1.  Loose connective tissue

The loose connective tissue, made up of a loose network of collagenous fibers and elastic fibers, is the most abundant connective tissue in the human body.  It binds a variety of structures together (i.e. connecting muscle fibers) and helps to make up layers of the skin.

2.  Fibrous connective tissue

The fibrous connective tissue, found most typically in tendons and ligaments, is a dense tissue made up primarily of collagenous fibers.  These fibers are made very small, strong collagen fibrils.  Interestingly enough, these collagen fibers are fluorescent under a UV light.

3.  Bone
Vertebrates have skeletons made up of bone tissue.  Bone is a very hard tissue, created by osteoblasts which initially produce a strong matrix of collagen.  Then calcium,magnesium, and phosphate ions mix together inside of the matrix, forming a hard mineral substance.  The ring shapes shown on the left are called "osteons" and are made up of circular layers of the matrix surrounding a passage of blood vessels and nerves.

4.  Adipose tissue
Adipose tissue is the substance more commonly known as "fat."  The cells make up a loosely connected tissue.  Each individual adipose cell stores fat droplets.  When fat is being stored the cells expand to hold more and when it is being used up they contract as the fuel is burned.  The adipose cells are found throughout out the body's matrix of connective tissue.  They are essential to well being.  They provide padding, insulate the human being to assist in maintaining body temperature, and help by storing energy in the form of fat that can be burned in times of famine or after great energy expenditure.

5.  Cartilage

Cartilage is a network of collagen-containing fibers within a mixture of proteins and carbohydrates commonly referred to as chondroitin sulfate.  Chondrocytes are the cells which create cartilage by producing both the collagen and the chondroitin sulfate.  The combination of these two substances is durable and pliable.  In fact, cartilage makes up the skeletons of many animals with vertebrae while they are still within the womb.  In an adult human it can be found throughout the body providing cushioning between bones, such as the knees and the vertebrae.

6.  Blood
Blood is the substance which flows throughout the vein sin the human body.  It is made up of plasma (a liquid ECM), erythrocytes (commonly known as red blood cells), leukocytes (known as white blood cells) and platelets.  Each of these substances serves a unique purpose.  The erythrocytes act as oxygen carriers, shuttling the substance throughout the body to be used in  cellular respiration.  The leukocytes are part of the immune system and defend the body from harmful pathogens.  The platelets allow the blood to clot, which is essential to healing cuts and abrasions.

The third tissue type is muscle tissue.  Muscle tissue is the tissue that allows animals and people to move.  It is composed if actin and myosin protein filaments that enable muscle contraction.

1.  Skeletal Muscle
Skeletal muscle, so named because it makes up the muscles that are attached to the skeleton, enables voluntary movement.  This muscle type is also known as striated muscle.  It is composed of muscle fibers (collections of long cells shown on the left).

2.  Smooth muscle

Smooth muscle differs from skeletal muscle in that it has no striations (this cal be seen by comparing the two images).  It is present in the digestive tract and other internal organs.  They are long and narrow, and cause involuntary movement, for example movement of the stomach and the arteries.

3.  Cardiac muscle

Cardiac muscle is the tissue that makes up large portions of the heart.  It is responsible for the heart beating.  The cells are interconnected and thereby organize their heart's "beats" or contractions within the contractile wall.

The fourth and final type of tissue is nervous tissue.  This is the tissue that makes up the nervous system.

1.  Neurons
Neurons are the most basic aspects of the nervous system.  It takes electrical impulses from the rest of the body's cells and the dendrites which stretch down from the neuron's body.  The transmit these impulses to each other or different cells through axons, which are gathered together to compose nerves.

2. Glial cells
The glial cells bring nutrients, provide insulation for, and restore the different neurons in the brain.  Occasionally they also monitor neuron function.

Sources: (accessed 1/5/13)
Campbell Biology Ninth Edition (Reece, Urry, Cain, Wasserman, Minorsky, Jackson) p. 856-859 (accessed 1/5/13) (accessed 1/5/13) (accessed 1/5/13) (accessed 1/5/13) (accessed 1/5/13) (accessed 1/5/13) (accessed 1/5/13)

Images courtesy of (in order as shown in the post):

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