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Leaf: How are animal cells arranged to form tissues?

How are cells arranged in multicellular organisms?

Cells are not arranged randomly in multicellular organisms. Even colonial algae and the simplest sponges have their cells arranged in specific patterns and groups, called tissues, organs, and organ systems.

  • Tissues are groups of differentiated cells that work together in a coordinated way to perform a specific set of functions. The cells in a tissue are not necessarily identical, but all the cells in a tissue at a particular location come from the same part of a developing embryo. Tissues are organized on a microscopic scale.
  • Organs and organ systems are functional groups of tissues that come together to perform specific functions. Organs and organ systems are composed of tissues that originated from many different parts of the embryo. Organs are arranged on a scale we can see with our eyes (the macroscopic scale).

This image shows the cell to organ system.
Cell to Organ System Organization.

What is the difference between tissues and organs?

The difference between tissues and organs might seem vague, since both are groups of cells that perform specific functions. Just remember that tissues are smaller, often microscopic. Also, one particular type of tissue can be found in many places in the body, while an organ has a specific location.

For example, capillaries are tiny thin blood vessels made up of a sheet-like layer of cells called epithelial tissue. All of the capillaries throughout your body are made of epithelium. By comparison, your heart is a single large structure that is always located in a particular location in the body.

This Leaf describes the general arrangement of animal cells into tissues. Other Leaves explore each type of tissue in more detail. [examples include Leaf: How are cells arranged in connective tissues? and Leaf: How are cells arranged in nervous tissues?]

How are animal cells arranged into tissues?

As a fertilized egg develops into a newborn human baby, the original single-celled zygote goes through about 42 rounds of mitosis to create 4-5 trillion cells (242 = 4.5 x 1012). These cells do not look like the original cell of the zygote. They undergo differentiation, a process where the general cells become more and more specialized. Some become blood cells, others muscle cells, and so on. In an adult animal, there may be more than 100 different types of differentiated cells.

Differentiated cells arrange themselves into 1 of 4 major tissue types. Each of these types can be subdivided further.

  1. Epithelial tissue covers body surfaces and lines body cavities. Other functions include lining, protecting, and forming glands. The cells are arranged into sheets, and may have more than one layer. Epithelial cells are tightly attached to each other, and there is only a small amount of extracellular matrix material surrounding the cells. Skin is an example of epithelium.

  2. Connective tissue serves many purposes in the body. It is the main space-filling tissue in an animal’s body, anchors other tissues in place, and forms the skeleton and supporting structures. Cells in connective tissue are surrounded by extracellular matrix. This material is not alive, but it contains proteins and other materials that connective tissue cells secrete. In connective tissue, the extracellular matrix is more important than the cells in determining the properties of the tissue. Very often the total mass of extracellular matrix is many times the mass of the cells that make it. Bone and tendon are examples of connective tissues.

  3. Muscle tissue is responsible for movements by an animal. Muscle is made up of cells that are nearly filled with specialized contractile proteins. Animals have 3 types of muscle: skeletal or striated, smooth, and cardiac. Skeletal muscle is what we think of most often when we say muscle in conversation. Cardiac muscle is found only in the heart. Smooth muscle surrounds all of the hollow organs, and most blood vessels. When your stomach rumbles before a meal, the movement you feel is caused by contractions of smooth muscle in the wall of your stomach. .

  4. Nervous tissue is specialized to transmit electrical signals long distances, and pass those signals to other cells, tissues, and organs. Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them. Some glial cells also transmit messages; others insulate and protect neurons.

Most animal organs are made up of multiple tissue types. For example, your stomach contains all 4 major tissue types. The inner lining is epithelium, covering a network of connective tissue that lets the stomach stretch and rebound. Around the connective tissue is smooth muscle that churns and mixes food. The muscle layer also has nerves, which control the muscle.

This diagram shows four types of tissues found in the stomach.
Types of tissues found in the stomach.


  1. http://home.comcast.net/~john.kimball1/BiologyPages/T/TOC.html
  2. http://www.emc.maricopa.edu/faculty/farabee/biobk/biobooktoc.html

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Page last modified on Tuesday 27 of January, 2015 20:49:38 EST

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