The Hierarchical Organization of Multicellular Organisms

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be exploring the awesome organization of living things. If you've ever wondered how microscopic cells could possibly produce a living human being, this lesson is for you.

What Are Multicellular Organisms?

If you look around and identify some living things, you might see plants, a pet or yourself. These organisms are all multicellular, meaning they are made of more than one cell. Cells are the basic units of life. They are microscopic and can't be seen by the naked eye. However, cells can work together, connecting to each other with layers of proteins and other organic molecules to form the organisms we can see, like ourselves and our pets. Today, we're going to explain how single cells form larger structures, and eventually, us.


Cells are the basic units of life, meaning that the smallest living thing is a single cell. Unlike the drawings of cells you might have seen, not all cells are alike. In a multicellular organism, there are many different types of cells in the same living thing. Each type of cell is specialized for a particular job or function. The structure of the cell is directly related to its function. Let's look at an example.

Neurons are cells of the nervous system. They make up your brain, spinal cord and peripheral nerves in the body. These cells are designed to send signals. All neurons have tiny projections to receive information and a cell body where the information is processed. Then, some neurons have extremely long tails called axons to send the information to the next cell. Axons can be incredibly long, reaching all the way from your neck to your feet. Only neurons have this specific structure because only neurons have the job of sending information around your body.

Different cells like this neuron have a specific structure for their job in the body


When a lot of the same type of cell comes together, they form a tissue. There are four main types of tissue in the human body: epithelial, muscle, connective and nervous.

Four types of tissues in the body
types of tissue

1. Epithelial Tissue

Epithelial tissue is made of cells that line our body cavities, or the outside of our body like our skin. Epithelial tissue protects us from the outside world and from foreign invaders. For example, epithelial tissue that lines our digestive system is specialized both to secrete chemicals needed to breakdown our food and to absorb nutrients needed by the body.

2. Muscle Tissue

Muscle tissue are groups of cells that give the body the ability to contract. We typically think of muscle as being attached to our arms and legs, giving us the ability to curl a dumbbell or run a mile. This type of muscle is called skeletal muscle. However, there are actually two more types of muscle in the body.

Cardiac muscle is the muscle of our heart. Only the heart contains cardiac muscle, which contracts involuntarily, without our conscious thought, for our entire life. Cardiac muscle keeps blood pumping around the body, supplying us with life-giving oxygen.

The last type of muscle is smooth muscle. Smooth muscle makes up our hollow internal organs, like the intestines, stomach and uterus. The contractions of smooth muscle allow us to churn our food in the stomach, and for the food to be pushed through the intestines where important nutrients can be absorbed.

3. Connective Tissue

Connective tissue holds our body together. Hair, nails, cartilage, bone and fat are all examples of connective tissue. They provide structure and support, like bone and cartilage, and cushioning for our organs, like fat. Nails and hair protect our body from the outside world.

4. Nervous Tissue

Nervous tissue is the type that makes up our nervous system. Nervous tissue is made of the cells we talked about previously, neurons. The job of neurons is to send information from the brain to the body, and to send information about the environment to the brain for processing.

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