Lesson Explainer: Animal Tissues Biology

In this explainer, we will learn how to describe the structure and function of different tissues found in animals.

Your heart is made of heart tissue, your brain is made of brain tissue, and your lungs are made of lung tissue. Put simply, tissues are what organs are made of. These tissues that make up our organs fall into certain categories according to their composition and function. There are four main types of tissues in complex, multicellular animals, such as humans: epithelial, nervous, connective, and muscular tissues.

We commonly define a tissue as a group of cells that work together to perform a specific function. Cells make up tissues, which make up organs, which make up organ systems, which make up organisms. Since tissues are made of cells, the types of cells found in a tissue determine the role that this tissue plays. In the same way, the type of tissue that makes up a particular organ determines the function of that organ.

Definition: Tissue

A tissue is a group of specialized cells that work together to carry out specific functions.

Let’s take a closer look at the four major types of tissues found in animals.

One of the four main types of animal tissues is epithelial tissues. Epithelial tissues are lining tissues whose primary function is to cover and protect the internal and external surfaces of our organs and bodies. They also function in absorption and secretion. For example, the epithelial tissue that lines our intestines is adapted to absorb nutrients from our food, whereas the epithelial tissue that lines our airways is adapted to secrete mucus to keep them moist.

Key Term: Epithelial Tissues

Epithelial tissues function as a lining tissue that protects the surfaces of our bodies and organs. The cells of epithelial tissues are closely joined together to form a continuous layer.

Epithelial tissues make up the outer layers of our skin, called the epidermis, which is shown in Figure 3. Epithelial tissues can be found lining our digestive organs. Epithelial tissues also line our endocrine glands, such as the adrenal glands, where they produce endocrine hormones. They also line our exocrine glands, such as the sweat glands. Epithelial tissues line ducts throughout our bodies, such as our ureters, which connect the kidneys to the urinary bladder. A special type of epithelial tissue lines the inside of our urinary bladder, which allows it to expand to large volumes.

Epithelial tissues have certain characteristics that make them well adapted to their function as a lining tissue. Epithelial cells are closely packed together, forming a continuous sheet. Since epithelial tissues act as a protective layer, these cells are frequently damaged (e.g., by a cut or scrape in the skin) and are replaced frequently.

Depending on the function of the tissue, it may possess specialized types of epithelial cells involved in secretion or absorption. Some examples of specialized epithelial cells include goblet cells, which are found in the lungs and lower digestive tract; ciliated cells, which can be found in the respiratory tract; and cells with microvilli in the small intestine. Goblet cells secrete mucus, ciliated cells move fluids in a sweeping motion, and cells with microvilli have an increased surface area, which is ideal for absorption.

An epithelial tissue is categorized by the number of layers it possesses and the shape of the epithelial cells. An epithelial tissue with one layer of cells is called a simple epithelial tissue, whereas an epithelial tissue with multiple layers of cells is called a stratified epithelial tissue, as can be seen in Figure 5.

The cell shape can be squamous, cuboidal, or columnar. These terms describe the outermost layer of epithelial cells in a particular epithelial tissue. Squamous cells are flat and irregular, cuboidal cells are about as wide as they are tall, and columnar epithelial cells are taller than they are wide. We can describe epithelial tissues using both the number of layers and the cell type, so an epithelial tissue with one layer of elongated cells, such as that shown in Figure 4, would be called a simple ciliated columnar epithelium.

Example 1: Identifying an Organ according to a Description of Tissues and Function

Which of the following organs has an outer layer of stratified squamous epithelial cells that function to protect the body from entry by pathogens and are routinely shed and replaced?

  1. Heart
  2. Kidneys
  3. Lungs
  4. Skin
  5. Liver

Answer

This question is asking us to identify an organ according to the description of one of the tissues it possesses as well as its function. We are told that this organ has an outer layer of stratified squamous epithelial cells. Squamous epithelial cells are flat and irregular. The fact that this tissue is described as stratified means that there are several layers of cells in this epithelial tissue. We are further told that these cells are routinely shed and replaced. This is another characteristic of epithelial tissues: The cells are frequently and easily replaced. However, many structures of the body are lined with epithelial tissues. The last clue in the question is that this organ is responsible for protecting the body from pathogens. While the heart, kidneys, lungs, and liver may all possess epithelial tissues, only the skin forms a barrier between our bodies and the pathogens in the outside world. We can further see in the following diagram of the skin that the outer layer has flat, stacked cells, which are characteristic of stratified squamous epithelia.

With this information, we can conclude that the skin is the organ described in the question.

Another type of animal tissue is the connective tissue. Connective tissues connect, attach, support, bind, and protect different structures in the human body. This type of tissue is called connective because it is found between other types of tissues, nearly everywhere in the human body. These tissues have a wide variety of functions, depending on their location and composition.

Key Term: Connective Tissues

Connective tissues connect, attach, support, bind, and protect different structures in the human body. This type of tissue is composed of living cells suspended within a nonliving matrix.

There are numerous examples of connective tissues in the body. For example, the layer of tough fibrous connective tissues around the heart protects the heart while at the same time giving it strength and structure. Blood contains multiple different cells suspended in a fluid matrix. It connects many parts of the body as it flows through the blood vessels. Tendons and ligaments are dense fibrous connective tissues that connect bones to muscles and bones to bones respectively. Bone is a strong connective tissue that protects our organs and produces red and white blood cells, among other functions. Adipose tissues are a type of tissue that is made up of numerous lipid-rich cells that together store energy in the form of fat. Cartilage, a smooth, impact-resistant type of connective tissue, cushions the joints.

As we can see, connective tissues are highly diverse and are found in many locations throughout the human body.

The properties of the various connective tissues are what make them adapted to their functions. Connective tissues are composed of living cells suspended within a nonliving matrix, or surrounding material. The types of cells and the matrix that they are suspended in give connective tissues their various properties. For example, blood consists of many blood cells, such as red blood cells and white blood cells, suspended in a liquid matrix that we call plasma. This allows the blood tissue to perform its function of carrying materials from one place to another. Another example is the bone tissue, which is made of bone cells suspended in a dense, hard, mineral-rich matrix. This allows the bone tissue to carry out its function of supporting the structure of the body.

You can see a micrograph of a sample of loose connective tissues below, where the dark spots are cells, the pink and purple strands are fibers, and the clear space in between is filled with ground substance.

Areolar connective tissue

Figure7

Connective tissues are often categorized by their structure as either specialized or proper. Proper connective tissues consist of cells and fibers within a gel or liquid called ground substance. Proper connective tissues can further be categorized as loose or dense, depending on the proportion of fibers to ground substance. Examples of proper connective tissues include tendons and ligaments. Specialized connective tissues are cells suspended in a matrix that may be composed of substances other than fibers and ground substance. Specialized connective tissues may not possess fibers at all. Examples of specialized connective tissues include blood tissues and bone tissues.

Sometimes connective tissues are categorized according to their function. For example, vascular connective tissues include blood and lymph fluid, which flow within blood vessels and lymph vessels respectively. Another example is skeletal connective tissues, which include the bones of the skeleton and the cartilage that is attached to their ends.

Muscular tissues are the third type of the four main types of animal tissues that we will discuss. The main function of muscular tissues is movement. Any part of our body that can move or change shape likely has some muscular tissues associated with it. There are three types of muscular tissues: skeletal, smooth, and cardiac muscle tissues. Each of these different types of muscle tissues has different features depending on its function.

Key Term: Muscular Tissues

Muscular tissues are tissues that can cause motion or movement. Muscle tissues possess muscle cells, which are full of contractile proteins.

Let’s have a look at the three types of muscle tissues in more detail.

Skeletal muscles are attached to the bones and are responsible for all voluntary motion. They move our bodies, allow us to make facial expressions, and even control the movement of our tongues. Smooth muscles are associated with our internal organs. These muscles are considered involuntary muscle tissues because they do not require conscious control to move. Smooth muscles are what allows our stomach to churn, what pushes food through our digestive tracts, and what opens and closes ducts throughout our bodies. They also change the diameters of our blood vessels to maintain constant blood pressure. Cardiac muscle tissues are found only in the heart. These are also involuntary muscle tissues, since we do not control them consciously. Cardiac muscle tissues are designed to pump rhythmically and continuously, without tiring, for our entire lifespan.

All muscular tissues have certain traits in common, which allow them to carry out their function of causing movement. Muscular tissues are primarily composed of muscle cells. Muscle cells are cells that are adapted to generate force, which causes contraction and movement. These cells are filled with special contractile proteins. These proteins can slide forward and backward along each other, as shown in Figure 8. They can slide closer together to shorten the muscle cell, or contract. When they slide back to their original positions, the muscle cell becomes longer, or relaxed.

It should be noted that the three different types of muscle tissues are different in their appearance.

Skeletal muscles are made of long, continuous, unbranched fibers. Under a microscope, skeletal muscle tissues have a striped, or striated, appearance due to the alignment of the myofibers. These fibers can be many centimeters long and possess several nuclei. When stimulated, the entire muscle fiber shortens to contract.

Cardiac muscles are striated like skeletal muscles. However, unlike skeletal muscles, the cells of cardiac muscle tissues possess only one nucleus. Cardiac muscle cells are branched, forming multiple connections, which allow them to contract in a coordinated, rhythmic fashion. Smooth muscles have elongated cells, which are narrow at each end. Under a microscope, smooth muscles are not striated, which is how they got their name. Each smooth muscle cell possesses only one nucleus. Smooth muscle cells contain a lattice of myofibers that allow them to contract in two dimensions.

Example 2: Determining the Location of Smooth Muscles

Where in the body is smooth muscle tissue found?

  1. Digestive canal and blood vessels
  2. The skin and pancreas
  3. Bones and cartilage
  4. Surrounding the heart

Answer

Muscle tissues are one of the four major types of animal tissues. They function throughout the body to cause movement by the contraction of muscle cells. There are three types of muscle tissues: cardiac, skeletal, and smooth muscle tissues. Cardiac muscle tissues are only found in the heart and are specially adapted to beat rhythmically and constantly, pumping the blood throughout the body. Skeletal muscle tissues are attached to the bones of the skeleton and control all our voluntary movements, including writing, typing, and speaking. Smooth muscles are the third type of muscle tissues, and they are associated with our organs. Like cardiac muscles, they move involuntarily. Smooth muscles cause our stomach to churn, our pupils to constrict, and our blood vessels to change diameter in response to our blood pressure.

Therefore, we know that smooth muscles are found in the digestive canal and blood vessels.

The last of the four tissue types is the nervous tissue. Nervous tissues have the primary function of communication. They allow the communication between parts of the body and between the body and the outside world. These tissues allow us to sense our surroundings through our sensory organs. They also allow us to process information in our central nervous system. Nervous tissues allow us to react to changes through the control of both our involuntary and voluntary responses. Almost every organ in the body has nervous tissues associated with it for the purposes of control, monitoring, or both.

Key Term: Nervous Tissues

Nervous tissues function in communication. They carry signals throughout the body and process information to provide appropriate responses within the central nervous system.

Nervous tissues are the primary tissue of the nervous system. Our nervous system is divided into a central nervous system and a peripheral nervous system. The central nervous system contains the brain and the spinal cord. The brain is the center of information processing. The spinal cord carries information between the brain and the rest of the body. The peripheral nervous system contains the peripheral nerves and sensory organs, such as our eyes. Our peripheral nerves carry signals between our body parts and organs from and to the spinal cord. All of the parts of the nervous system are mainly made of nervous tissues. You can see this in Figure 10.

Nervous tissues are mostly made of nerve cells called neurons. Neurons are cells that are specifically adapted to receive and transmit signals. They have special parts called dendrites, which receive signals, and special parts called axons, which carry signals away from the cell body toward the axon terminals, where these signal can be transmitted to other cells. The length of the axon and the length and number of dendrites that a neuron has vary depending on its function. Also, neurons may or may not possess what we call a myelin sheath. The myelin sheath is a fatty coating around the axons of some neurons, which speeds up the transmission of electrical signals along the axon of the neuron. You can see the structure of a neuron in Figure 11.

Besides neurons, nervous tissues possess many types of support cells called neuroglia. These neuroglia help the neurons do their job; for instance, they form the myelin sheath around the neurons, provide the neurons with nutrients, and act as immune cells to protect the neurons from disease. The structure and composition of cells within nervous tissues make them adapted to their specific function within the nervous system.

Example 3: Describing the Function of Nervous Tissues

What is the main function of nervous tissue?

  1. To contract and move parts of the body
  2. To receive information from stimuli and communicate with the spinal cord and brain via electrical impulses
  3. To bind, support, and protect structures within the body
  4. To provide a protective barrier for internal and external surfaces

Answer

Nervous tissues are one of the four major types of animal tissues. These four types are nervous tissues, muscle tissues, connective tissues, and epithelial tissues. Let’s look at the functions and structures of these tissues in more detail.

A tissue is commonly defined as a group of cells working together to carry out a specific function. The cells that tissues are made of, as well as their structure and arrangement, determine the function of the tissue. Nervous tissues mainly consist of neurons. Neurons are specialized cells that transmit impulses from one place to another in a network of rapid communication that spans the body. The brain and spinal cord are primarily composed of nervous tissues. Muscle tissues mainly consist of muscle cells. Muscle cells are adapted to contract, which generates force and causes movement. Muscle tissues move our body, shape our internal organs, and even cause our hearts to beat. Connective tissues consist of living cells suspended in a nonliving matrix. These tissues attach, bind, and support different parts of our bodies. Some examples of connective tissues are bone, blood, tendons, ligaments, and fat. Epithelial tissues are made of epithelial cells. Epithelial cells are closely joined into a continuous layer and are easily replaced. Epithelial tissues are a lining tissue and form internal and external barriers in and on organs. Epithelial tissues are found inside our mouths, as the outer layer of skin, and lining our digestive tracts and blood vessels.

This tells us that the main function of nervous tissues is to receive information from stimuli and communicate with the spinal cord and brain via electrical impulses.

The four major types of animal tissues are composed of different cells, which give them different structures that are adapted to fulfill different functions. Tissues work together in organs, and the tissues that an organ is composed of determine its function.

Example 4: Identifying the Major Types of Tissues in the Human Body

Which of the following is not a major type of tissue found in the human body?

  1. Skeletal tissue
  2. Nervous tissue
  3. Connective tissue
  4. Epithelial tissue
  5. Muscle tissue

Answer

Tissues are groups of cells working together to perform a certain function. All organs are made of tissues. For example, your lungs are made of lung tissues, and your kidneys are made of kidney tissues. There are four main categories, or types, of tissues found in the human body, and they each have special structures that support their functions. The tissue that lines and protects our internal and external surfaces is called the epithelial tissue. Epithelial tissues are made of closely joined cells that form a cohesive layer like in the skin. These cells are typically easily and frequently replaced since they are often damaged. Nervous tissues allow our bodies to communicate rapidly and respond to stimuli. These tissues contain neurons that help send and receive nerve impulses. The tissue that is adapted to cause movement is the muscle tissue. Muscle tissues have specialized muscle cells that are full of contractile proteins. Finally, the tissue that binds, supports, protects, connects, and attaches the different parts of the body to each other is called the connective tissue. Connective tissues are made of cells suspended in a nonliving matrix. There are many different types of connective tissue, which allow it to carry out these many different functions.

So, the option that is not one of the four major tissue types is skeletal tissue.

Let’s summarize what we have learned in this explainer.

Key Points

  • The level of organization in the human body is cells tissues organs organ systems organism.
  • There are four major types of animal tissues: epithelial, connective, muscular, and nervous.
  • Epithelial tissues may be simple or stratified and may be columnar, squamous, or cuboidal.
  • Connective tissues can be classified according to their structure (i.e., proper or specialized) or according to their function (i.e., vascular or skeletal).
  • There are three types of muscular tissues: smooth, skeletal, and cardiac.

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