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Biology LibreTexts

Animal Tissues

Levels of Organization


Multicellular (large) organisms function more efficiently if cells become specialized for specific functions.

A tissue is composed of cells that function together in a specialized activity.

There are four types of tissues found in animals: epithelial, connective, nerve, and muscle tissue.

Sponges do not have tissues.


Organs are composed of two or more tissues which function together to perform a common task. For example, the heart contains all 4 types of tissues.

Sponges and cnidarians do not have organs.

Organ systems

An organ system consists of two or more organs which perform a specific task.

Some organ systems are: the integumentary, nervous, sensory, endocrine, skeletal, muscular, circulatory, immune, digestive, respiratory, excretory, and reproductive systems.

Embryonic Tissues

Ectoderm, mesoderm, and endoderm are embryonic tissues that give rise to all of the tissues, organs, and organ systems in the body.

Ectoderm forms the outer layer of skin and nervous system.

Mesoderm forms the muscles, connective tissues, skeleton, kidneys, and circulatory and reproductive organs.

Endoderm forms the lining of the gut, respiratory tract, and urinary bladder. It also forms the glands associated with the gut and respiratory tract.


Cells are joined to each other by proteins. The point of connection between two cells is called a junction.

Junctions bind cells together. Some kinds of junctions prevent the passage of molecules between cells. Other kinds of junctions allow molecules to pass from one cell to another.

Epithelial Tissue

Epithelial tissue covers external surfaces and internal cavities and organs. Glands are also composed of epithelial tissue.

Epithelia forms boundaries. Most substances that move into or out of the body must pass through epithelial tissue.

One surface of the tissue is free and the other adheres to a basement membrane.

The photograph below shows kidney tubules. The cells lining the tubules are epithelial tissue. One surface is attached (the basal surface) and the other surface is free.

The apical surface of epithelial cells may have tiny projections called microvilli. These function to increase surface area. For example, microvilli on intestinal cells increase the surface area available for absorption.

Eipthelial cells may have cilia. Cilia can be seen on the cells lining the trachea in the photograph below. They function to move mucus and trapped particles upward toward the mouth where it will be swallowed, thus keeping the trachea clear of foreign particles.

Function of Epithelial Tissue


  • Epithelial tissue forms the skin of many animals.
  • Terrestrial vertebrates have keratin in their skin cells making them resistant to water loss.
  • Ciliated epithelium lines the respiratory tract. Numerous cilia on these cells sweep impurities toward the throat.


  • Absorption is an important function of epithelial tissue. For example, the gut is lined with epithelial tissue and it functions to absorb nutrients from food. The lungs are also lined with epithelial tissue and it functions to absorb oxygen.


  • Glandular epithelium secretes chemicals.
  • Endocrine glands secrete hormones directly into the extracellular space.
  • Exocrine glands often secrete through DUCTS; they secrete mucus, saliva, wax, milk, etc.

Simple epithelium is one cell thick.

Example: Respiratory surfaces such as the lining of the lungs or the skin of a frog (below) are only one cell thick so that gasses can pass through quickly.

Squamous Epithelium - Frog Skin Flat Mount

Click the photograph to view a larger photograph.

Stratified epithelium has more than one layer. It is found in areas of high abrasion such as the skin or the lining of the mouth.

Cell division occurs in cells near the basement membrane, pushing older cells toward the surface. Cells lost by abrasion at the surface are replaced by cells underneath. 

Example: the human skin shown below contains stratified epithelium.

Stratified Squamous Epithelium, Human sec. X 100

Pseudostratified epithelium appears to be layered but each cell touches the same basement membrane. Some cells are elongate; they extend from the basement membrane to the free surface. Other cells are smaller, causing the tissue to appear stratified. The nuclei in the cells shown below appear to form multiple layers but the cells are all attached to the same basement membrane.

Pseudostratified Ciliated Columnar Epithelium

Epithelial cells are flat (squamous), cube-shaped (cuboidal), or elongated (columnar).

The words that describe layers (previous slides) can be used with words that describe shape. For example, simple squamous epithelium is one layer of flat cells.


Simple Squamous Epithelium

Simple squamous epithelium is a single layer of flat cells. It is found in the walls of small blood vessels (capillaries) and in the air sacs of the lungs (alveoli). Because it is thin, it permits diffusion of substances from one side to the other. For example, materials can diffuse out of the capillaries. In the lungs, oxygen can diffuse across the alveoli and into the blood.

Below: The skin of a frog is used for gas exchange. The outer layer of skin is simple squamous epithelium. The thin, flattened cells promote rapid diffusion of gasses between the air and the blood vessels underneath the epithelium.

Squamous Epithelium - Frog Skin


The cells that line the tubules of the kidneys are cuboidal. They function in secretion and absorption. 

The ducts of some glands contain simple cuboidal epithelium.

Simple Cuboidal Epithelium

Simple Columnar

Simple columnar epithelium is a single layer of elongate cells. It is found in the lining of the gut and parts of the respiratory tract. It functions in secretion and absorption. The photograph below is a cross section of the small intestine.

Small Intestine (Jejunum) X 200

Connective Tissue

The cells of connective tissue are separated by non-living material.

Connective tissue binds and supports body parts, protects, fills spaces, stores fat (for energy), and transports materials.

Structure of Loose and Dense Connective Tissue

Loose connective tissue and dense connective tissue contain three kinds of fibers. Collagen fibers provide strength and flexibility. Collagen is the most abundant protein in animal bodies. Elastic fibers provide elasticity. When stretched, they return to their original shape. Reticular fibers are small and branched. They provide a support framework for organs such as the liver and lymph nodes.

The cells of loose and dense connective tissue are called fibroblasts. They produce the fibers and nonliving matrix material. Macrophages are cells specialized for phagocytizing foreign materials, bacteria, and cleaning up debris. Macrophages will be discussed in the chapter on the immune system.

Loose Connective Tissue

Loose connective tissue includes areolar, adipose, and reticular connective tissue.

Areolar Connective Tissue

The fibroblasts (cells) of areolar connective tissue are separated by a nonliving, jellylike matrix. The tissue contains collagen fibers for flexibility and strength, and numerous elastic fibers that enable it to be stretched.

Areolar connective tissue X 200

Areolar connective tissue is found in the skin and in most internal organs of vertebrates, where it allows the organs to expand; it also forms a protective covering for muscles, blood vessels, and nerves.

Adipose tissue is a type of loose connective tissue. It has reduced matrix material and contains enlarged fibroblasts (cells) that store fat. Adipose tissue functions to store energy, insulate, and provide padding, especially in the skin and around the kidneys and heart.

Adipose Tissue Human sec X 200
Reticular Connective Tissue

Reticular connective tissue contains an abundance of reticular fibers. It provides a supporting framework for organs such as the lymph nodes, spleen, and liver.

Dense (Fibrous) Connective Tissue

The collagen fibers of dense connective tissue are more closely packed than those of loose connective tissue.

White Fibrous Connective Tissue X 200

Regular dense connective tissue contains collagen fibers oriented in one direction to provide strength in that direction. It is found in tendons and ligaments. Tendons connect muscle to bone; ligaments connect bone to bone.

Irregular dense connective tissue (not shown) contains collagen fibers oriented in many different directions. It is found in the deep layers of the skin (dermis) and the tough capsules that surround many of the organs such as the kidneys, adrenal glands, nerves, bones, and the covering of muscles. It provides support and strength.


The cells of cartilage are embedded in a protein-containing matrix that is strong but flexible.

It contains collagen and elastic fibers.

Hyaline Cartilage X 200

It is resilient; it does not stretch and can resist compression. It is also flexible but maintains its shape.

It is found in the ends of bones where it prevents friction within the joints. In the nose, external ear, and the walls of the trachea it functions to support the softer tissues.

The intervertebral disks function as shock pads.

The fetal skeleton of vertebrate animals is composed of cartilage before bone forms. The skeleton of cartilaginous fish is composed of cartilage.


Bone forms when calcium salts are deposited around protein fibers. The calcium salts provide rigidity while the fibers provide elasticity and strength.

Bone, dry ground human c.s. X 100


Blood is a connective tissue. Like other kinds of connective tissues, it contains cells that are separated by a non-living material. In this case, the nonliving material is the plasma.

Human Blood, Wright Stain X 1000

Muscle Tissue

Muscle tissue contracts in response to stimulation. It cannot lengthen by itself but is lengthened by the contraction of other muscles..

Muscle tissue is used for locomotion, food movement in gut, and heat production.

Smooth Muscle

Smooth muscle is involuntary. 

It surrounds the gut and moves food through the digestive tract. 

It surrounds the blood vessels where it controls the distribution of blood. There is not enough blood in the body to fill all of the blood vessels so some must be contracted while others are filled. For example, after meals, the blood vessels of the gut are opened while many of those in the skeletal muscles contract. 

The ends of the cells are tapered.

Smooth Muscle, Human X 200
Skeletal Muscle

Skeletal muscle is voluntary. 

The cells are very long, extending the length of the muscle. They are multinucleate, and striated (striped).

Skeletal Muscle Teased X 200
Cardiac Muscle

Cardiac muscle is found in the heart.

It is striated and branched.

Cardiac Muscle X 200

Muscle tissue will be discussed in more detail in the chapter on motor systems.

Nervous Tissue

Nervous tissue responds to stimuli and transmits impulses from one body part to another.

Motor Neuron X 200

Nervous tissue will be discussed in more detail in these two chapters: 1) neurons, 2) nervous systems.


The skin is the largest organ in the body. It protects the tissues underneath, prevents invasion by foreign organisms, prevents dehydration, helps regulate body temperature, and contains receptors that provide information about the external environment.


The outer layer, the epidermis, is composed of stratified squamous epithelium. These cells prevent dehydration because they are filled with a waterproof protein called keratin.

The lower layers of the epidermis are basal cells that continuously divide to replace the layers above. The basal layer also contains melanocytes which produce melanin, the pigment that darkens skin and protects from ultraviolet radiation in sunlight.


The dermis lies underneath the epidermis and is composed of dense connective tissue.

Hair of mammals originates in hair follicles, which are embedded in the dermis. A smooth muscle called the arrector pili is attached to the hair follicle. When it contracts, the hair becomes erect. Sebaceous glands secrete an oily substance (sebum) into the follicle to lubricate the hair and skin.

Sweat glands are found in the dermis and subcutaneous layer. They secrete water to the surface, which cools the body when it evaporates.

The dermis contains blood vessels and neurons that control the flow of blood through the skin in order to regulate body temperature. When the body temperature is high, blood flow to these vessels increases. The increased blood flow to the surface helps the body lose excess heat. When the body temperature is low, blood flow to the dermis decreases. Shivering occurs as a mechanism to produce heat when the body temperature is low.

The dermis contains receptors for pressure, touch, temperature, vibration, and pain.

Subcutaneous layer

The subcutaneous layer is the deepest layer and is composed of loose connective tissue. 

This layer is actually not part of the skin. 

Adipose tissue in this layer insulates and stores energy in the form of fat.