Endoskeleton Definition

Endoskeletons are the structures that support and protect an organism’s internal organs and tissues within the interior of the body.

Endoskeletons can be complex, shaped and function differently depending on the animal’s requirements. Mineralized tissue in the form of bone and cartilage makes up the endoskeleton of most vertebrates. During embryogenesis, the mesoderm forms the ‘true skeleton’.

In the class chondrichthyes (sharks, rays, and chimaeras), the skeleton consists of cartilage, muscle, and connective tissues rather than bone.

A select few invertebrates, such as squid and octopus, and echinoderms such as starfish and sea urchins, have endoskeletons, unlike the majority.

Cnidarians (jellyfish) and porifera (sponges) have an endoskeleton called a hydrostatic skeleton. Instead of bone or cartilage, it consists of a cavity called the coelom, which is filled with gelatinous substance called mesohyl and supported by fluid pressure.

In contrast to exoskeletons, endoskeletons grow in tandem with the rest of the body since they are living tissues. To grow from infancy to adulthood, organisms with exoskeletons must shed their outer skeletons and grow a new one. Endoskeletons do not require this. As a result of being without an exoskeleton during molting, an animal is particularly vulnerable. Exoskeletons are also expensive to grow or acquire in terms of resources,

Despite being lightweight, endoskeletons can support greater body weights than exoskeletons. As a result, vertebrate organisms can grow much larger than insects with external skeletons.


There are two types of bone tissue within the endoskeleton of humans:

The Cortical Bone

The cortical bone—also called the ‘compact bone’— is the dense bone tissue that forms the hard exterior and gives long bones their strength.

Compact bone is formed of a calcified matrix containing very few spaces, although it does contain many small cylindrical columns of only a few millimeters wide called lamellae. These lamellae form the osteon or the haversian system.

Within the osteon is the haversian canal, the central canal which surrounds blood cells and nerves.

Surrounding the haversian canal are the osteocytes, which store the mineral tissue of bones such as calcium. These osteocytes are connected to each other in a network of tiny canals called canaliculi, which allows them to transport minerals, fatty acids and waste and between each other.

The Cancellous Bone

The cancellous bone, also known as trabecular bone or ‘spongy bone’, makes up the interior of the bone structure. Cancellous bone is typically found at the ends of the long bones as well as the rubs, skull, pelvic bones and the vertebrae of the spinal column.

It is a lightweight and porous bone with the tissue arranged into a honeycomb-like matrix with large spaces; these spaces are often filled with blood vessels and bone marrow. The main structure of the cancellous bone is formed of thin rod-like bones called trabeculae.

Functions of the Endoskeleton

Protection and Support

The endoskeleton provides the structural support for the body, enabling its owner to stand up; without it, the body would have no shape.

Although the skeleton does not necessarily prevent damage to outer organs such as the skin, it provides a great deal of protection for the inner organs.

The vertebrate skeleton is formed of two different parts:

The axial skeleton is the ‘inner skeleton’. This is comprised of the skull, the ribcage and the vertebral column. Its main protective function is for the central nervous system and the vital organs such as the lungs, heart, kidneys and liver.

The appendicular skeleton consists of the pelvic girdle, the shoulder blades and arm bones and the legs and feet. This part of the endoskeleton protects and supports the limbs.


Bones, when supported by the function of muscles, deliver the capacity of locomotion (movement). The muscles are attached to the bone via tendons or ligaments.

Since the structure of bones is mostly rigid, movement of the skeleton is made possible by connecting bones called joints. There are several different types of joint, allowing different ranges of movement.

The hip and shoulder have ‘ball and socket’ joints. The ‘ball’ part of the joint is a spherical bone, which fits within the ‘socket’, and can move in almost all directions.

The wrist has a ‘condyloid’ joint. This is similar in structure to the ball and socket, and although it has a wide range of movements, it does not allow the wrist to rotate 360-degrees.

A ‘saddle’ joint is the joint that allows movement in the thumb. This provides the same range of movements as the condyloid joints although cannot bend backwards.

The ‘hinge’ joint is found within the fingers and toes. This allows movement like the hinge of a door—bending in and straightening, although not backwards or sideways. The knee and ankle joints, although hinges, allow a degree of movement when the limb is held in a certain position.

A ‘pivot’ joint allows rotational movement. This joint can be found at the elbow, and at the vertebrae directly under the skull allowing the head to move in a rotation.


The osteocyte cells—star shaped cells that form a network surrounding the haversian canals—are the cells that are responsible for the maintenance of mature bone.

The bone is made up of calcium, phosphorus and other fatty acids, all of which are stored within the osteocytes in the compact bone. When the body is in need of these nutrients, they can be taken from these stores and utilized.


Within the cancellous bone is the flexible tissue called bone marrow. There are two types of bone marrow: yellow marrow and red marrow.

Within the bone marrow, there are special cells called ‘stem cells’. These are unique in that they have the ability to become any other type of cell.

Yellow bone marrow consists primarily of fat, which gives it the yellow color. This fat contains a source of energy that can be used in times of starvation. The yellow marrow contains stem cells called stroma, which can produce fat, cartilage and bone tissue).

Red bone marrow—also called myeloid tissue—contains hemopoietic stem cells, which produce an assortment of different blood cells through haematopoiesis. This system typically produces around 500 billion blood cells per day.

Some of these blood cells are the red blood cells associated with carrying oxygen around the body, while others, such as lymphocytes, are essential for support of the immune system.


The bones of the endoskeleton hold around 99% of the body’s calcium, so they play a key part in the regulation of calcium levels within the body through the process of homeostasis.

When blood calcium levels become too high, the hormone calcitonin is released from the thyroid gland. Calcitonin inhibits the osteoclast cells (those responsible for the break down of bone tissue) within the osteon, and stimulates the osteoblast cells (responsible for the building of bone tissue), thus absorbing calcium to the bone and decreasing the calcium levels in the blood.

When calcium levels are too high, the thyroid gland releases parathyroid hormone, which acts to inhibit osteoblasts and stimulate osteoclasts, as well as reducing the output of calcium from the kidneys and increasing the amount of calcium absorbed by the small intestine, thereby increasing the blood calcium levels.

Related Biology Terms

  • Exoskeleton – A rigid external skeleton, which acts to protect the soft interior of an animal.
  • Cartilage – A smooth malleable tissue with elastic qualities that forms part of the endoskeleton, or the whole endoskeleton in the case of chondricthyes.
  • Hydrostatic Skeleton – The endoskeleton of certain soft-bodied animals, which consists of fluid-filled cavities rather than bone or cartilage.
  • Notochord – The skeletal rod structure made of cartilage, which supports the body in chordate embryos and some adults.


What is an endoskeleton?

An endoskeleton is an internal structural framework found in certain organisms, including humans and other vertebrates. It provides support, protection, and attachment points for muscles, tendons, and ligaments.

How does an endoskeleton differ from an exoskeleton?

An endoskeleton is located inside the body, whereas an exoskeleton is located on the outside. Endoskeletons are made up of bones or cartilage, while exoskeletons are typically made of hard, chitinous material.

What are the advantages of an endoskeleton?

Endoskeletons offer several advantages, such as flexibility, allowing for a wider range of movement compared to organisms with exoskeletons. They also provide protection for internal organs and can grow and adapt as the organism develops.

Which organisms have an endoskeleton?

Endoskeletons are primarily found in vertebrates, including mammals, birds, reptiles, amphibians, and fish. Humans, for example, have an endoskeleton consisting of bones and cartilage.

What is the function of an endoskeleton?

The primary function of an endoskeleton is to provide structural support to the body, allowing for movement and protection of internal organs. It also serves as an attachment point for muscles, enabling locomotion and facilitating bodily functions such as respiration and circulation.

Leave a Comment