The skeletal structure consists of different types of bone, joints, and significant skeleton designs that support, protect, and provide movement to the bodies.

A skeletal system is made of 4 types of bones; these are long, short, flat, sesamoid, and irregular bones. A long bone is cylindrical one, being longer than it is wide. They are found in the arms and legs, as well as in the fingers and toes where they function as levers; they move when muscles contract. A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. They provide stability and support as well as some limited motion.

Flat bones are typically thin and often curved. Examples include the skull bones, the shoulder blades bone, the breastbone, and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs. A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons, where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person.

Bone types: This image shows the different bone classifications, based on shape, that are found in a human skeleton. These are flat bone, sutural bone, short bone, irregular, sesamoid bone, and long bone.

A joint is a connection that occurs between bones in the skeletal system. Joints provide the means for movement. They can be classified based on structure and function.

The structural joints include fibrous, cartilaginous, and synovial, each of which has its subgroups.

Fibrous joints are connected by dense, tough connective tissue that is rich in collagen fibers. These fixed or immovable joints are typically interlocked with irregular edges. Sutures, syndesmoses, and gomphoses are the three type of fibrous joints. Sutures are found only in the skull and possess short fibers of connective tissue that hold the skull bones tightly in place. Syndesmoses-joints in which the bones are connected by a band of connective tissue, allowing for more movement than in a suture. Gomphoses occur between teeth and their sockets; the term refers to the way the tooth fits into the socket like a peg.

Fibrous joints: Image demonstrating the three types of fibrous joints. (a) Sutures (b) Syndesmosis  (c) Gomphosis.

At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage. As the name indicates, at a cartilaginous joint, the adjacent bones are united by cartilage, a robust but flexible type of connective tissue. These types of joints lack a joint cavity and involve bones that are joined together by either hyaline cartilage or fibrocartilage. There are two types of cartilaginous joints: synchondrosis and symphysis.

Synchondrosis-a cartilaginous joint where the bones are joined by hyaline cartilage such as the epiphyseal plate.

Symphysis (“growing together”)-is where the bones are joined by fibrocartilage. The symphysis pubis and intervertebral discs are types of symphysis joints.

Cartilaginous Joints. The image demonstrates a synchondrosis joint with epiphyseal plate indicated (a) and a symphysis joint (b). During development, bones get longer at epiphyseal plates by the conversion of excess cartilage to bone through the process of ossification.

At a synovial joint, the articulating surfaces of the bones are not directly connected but instead come into contact with each other within a joint cavity that is filled with a lubricating fluid. Synovial joints allow for free movement between the bones and are the most common joints of the body.

Synovial Joint: The joint is surrounded by an articular capsule that defines a joint cavity filled with synovial fluid. The articulating surfaces of the bones are covered by a thin layer of articular cartilage.

Consequently, two significant skeleton designs support, protect, and provide movement to the bodies of different types of animals and humans. These are exoskeleton and endoskeleton.

An exoskeleton is an external, hard encasement on the surface of an organism. It provides defence against predators, supports the body, and allows for movement through the contraction of attached muscles.  For example, the shells of crabs and insects are exoskeletons.

Exoskeleton. Muscles attached to the exoskeleton of the Halloween crab (Gecarcinus quadratus) allow it to move.

On the contrary, an endoskeleton consists of hard, mineralized structures located within the soft tissue of organisms. It provides support for the body, protects internal organs, and allows for movement through contraction of muscles attached to the skeleton.

Endoskeletons. The skeletons of humans and horses are examples of endoskeletons.

Practice Questions

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MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Question 118

Biology Question Pack, Vol 2. Question 16

Key Points

• All the bones in the body can be described as long bones or flat bones; however, there are different types of bone; these are long, short, flat, sesamoid, and irregular bones.

• A joint, or an articulation, is a connection that occurs between bones in the skeletal system; the structural classification divides joints into fibrous, cartilaginous, and synovial joints.

• The exoskeleton and endoskeleton support, protect, and provide movement to the bodies of different types of animals.

• An exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through muscles attached on the inside.

• An endoskeleton is an internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles.

Key Terms

epiphyseal plate: a hyaline cartilage plate in the metaphysis, located at each end of a long bone where growth occurs in children and adolescents

hyaline cartilage: is the glass-like but translucent cartilage found on many joint surfaces

synovial cavity: the space found between bone segments and articular capsule

endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage

exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda

skeletal system: all of the bones and joints in the body

contract: when a muscle gets smaller in size

sesamoid bone: a small, round bone shaped like a sesame seed. These bones form in tendons

tendon: fibrous connective tissue that connects muscle to bone

collagen: the main structural protein in various connective tissues

joint: a connection that occurs between bones

sutures: found only in the skull and hold the skull bones tightly in place

syndesmoses: joints in which the bones are connected by a band of connective tissue

gomphoses: occur between teeth and their sockets; the term refers to the way the tooth fits into the socket like a peg

cartilage: rubber-like padding that covers and protects the ends of long bones at the joints

ossification: the process of bone formation

synchondrosis: a cartilaginous joint where the bones are joined by hyaline cartilage

symphysis: where the bones are joined by fibrocartilage.