This jellyfish, slugs, snails, and other soft-bodied animals

This lesson will discuss various types of connective tissue and bone found in animals. The structure of these connective tissues will also be reviewed.

Osseous Tissue

Most animals need to move and support their bodies either on land or in the ocean with a rigid internal support system.

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With few exceptions – like jellyfish, slugs, snails, and other soft-bodied animals – most animals have some sort of support system usually in the form of a skeleton. Bones, tendons, and ligaments all are classified as connective tissue that serve to support the body, allow movement, and protect internal organs. For the purpose of this lesson, we will primarily focus on human connective tissue; however, a few other species will be mentioned that have a unique anatomy.

Cellular Structure Bones

First, it is important to recognize the two different types of bone tissue: compact bone and spongy bone. Compact bone is the hard outer surface of bones, and spongy bone is found under the layer of compact bone.

In spongy bone, you will find blood vessels and bone marrow especially in the long bones and bones of the pelvis. There are three distinct classes of bone cells. They may sound similar but they have distinctly different functions.

  1. Osteoclasts: The word osteoclasts comes from two Greek words that translate to ‘bone break.’ These cells work to reabsorb and break down bone. These cells are useful in the maintenance and repair of the skeletal system.
  2. Osteoblasts: The word osteoblast comes from two Greek words that translate to ‘bone germinate.’ These cells do the opposite of osteoclasts – they work to build bone tissue.
  3. Osteocytes: The word osteocyte comes from two Greek words that translate to ‘bone cells.’ Osteocytes work to hold bone together and give bones strength.

Connective Tissue

The skeleton is useless if it isn’t held together in a way that also works with muscles. Suppose your bicep was not connected in anyway to you scapula or radius. This would mean that your muscle would have nothing to anchor itself to, and it would be the equivalent of a pulley system where the string was not connected to any kind of anchor.For this reason, we need both tendons and ligaments. What is the difference between the two? Tendons connect muscles to bone, and ligaments connect bones to other bones.

Ligaments are found at the ends of muscles where they have an insertion and origin in two different bones. For example, the biceps has an origin in the scapula and an insertion in the radius. Tendons anchor muscles to bones allowing them to serve as pulleys and levers. Tendons connect bones to other bones like the femur to the tibia and fibula or the humerus to the radius and ulna.A third connective tissue called cartilage is also worth mentioning, and it is found at the ends of long bones and between the vertebra of the spine.

Cartilage serves as a cushion between bones and is coated with a slippery substance called synovial fluid, which allows bones to rub against each other at joints with little to no friction. During the aging process, this cartilage wears thin and many elderly have pain in their joints due to bone on bone rubbing with higher than normal friction.

Comparative Anatomy

If we examine the skeletons of various mammals, birds, reptiles, amphibians and even fish we will notice they all have similar body plans.

Most of them have four limbs (and the same bones within those limbs like with fingers and toes), a vertebral column, a pelvis and a skull. Why are are the skeletons of these animals so similar? All of these animals evolved from a common ancestor with these bones, and although the shape and size of those bones may have changed through evolution, they are still there in the same configuration.Some animals, like sharks and sting rays, are interesting to note as they have evolved a cartilage skeleton not composed of bone.

As a result, the only things left behind from them in the fossil record are teeth. There are also animals, like snakes and whales, which have lost certain limbs over the course of evolution but still retain the pieces of the genes necessary to grow them. If we look closely at the image below we can see that the same bones have been conserved in all of the limbs through the course of evolution, they are just different sizes and shapes.

Comparative anatomy

Lesson Overview

Bones are necessary for support, movement and the protection of internal organs.

Bones are composed of several different types of cells that work to build bone (osteoblasts), break it down and reshape it (osteoclasts) and hold the bone together tightly (osteocytes). Tendons connect muscles to bones, while ligaments connect bones to other bones. Cartilage serves as a cushion for bones as well as a slippery surface for them to move over each other without a lot of friction.

Through the course of evolution, the skeletal system of most animals has remained relatively the same with four limbs, a vertebral structure, a pelvis and a skull.


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