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In this lesson, you’ll learn how to classify bones by their shape.

Additionally, you’ll discover the structural makeup of bone and the different bone markings.

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Bone Shape

Bones can be described in many ways, and one of the easiest ways to categorize them is by their shape. There are five main shapes of bones. These are long bones, short bones, flat bones, irregular bones, and sesamoid bones.Long bones are longer than they are wide and are primarily responsible for the structural support of our skeleton. Examples include the femur, or thigh bone, and the bones which make up our arms: the humerus, ulna, and radius.Short bones are as wide as they are long.

These provide support and stability with little movement. Examples of short bones are the tarsals in the foot and carpals in the hand. These are the bones that make up your ankle and wrist, respectively.

Flat bones are just that: flat. They provide surface area for protection or provide a flat surface for muscle to attach to. The best examples of flat bones are our ribs (think about all the organs that your rib cage protects!).Irregular bones are bones that don’t fall squarely into any other category. These bones tend to be oddly shaped. Vertebrae from our spinal column and most of the bones which make up our face are considered irregular bones.

Sesamoid bones are bones imbedded within a tendon. These are found where a tendon passes over a joint (a location where two or more bones connect, such as your elbow or your knee). They protect the tendon and increase the efficiency of the joint.

A great example of a sesamoid bone is the patella, or kneecap.

Types of Bone Tissue

So, now that we know how to classify bones by shape, let’s discuss the different types of osseous, or bone, tissue. There are two types of osseous tissue which form the bones of our skeleton. These are compact, or cortical bone, and spongy, or cancellous bone.Cortical, or compact bone, is what most people think of when you say the word ‘bone.’ This osseous tissue is what forms the outside shell, or cortex, of bones.

Think of this like the skin of an apple. Compact bone makes up about 80% of the human skeleton and is directly responsible for providing structural support for our muscles, protection for our internal organs, and the release of calcium to form new bone and repair damaged bone.Cancellous, or spongy bone, is typically found at the end of long bones.

This is a dense tissue which contains red bone marrow. It is ‘spongy’ because it has a lattice made up of spicules, or tiny needle-shaped pieces of bone, which resembles a sponge. This lattice allows for cancellous bone to have a greater surface area than cortical bone, so it is the location where metabolic activity of bones occurs.

Cancellous bone is spongy and has a lattice structure.
Cancellous Bone

Bone Tissue Layers

The two types of tissue layers that line bones
tendons attach.

The endosteum lines the medullary cavity, or the hollow portion in the middle of a long bone.

Bone Marrow

Bone marrow is found inside our bones, and it is part of the lymphatic system, which helps support our body’s immune system. There are two types of bone marrow, red and yellow. Red bone marrow is the site of hematopoiesis, or red blood cell formation. Platelets and lymphocytes, a type of white blood cell, are also produced in red bone marrow.

In addition to being found at the end of long bones, red bone marrow is primarily found in flat bones, such as the pelvis, sternum, and ribs.Yellow bone marrow is made up of fat, or adipose cells. Yellow marrow is only found in the medullary cavity.

When we’re young, all the bone marrow in our bodies is red bone marrow. As we get older, about half of our bone marrow is converted to yellow bone marrow. However, if we get severely injured or lose a lot of blood, we can temporarily convert yellow bone marrow to red in order to help restore the amount of red blood cells in our body.

Bone Markings

When you look at a bone, you may notice there are many different surfaces; there are bumps, protrusions, and grooves. We call these structures bone markings.A diaphysis is the main or mid-section of a long bone, also known as the shaft of the bone.

It is made up of cortical or compact bone. This is where primary ossification (or turning minerals into bone) occurs.

The diaphysis is the shaft of a long bone and is made up of compact bone.
Diaphysis Image

A fossa is a depression or hollow in bone; ‘fossa’ in Latin means ‘ditch’ or ‘trench.

‘ An example of a fossa is the glenoid cavity on the scapula, or shoulder blade. This concave depression is what attaches the scapula to the head of the humerus, the bone which makes up your upper arm.A trochanter is a protrusion off the femur, or thigh bone. In humans, we have the greater trochanter and the lesser trochanter. The trochanters help form our hips.

The trochanter in the human body

Condyles are found at the ends of bones, typically helping to form a joint. Condyles resemble knuckles. You have a pair of condyles on your femur, which make up the back of your knee. Take a moment and feel the inside of your elbow; feel that bump? That is the medial epicondyle of your humerus.

Lesson Summary

The easiest way to classify bones is by their shape.

There are long, short, flat, irregular, and sesamoid bones. No matter what their shape, bones are made up of compact, or cortical, bone, as well as spongy, or cancellous, bone. Our bones are lined on the outside with a tissue layer called the periosteum and on the inside by a tissue layer called the endosteum. The endosteum lines the medullary cavity, which is where bone marrow is found.All bones contain bone marrow, which is either red or yellow. Red bone marrow is where red blood cells are formed. Yellow bone marrow is made up of fat or adipose cells.

Each bone can have bone markings, or ways to describe the different projections and grooves in the bone. There are several types of bone markings; a few examples of these are the diaphysis, fossa, trochanter, and condyle.

Learning Outcomes

Following this lesson, you will be able to:

  • List the shape classifications of bones
  • Describe the two tissue layers of bones
  • Differentiate between red and yellow bone marrow
  • Explain examples of bone markings, including the diaphysis, a fossa, the trochanters and condyles

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