Bone Classification and Structure

1. Bone Classification by Shape

The adult skeleton is composed of 206 bones. These bones are divided into five categories based on their shapes, as shown in the image below.

A long bone is one that is cylindrical in shape and is longer than it is wide. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers, meaning they move when the muscles attached to them contract.

think about it

Keep in mind that the term describes the shape of a bone, not its size. Long bones come in a variety of lengths and are not always the largest bone when compared to other categories.

A short bone is one that is cube-like in shape, approximately equal in length, width, and thickness. The only short bones in the human skeleton are the bones in the wrists (carpals) and the ankles (tarsals). Short bones provide stability and support as well as some limited motion.

The term “flat bone” is somewhat of a misnomer because although a flat bone is typically thin, it is also often curved. Flat bones serve as points of attachment for muscles and often protect internal organs. Examples include the skull (cranial) bones, the shoulder blades (scapulae), the breastbone (sternum), and the ribs.

A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form inside tendons (the sheaths of tissue that connect bones to muscles) 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. Therefore, the human body usually contains more than 206 bones; however, the bones numbering 207 and beyond are unique to each individual. The kneecaps (patellae, singular = patella) are the only sesamoid bones found in common with every person.

An irregular bone is one that does not have any easily characterized shape and does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. The vertebrae and many bones of the face (facial bones), particularly the ones containing sinuses, are classified as irregular bones.

The table below provides an overview of the bone classifications.

terms to know

Long Bone

A bone that is longer than it is wide and functions as a lever.

Short Bone

A bone that is approximately equal in length, width, and thickness and provides stability, support, and limited movement.

Flat Bone

A bone that is thin, curved, and provides an attachment point for muscles as well as protection.

Sesamoid Bone

A bone that is small, round, and sesame seed-shaped and protects tendons from compressive forces.

Irregular Bone

A bone that has a complex shape and protects internal organs.

2. Gross Anatomy of Bone

The structure of a long bone (as opposed to the other four categories based on shape) allows for the best visualization of all of the parts of a bone. As a whole, a long bone can be divided into two parts: the diaphysis and the epiphysis. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow central region in the diaphysis is called the medullary cavity, which is filled with yellow marrow. The walls of the diaphysis are predominantly composed of compact bone, a dense osseous tissue able to sustain compressive forces.

The wider section at each end of the bone is called the epiphysis (plural, epiphyses), which is predominantly filled with spongy bone, a porous osseous tissue which provides strength along with storage of red bone marrow. In a growing bone, where each epiphysis meets the diaphysis, there is an epiphyseal plate (growth plate), a layer of hyaline cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18–21 years of age), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.

The medullary cavity has a thin connective tissue lining called the endosteum (endo, inside; osteo, bone), where bone growth, repair, and remodeling occur. The outer surface of the bone is covered with a fibrous membrane called the periosteum (peri, around or surrounding). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also are connected to bones by attaching to the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints. In this region, the epiphyses are covered with articular cartilage, a thin layer of cartilage that reduces friction and acts as a shock absorber.

Flat bones, like those of the cranium shown in the image below, consist of a layer of spongy bone (called diploë), lined on either side by a layer of compact bone. The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.

terms to know

Diaphysis

The tubular shaft that runs between the proximal and distal ends of a long bone.

Medullary Cavity

The hollow central region in the diaphysis.

Compact Bone

A dense osseous tissue able to sustain compressive forces.

Epiphysis

The wider section at the proximal and distal ends of a long bone (plural, epiphyses).

Spongy Bone

A porous osseous tissue that provides strength and red bone marrow storage.

Epiphyseal Plate

A layer of hyaline cartilage in a growing long bone where the diaphysis meets the epiphysis.

Epiphyseal Line

The ossified epiphyseal plate in a long bone that has completed its growth.

Endosteum

A thin connective tissue lining the medullary cavity.

Periosteum

A fibrous membrane that covers the superficial surface of a bone except for regions participating in a joint.

2a. Bony Landmarks

key concept

The surface features of bones vary considerably, depending on the function and location in the body. There are two general classes of bony landmarks (also referred to as markings): projections and depressions.

As the name implies, a projection is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape are an indication of the forces exerted through the attachment to the bone. Also implied by the name, a depression in the bone is an area of a bone that sinks below the surface of the bone. This is an opening or groove in the bone that allows other structures (i.e., tendons, ligaments, blood vessels, nerves) to enter the bone. As with the other landmarks, their size and shape reflect the size of the structures that penetrate the bone at these points. Additionally, certain projections and depressions also function together as an articulation, which is where two bone surfaces come together to form a joint (articulus, joint).

The tables below provide an abridged version of the general bony landmarks that you will find on many bones.

terms to know

Projection

An area of a bone that projects above the surface of the bone.

Depression

An area of a bone that projects below the surface of the bone.

Articulation

Where two bones come together to form a joint.

2b. Blood and Nerve Supply

Bone tissue receives nourishment from arteries that pass through the bone matrix. The arteries enter through the nutrient foramen (plural, foramina), which are small openings in the bone. Nutrients diffuse through bone marrow or canals in the bone matrix to reach bone cells. As wastes are generated, they are collected by veins that then pass out of the bone through the same foramina. The epiphysis contains its own series of blood vessels that enter through a separate foramen. In adults, these blood vessel networks merge to share supply between all portions of the bone.

In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and it appears the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.

term to know

Nutrient Foramen

Small openings in the bone where blood vessels enter and exit (plural, foramina).

3. Microanatomy of Bone

Osseous (bone) tissue has a rigid extracellular matrix formed by collagen fibers and inorganic salt crystals with a relatively small number of cells found within it. The calcium salt crystals found in bone are called hydroxyapatite. Its structure incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, within a mix of collagen fibers. The hydroxyapatite crystals give bones their hardness and strength, while the collagen fibers give them flexibility so that they are not brittle.

3a. Bone Cells and Tissue

Although bone cells compose a small amount of bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue:

• Osteogenic cells
• Osteoblasts
• Osteocytes
• Osteoclasts

Osteogenic cells are undifferentiated stem cells that undergo mitosis at high rates, generating new bone cells. They are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. Both of these groups differentiate and develop into osteoblasts on their respective sides of the bone.

When osteogenic cells divide, they form osteoblasts. The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it; as a result, it changes in structure and becomes an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna (plural, lacunae) and is surrounded by bone tissue. Recall that chondrocytes in cartilage are also located in spaces called lacunae. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes do not perform mitosis. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular, canaliculus), which are channels within the bone matrix.

The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone, while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. This process, along with changes in physical stress over time, is responsible for the remodeling of bones over time, causing certain bones—or even parts of bones—to become stronger and weaker than others. The following table reviews the bone cells, their functions, and their locations.

terms to know

Osteogenic Cell

The bone stem cell responsible for production of osteoblast.

Osteoblast

The bone cell responsible for formation of bone matrix.

Osteocyte

The bone cell responsible for maintenance of bone matrix; mature osteoblast.

Lacuna

A small space where chondrocytes are located in a cartilage tissue and osteocytes are located in bone tissue (plural, lacunae).

Canaliculi

Channels in the bone matrix extending between lacunae.

Osteoclast

The bone cell responsible for breakdown of bone matrix.

3b. Compact and Spongy Bone

Recall that compact bone is a dense osseous tissue able to sustain compressive forces. It is the denser, stronger of the two types of bone tissue. It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection.

The repeating microscopic structural unit of compact bone is called an osteon. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular, lamella). Over time, compact bone is naturally turned over—broken down and replaced. Evidence of this can be seen in the spaces between osteons where older bone tissue not part of any visible osteon is located.

Recall that spongy bone, which is also known as cancellous bone, is a porous type of osseous tissue. Spongy bone contains lamellae, osteocytes, and lacunae, but they are not arranged in tightly packed formations as compact bone is. Instead, they form a lattice-like network of matrix spikes called trabeculae (singular, trabecula) (see image below). The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where blood cells are created.

watch

View this video to learn more about bone microanatomy.

IN CONTEXT
Aging and the Skeletal System: Paget’s Disease

Paget’s disease is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more bone is resorbed (broken down) than is laid created. The osteoblasts try to compensate, but the new bone they lay down is weak and brittle and, therefore, prone to fracture.

Paget’s disease usually occurs in adults over age 40. While some people have no symptoms, others experience pain, bone fractures, and bone deformities. Bones of the pelvis, skull, spine, and legs are the most commonly affected. When occurring in the skull, Paget’s disease can cause headaches and hearing loss.

What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus.

Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease.

Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.

terms to know

Osteon

The repeating microscopic structural unit of compact bone.

Lamellae

Concentric rings of calcified matrix that compose an osteon.

Trabeculae

The spikes forming the lattice-like network of spongy bone (singular, trabecula).