Type and Classification of Bones

Types of Bones:

Recall that bones are composed of living cells called osteocytes and non-living cells (composed mainly of minerals salts and collagen fibres).

In a previous article, we explained the classification of bones based on shape. In case you missed our explanation on bones under the above classification; please read it here.

In this article, we will continue to explain the various types of bone based on their classifications.

• Classification of Bone based on Structure

We already know that all bones are composed of the same materials. These materials include:

1. Osteocytes or Bone Cells
   Please read on Bone and Bone Cells here.
2. Hydroxyapatite (Calcium phosphate)
3. Calcium Carbonate
4. Protein and Collagen fibres

Based on how the contents of a bone is structured, we can classify them into the followings:

1. Compact Bone

A compact bone has more of bone cells together with minerals (hydroxyapatite); and as a result, only little space or cavity is present within it.

Please read on Minerals, their Functions and Deficiencies in Humans here.

Compact bone forms the hard external layer of all bones and will typically surround the medullary cavity. The medullary cavity of a bone houses the bone marrow.

The bone marrow in a compact bone is referred to as the yellow marrow.

The compact bone function to strengthen and protect the inner matrix of the bone.

Compact bone contains osteons; and the haversian systems are present within them.

Note: Osteons are composed of osteocytes and minerals (hydroxyapatite) connected by a canaliculi which transports blood.

Please read on Arteries and its Layers here.

A compact bone is also referred to as a cortical bone.

The diagram below shows the Compact and Spongy bones:

2. Spongy Bone

The structure of a spongy bone is such that empty spaces or cavities are more when compared to the bony tissue; that is osteocytes, hydroxyapatite and fibres.

A spongy bone constitutes the internal layers of bones; unlike compact bone which forms the external layers.

Present within the cavities of the spongy is the red bone marrow. (Please see the diagram above).

The Red Blood Cells (alongside other blood cells) are produced in the red bone marrow of spongy bones.
Please read more on the Red Blood Cells here.

The tissues (group of cells) making up the spongy bone does not contain osteons that constitute a compact bone tissue, rather they are made of trabeculae. For this reason, the spongy is also termed as the trabecular bone.

When we examine the head of long bones (epiphysis), we will observe that within it are spongy bones. Here, they function to reduce the density of the epiphysis, thus allowing for compression when such bone is stretched.

Please read on the Functions of the Human Skeleton here.

Note: A spongy bone can be easily broken and chewed to extract its fluid. In Nigeria, it's fondly referred to as “biscuit bone”.

Another name for spongy bone is Cancellous bone or Trabecular bone.

3. Fibrous Bone

They are found only in fetus and are composed mainly of fibres.

4. Lamellar Bone

You can read on Cartilage and Types of Cartilage here.

• Classification of bone based on Development

We the 3 types of bones with regards to how they are developed; they are:

1. Cartilaginous Bones

Ossification of cartilaginous bone begins from the cartilage.

The bones of the upper limbs, lower limbs and rib cage are all cartilaginous bone.

Please read on the Differences between Bones and Cartilages here.

The upper limb is the region of the body that extends from the arm through the fingers.

The lower limb is the region that extends from the thigh through the toes

2. Membranous Bones or Dermal Bones

These bones are very likely to ossify in a fetus. (Think of a fetus or foetus as a baby still inside the mother's womb).

The process through which they ossify is termed intra-membranous ossification.

Note: The skull and face bones are membranous bones.

Please read on the Face and Skull Bones - with all the 206 Bones that comprises the Human Skeleton here.

3. Membrocartilaginous Bones

Examples are collar bone (clavicle), occipital bone and the mandible.

• Classification of Bone based on Bodily Region

These bones are classified based on whether they are medial (towards the midline) of the body or lateral (away from the midline of the body).

We can categorize them into 2:

1. Bones of the Axial Skeleton

The axial skeleton in humans is found along the midline of their body. They include the following bones:

1. Skull
2. Vertebral column
3. Sternum
4. Ribs

You can read on Cartilage and its Functions here.

The diagram below shows the axial skeleton (colored in yellow) and the appendicular skeleton (colored in green):

Note: The ribs, sternum and the thoracic vertebra form the rib cage or thoracic cage.

2. Bones of the Appendicular Skeleton

The appendicular skeleton is located lateral (away) from the body. They include the:

1. Bones of the Upper Limbs: The upper limb in humans consists of the region of the arm through the fingers.

   The bones of the upper limbs include

   • Humerus
   • Ulna
   • Radius
   • Wrist Bones (or Carpal Bones)
   • Metacarpals
   • Phalanges

   Please read on the Unique Characteristics of Man here.

2. Bones of the Lower Limb: The lower limb in humans consists of the region from the thigh through the toe nails.

   The bones of the lower limbs include:

   • Femur
   • Tibia
   • Fibula
   • Ankle Bones (or Tarsal Bones)
   • Metatarsals
   • Phalanges