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There are over 206 bones in the adult human skeleton, a number which varies between individuals and with age – newborn babies have over 300 bones, some of which fuse together into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.

There are four different types of bones where shape is the basis for categorization, flat, long, short and irregular.

Flat bones are like those found in the skull, ribs, sternum and scapula.

Long bones are the type found in the extremities, like the femur or thighbone in the leg or ulna in the arm.

Short bones are found in the wrists and ankles and some of the irregular bones are found in the vertebrae and at the sutures in the skull.

Early in gestation, a fetus has a cartilaginous skeleton, from which the long bones after birth form, in a process called endochondral ossification.

The flat bones of the skull and the clavicles are formed from connective tissue in a process known as intramembranous ossification, and ossification of the mandible occurs in the fibrous membrane covering the outer surfaces of Meckel's cartilages.


At birth, a newborn baby has over 300 bones, whereas an average adult human has 206 bones. (these numbers can vary slightly from individual to individual).

The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column.

When structure is the basis, there are two types of bone tissue.

Compact bone is like its’ name implies, hard/dense and composes the outer layer of all developed bones.

Cancellous, or spongy, bone is contained inside the compact bone and allows the bone to not only be hard but somewhat flexible. Without the cancellous bone our bones would be very brittle.

A typical long bone has a diaphysis or shaft and two epiphyses or ends.

Epiphysis are the growing ends of the body.

All the bones grow from birth until entering adulthood.

Epiphyses control the growth of bones.

When epiphyses are fused after puberty the growth of the person stops.

If the epiphyses are involved in trauma or fractures in growing age then the growth is defective or stunted.

If we were able to watch a bone develop and grow, we’d see that in the embryo the bone is completely cartilaginous and for each bone there is a temporary cartilage model.

Red blood cells and osteoblasts (an osteoblast is a cell that’s responsible for the formation, or ossification, of the bone) gradually change the model to permanent bone and periosteum (a layer of dense connective tissue that covers the bone surface), except at the joints also known as the articular surfaces.

The shaft is covered by a membrane called the periosteum and is what the muscles attach to. The shaft contains osteoblast, bone forming cells of the red bone marrow, and blood vessels that carry nutrients to the bone.

The inside of the shaft or medullary cavity, is lined with a membrane and contains a yellow bone marrow that’s made up primarily of fat cells. The interior of the epiphyses is where red blood cells are produced, called hematopoiesis, in the red bone marrow.

Bone production begins first in the centers of the bone and then in the ends.

Even after the ends are completely formed a thin layer of cartilage remains between the end and the shaft.

As the growth in length continues, thickness is achieved as the osteoblasts deposit new bone tissue in the periosteum.

Sometime between puberty and the 25th year, final bony fusion of all separate parts occurs ending the growth process.

A large part of the bone formed during the growth period is destroyed in the growth process.
The destruction is performed by specialized cells called osteoclasts.

The bone remodelling by osteoclasts continues throught the life.

Bones grow in length by:

A long bone, such as your femur (thigh bone), grows in length at either end in regions called growth plates.

Growth occurs when cartilage cells divide and increase in number in these growth plates.
These new cartilage cells push older, larger cartilage cells towards the middle of a bone. Eventually, these older cartilage cells die and the space they occupied is replaced with bone.

When a bone has reached its full size, its growth plates are converted into bone by adding calcium and other minerals.


Hope this info helps.

Please feel free to ask any further follow up questions. I will be happy to help.
Dr. Sudarshan

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