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Bone Development During Fetal Development

By Karen Hellesvig-Gaskell ; Updated June 13, 2017

Bone Formation

The skeleton of the fetus begins developing bones about 13 weeks following conception. The bones gradually become harder and muscle tissue begins to develop. At birth, a newborn's body has approximately 300 bones. Over time, these bones grow together to form the 206 bones in adults.

Cartilage to Bone

According to the National Institute of Dental and Craniofacial Research (NIDCR), a human embryo starts off with cartilage that is transformed into bone. Cartilage is a flexible, rubbery substance. The NIDCR says the transition from cartilage to bone is coordinated by an intricate system of proteins called transcription factors. This network ignites the proper genes at the precise time to complete the ossification (bone tissue formation) process. The bones of newborns and young children are not as strong as in adults since they have yet to be ossified. During childhood, the cartilage grows and is gradually replaced by bone with assistance from calcium.

Bone Matter

Bones consist of primarily of collagen, phosphorus, sodium and calcium. Calcium hardens the bones so they are strong enough to endure body weight. Bones contain calcium and will discharge some into the bloodstream when it's required in other areas of the body. The majority of blood cells are produced by the bone marrow that’s located on the interior of the bone. The stem cells contained in bone marrow manufacture red blood cells (provide oxygen to tissues), platelets (assist with blood clotting), and certain kinds of white blood cells (infection fighters). Bones are connected to other bones by ligaments (lengthy, fibrous straps).

Types of Bones

The two types of bones in the human body are compact bone and cancellous bone. Compact bone resembles ivory. It is the strong, smooth and solid exterior portion of the bone. Inside the compact bone are several layers of cancellous bone, which is similar in appearance to a sponge. While cancellous bone is not as durable as compact bone, it is very sturdy.

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