Skeletal formation

A baby’s skeleton begins as fragile membranes and cartilage, but after three months, the membranes and cartilage start turning into bone, providing protection for the internal organs, and a solid framework for the muscles.

Late in the second month of fetal development, a fetus’ skeleton is made up of thin membranes, which are about the thickness of paper tissue, and soft, flexible cartilage, like the kind you find in your ear. Over time, both types of tissue will turn into bone in a process called ossification.

Ossification occurs in two ways...the first is when membranes turn into bone.

If we look at a fetus during the third month, we can see that the membranes on the side and back of the skull are starting to ossify. That means that the bone tissue is slowly growing over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

As the baby’s development is close to birth, you can see the bones of the skull still have gaps between them. These gaps, called fontanelles, allow room for the baby’s brain to grow, and also enable the head to be compressed during delivery.

The fontanelles will remain open until the end of the second year. And even though they’re commonly known as the baby’s soft spot, the fontanelles are actually about the thickness and strength of a piece of canvas. Which kind of makes them a soft, but tough, spot.

The bones of the skull won’t stop growing until a child reaches adulthood. That’s when the joints between the bones, called the sutures, will fuse together.

Now let’s go back once again and watch the second type of ossification when cartilage turns into bone. This time we’ll look at the hand. Most of the bones of the skeleton, like the arms, legs, ribs, fingers, and backbone, start off as cartilage.

We can get a good idea of how cartilage turns into bone by looking at this portion of the hand. Here’s what it looks like on the inside.

From the second month until the end of the third month, remarkable changes take place. Watch the middle of the cartilage: both the inside and the outside turn into bone, or ossifies.

This is how the bones will continue to grow until adulthood---from the middle of the bone outward. That way they can continue to increase in their length and width.

Review Date: 8/23/2023

Reviewed By: LaQuita Martinez, MD, Department of Obstetrics and Gynecology, Emory Johns Creek Hospital, Alpharetta, GA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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