The Skeletal System

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The bones of the skeleton also serve to store fat and as a reservoir for calcium for the body. [3] Fat is stored in the yellow bone marrow. [2] Calcium is stored in the extracellular matrix. New blood cells are formed in the marrow of flat bones.

Where two bones meet at a joint, ligaments hold them together. Ligaments are flexible bands of connective tissue. [3] Similar bands of connective tissues that attach the muscles to bones are the tendons.

Bone Formation

In embryonic development of the human fetus, parts of the skeleton, such as arms and legs, are formed of cartilage. Eventually, the cartilage calcifies into bone. Biologists state that the bones are first "modeled in cartilage" during fetal development before converting to bone.

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As a child grows, cartilage thickens and is gradually replaced with bone. By late adolescence, cartilage is replaced by bone except at the joint surfaces of bones.


Back view illustration of the human skeleton.

Cells

Bone is connective tissue made up of cells in a matrix of mineral salts including calcium phosphate which provides solidity and collagen fibers, which provide strength. In the human skeleton bones have three types of cells -- osteoblasts, osteoclasts, and osteocytes. Together the osteoclasts and osteoblasts are constantly at work "remodeling" the bones. Bones form in two ways. The long bones are first modeled in cartilage, as stated above. In flat bones, such as cranial bones, cells produce bone growth.

Osteoclasts dissolve bone by secreting enzymes that digest the matrix and release the calcium into the blood stream. This resorbing creates tunnels and cavities within the bone. Osteoblasts follow and deposit new matrix material on the surfaces of bones. Once the osteoblasts become surrounded and enclosed by new matrix, they are called osteocytes. Osteocytes remain alive within the small cavities of the bones. The ongoing work of the osteoblasts and osteoclasts constantly replaces the bone matrix. [1, 2, 3]

Back to page 1.

See a list of the bones in the human body.

References
1 Biology, Seventh Edition; Peter H. Raven, George B. Johnson, Jonathan B. Losos, Susan R. Singer; McGraw-Hill, New York, New York; 2005.
2 Essentials in Biology; Sylvia S. Mader; McGraw-Hill, New York, New York; 2007.
3 Life: The Science of Biology, Seventh Edition; William K. Purves, David Sadava, Gordon H. Orians, H. Craig Heller; Sinauer Associates, Sunderland, Massachusetts; 2004.
4 Science 101: Biology; George Ochoa; HarperCollins, New York, New York; 2007.

Fun Fact

All vertebrates except sharks and rays replace the cartilage of the embryo with bone. Sharks and rays have skeletons compose entirely of cartilage and are referred to as cartilaginous fishes. [3]

Can you put a human skeleton back together? Try the interactive exercise at the Web site of the University of California - Berkley's FOSS Human Body.

Test your knowledge of the skeletal system. Play the puzzle Skeletal System 34 Box and solve the Bones word search puzzle with clues.