Spongy Bone Beginnings

What am I looking at?

This is a stained cross section of spongy bone from a human fetus. The spongy bone is blue (1). You can also see the connective tissue between the bones in light blue and gold (2). The outer layers of the spongy bone, in red (3), are where it begins to intermingle with the connective tissue that’s starting to become ossified – that is, harder and less flexible – and that will eventually become condensed bone.

Biology in the background

Bones give our bodies structure and protect our internal organs. But they also contain marrow, which produces a variety of different blood cells. Therefore, bones are not solid all the way through or there would be no place for the marrow to reside. The outer layer of bone, known as condensed bone, is hard and dense to provide the structure and protection the body needs. However, bones’ inner layer, known as spongy bone, is softer and porous – filled with tiny holes – to allow space for the marrow and to give the bones more flexibility than if they were solid all the way through.

The sections of spongy bone in this image are about 50 micrometers across, or roughly two-thirds the width of a human hair.

Technique

This image was created using a form of light microscopy called darkfield illumination.