Thymus, human fetal - 40X
 
Sometimes the thymus can be mistaken for cerebrum or cerebellum because there are distinct layers--outer cortex and inner medulla--in each lobule. On some slides (not the one from which this image was made) the two layers of thymic tissue look just like gray and white matter. Fortunately, not all slides of the thymus are so similar to brain tissue. As you will see below, the thymus has a unique characteristic called Hassall's corpuscles. When you find them, you can be sure that you are looking at the thymus.

Thymus, human fetal - 100X
 
The dark areas around the outside of this image are portions of thymic cortex. The large light area in the center is thymic medulla.









Thymus, human fetal - 400X
 
Hassall's corpuscles are made of layers of flattened cells that are degenerating. No one knows why they exist.











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