VM8054 Veterinary Histology

Example: Bursa of Fabricius in Birds

Author: Dr. Thomas Caceci
This low magnification image shows the layout of the bursa. This organ is sometimes called the "cloacal tonsil"; its structure does in fact resemble that of the tonsils of mammals, being a set of blind invaginations, filled with lymphatic cells and overlain by epithelium.

At this magnification it could also easily be mistaken for a mammalian thymus. But the lack of extensive primary lobulation and the scanty CT investment would rule out either of these two "look alikes" if you were trying to identify it on a microscope slide. So would the overlying epithelium, which is a different type than that found on the tonsil. The thymus has no overlying epithelium.

The bursa has deep crypts, which end in blind pockets like those you see here. The stratification of the lobes into a cortex and a medulla isn't nearly so pronounced as it is in many other organs.

Notice the epithelium lying over the crypts: it's a stratified columnar type (it may be pseudostratified in some instance). There are no goblet cells in it. This epithelium is unlike anything found lining the crypts of mammalian lymphatic organs.



At high magnification the nature of the epithelium becomes more even evident. Some texts will report stratified squamous epithelium in this site. Actually, there are species differences, and a transition from stratified squamous on the surface facing the cloaca to the deep crypts.

In this image you can see some nucleated erythrocytes, a dead giveaway that this sample is not from a mammal.



Avian bursa (Turkey); H&E stain, paraffin section, 20x, 100x, and 400x

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