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
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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