CRR Histology Lab 6
Lymphatic System
CRR Week 8
Lymphoid tissues are organized accumulations of lymphocytes, which appear at various sites distributed throughout the
body. Wherever lymphocytes accumulate, there may be sites of lymphocyte proliferation called lymph nodules or
lymphoid follicles. Each lymph nodule is characterized by a relatively pale "germinal center" and by a darker "cap".
NOTE: Under the microscope, lymphoid tissue is most readily appreciated at low magnification.
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Slide 40, Peyer's patches in intestine.
Small sites of lymphocyte accumulation, often
including organized lymph nodules, may occur anywhere
along the mucosa of any hollow organ. Particularly large
patches, each with several lymph nodules, are
characteristic of ileum and are called Peyer's patches.
On this slide of three regions of intestine, only one
section includes conspicuous lymphoid tissue. The
accumulated lymphocytes, crowded together into sizeable
masses of small, intensely-basophilic cells, may even be
visible without the microscope.
Slide 44, Appendix.
The appendix might be described as an "intestinal
tonsil", a pocket lined by intestinal epithelium with
abundant lymphoid tissue, including lymph nodules, in
the mucosa. See tonsils (below) for more description.
Slide 92, Thymus.
Superficially, the thymus resembles a gland
subdivided into numerous small lobules. But the thymus
has no ducts and no proper secretory tissue. Its principle
"secretion" is lymphocytes. In an active (i.e., young)
thymus, the cortex of each lobule is densely packed with
lymphocytes while each lobule has a paler medullary core
where cells are dividing. Small patches of peculiar
epithelial tissue ("Hassall's corpuscles") may be visible.
Slides 93 and 94, Tonsils.
Crypts -- Tonsils are lymphoid tissue of the
oropharynx. Tonsilar lymphoid tissue is arranged around
crevices or pockets ("crypts") which are lined by
epithelium. In pharyngeal tonsils, these crevices are lined
by respiratory epithelium (ciliated pseudostratified
columnar). In palatine and lingual tonsils, crypts are lined
by oral epithelium (nonkeratinized stratified squamous).
Lymphocytes wander freely across this epithelium and
typically obscure its appearance.
Lymph nodules -- Lymphocytes are organized into
oval clusters called lymph nodules (or lymphoid follicles).
These are most conspicuous at low power, where the
germinal center of each nodule (where lymphocytes are
proliferating) appears paler than the surrounding mass of
lymphocytes migrating in and out of the germinal centers.
parenchyma, called "red pulp", is rich in blood where
lymphoid tissue and blood are intermixed and served by a
network of vascular sinusoids.
"Recognition characters" for spleen are large size
(compared to other lymphoid tissues), patches of dense
lymphoid tissue ("white pulp") surrounding small arteries,
remaining tissue ("red pulp") with intermixed RBCs and
lymphocytes, and occasional small patches of collagen
(trabeculae).
Slide 96, Lymph Node.
Lymph nodes are small organs scattered throughout
the body at sites you should learn (someday) in gross
anatomy.
Lymph typically enters a lymph node through several
small lymphatic vessels that penetrate the capsule and
open into the cortex. Lymph percolates through the
cortex and medulla and then exits through a larger
lymphatic vessel. Blood vessels serving lymph nodes
may include "high-endothelium venules", with cuboidal
rather than squamous endothelium.
"Recognition characteristics" for lymph nodes include
small size (compared to spleen), oval or kidney-bean
shape, a thin capsule of collagen, and a cortex containing
lymph nodules wrapping part way around a medulla in
which lymphocytes are not organized into lymph nodules.
Slide 97, Reticular tissue.
"Reticular" describes connective tissues, including
spleen and lymph nodes, whose matrix is supported by
reticular fibers. Reticular fibers are one of the several
special forms of collagen (type III collagen). They are
unstained by H&E, but they can be visualized with silver
stains, as in this specimen.
To determine the source of this specimen, compare it
with your slides of spleen and lymph node.
SAQ slides.
At least one (maybe more) of the CRR SAQ slides
should recognizably represent some region(s) of the
lymphatic system. Find any such slide(s), identify the
region(s), and repeat appropriate observation of detailed
features as described above.
Slide 95, Spleen.
In the spleen, loose lymphoid cells and blood cells are
contained within a fibrous connective tissue capsule and
supported by strands of fibrous connective tissue
("trabeculae"). Arterioles entering the spleen are
At your discretion, you may notify an instructor
surrounded by lymphoid tissue (which may contain lymph
for a brief oral evaluation on this material.
nodules) forming the so-called "white pulp". In between
patches of white pulp, the majority of the splenic
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