Veterinary Histology UFF
Department of Morphology - Biomedic Institute
LaBEc - Laboratory of Cellular and Extracellular Biomorphology
Veterinary Histology Atlas
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Imune System

Lymph node
• It has a capsule and a kidney-like or round aspect
The capsule is formed by dense connective tissue that sends septa into the node, dividing it into incomplete compartments.
• It presents a incurvature called hilum
• The parenchyma is divided into a cortical region, below the capsule, and a medullar region, at the center of the organ and the hilum.
• The region between the cortical and medullar region is called the paracortical region and this is where we find the T lymphocytes


Cortical Region

• Beneath the capsule and around the trabeculae we find the loose lymphoid tissue forming the subcapsular and peritrabecular sinuses
• The lymph runs inside the sub-capsular and peritrabecular sinuses
• The loose lymphoid tissue presents predominantly reticular cells, reticular fibers and fixed macrophages
The rest of the cortical region is formed by dense lymphoid tissue with lymphatic nodules that can present a germinal center (central region that is less stained)
• The dense lymphoid tissue is formed by reticular cells, reticular fibers, fixed macrophages and mostly free cells(lymphocytes).


Lymphatic Nodules
• Formed by reticular cells, reticular fibers, fixed macrophages and free cells.
• The germinal center is formed by lymphoblasts, large, medium and small lymphocytes, and plasmacytes in various developmental stages.


Paracortical Region

• Formed by dense lymphoid tissue that is populated by T lymphocytes


Medullar Region

• Presents medullary cords and medullary sinuses
• The medullary cords are formed by dense lymphoid tissue
• • The medullary sinuses are formed by loose lymphoid tissue


• Presents a dense connective tissue capsule that presents smooth muscle fibers.
From this capsule, septa are sent into the organ dividing it into incomplete compartments.
• The parenchyma is divided into the White Pulp and the Red Pulp.



White Pulp

• Formed by dense lymphatic tissue that forms a supporting net of blood vessels.
• It possesses reticular fibers, mesenchymal reticular cells, lymphoblasts and plasma cells.
• In some points of the parenchyma, a network of blood vessels form nodules that contain, internally, an arteriole called the central arteriole, however it is normally observed in an excentric position in histological sections.
The structure represented by a nodule with an excentric arteriole constitutes the characteristic elements for the histological diagnosis of the organ, the Malpighian corpuscule.
• The dense lymphatic tissue is arranged in cords that are called the Billroth cords.



Red Pulp

• Composed of loose lymphatic tissue that form splenic sinuses that are set around the entire parenchyma surrounding the Malpighian corpuscles and Billroth cords.
• There are also the sinusoid capillaries whose discontinuous walls allow free passage of the blood which will occupy all of the loose lymphatic tissue and therefore in anatomical sections a redish color is observed.
• We also find the penicillate arteries, the reticular fibers and characteristic cells of the loose lymphatic tissue
• It possesses reticular fibers, mesenchymal reticular cells, lymphoblasts, lymphocytes and plasma cells
• The Billroth cords, though formed by dense lymphatic tissue, can also be considered part of the red pulp due to its location.


• It has 2 lobes surrounded by a capsule of dense connective tissue that emits septa dividing the organ into lobules, this septation is incomplete, and therefore the separation of the lobules is irregular.
The lobules present a cortical zone of dense lymphatic tissue and a central zone( medullar) of diffuse lymphatic tissue with mostly young cells (lymphoblasts)
• Lymphoid nodules cannot be found in the thymus, the cortical and medullar region have the same types of cells, however in different amounts, which are: the T lymphocytes in different phases of maturation and reticular epithelial cells.

Cortical Region

• Small lymphocytes predominate.
• Epithelial reticular cells are found in small amounts


Medullar Region

• We find lymphoblasts, young lymphocytes and epithelial reticular cells.
• We also observe epithelial reticular cells in concentric layers, forming the corpuscles of Hassal.
• We can find at the center of the Hassal Corpuscles, cell debris that are many times calcified


Bursa of Fabricius
• It is a dorsal saccular diverticulum of the proctodeum, exclusive to birds.
• Characterized by tall and thick mucous folds (plicae) filled with numerous polyhedral follicles
• Each follicle, which is composed of lymphatic tissue, is divided into a cortex and medulla.
• A layer of undifferentiated epithelial cells occupies the periphery of the medulla that is separated from the cortex by a capillary layer.
The bursa is lined by pseudo-stratified cylindrical epithelium, except at the apex of each follicle where it is lined by simple cylindrical epithelium.




Accessory Gls.
Male Reprod.
Female Reprod.
Birds Histology