lymphoid nodules

Lymphoid tissues and lymphoid organs
Lymphoid tissues is a type of connective tissue characterized by a rich
supply of lymphocytes. It exists free within the regular connective tissue or
is surrounded by capsules, forming the lymphoid organs. Because
lymphocytes have very little cytoplasm, lymphoid tissue stains dark blue in
hematoxylin and eosin stained sections. Lymphoid tissues are basically
made of free cells as a result, they typically have a rich network of reticular
fibers that support the cells, the thymus gland is an exception, its cells are
supported by a reticulum of epithelial cells of endodermic origin.
Lymph nodules
They are found free in connective tissues anywhere in the body or within
lymphoid organs (lymph nodes, spleen, tonsils, but not the thymus), they are
never covered with a capsule. Free lymphoid nodules are commonly present
in the lamina propria of several mucosal linings, where they constitute the
mucosa- associated lymphoid tissue (because the digestive, respiratory, and
genitourinary tracts are common sites of microbial invasions because their
lumens are open to the external environment, and to protect the organism,
the mucosa and submucosa of these tracts contain a large amount of diffuse
collections of lymphocytes.)
Lymph nodules (such the nodules that found as groups in the ileum called
Peyer’s patches ), the nodule is spherical in shape, contains dark cortex
contain small lymphocytes close to each others, and a central region called
the germinal center (germinative center) looks light in color contains
medium sized lymphocytes and reticular cells and few large lymphocytes,
there are more spaces between the cells. Primarily the nodules contains B
lymphocytes, when the lymphoid nodule become activated (due to an
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infection) the lymphocytes proliferate in the central portion (the germinative
center).
Lymph node
The lymph node has oval or bean shape, has a convex surface and the
other surface with little concave called the hilum or hilus. The blood vessels
enter and out from the node through the hilus, the afferent lymphatic vessels
enter the node from the convex surface, and out from the hilus as efferent
lymphatic vessels.
Lymph node covered with capsule composed of connective tissue contains
few smooth muscle cells, trabeculae (or septa)extend from the capsule inside
the node. The node composed of two regions, the outer is the cortex and the
inner is the medulla.
The cortex characterized by the presence of the lymph nodules composed
of dense lymphoid tissue and contain germinative centers. The space
between the capsule and the nodules called subcapsular sinus. The spaces
between trabecula and the nodules called cortical sinuses, the sinuses
composed of reticular connective tissue.
The lymphoid tissue of the medulla composed of cords called medullary
cords separated from the medullary trabeculae by medullary sinuses which
connect with the corticular sinuses.
The Thymus
The thymus surrounded by capsule composed of connective tissue with
trabeculae divide the thymus gland into lobules, each lobule consists of dark
cortex has dense lymphoid tissue and a light medulla composed of loose
lymphoid tissue, in addition to T lymphocytes the medulla contains several
oval or spherical corpuscles called thymic corpuscles or Hassall corpuscles,
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which contain flattened epithelial reticular cells that are arranged
concentrically and are filled with keratin filaments.
The Spleen
This organ covered with a capsule composed of dense connective tissue
contain few smooth muscle cells, the capsule has trabeculae extends inside
the organ and divide it into lobules, the spaces between trabeculae filled with
connective tissue called splenic pulps, which found in two types:
White pulp: which surrounded the arteries that enter the spleen, and splenic
nodules, each splenic nodule contains many germinative center and a central
artery (although its name this artery is eccentric in the nodule).
The nodules of white pulp surrounded by marginal zone consisting of many
blood sinuses and loose lymphoid tissue contain few lymphocyte but many
active macrophages), this zone contains a bundance of blood antigens and
plays a major role in the immunological activities of the spleen.
Red pulp: fills the spaces between the trabeculae and white pulps, it is
looser than the white pulps, contain many venous sinuses. This pulp formed
as loose cords called splenic cords or cords of Billroth, these cords contain a
network of reticular cells supported by reticular fibers. The splenic cords
contain T and B lymphocytes, macrophages, plasma cells, and many blood
cells (erythrocytes, platelets, and granulocytes).
The Tonsils
Tonsils belong to the mucosa-associated lymphoid tissues, but, because
they are incompletely encapsulated, they are considered as lymphoid organs.
They constitute a lymphoid tissue that lies beneath and in contact with the
epithelium of the initial portion of the digestive tract. They are palatine,
pharangeal, and lingual tonsils.
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Two palatine tonsils are located in the lateral walls of the oral part of the
pharynx. They lined with squamous strastified epithelium.
The lymphoid tissue of these tonsils form a band that contains free
lymphocytes and lymphoid nodules generally with germinative centers. Each
tonsil has 10-20 epithelial invaginations that penetrate the tonsil deeply,
forming crypt, whose lumens contain desquamated epithelial cells, live and
dead lymphocytes, bacteria. Crypts may appear as purulent spots in
tonsillitis.
A band of dense connective tissue separating the lymphoid tissue from
subjacent structures, this band called the capsule of the tonsil. This capsule
usually acts as a barriers against spreading infections.
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