Dr. Maryam Mohammed 2
nd
stage College of Medicine
Dep. of anatomy & histology
1
Lymphatic system
Diffuse lymphoid tissue is especially prominent in the mucosa of the
gastrointestinal and respiratory systems, it is organized as nonencapsulated
clusters of lymphoid cells or as lymphoid (lymphatic) nodules. Diffuse lymphoid
tissue is collectively called mucosa‐associated lymphoid tissue (MALT).
A. MALT consists of two major types, bronchus‐associated lymphoid tissue
(BALT) and gutassociated lymphoid tissue (GALT) . Both types possess lymphoid
nodules that are isolated from one another, except in the case of Peyer patches.
B. Peyer patches are aggregates of lymphoid nodules found in the ileum, they are
components of the GALT.
C. Lymphoid (lymphatic) nodules are transitory dense spherical accumulations
of lymphocytes (mostly B cells). The dark, peripheral region of nodules (corona)
is composed mainly of small, newly formed lymphocytes. Lymphoid nodules of
the GALT are isolated from the lumina of their respective tracts by microfold (M)
cells, which transfer antigens from the lumen and present them (without
processing them into epitopes) to lymphocytes and macrophages lying in deep
invaginations of their basal cell surfaces. From here, an appropriate immune
response is mounted by lymphoid tissue in the underlying lamina propria.
1. Secondary nodules, formed in response to an antigenic challenge, have a
lightly staining central area called the germinal center, which is composed of B
lymphocytes (lymphoblasts [centroblasts] as well as centrocytes). A darker
region, known as the mantle (corona), is composed of resting B cells that are
being displaced from the germinal center by the newly formed B cells. In
addition to centroblasts and centrocytes, the germinal center houses B memory
cells, plasma cells, migrating dendritic cells, follicular dendritic cells,
macrophages, and reticular cells.
a. Centroblasts do not display surface immunoglobulins (sIgs).
b. Centrocytes that express sIgs against self are forced into apoptosis.
c. Surviving centrocytes become B memory cells or plasma cells.
d. Migrating dendritic cells, derived from the bone marrow.
e. Follicular dendritic cells are resident cells of lymph nodes or lymphoid
nodules.
f. Reticular cells are fibroblast like cells that manufacture reticular fibers (type III
collagen) to form the supporting skeleton of the lymphoid nodule and lymph
node.
2. Primary nodules lack germinal centers and are composed of resting B memory
cells, plasma cells, migrating dendritic cells, follicular dendritic cells,
macrophages, and reticular cells.
Dr. Maryam Mohammed 2
nd
stage College of Medicine
Dep. of anatomy & histology
2
Lymph nodes
A lymph node is a small, encapsulated ovoid to kidney‐shaped structure with
a capsule that sends trabeculae into the substance of the node ,the convex
surface of a lymph node receives afferent lymphatic vessels, whereas the concave
surface (the hilum) is the site where arterioles enter and venules and efferent
lymphatic vessels exit, lymph nodes possess a stroma composed of a supportive
framework rich in reticular fibers.
Function. Lymph nodes filter lymph, maintain and produce T and B cells, and
possess memory cells (especially T memory cells). Antigens delivered to lymph
nodes by APCs are recognized by T cells, and an immune response is initiated.
Structure Lymph nodes.
Lymph nodes are divided into three regions, the outermost cortex, the middle
paracortex, and the innermost medulla .
1‐The cortex of lymph nodes
lies deep to the capsule, from which it is separated by a subcapsular sinus , is
incompletely subdivided into compartments by connective tissue septa derived
from the capsule, contains lymphoid nodules and sinusoids.
(a) Lymphoid nodules are composed mainly of B cells but also of some T cells,
follicular dendritic cells, macrophages, and reticular cells. They may possess a
germinal center.
(b) Sinusoids are endothelium‐lined lymphatic spaces that extend along the
capsule and trabeculae and are known as subcapsular and cortical sinusoids,
respectively.
2‐The paracortex of the lymph node lies between the cortex and the medulla.
(a) It is composed of a nonnodular arrangement of mostly T lymphocytes (the
thymus dependent area of the lymph node).
(b) The paracortex is the region where circulating lymphocytes gain access to
lymph nodes via postcapillary (high endothelial) venules.
3‐The medulla of a lymph node lies deep to the paracortex and cortex, except at
the region of the hilum. It is composed of medullary sinusoids and medullary
cords.
(a) Medullary sinusoids are endothelium‐lined spaces supported by reticular
fibers and reticular cells. They frequently contain macrophages. Medullary
sinusoids receive lymph from the cortical sinuses.
(b) Medullary cords are composed of lymphocytes and plasma cells.
Dr. Maryam Mohammed 2
nd
stage College of Medicine
Dep. of anatomy & histology
3
Thymus
The thymus is derived from both endoderm (epithelial reticular cells) and
mesoderm (lymphocytes). It begins to involute near the time of puberty. A
connective tissue capsule surrounds the thymus, the septa of this capsule divide
the parenchyma into incomplete lobules, each of which contains a cortical and
medullary region. The thymus does not possess lymphoid nodules.
Structure—Thymus
A. The thymic cortex is supplied by arterioles in the septa; these arterioles
provide capillary loops that enter the substance of the cortex. The cortex is the
region in which T‐cell maturation occurs.
(1) Epithelial reticular cells.
(a) These cells originate from endoderm and form a meshwork with interstices
in which T cells are tightly packed.
(b) They possess long processes that surround the thymic cortex, isolating it
from both the connective tissue septa and the medulla.
(d) They manufacture thymosin, serum thymic factor, and thymopoietin,
hormones that function in the transformation of immature T lymphocytes into
immunocompetent T cells.
(2) Thymocytes
(a) Thymocyte plasmalemma possesses Notch‐1 receptors that permit these
cells to respond to cytokines released by epithelial reticular cells to become T
cells. Once committed to the T‐cell lineage, they are known as immature T
lymphocytes and are noted to be present within the thymic cortex in different
stages of differentiation.
(b) Thymocytes are surrounded by processes of epithelial reticular cells .
(c) They migrate toward the medulla as they mature; most T cells die in the
cortex, and the dead cells are phagocytosed by macrophages.
(d) Surviving T cells are naïve, they leave the thymus and are distributed to
secondary lymphoid organs by the vascular system.
(3) Blood–thymus barrier
(a) This barrier exists in the cortex only, making it an immunologically protected
region.
(b) It ensures that antigens escaping from the blood stream do not reach
developing T cells in the thymic cortex.
(c) It consists of the following layers: endothelium of the thymic capillaries and
the associated basal lamina, perivascular connective tissue and cells (e.g.,
pericytes and macrophages), and epithelial reticular cells and their basal
laminae.
Dr. Maryam Mohammed 2
nd
stage College of Medicine
Dep. of anatomy & histology
4
B. Thymic medulla
(1) The thymic medulla is continuous between adjacent lobules and contains
large numbers of epithelial reticular cells and mature T cells, which are loosely
packed, causing the medulla to stain lighter than the cortex .
(2) It also contains whorl‐like accretions of epithelial reticular cells called
Hassall corpuscles (thymic corpuscles), these structures display various stages of
keratinization and increase in number with age,.
(3) Mature T cells exit the thymus via venules and efferent lymphatic vessels
from the thymic medulla. The T cells then migrate to secondary lymphoid
structures.
Spleen
a. A simple squamous epithelium (peritoneum) covers the dense irregular
collagenous connective tissue capsule of the spleen, which sends trabeculae into
the substance of the spleen to form a supportive framework.
b. The spleen is similar to lymph nodes in that it possesses a hilum but differs
from both the thymus and lymph nodes in that it lacks a cortex and medulla. It
further differs from lymph nodes because it has no afferent lymphatic vessels.
c. The spleen is divided into red pulp and white pulp; the latter contains
lymphoid elements, these two regions are separated from each other by the
marginal zone.
Function—Spleen. The spleen filters blood, stores erythrocytes, phagocytoses
damaged and aged erythrocytes, and is a site of proliferation of B and T
lymphocytes and the production of antibodies by plasma cells.
Vascularization of the spleen
Is derived from the splenic artery, which enters the hilum and gives rise to
trabecular arteries.
a. Trabecular arteries leave the trabeculae, become invested by a periarterial
lymphatic sheath (PALS, described later), and are known as central arteries.
b. Central arteries branch but maintain their lymphatic sheath until they leave
the white pulp to form several straight penicillar arteries.
c. Penicillar arteries enter the red pulp. They have three regions: pulp arterioles,
macrophage‐sheathed arterioles, and terminal arterial capillaries. These last
named vessels either drain directly into the splenic sinusoids (closed circulation)
or terminate as open‐ended vessels within the splenic cords of the red pulp
(open circulation).
d. Splenic sinusoids are drained by pulp veins, which are tributaries of the
trabecular veins; these in turn drain into the splenic vein, which exits the spleen
at the hilum.
Dr. Maryam Mohammed 2
nd
stage College of Medicine
Dep. of anatomy & histology
5
Structure—Spleen
a. White pulp of the spleen includes all of the organ’s lymphoid tissue (diffuse
and nodular),such as lymphoid nodules (mostly B cells) and PALS (mostly T
cells) around the central arteries. It also contains macrophages and other APCs.
b. The marginal zone of the spleen
(1) is a sinusoidal region between the red and white pulps at the periphery of
the PALS.
(2) receives blood from capillary loops derived from the central artery and thus
is the first site where blood contacts the splenic parenchyma.
(3) is richly supplied by avidly phagocytic macrophages and other APCs.
(4) is the region where circulating T and B lymphocytes enter the spleen before
becoming segregated to their specific locations within the organ and where inter
digitating dendritic cells .
c. Red pulp of the spleen is composed of an interconnected network of sinusoids
supported by a loose type of reticular tissue (splenic cords).
(1) Sinusoids
(a) are lined by long fusiform endothelial cells separated by relatively large
blood containing intercellular spaces.
(b) have a discontinuous basal lamina underlying the endothelium and
circumferentially arranged ribs of reticular fibrils.
(2) Splenic cords (cords of Billroth) contain plasma cells, stellate reticular cells,
blood cells, and macrophages enmeshed within the spaces of the reticular fiber
network. Processes of the macrophages enter the lumina of the sinusoids
through the spaces between the endothelial cells.
Tonsils
They are aggregates of lymphoid tissue, which sometimes lack a capsule. All
tonsils are in the upper section of the digestive tract, lying beneath but in contact
with the epithelium. Tonsils assist in combating antigens entering via the nasal
and oral epithelia.
1. Palatine tonsils
a. possess crypts, deep invaginations of the stratified squamous epithelium
covering of the tonsils, frequently containing debris.
b. possess primary and secondary (with germinal centers) lymphoid nodules.
c. are separated from subjacent structures by a connective tissue capsule.
2. The pharyngeal tonsil is a single tonsil in the posterior wall of the
nasopharynx.
a. It is covered by a pseudostratified ciliated columnar epithelium.
b. Instead of crypts, it has longitudinal pleats (infoldings).
Dr. Maryam Mohammed 2
nd
stage College of Medicine
Dep. of anatomy & histology
6
3. Lingual tonsil
a. is on the dorsum of the posterior third of the tongue and is covered by a
stratified squamous nonkeratinized epithelium.
b. possesses deep crypts, which frequently contain cellular debris. Ducts