LYMPHATICS HISTOLOGY, LYMPHATICS HISTOLOGY

LYMPHATICS HISTOLOGY

1. AL4 LYMPHATICS HISTOLOGY
LYMPH NODES
GROSS STRUCTURE HISTOLOGY
-Situated along the lymph vessels, most commonly in axilla, inguinal region, neck,
body cavities and along major blood vessels.
-Flattened, bean shaped structure that consists of frame work of reticular network
of fibers, afferent lymph vessels, efferent lymph vessels, lymphatic sinuses, capsule
and hilum.
-It is subdivided into Cortex & Medulla.
Capsule (C) contains collagen fibers & elastic fibers. It sends in trabeculae (T),
dividing the cortex into irregular compartments.
Cx- Cortex, P- paracortex (deeper part of cortex), M- Medulla, MC – medullary
cords, H- hilum
LYMPHATICS SINUSES
-Consist of a system of channels like spaces through which lymph filter.
-As lymph passes through the sinuses 99% or more of antigens & other debris are
removed by the macrophages that line the sinus.
-Lymph flow: The afferent vessels lead to sub capsular sinuses, then to cortical
sinuses, then to medullary sinuses, and finally to efferent vessels.
CORTEX
-darker in color due to abundant of lymphocytes.
-It consists of – Outer cortex & Inner cortex (paracortex)
-Outer Cortex has nodular (lymphatic nodules) & diffuse lymphatic tissue.
-Primary Lymphatic nodule consists of mainly of B cells, few plasma cells & no
pale center.
-Secondary lymphatic nodule - Germinal (pale) center is present. Develops in
response to antigenic challenge (eg. infection).
-Cells in the germinal centre are larger & possess more cytoplasm & paler nuclei
than do the small lymphocytes.
The cells in GC: lymphoblasts, medium sized lymphocytes, macrophages & T
lymphocytes.
INNER CORTEX (PARACORTEX)
- consists of diffuse lymphatic tissue, majority of cells being, T lymphocytes &
follicular dendritic cells (has pale staining nuclei aka antigen presenting cell).
This part of cortex is also k/a thymic dependent area (populated with T
lymphocytes)
MEDULLA
- is paler, compared to cortex (due to less lymphocytes), consists of (1) medullary
sinuses (space between medullary cords) and irregular (2) medullary cords (cords
of lymphatic tissue) that branch & anastomose. There are a lot plasma cells,
macrophages.

2. SPLEEN
GROSS STRUCTURE HISTOLOGY
- Spleen is found in the left hypochondrium in the abdominal cavity.
- Has a well-developed Capsule with some smooth muscle. Capsule sends in
trabeculae, which divide the parenchyma into incomplete compartments. It has a
hilum through which blood vessels enter.
-It has a framework of reticular fibers. It consists of White pulp & Red Pulp
FUNCTIONS
-Filtration of blood
-Formation of lymphoid cells
-Inactivation of antigens and foreign bodies
-Destruction of erythrocytes (graveyard of rbc)
-Blood storage & hematopoiesis (if necessary).
WHITE PULP
-consists of lymphatic nodules and periarterial sheath. The lymphatic nodules are
also called as Splenic nodules.
-It is associated with an arteriole, which is ensheathed by lymphoid tissue & then it
expands to form lymphatic nodules.
-Arteriole is known as central artery which is an important characteristic feature.
-T lymphocytes are abundant in periarterial sheath & B lymphocytes are abundant
in peripheral part of lymphatic nodules. The germinal centers are also present.
Reticular fibers delimit the white pulp from the red pulp
(PL) forming the marginal zone.
Marginal zone (Mgz) consists of
(1) B lymphocyte
(2) Macrophage
(3) Dendritic cells
(4) Plasma cells.
GC – germinal center
RED PULP
- consists of; Splenic cords & Splenic
sinusoids (venous sinuses (VS)), lined by
endothelial cells (E).
Splenic cords consists of;
Lymphocytes, Macrophages
RBCs, WBCs in the parenchyma (P) or
supporting framework
-Splenic sinusoids have wide lumen (20- 40
micron), made up of elongated, fusiform
endothelial cells that lie parallel to the long axis of the vessel. Slit like spaces are
present between the endothelial cells. Reticular fibers are wrapped around the
sinusoid.
FROM WHITE PULP TO RED PULP
The splenic artery at the hilum, divides in to trabecular arteries-central arteries C
(white pulp arteries)--enveloped by periarterial lymphatic sheath (PALS), then after
leaving white pulp--penicillar arteries (PA)--sheath cappillaries (SC)--non sheath
cappillaries in red pulp(RP)—sinusoids(S)--pulp vein-- trabecular veins--then leaves
the hilum as splenic vein.

3. THYMUS
GROSS STRUCTURE HISTOLOGY
-Is situated in the superior mediastinum, in front of the great vessels of the heart. It
grows until puberty, but later it undergoes degeneration & replaced by fatty tissue.
-It has a thin Capsule that provides septa dividing it into irregular lobules. Each
lobule has Cortex & Medulla.
FUNCTIONS
-Developing T lymphocytes were tested. Those cells that mount reaction against
self-antigen (produced by epithelial reticular cells) were removed and killed.
Therefore 95 %- 98% of developing T cells die in the cortex.
-The surviving T cells are distributed via the medullary blood vessels to the
secondary lymphoid organs.
CORTEX
-is made up of meshes of reticular network,
consisting of reticular fibers.
-The cells are (1) epithelial reticular cells,
lightly stained, with large round or oval
euchromatic nuclei, one or more prominent
nucleoli, the cytoplasm contains bundles of
keratohyaline fibrils.
- (2) Densely packed lymphocytes.
-Cortex (dark) is deeply stained basophilic due
the presence of closely packed T lymphocytes also k/a Thymocyte ).
There is intense proliferations of T lymphocyte during embryonic & prepubertal
development pushes the epithelial reticular cells apart.
- (3) Macrophages are also present.
MEDULLA
-Medulla forms the central region, broad & pale within a lobule.
-Medulla is acidophilic due the presence of less lymphocyte and more epithelial
reticular cells.
-There are other cells such as: antigen presenting cell, plasma cells, mast cell and
eosinophils.
THYMIC CORPUSCLES OR HASSALL’S
CORPUSCLES
Consists of
1. Flattened epithelial reticular cells arranged in
concentric lamellations
2. Centrally located hyalinized mass or
degeneration center (eosinophilic or pink)
derived from degenerated epithelial reticular
cells.
-form the characteristic feature of Thymus.

4. TONSILS
GROSS STRUCTURE HISTOLOGY
-Aggregates of lymphoid nodules associated with oropharynx, lateral & posterior
wall of nasopharynx & tongue.
-Palatine tonsils, Pharyngeal Tonsils, Lingual Tonsils, Tubal Tonsils
-These tonsils together form the Waldeyer’s Ring
FUNCTIONS
Tonsils do not filter lymph. Tonsils contribute to formation of lymphocytes, many of
which migrate through the covering epithelium and appear in the sputum as salivary
corpuscles.
PALATINE TONSILS
-paired, situated on the lateral wall of the oral part of
pharynx.
-Dense lymphoid tissue is present under the stratified
squamous epithelium.
-Lymphatic nodules with germinal centers
-It has deep epithelial invagination or Crypts, whose
lumens contain desquamated epithelial cells,
lymphocytes & bacteria.
-It has a capsule on one side.
LINGUAL TONSILS
-Nodular bulges present at the root of the
tongue (posterior one third).
-It is covered by stratified squamous
epithelium. Lymphatic nodules are present. -
Crypts are deep & branched. Mucous glands
present in the underlying muscles, also drain
into the crypts.
PHARYNGEAL TONSILS
-single, situated on the posterior wall of the
nasopharynx.
-It is covered by ciliated pseudostratified columnar epithelium with goblet cells
-Patches of stratified squamous epithelium may also be seen.
-Lymphatic nodules are present. Crypts are shallow
PEYER’S PATCHES
-Another example of mucosa associated lymphatic tissue is Peyer’s patches
(aggregations of lymphoid follicle LF or lymphatic nodules) which is present in the
mucosa of Ileum.
-Germinal centers contain proliferating B lymphocytes & the areas between germinal
centres are occupied by T lymphocytes.