histology of lymphatic system

Dr Prennie VidieraFr. Muller Homoeopathic Medical College & Hospital

The lymphatic system consists of organs, ducts, and nodes.

It transports a watery clear fluid called LYMPH

distributes immune cells and other factors throughout the body.

When circulating blood reaches the capillaries

part of its fluid

passes into surrounding tissues

as a

Tissue fluid

most of the fluid

re-enters the capillaries at their venous ends

some of it

return to the circulation through a separate system

Lymphatic vessels/ Lymphatics

• The fluid passing through the lymphatics is called lymph.

• The smallest lymphatic vessels are lymphatic capillaries which join together to form larger lymphatic vessels.

• Along the course of lymphatic vessels there are numerous small bean shaped structures called LYMPH NODES

• Usually present in groups

• Lymph from any part of the body passes through one or more lymph nodes before entering the blood stream

• Lymph nodes act as filter removing bacteria and other particulate matter from lymph

• Lymphocytes are added to lymph in these nodes

Each group of lymph nodes has a specific area of drainage.

Two Types

MALT Mucosa Associated Lymphoid Tissue ( non encapsulated)

Solitary Nodules

Aggregated nodules (Peyer’spatches)

Lymphoid nodules in vermiform appendix

DICRETE LYMPHOID ORGANS (Encapsulated)

Thymus

Lymph node

Spleen

Tonsil

• Thymus and the spleen are almost entirely made up of lymphoid tissue

• Prominent aggregations of lymphoid tissues are present in close relation to the lining epithelium of the gut

• Lymphoid tissue is seen in the mucosa of the large intestine, walls of the trachea and larger bronchi and urinary tract

LYMPH

• Lymph is a transudate from blood and contains the same protein as in plasma, but in larger amount

• Contains lymphocytes

• Large molecules of fat absorbed from the intestine

Lymphatic Vessels

Lymph Capillaries

• These begin blindly in tissues where they form a network

• The structure is basically similar to that of blood capillaries but has the greater permeability

• There is an inner lining of endothelium

• Basal lamina is absent or poorly developed

• Pericytes or connective tissue are not present around the capillary

• These are present in most tissues of the body

• Absent in,

– avascular tissue

– splenic pulp

– bone marrow

Larger Lymph Vessels

• The structure is similar to that of veins.

• Tunica intima, Tunica media and Tunica adventitia can be distinguished

• Elastic fibres are prominent and seen in all three layers

• Media and adventitia contains smooth muscles, arranged circularly.

• Numerous valves, similar to those in veins, are present in small and large lymphatic vessels and give lymph vessels a beaded appearance

Lymph Nodes

• Each lymph node consists of,

– Connective tissue framework

– Lymphocytes

• Entire node is bean shaped

• The concavity constituting a hilum through which blood vessels enter and leave the node

• Several lymph vessels enter the node on its convex aspect

• Usually a single lymph vessel leaves the node through its hilum.

• On section,

– The node has an outer zone containing densely packed lymphocytes, stains darkly

• Cortex

• Cortex does not extend into the hilum

• Surrounded by the cortex, there is a lighter staining zone in which lymphocytes are fewer

• Medulla

• Within the cortex,

several surrounded areas called lymphatic follicles or lymphatic nodules.

Each nodule has a paler staining germinal centre surrounded by a zone of densely packed lymphocytes.

• Within the medulla,

the lymphocytes are arranged in the form of branching and anastomosing cords.

The connective tissue framework

The lymph node is covered by a capsule consisting mainly of collagen fibres.

Some elastic fibres and some smooth muscles may be present

A number of septa/trabeculae extend into the node from the capsule and divide the node into the lobules

The hilum is occupied by a mass of dense fibrous tissue

The remaining space within the node is filled by a delicate network of reticular fibres.

Associated with the network there are reticular cells Macrophages

Blood supply of lymph nodes

Arteries enter the lymph node at the hilum

Pass through the medulla & reach cortex

They end in arterioles and capillaries which are arranged as loops that drain into venules

Post capillary venules in lymph nodes are lined by cuboidal endothelium

Called high endothelial venules

Readily allows the passage of lymphocytes between the blood stream and surrounding tissue

These endothelial cells bear receptors that are recognized by circulating lymphocytes.

Contact with these receptors facilitates passage of lymphocytes through the vessel valve

Applied Anatomy

• Inflammation of lymph nodes, lymphadenitis

• Cancer spreads from the primary sites either by growth of malignant cells along lymph vessels

• Largest lymphoid organ

• Present in the upper part of abdominal cavity behind the stomach

• Covered by peritoneum

Blood forming organ in foetal life

Blood destroying organ in post natal life (graveyard of RBC’s)

Filters the blood from blood-borne antigens and micro organisms

COMPONENTS

Connective Tissue Framework

• Capsule

• Trabeculae

• Reticular Stroma (Fibres)

Parenchyma

• White pulp

• Red pulp

Connective Tissue

Framework

• Covers the spleen

• Lies deep to the peritoneum

• Formed by dense collagenous connective tissue and few smooth muscle fibre

Trabeculae

• These are given off from the capsule into the spleen

• thick and robust

• Carry trabecular vessels

Reticular Stroma

• Made of reticular fibres

• Forms a mesh work

• Supports the cells of parenchyma

Parenchyma

• The interior of spleen shows

Rounded white/grey areas surrounded by red matrix White Pulp

Dark red matrix Red Pulp

White Pulp

• Made up of aggregation of lymphoid tissue around a small artery

• This artery is a branch of trabecular artery that leaves the trabeculum and enters the pulp

• On entering the pulp it is surrounded by lymphoid tissue, the periarterial lymphatic sheath (PALS)

• Populated by T lymphocytes, to become central artery or white pulp artery

• Around the course of PALS, there are large collection of B lymphocytes forming lymphatic nodules with germinal centres (white pulp)

• In these nodules the central artery occupies an eccentric position

• The lymphatic nodules (white pulp) are surrounded by an immunologically active zone containing few T lymphocytes

• This functional zone between the white and red pulp is called marginal zone

• The central artery leaves the lymphatic sheath of white pulp and enters the red pulp where it divides to form straight penicillar arterioles

• Some of the penicillar arterioles may show thickening of the wall due to aggregation of macrophages, reticular cells and lymphoid cells Ellipsoids

• The penicilli terminate as arterial capillaries

Red Pulp

• Modified lymphoid tissue

• Infiltrated with all the cells of circulating blood dark red colour

• It is composed of anastomosing splenic cords of Bilroth and broad splenic venous sinuses in between the cords

• The splenic cords consists of spongy network of reticular fibres infiltrated with reticular cells, lymphocytes, macrophages, plasma cells and all elements of the circulating blood

• The splenic venous sinuses are lined by endothelial cells

• Externally the sinuses are encircled by reticular fibres

• Central lymphoid organ

• Present in the superior and anterior mediastinum of thorax

It is bilobed organ

Each lobe has a connective tissue capsule

Connective tissue septa

passes inward

Subdivide the lobe

number of

Lobule

2mm in diameter

has an outer cortex & inner medulla

Epithelial cells (Epitheliocytes)

• Embryologically- derived from endoderm

• The cells join to form sheets that cover the internal surface of the capsule, the surface of the septa, & blood vessels

• Blood-thymus barrier

• The epithelial cells lining deeper in the lobule develop processes that join similar processes of other cells to form a reticulum

Lymphocytes (Thymocytes)

• In the cortex of each lobule the reticulum formed by epithelial cells is densely packed with lymphocytes

• Lymphatic nodules are not present in the normal thymus

• Stem cells formed in bone marrow travel to the thymus

• Here they come to lie in the superficial part of the cortex, & divide repeatedly to form small lymphocytes.

• The medulla also contains lymphocytes

• Less densely packed

• The epithelial reticulum is more obvious

• As lymphocytes divide they pass into the cortex, & into the medulla

• They leave the thymus by passing into blood vessel and lymphatics

Corpuscles of Hassall

• Small rounded structures present in the medulla of the thymus

• Has a central core formed by epithelial cells that have undergone degeneration

• They form a pink stains hyaline mass

• Around this mass there is a wall formed by concentrically arranged epithelial cells

• The central mass also contains degenerating macrophages

Mucosa Associated lymphoid tissue

Digestive system: in the wall of the pharynx - tonsils (palatine, lingual,

pharyngeal)

in the wall of the small intestine - aggregate lymphoid nodules

in the wall of the colon-aggregate lymphoid nodules

in the walls of the appendix

Reproductive system: in the wall of the vagina

PALATINE

TONSILS

• Each palatine tonsil consists of diffused lymphoid tissue in which lymphatic nodules are present

• The lymphoid tissue is covered by stratified squamous epithelium

• This epithelium extends into the substance of the tonsil in the form of several tonsillarcrypts

• Numerous mucous glands open into the crypts

• The lumen of the crypt usually contains some lymphocytes that have crossed through the epithelium

dense lymphatic tissue that is accumulated in the connective tissue underlying the epithelium.

lymphoid nodules and germinal centers

PHARYNGEAL TONSIL

• This is a mass of lymphoid tissue present on the posterior wall of the nasopharynx in the midline.

• Covered by epithelium