Lymphatic system | Histology | Anjani Mishra

LYMPHATIC SYSTEM

                               Written By Anjani Mishra

Lymphatic system 

This is a close system of lymphatic vessels and associated lymphoid tissues and cells which conveys tissue fluid(lymph) into the blood. 
The system is also involved in phagocytosis and in raising immune responses. 
The lymphoid system is a part of immune system.

Components of Lymphoid (immune) system

1.  Lymphatic vessels

2.  Lymphoid organs

i) Primary or central lymphoid organs

• Bone marrow
• Thymus

ii)  Secondary or peripheral lymphoid organs

• Lymphnode
• Spleen
• Tonsils
• Peyer's patches of the ileum 

3.  Lymphoid cells (free cells)

·          Lymphocytes;

•       B-Lymphocyte (B cells)
•       T-Lymphocyte (T cells)
•       Natural killer cell (NK cells )

·         Macrophages;

•       Fixed cell 
•       Free cells

     Functions of lymphoid system

• Defense
• Fluid balance
• Fat absorption 

Lymphatic capillaries are absent in;

• Avascular structures (Epidermis, hair, nails, cornea, lens, vitreous body and articular hyaline cartilage)    
• Brain and spinal cord
• Bone marrow

LYMPHNODE

• These are oval or bean shaped nodular brown or reddish black structures of variable sizes ranging from very minute bodies to the size of a lemon.
• Lymph nodes are encapsulated structures, distributed throughout the body and are inserted in the course of lymphatic vessels, so that the lymph must flow through them. 

HISTOLOGY

Structure

1. Capsule: 

•   Lymph nodes are surrounded by a connective tissue capsule, from which a number of  trabeculae extend interior to the organ.
•     These capsules are composed of collagen fiber, few elastic fiber and may be smooth muscle fiber. 
•      The trabeculae are composed of reticular fiber with some smooth muscle fibers.
•     Immediately below the capsule, there is a canal like space, called sub-capsular sinus, that  extends within the cortex at regular intervals as trabeculae forming a cortical sinuses.
•      These sinuses are also called lymphatic sinuses/trabecular sinuses.

(Note:-Trabeculae- Fibrous cord of connective tissue that serves as supporting fibers by forming a septum that extends into an organ from its wall or capsule.

Figure: Lymphnode of sheep, showing subcapular and cortical sinus

1. Capsule,  2. Cortical sinus,  3. Diffuse lymphatic tissue,  7. subcapular sinus, 8. Trabeculae
 

Figure: Lymphnode of cow, showing capsule and cortex 

1.Capsule,  4.Lymphatic nodule,  6.Smooth muscle,  7.Subcapsular sinus

2.  Lobule:

The space between the two connective tissue septa is known as lobule. The lobule has two parts;

• Cortex- the outer peripheral part
•  Medulla- the inner central part

Cortex

It is situated under the capsule. It consists of the following components;

• Lymphatic nodules, with or without germinal centre, composed mainly of B-lymphocytes embedded within the diffused population of other cells.
• Many reticular cells, macrophages and lymphocytes are also present.

Medulla

It is the central part of the lymphnode. It is composed of following components;

• Medullary cords- are branched cord like extensions of lymphoid tissue, that contain primarily B-lymphocyte and often plasma cells and macrophages.
• Medullary sinuses- are dilated spaces separating the medullary cords, and are frequently bridged by reticular cells and fibers.

 Fig: Lymphnode of cow showing large lymphatic nodules

1.Capsule,  6.Lymphatic nodule,  8.Medullary cord,  9.Medullary sinus  

Lymph circulation

Afferent lymph vessels-- crossing the capsule-- pour lymph into sub-capsular space or sinus-- cortical sinus-- medullary sinus-- Efferent Lymph vessels

SPLEEN

Structure

Capsule 

• The spleen is surrounded by a capsule composed of dense connective tissue and trabeculae.
• The capsule has two covering; the outer mesothelial layer(peritoneal covering) and the inner connective tissue layer composed of collagen fiber, reticular fiber, but rich in elastic fiber and smooth muscle layer in deeper parts.
• Trabeculae emerge from the capsule composed of collagen fiber, few elastic fiber and may be smooth muscle fiber, which partially subdivide the parenchyma of spleen into white pulp and red pulp, called splenic pulp.
• There is no cortex and medulla in the spleen.

Figure: Spleen of dog showing capsule and trabeculae

1.Capsule,  2. Cortical sinus,  3. Diffuse lymphatic tissue,  7. subcapular sinus, 8. Trabeculae

White pulp

•  It consists of lymphatic nodules and the periarteriolar lymphoid sheaths.
•  Lymphatic nodules consists mainly of B-lymphocytes. Each nodule has a germinal centre.
• Surrounding the central arterioles are the T-lymphocytes mainly.
• Surrounding the lymphatic nodules, there is a marginal zone consists of lymphocytes, macrophages and blood antigens.

Red pulp

·         Contains splenic cord and venous sinusoids

·         Splenic cord 
      It contain networks of reticular cells or fibers that support T & B lymphocyte, macrophages, plasma cells & many blood cells (erythrocytes, granulocytes, platelets)

Splenic sinusoid
It separate the splenic cords. It is lined by elongated endothelial cells called stave cells, supported by incomplete basal lamina.

Fig: Spleen of dog showing splenic pulp, trabeculae, lymphatic nodule 

1.Capsule,  2.Central artery,  5.Lymphatic nodule,  8.Red pulp,  9.Trabeculae,  11.White pulp


THYMUS
Structure
 1. Capsule:

• The thymus consists of two lobes and is surrounded by a capsule composed of loose areolar connective tissue and trabeculae without muscle fiber.
• From the capsule, the trabeculae enter the gland that divide the parenchyma of each lobe into pyramidal or polygonal lobules.
• These lobules vary in size, depending upon the species, from 5 to 13mm in width.
• A large number of lobules are interconnected in varying degree through continuity of their medullary portion.

Fig: Gross appearance of thymus of ox showing lobules and capsule

 Fig: Thymus of puppy showing capsule, cortex, medulla 

1.Capsule,  2.Cortex,  6.Medulla,  8. Septum

2.  Lobule:

In stained section, the parenchyma of the thymus consists of a well developed darker cortex and a lighter medulla.

• Cortex- the outer peripheral darker zone
•  Medulla- the inner central lighter zone

  Fig: Thymus of chicken showing cortex and medulla(reticular structure)

3.Cortex,  7.Medulla,  11.Reticular structure,  12.Septum 

Cortex

• The cortex is characterized by the presence of large numbers of small lymphocytes like cells called T-lymphocytes or thymocytes.
• A few scattered primitive reticular cells are easily recognized by their large pale nuclei.
• Eosinophils and plasma cells are also present.
• The lymphocytes migrate from the neighbouring mesenchymal tissue into the thymus primordium, pushing the epithelial cells apart.
• The epithelial cells are thus transformed into stellate cells, connected by their processes to form a reticulum.

Medulla

• The medullary substances of the lobules differs from the cortical substance in that the reticular   cells have more cytoplasm, the thymocytes are fewer and show no mitotic figures.
• Eosinophils, myelocytes(immature neutrophil) and plasma cells are normally observed.
• The characteristic features of medulla is the Hassall's corpuscles/bodies.
• The Hassall's corpuscles are spherical or oval structures varying in size ranging from 30 to 100um in diameter.
• Generally the central portion of the corpuscle consists of keratin and non-living hyaline material.
• Their periphery consists of a few layers of concentrically arranged flattened or polygonal epithelial cells.

                                      Fig: Thymus of puppy showing Hassall's corpuscle

2.Cortex,  4.Hassall's corpuscle,  6.Medulla


 TONSIL
Structure

• Tonsils are incompletely encapsulated aggregates of lymphatic organs that form, with solitary lymphatic nodules and diffused lymphatic tissue.
• Depending on their location, tonsils in the mouth and pharynx are called palatine, pharyngeal or lingual tonsils.

Fig: gross appearance of palatine tonsil of ox showing incomplete capsule

Fig: Palatine tonsil of horse showing tonsillar crypt

3. Crypt,  7. Lymphatic nodule,  10. Salivary gland,  11. Stratified squamous epithelium

Palatine tonsils

• These are two in number and is located in the lateral walls of the oral part of the pharynx.

Structure

• Crypts are deep invaginations of  the stratified squamous epithelium covering the tonsils.
• The crypt contain desquamated epithelial cells, live and dead lymphocytes and bacteria.
• The tonsils are separated from underlying tissues by a dense, fibrous capsule.
• The parenchyma of the tonsil is composed of numerous lymphoid nodules, many of which display germinal centres, indicative of B cell formation.

Fig: Palatine tonsil of horse showing tonsillar crypt

3.Crypt,  5.Diffuse lymphatic tissue,  7.Lymphatic nodule,  11.Stratified squamous epithelium

Pharyngeal tonsils

• A single tonsil, located in the posterior wall of the nasopharynx.

Structure

• It is covered by a pseudostratified ciliated columnar epithelium.
• Instead of crypts, the praryngeal tonsil is composed of pleats (infoldings) of mucosa and contains diffuse lymphoid tissue and nodules.

 Lingual tonsils 

• Small and more numerous.
• They are located at the base of the tongue.

Structure

• They are covered by stratified squamous  epithelium. 
• Each lingual tonsil has a single crypt, which contains cellular debris.

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