Histology of lymphoid organs

Histology of Lymphoid organs :- 

Skin, Spleen, Skin Thymus, Ileum, Lymph Node & Bone Marrow

Skin:-

Type:- Skin is a organ of first line of defence against germ entering from outside the body. It is described as anatomical barriers. It is a secondary Lymphoid organ. 

Anatomy & Morphology :- It is impermeable for pathogens due to epidermis & dermis layer as well as low pH due to sebum secretion from sebaceous gland.

Histology :- Skin is made up of epidermis & dermis.

Epidermis :- It is the uppermost layer & rejuvenates after every 28 days. It consists of 5 different layers. 

a] Stratum basal / germinativum :- single row of cells attach to dermis & youngest cell. 

b] Stratum spinosum /prickle :- Made up of bundles of Proteins that resist tension.

c] Stratum granulosum :- Layer of flattened keratinocytes produces keratin.

d] Stratum lucidum / clear :- Layer present only on palm & soles.

e] Stratum Corneum :- Horny cornified superficial layer.

●Epidermis is made up of variety of cell types :- 

Keratinocytes :- Cells containing keratin, these are stratified Squamous epithelial cells. They arise from basal layer calls. 

Non-keratinocytes :- Includes melanocytes, lymphocytes & Langerhan's cells. Melanocytes synthesized & stores melanin. Langerhan's cells serves as antigen presenting cells.

Dermis :- Below the epidermis, dermis is present. Irregularly arranged & filled mostly with connective tissue. It contains two types of cells :-

i)Permanent cells :- like arrector pilli muscles, Vessels & nerves. 

ii)Migratory cells-Lymphocytes, mast cells, NK cells. 

Spleen : 

Type :- secondary Lymphoid organ.

Location :- Situated in left side of abdominal cavity below Pancreas. 

Morphology :- Bean shaped, red coloured encapsulated organ. It is the largest lymphoid organ with a depression called hilum. Entire spleen is covered by serosa membrane peritoneum except at hilum.

Histology :- 

(1)Beneath Peritoneum, elastic fibrous connective tissue & smooth muscles collectively forms the capsule.

(2) From interior of capsule many trabaculae arises. 

(3) From hilum spleenic vessels (artery & vein) make the entrance & exit. 

(4) The parenchyma contain the (a) White pulp & (b) Red pulp.

a] White pulp:- surrounds branches of spleenic vessels (artery) forming a periarteriole lymphoid sheath (PALS) mainly consisting of T-Lymphocytes. Primary lymphoid follicles are attached. To PALS rich in B cells. Secondary lymphoid follicles present deep interior & contains germinal centre along 'B'cells - developed when antigenic challenges to primary follicles are seen. 

Red pulp :- It Consists of network of sinusoid with macrophages red blood cells & few lymphocytes. It is a site for destruction of old & defective RBC. 

Functions :- (1)Spleen macrophages destroy blood born pathogen via phagocytosis. (2)Within red pulp removal of ruptured defective blood cells & Platelets; storage of Platelets (1/3rd) (3)Haematopoiesis during foetal life.

Fig.(b)L.S. of Spleen

 

Thymus :- Type - It is a primary lymphoid organ. 

Location :- It is located in the thorax immediately below the sternum & above the heart with its upper narrower part extended into the neck, Extends between thyroid gland fourth costal cartilage within superior mediasternum & anterior part of inferior mediasternum. 

Anatomy & Morphology :- In the early stage of development it is entirely a epithelial structure derived from third & fourth pharyngeal pouches. 

        The epithelial structure gradually grows & descends to thorax loosing the connection with the pharyngeal pouches. Progenitor lymphocyte of haematopoietic cells migrate from bone marrow to the thymus through blood stream fot differentiation & maturation into immunocompetant lymphocytes. 

Histology :- The fully developed thymus is encapsulated by connective tissue & becomes a bilobed structure. The number of lobes in thymus is varies in different animals. The Connective tissues of capsule penetrates deep into the lobes dividing each lobe incompletely into many lobes. The strands of Connective tissue in between the lobes are known as "trabacules".

        Each lobules in thymus organized into an outer cortese & inner medulla. Cortex or peripheral part of each lobule is diversely populated with differentiated but immature 'T' cells or thymocytes & the medulla or central portion of lobule is Packed with loosely arranged mature lymphocytes. The hormone thymosine secreted by thymus epithelial Calls might be helping in the differentiation & maturation of the 'T'- lymphocytes.

Ileum :- 

Type- It is a type of secondary lymphoid organ. 

Anatomy & Morphology- last part of small intestine which connects to caecum separately duodenum and jejunum  by ileocecal valve(ICV). Ileum along with jejunum, is suspended inside. mesentry lymphatic vessels & nerve fibres. Ileum has abundant payer's patches unencapsulated lymphoid nodule that contains large number of lymphocytes and cells of immune system.

Histology :- It is made up of four layers, which make up the wall of ileum that are mucosa membrane, Submucosa, external muscularis layer & serosa. 

i] Mucasa membrane :- Formed by three layers. 

(a) single layer of tall cells lining the lumen of organ. Epithelium the innermost layer has 5 distinct type of cells introcytes with microvilli, goblet cells, Paneth cells (release antimicrobial substance like &- defensive & lysozymes) most common in terminal part, microfold cells(take up & transport Ag from lumen to lymphatic cells of lamina Propria) & entero endocrine, cells hormone).

(b) Lamina propria - Composed of loose connective tissue large aggregate of lymphoid tissue called payer's  patches.

(c) Thin layer of smooth muscle called muscularis mucosa. 

ii] Submucosa :- Formed by dense irregular connective tissue caries large blood vessels & nervous component Called submucosal plexus, which is part of enteric nervous system.

iii] External muscular layer :- Formed by a layers of Smooth muscles arranged in circular bundles in inner layer and longitudinal bundles in outer layer. 

iv] Serosa :- Composed of mesothelium, a single layer of flat cells with varying quantities of underlying connective and adipose tissues.

Functions :- 

1]Enzymatic digestion of nutrients. 

2] Absorption of Vit B 12, fats & bile salts. 

3] Immunological Functions. 

Lymph Node:-

Type :- It is a secondary lymphoid organ.

Anatomy & Morphology :-

Location :- Found all over the body including neck, armpits, brain, around the gut & between the lungs. 

Structure :- Lymph node are fully commited to regulating an immune response. They are encapsulated bean shaped structure that includes network of stromal cells. (ie. supportive tissue) packed with lymphocytes, macro Phages & dendritic cells.

Connected to both blood vessels & lymphatic vessels, lymph nodes are the first organised lymphoid Structure to encounters antigens that enter tissue space. 

Histology :- It has three concentric regions cortex, Para cortex & medulla.

Cortex :- It is the outermost layer. It contains lymphocytes (mostly B cells), macrophages, follicular, dendritic cells arranged in follicles.

Para Cortex :- It is present beneath the cortex. Para cortex is populated largely by T. lymphocytes but also contains dendritic cells. Dendritic cells are migrated from surrounding tissue.

Medulla :- It is the innermost layer. It is the site where lymphocyte unit the lymph node through the outgoing lymphocytes. 

Functions :- Lymph node Provides idea microenvironment for encounter between antigens & lymphocytes & protective. Organized cellular & humoral immune response.

Bone Marrow :-

Type :- It is a primary lymphoid organ. 

Location :- It occurs in long bones like femur, tibia, Fibula, etc. 

Structure :- It is soft, spongy, gelatinous tissue. It is highly cellular composed of haematopoietic cells in various stages of development, supportive reticular connective tissue and sinusoid complex, capillaries. It Consists of delicate reticular connective tissue mesh with various cells in between.

Cell type :- Conspicuous in marrow are large adipose cells with large vacuoles & small peripheral cytoplasm are seen. other cells include megakaryocytes, mylerocytes, granulocytes and many erythrocytes. Bone marrow smear- pleuripotent stem cells are capable of differentiating into stem cells for different haematopoietic on the basis of colonies in spleen, thus are called colony forming units or CFU. In adults greatest concentration is found in bone marrow.