LYMPHOID SYSTEM

Consists of cells, tissues and organs

Protects the body against damage by foreign substances

Immuno competent cells in the lymphoid system distinguish between the bodies own molecules and foreign molecules.

The response is immunity.

lymphoid tissues have a: - reticular framework (collagen III) consisting of:  reticular cells , (indistinguishable from fibroblasts) , lymphocytes, macrophages,  antigen presenting cells, plasma cells

Each organ has special features:

Capsulated – spleen, lymph nodes, thymus

Unencapsulated – tonsils,  Peyers patches. lymphoid nodules in: - alimentary canal

- Nodules in: respiratory tract,  urinary tract, reproductive tracts

2 Types of immunity:

- Cellular: Macrophages - destroy foreign cells

- Humeral – immunoglobulins and antibodies (glycoproteins) interact with foreign substances

- cellular and humeral immune system require accessory cells like: macrophages, antigen presenting cells

Thymus

Lymphocytes develop from mesenchym. The lymphocytes then invade an epithelial premordium .The epithelial cells are pushed apart by lymphocytes. Epithelial cells remain connected through desmosomes to form the epithelial reticular cells.  Septae from the capsule divide the thymus up into incomplete lobules (0,5-2 mm ). Each lobule has a cortex which is packed with lymphocytes. In the middle of the lobule is the lighter staining medulla. The cortex and medulla are continuous. Hassall's corpuscles, consisting of flat epithelial cells, lie in the medulla .The corpuscles increase in size and number through life

Thymus cells:

- Cortex and medulla have the same cells – only their proportions differ

- The predominant cell is the T lymphocytes and precursors

- There are also epithelial reticular cells with large oval nuclei. The cells are joined by desmosomes.

- A few mesenchymal reticular cells are also present.

- There are many macrophages.

Cortex:

- Only capillaries (no other vessels)

- small lymphocytes predominate

- here they do not form nodules

- epithelial cells surround groups of lymphocytes and blood vessels

- around the capillary is a space

- forms blood thymus barrier

- Layers of the blood thymus barrier:

- capillary wall endothelium

basal lamina

little CT with macrophages

- epithelial reticular cells - basal lamina

- cytoplasm of epithelial reticular cells

Medulla:

- Stains light because of many epithelial reticular cells

- 5% of thymic lymphocytes found in medulla

- mature lymphocytes - smaller than that of cortex

- leave through venules to populate organs such as the spleen and lymph nodes

- In the medulla the covering of capillaries by epithelial reticular cells is incomplete - no barrier

- Hassall's corpuscles

- 30 - 150µm .

- consists of layers of epithelial reticular cells

- the central part of the corpuscle may only be cell remnants

- unknown function

Lymph nodes

- Encapsulated

- found throughout the body

- form filters in the lymph tracts

- lymph penetrate through afferent lymph vessels on the convex surface

- exit through efferent lymph vessels of the hilum

- capsule send trabeculae into the node to divide it up into incomplete compartments

- reticular tissue provide the super structure

- under the capsule is a cortex – the cortex is absent at the hilum

- At the centre of the node and at the hilum is a medulla

- The cortex has a subcapsular sinus and peritrabecular sinuses

The sinuses:-

- Incompletely lined by reticular cells

- Have numerous macrophages

- fibres cross the sinuses

- they slow the flow of lymph down -

- so that the macrophages can get a chance to perform their function.

Primary and secondary lymphoid nodules

- Some lymphocytes in the cortex form spherical aggregations 0,2-1 mm Ø called primary nodules (or follicles)

- They contain mainly B lymphocytes but some T- lymphocytes are also present

- A germinal centre may develop in the middle of the nodule when an antigen is present. The nodule then becomes a secondary nodule, which is:

- light staining in the centre because:

- many B lymphocytes increase in size to become plasmablasts

- plasmablasts undergo mitosis to become plasmacytes

- plasmacytes migrate to the follicular periphery and then to the medullary cords where they mature

into plasma cells that secrete antibodies into the efferent lymph.

- lymphocytes that don’t differentiate into plasma cells remain small lymphocytes and are called memory

cells – which migrate to different parts of the body

- memory cells are capable of mounting a rapid humoral response on subsequent contact with the same antigen.

- In the nodules there are also follicular dendritic cells which are:

- non phagocytic

- with cytoplasmic extensions

- trap antigens on their surface

- present it to B and T lymphocytes which then respond

Paracortical Zone

- Between adjacent nodules and between the nodules and the medulla are loosely arranged lymphocytes which form the paracortical area or deep cortical area.

- The main cell type in this area is the T lymphocyte.

- They enter the lymph node with the blood and migrate into the paracortical zone.

- T lymphocytes are stimulated when presented with an antigen by the follicular dendritic cells.

- They transform into large lymphobasts which undergo mitosis to produce activated T lymphocytes.

- These activated T lymphocytes must go to the area of antigen stimulation to perform its function.

- When this happens the paracortex expand greatly.

- Later they join the efferent lymph to leave the lymph node.

- These lymphocytes disappear when the thymus is removed - especially if done at birth

The medulla

- Consists of medulla with branching cords separated by medullary sinusses.

- Througout the medulla are trabeculae.

- The cords contain numerous B lymphocytes and plasma cells.

- A few macrophages and T lymphocytes may also be present.

- Receive and circulate lymph from the cortical sinuses.

- Medullary sinuses communicate with efferent lymph vessels.

Spleen

- Largest lymphatic organ

- Many phagocytic cells

- Filters blood

- Form activated lymphocytes which go into the blood

- Form antibodies

General structures:

- Dense CT capsule with a few smooth muscle fibres encapsulate the spleen

- The capsule is thickened at the hilum.

- Trabeculae from the hilum carry blood vessels and nerves in and out of the spleen.

- The capsule divide the spleen into incomplete compartments.

- The spleen has no lymph vessels because it is a blood filter and not a lymph filter like the lymph nodes.

Splenic pulp

- The lymph nodules are called the white pulp

- The white pulp lies in dark red tissue called red pulp

- Red pulp is composed of splenic cords (Billroth cords) which lie between sinusoids

- Reticular tissue forms the superstructure for the spleen and contains:

- reticular cells

- macrophages

Blood circulation

- The splenic artery divide as it enters the hilum

- The arteries in the trabeculae are called trabecular arteries

- The trabecular arteries give of braches into the white pulp (central arteries).

- The artery may not lie in center but is still called a central artery.

- The central arteries give off branches to the white pulp which go through the white pulp to end in the marginal sinuses on the perimeter of the white pulp.

- The central artery continues into the red pulp (called the pulp artery) where it branches into straight arteries called penicilli.

- The penicilli continue as arterial capillaries some of which are sheated by macrophages.

- The blood from the arterial capillaries flow into the red pulp sinuses that lie between the red pulp cords.

- The way the blood gets from the capillaries into the sinuses is uncertain. It can either:

- Flow directly into the sinuses - closed theory

- Or flow through the spaces between the red pulp cord cells and then enter the sinusoid - open theory.

- Presently the open theory is popular.

- From the sinusoids the blood flow into the: - Red pulp veins

- which join the trabecular veins

- to form form the splenic vein

(Trabecular veins form channels without a wall lined by endothelium in the trabeculae.)

White pulp:

- Forms a lymph tissue sheath around the central artery

- The lymphocytes around the central artery is called the periarterial lymphatic sheath (PALS).

- Which contains mainly T lymphocytes

- So the PALS is chracterized by a central artery.

- True nodules may also be present as an extension of the PALS.

- They displace the central artery so that it lies eccentric.

- Nodules normally have a germinal center and consists mainly of B lymphocytes

- Between the red and white pulp there is a marginal zone consisting of:

- Many sinuses and of  loose lymphoid tissue.

- There are few lymphocytes

- many macrophages

- lots of blood antigens which

- play a major role in immunologic activity.

Red Pulp:

- In the fresh state this tissue has a red colour because of the many erythrocytes.

- Red pulp consists of splenic sinusses separated by splenic cords (cords of Billroth).

- Between reticular cells are macrophages, lymphocytes, granulocytes and plasma cells.

- Many of the macrophages are in the process of phagocytosing damaged erythrocytes.

- The splenic sinusoids are special sinusoidal vessels in the following ways:

- It has a dilated large irregular lumen

- Spaces between unusually shaped endothelial cells permit exchange between sinusoids and adjacent tissues. (The endothelial cells are very long arranged parallel to the direction of the vessel)

- The basal lamina of the sinusoid is not continuous but form rings.

Tonsils

- Tonsils are incompletely encapsulated lymphoid tissues

- There are - Palatine tonsils

- pharyngeal tonsils

- lingual tonsils

Palatine Tonsil

- Contains dense lymphoid tissue.

- Covered by stratified squamous non-keratinized epithelium

- with an underlying CT capsule

- Crypts that enter the tissue end blind.

Lingual Tonsil

- Lie on the posterior 1/3 of the tongue.

- Crypts link up with underlying glands that flush them.

- Epithelial covering is the same as that of the palatine tonsil.

The Cheeks

• The cheeks (L. buccae) form the lateral wall of the vestibule of the oral cavity.
• They have essentially the same structure as the lips with which they are continuous.

• The principal muscular component of the cheeks is the buccinator muscle.
• Superficial to the fascia covering this muscle is the buccal fatpad that gives cheeks their rounded contour, especially in infants.

• The lips and cheeks act as a functional unit (e.g. during sucking, blowing, eating, etc.).
• They act as an oral sphincter in pushing food from the vestibule to the oral cavity proper.
• The tongue and buccinator muscle keep the food between the molar teeth during chewing.

Sensory Nerves of the Cheeks

• These are branches of the maxillary and mandibular nerves.
• They supply the skin of the cheeks and the mucous membrane lining the cheeks.

Connective Tissue

Functions of Connective tissue:

→ joins together other tissues

→ supporting framework for the body (bone)

→ fat stores energy

→ blood transports substances

Connective tissue is usually characterized by large amounts of extracellular materials that separate cells from each other, whereas epithelial tissue is mostly cells with very little extracellular material. The extracellular substance of connective tissue consists of protein fibers which are embedded in ground substance containing tissue fluid.

Fibers in connective tissue can be divided into three types:

→ Collagen fibers are the most abundant protein fibers in the body.

→ Elastic fibers are made of elastin and have the ability to recoil to original shape.

→ Reticular fibers are very fine collagen fibers that join connective tissues to other tissues.

Connective tissue cells are named according to their functions:

 → Blast cells produce the matrix of connective tissues

→ Cyte cells maintains the matrix of connective tissues

→ Clast cells breaks down the matrix for remodeling (found in bone)

Structure of the Nasal Septum

• This part bony, part cartilaginous septum divides the chamber of the nose into two narrow nasal cavities.
• The bony part of the septum is usually located in the median plane until age 7; thereafter, it often deviates to one side, usually the right.

• The nasal septum has three main components: (1) the perpendicular plate of the ethmoid bone; (2) the vomer, and (3) the septal cartilage.

• The perpendicular plate, which forms the superior part of the septum, is very thin and descends from the cribiform plate of the ethmoid bone.
• The vomer, which forms the posteroinferior part of the septum, is a thin, flat bone. It articulates with the sphenoid, maxilla and palatine bones.