Lymph Nodes, Spleen, and Thymus Histology

Question 1

Functions of Lymph Tissue

Answer 1

Start with lymphatic vessels (blind ended capillaries) and allow Ag, macromolecules, and excess fluid through and lymph nodes filter the lymph and elicit immune response if something present
Thoracic duct: larger lymph vessel; enter blood circulation at junction between internal jugular and subclavian veins on the left side, and on the right have the right lymphatic duct that enters at junction between
internal jugular and subclavian vein

Macrophages: APC
Dendritic cell: very efficient APC
Reticular cells: are essentially fibroblasts that make reticular fibers (type III collagen)

Question 2

Diffuse Lymphatic Tissue

Answer 2

Disbursed tissue
Lining of columnar in the gut
LCT under epithelium will contain macrophages, eosinophils, etc.
Find under epithelium so if it breaks can have an immune response
Mostly T lymphocytes in lamina propria of gut & respiratory tree

Question 3

Isolated Lymphatic Nodule

Answer 3

MALT: mucosal associated lymphoid tissue with epithelium and LCT underneath it; divided into two categories:
1. GALT: gut associated lymphatic tissue; line the alimentary canal = stomach, small and large intestine, esophagus
2. BALT: bronchus associated lymphoid tissue lining the respiratory tree
Find diffuse lymphatic tissue in UG tract and female productive tract

Question 4

Aggregations of Nodules: Peyer’s Patches

Answer 4

Peyer’s Patches (ileum)
Aggregations of lymphatic nodules are also found in the appendix.
Found in ileum of large intestine, appendix, etc.
Peyer’s patches with lighter germinal center containing lymphocytes undergoing mitosis and dividing and Ag are responding to it
These are not encapsulated

Question 5

Aggregating Nodules: Germinal Centers

Answer 5

Germinal centers (light center) of lymphatic nodules develop after B cells become activated by antigen and begin to proliferate.
Lymphocytic caps contain differentiated B cells, usually B-memory cells.
Connective tissue stained blue. Since CT does not entirely surround the tonsil, this organ is NOT encapsulated.

Some nodules will have germinal centers, and others have lymphocytic caps

Question 6

Encapsulated Lymph Nodes

Answer 6

Lymph - tissue fluid (similar to plasma but usually with a lower protein content than plasma)
Can be locally rich in protein and lipids (e.g., small intestine).
When node is inactive, it’s small. Enlarges when lymphocytes have recognized antigen and are dividing.

Lymph nodes are completely encapsulated with dense irregular CT, and occur at different intervals going up lymphatic vessels, but concentrated in certain areas such as the neck, groin, axilla, and mesenteries

Question 7

Lymph Node Flow

Answer 7

On the convex surface, have lymph vessels that penetrate the capsules = afferent lymphatic vessels; lymph nodes are the only organs of the lymphatic tissue that have afferent vessels
Lymph is flowing through sinuses which merge into efferent vessels and goes onto the next node
Blood comes from arteries and leaves via veins; capillary beds are also present throughout the node

Cortex: nodular cortex and most superficial
Paracortex middle layer
Medulla is innermost layer

CT capsule surrounding node and inserting into it is the trabeculae and along them are lymphatic sinuses/channels (open) and drain into sinuses within the medulla and go to the efferent vessels

Question 8

Lymph Node Structure

Answer 8

99% of antigens are removed from lymph by the time it gets to the medullary sinus, thus, a good filtering mechanism and response.
Cytoplasmic processes of macrophages along with reticular fibers made by reticular cells= forms meshwork that is a filter to trap Ag and whatever else that are coming into the trabeculae and sinuses

Paracortex: contain T lymphocytes; thymus dependent area
Medulla: have different cells that are in cords of cells (B cells, macrophages) and surround the medullary sinuses and go to efferent vessels and all leave via the hilum

Question 9

Active vs. Not Active Nodules

Answer 9

Medullary sinuses carry lymph and lymphocytes.
Medullary cords consist mainly of plasma cells, macrophages, and dendritic cells.
Dark nodules with small lymphocytes are primary nodule (not active)
Light nodules are secondary nodules and are the ones with germinal centers from B cell proliferation (active)
Cords: collection of B cells and macrophages that are in cord like structures
Germinal center are only found in SECONDARY NODULES
Mantle zone: contains small resting B cells around the nodules

Question 10

Lymph Node Capsule and Sinuses

Answer 10

Lymph Node Capsule invaginates as a trabecula
Sinuses contain APCs, macrophages, lymphocytes, lymph, reticular cells, and reticular fibers

Sinuses will slow down flow of fluid with the macrophages and reticular fibers meshwork to pick off particulate matter and Ag

Afferent vessels are present ONLY in lymph nodes

Question 11

Lymphocyte Circulation

Answer 11

Blood coming into lymph node to supply the organs
Only about 10% of lymphocytes are coming from afferent vessels, but most come from blood supply
After blood has made capillary bed, have post capillary venules which have endothelial cells that are unique (instead of flat they are cuboidal cells) called high endothelial cells
AKA most lymphocytes come from blood and not lymph

Question 12

High Endothelial Venules

Answer 12

Direction of Lymph Flow:
Afferent lymphatic vessels to subcapsular sinus to trabecular sinus to medullary sinus to efferent lymphatic vessel
Lymhocytes go to deep cortex or medulla depending on if B or T cells
Endothelial cells have markers/receptor specific for tagging Ag primed lymphocytes (have seen Ag), and those lymphocytes will adhere to endothelial cells and move through via diapedesis (uninjured vessels)

Question 13

Reticular Meshwork

Answer 13

Reticular fibers (Type III collagen) for support

Cells:

1. Reticular cells (fibroblasts)
2. Antigen-presenting cells: dendritic, macrophages
3. Follicular dendritic cells – between B cells in germinal centers; Ag-Ab complexes adhere to them; NOT APCs!

Question 14

Cell Types According to Layer of Lymph Node

Answer 14

Germinal centers: proliferating B cells
Mantle zone: has resting B cells or memory B cells
Medullary sinuses: has plasma cells that are making Ab, but not proliferating
Paracortex: site of T cells

Question 15

Thymus

Answer 15

Invaded by T cell precursors from the bone marrow and then become immunocompetent

Functional at birth and disappears by puberty and replaced by fat

Develop from 3rd brachial pouch from endoderm epithelium

Question 16

Encapsulated Thymus

Answer 16

Cortex: outer layer; white specs are masses of macrophages that are involved in eliminating misprogrammed T cells; contains closely packed developing T cells (thymocytes), macrophages, and epitheliorecticular cells

Septa or trabeculae form incomplete lobules and are connected by internal medulla

Question 17

Cells of the Thymus

Answer 17

Epithelioreticular cells = epithelial cells; form a reticulum or network; do NOT make reticular fibers. Produce thymic hormones and growth factors for the differentiation and education of T-lymphocytes.

T lymphocyte precursors from the bone marrow invade the developing thymus; reside in between the epithelioreticular cells already there

Cells that form framework are epithelial cells from primitive pharynx

With time, epithelial cells will condense to form thymic corpuscle = characteristic of thymus

Question 18

Thymic Medulla

Answer 18

1. mature T cells - large, loosely packed
2. Macrophages
3. Epithelioreticular cells
4. Hassall’s corpuscle: closely packed, concentrically arranged Ep cells may be keratinized; characteristic of medulla

Question 19

T Cell Education

Answer 19

T cell education involves expression and deletion of specific surface cluster of differentiation (CD) antigens.
As T cells mature, they leave the cortex and enter the medulla. Once fully immunocompetent, they leave the medulla and pass into the blood circulation.

T cell precursors from bone marrow enter the thymic medulla via post-capillary venule

Positive selection in cortex: can you recognize antigen?

If yes, pass to medulla for negative selection: cells directed against self-antigen are eliminated

Question 20

Blood-Thymus Barrier

Answer 20

When T cells come from marrow, they are immature and we don’t want them to be exposed to Ag in blood until go through maturation process, so blood thymus barrier is made around blood capillary that enter the thymus and contain simple squamous epithelial with occludens tight junctions with series of macrophages in thymus and epithelialreticular cells (founding fathers of thymus) = all form the barrier

Type of capillary: continuous capillary because tight junction with continuous basement membrane around

Question 21

Spleen Functions

Answer 21

Produces immune response to blood-borne Ags
Filters blood of microorganisms
Destroys old & damaged RBCs
Produces RBCs (early fetus)

Question 22

Spleen Structure

Answer 22

Hilum: entry for Splenic artery, vein, nerves, and lymphatic vessels

Hilum DOES NOT have afferent vessels
Capsule: dense irregular CT
Contains white and red pulp
Marginal center: proliferating lymphocytes from response to Ag

Central artery: gives off branch to white pulp and go into sinuses and drain into splenic vein to leave the spleen
Central arteriole surrounded by periarteriole lymphatic sheath PALS (T cells)

Question 23

Splenic White Pulp Structure

Answer 23

Nodule with germinal center with proliferating cells

Marginal zone with surrounding sinuses that blood enters

Arterioles are surrounded by T cells = PALS
Several nodules with germinal centers along the artery

Mostly involved in immune response

Question 24

Splenic Red Pulp

Answer 24

Splenic cords: reticular cells, macrophages (APC and phagocytosis), dendritic cells (APC), and plasma cells (B cells that are making Ab)

Red pulp: have venous sinuses that blood goes through to pick out blood borne Ag; reticular cells that make up meshwork to trap things

Mostly involved in filtering the blood

Question 25

Splenic Blood Flow

Answer 25

As the blood comes through central artery, which gives off branches to nodule
Marginal sinuses are the first place where Ag in the blood can be exposed to B cells that are in this nodule aka cause an immune response
Once blood can continue on and go onto red pulp for filtering, and central arteriole will be called a penicullar arteriole, which are only found in red pulp and enter sinusoids, and sometimes ensheathed with macrophages; merge with vein and go out through hepatic vein

Question 26

Closed vs. Open Circulation

Answer 26

Closed circulation: continuous vessel with no gaps that goes into splenic sinus and going into vein

Open circulation: capillaries dump blood into cords of macrophages, plasma cells, and macrophages and must find way back to splenic sinus; blood gets dumped into capillary into the red pulp with meshwork of cells and blood will go through that to expose cells to macrophages to ensure filtering process is best

Question 27

Specialized Endothelium Lines Splenic Sinuses

Answer 27

Rod-shaped endothelial cells; long discontinuous basal lamina

Cytoplasmic extensions of macrophages extend between endothelial cells into the lumen of the sinuses (sinusoids) to monitor the passing blood for foreign antigens.
Reticular fibers surround the sinusoid and are mainly perpendicular to the long axis of the endothelial cells.