8: Lymphatic vessels

and lymphoid tissue

nur

Lymphatic vascular system

Functions

return to the blood extracellular fluid (Lymph) from connective tissue spaces.

ensures the return of water, electrolytes and plasma proteins to the blood.

i.e plays a key role in homeostasis of the volume of extracellular fluid.

also returns lymphocytes from the lymph nodes to the blood

transports immunoglobulins (antibodies) from the lymphnodes to the blood.

Lymph capillaries

in most organs close to blood capillaries

(an exception is the CNS, cartilage, bone,

bone marrow, placenta, teeth and thymus)

begin as small blind-ending tubes.

Have numerous valves

Similar in structure to blood capillaries but

thinner

lack a basal lamina

lack erythrocytes

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Lymph vessels drain into lymph nodes (for

filtration)

ultimately drain into two large lymphatic

ducts in the area at the base of the neck

thoracic duct,

right lymphatic duct)

to return the lymph to the blood.

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Lymphatic tissues

characterized by an abundance of lymphocytes

no lymphatic tissue in myeloid tissue

involved with lymphocyte production and

immune responses.

constant recirculation of lymphocytes

(especially T-lymphocytes) between the

lymph and blood.

When they encounter foreign molecules or cells

they activate a series of immune responses to

neutralize and destroy the invader.

TYPES OF LYMPHATIC TISSUE

3 basic types:

Loose lymphatic tissue

dominated by reticular cells and fibers

forms a loose, spongy network through which lymph

flow is slow and cells are temporarily trapped

Dense lymphatic tissue

dominated by large populations of closely aggregated

lymphocytes

Nodular lymphatic tissue (lymphatic nodules)

found in all the lymphatic organs apart from the

thymus.

Thymus

site where T-lymphocytes develop

Located in the anterior neck

First lymphoid tissue in embryonic dev’t

from epithelial outgrowths of primitive

pharynx

Lobulated and encapsulated

2 zones: cortex and medulla – density of

lymphocytes

Low mag of the thymus

Cortex

(C)

Medull

a

(M) C

C

M

M

Dark nuclei

= T-cells

Larger

nuclei =

endothelial

reticular

cells

Arrow =

Hassell’s

Corpuscle

High mag of the medulla of the thymus

Lymph Nodes

kidney-shaped structures situated along

lymphatic vessels

2 key functions

serve as in-line mechanical filters of lymph by

macrophages

Storage of B and T-lymphocytes

Encapsulated

capsule forms radial extensions: trabeculae - to

form incomplete compartments

lymph node is divided into two regions:

An outer cortical region

An inner medullary region

2 key functions of lypmhnodes

Filtration of lymph by the macrophages

Storage of B and T-lymphocytes

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Diagram of a lymph node

Low mag of a lymph node

Cx = cortex w/ lymphatic nodules (F); M = medulla; C = CT capsule

A lymphatic nodule with germinal center (GC)

Non-encapsulated Lymphatic

Nodules

temporary structures, which may appear

and disappear in the same site

lack a connective tissue capsule

mainly composed of dense aggregates of

small B-lymphocytes

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Structure cells at the center of the nodule are much

larger, with more cytoplasm

less-stained central area is known as the

germinal center - develops according to

the functional state of the nodule

ring of small B-lymphocytes surrounding

the germinal center is referred to as the

corona or mantle zone

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Cell types of the germinal center include

activated B-lymphocytes (B-immunoblasts)

mitotic cells

plasma cells

dendritic reticular cells

macrophages

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Peyers Patches = lymphatic nodules in the ileum

Low mag of a palatine or lingual tonsil

Palatine Tonsil w/ SS

non-K (arrow)

Pharyngeal Tonsil w/ Resp

epith (arrow = nodule)

The Spleen

largest lymphatic organ of the circulatory system

acts as a blood filter to defend against

microorganisms that succeeded to penetrate the

blood system

aged erythrocytes are broken down and destroyed by

macrophages

site of formation of activated lymphocytes

responds promptly to antigens carried in the

blood. Infection can result in an enlarged spleen

(splenomegaly)

fetal spleen (months 4-6) is hematopoietic

(producing granulocytes and erythrocytes)

Structure

dense connective tissue capsule, with

connective tissue trabeculae

White pulp

lymphatic tissue surrounding arteries and

nodules

Red pulp

sponge composed of cords of cells (splenic

cords) and splenic sinusoids (venous

sinusoids)

Splenic cords (Billroth cords) are

composed of :

reticular cells and fibers

fixed and wandering macrophages

lymphocytes

plasma cells

blood cells (erythrocytes, granulocytes) and

platelets

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Diagram of the Spleen

White pulp

(lymphatic nodules)

Thick CT capsule

CT Trabeculae

Red Pulp

CT

Trabeculae

Low magnification of the Spleen

CT Trabeculae

Lymphatic

nodules

White pulp

Red

Pulp

Splenic circulation

Splenic artery

enters at the hilum and divides into trabecular

arteries

Trabecular arteries

completely surrounded by a sheath of T-

lymphocytes (periarterial lymphatic sheath or

PALS)

Central arteries (or central arterioles)

penetrate the lymphatic nodules, usually in

the periphery

Lead to sinusoids

Circulatory System in the Spleen

White Pulp

White Pulp

Red Pulp

Red Pulp

High mag of the splenic red pulp

Break

1. In the next session:

Gastro-intestinal system and

Respiratory system

2. Review : lymphatic system nur