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.

Blood Supply to the Head and Neck

• Most arteries in the anterior cervical triangle arise from the common carotid artery or one of the branches of the external carotid artery.
• Most veins in the anterior cervical triangle are tributaries of the large internal jugular vein.

The Common Carotid Arteries

• The right common carotid artery begins at the bifurcation of the brachiocephalic trunk, posterior to the right sternoclavicular joint.
• The left common carotid artery begins arises from the arch of the aorta and ascends into the neck, posterior to the left sternoclavicular joint.

• Each common carotid artery ascends into the neck within the carotid sheath to the level of the superior border of the thyroid cartilage.
• Here it terminates by dividing into the internal and external carotid arteries. 

The Internal Carotid Artery

• This is the direct continuation of the common carotid artery and it has no branches in the neck.
• It supplies structures inside the skull.
• The internal carotid arteries are two of the four main arteries that supply blood to the brain.

• Each artery arises from the common carotid at the level of the superior border of the thyroid cartilage.
• It then passes superiorly, almost in a vertical plane, to enter the carotid canal in the petrous part of the temporal bone.

• A plexus of sympathetic fibres accompany it.

• During its course through the neck, the internal carotid artery lies on the longus capitis muscle and the sympathetic trunk.
• The vagus nerve (CN X) lies posterolateral to it.

• The internal carotid artery enters the middle cranial fossa beside the dorsum sellae of the sphenoid bone.
• Within the cranial cavity, the internal carotid artery and its branches supply the hypophysis cerebri (pituitary gland), the orbit, and most of the supratentorial part of the brain. 

The External Carotid Arteries 

• This vessel begins at the bifurcation of the common carotid, at the level of the superior border of the thyroid cartilage.
• It supplies structures external to the skull.

• The external carotid artery runs posterosuperiorly to the region between the neck of the mandible and the lobule of the auricle.
• It terminates by dividing into two branches, the maxillary and superficial temporal arteries.
• The stems of most of the six branches of the external carotid artery are in the carotid triangle. 

The Superior Thyroid Artery

• This is the most inferior of the 3 anterior branches of the external carotid.
• It arises close to the origin of the vessel, just inferior to the greater horn of the hyoid.
• The superior thyroid artery runs anteroinferiorly, deep to the infrahyoid muscles and gives off the superior laryngeal artery. This artery pierces the thyrohyoid membrane in company with the internal laryngeal nerve and supplies the larynx.

The Lingual Artery

• This arises from the external carotid artery as it lies on the middle constrictor muscle of the pharynx.
• It arches superoanteriorly, about 5 mm superior to the tip of the greater horn of the hyoid bone, and then passes deep to the hypoglossal nerve, the stylohyoid muscle, and the posterior belly of digastric muscle.

• It disappears deep to the hyoglossus muscle.
• At the anterior border of this muscle, it turns superiorly and ends by becoming the deep lingual artery.

The Facial Artery

• This arises from the carotid artery either, in common with the lingual artery, or immediately superior to it.
• In the neck the facial artery gives off its important tonsillar branch and branches to the palate and submandibular gland.
• The facial artery then passes superiorly under the cover of the digastric and stylohyoid muscles and the angle of the mandible.

• It loops anteriorly and enters a deep groove in the submandibular gland.

• The facial artery hooks around the inferior border of the mandible and enters the face. Here the pulsation of this artery can be felt (anterior to the masseter muscle).

The Ascending Pharyngeal Artery

• This is the 1^st or 2^nd branch of the external carotid artery.
• This small vessel ascends on the pharynx, deep to the internal carotid artery.
• It sends branches to the pharynx, prevertebral muscles, middle ear and meninges.

The Occipital Artery

• This arises from the posterior surface of the external carotid near the level of the facial artery.
• It passes posteriorly along the inferior border of the posterior belly of digastric.
• It ends in the posterior part of the scalp.
• During its course, it is superficial to the internal carotid artery and three cranial nerves (CN IX, CN X and CN XI).

The Posterior Auricular Artery

• This is a small posterior branch of the external carotid artery.
• It arises from it at the superior border of the posterior belly of the digastric muscle.
• It ascends posteriorly to the external acoustic meatus and supplies adjacent muscles, the parotid gland, the facial nerve, structures in the temporal bone, the auricle, and the scalp. 

The Internal Jugular Vein

• This is usually the largest vein in the neck.
• The internal jugular vein drains blood from the brain and superficial parts of the face and neck.
• Its course corresponds to a line drawn from a point immediately inferior to the external acoustic meatus to the medial end of the clavicle.

• This large vein commences at the jugular foramen in the posterior cranial fossa, as the direct continuation of the sigmoid sinus.

• The dilation at its origin is called the superior bulb of the internal jugular vein.
• From here it runs inferiorly through the neck in the carotid sheath.

• The internal jugular vein leaves the anterior triangle of the neck by passing deep to the SCM muscle.
• Posterior to the sternal end of the clavicle, it unites with the subclavian vein to form the brachiocephalic vein.
• Near its termination is the inferior bulb of the jugular vein contains a bicuspid valve similar to that of the subclavian vein.

• The deep cervical lymph nodes lie along the course of the internal jugular vein, mostly lateral and posterior.

Tributaries of the Internal Jugular Vein

• This large vein is joined at its origin by the: inferior petrosal sinus, the facial, lingual, pharyngeal, superior and middle thyroid veins, and often the occipital vein.

The Nasopharynx

• The nasal part of the pharynx has a respiratory function.
• It lies superior to the soft palate and is a posterior extension of the nasal cavity.
• The nose opens into the nasopharynx via to large posterior apertures called choanae.

• The roof and posterior wall of the nasopharynx form a continuous surface that lies inferior to the body of the sphenoid bone and the basilar part of the occipital bone.
• In the mucous membrane of the roof of the posterior wall of the nasopharynx is a collection of lymphoid tissue, known as the pharyngeal tonsil (commonly known as the adenoids).

• The pharyngeal orifice of the auditory tube is on the lateral wall of the nasopharynx, 1 to 1.5 cm posterior to the inferior concha, and level with the superior border of the palate.
• The orifice is directed inferiorly and has a hood-like tubal elevation over it called the torus of the auditory tube or the torus tubarius (L. torus, swelling).

• Extending inferiorly from the torus is a vertical fold of mucous membrane, known as the salpingopharyngeal fold.

• The collection of lymphoid tissue in the submucosa of the pharynx, posterior to the orifice of the auditory tube, is known as the tubal tonsil.

• Posterior to the torus and the salpingopharyngeal fold, there is a slit-like lateral projection of the pharynx called the pharyngeal recess.
• It extends laterally and posteriorly.

The Orbital Vessels

• The orbital contents are supplied chiefly by the ophthalmic artery.
• The infraorbital artery, the continuation of the maxillary, also contributes blood to this region.
• Venous drainage is through the superior orbital fissure to enter the cavernous sinus.

The Ophthalmic Artery

• This artery arises from the internal carotid artery as it emerges from the cavernous sinus.
• It passes through the optic foramen within the dural sheath of the optic nerve and runs anteriorly, close to the superomedial wall of the orbit.

The Central Artery of the Retina

• This is the one of the smallest but most important branches of the ophthalmic artery.
• It arises inferior to the optic nerve until it approaches the eyeball.
• It then pierces the optic nerve and runs within it to emerge through the optic disc.
• The central artery of the retina spreads over the internal surface of the retina and supplies it.

The Ophthalmic Veins

The Superior Ophthalmic Vein

• The superior ophthalmic vein anastomoses with the facial vein.
• It has no valves and blood can flow in either direction.
• It crosses superior to the optic nerve, passes through the superior orbital fissure and ends in the cavernous sinus.

The Inferior Ophthalmic Vein

• This begins as a plexus on the floor of the orbit.
• It communicates with the inferior orbital fissure with the pterygoid plexus, crosses inferior to the optic nerve, and ends in either the superior ophthalmic vein or the cavernous sinus.

Hip

Constitutes the pelvic girdle
United with the vertebral column
Union of three parts that is marked by a cup shaped cavity (acetabulum) Ilium
•    Prominence of the hip
•    Superior border is the crest
•    Anterosuperior spine-projection at the anterior tip of the crest
•    Corresponding projections on the posterior part are the posterosuperior and posteroinferior iliac spines
•    Greater sciatic notch-located beneath the posterior part
•    Most is a smooth concavity (iliac fossa)
•    Posteriorly it is rough and articulates with the sacrum in the formation of the sacroiliac joint

Pubic bone

Anterior part of the innominate bone
Symphysis pubic-joining of the two pubic bones at the midline
Body and two rami 
•    Body forms one fifth of the acetabulum
•    Superior ramis extends from the body to the median plane: superior border forms the pubic crest
•    Inferior ramus extends downward and meets with the ischium
•    Pubic arch is formed by the inferior rami of both pubic hones

Ischium
    Forms the lower and back part of the innominate bone
    Body
•    Forms two fifths of the accrabulum
•    Ischial tuberosiry-supports the body in a sitting position
•    Ramus-passes upward to join the inferior ramus of rhe pubis; known as rhe obturator foramen

Pelvis

Fanned by the right and left hip bones, sacrum, and coccyx

Greater pelvis

o    Bounded by the ilia and lower lumbar vertebrae
o    Gives support to the abdominal viscera

Lesser pelvis

o    Brim of the pelvis corresponds to the sacral promontory
o    Inferior outlet is bounded by the tip of the coccyx, ischial tuberosities, and inferior rami of the pubic bones

Female pelvis

o    Shows adaptations related to functions as a birth canal Wide outlet
o    Angle of the pubic arch is obtuse

Male pelvis

o    Shows adaptations that contribute to power and speed
o    Heart-shaped outlet
o    Angle of the pubic arch is acute

Thigh

Femur-longest and strongest bone of the body
Proximal end has a rounded head that articulates with the acetabulum
Constricted portion-the neck
Greater and lesser trochanters
Slightly arched shaft; is concave posteriorly
o    Linea aspera-strengthened by this prominent ridge
o    Site of attachment for several muscles
Distal end has two condyles separated on the posterior side by the intercondyloid notch

Knee cap

Patella-sesamoid bone
Embedded in the tendon of the quadriceps muscle
Articulates with the femur

Leg

Tibia-medial bone
o    Proximal end has two condyles that articulate with the femur
o    Triangular shaft

    Anterior-shin
    Posterior-soleal line
    Distal-medial malleolus that articulates with the latus to form the ankle joint
Fibula-lateral bone
o    Articulates with the lateral condyle of the tibia but does not enter the knee joint
o    Distal end projects as the lateral malleolus

Ankle, foot, and toes

Adapted for supporting weight but similar in structure to the hand

Talus

o    Occupies the uppennost and central position in the tarsus
o    Distributes the body weight from the tibia above to the other tarsal bones
Calcaneus (heel)-Iocated beneath the talus
Navicular-located in front of the talus on the medial side; articulates with three cuneifonn bones distally
Cuboid-lies along the lateral border of the navicular bone

Metatarsals
o    First, second, and third p1etatarsals lie in front of the three cuneifonn bones
o    (2) Fourth and fifth metatarsals lie in front of the cuboid bone

Phalanges

o    Distal to the metatarsals
o    (2) Two in the great toe; three in each of the other four toes .

Longitudinal arches in the foot (2)

o    Lateral-fonned by the calcaneus, talus, cuboid, and fourth and fifth metatarsal bones
o    Medial-fonned by the calcaneus, talus, navicular, cuneifonn, and first, second, and third metatarsal bones

Transverse arches-formed by the tarsal and metatarsal bones