NEET MDS Lessons
Anatomy
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.
Genioglossus Muscle
- Origin: Mental spine of the mandible.
- Insertion: Dorsum of the tongue and hyoid bone.
- Nerve Supply: Hypoglossal nerve (CN XII).
- Arterial Supply: Sublingual and submental arteries.
- Action: Depresses and protrudes the tongue.
The Auditory Ossicles
The Malleus
- Its superior part, the head, lies in the epitympanic recess.
- The head articulates with the incus.
- The neck, lies against the flaccid part of the tympanic membrane.
- The chorda tympani nerve crosses the medial surface of the neck of the malleus.
- The handle of the malleus (L. hammer) is embedded in the tympanic membrane and moves with it.
- The tendon of the tensor tympani muscle inserts into the handle.
The Incus
- Its large body lies in the epitympanic recess where it articulates with the head of the malleus.
- The long process of the incus (L. an anvil) articulates with the stapes.
- The short process is connected by a ligament to the posterior wall of the tympanic cavity.
The Stapes
- The base (footplate) of the stapes (L. a stirrup), the smallest ossicle, fits into the fenestra vestibuli or oval window on the medial wall of the tympanic cavity.
Functions of the Auditory Ossicles
- The auditory ossicles increase the force but decrease the amplitude of the vibrations transmitted from the tympanic membrane.
BONE
A rigid form of CT, Consists of matrix and cells
Matrix contains:
organic component 35% collagen fibres
inorganic salts 65% calcium phosphate (58,5%), calcium carbonate (6,5%)
2 types of bone - spongy (concellous)
compact (dense)
Microscopic elements are the same
Spongy bone consists of bars (trabeculae) which branch and unite to form a meshwork
Spaces are filled with bone marrow
Compact bone appears solid but has microscopic spaces
In long bones the shaft is compact bone
And the ends (epiphysis) consists of spongy bone covered with compact bone
Flat bones consists of 2 plates of compact bone with spongy bone in-between
Periosteum covers the bone
Endosteum lines marrow cavity and spaces
These 2 layers play a role in the nutrition of bone tissue
They constantly supply the bone with new osteoblasts for the repair and growth of bone
Microscopically
The basic structural unit of bone is the Haversian system or osteon
An osteon consists of a central Haversian canal
- In which lies vessels nerves and loose CT
- Around the central canal lies rings of lacunae
- A lacuna is a space in the matrix in which lies the osteocyte
- The lacunae are connected through canaliculi which radiate from the lacunae
- In the canaliculi are the processes of the osteocytes
- The canaliculi link up with one another and also with the Haversian canal
- The processes communicate with one another in the canaliculi through gap junctions
- Between two adjacent rows of lacunae lie the lamellae, 5-7µm thick
- In three dimensions the Haversian systems are cylindrical
- The collagen fibres lie in a spiral in the lamellae
- Perpendicular to the Haversian canals are the Volkman's canals
- They link up with the marrow cavity and the Haversian canals
- Some lamellae do not form part of a Haversian system
- They are the:
- Inner circumferential lamellae - around the marrow cavity
- Outer circumferential lamellae - underneath the outer surface of the bone
- Interstitial lamellae - between the osteons
Endosteum
Lines all cavities like marrow spaces, Haversian- and Volkman's canals
Consists of a single layer of squamous osteoprogenitor cells with a thin reticular CT layer underneath it
Continuous with the inner layer of periosteum
Covers the trabeculae of spongy bone
Cells differentiate into osteoblasts (like the cells of the periosteum)
Periosteum
Formed by tough CT
2 layers
Outer fibrous layer: Thickest, Contains collagen fibres,
Some fibres enter the bone - called Sharpey's fibres
Contains blood vessels.
Also fibrocytes and the other cells found in common CT
Inner cellular layer
Flattened cells (continuous with the endosteum)
Can divide and differentiate into osteoprogenitor cells
spindle shaped
little amount of rough EPR
poorly developed Golgi complex
play a prominent role in bone growth and repair
Osteoblasts
Oval in shape, Have thin processes, Rough EPR in one part of the cell (basophilic)
On the other side is the nucleus, Golgi and the centrioles in the middle, Form matrix
Become trapped in the matrix
Osteocytes
Mature cells, Less basophilic than the osteoblasts, Lie trapped in the lacunae, Their processes lie in the canaliculi, Processes communicate with one another through gap junctions, Substances (nutrients, waste products) are passed on from cell to cell
Osteoclasts
Very large, Multinucleate (up to 50), On inner and outer surface of bone, Lie in depressions on the surface called Howships lacunae, The cell surface facing the bone has short irregular processes
Acidophylic
Has many lysosomes, polyribosomes and rough EPR
Lysosomal enzymes are secreted to digest the bone
Resorbs the organic part of bone
Histogenesis
Two types of bone development.
- intramembranous ossification
- endochondral ossification
In both these types of bone development temporary primary bone is deposited which is soon replaced by secondary bone. Primary bone has more osteocytes and the mineral content is lower.
Geniohyoid Muscle
- Origin: Inferior genial tubercles of the mandible.
- Insertion: Anterior surface of the body of the hyoid bone.
- Nerve Supply: Branch of C1 through the hypoglossal nerve (CN XII).
- Arterial Supply: Sublingual branch of the lingual artery.
- Action: Elevates the hyoid bone and depresses the mandible.
The Layers of the Pharyngeal Wall
- The pharyngeal wall is composed of 5 layers. From internal to external, they are as follows.
- Mucous membrane: this lines the pharynx and is continuous with all chambers with which it communicates.
- Submucosa
- Pharyngobasilar fascia: this is a fibrous layer that is attached to the skull.
- Muscular layer: this is composed of inner longitudinal and outer circular parts.
- Buccopharyngeal fascia: this is a loose connective tissue layer.
- This fascia is continuous with the fascia covering the buccinator and pharyngeal muscle.
- It contains the pharyngeal plexus of nerves and veins.
->The sides and base of the skull are formed partly by these bones.
->Each bone consists of four morphologically distinct parts that fuse during development (squamous, petromastoid, and tympanic parts and the styloid process).
->The flat squamous part is external to the lateral surface of the temporal lobe of the brain.
->The petromastoid part encloses the internal ear and mastoid cells and forms part of the base of the skull.
->The tympanic part contains the bony passage from the auricle (external ear), called the external acoustic meatus. The petromastoid part also forms a portion of the bony wall of the tympanic cavity (middle ear). The meatus and tympanic cavity are concerned with the transmission of sound waves.
->The slender, pointed styloid process of the temporal bone gives attachment to certain ligaments and muscles (e.g., the stylohyoid muscle that elevates the hyoid bone).
->The temporal bone articulates at sutures with the parietal, occipital, sphenoid, and zygomatic bones.
->The zygomatic process of the temporal bone unites with the temporal process of the zygomatic bone to form the zygomatic arch. The zygomatic arches form the widest part of the face.
->The head of the mandible articulates with the mandibular fossa on the inferior surface of the zygomatic process of the temporal bone.
->Anterior to the mandibular fossa is the articular tubercle.
->Because the zygomatic arches are the widest parts of the face and are such prominent facial features, they are commonly fractured and depressed. A fracture of the temporal process of the zygomatic bone would likely involve the lateral wall of the orbit and could injure the eye.