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.
EPITHELIUMS
Epithelial Tissue Epithelial tissue covers surfaces, usually has a basement membrane, has little extracellular material, and has no blood vessels. A basement membrane attaches the epithelial cells to underlying tissues. Most epithelia have a free surface, which is not in contact with other cells. Epithelia are classified according to the number of cell layers and the shape of the cells.
- Epitheliums contain no blood vessels. There is normally an underlying layer of connective tissue
- Almost all epitheliums lie on a basement membrane.The basement membrane consists of a basal lamina and reticular lamina. The reticular lamina is connected to the basal lamina by anchoring fibrils. The reticular lamina may be absent in which case the basement membrane consist only of a basal lamina. The basal lamina consists of a - lamina densa in the middle (physical barrier) with a lamina lucida on both sides (+charge barrier),The basement membrane is absent in ependymal cells.The basement membrane is not continuous in sinusoidal capillaries.
- Epitheliums always line or cover something
- Epithelial cells lie close together with little intercellular space
- Epithelial cells are strongly connected to one another especially those epitheliums that are subjected to mechanical forces.
Functions of Epithelium:
→ Simple epithelium involved with diffusion, filtration, secretion, or absorption
→ Stratified epithelium protects from abrasion
→ Squamous cells function in diffusion or filtration
Tongue
Appears at 4th week.
Musculature derived from mesoderm of occipital somites. Precursor muscles cells migrate to region of tongue and are innervated by general sensory efferent fibers of CN XII.
Mucosa derived from anterior endoderm lining arches 1-4; accordingly, innervation depends on arch derivation:
Mucosa of anterior 2/3 of tongue comes from the first arch -> CN V
Mucosa of posterior 1/3 of tongue comes from third and forth arch -> CN IX, X
Special taste of anterior 2/3 of tongue comes from CN VII.
Special taste of posterior 1/3 of tongue comes from CN X.
Tongue freed from floor of mouth by extensive degeneration of underlying tissue. Midline frenulum continues to anchor tongue to floor of mouth.
Thyroid Gland
Develops as in growth of mucosal epithelium located in the midline of the tongue (at foramen cecum). It descends along front of pharyngeal gut, but remains connected to tongue by thyrooglossal duct, which is obliterated later in development. Thyroid gland descends to a point just caudal to laryngeal cartilages.
Facial structures (general)
a) medial nasal prominence forms midline of nose, philtrum and primary palate
b) lateral nasal prominence forms alae of nose
c) maxillary prominence forms cheek region and lateral lip
d) clefts can form at inter-prominence fusion lines
Nose
At the time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour. Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP. Mesenchyme swells around nasal placode producing a medial and lateral nasal prominence (nasomedial and nasolateral processes). These nasal prominences form the nose.
Mouth
Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch. The first pharyngeal arch forms the dorsal maxillary prominence and ventral mandibular prominence. The maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion. Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (first 4 teeth).
Nasolacrimal structures
Maxillary and lateral nasal prominences are separated by deep furrow, the nasolacrimal groove. Ectoderm in floor of groove forms epithelial cord, which detaches from overlying ectoderm. The epithelial cord canalizes to form the nasolacrimal duct. The upper end of the duct widens to form the lacrimal sac. After detachment of the cord, the maxillary and lateral nasal prominences merge with each other, resulting in the formation of a nasolacrimal duct that runs from the medial corner of the eye to the inferior meatus of the nasal cavity.
The maxillary prominences enlarge to form the cheeks and maxillae.
The lateral nasal prominences form the alae of the nose.
Secondary (hard) palate
Main part of definitive palate formed by two palatine shelves derived from intraoral bilateral extensions of the maxillary prominences. These appear at the 6th week. They are directed obliquely downward on each side of the tongue; they move down when mandible gets bigger.
At the seventh week, they ascend to attain a horizontal position, then fuse to form the secondary palate. At the time the palatine shelves fuse, the nasal septum (an outgrowth of median tissue of the frontonasal prominence) grows down and joins the cephalic aspect of the newly formed palate
Anteriorly, shelves fuse with triangular primary palate. The incisive foramen marks the midline between the primary and secondary palate.
External Ear
The auricle is derived from 6 auricular hillocks (mesenchymal proliferations) along the dorsal aspect of arches 1 (top of ear) and 2 (bottom of ear). These fuse to form the definitive auricle. At the mandible grows, the ear is pushed upward and backward from its initial horizontal position on the neck.
The EAM is derived from the 1st pharyngeal arch.
The eardrum (tympanic membrane) is composed of 3 layers of cells: 1) ectodermal epithelial lining of bottom of EAM; 2) endodermal epithelium lining of tympanic cavity; 3) intermediate layer of connective tissue.
The eardrum is composed of multiple cell layers because it represents the first pharyngeal membrane, and thus lies at the junction of the first pharyngeal pouch and cleft.
Middle Ear
The middle ear consists of an auditory tube (from the 1st pharyngeal pouch, along with tympanic cavity) and the ossicles (from pharyngeal arches 1 and 2 cartilage).
The first arch cartilage forms the malleus and incus. The tensor tympani (muscle of the malleus) is derived from the fourth somitomere (associated with the first arch) and is therefore innervated by CN V.
The second arch cartilage forms the stapes. The stapedius (muscles of the stapes) is derived from the sixth somitomere (associated with the second arch) and is therefore innervated by CN VII.
The ossicles are initially embedded in mesenchyme, but in the 8th month, the mesenchyme degenerates and an endodermal epithelial lining of the tympanic cavity envelops the ossicles and connects them to the wall of the cavity in a mesentery-like fashion.
Inner Ear
The inner ear is derived thickening of surface ectoderm on both sides of the hindbrain (otic placodes). The placodes invaginate to form otic vesicles (otocytes). The vesicles then divide into ventral and dorsal components.
The ventral component forms the saccule and cochlear duct.
The dorsal component forms the utricle and semicircular canals and endolymphatic duct.
Cochlear Duct
Derived from an outgrowth of the saccule during the 6th week. The outgrowth penetrates the surrounding mesenchyme in a spiral fashion. The surrounding mesenchyme forms the cartilage and undergoes vacuolization.
The scala vestibule and scale tympani form and surround the cochlear duct. They are filled with periplymp to receive mechanical vibrations of ossicles. The mechanical stimuli activates sensory (ciliary) cells in the cochlear duct.
Semicircular canals
The utricle is initially three flattened outpocketings, which lose the central core. From this three semicircular canals are forms, each at 90 degree angles from one another. Sensory cells arise in the ampulla at one end of each canal, in the utricle and saccule.
Appendicular Skeleton
Upper extremity
• Shoulder-clavicle and scapula
Clavicle
Articulates with the manubrium at the sternal end
Articulates with the scapula at the lateral end
Slender S-shaped bone that extends horizontally across the upper part of the thorax
Scapula
Triangular bone with the base upward and the apex downward
Lateral aspect contains the glenoid cavity that articulates with the head of the humerus
Spine extends across the upper part of the posterior surface; expands laterally and
forms the acromion (forms point of shoulder)
Coracoid process projects anteriorly from the upper part of the neck of the scapula
Arm (humerus)
Consists of a shaft (diaphysis) and two ends (epiphyses)
Proximal end has a head that articulates with the glenoid fossa of the scapula
Greater and lesser tubercles lie below the head
Intertubercular groove is located between them; long tendon of the biceps attaches here
Surgical neck is located below the tubercles
o Radial groove runs obliquely on the posterior surface; radial nerve is located here
o Deltoid muscles attaches in a V-shaped area in the middle of the shaft. called the deltoid tuberosity
Distal end has two projections. the medial and lateral epicondyles
Capitulum-articulates with the radius
Trochlea-articulates with the ulqa
Forearm
Radius
Lateral bone of the forearm
Radial tuberosity is located below the head on the medial side
Distal end is broad for articulation with the wrist: has a styloid process on its lateral side
Ulna
Medial side of the forearm
Conspicuous part of the elbow joint (olecranon)
Curved surface that articulates with the trochlea of the humerus is the trochlearnotch
Lateral ide is concave (radial notch); articulates with the head of the radius Distal end contains the styloid process
Distal end contains the styloid process
Hand
Carpal bones (8)
Aranged in two rows of four
Scaphoid. lunate. triquetral. and pisiform proximal row); trapezium. trapezoid.
capitate. and hamate (distal row)
Metacarpal bones (5)
Framework of the hand
Numbered 1 to 5 beginning on the lateral side
Phalanges (14)
Fingers
Three phalanges in each finger; two phalanges in the thumb
Extrinsic Muscles of the Tongue (p. 746)
The Genioglossus Muscle
- This is a bulky, fan-shaped muscle that contributes to most of the bulk of the tongue.
- It arises from a short tendon from the genial tubercle (mental spine) of the mandible.
- It fans out as it enters the tongue inferiorly and its fibres attach to the entire dorsum of the tongue.
- Its most inferior fibres insert into the body of the hyoid bone.
- The genioglossus muscle depresses the tongue and its posterior part protrudes it.
The Hyoglossus Muscle
- This is a thin, quadrilateral muscle.
- It arises from the body and greater horn of the hyoid bone and passes superoanteriorly to insert into the side and inferior aspect of the tongue.
- It depresses the tongue, pulling its sides inferiorly; it also aids in retrusion of the tongue.
The Styloglossus Muscle
- This small, short muscle arises from the anterior border of the styloid process near its tip and from the stylohyoid ligament.
- It passes inferoanteriorly to insert into the side and inferior aspect of the tongue.
- The styloglossus retrudes the tongue and curls its sides to create a trough during swallowing.
The Palatoglossus Muscle
- Superior attachment: palatine aponeurosis.
- Inferior attachment: side of tongue.
- Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.
- This muscle, covered by mucous membrane, forms the palatoglossal arch.
- The palatoglossus elevates the posterior part of the tongue and draws the soft palate inferiorly onto the tongue.
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BONES OF THE CRANIUM
Occipital (1)
Frontal (1)
Sphenoid (1)
Ethmoid (1)
Parietal (2)
Temporal (2)
BONES OF THE FACE
Mandible (1)
Vomer (1)
Maxillae (2)
Zygomae (2)
Lacrimal (2)
Nasal (2)
Inferior nasal conchae (2)
Palatine (2)
- The tongue is divided into halves by a medial fibrous lingual septum that lies deep to the medial groove.
- In each half of the tongue there are four extrinsic and four intrinsic muscles.
- The lingual muscles are all supplied by the hypoglossal nerve (CN XII).
- The only exception is palatoglossus, which is supplied by the pharyngeal branch of the vagus nerve, via the pharyngeal plexus.