NEET MDS Synopsis
Tooth development
Dental Anatomy
Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth.. For human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero.
Overview
The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth.These cells are derived from the ectoderm of the first branchial arch and the ectomesenchyme of the neural crest.The tooth bud is organized into three parts: the enamel organ, the dental papilla and the dental follicle.
The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The location where the outer enamel epithelium and inner enamel epithelium join is called the cervical loop. The growth of cervical loop cells into the deeper tissues forms Hertwig's Epithelial Root Sheath, which determines the root shape of the tooth.
The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. Mesenchymal cells within the dental papilla are responsible for formation of tooth pulp.
The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.
Molecular techniques
General Pathology
Molecular techniques
Different molecular techniques such as fluorescent in situ hybridization, Southern blot, etc... can be used to detect genetic diseases.
PHYSICAL AGENTS
General Microbiology
PHYSICAL AGENTS
Heat occupies the most important place as a physical agent.
Moist Heat : This is heating in the presence of water and can be employed in the following ways:
Temperature below 100°C: This includes holder method of Pasteurization where 60°C for 30 minutes is employed for sterilization and in its flash modification where in objects are subjected to a temperature of 71.1°C for 15 seconds. This method does not destroy spores.
Temperatures Around 100°C : Tyndallization is an example of this methodology in which steaming of the object is done for 30 minutes on each of three consecutive days. Spores which survive the heating process would germinate before the next thermal exposure and would then be killed.
Temperatures Above 100°C : Dry saturated steam acts as an excellent agent for sterilization. Autoclaves have been designed on the principles of moist heat.
Time-temperature relationship in heat sterilization
Moist heat (autoclaving)
121°C 15 minutes
126°C 10 minutes
134 C 3 minutes
Dry heat
>160°C >120 minutes
>170°C >60minutes
>180°C >30 minutes
Mechanism of microbial inactivation
The autoclaving is in use for the sterilization of many ophthalmic and parentral products. surgical dressings, rubber gloves, bacteriological media as well a of lab and hospital reusable goods.
Dry Heat: Less efficient, bacterial spores are most resistant. Spores may require a temperature of 140° C for three hours to get killed.
Dry heat sterilization is usually carried out by flaming as is done in microbiology laboratory to sterilize the inoculating loop and in hot air ovens in which a number of time-temperature combinations can be used. It is essential that hot air should circulate between the objects to be sterilized. Microbial inactivation by dry heat is primarily an oxidation process.
Dry heat is employed for sterilization of glassware glass syringes, oils and oily injections as well as metal instruments. -
Indicators of Sterilization:
These determine the efficacy of heat sterilization and can be in the form of spores of Bacillus stearothermophilus (killed at 121C in 12 minutes) or in the form of chemical indicators, autoclave tapes and thermocouples.
Ionizing Radiations
Ionizing radiations include X-rays, gamma rays and beta rays, and these induce defects in the microbial DNA synthesis is inhibited resulting in cell death. Spores are more resistant to ionizing radiations than nonsporulating bacteria.
The ionizing radiations are used for the sterilization of single use disposable medical items.
Mechanism of microbial inactivation by moist heat
Bacterial spores
• Denaturation of spore_epzymes
• Impairment of germination
• Damage to cell membrane
• Increased sensitivity to inhibitory agents
• Structural damage
• Damage to chromosome
Nonsporulating bacteria
• Damage to cytoplasmic membrane
• Breakdown of RNA
• Coagulation of proteins
• Damage to bacterial chromosome
Ultraviolet Radiations :
wave length 240-280 nm have been found to be most efficient in sterilizing. Bacterial spores are more resistant to U.V. rays than the vegetative forms. Even viruses are sometimes more resistant than vegetative bacteria.
Mechanism of Action :
Exposure to UV rays results in the formation of purine and pyrimidine diamers between adjacent molecules in the same strand of DNA. This results into noncoding lesions in DNA and bacterial death.
Used to disinfect drinking water, obtaining pyrogen free water, air disinfection (especially in safety laboratories, hospitals, operation theatres) and in places where dangerous microorganisms are being handled.
Filteration
Type of Filters
Various types of filters that are available are /
Unglazed ceramic filter (Chamberland and Doulton filters)
Asbestos filters (Seitz, Carlson and Sterimat filters)
Sintered glass filters
Membrane filters
Membrane filters are widely used now a days. Made up of cellulose ester and are most suitable for preparing_sterile solutions. The range of pore size in which these are available is 0.05-12 µm whereas the required pore size for sterlization is in range of 0.2-0.22 p.m.
Porosity defects in Dental casting
Prosthodontics
Porosity
Porosity refers to the presence of voids or spaces within a solid material. In
the context of prosthodontics, it specifically pertains to the presence of small
cavities or air bubbles within a cast metal alloy. These defects can vary in
size, distribution, and number, and are generally undesirable because they
compromise the integrity and mechanical properties of the cast restoration.
Causes of Porosity Defects
Porosity in castings can arise from several factors, including:
1. Incomplete Burnout of the Investment Material: If the wax pattern used to
create the mold is not completely removed by the investment material during the
burnout process, gases can become trapped and leave pores as the metal cools and
solidifies.
2. Trapped Air Bubbles: Air can become trapped in the investment mold during the
mixing and pouring of the casting material. If not properly eliminated, these
air bubbles can lead to porosity when the metal is cast.
3. Rapid Cooling: If the metal cools too quickly, the solidification process may
not be complete, leaving small pockets of unsolidified metal that shrink and
form pores as they solidify.
4. Contamination: The presence of contaminants in the metal alloy or investment
material can also lead to porosity. These contaminants can react with the metal,
forming gases that become trapped and create pores.
5. Insufficient Investment Compaction: If the investment material is not packed
tightly around the wax pattern, small air spaces may remain, which can become
pores when the metal is cast.
6. Gas Formation During Casting: Certain reactions between the metal alloy and
the investment material or other substances in the casting environment can
produce gases that become trapped in the metal.
7. Metal-Mold Interactions: Sometimes, the metal can react with the mold
material, resulting in gas formation or the entrapment of mold material within
the metal, which then appears as porosity.
8. Incorrect Spruing and Casting Design: Poorly designed sprues can lead to
turbulent metal flow, causing air entrapment and subsequent porosity.
Additionally, a complex casting design may result in areas where metal cannot
flow properly, leading to incomplete filling of the mold and the formation of
pores.
Consequences of Porosity Defects
The presence of porosity in a cast restoration can have several negative
consequences:
1. Reduced Strength: The pores within the metal act as stress concentrators,
weakening the material and making it more prone to fracture or breakage under
functional loads.
2. Poor Fit: The pores can prevent the metal from fitting snugly against the
prepared tooth, leading to a poor marginal fit and potential for recurrent decay
or gum irritation.
3. Reduced Biocompatibility: The roughened surfaces and irregularities created
by porosity can harbor plaque and bacteria, which can lead to peri-implant or
periodontal disease.
4. Aesthetic Issues: In visible areas, porosity can be unsightly, affecting the
overall appearance of the restoration.
5. Shortened Service Life: Prosthodontic restorations with porosity defects are
more likely to fail prematurely, requiring earlier replacement.
6. Difficulty in Polishing and Finishing: The presence of porosity makes it
challenging to achieve a smooth, polished finish, which can affect the comfort
and longevity of the restoration.
Prevention and Management of Porosity
To minimize porosity defects in prosthodontic castings, the following steps can
be taken:
1. Proper Investment Technique: Carefully follow the manufacturer's instructions
for mixing and investing the wax pattern to ensure complete burnout and minimize
trapped air bubbles.
2. Slow and Controlled Cooling: Allowing the metal to cool slowly and uniformly
can help to reduce the formation of pores by allowing gases to escape more
easily.
3. Pre-casting De-gassing: Some techniques involve degassing the investment mold
before casting to remove any trapped gases.
4. Cleanliness: Ensure that the metal alloy and investment materials are free
from contaminants.
5. Correct Casting Procedure: Use proper casting techniques to reduce turbulence
and ensure a smooth flow of metal into the mold.
6. Appropriate Casting Design: Design the restoration with proper spruing and a
simple, well-thought-out pattern to allow for even metal flow and minimize
trapped air.
7. Proper Casting Conditions: Control the casting environment to reduce the
likelihood of gas formation during the casting process.
8. Inspection and Quality Control: Carefully inspect the cast restoration for
porosity under magnification and radiographs before it is delivered to the
patient.
9. Repair or Replacement: When porosity defects are detected, they may be
repairable through techniques such as metal condensation, spot welding, or
adding metal with a pin connector. However, in some cases, the restoration may
need to be recast to ensure optimal quality.
Tubular reabsorption
Physiology
Remember the following principles before proceeding :
- Reabsorption occurs for most of substances that have been previously filterd .
- The direction of reabsorption is from the tubules to the peritubular capillaries
- All of transport mechanism are used here.
- Different morphology of the cells of different parts of the tubules contribute to reabsorption of different substances .
- There are two routes of reabsorption: Paracellular and transcellular : Paracellular reabsorption depends on the tightness of the tight junction which varies from regeon to region in the nephrons .Transcellular depends on presence of transporters ( carriers and channels for example).
1. Reabsorption of glucose , amino acids , and proteins :
Transport of glucose occurs in the proximal tubule . Cells of proximal tubules are similar to those of the intestinal mucosa as the apical membrane has brush border form to increase the surface area for reabsorption , the cells have plenty of mitochondria which inform us that high amount of energy is required for active transport , and the basolateral membrane of the cells contain sodium -potassium pumps , while the apical membrane contains a lot of carrier and channels .
The tight junction between the tubular cells of the proximal tubules are not that (tight) which allow paracellular transport.
Reabsorption of glucose starts by active transport of Na by the pumps on the basolateral membrane . This will create Na gradient which will cause Na to pass the apical membrane down its concentration gradient . Glucose also passes the membrane up its concentration gradient using sodium -glucose symporter as a secondary active transport.
The concentration of glucose will be increased in the cell and this will enable the glucose to pass down concentration gradient to the interstitium by glucose uniporter . Glucose will then pass to the peritubular capillaries by simple bulk flow.
Remember: Glucose reabsorption occurs via transcellular route .
Glucose transport has transport maximum . In normal situation there is no glucose in the urine , but in uncontrolled diabetes mellitus patients glucose level exceeds its transport maximum (390 mg/dl) and thus will appear in urine .
2. Reabsorption of Amino acids : Use secondary active transport mechanism like glucose.
3. Reabsorption of proteins :
Plasma proteins are not filtered in Bowman capsule but some proteins and peptides in blood may pass the filtration membrane and then reabsorbed . Some peptides are reabsorbed paracellulary , while the others bind to the apical membrane and then enter the cells by endocytosis , where they will degraded by peptidase enzymes to amino acids .
4. Reabsorption of sodium , water , and chloride:
65 % of sodium is reabsorbed in the proximal tubules , while 25% are reabsorbed in the thick ascending limb of loob of Henle , 9% in the distal and collecting tubules and collecting ducts .
90% of sodium reabsorption occurs independently from its plasma level (unregulated) , This is true for sodium reabsorbed in proximal tubule and loop of Henle , while the 9% that is reabsorbed in distal ,collecting tubules and collecting ducts is regulated by Aldosterone.
In proximal tubules : 65% of sodium is reabsorbed . The initial step occurs by creating sodium gradient by sodium-potassium pump on the basolateral membrane . then the sodium will pass from the lumen into the cells down concentration gradient by sodium -glucose symporter , sodium -phosphate symporter and by sodium- hydrogen antiporter and others
After reabsorption of sodium , an electrical gradient will be created , then chloride is reabsorbed following the sodium . Thus the major cation and anion leave the lumen to the the interstitium and thus the water follows by osmosis . 65% of water is reabsorbed in the proximal tubule.
Discending limb of loop of Henle is impermeable to electrolytes but avidly permeable to water . 10 % of water is reabsorbed in the discending thin limb of loob of Henle .
The thick ascending limb of loop of Henly is permeable to electrolytes , due to the presence of Na2ClK syporter . 25% of sodium is reabsorbed here .
In the distal and collecting tubules and the collecting ducts 9% of sodium is reabsorbed .this occurs under aldosterone control depending on sodium plasma level. 1% of sodium is excreted .
Water is not reabsorbed from distal tubule but 5-25% of water is reabsorbed in collecting tubules .
Conductivity
Physiology
Conductivity :
Means ability of cardiac muscle to propagate electrical impulses through the entire heart ( from one part of the heart to another) by the excitatory -conductive system of the heart.
Excitatory conductive system of the heart involves:
1. Sinoatrial node ( SA node) : Here the initial impulses start and then conducted to the atria through the anterior inter-atrial pathway ( to the left atrium) , to the atrial muscle mass through the gap junction, and to the Atrioventricular node ( AV node ) through anterior, middle , and posterior inter-nodal pathways.
The average conductive velocity in the atria is 1m/s.
2- AV node : The electrical impulses can not be conducted directly from the atria to the ventricles , because of the fibrous skeleton , which is an electrical isolator , located between the atria and ventricles. So the only conductive way is the AV node . But there is a delay in the conduction occurs in the AV node .
This delay is due to:
- the smaller size of the nodal fiber.
- The less negative resting membrane potential
- fewer gap junctions.
There are three sites for delay:
- In the transitional fibers , that connect inter-nodal pathways with the AV node ( 0.03 ) .
- AV node itself ( 0.09 s) .
- In the penetrating portion of Bundle of Hiss ( 0.04 s) .
This delay actually allows atria to empty blood in ventricles during the cardiac cycle before the beginning of ventricular contraction , as it prevents the ventricles from the pathological high atrial rhythm.
The average velocity of conduction in the AV node is 0.02-0.05 m/s
3- Bundle of Hiss : A continuous with the AV node that passes to the ventricles through the inter-ventricular septum. It is subdivided into : Right and left bundle. The left bundle is also subdivided into two branches: anterior and posterior branches .
4- Purkinje`s fibers: large fibers with velocity of conduction 1.5-4 m/s.
the high velocity of these fibers is due to the abundant gap junctions , and to their nature as very large fibers as well.
The conduction from AV node is a one-way conduction . This prevents the re-entry of cardiac impulses from the ventricles to the atria.
Lastly: The conduction through the ventricular fibers has a velocity of 0.3-0.5 m/s.
Factors , affecting conductivity ( dromotropism) :
I. Positive dromotropic factors :
1. Sympathetic stimulation : it accelerates conduction and decrease AV delay .
2. Mild warming
3. mild hyperkalemia
4. mild ischemia
5. alkalosis
II. Negative dromotropic factors :
1. Parasympathetic stimulation
2. severe warming
3. cooling
4. Severe hyperkalemia
5. hypokalemia
6. Severe ischemia
7. acidosis
8. digitalis drugs.
Propofol
Pharmacology
Propofol -Intravenous Anesthetics
- A nonbarbiturate anesthetic
- It is very lipid-soluble, acts rapidly and has a short recovery time.
- It is associated with less nausea and vomiting than some of the other IV anesthetics.
- Propofol is very similar to thiopental in its effects on the cardiorespiratory system.
- It does not have any analgesic properties but lowers the dose of opioid needed when the two agents are used in combination.
- The most significant adverse cardiovascular effect associated with propofol administration is hypotension. It should be used with caution in patients with cardiac disease.
Contractility
Physiology
Contractility : Means ability of cardiac muscle to convert electrical energy of action potential into mechanical energy ( work).
The excitation- contraction coupling of cardiac muscle is similar to that of skeletal muscle , except the lack of motor nerve stimulation.
Cardiac muscle is a self-excited muscle , but the principles of contraction are the same . There are many rules that control the contractility of the cardiac muscles, which are:
1. All or none rule: due to the syncytial nature of the cardiac muscle.There are atrial syncytium and ventricular syncytium . This rule makes the heart an efficient pump.
2. Staircase phenomenon : means gradual increase in muscle contraction following rapidly repeated stimulation..
3. Starling`s law of the heart: The greater the initial length of cardiac muscle fiber , the greater the force of contraction. The initial length is determined by the degree of diastolic filling .The pericardium prevents overstretching of heart , and allows optimal increase in diastolic volume.
Thankful to this law , the heart is able to pump any amount of blood that it receives. But overstretching of cardiac muscle fibers may cause heart failure.
Factors affecting contractility ( inotropism)
I. Positive inotropic factors:
1. sympathetic stimulation: by increasing the permeability of sarcolemma to calcium.
2. moderate increase in temperature . This due to increase metabolism to increase ATP , decrease viscosity of myocardial structures, and increasing calcium influx.
3. Catecholamines , thyroid hormone, and glucagon hormones.
4. mild alkalosis
5. digitalis
6. Xanthines ( caffeine and theophylline )
II. Negative inotropic factors:
1. Parasympathetic stimulation : ( limited to atrial contraction)
2. Acidosis
3. Severe alkalosis
4. excessive warming and cooling .
5. Drugs ;like : Quinidine , Procainamide , and barbiturates .
6. Diphtheria and typhoid toxins.