Author: Anisha Valli

Lymph Nodes

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LYMPH NODES

  • Secondary organs of the immune system. These small bean-shaped structures are found where blood and lymph vessels converge, such as the axillary and groin areas.

Overview of Structures:

Capsule

  • Gives rise to trabeculae, which divide the node into sections and provide passage for blood vessels.
    Afferent lymphatic vessels
  • Pierce the capsule to deliver lymphatic fluid to the node; valves promote unidirectional flow.
    Hilum
  • Area of indentation of the node; the efferent lymphatic vessel drains lymph from the node at the hilum.
  • Blood vessels also enter and exit at the hilum.
    Medulla
  • Open to the hilum
    Cortex
  • Lies just beneath the capsule.

Stroma

Reticular fibers and cells

Lymphatic tissues of Lymph Node

  • Lymphatic tissues are responsible for filtering and processing antigens present in the lymphatic fluid as it travels from the afferent vessels to the efferent vessel.
  • Lymphatic tissues are densely packed in the cortex
  • Reside more loosely in the medulla as the medullary cords.

Cortex

  • Outer cortex
    • Aggregations of B cells form primary follicles.
    • When the B cells proliferate, they produce secondary follicles, which comprise a lighter-staining germinal center and the darker-staining mantle.
    • The light and dark areas reflect lymphocyte size: the germinal centers comprise active medium-sized loosely organized lymphocytes, and the mantle comprises smaller lymphocytes with condensed chromatin.
    • In addition to lymphocytes, macrophages and follicular dendritic cells (FDC) reside within the follicles;
      Dendritic cells are the primary antigen-presenting cells (APC); they present microbial antigens on their surfaces to trigger T cell activation.
  • Inner cortex
    • T cells and dendritic cells

Sinuses and Medullary cords

  • Macrophages
  • Additional lymphocytes

Route of lymph fluid through Sinuses

  • Sinuses are lined by endothelial cells
  1. Lymph fluid passes through the afferent lymphatic vessel into the sub-capsular, aka, marginal sinus
  2. Travels through the cortical, aka, trabecular or para-trabecular sinus, to the medullary sinus
  3. Exits medullary sinus through efferent lymphatic vessel.
  4. From there, it is transported in the lymphatic vessels to other lymph nodes and eventually returned to the blood.

Hindbrain Malformations (eg, Chiari Malformation)

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THE MAJOR HINDBRAIN MALFORMATIONS

Chiari malformation

A syndrome of cerebellar herniation through the foramen magnum, which subdivides into three types: 1 through 3, from most mild to most severe.

CHIARI MALFORMATION

Type 1 Chiari Malformation

  • Chiari malformation always involves downward displacement of the cerebellar tonsils through the foramen magnum but what determines its morbidity is the degree of cerebellar displacement, the degree of displacement of additional brainstem structures, and the associated pathologic involvement of other areas of the central nervous system.
  • There is typically a normal ventricular system: 3rd ventricle and cerebral aqueduct, and 4th ventricle.
  • There is an associated syringomyelia (a central cavitation of the spinal cord).

Syringomyelia (Cervical)

  • The spinothalamic tract fibers cross at the ventral commissure.
  • A fluid-filled cavity (a syrinx) expands outward from the central canal and disrupts the crossing fibers and ascending spinothalamic tracts.
  • The clinical exam findings of a cervical syringomyelia include a suspended-sensory level wherein small fiber sensation is lost in a cape-like distribution in the arms and upper trunk but preserved in the legs, upper neck, and face.
  • The location of the syrinx in the at the cervical level affects the cervical crossing fibers; hence the cape-like distribution.
  • This upper limb distribution of deficit also occurs because the inner (arm) spinothalamic fibers lie more central than the outer white matter leg fibers (and, thus, are affected first).

Although the literature suggests that the association between syringomyelia and Chiari Type 1 malformation is quite common, frequent MRI imaging has taught us about the gross underestimation of the prevalence of more mild, asymptomatic Chiari Type 1 malformation cases.

Type 2 Chiari Malformation (aka Arnold-Chiari malformation)

  • There is cerebellar tonsillar herniation and kinking below the foramen magnum.
  • This kinking can block the output of CSF, which can cause hydrocephalus, which manifests with a dilated 3rd ventricle, cerebral aqueductal stenosis (narrowing) or atresia, and downward displacement of the 4th ventricle.
  • The effect is to cause callosal dysgenesis (thinning) and enlargement of the diencephalon with often an absent septum pellucidum.
  • Type 2 malformation is most often associated with a lumbar meningomyelocele (a protrusion of the spinal cord and meninges through a defect in the posterior vertebral column).

We can imagine that this tethering of the lumbar cord could serve as the nidus of the downward displacement of the posterior fossa structures by the tethering effect of the meningomyelocele but however logical this theory, it hasn’t been substantiated in the scientific literature.

Type 3 Chiari Malformation

The severest, rarest form.

  • There is a high cervical and occipital encephalocele, which involves:
    -Occipital lobe herniation
    -Cerebellar herniation
    -Spinal cord herniation
    -Cystic dilatation of the 4th ventricle
    -And squashing of the corpus callosum and diencephalon from the caudal dragging of the occipital and cerebellar herniations.

*DANDY-WALKER MALFORMATION*

Cerebellar agenesis and a grossly dilated 4th ventricle.

  • Hydrocephalic 3rd ventricle
  • Enlarged diencephalon
  • Dysgenic corpus callosum (much like in the Chiari Type 2).
  • Cerebellar agenesis and a grossly dilated 4th ventricle.
  • Dandy Walker malformation involves cystic dilatation of the fourth ventricle, agenesis of the cerebellar vermis, and hydrocephalus with thinning or absence of the corpus callosum may be absent.
  • Dandy Walker malformation may cause impaired motor development, macrocephaly (enlargement of the skull), and indicate that it can be associated with other syndromes such as Walker Warburg syndrome, which combines a Type 2 Lissencephaly with Dandy Walker malformation, or other syndromes involving cardiac, neural, and limb developmental anomalies.

Phenylalanine & Tyrosine Metabolic Disorders

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PHENYLKETONURIA

Pathophysiology: Toxic Metabolites of Phenylalanine

To understand the pathophysiology of phenylketonuria, show that when phenylalanine accumulates at toxic levels, it transaminates into:

  • Phenylpyruvate (aka phenyl ketone); hence, “phenylketonuria” describes the presence of phenylpyruvate, phenylalanine, and two key other derivatives in the urine and blood:
    • Phenylacetate which has a distinct “must/mousy odor”.
    • Phenyllactate.

Phenylalanine Excess / Tyrosine Deficiency

  • Thus, overall indicate that in phenylketonuria, there is an:
    • Excess of phenylalanine
    • Deficiency of tyrosine

So the goal of therapy is to reduce phenylalanine intake and to supplement tyrosine deficiency via the diet. Remember the sparing action of tyrosine on the requirements of phenylalanine

Clinical Presentation of PKU

  • Hypopigmentation
    • Indicate that hypopigmentation (of the skin and iris) is a finding in this disorder (remember: melanin is a derivative of tyrosine and tyrosine is deficient in PKU).
  • Neuropsychiatric disorder
    • And because the toxic levels of phenylalanine and its derivatives are neurotoxic, this disorder causes tremo r, psychosis, seizures, and cognitive dysfunction.

PHEOCHROMOCYTOMA

Clinical Presentation of Pheochromocytoma

  • Symptoms
    • Spontaneous severe anxiety: palpitations, sweating, panic
  • Physical Exam Signs
    • Tachycardia (Rapid heart rate)
    • Hypertension (High blood pressure)

Biochemical Pathophysiology

As a simplification…

  • Dopamine
    • We can attribute the agitation and possible psychosis to the surge in Dopamine.
  • Norephine & Epinephrine
    • The sympathetic nervous system “fight or flight” symptoms relate to the surge in norepinephrine and epinephrine.

Laboratory Testing

  • We test for pheochromocytoma in patients with unexplained episodic hypertension (high blood pressure) with blood and urine collection of:
    • Catecholamine metabolite levels
    • Metanephrine levels
  • Specifically, we typically order:
    • Urine and plasma free metanephrines
    • Urine and plasma free catecholamines
    • Urine homovanillic acid (HVA)
    • Urine vanillylmandelic acid (VMA)
  • The tests are highly sensitive, which leads to false positives. As anticipated, causes of false positives include:
    • Sympathetic nervous system agitation (ie, psychophysiological stress)
    • Exogenous triggers of catecholamines: pharmaceuticals, tobacco, caffeine, and illicit drugs.

Tumor Appearance

  • Pheochromocytomas are black staining tumors (remember this was the color of melanin, another tyrosine derivative, as well) that classically grow out of the medullary layer of the adrenal gland.
    • For a better understanding of the difference between the adrenal medulla and the adrenal cortex, see adrenal gland hormone production.
  • Paragangliomas are essentially extra-adrenal pheochromocytomas
    • They derive from cancerous autonomic nervous system tissue.
    • True to the anatomy of the autonomic nervous system – head/neck ANS paragangliomas are parasympathetic whereas thorax and abdominal paragangliomas are sympathetic.

PHARMACOTHERAPEUTICS IN PARKINSON’S DISEASE

Parkinson’s disease (and for that matter, all Parksinonism syndromes) are Dopamine deficiency syndromes within the brain.

Carbidopa

  • Indicate that the pharmaceutical carbidopa is used to block the decarboxylation of DOPA to dopamine, peripherally, and increase the bioavailability of dopamine centrally (in the central nervous system – where it is intended to treat Parkinson’s disease).
  • Dopamine cannot cross the blood brain barrier but DOPA can, so Dopamine is administered systemically as L-DOPA. However, if it were administered without a decarboxylase inhibitor (such as carbidopa) it would be decarboxylated peripherally into Dopamine and patients would simply become nauseated.
  • In the presence of a peripheral decarboxylase inhibitor, DOPA is taken up in the CNS and THEN decarboxylated to Dopamine (in the basal ganglia where it serves to replenish the deficient stores of Dopamine). Carbidopa (itself) doesn’t cross the blood brain barrier.

Therapeutics

  • Levodopa: Dopamine Precursor
    • So to treat a patient with Parkinson’s disease, let’s add levodopa as a Dopamine precursor.
  • Carbidopa: DOPA decarboxylation Inhibitor
    • At the same time, we need to add Carbidopa to block the peripheral DOPA decarboxylation of DOPA – we need to ensure that the levodopa makes it through the systemic circulation and enters the brain, otherwise it will simply act like any catecholamine within the periphery and increase blood pressure and heart rate but fail to impact the central nervous system Dopamine deficiency state.
  • Ropinirole & Pramipexole: Dopamine agonists
    • We can also add ropinirole or pramipexole, which are Dopamine agonists that optimize the release of Dopamine from the remaining Dopaminergic neurons within the substantia nigra.
  • Entacapone: COMT Inhibitor
    • We can add entacapone, which is a COMT inhibitor to increase the circulation of the Dopamine that we’ve stimulated or replaced (ie, we can inhibit catecholamine metabolism).
  • Selegeline or Rasagaline: MAO-B Inhibitors
  • And we can add selegeline or rasagaline, which are MAO-B (specifically) inihibitors which also increase Dopamine but via MAO inhibition (catecholamine metabolism inhibition).
  • The B subunit is specific to Dopamine catalysis, whereas the MAO-A enzyme is less specific and also metabolizes norepinephrine and serotonin, thus drugs that inhibit MAO-A are potentially much more hazardous to use.

HYPERTHYROIDISM

Clinical Presentation

  • Hypermetabolic state that manifests with:
    • Weight loss, sweats, fevers, rapid heart rate.
  • Skin and hair thinning
  • Grave’s Ophthalmopathy
    • Ocular protrusion and reddening

ALKAPTONURIA

  • Show alkaptonuria, which we can think of as a melanin-like substance in the urine and joints.

Pathogenesis

  • It occurs from a deficiency in homogentisate 1,2 dioxygenase.
    • This results in a build-up of a melanin-like polymer called benzoquinone acetic acid (a product of the oxidation of homogentisic acid), which binds connective tissue and causes dark pigmentation or ochronosis (arthritis).
  • Thus, we can think of alkaptonuria as the opposite of albinism + the build-up of acetic acid in the tissues irritates the joints and causes joint pain.

Presenting Symptoms

  • Children: Dark Urine
    • The presenting manifestation in children is typically urine that darkens when it sits for awhile (not common now with disposable diapers). The darkening occurs from the excess homogentisate in the urine (5,000 mg vs 20-30mg (normally).
  • Adults: Join Pain
    • In adults, the disease presents, typically, from joint pain from the build-up of acetic acid in the tissues, which irritates the joints.

TYROSINEMIA TYPE I

Pathogenesis: fumarylacetoacetate hydrolase deficiency

  • Indicate that tyrosinemia type I (aka hereditary tyrosinemia, tyrosinosis) results from fumarylacetoacetate hydrolase deficiency.

Presenting symptom: “cabbage-like odor”

  • Indicate that it characteristically causes a “cabbage-like odor” but importantly causes liver and kidney failure, polyneuropathy, and bone dysplasia (rickets), manifesting early-on with diarrhea, vomiting and tyrosine and its metabolites in the urine.
  • Also consider that transient tyrosinemia (elevated blood levels of tyrosine) occurs in ~ 10% of newborns, most often due to vitamin C deficiency or immature liver enzymes due to premature birth.

Congenital Left to Right Shunts

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HEALTHY HEART

Typically, post-natal systemic and pulmonary circulation run in parallel and maintain separation between low and high oxygen blood. When separation is incomplete, low and high oxygen blood mix, which produces systemic blood with insufficient oxygen concentrations.

The Great Vessels

  • The aorta carries blood with a high oxygen concentration to the body tissues.
  • The pulmonary trunk carries blood with a low oxygen concentration to the lungs.
  • Ductus arteriosus
    • Allows blood to flow from pulmonary trunk to aorta, bypassing the lungs.
    • The ligamentum arteriosus is the adult remnant of the ductus arteriosus.

Left to right shunts
In conditions with left to right blood shunting, oxygenated blood recirculates through the lungs rather reaching the body tissues.

Ventricular septal defects

Characterized by an incomplete ventricular septum, which allows oxygenated blood to pass from the left ventricle to the right, which then returns it to the lungs.

Atrial septal defects

Allow oxygenated blood from the left atrium to pass into the right heart and, ultimately, back through the pulmonary circulation.

Patent ductus arteriosus

Typically, the ductus arteriosus regresses after birth and becomes the ligamentum arteriosus; however, in some cases, the channel persists.

  • Low pulmonary vascular pressure allows oxygenated blood to “backflow” from the aorta into the pulmonary trunk, which then recirculates it through the lungs.

Eisenmenger syndrome

Occurs when an uncorrected congenital left to right shunt induces anatomical changes that reverse the shunt.

  • A congenital left to right shunt increases pulmonary blood flow.
  • Increased pulmonary blood flow work induces hypertrophy of the right ventricle.
  • Hypertrophic right ventricle becomes so powerful that it eventually overwhelms the pressure from the left ventricle and sends deoxygenated blood directly into the left ventricle, causing hypoxemia.

Congenital Right to Left Shunts

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Healthy Heart
Typically, post-natal systemic and pulmonary circulation run in parallel and maintain separation between low and high oxygen blood. When separation is incomplete, low and high oxygen blood mix, which produces systemic blood with insufficient oxygen concentrations.

Review fetal circulation & adult circulation

Right to Left Shunts

  • Diagnosed prenatally or soon after birth.
  • Are characterized by early cyanosis.
    • So-called “blue babies” have hypoxemia because systemic blood bypasses the lungs.

The Great Vessels

  • The aorta carries blood with a high oxygen concentration to the body tissues.
  • The pulmonary trunk carries blood with a low oxygen concentration to the lungs.
  • Ductus arteriosus
    • Allows blood to flow from pulmonary trunk to aorta, bypassing the lungs.
    • The ligamentum arteriosus is the adult remnant of the ductus arteriosus.

Persistent Truncus Arteriosus

  • Caused by a malformed or absent aortico-pulmonary septum that fails to form separate outflow tracts for systemic and pulmonary circulations.
  • Thus, blood from each ventricle enters the common vessel, largely bypassing the lungs.
  • The defect is attributed to disturbances in secondary heart field or cardiac neural crest formation, and, therefore, is often associated with ventricular septal defect (recall the aorticopulmonary septum contributes to the membranous portion of the interventricular septum.

Transposition of the Great Vessels

  • Occurs when the conotruncal ridges fail to spiral, and the aorticopulmonary septum creates two distinct vessels.
  • The right ventricle continuously pumps low-oxygen blood to the body through the aorta, while the left ventricle continuously pumps high oxygen blood to the lungs through the pulmonary trunk.
  • Thus, blood from the right heart never enters the lungs for re-oxygenation, and blood from the left heart never reaches body tissues.
  • Newborn viability depends on accompanying septal defects and patent ductus arteriosus to provide opportunities for blood mixing.

Tricuspid Atresia

  • The right atrioventricular valve, aka, tricuspid valve, doesn’t properly form. Instead of ensuring unidirectional blood flow from the right atrium to the right ventricle, it blocks flow.
  • Associated characteristics are septal defects and/or patent ductus arteriosus that allow blood mixture, and a hypoplastic (smaller than usual) right ventricle.
  • Corrective surgeries are necessary to establish healthy blood flow.

Tetralogy of Fallot

  • Venous blood from the right heart and mixed blood from the left heart are pumped through the pulmonary trunk and aorta.
    Characterized by 4 defects:
    • Pulmonary stenosis (narrowing of pulmonary trunk)
    • Rightward displaced aorta, with opening over right ventricle (aka, overriding aorta)
    • Ventricular septal defect, specifically, of the membranous portion
    • Right ventricular hypertrophy, which occurs in response to increased work load to pump blood through stenotic pulmonary trunk

Total Anomalous Pulmonary Venous Return

  • Characterized by pulmonary veins that drain into the right atrium via the coronary sinus, superior vena cava, brachiocephalic vein, etc.
    Example:
  • Paired right and left pulmonary veins also draining into the right atrium via the coronary sinus.
    • In this case, viability requires an accompanying shunt that allows oxygenated blood to reach the left side of the heart.

Vesicular Budding and Fusion

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STEPS OF VESICULAR BUDDING AND FUSION (+ PROTEINS INVOLVED)

  • Cargo selection (cargo receptor, adaptor protein)
  • Vesicular budding (adaptor proteins, coat proteins)
  • Fission from donor membrane (dynamin)
  • Vesicular coat dissociates
  • Vesicular targeting and transport (Rab-GTPase, tethering protein)
  • Fusion with target membrane (V-snare and T-snare)

PROTEINS OF VESICULAR BUDDING AND FUSION

  • Cargo receptors – select and concentrate molecules to be transported in vesicle
  • Adaptor proteins – bind cargo receptor and coat proteins
  • Coat proteins – form protein scaffold around vesicle that facilitate facilitate vesicular budding
  • Dynamin – GTPase involved in vesicular fission
  • RabGTPase – associates with vesicle after coat has dissociated. Facilitates transport of vesicle to appropriate target membrane. Locks vesicle to target membrane by attaching tethering proteins
  • Tethering proteins – anchored in target membrane, attach rabGTPase. Move vesicle close to target membrane for vesicular fusion.
  • V-snares(vesicular) and T-snares(target membrane) – Play role in vesicular fusion

COAT PROTEINS DIRECT VESICLE TRANSPORT

  • Clathrin +adaptin 1: Golgi → Lysosome
  • Clathrin + adaptin 2: Plasma membrane → Endosomes (endocytosis)
  • COP 1: Cis golgi → ER AND Later cisternae → Earlier ones (retrograde transport)
  • COP II: ER → Cis golgi

Vesicular Transport Overview

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3 PATHWAYS OF VESICULAR TRANSPORT

  • Secretory pathway: delivers cargo to the plasma membrane.
  • Endocytic pathway: uptake cargo from the plasma membrane.
  • Retrieval pathway: recycles cellular molecules.

KEY FACTS ABOUT VESICULAR TRANSPORT:

  • Compartment lumens mix via the transport intermediate.
  • The membrane of each vesicle maintains its orientation.
  • If the cell is growing, the secretory pathway is more active than the endocytic pathway.

STEPS IN SECRETORY PATHWAY

  • Transport vesicles bud from the ER and carry content away from it to cis side of Golgi.
  • Vesicular budding and fusion mediates the transport of cargo through the Golgi stacks, from cis to trans side.
  • Cargo exits the Golgi via a transport vesicle on trans side.
  • Transport vesicles fuse with plasma membrane or with endosomes (and then lysosomes).

STEPS IN ENDOCYTOTIC PATHWAY

  • Early endosome forms from plasma membrane and extracellular materials.
  • Early endosome targets cargo to late endosomes.
  • Late endosomes then deliver cargo to lysosomes, which degrade cargo.

THE RETRIEVAL PATHWAY TAKES SEVERAL FORMS

  • Endosomes can return cargo to the cell surface via recycling endosomes.
  • Cargo in early and late endosomes can also return to the Golgi for reuse.
  • Vesicles can deliver proteins from the trans face to the cis face of the Golgi.
  • Vesicles can return proteins from the golgi to the ER as well.

3 STEPS OF VESICULAR FORMATION

  • Cargo selection. Incorporation of cargo into a vesicle is carefully regulated to ensure that only the correct cargo gets transported.
  • Vesicular budding. deformation of the hydrophobic membrane bilayer and breaking off of the membrane into a vesicle
  • Vesicular targeting and fusion. Highly regulated just like cargo selection.

Cellular compartments are topologically equivalent when:

• Molecules can get from one to another without having to cross a membrane.
• Nuclear envelope, ER, Golgi, transport vesicles, endosomes, lysosomes, and extracellular space = topologically equivalent

Protein Folding and Glycosylation in the Endoplasmic Reticulum

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KEY PROCESSES IN ER

• Protein glycosylation
• Protein folding

N-GLYCOSYLATION

• Is an ER event
• Is a Co-translational event.
• Occurs on Asn-X-Ser/Thr residues

O-linked glycosylation occurs in the Golgi apparatus

EVENTS OF N-GLYCOSYLATION

• Nascent protein imports co-translationally through translocon.
• Oligosaccharyl transferase transfers oligossacharide from dolichol to N-side group of Asn residue
• 3 sugars trimmed from the 14 sugar oligossacharide as protein exits translocon.
• Chaperones help protein fold in lumen

Protein Insertion into the ER Membrane

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4 TYPES OF TRANSMEMBRANE PROTEINS

  • Type I membrane proteins have a signal sequence at their N-terminus and an internal stop-transfer sequence. They have no cytosolic tail because signal peptidase cleaves it.
  • Type II have an internal signal. Their C term is luminal, and their N term is cytosolic.
  • Type III have an internal signal sequence. Their C term is cytosolic, and their N term is luminal.
  • Type IV are multipass membrane proteins that have multiple internal stop transfer sequences and start transfer sequences.

Protein Import into the Endoplasmic Reticulum

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PROTEINS FOR IMPORT
• Water-soluble proteins
• Transmembrane proteins

KEY MECHANISMS
• Cotranslational: ribosomes continue synthesizing protein as it crosses membrane
• Post-translational: protein imports after it is completely synthesized by cytosolic ribosomes

SIGNAL SEQUENCE
• Hydrophobic sequence (15-60 residues): directs proteins to specific organelles (i.e. ER membrane)

COTRANSLATIONAL IMPORT OF WATER-SOLUBLE PROTEINS

  1. SRP (signal recognition particle) recognizes and binds signal sequence on a nascent protein in the cytosol; halts translation
  2. SRP (with ribosomal complex) binds SRP receptor
  3. SRP released from complex
  4. Signal sequence inserts into translocon, translocon opens and translation resumes
  5. Signal peptidase cleaves signal sequence and releases protein in ER lumen
  6. Chaperones help protein fold correctly

POST-TRANSLATIONAL IMPORT OF WATER-SOLUBLE PROTEINS

  1. Chaperones maintain newly synthesized protein’s unfolded conformation in cytosol
  2. Translocon/SRP receptor complex recognizes signal sequence
  3. Protein enters translocon; chaperone in lumen prevents peptide from sliding back through translocon

CLINICAL CORRELATION

Alzheimer’s and Parkinson’s

• Neurodegenerative diseases that involve improper folding of proteins in endoplasmic reticulum