Intro_embryo & Cases Mhs Rs 08-09

8/13/2019 Intro_embryo & Cases Mhs Rs 08-09

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EMBRYOLOGIOF

THE RESPIRATORY SYSTEM

Arti Rosaria Dewi, dr


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Embryonic disc begins to fold endodermlined cavity primitive gut

- the anterior end: fore gut,

- the posterior end: hind gut- mid gut connected to yolk-sac

The opening between fore gut and mid gut :anterior intestinal portalBetween hind gut and mid gut :posterior intestinal portal


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Primitive gut or tubular gut mouth, pharynx,digestive tube

Fore gut specializes into: mouth, pharynx, partof digestive tube

Hind gut rest of small, intestine, colon, rectum

Mid gut will: disappear

Derivatives of the endodermal digestive tube:

- respiratory system- thyroid, parathyroid, thymus- liver, vesica felea and pancreas


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The epithelial lining of digestive andrespiratory tracts : endoderm

Accessory coats : muscle and connective tissuefrom splanchnic mesoderm tunica mucosa


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THE NOSE

Ventrolateral of the head thickened

olfactory placode olfactory pit.Peripheral of the pit median andlateral nasal process nasal septum.

- Maxillary and nasal process nostril- Olfactory epithelium olfactory nerve


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Formation of the Lung Buds

+ 4 weeks old the embryo :

the respiratory diverticulum

(laryngotrakheal diverticulum)- An outgrowth from the ventral wall of the

foregut.

- The location is determined by signals fromthe surrounding mesenchyme, includingfibroblast growth factors (FGFs) thatinstructs the endoderm.


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Epithelium of the internal lining of the larynx,trachea, bronchi, and the lungs, endodermal origin.

The cartilaginous, muscular, and connectivetissue components of the trachea and lungs

splanchnic mesoderm


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Initially the lung bud is in opencommunication with the foregut.When the diverticulum expands caudally :the tracheoesophageal ridges

the tracheoesophageal septum

the foregut is divided into :

- a dorsal portion, the esophagus,- a ventral portion, the trachea and lung buds.


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Abnormalities in partitioning of theesophagus and trachea

Result in esophageal atresia +/-tracheoesophageal fistula (TEFs).1/3000 births90 % result in the upper portion of theesophagus ending in a blind pouch and thelower segment forming a fistula with the

trachea (A).


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Isolated esophageal atresia (B): 4 %H-type TEF without esophageal atresia : 4%.Other variations (D & E) : 1 %.

Associated with other birth defects, cardiacabnormalities : 33 %.


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TEFs are a component of the VACTERL(Vertebral anomalies, Anal atresia, Cardiacdefects, Tracheoesophageal fistula,Esophageal atresia, Renal anomalies, and

Limb defects).

Complication TEFs :

- polyhydramnion- Pneumonitis


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The respiratory primordium maintains itscommunication with the pharynx through the

laryngeal orifice.


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LARYNX

The internal lining of the larynx : endoderm.The cartilages & muscles : mesenchyme ofthe 4 th & 6 th pharyngeal arches.Rapid proliferation of this mesenchyme

changes the laryngeal orifice( a sagittal slit T-shaped opening)


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mesenchyme of the two arches transformsinto : - the thyroid

- cricoid, and- arytenoids cartilages

(the characteristics adult shape)


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At about the time that the cartilages areformed, the laryngeal epithelium alsoproliferates rapidly a temporary occlusion ofthe lumen.Subsequently, vacuolization andrecanalization a pair of lateral recesses, thelaryngeal ventricles.

These recesses are bounded by folds oftissue that differentiate into the false and truevocal cords.


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All laryngeal muscles are innervated bybranches of the 10 th cranial nerve, the vagusnerve.

- The superior laryngeal : derivatives of the4 th pharyngeal arch, and

-The recurrent laryngeal : derivatives of the

6 th pharyngeal arch.


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Trachea, Bronchi, and Lungs

During its separation from the foregut, thelung bud forms :- the trachea and- two lateral outpocketings, the bronchial buds. At the beginning of the 5 th week, each of

these buds enlarges to form right and leftmain bronchi.


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The right forms three secondary bronchi the three lobes, and

The left forms two secondary bronchi thetwo lobes.


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Lung buds lie on each side of the foregut andare gradually filled the pericardioperitoneal bythe expanding lung buds.


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The pleuroperitoneal and pleuropericardialfolds separate the pericardioperitoneal canalsfrom the peritoneal and pericardial cavities,respectively, and the remaining spaces formthe primitive pleural cavities.


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The mesoderm, which covers the outside ofthe lung, develops into the visceral pleura .The somatic mesoderm layer, covering thebody wall from the inside, becomes theparietal pleura .The space between the parietal and visceral

pleura is the pleural cavity .


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Secondary bronchi forming 10 tertiary

(segmental) bronchi in the right lung and 8 inthe left, creating the bronchopulmonarysegments of the adult lung.The end of the sixth month 17 generations

of subdivisions have formed. An additional 6 divisions form duringpostnatal life.


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Branching is regulated by epithelial-mesenchymal interactions ( the endoderm ofthe lung buds - splanchnic mesoderm that

surrounds them).Signals for branching : members of thefibroblast growth factor (FGF) family.by the time of birth the bifurcation of thetrachea is opposite the fourth thoracicvertebra .


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Maturation of the Lungs

Up to the seventh prenatal month, thebronchioles divide continuously into more andsmaller canals (canalicular phase), and the

vascular supply increases steadily.Respiration becomes possible when some ofthe cells of the cuboidal respiratorybronchioles change into thin, flat cells.


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These cells are intimately associated withnumerous blood and lymph capillaries, andthe surrounding spaces are now known asterminal sacs or primitive alveoli .During the seventh month, sufficient numbersof capillaries are presents to guaranteeadequate gas exchange, and the prematureinfant is able to survive.


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During the last 2 months of prenatal life andfor several years thereafter, the number ofterminal sacs increases steadily.In addition, cells lining the sacs.known as type

1 alveolar epithelial cells , become thinner,sothat surrounding capillaries protrude into thealveolar sacs.This intimate contact between epithelial and

endothelial cells makes up the blood-airbarrier.Mature alveoli are not present before birth.


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In addition to endothelial cells and flatalveolar epithelial cells, another cell typedevelops at the end of the sixth month.

These cells, type II alveolar epithelial cells ,produce surfactant, a phospolipid-rich fluidcapable of lowering surface tension at the air-alveolar interface.


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Before birth the lungs are full of fluid thatcontains a high chloride concentration, littleprotein, some mucus from the bronchial

glands, and surfactant from the alveolarepithelial cells (type II).The amount of surfactant in the fluidincreases, particularly during the last 2 weeksbefore birth.


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Fetal breathing movements begin before birthand cause aspiration of amniotic fluid.These movements are important for

stimulating lung development andconditioning respiratory muscles.When respiration begins at birth, most of thelung fluid is rapidly resorbed by the blood and

lymph capillaries, and a small amount isprobably expelled via the trachea and bronchiduring delivery.


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When the fluid is resorbed from alveolar sacs,surfactant remains deposited as a thinphospholipids coat on alveolar cellmembranes.With air entering alveoli during the firstbreath, the surfactant coat presentsdevelopment of an air-water (blood) interfacewith high surface tension.Without the fatty surfactant layer, the alveoliwould collapse during expiration (atelectasis).


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Respiratory movements after birth bring airinto the lungs, which expand and fill the

pleural cavity. Although the alveoli increase somewhat insize, growth of the lungs after birth is dueprimarily to an increase in the number of

respiratory bronchioles and alveoli.It is estimated that only one-sixth of the adultnumber of alveoli are present at birth.The remaining alveoli are formed during thefirst 10 years of postnatal life through thecontinuous formation of new primitive alveoli.


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Terminal Respiratory Unit (gas-exchangeunit/acinus)- the basic functional unit of the lung.

- consist : the structures distal to theterminal bronchiole

- the respiratory bronchiale

- alveolar ducts- alveoli


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In the adult lung :- total gas volume 2500 ml- surface area 80 m 2

Within the terminal respiratory units : 2 typeintercommunicating channels providecollateral ventilation for the gas-exchangingunits :

1. the alveolar pores of Kohn2. the canals of Lambert


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Add. 1 :- are holes in the alveolar wall of 3-13 um indiameter that provide channels for gas

movement between contiguous alveoli.- are not present in newborn lungs.


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Add.2 :- are accessory channels that connect a smallairway to an air space normally supplied by a

different airway.


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Collateral VentilationIn the adult collateral ventilation themovement of gas from one acinus toanother-occurs through holes in thealveoli, the pores of Kohn andepithelium-lined channels betweenterminal bronchioles & adjacent alveoli

called the canals of Lambert.


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These structures may be present in the infantlung, but they are probably not of sufficientsize to allow for air drift.

Although collateral pathway in the adult areprobably not of great significance forventilation they do prevent absorption of gasin regions distal to airway obstruction.


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The relative absence of collateral pathwayprobably contribute to the patchy atelectasis so common in airway disease of infants &

young children.The product of resistance & compliance thetime constants , of these collateral pathwayshave been investigated in maturing sheep &in young adults.


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The alveoli are lined by 2 types of epithelial cells:*the type I epithelial cell

- extremely broad,thin cell(0.1-0.5 um)- covers 95% of the alveolar surface- is markedly differentiated cell possessing feworganelles.

*the type II epithelial cell - more numerous than type I cell.

- owing to their cuboidal shape.- occupy only 5 % of the alveolar surfacearea(table 3-1).


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-characterized histologically by microvilli andosmophilic inclusion bodies.-the type II epithelial cell:maintainshomeostasis within the alveolar space in

several ways:1.it is the source of pulmonary surfactant andas such indicates maturity of the lung;itdecreases the surface tension at the alveolar

air-liquid interface.2.it cell is likely the precursor of the alveolartype I cell and thus plays a key role in therepair process following lung injury


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3.it is capable of actively transporting ionsagainst an electrochemical gradientand likely

is involved in both fetal lung liquid secretionand, post natally, the reabsorption of fluidfrom the air space following the developmentof alveolar pulmonary edema.

Two pediatric disorders associated with thetype II epithelial cell:1.hyaline membrane disease:its lack of

maturity and surfactant secretion2.alveolar proteinosis:its excessive and

disordered secretion of surfactant


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The cell junctions between alveolar type I andII cells are very tight and restrict the

movement of both macromolecules and smallions such as sodium and chloride.This tightness is an essential characteristic ofthe cells lining the alveolar space; its enablesthe active transport of ions.Tight junctions provide a margin of safety forpatients susceptible to pulmonaryedema:significant interstitial pulmonaryedema can be present without alveolarflooding occurring, thus preserving gasexchange.


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CASE 2 A premature infant developed rapid, shallowrespiration shortly after birth. A diagnosis ofrespiratory distress syndrome (RDS) wasmade.

1. How do you think the infant might attempt toovercome his or her inadequate exchange ofoxygen and carbondioxide/

2. what usually causes RDS?

3. what treatment is currently used clinically toprevent rds?4. a deficiency of what substance is associated

with RDS?


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CASE 3The parents of a new born infant were toldthat their son had a fistula between histrachea and asophagus.

1.What is the most common type oftracheosophageal fistula?

2. What is its embryological basis?3. What anomaly of the digestive tract is

frequently associated with this abnormality/


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Case 4 A new born infant with esophageal atresiaexperienced respiratory distress withcyanosis shortly after birth. Radiographsdemonstrated air in the infants stomach.

1. How did the air enter the stomach?2. What other problem might result in an infant

with this fairly common type of congenital

anomaly?

Intro_embryo & Cases Mhs Rs 08-09

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