DEVELOPMENT OF LUNGS AND RELATED

ABNORMALITIES

AYUSH JAIN JR 2RESPIRATORY MEDICINE

INTRODUCTION

Lung is designed to provide a large

internal surface area so inspired air and capillary blood

can get close to each other for exchange of

gases.

RESPIRATORY EMBRYOLOGY

•Lower respiratory system (from pharynx down) start develops during 4th week

•Respiratory diverticulum appears as an outgrowth from ventral wall of foregut

•Respiratory diverticulum expands caudally and tracheoesophageal septum seprates it from foregut.

• Connective tissue, cartilage and smooth muscles - develop from splanchnic mesenchyme surrounding foregut

STAGES OF DEVELOPMENT OF

LUNG

Stages of human lung development and their timing. note overlapping between

stages, particularly between the alveolar stage and the stage of microvascular maturation.open ended bars indicate

uncertainity of the exact timing.

`Development of the airways and arteries. The stages of lung development

(blue) are correlated to thedevelopment of the bronchial tree (black) and the arteries (red). An average

value of the generations formed by eachcategory of airways is given in parentheses. On average an airway of the

human lung ends after 23 generations in an alveolar saccule.

EMBRYONIC STAGE

•Dorsal part forms esophagus

•Ventral forms trachea and lung bud

•Respiratory primordium communicates with pharynx by laryngeal orifice.

•The lung bud develops into trachea and 2 endodermal bronchial buds grow into pericardioperitoneal cavities

• By 5th week , each bronchial bud enlarges and forms primordium of a primary bronchus .Pleuroperitoneal canal narrows & pleuroperitoneal and pleuropericardial fold separate peritoneal and pericardial cavities form primitive pleural cavity

•At 7th week :primary bronchi subsequently –secondary-tertiary bronchi develop

• Tertiary bronchi divide an additional 7 more times before birth and 14 times after birth to form respiratory bronchioles .

PRENATAL LUNG DEVELOPMENT----FETAL PERIOD (WEEK 5th -TERM)

PSEUDOGLANDULAR STAGE

•At this stage developing lung is having characteristics of tubular gland.

•No respiratory bronchiole or alveoli

•BRANCHING CONTINUED TO FORM TERMINAL BRONCHIOLE

The stage of conductive airway formation.

During the pseudoglandular stage, •epithelial tubules branch constantly penetrate to surrounding mesenchyme

•loose three-dimensional capillary network is located in the mesenchyme

CANALICULAR STAGE (Weeks 16th-26th)

•At canalicular stage differentiation of the pulmonary epithelium and formation of the typical air-blood barrier; •The beginning of surfactant synthesis and secretion;

•The “canalization” of the lung parenchyma by capillaries.

The canalicular stage

a widening of the future airways;

a differentiation of the tall columnar

•Epithelial cells of the pseudoglandular stage divide into type I and II epithelial cells. •The formation of first air-blood barriers

•Each terminal bronchiole divide into 2 more respiratory bronchiole which divide into 3-6 alveolar duct

•At the end of canalicular stage: lung gas exchange is possible

•There is no chance of survival ,if a premature child born before this stage

Epithelial Differentiation: Formationof the Air-Blood Barrier The cuboidal glycogen-rich epithelium of the bronchialtree differentiates into two types of epithelial cells: (1) type II epithelial cell : produce surfactant; and formation of the first thin air-blood barrier (2)type I epithelial cells-become thinner –surrounding capillaries protrude into alveolar sac cause .intimate contact between epithelial & endothelial cell

SACCULAR STAGE (WEEK 24 TO TERM)

The mesenchyme condenses to formthick inter-airway septa that contain a capillary layer on either side of the septum. The widened terminal ends of the bronchial tree are recognized as saccules.

•Beginning of saccular stage: peripheral airways form typical clusters of widened airspaces termed saccules or terminal sacs By widening and lengthening of all airspace generations. Distal to the terminal bronchioles & last generations of airspaces, future gas exchange region expands massively.

• Each new generation formed as blind-ending Saccule & open-ended channel

• Morphology of all these channels and saccules undergoes change until the formation of the alveoli is completedin the postnatal period( transitory ducts and transitory saccules)

• Morphology changes to form primitive alevoli& capillaries which establish close contact

POSTNATAL LUNG DEVELOPMENT

•Human babies are born in the early alveolar stage.

•At birth the replacement of lung liquid by air and the onsetof respiration represents a major change in pulmonary development.

•In humans, formation of alveoli starts during late intrauterine life, most alveoli (more than 85 percent) are formed after birth.

•Alveolar Stage (Week 36 to 1-2 Years)

•Stage of Microvascular Maturation (Several months to 2–3 Years)

visceral pleura: splanchnic mesoderm covering outside of lung. Parietal pleura: by somatic mesoderm layer covering inner surface of body wall.

FORMATION OF PLEURA

TRACHEOESOPHAGEAL FISTULA

Incidence 3000-4500 live birth

•Incomplete division of cranial part of forgut into respiratory and esophageal part

•A fistula may exist connecting trachea and esophagus

•Infant present as coughing and choking upon swallowing. Gastric contents reflux into trachea and lungs -pneumonia or pneumonitis.

•Associated with Polyhydramnios

90%case :upper oesophagus ending in blind pouch and lower segment form fistula

4% case isolated esophageal atresia

4% case H-type TEF TEF component of VACTERL

A. Posteroanterior radiograph of a smooth-walled sub carinal bronchogenic cyst. Note that the cyst isdistinct from the right heart border (arrows). B. Contrast-enhanced axial CT image of the homogenous, subcarinal

bronchogenic cyst.

Formed due to abnormal dilation of terminal larger bronchi

May be small or multiple onradiograph(honeycomb appearance)

Cystic structures drain poorly-cause frequent chronic infection

CONGENITAL CYST OF LUNG

OTHER ABNORMALITIES•Surfactant formed by type 2 epithelial cell important for survival of premature infant

Decrease surfactant cause high air water surface membrane tension so alveoli become collapse during expiration

Called as respiratory distress syndrome

Blind ending trachea,absence of lung,agensis of lung are rare conditions

Abnormal division of bronchial tree common and little clinical significance but can cause difficulties during bronchoscopy

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