Anatomy And Development Of The Respiratory System

Question 1

Summarize development of larynx.

Answer 1

Laryngeal epithelium develops from endoderm of cranial part of the Laryngotracheal tube

Laryngeal cartilages are derived from migration of neural crest into the mesenchymal of 4th & 6th pairs of pharyngeal arches

Laryngeal muscles from mesenchymal myoblasts from the 4th and 6th pharyngeal arches

Arytenoid swellings-mesenchyme at cranial end of Laryngotracheal tube

Epigottal swelling- derived from the caudal part of hypopharyngeal eminence (3rd and 4th pharyngeal arches)

Laryngeal ventricles- formed by recanalozation of the laryngeal lumen

Question 2

Summarize development of larynx and trachea

Answer 2

The respiratory system develops from endodermal and mesodermal germ layers of the developing human

Laryngotracheal groove envaginates forming Laryngotracheal diverticulum anterior to the foregut by the end of week 4.

• As the laryngotracheal diverticulum elongates, it becomes the Laryngotracheal tube and it’s distal end enlarges to form the Respiratory bud- then bronchial buds
• Splachnic mesoderm surrounding the Laryngotracheal diverticulum develop into the cartilage, connective tissue and muscles of the trachea.

Tracheoesophageal septum divides the fire gut into:

Ventral- Laryngotracheal tube
Dorsal- oropharynx and esophagus

Question 3

Outline formation of trachea and lungs

Answer 3

Starts at beginning of 4th week—> tracheal buds form from Laryngotracheal tube—> esophagus and trachea form—> bronchial bud forms under trachea —> trachea bifurcated—> bronchial buds develop—>8 weeks shows

Question 4

What is the significance of respiratory bud ?

Answer 4

A respiratory bud (lung bud) develops at the caudal end of the Laryngotracheal diverticulum during the 4th week

As the lung bud develops it divides into 2 primary bronchial buds.

The secondary and tertiary buds grow laterally from the primary buds into the pericardioperitoneal canals

Bronchial buds develop into bronchi

• primary buds- primary/main bronchi
• secondary bronchi-lobar, segmental and intrasegmental branches

Segmental bronchus with surrounding mesenchyme- bronchopulmonary segments
-By the 24th week, about 17 orders of branches are formed and the respiratory bronchioles develop

7 more generations of branches are formed after birth- 8 years

Question 5

What is atresia?

Answer 5

Obstruction/blockage/abnormal closure or development of a passage. Here it is due to failure of recanalization of the lumen or incomplete division of the foregut

Question 6

What is stenosis ?

Answer 6

Narrowing/partial closure of a passage. Here it is mainly due to unequal partitioning of the foregut

Question 7

What is Fistula?

Answer 7

Abnormal communication / passage due to incomplete division of a tube (e.g. incomplete division of cranial part of foregut into respiratory system and esophagus)

Question 8

What is a tracheo-esophageal fistula (TEF)?

Answer 8

Abnormal communication between trachea & esophagus due to defective development (incomplete division or abnormal development of the upper foregut) of the tracheo-esophageal septum [1 in 3,000 births; common in males] may be associated with esophageal atresia

Question 9

What is esophageal atresia?

Answer 9

Abnormal blockage/ development of the esophagus mainly due to failure of recanalization of lumen/incomplete division of foregut (forming a blind pouch). More common

In about 30% of cases TEF may be associated with other anomalies. TEF may be a component of VECTERL- Vertebral anomalies, Anal atresia, Cardiac defects, TEF, Esophageal atresia, Renal anomalies & Limb defects

TEF. May be associated with Poly-hydramnios [excessive quantity of amniotic fluid in fetal life] - as esophageal atresia inhibits the free passage of swallowed amniotic fluid to the intestines

Question 10

What are the signs and symptoms of trachea-esophageal fistula?

Answer 10

Coughing and choking during feeding

Pneumonia

Pneumonitis

Poly hydramnios in fetal life

Question 11

How does TEF manifest physically?

Answer 11

Upper esophageal segment ends blindly

Esophagus ends blindly, inferior end joins the trachea (most common)

Question 12

Explain the stages of Lung development

Answer 12

A. Pseudo-glandular stage (week 5-week 17)- major elements of the lung are formed but no respiratory bronchioles

B. Canalicular period (week 16-25)-terminal bronchiole divides into 2 or more respiratory bronchioles, which in turn divide into 3-6 alveolar ducts. Type II Alveolar cells appear which produce surfactant.

-Surfactant secretion starts during the 20th week, increases gradually with a sharp increase during last 2 weeks of pregnancy

C. Terminal Sac period(week 24 to birth)-terminal Sacs formed lined by squamous epithelium; capillaries establish close contact

D. Alveolar period ( week 32(late fetal period) to 8-10 years)-Primitive alveoli with well developed epithelial- endothelial capillary contacts (blood-air barrier after birth8

-Mature alveoli are formed after birth as the lungs expand (alveoli formation continues up to 8- 10 years)

Question 13

Explain the production of surfactant

Answer 13

-Surfactant (Surface active agent, lipoprotein produced by Type II alveolar cells) reduces the surface tension and facilitates expansion of the terminal sacs

Production begins around week 20, but only around weeks 22-26 it’s production is sufficient for survival (can survive in an intensive care unit)

By. Weeks 26-28- adequate terminal sacs & surfactant (can survive if born prematurely)

Question 14

Explain the significance of surfactant deficiency

Answer 14

This is a major cause of respiratory distress syndrome (RSD)/hyaline membrane disease-1-2% of newborns, (higher in premature babies). Infants develop rapid labored breathing shortly after birth

Prolonged intrauterine asphyxia- leads to irreversible changes in type II cells rendering them incapable of producing surfactant

Question 15

What is the significance of glucocorticoids for RDS?

Answer 15

Glucocorticoids during pregnancy accelerates fetal lung movements & surfactant production (corticosteroids or exogenous surfactant are indicated in RDS)

Question 16

What are the signs and symptoms of surfactant deficiency?

Answer 16

Under inflated lungs

Amorphous protein deposits in lungs,

Respiratory distress

Question 17

What factors influence the normal development of lungs?

Answer 17

1. Thoracic space for growth
2. Fetal breathing movements
3. Amniotic fluid movements

Question 18

Explain changes in changes in the respiratory system at birth

Answer 18

• In infants with congenital diaphragmatic hernia the lung is unable to develop normally as the thoracic space is reduced due to the presence of intestinal content in the thoracic space. Therefore, reducing the thoracic space
• Fetal breathing movements stimulate lung development(allows for the aspiration of amniotic fluid into the lungs). At birth lungs are partially filled with amniotic fluid, which is later absorbed by the capillaries & lymphatics. [A still born infant’s lungs are firm and sinks in water due to them containing more fluid and no air]
• Adequate Amniotic fluid levels allows for proper lung development. Oligohydroamnios (insufficient amount of amniotic fluid) may lead to retarded development of lungs (pulmonary hypoplasia)

Question 19

What is the significance of intra embryonic coelom (IEC) and body cavities?

Answer 19

Intraembryonic coelom is the primordium of future body cavities

Develops in the lateral mesoderm

Divides the lateral mesoderm into 2 layers

Somatopleura- parietal layer of. Serous pericardium, pleura and peritoneum

Splanchnopleure- visceral pericardium, pleura and peritoneum

Question 20

Describe formation of pleural and pericardial cavities

Answer 20

Bronchial buds grow into the pericardioperitoneal canal

Membranous ridges develop from lateral wall.
-Cranial ridge-
Pleuropericardial folds
-Caudal ridge-
Pleuroparietal folds

Pleuropericardial membrane

• separate pleural cavity from pericardial cavity
  
  • will fuse to form the fibrous pericardium

Question 21

Describe formation of the diaphragm

Answer 21

Closure of the pleeuroperitoneal openings (development of diaphragm)-separate the thoracic cavity from the abdominal cavity from the abdominal cavity

Septum transversum-grows from the anterior body wall and partially separates the pericardial and peritoneal cavities during lateral folding of the body wall

Dorsal mesentery of esophagus- forms the median structure where the other parts of the diaphragm will attach

Migration of myoblasts- into the peritoneal membrane- results in final closure of the pleuroperitoneal canal

Question 22

What are the sites of common diaphragmatic hernias, and defects?

Answer 22

1. Anteromedial defects
   - Rare about 2-5%
   - Defect between costal and sternal musculature
2. Central Esophgeal hiatus may be abnormally large- congenital hiatal hernia
3. Posterolateral defects
   - defect of pleuroperitoneal membrane-congenital diaphragmatic hernia

• abdominal contents herniate into thorax
• more common on the left side

Question 23

What are the signs and symptoms of congenital diaphragmatic hernia?

Answer 23

The abdomen is empty when lying flat as abdominal contents herniate into the thorax

The lung is compressed & hypoplastic (high mortality rates)- requires urgent surgery

Question 24

What causes event ration of the diaphragm?

Answer 24

Caused by a defective mechanism in one/both halves of the diaphragm due to failure of the muscle tissue fro’ the body wall to extend into the pleuroperitoneal membrane. The affected side moves up with the contraction of diaphragm during respiration- paradoxical respiration

Question 25

Explain the clinical presentation of eventration of diaphragm

Answer 25

Has similar clinical presentation as a diaphragmatic hernia as intestinal content moves into the thoracic cavity. Difference being the type of defect (muscular in evaluation and pleuroperitoneal membrane in hernia)