SN4 SHORT NOTE 5M ★ High-yield

Describe the development of the lungs and the body cavities.

Anatomy · 1st Professional — Pre-Clinical

Stage	Weeks	Events
Pseudoglandular	5–16	Branching to the terminal bronchioles; gland-like appearance; respiration is not possible
Canalicular	16–26	Respiratory bronchioles form; capillaries proliferate; survival just possible at the end
Terminal sac (saccular)	26–birth	Terminal sacs form; type I and type II pneumocytes differentiate; surfactant production begins
Alveolar	32 weeks to about 8 years	Mature alveoli form; most alveoli develop after birth

Only about 15% of the adult number of alveoli is present at birth; the rest form in childhood, which is why lung damage in early life has lifelong consequences
Surfactant appears from about 24 weeks and is adequate by 34 to 36

Oesophageal atresia with tracheo-oesophageal fistula — failure of the tracheo-oesophageal septum; the commonest variety has a blind upper pouch with a distal fistula. Suspect it with polyhydramnios, frothing and choking on the first feed
Respiratory distress syndrome — surfactant deficiency; antenatal corticosteroids between 24 and 34 weeks and exogenous surfactant have transformed survival
Bronchopulmonary sequestration — a mass of lung tissue with no connection to the airway and a systemic arterial supply, usually from the aorta
Congenital lobar emphysema and congenital cystic adenomatoid malformation present with respiratory distress or recurrent infection

The intraembryonic coelom appears in the lateral plate mesoderm as a horseshoe-shaped cavity
It is divided by the pleuropericardial folds into the pericardial cavity and the two pleural cavities
And by the pleuroperitoneal membranes from the peritoneal cavity
The pleuropericardial folds carry the phrenic nerves, which is why the phrenic nerve lies in the fibrous pericardium in the adult

Congenital diaphragmatic hernia — from failure of the pleuroperitoneal membrane, usually on the left; the danger is pulmonary hypoplasia rather than the hernia
The phrenic nerve's position on the fibrous pericardium is a direct consequence of the pleuropericardial fold, and explains why it is at risk in cardiac and mediastinal surgery
Pericardial defects are rare and usually asymptomatic, but can allow herniation of the heart
Understanding the septum transversum ties together the diaphragm, the liver, the lesser omentum and the referred pain of the shoulder — one structure explaining several apparently unrelated facts

All questions in Systemic Embryology