Embryonic Development of the Respiratory System

Core Concepts

Origins: the lung bud and the foregut

Around the fourth week, the respiratory system begins as an outgrowth of the endoderm — the respiratory diverticulum (lung bud) appears on the ventral wall of the foregut, just caudal to the pharynx. Endoderm forms the epithelial lining of the airways; the surrounding splanchnic mesoderm forms connective tissue, cartilage, smooth muscle, and capillaries. The foregut and lung bud initially share a channel; the tracheoesophageal septum grows between them, dividing the tube into the ventral laryngotracheal tube (larynx, trachea, lungs) and the dorsal esophagus. Incomplete separation causes a tracheoesophageal fistula — an abnormal trachea–esophagus connection managed surgically after birth.

Stage 1 — Embryonic (weeks 4–7): the tree begins

During the embryonic stage the lung bud elongates into the trachea and splits into right and left primary bronchi. The right bronchus branches into three secondary (lobar) bronchi (for the right lung's three lobes) and the left into two (for the left lung's two lobes); continued branching produces the segmental bronchi. Even at this early stage, the pattern of the adult bronchial tree is laid down — an asymmetric tree mirroring the adult arrangement (three right lobes, two left).

Stage 2 — Pseudoglandular (weeks 5–17): the conducting airways

The name "pseudoglandular" comes from the microscopic appearance: branching tubules lined with cuboidal epithelium look like a developing gland. The bronchial tree branches repeatedly, generating the complete conducting airway system down to the terminal bronchioles — all the airways that will conduct air, but none that will exchange it. Because these generations form so early, disruption during this window can impair branching.

Stage 3 — Canalicular (weeks 16–25): the first gas-exchange surfaces

This is the turning point. The terminal bronchioles divide into respiratory bronchioles, which bud into terminal sacs (primitive alveoli) — the first structures capable of gas exchange. The epithelium thins and capillaries approach the sac walls, forming the first thin respiratory membranes. Type II alveolar cells begin producing surfactant near the end of this stage (around week 20 and beyond). This is also when a fetus first becomes potentially viable: survival outside the womb becomes possible near the canalicular-to-saccular boundary, with intensive neonatal care and significant risk — viability depends on medical capability and varies over time and place.

Stage 4 — Saccular (weeks 24 to birth): saccules multiply, surfactant matures

The terminal sacs multiply and enlarge, greatly increasing the future gas-exchange surface. Type II cells step up surfactant production, and the surfactant system matures — the period when the lungs become progressively able to stay open after birth. Surfactant output rises steeply late in gestation, which is why lung maturity and gestational age are so closely linked.

Stage 5 — Alveolar (week 36 to about 8 years): the final expansion

True alveoli — the definitive gas-exchange units — begin to appear at about 36 weeks. Most alveolar formation, however, happens after birth: the newborn has only a fraction of the adult's roughly 300 million alveoli (commonly taught estimate), and new alveoli continue to form through early childhood, with the greatest multiplication in the first few years. Lung development is not finished at birth.

Surfactant and the premature lung

Surfactant — produced by type II alveolar cells — is a mix of phospholipids and proteins that lowers surface tension at the air-liquid interface of the alveoli (see the breathing topic). In a premature infant whose surfactant has not matured, the alveoli have high surface tension, collapse easily (atelectasis), and are hard to reinflate; the infant works hard to breathe and oxygenates poorly. This is neonatal respiratory distress syndrome (NRDS) — a condition of prematurity, distinct from adult respiratory distress syndrome (ARDS), though both impair gas exchange. Management belongs to neonatal intensive care teams; the educational point is that the surfactant timeline is why gestational age predicts lung function at birth.

Before birth: the fetal circulation and the first breath

The fetus does not use its lungs for gas exchange; the placenta does, so the fetal circulation is arranged to bypass the lungs. Oxygenated blood returns from the placenta via the umbilical vein, much of it shunted past the liver through the ductus venosus; in the heart, the foramen ovale lets blood pass from right atrium to left atrium, bypassing the right ventricle and lungs; and the ductus arteriosus connects the pulmonary trunk to the aorta, diverting blood away from the unexpanded lungs. At birth, several things happen at once: the first breath expands the lungs, pulmonary vascular resistance falls, blood flow to the lungs surges, and the shunts begin to close — the foramen ovale seals functionally within minutes (left atrial pressure rises), and the ductus arteriosus constricts within the first day or two (becoming the ligamentum arteriosum). The first cry is not just a sound: it is the mechanical event that inflates the lungs and triggers the transition from placental to pulmonary life. (Closure timing varies and is verified in current texts; closure is functional before it is anatomical.)

Common Confusions