FORMATION OF THE LUNG BUDS
When the embryo is approximately 4 weeks old,
the respiratory diverticulum (lung bud) appears
as an outgrowth from the ventral wall of the foregut (Fig. 14.1A). The appearance and location of
the lung bud are dependent upon an increase in
retinoic acid (RA) produced by adjacent mesoderm. This increase in RA causes upregulation of
the transcription factor TBX4 expressed in the
endoderm of the gut tube at the site of the respiratory diverticulum. TBX4 induces formation of the
bud and the continued growth and differentiation
of the lungs. Hence, epithelium of the internal lining of the larynx, trachea, and bronchi, as well as
that of the lungs, is entirely of endodermal origin.
The cartilaginous, muscular, and connective
tissue components of the trachea and lungs are
derived from Visceral (splanchnic) mesoderm
surrounding the foregut.
Initially, the lung bud is in open communication
with the foregut (Fig. 14.1B). When the diverticulum expands caudally, however, two longitudinal
ridges, the tracheoesophageal ridges, separate it
from the foregut (Fig. 14.2A). Subsequently, when
these ridges fuse to form the tracheoesophageal
septum, the foregut is divided into a dorsal portion, the esophagus, and a ventral portion, the trachea (Fig. 14.2B,C). The respiratory primordium
maintains its communication with the pharynx
through the laryngeal orifice (Fig. 14.2D).
FIGURE 14.1 A. Embryo of approximately 25 days’ gestation showing the relation of the respiratory diverticulum to the heart, stomach, and liver. B. Sagittal section through the cephalic end of a 5—week embryo
showing the openings of the pharyngeal pouches and the laryngotracheal orifice.
FIGURE 14.2 A-C. Successive stages in development of the respiratory diverticulum (lung bud) showing the tracheoesophageal ridges and formation of the septum, splitting the foregut into esophagus and trachea with bronchial buds. D. The ventral portion of the pharynx seen from above showing the laryngeal orifice and surrounding swelling.
