The Questions of Developmental Biology

Liver, pancreas, and gallbladder
The endoderm also forms the lining of three accessory organs that develop immediately
caudal to the stomach. The hepatic diverticulum is the tube of endoderm that extends out from
the foregut into the surrounding mesenchyme. The mesenchyme induces this endoderm to
proliferate, to branch, and to form the glandular epithelium of the liver. A portion of the hepatic
diverticulum (that region closest to the digestive tube) continues to function as the drainage duct
of the liver, and a branch from this duct produces the gallbladder (Figure 15.29).
The pancreas develops from the fusion of distinct dorsal and ventral diverticula. Both of
these primordia arise from the endoderm immediately caudal to the stomach, and as they grow,
they come closer together and eventually fuse. In humans, only the ventral duct survives to carry
digestive enzymes into the intestine. In other species (such as the dog), both the dorsal and ventral
ducts empty into the intestine.
The Respiratory Tube
The lungs are a derivative of
the digestive tube, even though they
serve no role in digestion. In the center
of the pharyngeal floor, between the
fourth pair of pharyngeal pouches, the
laryngotracheal groove extends
ventrally (Figure 15.31).
This groove then bifurcates into
the two branches that form the
paired bronchi and lungs. The laryngotracheal
endoderm becomes the lining of the trachea, the
two bronchi, and the air sacs (alveoli) of the
lungs.
The lungs are an evolutionary novelty, and they are among the last of the mammalian
organs to fully differentiate. The lungs must be able to draw in oxygen at the newborn's first
breath. To accomplish this, the alveolar cells secrete a surfactant into the fluid bathing the lungs.
This surfactant, consisting of phospholipids such as sphingomyelin and lecithin, is secreted very
late in gestation, and it usually reaches physiologically useful levels at about week 34 of human
gestation.