Peristalsis begins in week 10 when neural crest cells invade the mus-
cular layer to form the enteric nervous (autonomic) system. Failure of
neural crest cell migration to the distal hindgut results in aganglionic
megacolon (Hirschsprung disease),which may cause fatal intestinal
obstruction.
The adult pattern of GI organ distribution is achieved by physiologic
herniationand then retraction of the midgut during the second month.
Failure of the midgut loop to return to the abdominal cavity may result
in an omphaloceleorumbilical hernia.
Associated digestive organs (liver, gallbladder, and pancreas) originate
as outgrowths of the endodermal tube. Connective tissue components of
the liver are derived from both splanchnic and somatic (septum transver-
sum)lateral plate mesoderm. Lateral plate mesoderm also forms the peri-
toneum and mesenteries of the abdominal cavity.
FORMATION OF THE RESPIRATORY SYSTEMThe first part of the respiratory system is lined by ectoderm derived from
the nasal ectodermal placodes.
In the fourth week, a respiratory diverticulumarises as an outgrowth
of the ventral endodermal tube.
Endoderm will form the respiratory epithelium, whereas splanchnic
lateral plate mesodermwill form connective tissue elements including
cartilage, smooth muscle, and blood vessels.
Mesodermdirects the branching pattern of the developing airways.
Thediaphragmforms from the septum transversum,the two pleu-
roperitoneal membranes,thedorsal mesenteryof the esophagus(where
thecruradevelop), and the muscular parts of the dorsal and lateral body
wall.
Although most alveolido not form until after birth, the lungs are capa-
ble of sufficient gas exchange after 6.5 months of gestation. Respiratory dis-
tress syndrome (RDS)develops in premature births because of immaturity
of the Type II pneumocytes that produce surfactant.Surfactant is essential
for expansion of the pulmonary alveoli; it lowers the air-interface surface
tensionand prevents the alveoli from collapsing at the end of expiration.
Without surfactant, premature babies suffer from RDS with rapid breathing,
chest wall retractions, grunting noise with each breath, and nasal flaring.
Abnormal septation of the trachea and esophagus can result in steno-
sis, atresia, or tracheoesophageal fistulas (TEFs).
10 Anatomy, Histology, and Cell Biology