Understanding the key steps in gastrointestinal development offers insight into the pathophysiology of both acquired and congenital gastrointestinal diseases. Here we review early patterning, later differentiation, and the role of stem cells in the maintenance of the mature epithelium. Because many developmental paradigms are conserved across vertebrate species, we use examples from mammalian and nonmammalian models to illustrate much of what is currently understood about the molecular control of gastrointestinal development.
Early Development: Patterning the Gastrointestinal Tract
Three Germ Layers Contribute to Gastrointestinal Development
Development of the human gastrointestinal tract begins early in gestation. At 4 weeks, a tube of endoderm, the primitive gut, is formed by cephalocaudal and lateral folding. At this stage, the gastrointestinal tract includes a cranial part known as the foregut, a caudal part known as the hindgut, and a midgut, which lies between the two and is temporarily connected to yolk sac by the vitelline duct.
During the next 12 weeks of gestation, the gut tube extends lengthwise, and turns on itself as differentiation proceeds along the craniocaudal axis. In addition, the foregut endoderm buds into the adjacent lateral plate mesoderm (LPM) to form the liver, pancreas, lung, and thyroid. In cooperation with the LPM, the foregut endoderm also gives rise to the pharynx, esophagus, stomach, and part of the duodenum. The distal duodenum, jejunum, ileum, and proximal portions of the colon derive from the mesoderm-endoderm bilayer of the midgut. Finally, the hindgut will yield the distal two-thirds of the colon and the rectum ( Figure 1-1 and Figure 1-2 ). Concurrent with these early steps, neural crest cells migrate along the developing gut to eventually form the enteric nervous system. Thus, in total, all three embryonic germ layers contribute to the gastrointestinal tract, which at birth comprises a series of functional segments along the craniocaudal axis synchronized to digest and assimilate food, while eliminating nondigestible matter.
The Gastrointestinal Tract Develops Along Four Axes
Early gastrointestinal development is orchestrated along four axes : anteroposterior, radial, dorsoventral, and left-right axes (reviewed in de Santa Barbara et al ).
Anteroposterior and Dorsoventral Axis
Anteroposterior (AP) and dorsoventral (DV) cell fate decisions begin as early as the fourth week of human gestation resulting in the characteristic regionalization into foregut, midgut, and hindgut.
It is commonly accepted that the gastrointestinal system develops from the embryonal germ layers through a series of reciprocal interactions between the endoderm and mesoderm. Although the exact molecular signals are not yet understood in humans, the mouse, zebrafish, and Xenopus laevis models have contributed significantly to advancing our understanding of vertebrate gastrointestinal development at the genetic and molecular levels.
Temporally regulated expression of tissue-specific transcription factors and signaling pathways interact to ensure gut differentiation along the AP axis. The major transcription factors involved in early regionalization are summarized in Figure 1-3 . In brief, the anterior endoderm, which will give rise to the epithelium of the cranial part of the gastrointestinal tract and the trachea, expresses NK2 homeobox 1 (Nkx2.1). The midgut is characterized by the expression of Pancreatic and duodenal homeobox 1 ( Pdx1) in precursors of the pancreas, gallbladder, cystic duct, and common bile duct. The hindgut is characterized by the expression of Caudal type homeobox 2 ( CDX2). Its expression is strongest in the presumptive distal jejunum, ileum, and proximal colon, and it is also a key proliferation and differentiation factor along the crypt-villus axis.
These endodermal transcription factors respond to local endodermal signals and to signals from the surrounding mesoderm. Very early anterior endoderm patterning, prior to the development of the primitive gut tube, is under the molecular control of mesodermal Wingless-In (Wnt), bone morphogenetic protein (BMP), and fibroblast growth factor (FGF) signaling ( Figure 1-4 ). The secreted protein Sonic hedgehog (SHH) is secreted by the endoderm in mouse and chick to contribute to the patterning and differentiation of the adjacent mesoderm throughout the AP axis. SHH inhibits pancreas formation and promotes intestinal development. Likewise, Wnts promote hindgut, whereas Wnt repression is required for liver development. Taken together, these examples highlight two important developmental aspects: the importance of crosstalk between the layers of the developing gastrointestinal tract and the role of very tight regional and temporal regulation of developmental programs to yield specialized tissues.
Left–Right Axis
As gastrointestinal development proceeds, both visceral organs and vasculature begin to show the characteristic abdominal asymmetry known to most vertebrates. Left–right (LR) axis patterning begins during neurulation. Although the heart is the first organ to display visible asymmetry around 6 weeks (reviewed in Babu et al. ), shortly thereafter, the spleen, stomach, liver, and gallbladder localize to their respective sides of the abdomen and the lungs exhibit structural differences between the left and right organs.
In mouse and chick models, it has been shown that laterality is under the control of Nodal and Hedgehog (Hh) signaling upstream of a complex molecular cascade in the left LPM. Figure 1-5 summarizes the complex molecular process preceding the differential mesenchymal aggregation of the mesentery, which initiates chirality.
Radial Axis
The intestine is a long tube organized in concentric layers: the epithelium, mesenchyme, immune cells, and smooth muscle layers. By radial patterning, we refer to the differentiation from the lumen (epithelium) to the serosa surrounding the outer layer of muscularis. The local specialization of each segment of the gastrointestinal tract lies in its radial organization ( Figure 1-6 ).
As for the others axes, radial axis development depends on mesoderm-endoderm interactions. SHH has several functions in this process affecting both cell fate and proliferation in the mesenchyme and epithelium. In mouse and chick models, Shh regulates the development of the smooth muscle layers, controls epithelial proliferation and differentiation, and ensures the positioning of crypts. In addition, SHH signals via BMP4 to ensure survival and proliferation of the neural crest cells, which migrate into the primitive gut where they differentiate into the enteric nervous system. The development of the enteric nervous system is reviewed extensively elsewhere.
Later Morphologic and Functional Development: Differentiation
Esophagus
Esophageal development is tightly linked to that of other foregut structures, especially the trachea, which emerges at around the fourth week of gestation. The foregut separates into trachea and esophagus. Although the mechanisms underlying the separation remain controversial, focal apoptosis of early epithelial cells may be involved. Incomplete separation may lead to anomalous communications between the trachea and esophagus, diagnosed immediately after birth under the name trachea-esophageal fistula. Clear candidate genes associated with this spectrum of disorders have not been identified, but studies in mice suggest that the absence of Sox2 or Nkx2.1, respectively, in the dorsal and ventral foregut, leads to incomplete separation of the esophagus and trachea.
Stomach and Duodenum
Stomach development takes place over a 4-week interval. Epithelial stomach morphogenesis is regulated by mesodermal homeobox genes under the control of mesodermal signaling (summarized in Figure 1-3 ). At the fourth week of human development, the proximal foregut dilates to adopt a fusiform shape. During the fifth week, differential growth between the dorsal and ventral walls of the stomach yields the greater and lesser curvatures. Concurrent LR patterning places the stomach in the upper left side of the abdomen, thereby forcing the midgut to loop in a counterclockwise manner. This loop first rotates anticlockwise by 90° at the seventh week, and at the eighth week, the stomach acquires its characteristic shape well suited to mixing and stirring swallowed food.
The stomach and duodenum are separated by the pyloric sphincter. Under normal conditions, the pyloric sphincter is slightly hypertrophied to limit luminal flow into duodenum. In some cases hypertrophy is more severe and obstructs chyme movement; this partial obstruction is known as infantile hypertrophic pyloric stenosis (IHPS).
Small Intestine and Colon
The main function of the small intestine is the absorption of nutrients and minerals in food. Then water and sodium, together with some vitamins and micronutrients, are absorbed in the colon.
From the fifth week, rapid growth causes intestine elongation and rotation. At the beginning of the sixth week, the middle portion of the growing intestine begins to herniate into the body stalk and undergoes a 90° counterclockwise twist around the axis of the mesenteric vessels. At the 10th week, another rotation in a 180° anticlockwise direction ensures the retraction of the extraembryonic segment of the gut back into the abdominal cavity.
Intestinal elongation is limited by the size of the coelomic cavity, which may in part contribute to the extraembryonic herniation. Once the developing small bowel returns to the abdominal cavity, spatial constraints dictate economical organization and folding. A recent study suggests that the size and number of intestinal loops can be predicted based on physical properties, which include the geometry, relative growth rates, and mechanical properties of the mesentery and gut tube.
Development of the Gastrointestinal Epithelium
The epithelium of the postnatal and adult gastrointestinal tract is metabolically active to ensure digestion and absorption of nutrients. It is characterized by a high cell turnover. Throughout life, the intestinal epithelium is renewed every 5 days by an intricate system of progenitor-cells. This feature is important because repetitive exposure to the external environment via the passage of food contributes to the enzymatic and cellular wear and tear of the intestinal epithelium.
Epithelial morphogenesis begins with endoderm formation. The primitive intestinal epithelium is initially pseudostratified. A crucial developmental step occurs during the vacuolization and recanalization of the intestinal lumen ( Figure 1-7 ): mesoderm-derived mesenchymal expansions project into the remodeling lumen to form villi, the length of which will depend on their position along the craniocaudal axis. At 30 weeks of gestation, adult-type crypt epithelium is present.
The intestinal epithelium relies on several types of intercellular junctions for normal function. These junctions appear from the 10th week onward. Their role is to maintain the mechanical and chemical integrity of the epithelium while allowing for intercellular signaling. Desmosomes are located on the lateral side of the enterocyte; they provide mechanical strength by distributing epithelial forces. Adherens junctions are located at the apical pole of the enterocyte and contribute to epithelial cohesion. Gap junctions are located between enterocytes; their role is intercellular communication and electrical coupling. Finally, tight junctions constitute the barrier between the external milieu in the lumen and the internal environment of the submucosa. Together, these intercellular junctions collaborate to maintain a barrier to pathogens, toxins, and antigens while allowing the absorption of nutrients and water (reviewed in Zeissig et al. ). Disruption of the intestinal tight junction barrier leads to inflammation and secondary changes in the mucosal immune system, which is accepted to contribute to the development of inflammatory bowel disease, celiac disease, and type 1 diabetes.
Crypts and Villi: Complementary Roles in Development and Homeostasis
The crypt is the proliferative compartment of the intestinal epithelium. Intestinal stem cells reside in the crypt from which multipotent cells emerge. These multipotent cells then progressively specialize into enterocytes (absorptive cells), enteroendocrine cells (which secrete peptide hormones), Paneth cells (which secrete a number of antimicrobial molecules and contribute to the stem cell niche ), and mucous cells. Unlike the other cell types, Paneth cells remain at the base of the crypts where they are removed after 21 days by phagocytosis. Conversely, enterocytes, enteroendocrine, and mucous cells continue to proliferate, differentiate, and migrate to the apex of the villi where they are eventually eliminated in the lumen. Long and thin villi are characteristic of small intestine epithelium, whereas colonic epithelium is typically flat.
Although much is understood about the molecular regulation of gastrointestinal epithelial development, little is known about what drives morphologic changes, such as the development of villi. Recent work in chick, mouse, and Xenopus laevis , suggests that the mechanical constraints imposed by the development of concentric muscle layers contribute to morphologic changes in the adjacent epithelium and submucosa.
Nutrient absorption in the gastrointestinal tract is ensured by microvilli on the enterocyte apical membrane, known as the brush border. Microvilli have multiple functions: absorption, secretion, cellular adhesion, and mechanotransduction. Between the 10th to 14th weeks, the brush border develops at the apical pole of the enterocyte through the appearance of finger-like projections called microvilli, which are the seat of digestive enzyme activity. Brush-border enzyme activity begins during the eighth week of human development. The activity of sucrase, maltase, alkaline phosphatase, and aminopeptidase increases little by little, and by 14 weeks of gestation, it is comparable to that of adult intestine.
Later Development and Postnatal Maturation of the Intestine
Although complex, intestinal development is relatively rapid and occurs early in gestation. But a number of important gastrointestinal functions appear later and even after birth. In addition to the absorption and digestion of nutrients, the gut must also maintain a protective barrier against the external environment. The barrier is composed of the epithelium, the intestinal microbiota, and the immune system.
The Microbiota
During fetal life, the gastrointestinal environment is sterile, but at birth and rapidly thereafter, bacteria from the mother and the surrounding environment colonize the infant gut. Within a few minutes after birth, the intestine becomes an organ permanently colonized by a rich and complex community of microorganisms: the intestinal microbiota. Gastrointestinal colonization continues during the first 2 years of life ( Figure 1-8 ), and is in permanent flux owing to epithelial turnover.
