GIT Development: Video 1 Notes

Learning objectives and video overview

This is the first of three videos on the development of the gastrointestinal tract (GIT). The presenter, Katie Dixon, outlines the learning objectives for the three-part series and states that in this first video she will focus on three main points: (1) outlining the main features and events in the early development of the GIT, (2) explaining how the three-dimensional (3D) gut develops from a two-dimensional (2D) embryonic disc that forms on the yolk sac, and (3) tracing the formation of the GIT, its major regional divisions (foregut, midgut, hindgut), and the development of the major vessels and accessory organs. The slide also emphasizes the locations of the foregut, midgut, and hindgut, their arterial supply, and the mesenteries as key themes for this video.

Embryonic disc and origin of the GIT

Development begins with a flat, three-layer embryonic disc by the third week, consisting of ectoderm, mesoderm, and endoderm. The GIT arises from the endoderm and the lateral plate mesoderm. The lateral plate mesoderm splits into two components: the somatic (parietal) mesoderm and the splanchnic (visceral) mesoderm. The splanchnic component forms the primitive lining of the GIT. Over the next days, the lateral plate mesoderm hollows out, and its expansion partitions into the somatic and splanchnic layers. The endoderm and the lateral plate mesoderm together establish the earliest gut lining and the intraembryonic coelom. The gut tube is formed during the fourth week as differential growth rates cause the embryonic disc to enlarge more rapidly in length than the yolk sac. This differential growth drives the disc to fold into a cylinder, creating a tube with a foregut (head region), a midgut (in continuity with the yolk sac), and a hindgut (tail region). In cross-sectional views, the gut tube is suspended by dorsal and ventral mesenteries and flanked by coelomic cavities. By the end of the first month, organ buds—such as the stomach and the beginnings of the liver and pancreas—grow out from the gut tube, with organs initially supported by both dorsal and ventral mesenteries. A key point noted is that the ventral mesentery of the midgut and hindgut regresses, whereas the dorsal mesentery persists, which has important consequences for subsequent gut positioning and fixation.

Gut tube formation, embryonic folding, and regional plan

The formation of the gut tube is intimately tied to embryonic folding, a process that unfolds during the fourth week in two simultaneous planes: the horizontal plane and the median plane. Horizontal folding produces two lateral body folds, while median plane folding produces cranial and caudal folds. The rapid, concurrent folding transforms the trilaminar disc into a cylinder composed of endoderm (internal), mesoderm (middle), and ectoderm (external). As folding proceeds, the endoderm moves toward the midline and fuses, incorporating the dorsal part of the yolk sac to form the primitive gut tube. This tube differentiates into three regions—the foregut, midgut, and hindgut. The foregut lies toward the cranial end and is temporarily closed by the oropharyngeal membrane, which ruptures at the end of the fourth week to form the mouth. The midgut remains connected to the yolk sac until around the fifth week, with the yolk sac connection narrowing into a vitelline (yolk stalk) duct. The hindgut lies at the caudal end and is temporarily closed by the cloacal membrane, which ruptures during the seventh week to form the urogenital and anal openings. The three main germ layers continue to differentiate, giving rise to their respective tissues and organ systems, with the gut tube present by week four.

Foregut, midgut, and hindgut: regional anatomy and arterial supply

The three gut regions have distinct arterial supplies and derivatives:

Foregut: includes the pharynx, thoracic esophagus, and abdominal foregut. The abdominal foregut gives rise to the abdominal esophagus, stomach, and the proximal part of the duodenum, as well as the liver, gallbladder, pancreas, and their ducts. The thoracic foregut (pharynx and thoracic esophagus) is supplied by multiple thoracic arteries.
Midgut: forms the remainder of the duodenum, jejunum and ileum, the ascending colon, and about two-thirds of the transverse colon.
Hindgut: forms the remaining one-third of the transverse colon, the descending colon, the sigmoid colon, and the upper two-thirds of the anorectal canal (the lower third of the anorectal canal is not derived from the gut tube).
Governing arterial supplies align with these regions: the celiac artery serves the abdominal foregut; the superior mesenteric artery (SMA) supplies the midgut; and the inferior mesenteric artery (IMA) supplies the hindgut. The foregut’s thoracic portion is supplied by five aortic branches in the thorax.

Mesenteries and gut suspension in the peritoneal cavity

By the end of the fourth week, nearly the entire gut tube is suspended from the posterior abdominal wall by the dorsal mesentery, with the ventral mesentery regressing along the midgut. The visceral organs, including the stomach and abdominal foregut, are initially suspended by both dorsal and ventral mesenteries, whereas much of the gut tube below the stomach (and the foregut up to the abdominal segment) is suspended only by the dorsal mesentery. The dorsal and ventral mesenteries arise from the lateral plate mesoderm and contribute to the formation of the peritoneum, the serous lining of the abdominal cavity. The outer lining of the peritoneum (the simple squamous epithelium) is derived from the lateral plate mesoderm. The ventral mesentery disappears in most parts of the gut tube as folding progresses, particularly at the level of the midgut, while the dorsal mesentery remains intact and continues to anchor most of the gut to the posterior abdominal wall.

Peritoneal classification: intraperitoneal, retroperitoneal, and secondary retroperitoneal

Organs within the peritoneal cavity that are suspended by a mesentery are described as intraperitoneal. Other organs are embedded in the body wall and covered by peritoneum and are termed retroperitoneal, such as the bladder and kidneys. Some organs become secondarily retroperitoneal as their mesentery disappears and they fuse with the posterior abdominal wall, losing their intraperitoneal mesenteric attachments. This dynamic arrangement explains why some organs (e.g., pancreas) acquire a particular retroperitoneal position and others remain intraperitoneal (e.g., stomach) with their mesenteries.

Summary connections and implications

The development of the GIT is governed by three major processes: (1) the formation of a gut tube from the endoderm with supporting mesodermal structures, (2) the systematic folding of the embryo in two planes that creates the foregut, midgut, and hindgut and establishes their relationships to the yolk sac and intraembryonic coelom, and (3) the differential development of mesenteries that suspends the gut and defines peritoneal relationships (intra-, retro-, and secondary retroperitoneal). The ventral mesentery’s disappearance at most levels contrasts with the persistence of the dorsal mesentery, shaping the spatial arrangement of abdominal organs and their peritoneal coverings. The early organ buds (stomach, liver, pancreas) appear by the end of the first month, setting the stage for subsequent regional specialization and vascularization by the three primary arteries (celiac, SMA, IMA).

Key terms to review

Endoderm, ectoderm, mesoderm; lateral plate mesoderm; somatic (parietal) vs splanchnic (visceral) mesoderm
Intraembryonic coelom; coelomic cavities
Foregut, midgut, hindgut; oropharyngeal membrane; cloacal membrane; vitelline (yolk stalk) duct
Dorsal vs ventral mesentery; peritoneum; intraperitoneal vs retroperitoneal vs secondarily retroperitoneal
Major arterial supplies: celiac artery, superior mesenteric artery, inferior mesenteric artery
Weeks of development: gut tube present by week 4; oropharyngeal membrane ruptures by end of week 4; cloacal membrane ruptures by week 7
Organ buds: stomach, liver, pancreas, gallbladder, ducts