OESOPHAGUS

The oesophagus can be distinguished from the stomach at stage 13 (embryo 5 mm). It elongates during successive stages and its absolute length increases more rapidly than the embryo as a whole. Cranially it is invested by splanchnopleuric mesenchyme posterior to the developing trachea, and more caudally between the developing lungs and pericardioperitoneal canals posterior to the pericardium. (For details of tracheo-oesophageal fistulae see p. 1037). Caudal to the pericardium, the terminal, pregastric segment of the oesophagus has a short thick dorsal meso-oesophagus (from splanchnopleuric mesenchyme), while ventrally it is enclosed in the cranial stratum of the septum transversum mesenchyme. Each of the above are continuous caudally with their respective primitive dorsal and ventral mesogastria. Thus the oesophagus has only limited areas related to a primary coelomic epithelium. However, note the subsequent development of the para-oesophageal right and left pneumatoenteric recesses (see Fig. 73.7), the relation of the ventral aspect of the middle third of the oesophagus to the oblique sinus of the pericardium, and the relation of its lateral walls in the lower thorax to the mediastinal pleura. All the foregoing are secondary extensions from the primary coelom.

Fig. 73.7 A–C, Stages of development of the subdiaphragmatic foregut and the right and left pericardioperitoneal/pleuroperitoneal canals, with particular reference to the terminal oesophagus, stomach, duodenum, spleen, the lesser sac of peritoneum and omenta: seen in semicoronal section (left column) and transverse section at the levels indicated by the arrows (right column). D, E, Three dimensional schematization of the lesser sac, dorsal and ventral mesogastria.

The oesophageal mucosa consists of two layers of cells by stage 15 (week 5), but the proliferation of the mucosa does not occlude the lumen at any time. The mucosa becomes ciliated at 10 weeks, and stratified squamous epithelium is present at the end of the 5th month: occasionally patches of ciliated epithelium may be present at birth. Circular muscle can be seen at stage 15 but longitudinal muscle has not been identified until stage 21. Neuroblasts can be demonstrated in the early stages; the myenteric plexuses have cholinesterase activity by 9.5 weeks and ganglion cells are differentiated by 13 weeks. It has been suggested that the oesophagus is capable of peristalsis in the first trimester. Oesophageal atresia is one of the more common obstructive conditions of the alimentary tract: fetuses swallow amniotic fluid, and so the condition may be indicated by polyhydramnios.

STOMACH

At the end of the fourth and beginning of the fifth week the stomach can be recognized as a fusiform dilation cranial to the wide opening of the midgut into the yolk sac (Figs 73.2 and 73.3). By the fifth week this opening has narrowed into a tubular vitelline intestinal duct, which soon loses its connection with the digestive tube. At this time the stomach is median in position and separated cranially from the pericardium by the septum transversum (see Fig. 73.5A), which extends caudally on to the cranial side of the vitelline intestinal duct and ventrally to the somatopleure. Dorsally, the stomach is related to the aorta and, reflecting the presence of the pleuroperitoneal canals on each side, is connected to the body wall by a short dorsal mesentery, the dorsal mesogastrium (see Fig. 73.6). The latter is directly continuous with the dorsal mesentery (mesenteron) of almost all of the remainder of the intestine, except its caudal short segment.

Fig. 73.5 Early development of the liver and the supra-umbilical peritoneal cavities. A, The hepatic endodermal primordium proliferates ventrally into the septum transversum mesenchyme. The endodermal cells forming the hepatic trabeculae will become hepatocytes; the septum transversum mesenchymal cells will become the endothelium of the liver sinusoids and early blood cells. The developing lung buds can be seen expanding into the pericardioperitoneal cavities. B, The septum transversum mesenchyme and the stomach become enclosed by the right and left pericardioperitoneal canals (shown on transverse section). The apposition of the medial pericardioperitoneal walls form the dorsal and ventral mesogastria. The proximity of the lung buds to the developing stomach can be seen. The pleural and supra-umbilical peritoneal cavities are transiently, and bilaterally, symmetrical above the umbilicus. C, The lower border of the ventral mesogastrium denotes the connection between the supra-umbilical peritoneal cavities (pleuro-peritoneal canals) can be identified by the position of the ventral pancreatic bud and common bile duct. White arrows indicate the direction of movement of the dorsal and ventral mesogastria.

Fig. 73.6 Three-dimensional schematization of the major developmental sequences of the subdiaphragmatic embryonic and fetal gut, together with its associated major glands, peritoneum and mesenteries: left anterolateral aspect. The development sequence A–F spans 1.5 months to the perinatal period. Three-dimensional schematization of the major developmental sequences of the subdiaphragmatic embryonic and fetal gut, together with its associated major glands, peritoneum and mesenteries: left anterolateral aspect. The development sequence A–F spans 1.5 months to the perinatal period. H denotes the general disposition of the remaining viscera and mesenteric roots with their lines of attachment and principal contained vessels, which approximate to the adult state for comparison.

In human embryos of 10 mm (stage 15–16), the characteristic gastric curvatures are already recognizable. Growth is more active along the dorsal border of the viscus: its convexity markedly increases and the rudimentary fundus appears. Because of more rapid growth along the dorsal border, the pyloric end of the stomach turns ventrally and the concave lesser curvature becomes apparent (Fig. 73.3; see also Fig. 73.6). The stomach is now displaced to the left of the median plane and apparently becomes physically rotated, which means that its original right surface becomes dorsal and its left surface becomes ventral. Accordingly the right vagus is distributed mainly to the dorsal, and the left vagus mainly to the ventral, surfaces of the stomach. The dorsal mesogastrium increases in depth and becomes folded on itself. The ventral mesogastrium becomes more coronal than sagittal. The pancreaticoenteric recess (see Fig. 73.7B(ii)), hitherto usually described as a simple depression on the right side of the dorsal mesogastrium, becomes dorsal to the stomach and excavates downwards and to the left between the folded layers. It may now be termed the inferior recess of the bursa omentalis. Put simply, the stomach has undergone two ‘rotations’. The first is 90° clockwise, viewed from the cranial end, the second is 90° clockwise, about an anteroposterior axis. The displacement, morphological changes and apparent ‘rotation’ of the stomach have been attributed variously to its own and surrounding differential growth changes, extension of the pancreaticoenteric recess with changes in its mesenchymal walls, and pressure, particularly that exerted by the rapidly growing liver.

DUODENUM

The duodenum develops from the caudal part of the foregut and the cranial part of the midgut. A ventral mesoduodenum, which is continuous cranially with the ventral mesogastrium, is attached only to the foregut portion. Posteriorly the duodenum has a thick dorsal mesoduodenum which is continuous with the dorsal mesogastrium cranially and the dorsal mesentery of the midgut caudally. Anteriorly the extreme caudal edge of the ventral mesentery of the foregut extends onto the short initial segment of the duodenum. The liver arises as a diverticulum from the ventral surface of the duodenum at the foregut–midgut junction, i.e. where the midgut is continuous with the yolk sac wall (the cranial intestinal portal). The ventral pancreatic bud also arises from this diverticulum. The dorsal pancreatic bud evaginates posteriorly into the dorsal mesoduodenum slightly more cranially than the hepatic diverticulum. The rotation, differential growth, and cavitations related to the developing stomach and omenta cause corresponding movements in the duodenum, which forms a loop directed to the right, with its original right side now adjacent to the posterior abdominal wall (see Fig. 73.6). This shift is compounded by the migration of the bile duct and ventral pancreatic duct around the duodenal wall. Their origin shifts until it reaches the medial wall of the second part of the fully formed duodenum: the bile duct passes posteriorly to the duodenum and travels in the free edge of the ventral duodenum and ventral mesogastrium. Local adherence and subsequent absorption of part of the duodenal serosa and the parietal peritoneum results in almost the whole of the duodenum, other than a short initial segment, becoming retroperitoneal (sessile).

Duodenal atresia is a developmental defect found in 1 in 5000 live births (Whittle 1999). It may be associated with an annular pancreas which may compress the duodenum externally (20% of duodenal atresia), or with abnormalities of the bile duct. In 40–60% of cases the atresia is complete and pancreatic tissue fills the lumen. The condition can be diagnosed on ultrasound examination, which reveals a typical double bubble appearance caused by fluid enlarging the stomach and the proximal duodenum. Polyhydramnios is invariably present and often the indication for the scan. Duodenal atresia commonly occurs with other developmental defects, e.g. cardiac and skeletal anomalies and in Down’s syndrome.

DORSAL AND VENTRAL MESENTERIES OF THE FOREGUT

The epithelium of the stomach and duodenum does not rotate relative to its investing mesenchyme. The rotation includes the coelomic epithelial walls of the pericardioperitoneal canals, which are on each side of the stomach and duodenum and form its serosa, and the elongating dorsal mesogastrium or the much shorter dorsal mesoduodenum. A ventral mesogastrium can be seen when the distance between the stomach and liver increases. Whereas the dorsal mesogastrium takes origin from the posterior body wall in the midline, its connection to the greater curvature of the stomach, which lengthens as the stomach grows, becomes directed to the left as the stomach undergoes its first rotation. With the second rotation a portion of the dorsal mesogastrium now faces caudally (see Fig. 73.6). The ventral mesogastrium remains as a double layer of coelomic epithelium which encloses mesenchyme and forms the lesser omentum (see Fig. 73.7).

Movement of the stomach is associated with an extensive lengthening of the dorsal mesogastrium, which becomes the greater omentum, and which now, from its posterior origin, droops caudally over the small intestine, then folds back anteriorly and ascends to the greater curvature of the stomach. The greater omentum is therefore composed of a fold containing, technically, four layers of peritoneum. The dorsal mesoduodenum, or suspensory ligament of the duodenum, is a much thicker structure, and it fixes the position of the duodenum when the rest of the midgut and its dorsal mesentery elongate and pass into the umbilical cord. For a more detailed account of this process see page 1216.

SPECIAL GLANDS OF THE POSTPHARYNGEAL FOREGUT

Pancreas

The pancreas develops from two evaginations of the foregut which fuse to form a single organ. A dorsal pancreatic bud can be seen in stage 13 embryos as a thickening of the endodermal tube which proliferates into the dorsal mesogastrium (Figs 73.3 and 73.4). A ventral pancreatic bud evaginates in close proximity to the liver primordium but cannot be clearly identified until stage 14, when it appears as an evagination of the bile duct itself. At stage 16 (5 weeks) differential growth of the wall of the duodenum results in movement of the ventral pancreatic bud and the bile duct to the right side and ultimately to a dorsal position. It is not clear whether there is a corresponding shift of mesenchyme during this rotation. However, the ventral pancreatic bud and the bile duct rotate from a position within the ventral mesogastrium (ventral mesoduodenum) to one in the dorsal mesogastrium (dorsal mesoduodenum) which is destined to become fixed onto the posterior abdominal wall. By stage 17 the ventral and dorsal pancreatic buds have fused, although the origin of the ventral bud from the bile duct is still obvious. Three-dimensional reconstruction of the ventral and dorsal pancreatic buds have confirmed that the dorsal pancreatic bud forms the anterior part of the head, the body and the tail of the pancreas and the ventral pancreatic bud forms the posterior part of the head and the posterior part of the uncinate process. The ventral pancreatic bud does not form all of the uncinate process (Collins 2002).

Fig. 73.4 Development of the pancreas in a human embryo. A, An early stage, 7.5 mm embryo; lateral view. B, A later stage, 14.5 mm embryo; ventral view.

The developing pancreatic ducts usually fuse in such a way that most of the dorsal duct drains into the proximal part of the ventral duct (Figs 73.3 and 73.4). The proximal portion of the dorsal duct usually persists as an accessory duct. The fusion of the ducts takes place late in development or in the postnatal period: 85% of infants have patent accessory ducts as compared to 40% of adults. Fusion may not occur in 10% of individuals, in which case separate drainage into the duodenum is maintained, so-called pancreatic divisum. Failure of the ventral pancreatic diverticulum to migrate will result in an annular pancreas which may constrict the duodenum locally.

The ventral pancreas does not always extend anterior to the superior mesenteric vein but remains related to its right lateral surface. Initially the body of the pancreas extends into the dorsal mesoduodenum and then cranially into the dorsal mesogastrium. As the stomach rotates, this portion of the dorsal mesogastrium is directed to the left forming the posterior wall of the lesser sac. The posterior layer of this portion of dorsal mesogastrium fuses with the parietal layer of the coelom wall (peritoneum) and the pancreas becomes mainly retroperitoneal (see Fig. 73.7C). The region of fusion of the dorsal mesogastrium does not extend so far left as to include the tail of the pancreas which passes into the lienorenal (splenorenal) ligament. The anterior border of the pancreas later provides the main line of attachment for the posterior leaves of the greater omentum.

Cellular development of the pancreas

The early specification of pancreatic endoderm involves the proximity of the notochord to the dorsal endoderm, which locally represses the expression of Shh transcription factor. Endoderm caudal to the pancreatic region does not respond to notochordal signals. The ventral pancreatic endoderm does not seem to undergo the same induction. Pancreatic mesenchyme is derived from two regions. The mesenchyme which surrounds the dorsal pancreatic bud proliferates from the splanchnopleuric coelomic epithelium of the medial walls of the pericardioperitoneal canals, whereas the ventral pancreatic bud is invested by septum transversum mesenchyme and by mesenchyme derived from the lower ventral walls of the pericardioperitoneal canals.

The primitive endodermal ductal epithelium provides the stem cell population for all the secretory cells of the pancreas. Initially these endocrine cells are located in the duct walls or in buds developing from them, and later they accumulate in pancreatic islets. The remaining primitive duct cells will differentiate into definitive ductal cells. In the fetus they develop microvilli and cilia but lack the lateral interdigitations seen in the adult. Branches of the main duct become interlobular ductules which terminate as blind ending acini or as tubular, acinar elements.

The ductal branching pattern and acinar structure of the pancreas is determined by the pancreatic mesenchyme. This mesenchyme gives rise to connective tissue between the ducts which, in the fetus, appears to be important in stimulating pancreatic proliferation and maintaining the relative proportions of acinar, α and β cells during development. It also provides cell lines for smooth muscle within the pancreas. Angiogenic mesenchyme invades the developing gland to produce blood and lymphatic vessels.

The process of islet differentiation is divided into two phases (Collins 2002). Phase I, characterized by proliferation of polyhormonal cells, occurs from weeks 9–15. Phase II, characterized by differentiation of monohormonal cells, is seen from week 16 onwards. The β cells, producing insulin and amylin, differentiate first, followed by α-cells which produce glucagon. The δ cells which produce somatostatin are seen after 30 weeks. The dorsal bud gives rise mostly to α cells, and the ventral bud to most of the pancreatic polypeptide producing cells. The β cells develop from the duct epithelium throughout development and into the neonatal period. Later, in weeks 10–15, some of the primitive ducts differentiate into acinar cells in which zymogen granules or acinar cell markers can be detected at 12–16 weeks.

The pancreas in the neonate has all of the normal subdivisions of the adult. The head is proportionately large in the newborn and there is a smooth continuation between the body and the tail. The inferior border of the head of the pancreas is found at the level of the second lumbar vertebra. The body and tail pass cranially and to the left, and the tail is in contact with the spleen (see Fig. 14.4).

Liver

The liver is one of the most precocious embryonic organs and is the main centre for haemopoiesis in the fetus. It develops from an endodermal evagination of the foregut and from septum transversum mesenchyme, which is derived from the proliferating coelomic epithelium in the protocardiac region. The development of the liver is intimately related to the development of the heart. The vitelline veins, succeeded by the umbilical veins passing to the sinus venosus are disrupted by the enlarging septum transversum to form a hepatic plexus, the forerunner of the hepatic sinusoids. (See Collins, 2002 for a detailed account of hepatic development.)

Early liver development

As the head fold and early intraembryonic coelom form, the ventral parietal wall of the pericardial cavity gives rise to populations of cells termed precardiac or cardiac mesenchyme. Hepatic endoderm is induced to proliferate by this mesenchyme, although all portions of the early heart tube, truncus arteriosus, atria, ventricle, both endocardium and myocardium, have hepatic induction potency which is tissue-specific, but not species-specific. As the heart and foregut become separated by the accumulation of the cardiac mesenchyme, the mesenchyme itself is renamed septum transversum. It is seen as a ventral mass, caudal to the heart, which passes dorsally on each side of the developing gut to join the mesenchyme proliferating from the walls of the pericardioperitoneal canals. The majority of the cells within the septum transversum are destined to become hepatic mesenchyme.

In the stage 11 embryo the location of the hepatic endoderm has been identified at the superior boundary of the rostral intestinal portal. By stage 12, the hepatic endodermal primordium is directed ventrally and begins to proliferate as a diverticulum. There are two parts: a caudal part, which will produce the cystic duct and gallbladder, and a cranial part which forms the liver biliary system (Figs 73.3, 73.5A). The cells start to express liver-specific molecular markers and glycogen storage.

Around the cranial portion of the hepatic diverticulum the basal lamina is progressively disrupted and individual epithelial cells migrate into the surrounding septum transversum mesenchyme. The previously smooth contour of the diverticulum merges into columnar extensions of endoderm, the epithelial trabeculae, which stimulate the hepatic mesenchymal cells to form blood islands and endothelium. The advance of the endodermal epithelial cells promotes the conversion of progressively more hepatic mesenchyme into endothelium and blood cells, and only a little remains to form the scanty liver capsule and interlobular connective tissue. This invasion by the hepatic epithelium is completed in stage 13, when it approaches the caudal surface of the pericardial cavity, and is separated from it only by a thin lamina of mesenchyme which will give rise to part of the diaphragm.

During this early phase of development the liver is far more highly vascularized than the rest of the gut. The hepatic capillary plexus is connected bilaterally with the right and left vitelline veins. Dorsolaterally they empty by multiple channels into enlarged hepatocardiac channels, which lead to the right and left horns of the sinus venosus (see Fig. 73.9); usually the channel on the right side is most developed. Both left and right channels bulge into the pericardioperitoneal canals, forming sites for the exchange of fluid from the coelom into the vascular channels. The growth of the hepatic tissue in these regions is sometimes referred to as the left and right horns of the liver.

Fig. 73.9 Development of the vitelline, umbilical and terminal cardinal vein complexes: the early symmetric condition. A, Early symmetrical arrangement of cardinal and umbilical veins entering the sinus venosus, and blood from the yolk sac passing via vitelline veins through anastomoses forming in the septum transversum mesenchyme (green). B, The developing duodenum is surrounded by transverse duodenal intervitelline anastomoses. Changes in the developing gut and umbilical veins cause shifts in the routes taken by venous blood flowing to the sinus venosus.

The liver remains proportionately large during its development and constitutes a sizeable organ dorsal to the heart at stage 14 then more caudally placed by stage 16. By this stage hepatic ducts can be seen separating the hepatic epithelium from the extrahepatic biliary system, but even at stage 17 the ducts do not penetrate far into the liver.

Maturation of the liver

At 3 months’ gestation, the liver almost fills the abdominal cavity and its left lobe is nearly as large as its right. When the haematopoietic activity of the liver is assumed by the spleen and bone marrow, the left lobe undergoes some degeneration and becomes smaller than the right. The liver remains relatively larger than in the adult throughout the remainder of gestation. In the neonate it constitutes 4% of the body weight, compared to 2.5–3.5% in adults. It is in contact with the greater part of the diaphragm and extends below the costal margin anteriorly, and in some cases to within 1 cm of the iliac crest posteriorly. The left lobe covers much of the anterior surface of the stomach and constitutes nearly one-third of the liver (see Fig. 14.4). Although its haemopoietic functions cease before birth its enzymatic and synthetic functions are not completely mature at birth.

Development of intrahepatic biliary ducts

The development of the intrahepatic biliary ducts follows the branching pattern of the portal vein radicles (Collins 2002). The cranial hepatic diverticulum gives rise to the liver hepatocytes, the intrahepatic large bile ducts (right and left hepatic ducts, segmental ducts, area ducts and their first branches) and the small bile ducts (septal bile ducts, interlobular ducts and bile ductules). The portal and hepatic veins arise together from the vitelline veins. Early in development the accumulation of mesenchyme around these veins is similar, whereas later mesenchyme increases around the portal veins. This is a prerequisite for bile duct development. Primitive hepatocytes surround the portal vein branches and associated mesenchyme and form a sleeve of cells termed the ductal plate. Portions of the ductal plate divide to produce lines of epithelial cells which migrate close to a portal vein branch where they differentiate into bile ducts. As the bile ducts develop, angiogenic mesenchymal cells form blood vessels which connect to the hepatic artery from 10 weeks. Thus the portal triads are patterned by the portal vein radicles which initially induce bile duct formation and then artery formation. The development of the biliary system extends from the hilum to the periphery. Abnormalities of the biliary tree are associated with abnormalities of the branching pattern of the portal vein. The developing bile ducts remain patent throughout development; the solid stage of ductal development previously promulgated has been refuted. Atresia of the extrahepatic bile ducts has been noted, often in association with extrahepatic atresia. The cause of this condition is not clear; inflammatory process may be involved, although some cases have features of ductal plate malformation (Howard 2002).

Development of extrahepatic biliary ducts

The caudal part of the hepatic endodermal diverticulum forms the extrahepatic biliary system, the common hepatic duct, gallbladder, cystic duct and common bile duct.

The bile duct, which originated from the ventral wall of the foregut (now duodenum), migrates with the ventral pancreatic bud first to the right and then dorsomedially into the dorsal mesoduodenum. The right and left hepatic ducts arise from the cranial end of the common hepatic duct from 12 weeks’ gestation.

Atresia of the extrahepatic bile ducts in neonates occurs alone or in conjunction with a range of other anomalies, including situs inversus, malrotation, polysplenia and cardiac defects. In such cases the intrahepatic bile ducts have a mature tubular shape but also show features of ductal plate malformation.

In the neonate the gallbladder has a smaller peritoneal surface than in the adult, and its fundus often does not extend to the liver margin. It is generally embedded in the liver and in some cases may be covered by bands of liver. After the second year the gallbladder assumes the relative size it has in the adult.

PRIMARY INTESTINAL (OR MIDGUT) LOOP

The midgut loop can first be seen at stage 15 when a bulge, the caecal bud, can be discerned on the lower limb of the loop, caudal to the yolk stalk (which arises from the apex or summit of the loop) (Fig. 73.3). Later, the original proximal limb of the loop moves to the right and the distal limb to the left (Fig. 73.3C). The longest portion of the dorsal mesentery is at the level of the yolk stalk: there is less relative lengthening near the caudal end of the duodenum or the cranial half of the colon. The midgut extends into the umbilical coelom having already rotated through an angle of 90° (anticlockwise viewed from the ventral aspect). This relative position is approximately maintained so long as the protrusion persists, during which time the proximal limb which forms the small intestine elongates greatly. It becomes coiled, and its adjacent mesentery adopts a pleated appearance. The origin of the root of the mesentery is initially both median and vertical, while at its intestinal attachment it is elongated like a ruffle and folded along a horizontal zone. The mesenteric sheet and its contained vessels has spiralled through 90°. The distal, colic, part of the loop elongates less rapidly and has no tendency to become coiled. By the time the fetus has attained a length of 40 mm (10 weeks), the peritoneal cavity has enlarged and the relative size of the liver and mesonephros is much less. The re-entry of the gut occurs rapidly and in a particular sequence during which it continues the process of rotation. The proximal loop returns first, with the jejunum mainly on the left and the ileum mainly on the right of the subhepatic abdominal cavity. As they re-enter the abdominal cavity the coils of jejunum and ileum slide inwards over the right aspect of the descending mesocolon, and so displace the descending colon to the left. The transverse colon passes superiorly to the origin of the root of the mesentery (Fig. 73.6). The caecum is the last to re-enter and at first lies on coils of ileum on the right. Later development of the colon leads to its elongation and to the establishment of the hepatic and splenic flexures. A timetable for intestinal rotation in staged human embryos is given by Kim et al (2003).

UMBILICAL CORD

During the period when the midgut loop protrudes into the umbilical coelom, the edges of the ventral body wall are becoming relatively closer, forming a more discrete root for the umbilical cord. Somatic mesenchyme, which will form the ventral body wall musculature, migrates into the somatopleuric mesenchyme and passes ventrally toward the midline. The umbilical cord (see p. 181

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