Clinical Embryology of the Abdomen: Normal and Pathologic Anatomy

47 Pages·2006·17.44 MB·English

2
Clinical Embryology of
the Abdomen: Normal
and Pathologic Anatomy
Bruce R. Javors‚ M.D.
Hiromu Mori‚ M.D.
Morton A. Meyers‚ M.D.
Ronald H. Wachsberg‚ M.D.
An understanding of the rudiments of embryologic de- and ectoderm. The endoderm becomes the lining of the
velopment is essential not only to a fuller appreciation gastrointestinal tract‚ as well as the liver and pancreatic
of the anatomic structures and their dynamic relation- glandular tissue. The ectoderm gives rise to the epider-
ships in the abdomen and pelvis but also to the critical mis and nervous system. Most other tissue is derived
awareness of congenitally based disorders that may be from the mesoderm.
initially manifested into adulthood. These disorders may The mesodermal middle layer of this disc develops a
be clinically encountered in a wide spectrum involving lateral cleft in connection with the yolk sac1 (Fig. 2–1).
the digestive and urogenital tracts‚ including the peri- Eventually‚ the lateral margins move ventrally and me-
toneal cavity and its mural structures. Clinical presen- dially to encompass the yolk sac (Fig. 2–2). This incor-
tations range from an asymptomatic condition in which porates the intraembryonic coelom to form a tube
the findings might even be misinterpreted or vague non- within a tube. The outer tube is the body cavity while
specific symptoms to episodic distress or an acute ful- the inner tube is the primitive gastrointestinal tract. The
minating crisis. Imaging plays a critical role in their di-
inner tube maintains a posterior attachment to the body
agnosis.
cavity via a dorsal mesentery. Most of its ventral attach-
The final positions and relationships of the abdominal ment involutes‚ except at the level of the distal foregut.2
organs and structures can be traced in large part to the
branching‚ growth‚ and rotation of an originally straight
tubular gastrointestinal tract as well as to three genera- Diaphragm
tions of renal primordia. Although their development
may be conceived as individual events‚ many of these
organs develop and even regress simultaneously. The fourth to sixth weeks of development mark the
division of the coelom into the definitive pericardial‚
pleural‚ and peritoneal spaces. The pleuropericardial
Early Development of
membranes‚ mesenchyme derived from the septum
the Embryo transversum‚ the dorsal mesentery of the esophagus‚ and
myoblasts from the abdominal wall all contribute to the
formation of the diaphragm (Fig. 2–3).
After fertilization‚ the zygote undergoes a rapid trans- Failure of the diaphragmatic components to properly
formation into a ball of cells and then into a trilaminar unite may leave an opening‚ especially on the left‚ for
disc with three distinct layers: endoderm‚ mesoderm‚ abdominal contents to pass into the thorax (foramen of
10 2. Clinical Embryology of the Abdomen: Normal and Pathologic Anatomy
both its ventral and dorsal mesenteries. The ventral mes-
entery that lies between the distal foregut (stomach and
duodenum) and the liver forms the gastrohepatic and‚
more distally, the hepatoduodenal ligaments.5 That por-
tion of the ventral mesentery that lies between the de-
veloping liver and the anterior abdominal wall persists
as the falciform ligament‚ with the obliterated umbilical
vein (ligamentum teres) lying in its free edge. However‚
during the second month of development‚ the distal
foregut undergoes asymmetric growth with the dorsal
aspect growing much more rapidly. Along with this dor-
sal bulge‚ the distal foregut rotates clockwise (as seen
from the front) about its anteroposterior axis and as
clockwise (as seen from below) about its longitudinal
axis. Therefore‚ the dorsal bulge presents to the left and
is convex inferolateral‚ forming the greater curvature of
the stomach. The original ventral concavity of the de-
veloping stomach is carried to the right‚ forming the
Fig. 2–1. Cross-sectional schematic through the
definitive lesser curvature (Fig. 2–5).
midportion of the embryo early in the fourth week
shows infolding of the ectoderm and mesoderm The change from the foregut to the midgut is marked
(somatopleure) as it begins to encase the intraembryonic by a change in arterial supply‚ from the celiac to the
coelom. This will eventually encompass the body cavity. superior mesenteric arteries‚ respectively. This occurs at
The splanchnopleure’s (endoderm and mesoderm) the level of the duodenal papilla. As the stomach rotates
contribution to the formation of the midgut is evident as to the left‚ the duodenum‚ which had buckled ventrally
well. (completing an S-shaped configuration with the dorsal
(From Javors BR, Sloves JH.4)
gastric bulge)‚ is carried to the right into its definitive
position. Its dorsal mesentery is eventually resorbed‚ re-
Bochdalek hernia)2‚3 (Fig. 2–4). A retrosternal weakness sulting in its “retroperitoneal” location‚ although its an-
(more frequently on the right) at the defect through terior surface is still covered by peritoneum. Failure of
which the superior epigastric vessels pass may allow her- the mesentery to be completely absorbed results in an
niation of omentum and‚ less likely‚ colon through the elongated and redundant appearance of the proximal
resultant foramen of Morgagni.4 duodenum (Fig. 2–6).
Gastrointestinal Tract
Duodenal Web and Bowel Duplication
The gastrointestinal tract starts as a hollow tube. Prolif-
The primitive gastrointestinal tract starts out as a rela- eration of the lining endothelium results in obliteration
tively straight tube. In the distal foregut‚ it maintains of the lumen. With time‚ vacuoles form within this cel-
Fig. 2–2. Cross-section at the
end of the fourth week of
embryonic development.
The envelopment of the in-
traembryonic coelom is almost
complete. The yolk sac has oth-
erwise separated into a more de-
finitive yolk stalk and midgut as
well.
(From Javors BR, Sloves JH.4)
Gastrointestinal Tract 11
lular plug; they coalesce and a normal diameter lumen
is reestablished. Failure of complete resorption may lead
to either atresia or stenosis. Stenosis may range from a
conical narrowing to a weblike constriction (Fig. 2–7).
An alternative hypothesis for atresia and stenosis of the
intestinal tract is intrauterine ischemia.6‚7 Very rarely‚ es-
pecially in the duodenum‚ ongoing peristalsis may pro-
pel intestinal contents against a web‚ stretching it out
until it resembles a wind sock‚ forming an intraluminal
diverticulum8 (Fig. 2–8). In addition‚ incomplete merg-
ing of the vacuoles may result in intestinal duplica-
tion.9–11 This is almost invariably seen along the mes-
enteric border‚ sharing the blood supply of the normal
intestinal lumen. In the stomach‚ a duplication lies along
the greater curvature (site of the initial dorsal mesentery‚
later the greater omentum)12‚13 (Fig. 2–9)‚ and in the
colon along the medial wall of the ascending and de-
scending colon and the superior wall of the transverse
Fig. 2–3. Diagram of a five-week embryo.
colon11‚14 (Fig. 2–10).
The relative contributions of the septum transversum,
esophageal mesentery, and pleuroperitoneal membranes will
change with further development.
(From Javors BR, Sloves JH.4)
Fig. 2–4. Foramen of Bochdalek hernia.
(a) Upper gastrointestinal series shows a normal subdiaphragmatic position of the stomach. The jejunum enters the
thoracic cavity through a diaphragmatic opening (foramen of Bochdalek) that is situated on the left and is more
lateral than the position of a conventional hiatal or foramen of Morgagni hernia.
(b) Lateral chest radiograph reveals the barium-filled colon reaching almost to the apex of the chest. The colon is
seen to enter the chest through a diaphragmatic opening that is far posterior.
(From Javors BR.3)
12 2. Clinical Embryology of the Abdomen: Normal and Pathologic Anatomy
Fig. 2–5. Cross-section
through a five-week embryo
at the level of the liver shows
the paired superior peritoneum
(right and left) separated by both
the ventral and dorsal mesenter-
ies. Even at this stage of devel-
opment, the origins of many of
the suspensory ligaments of the
adult are clearly demonstrated.
(From Javors BR, Sloves JH.4)
Fig. 2–6. “Hammock” duodenum.
Left lateral film from an upper GI series shows an
elongated‚ redundant postbulbar segment of duodenum
proximal to the descending portion. This represents a
persistent duodenal mesentery.
Fig. 2–7. Duodenal web.
Left posterior oblique film from a double-contrast
enteroclysis demonstrates a very thin annular constriction in
the descending duodenum (curved arrows) from a duodenal
web.
Gastrointestinal Tract 13
Embryologic Rotation and Fixation
of Gut
The suspending dorsal mesentery of the distal foregut
and midgut elongates considerably as the stomach and
duodenum go through their complex rotation. This
leads to the development of the lesser peritoneal sac and
the greater omentum. As the dorsal bulge of the stomach
becomes more marked‚ it carries the mesentery along
with it to the left side of the abdomen. As this rotation
takes place‚ the peritoneal space that originally lay to the
right of the mesentery extends posterior to the stomach
into thelefthemiabdomen15 (Fig. 2–11). This eventually
becomes the lesser sac. The elongated mesentery also
doubles back on itself to form an apron that hangs down
from the greater curvature covering the peritoneal cav-
ity (Fig. 2–12). Eventually‚ the potential space within
the omentum is obliterated. If the fusion is incomplete‚
an omental cyst may form. The differentiation of omen-
Fig. 2–8. Intraluminal diverticulum.
tal‚ enteric‚ mesenteric‚ and neurenteric cysts and/or
Spot film from a single-contrast upper GI series reveals a
dissectingpancreatic pseudocysts depends on their lining
saclike structure filled with debris and barium separated
from the remainder of the duodenal lumen by a thin wall cell elements and wall constituents.16 The suspending
(arrows). Although termed an intraluminal diverticulum‚ this dorsal mesentery of the transverse colon fuses with the
is actually a “ballooned out” duodenal web. greater omentum‚ forming the definitive transverse
(Courtesy of N. Spier‚ M.D.) mesocolon5‚9 (Fig. 2–12).
Relatively early in its development‚ the intestinal tract
markedly elongates‚ reaching a length too great to be
contained within the abdominal cavity. It therefore her-
Fig. 2–9. Gastric duplication cyst.
(a) Contrast-enhanced CT reveals a bilobate low-density mass (D) along the greater curvature of the stomach.
(b) Sagittal sonogram confirms the cystic nature of this gastric duplication cyst.
14 2. Clinical Embryology of the Abdomen: Normal and Pathologic Anatomy
Fig. 2–10. Duplication of the colon.
(a) Barium enema study demonstrates a communicating colonic duplication (D) within the mesentery
of the transverse colon and descending colon.
(b) Spot film clearly shows the features of the opacified duplication along the medial wall of the
descending colon.
niates into the yolk sac. The superior mesenteric artery
acts as the axis of this physiologic herniation. At the apex
of this loop is the omphalomesenteric (vitelline) duct.
This midgut elongation is predominantly composed of
that segment that lies proximal to the duct‚ the prear-
terial limb. In order to accommodate this increase in
length‚ the small bowel is thrown into a serpentine pat-
tern‚ an appearance it maintains into adulthood. The
more distal segment‚ distal to the omphalomesenteric
duct‚ is labeled postarterial and becomes the distalileum‚
appendix‚ and large bowel proximal to the splenic flex-
ure. The cecum starts as a small bud just distal to the
apex of the loop and plays an important role in the re-
duction of the physiologic herniation (Fig. 2–13).
The herniated loop of midgut undergoes a 270°
counterclockwise rotation (as seen from the front). The
Fig. 2–11. Diagram of the upper abdomen (as seen
prearterial limb‚ which starts out superiorly‚ is carried
from below) during gastric rotation.
first to the right and then inferiorly. Conversely‚ the
The suspending dorsal mesentery (mesogastrium) has
elongated and is carried to the left of midline. This allows distalpostarterial limb is carried first to the left and then
the right hemiperitoneum to extend posterior to the superiorly. Thus‚ the two limbs find themselves located
stomach‚ starting the formation of the lesser sac. 180° opposite to their original locations.
Gastrointestinal Tract 15
Fig. 2–12. Greater omentum and transverse mesocolon.
(a) Longitudinal schematic drawing showing the fusion of the two leaves of the greater omentum with
obliteration of the inferior recess of the lesser sac.
(b) Fusion of the greater omentum with the transverse colon and its dorsal mesentery gives rise to the
definitive gastrocolic ligament and transverse mesocolon.
(From Javors BR, Sloves JH.4)
Eventually‚ the body cavity enlarges sufficiently to al-
low the herniated bowel to return (Fig. 2–14). As it does
so‚ the final 90° rotation is completed. The developing
cecal bud hinders the return of the postarterial midgut‚
and therefore the prearterial limb returns to the abdo-
men first.9‚17–19 As the final part of the rotation occurs‚
the prearterial limb is carried into the left upper quad-
rant‚ crossing beneath the superior mesenteric arterial
axis. Therefore‚ the transverse duodenum is carried in-
ferior to the superior mesenteric artery.
The postarterial midgut and hindgut are now forced
to the periphery of the abdomen. The right colon‚
which develops from the distal limb of midgut‚ is carried
in front of the superior mesenteric artery into the right
upper quadrant (Fig. 2–15). It is further growth of the
right colon‚ rather than any additional rotation‚ that car-
ries it into the right lower quadrant.9 The suspending
dorsal mesentery of the ascending and descending co-
lons is eventually resorbed and united with the posterior
abdominal wall. This results in the so-called retroperi- Fig. 2–13. Longitudinal view of the intestinal tract at
tonealization of those structures. In actuality‚ they main- 6 weeks of development.
tain a peritoneal surface along their anterior aspect‚ The superior mesenteric artery (SMA) acts as the axis for
similar to that of the duodenum. midgut rotation. The omphalomesenteric duct (OMD)
divides the midgut into pre- and postarterial limbs. Also
The appendix develops from the cecal bud. It origi-
seen is the physiologic herniation of the midgut through the
nates as a triangular projection with a wide orifice that
umbilical orifice (UO). Heavy lines mark the foregut-
is in line with the longitudinal axis of the right colon.
midgut (/) and the midgut-hindgut (//) junctions. The
The ileocecal valve impedes the growth of the colonic
celiac axis (CA) is the major artery of the foregut; the
wall at its entry site‚ and the opposite wall continues to inferior mesenteric artery (IMA) supplies the hindgut.
grow‚ effectively moving the appendix to the same side CB = cecal bud.
of the cecum as the valve.20 In addition‚ further increase (From Javors BR, Sloves JH.4)
16 2. Clinical Embryology of the Abdomen: Normal and Pathologic Anatomy
Fig.2–15. Reduction of the physiologic herniation.
Fig. 2–14. Frontal view of a 10-week fetus. This is complete one week after the configuration shown in
The elongated redundant prearterial limb has reentered the Figure 2–14. The postarterial limb has partially completed
abdomen and crossed to the left of and behind the SMA. its 180° rotation. The cecum now lies in the upper
This displaces the hindgut to the left. Heavy lines mark the abdomen on its way to the right side. CB = cecal bud.
foregut–midgut (/) and the midgut–hindgut (//) junctions. (From Javors BR, Sloves JH.4)
CB = cecal bud; OMD = omphalomesenteric duct;
UP = umbilical orifice.
(From Javors BR, Sloves JH.4)
in the transverse diameter of the cecum‚ without further current‚ intermittent‚ or resolve spontaneously or by
growth of the appendiceal lumen‚ brings about the more placement of a nasogastric tube.
familiar vermiform (wormlike) appearance of the ap- Cecal volvulus is a rare cause of cecal distention and
pendix. accounts for 11% of all intestinal volvulus.26 It develops
This complex series of twists and turns and subse- in association with an abnormal fixation of the cecum
quent mesenteric resorption leaves the gastrointestinal to the posterior parietal peritoneum; a freely mobile ce-
tract prone to many and often complex errors of rotation cum is a prerequisite for it. An abnormally distended
and fixation.9‚10‚21‚22 cecum is demonstrated in the midabdomen on radio-
graphs‚ sometimes recognizable as the coffee bean sign.
Contrast enema is usually diagnostic‚ but a whirl sign‚
Volvulus which is composed of the twisted portion of the cecum
and mesentery‚ may be shown on CT (Fig. 2–17).27
Gastric volvulus is a rare condition encountered in the Nonrotation‚ often called malrotation‚ is the most
adult as well as the pediatric age group. The majority of commonly encountered major anomaly of rotation. In
cases are of the mesenteroaxial type rather than the or- actuality‚ it is incomplete rotation that stops after the first
ganoaxial type.23 Anomalies associated with acute gastric 90°9‚10‚22 At this point‚ the prearterial limb lies in the
volvulus include diaphragmatic defects‚ intestinal mal- right hemiabdomen‚ and the postarterial lies in the left.
rotation‚ and wandering spleen.24 Most cases of gastric In addition‚ the order of the returning loops of midgut
volvulus seem to be secondary to deficient fixation. Ab- is reversed‚ with the distal limb leading the proximal.
sence of the gastrophrenic ligament and the gastrosplenic This results in the jejunum lying in the right upper ab-
ligament as well as an absence of the spleen may lead to domen‚ whereas the colon lies to the left of midline (Fig.
gastric volvulus in asplenic patients.25 Elongation or ab- 2–18). The duodenal sweep is not formed‚ and there
sence of the splenorenal ligament even with preservation may be an unusual redundancy to the duodenum on the
of the gastrosplenic ligament may lead to mesenteroaxial right side of the spine. The duodenal–jejunal junction
gastric volvulus (Fig. 2–16). Gastric volvulus can be re- then usually lies medial to the left pedicle of the spine.28
Gastrointestinal Tract 17
Fig. 2–16. Acute gastric volvulus
(mesenteroaxial type) with wandering spleen.
A 64-year-old female presented with recurrent
vomiting.
(a) Supine radiograph shows a gas-filled abdominal
mass (arrows).
(b) Supine barium examination shows a lower
gastric fundus (F) and a high gastric antrum (A).
(c and d) Two T2-weighted axial MR views and
sagittal view (e) show a distended stomach
containing a large amount of gas and fluid (St)‚ a
posterior location of the gastric antrum (A) between
gastric fundus (F) and gastric body (B)‚ and medially
positioned spleen (Sp) along the greater curvature of
the stomach. P = pancreatic tail; LK = left kidney.
(Courtesy of Satoru Hosoi‚ M.D.)
18 2. Clinical Embryology of the Abdomen: Normal and Pathologic Anatomy
Most of the bowel shares a common mesentery that is of mesenteric vessels‚ thrombosed or patent‚ wrapping
prone to volvulus‚ particularly with a short mesenteric around the superior mesenteric artery‚ which may be
root. Patients with malrotation in whom the cecum is thrombosed‚ calcified‚ or patent‚ can be diagnostic. Di-
found in either upper quadrant are the most prone to lated collateral veins and/or congested or edematous
develop a volvulus. Midgut volvulus is usually consid- mesentery may be present distally (Fig. 2–19). Because
ered a surgical emergency‚ generally occurring in infants the proximal limb returns to the abdomen first‚ an ab-
during the first weeks of life. It has been recently rec- normally rotated proximal limb does not always result
ognized that midgut volvulus may occur in adults‚ and in an abnormally positioned distal limb. However‚ an
its clinical presentations may be vague and chronic or abnormally positioned distal limb is almost invariably
recurrent.28‚29 On CT‚ the whirled appearance repre- associated with an abnormal proximal one.4 Faulty re-
senting intestinal loops and mesenteric fat with branches sorption of the suspending dorsal mesentery of the colon
Fig. 2–17. Cecal volvulus.
A 65-year-old male presented
with abdominal pain and
vomiting.
(a) Contrast enema shows a
twisting (curved arrows) of the ab-
normally located cecum (C) that
is markedly distended.
(b) Postcontrast CT shows a dis-
tended air-filled cecum (C) and
twisted portion of cecum and
mesentery (curved arrows).
Emergency surgery revealed a
cecal volvulus with an elongated‚
nonattached ascending colon and
cecum.
(Courtesy of Yoshiki Senba‚
M.D.)