Appendix And Cecum

SURGICAL ANATOMY AND EMBRYOLOGY

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APPENDIX AND CECUM
Embryology Anatomy and Surgical Applications
Volker Schumpelick, MD, Bernhard Dreuw, MD, Kerstin Ophoff, MD, and Andreas Prescher, MD

DEVELOPMENT OF THE CECUM AND THE VERMIFORM APPENDIX
The development of the cecum and of the vermiform appendix (Fig. 1) is closely related to the development of the midgut. Four structures are considered as intestinal derivations of the midgut: the (1)small intestine (without the upper duodenal part), (2) cecum and vermiform appendix, (3)ascending colon, and (4) right half of the transverse colon. All parts of the midgut are supplied by the superior mesenteric artery.
During the early stages of development, the midgut is attached to the dorsal wall of the body by means of a short dorsal mesenterium. A ventral mesenterium is not present. A broad connection with the yolk sac exists ventrally and becomes strongly narrowed by the developing bending of the embryo. Consequently, the omphaloenteric (i.e., vitelline) duct is formed, which vanishes during the sixth week of gestation. If this reduction is disturbed, one of several types of Meckel's diverticulum may develop, depending on the type and mode of persistence of the duct. For a short time after the elimination of the omphaloenteric duct, a small bulge remains where the duct has inserted at the intestine. This remnant completely disappears during subsequent development. Because the midgut grows considerably faster than does the rest of the embryonal body, it experiences various regular movements and rotations, which can be divided into three phases: the (1)physiologic umbilical hernia (6th gestational week; phase l),(2) closure of the physiologic umbilical hernia (10th gestational week; phase 2), and (3) peritoneal fixation of the midgut (12th gestational week; phase 3).

From the Departments of Surgery (VS, BD, KO) and Anatomy (AP), University Hospital, University of Technology at Aachen, Aachen, Germany

SURGICAL CLINICS OF NORTH AMERICA

VOLUME 80 NUMBER 1- FEBRUARY 2000

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Figure 1. Normal development and rotation of the midgut. A, The midgut forms a loop with a superior or prearterial (1) and an inferior or postarterial (2) segment. Where these segments meet the dorsal wall, the duodenojejunal flexure (3) and the left colic flexure (4) are formed. The loop is located in the base of the umbilical cord (physiologic herniation) with the superior mesenteric artery (5) as an axis. The vitelline duct (6) inserts at the apex of the loop. B, The loop undergoes a counterclockwise twist, first of 90" and then of 180"
(total: 270"). The cecal pouch (7) becomes visible. C,The cecal pouch is located directly
below the liver, and grows out caudally. Descensus of the cecum (arrow). D, At last, the cecum (7) with the appendix (9) is positioned in the right iliacal fossa and the right flexure (8) is formed.
Phase 1
During phase 1, the midgut experiences considerable elongation, resulting in a hairpin-shaped loop. Cranially and caudally, h s loop joins the intestinal sections located at the dorsal wall of the body (i.e., later duodenum and later descending colon) in a sharp bend ("cranial" and "caudal" basepoints, according to Hinrichsenz6)T. he cranial basepoint corresponds to the later duodenojejunal flexure, and the caudal basepoint, to the left colic flexure.
As in the embryonal cavity, because of the enormous size of the liver and the kidneys, room to accommodate the intestinal loop is unavailable, and the loop extends into the extraembryonal coelom of the umbilical cord. This configuration, first noticed by MeckePgin 1817, is considered the physiologic umbilical hernia and develops during the sixth week of gestation (length of the embryo, 10 mm). The intestinal loop is subdivided into proximal (prearterial) and distal (postarterial) segments. The vitelline duct inserts at the vertex of the loop and marks the dividing point of the two segments.
The proximal segment grows strongly in length and forms six primary

APPENDIX AND CECUM 297
intestinal loops, whereas the cecum starts from the distal loop like a bud. This early structure of the cecum simultaneously indicates the border between the later colon and the later ileum. Accordingly, the later ileum is located cranially, and the later colon, caudally. Within the intestinal loop, as an axis, lies the superior mesenteric artery, around which the intestinal loop rotates 90" counterclockwise. As a result of this rotation, the proximal segment of the loop assumes a right-hand position, and the caudal segment, a left-hand position.
Phase 2
During the 10th week of gestation (length of the embryo, 4-5 cm), the intestinal convolute starts to relocate into the embryonal body cavity. This process is completed fairly quickly2',22, 35 and is difficult to observe. The forces responsible for the back-positioning of the intestinal convolute are unknown. The decrease in size of the mesonephric bodies, the general enlargement of the embryonal cavity, and the relative decrease in size of the liver are probably contributing factors. Because the relocation is accomplished comparatively fast, tensional forces resulting from the rapid enlargement of the liver are assumed. According to Broman? the caudal border of the enlarging liver presses the umbilical loop caudally and produces a force that pulls the intestinal loop into the body cavity. The problems of the formation of the umbilical loop and of the repositioning of the physiologic umbilical hernia are discussed in detail by Estrada14and Kie~selbach.~'
In the course of the repositioning of the umbilical hernia, an additional counterclockwise rotation of the midgut by 180" occurs, resulting in a total rotation of 270". Following this additional rotation, the budlike complex of the early cecum is now located on the right-hand side of the upper abdominal cavity, directly below the liver and at the level of the iliac crest. The beginning elongation of the transverse colon pushes the cecum toward the right wall of the body. As the colon continues to grow, it must slide in the caudal direction ("descensus"). In the course of this process, the right colic flexure is formed. Subsequent growth completes this descensus, and the cecum finally lies in the right iliac fossa. During this stage of development, the entire intestine still features a mesenterium.
Phase 3
After the intestine has reached its final length, the mesenterium of the ascending and descending colon is pushed against the dorsal body wall and fuses completely with the parietal peritoneum. The resulting layer of connective tissue between the gut and the dorsal wall lacks blood vessels and is known as Toldt's fascia. Toldt's fascia can be used for the detachment of the colon without bleeding. By the process just described, the ascending and the descending colon assume a secondary retroperitoneal position.
Development of the Appendix and lleocecal Valve
The initial structures of the cecum and vermiform appendix become visible as the so-called "bud of the cecum." In this stage, the conic bud is located in the distal segment of the umbilical loop, directly in the vicinity of the apex of

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the loop. Stretching and elongation of the colon result in the descensus of the cecum and of the appendix. As the complex of the vermiform appendix is pushed ahead of the cecum, it might assume various positions. The retrocecal, retrocolic, or pelvine position of the appendix must be considered to have occurred by chance, not based on immutable laws of development.
Postpartum, the cecum exhibits increased growth laterally, resulting in the dislocation of the vermiform appendix in the medial direction. According to Broman,sthe increasing accumulationof meconium within the colon is the cause of the increase in diameter of this section of the intestine. Because of a mucosal fold, the distal cecum cannot be filled completely with meconium, so that growth is not stimulated there and the vermiform appendix borders against the cecum as a thin structure.
In addition to these general details concerning the development of the intestine, the development of the ileocecal valve of Bauhin merits attention. During the third month of gestation (length of the embryo, 5-6 cm), the initial cecal complex bends sharply against the colon, so that the terminal part of the ileum is pressed together in the craniocaudal direction and acquires a flattened, wedge-shaped config~rationI.n~~the next step, the terminal part of the ileum becomes invaginated into the colon, during which, cranially and caudally, the walls of the colon and terminal part of the ileum are pressed together and fuse completely, so the two lips of the ileocecal valve are composed of two complete intestinal walls and are not simply mucosal folds.
The vermiform appendix becomes visible in the eighth week of gestation (length of the fetus 10-12 cm), and the first accumulations of lymphatic tissue develop during the 14th and 15th weeks of gestation.32The first minute accumulations of lymphatic cells are located directly below the epithelium and influence the later development of these epithelial cells. Some lymphocytes penetrate into the epithelial layer of the vermiform appendix, wluch distinctly contains fewer goblet cells than the other colic mucosa. The vermiform appendix, tonsils, and Peyer’s patches possess no lymphatic vessels draining to their lymphatic apparatus and are different from lymphatic nodes.
SURGICAL ANATOMY OF THE CECUM-APPENDIX
COMPLEX
Cecum
The cecum, with the vermiform appendix, forms the first section of the colon (Figs. 2 4 ) . The pouchlike cecum is approximately 6 cm in length and approximately 8 cm in width and is variable in shape. The cecum shows all distinctive features of the colon, such as teniae, haustra, and fatty epiploic appendices. It is located below the point where the ileum joins the colon. Two incisures, one on the ventral side and one on the dorsal side, are called the ventral and dorsal cecocolic incisures. These incisures delineate the cecum against the ascending colon. Inside of the cecum, these structures correspond to the frenula of the ileocecal valve of Bauhin.
The dome-shaped haustrum, located at the lowest position of the cecum in the standing position, forms the bottom of the cecum, the cecal fundus. The teniae join each other at the base of the appendix and form a continuous longitudinal muscular layer on the outside of the appendix (the free tenia leads to the hidden appendix and functions as an appendix pointer). In the range of the orifice of the vermiform appendix, lateral trains of muscle fibers branch from

APPENDIX AND CECUM 299
Figure 2. Anterior view of the ileocecal complex. Pars terminalis ilealis (1). Mesenteriolurn (2). Plica ileocecalis inferior (bloodless fold of Treves) (3). Recessus ileocecalis inferior (4). Plica ileocecalis superior (5). Recessus ileocecalis superior (6). Taenia libera (7). Haustra coli (8). Appendix vermiforrnis (9).
Figure 3. Internal structure of the ileocecal complex. Pars terminalis ilealis (1). Papilla ilealis (2). Ostiurn ileale (3). Labrum ileocecale (sive inferius) (4). Labrum ileocolicurn (sive superius) (5). Frenulurn ostii ilealis (6). Plica sernilunaris (7). Haustra coli (8). Ostiurn appendicis vermiformis with Gerlach’s valve (9).

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Figure 4. Positions of the verrniforrn appendix. A, Retrocecal (65%). B, Descending,
pelvine (31Yo).C, Transverse, retrocecal (2.5%). 0,Ascending, paracecal, preileal (1YO).€,
Ascending, paracecal, postileal (0.5%). (Data from Wakeley CPG: Position of the vermiforrn appendix as ascertained by analysis of 10,000 cases. J Anat 67:277, 1933.)
each tenia and mesh intensively with the inner annular muscle layer. Such a meshing of muscle fibers is also present at the apex of the appendix.6
The cecocolic junction of various mammals presents a distinctly developed sphincter system. Such a sphincter is pronounced in herbivores but also present in carnivores. According to endoscopic, radiologic, and anatomic observations,43 such a cecocolic sphincter system is also established in humans.
Within the lumen of the cecum, the ileal orifice is located at the ileal papilla. The ileal orifice is lined by two lips, called the ileocolic (superior) and ileocecal (inferior) lips. From these lips, the frenulum of the ileal orifice arises and is a prominent ridge extending to the left border of the cecum. In the 1998 issue of Terminologia Anatorni~a,'t~he common expression ileocecal valve is no longer listed because, according to Rosenberg and DiDi0,4~this part does not constitute a valve and the closing mechanism is supposed to be located in the terminal ileum.
In the right fossa, the cecum rests on the iliac muscle, sometimes extending onto the psoas major muscle or hanging over the linea terminalis into the pelvis.

APPENDIX AND CECUM 301
These varieties in location are caused by the variable means of attachment of the cecum. Different degrees and forms of the cecal attachment are responsible for these various positions. The main configurations of the cecal attachment can be classified as fixed cecum, mobile cecum with secondary fixation, and mobile cecum.23Lateral to the psoas major muscle, a flat depression, termed the cecal fossa, can be seen by lifting up a mobile cecum. This distinct fossa is delineated laterally by the cecal fold and can communicate continuously with the retrocecal recess. The tight-filled cecum contacts the anterior wall of the body, but, if emptied, it collapses so that the small intestinal loops can interponate between the cecum and anterior body wall.
According to general opinion, the ileal orifice projects on McBurney's point (denoting the right third point of the right spinoumbilical line, also called Monro's line), whereas the origin of the appendix projects either on McBurney's point, Lanz's point (right third point of the interspinal line), or Kummell's point (right side and somewhat below the umbilicus). The position of the cecum varies and depends on the cecal filling and on the filling stage of the adjacent organs. The position of the vermiform appendix also varies and depends on many other factors. McBurney's point, Lanz's point, and Kummell's point cannot be considered to be more than preliminary points of orientation.
In the region of the ileocecal complex, structures move from an intraperitoneal to an extraperitoneal position. Sometimes, typical peritoneal dislocations occur, resulting in characteristic peritoneal folds and recesses. In the angle between the terminal part of the ileum and the cecum, the superior ileocecal fold extends on the ventral side of the cecum. This fold contains the anterior cecal artery and typically does not reach the vermiform appendix. The superior ileocecal recess, with its caudally directed entry, lies below this fold. The inferior ileocecal fold extends from the antimesenterial border of the terminal part of the ileum to the mesenteriolum of the vermiform appendix and spreads on its ventral area. This fold contains neither adipose tissue nor blood vessels, but some individual fibers of smooth muscles may be present. Because the vessels in this peritoneal fold are lacking, it is commonly called the "bloodless fold of Treves." Behind the inferior ileocecal fold, the minor and inconspicuous inferior ileocecal recess is hidden, with the entry to this recess directed caudally.
The entire ileocecal complex is vascularized by the ileocolic artery, which branches from the superior mesenteric artery. The ileocolic artery reaches the cecum in the corner between the cecum and the terminal part of the ileum and divides into five branches: (1)several ileal rami, which supply the terminal part of the ileum; (2) the anterior cecal artery, which supplies the anterior circumference of the cecum; (3) the posterior cecal artery, which supplies the posterior circumference of the cecum; (4) the colic ramus (also called the ascending ramus), which ascends at the medial border of the cecum, supplying this area and the beginning of the ascending colon; and (5) the appendicular artery, which runs dorsally (rarely, ventrally) to the terminal part of the ileum into the mesenteriolum of the vermiform appendix.
According to the l i t e r a t ~ r e ,t~he~ appendicular artery originates from the iliac ramus in 35% of cases, from the division of the ileocolic artery in 28% of cases, from the anterior cecal artery in 20% of cases, from the posterior cecal artery in 12% of cases, from the ileocecal artery in 3% of cases, and from the ascending colic ramus in 2% of cases (Fig. 5). The veins accompany the arteries. Drainage of the lymph is provided by the ileocolic lymph nodes, located along the superior mesenteric artery, and by celiac nodes into the cisterna chyli (Pecquet's reservoir). Near the ileocolic valve, some minor lymphatic nodes, called the prececal and retrocecal nodes, lie directly under the serous membrane of the cecum.

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Figure 5. Variations of the origin of the appendicular artery. A. ileocolica (1). A. cecalis anterior (2). A. cecalis posterior (3). R. ilealis (4). R. colicus (5). A. appendicularis (6). A = artery; R = ramus. (Data from Lippert H, Papst R: Arterial variations in man. Munchen, Bergmann JF, 1995.) Malformationsof the Cecum
Malformations of the cecum are primarily malpositions resulting from a disturbed gut rotation. The many manifestations of this entity can be classified as follows.

APPENDIX AND CECUM 303
Nonrotation
Nonrotation develops if the umbilical loop does not fullfill the last step of the gut rotation of 1804 so that the inferior segment of the umbilical loop is relocated into the abdominal cavity first during the repositioning of the physiologic umbilical hernia. Consequently, the entire colon remains double-foldedly positioned in the left part of the abdominal cavity (a so-called "left colon"), without any retroperitoneal fixation. The entire small intestine lies in the right part of the abdominal cavity. Many of these malformations are symptom free, but volvulus commonly accompanies nonrotation.
Malrotation
Malrotation occurs if the umbilical loop does not complete the last 90" of the rotation. As a result, the cecum remains below the pylorus, becoming attached by Ladd's ligaments at the dorsal body wall. These ligaments cross the duodenum and are able to compress this structure, with the result being duodenal stenosis. They can also cause a strangulation ileus.
Subhepatic Cecum
In approximately 6% of cases, the elongation of the proximal colon in the third phase of the gut rotation does not occur, omitting the descensus of the cecum. In these cases, the cecum-appendix complex remains directly below the liver. Many transitional forms exist between the normal position in the right iliac fossa and the subhepatic position.
Mobile Cecum
Mobile cecum develops if the beginning of the ascending colon does not achieve retroperitoneal fixation. In an extreme variant of this embryologic abnormality, the colon lacks retroperitoneal fixation to the extent that the whole gut (i.e., small intestines, ascending colon, transverse colon, and descending colon) possesses a collective mesenterium, called the commune mesenterium. The mobile cecum and common mesenterium predispose to a volvulus or to malposition of the appendix.
Hyper-rotation
Hyper-rotation, a rare malformation, is a gut rotation of 450" causing the cecum to lie directly at the left colic flexure. Another hypothesis explains this anomaly with an unlimited descensus of the cecum, which pushes the cecum at first into .the pelvis and subsequently cranially to the dorsal abdominal wall. A typical case was reported by Low and Hildermam~.~~
Inverse Cecum
Inverse cecum, a rare occurrence, is characterized by an early subhepatic fixation of the cecum below the liver directly after a normal gut rotation. During the elongation of the transverse colon, the cecum bends upwards (Fig. 6).
Retroperitoneal Cecum
The cecum-appendix complex and ascending colon become enclosed by a peritoneal membrane (i.e.,Jackson's paracolic membrane).This membrane devel-

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Figure6. Incidentalfinding of a cecum inversumin a patient with uncharacteristicabdominal symptoms. Note the inverse cecum and the enlarged ascending colon.

ops if the cecum is pushed under the laterodorsal peritoneal fixation. Jackson’s membrane contains parallel blood vessels that allow for the differentiation from bloodless peritoneal adhesions.
Internal Hernias
In the paracecal region, internal hernias may develop. According to a review by Schumpelick and Klir1ge,4~they are the second most common group after paraduodenal hernias. Most paracecal hernias enter at the recesses of the left paracecal region and are located within the retrocecal recess (also called Rieux’s hernias). A special type of paracecal hernia is Short’s hernia, with an entry on the right side of the cecum?9

SURGICAL ANATOMY OF THE VERMIFORM APPENDIX

The appendices of adults are pencil-shaped structures with a length o f ,

approximately 9 cm. Short forms (5 cm) and long forms (35 cm)

Ac-

cording to anatomy textbooks, the appendix originates at the posteromedial

border of the cecum, on average 1.7 to 2.5 cm below the terminal part of the