Book - Developmental Anatomy 1924-7
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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.
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Chapter VII The Mesenteries and Coelom
I. The Mesenteries
The Primitive Mesentery
The gut arises when the entoderm is folded into a tube (Fig. 165). At the same time, the lateral expanse of superposed splanchnic mesoderm swings inward from each side toward the midplane and forms a double-layered sheet, extending from the roof of the coelom to the midventral body wall and containing the gut between its layers (Figs, no and in). This membrane is the primitive mesentery. The covering layers of the gut (and other viscera), mesenteries, and body wall are continuous with each other and consist of a mesothelium, overlying connective tissue (Fig. 111). The parietal lining is derived from the somatic layer of mesoderm and the visceral covering from the splanchnic layer (Fig. 165).
Fig. 110. Diagram showing the primitive mesenteries of an early human embryo in median sagittal section (Prentiss). The broken lines (A, B, and C) indicate the level of sections A , B, and C, in Fig. 111.
Fig. 111. Diagrammatic transverse sections of an early human embryo (Prentiss). A, Through the heart and pericardial cavities; B, through the fore-gut and liver; C, through the intestine and peritoneal cavity. (Compare guide lines A, B, C, Fig. 110.).
Differentiation of the Dorsal Mesentery
At first, the gut is broadly attached dorsad and its roof lies directly beneath the notochord and descending aortee (Fig. 165). Presently this region of attachment becomes relatively narrow, and the gut is then suspended throughout most its length by a definite dorsal mesentery which extends like a curtain in the midplane. The esophagus lies in the mediastinum and has no typical mesentery in the adult (Fig. 124). On the contrary, that portion of the digestive canal which passes through the peritoneal cavity is contained in an originally continuous dorsal mesentery. Later, distinctive names are given to its several regions (Fig. no): thus, there is the dorsal mesogastrium (or greater omentum), of the stomach, the mesoduodenum, the mesentery proper of the small intestine, the mesocolon, and the mesorecinm.
Fig. 112. Diagrammatic model cf an 8 mm. human embryo, showing the primitive omental bursa (Prentiss). The model is sectioned transversely, caudal to the liver, so the observer looks cephalad at the caudal surfaces of the liver and sectioned body.
The Omental Bursa
The history of the mesogastrium is chiefly concerned with the development of a huge sacculation known as the omental bursa, or lesser peritoneal sac. According to Broman, its first indication in a 3 mm. embryo is a peritoneal pocket which extends cranially into the dorsal mesentery, to the right of the esophagus. A similar pocket, present on the left side, has disappeared in 4 mm. embryos. Lateral to the opening of the primitive bursa, a lip-like fold of the mesentery is continued caudally along the dorsal body wall into the mesonephric fold as the caval mesentery, in which the inferior vena cava develops later (Fig. 1 12). Furthermore, it will be remembered that the liver grows out into the ventral mesentery from the fore-gut, and, expanding laterally and ventrally, takes the form of a cresent. Its right lobe comes into relation with the caval mesentery, and, growing rapidly caudad, forms with this fold a partition between the lesser sac and the peritoneal cavity. Thus, the cavity of the omental bursa is extended caudally from a point opposite the bifurcation of the lungs to the level of the pyloric end of the stomach. In 5 to 10 mm. embryos, it is crescent-shaped in cross section (cf. Fig. Ill) and is bounded mesially by the greater omentum (dorsal mesentery) and the right wall of the stomach, laterally by the liver and caval mesentery, and ventrally by the lesser omentum (ventral mesentery) (Fig. 1 14). It communicates to the right with the peritoneal cavity through an opening between the liver ventrally and the caval mesentery dorsally (Figs. 114 and 116). This aperture is the epiploic foramen (of Winslow). When the dorsal wall of the stomach rotates to the left, the greater omentum is carried with it to the left of its dorsal attachment. The omental tissue grows actively to this side and caudally, and gives the omentum an appearance of being folded on itself between the stomach and the dorsal body wall (Fig. 113). The cavity of the omental bursa is carried out between the folds of the greater omentum as the inferior recess (Figs. 97 and 116).
From the cranial end of the sac there is constricted off a small closed cavity which is frequently persistent in the adult. This is the infra-cardiac bursa and may be regarded as a third pleural cavity. It lies at the right of the esophagus in the mediastinum when the stomach changes its position and form so that its midventral line becomes the lesser curvature and lies to the right, the position of the lesser omentum is also shifted. From its primitive location in a median sagittal plane, with its free edge directed caudally, the lesser omentum is rotated through 90Â° until it lies in a coronal plane with its free margin facing to the right. The epiploic foramen then forms a slitlike opening leading from the peritoneal cavity into the vestibule of the omental bursa (Fig. 114). The foramen is bounded ventrally by the edge of the lesser omentum, dorsally by the inferior vena cava, cranially by the caudate process of the liver, and caudally by the wall of the duodenum.
Fig. 113. Diagrammatic model of a 14 mm. human embryo (adapted by Prentiss). The figure shows the caudal surface of a section through the stomach and spleen, a ventral view of the stomach, the liver having been cut away to leave the sectioned edges of the omental bursa and caval mesentery, and the caudal surface of the septum transversum and pleuro-peritoneal membrane. Upon the surface of the septum is indicated the attachment of the liver.
During fetal life the greater omentum grows rapidly to the left and caudad, in the form of a sac, flattened dorso-ventrally. It overlies the intestines ventrally and contains the inferior recess of the omental bursa (Fig. 1 1 5). In the fourth month, the dorsal wall of the sac usually fuses with the transverse colon and mesocolon where it overlies them (Fig 115 B). The transverse mesocolon of the adult is conseciuently a double structure and the omental connection between stomach and colon becomes the gastro-coUc ligament. Caudal to this attachment, the walls of the omental bursa commonly unite and obliterate its cavity. The inferior recess of the omental bursa thus may be limited in the adult chiefly to a space between the stomach and the dorsal fold of the great omentum, which latter is largely fused to the peritoneum of the dorsal body wall. The s[)leen develops in the cranial portion of the great omentum; that stretch of the omentum extending between the stomach and spleen is known as the gastro-splenic ligament (Fig. 113), while its continuation beyond the spleen is the spleno-renal ligament.
Fig. 114. Transverse section through a 10 mm. human embryo at the level of the stomach and epiploic foramen (Prentiss). X 33.
Fig. 115. Diagrams showing the developmental relations of the greater omentum (Hertwig). A, Illustrates the beginning of the greater omentum and its independence of the transverse mesocolon: in B the two come into contact; in C they have fused. A, Stomach; B, transverse colon; C, small intestine; D, duodenum; E, pancreas; F, greater omentum; G, greater sac; H, omental bursa.
Other Changes in the Dorsal Mesentery
As long as the gut remains a straight tube, the dorsal mesentery is a simple sheet whose two attached edges are equal in length. But when the intestine starts to elongate faster than the body wall, and forms first a loop and then coils, the intestinal attachment of the mesentery grows correspondingly and is carried out into the umbilical cord between the intestinal limbs. Even before this herniation of the intestine occurs, its limbs are so shifted that the cecal end of the large intestine comes to lie cranially and to the left, and the small intestine caudally and to the right; in this position the future duodenum and colon cross in close proximity to each other (Fig. 95). On the return of the intestinal loop into the abdomen, the cecal end of the colon is carried over to the right, and the transverse colon crosses the duodenum ventrally and cranially (Fig. 116 A). The primary loops of the small intestine lie caudal and to the left of the ascending colon. There has thus been a torsion of the mesentery about the origin of the superior mesenteric artery as an axis, which is accentuated as the limb of the ascending colon elongates toward the pelvis (Fig. 116 B). From this focal point, the mesentery of the small intestine and colon spreads out like a fan or funnel.
Fig. 116. Diagrams showing the later history of the dorsal mesentery in ventral view (Tourneux-Prentiss). *, Cut edge of greater omentum; a, h, area of ascending and descending mesocolon fused to dorsal body wall. The arrow emerges from the omental bursa.
Previous to the middle of the fourth month, the gut is freely movable within the scope of its restraining mesentery, but soon secondary fusions occur which attach certain segments. The ascending and descending colon are applied against the body wall on the right and left side respectively. The flat surfaces of their mesenteries fuse with the adjacent dorsal peritoneum, and these two limbs of the colon become permanently anchored (Fig. 1 16). vSince the transverse colon passes ventral to the duodenum (Fig. 1 16), its mesentery remains distinct; but in the region of crossing, the base of the mesocolon fuses with the surface of the duodenum and pancreas. In accordance with its final position, this mesentery is now known as the transverse mesocolon. The line of attachment of the mesocolon presses the duodenum firmly against the dorsal body wall and obliterates its mesentery, thereby fixing this portion of the small intestine. The pancreas, which primarily is an outgrowth of the duodenum into the mesoduodenum, necessarily assumes also a retroperitoneal position behind the root of the transverse mesocolon. The mutual union of the lamellae of the greater omentum and its fusion to the transverse colon and the dorsal body wall have been mentioned. The mesentery proper of the small intestine is thrown into numerous folds, corresponding to the loops of the intestine, but normally does not exhibit secondary attachments; the sigmoid mesocolon likewise remains free.
Differentiation of the Ventral Mesentery
The same splanchnic mesodermal layers that invest the entoderm and form the dorsal mesentery, also combine beneath the gut as the ventral mesentery (Figs, no and uI). The ventral mesentery is associated intimately with the development of two important organs. One is the heart, which becomes a single tube by the union of paired anlages lying one in each lateral fold of splanchnic mesoderm (Fig. 165). Hence the heart is supported by the ventral mesentery both above and below (Figs, no and in A). The other organ, the liver, grows downward into the ventral mesogastrium, splitting apart its component lamellae and then having similar mesenterial relations as the primitive heart (Figs, no and in B). Caudal to the yolk sac the ventral mesentery does not persist, even in young embryos (Figs, no and in C).
Most cephalad, the heart is suspended in the ventral mesentery which is there designated the dorsal and ventral mesocardium (Figs, -no and uI * 4 ). The latter is transitory and the dorsal mesocardium also disapjiears somewhat later, leaving the heart unsupported in the pericardial cavity (Fig. i66).
Ligaments of the Liver
From the first, the liver is enclosed by the lamelte of the ventral mesogastrium, which, as the liver increases in size, give rise to its capsule and ligaments (Figs, no and in B). Wherever the liver is unattached, the enveloping mesodermal layers form the capsule (of Glisson), a fibrous layer covered by mesothelium, continuous with that of the jjeritoneum (Fig. in B). Along its mid-dorsal and midventral line, the liver remains connected to the ventral mesentery. That portion of the mesentery between the liver, stomach, and duodenum is the lesser omentum (Fig. 113). This in the adult is differentiated into the hepato-gastric and he pato-duodcnal ligaments. The mesogastrial attachment of the liver to the ventral body wall extends caudally from diaphragm to umbilicus and constitutes the falciform ligament.
In its early development, the liver abuts upon the primitive diaphragm, and, in 4 to 5 mm. embryos, is attached to it along its cephalic and ventral surfaces. Soon, dorsal prolongations of the lateral liver lobes, the coronary appendages, come into relation with the diaphragm dorsally and laterally (Fig. 124). The attachment of the liver to the septum transversum now has the form of a crescent, the dorsal horns of which are the coronary appendages (Fig. 113). This union becomes the coronary ligament of the adult liver. The dorso-ventral extent of the coronary ligament is reduced during development, and, at five months, its lateral extensions upon the diaphragm give rise to the triangular ligaments of each side.
The right lobe of the liver, comes into relation along its dorsal surface with the caval mesentery of 9 mm. embryos (Figs. 112 and 113). This attachment extends the coronary ligament caudally on the right side and makes possible the connection between the veins of the liver and mesonephros which contributes to the formation of the inferior vena cava. The portion of the liver included between the caval mesentery and the lesser omentum is the caudate lobe. The umbilical vein (later the ligamentum teres) courses in a deep groove along the ventral surface of the liver, and, with the portal vein and gall bladder, bounds the quadrate lobe.
In general, the several displacements and secondary fusions of the primitive mesentery cause its line of peritoneal attachment to depart throughout most of its extent from the original midsagittal position.
The special mesenterial supports of the urogenital organs will be described in the next chapter.
The mesenteries may show malformations, due to the persistence of the simpler embryonic conditions, usually correlated with the defective development of the intestinal canal. In about 30 per cent of cases the ascending and descending mesocolon are more or less free, having failed to fuse with the dorsal peritoneum. The primary sheets of the greater omentum may also fail to unite, so that the inferior recess extends to the caudal end of the greater omentum, as is normal in many mammals.
II. The Coelom
Pericardial cavity surface of fore-gut.
The Primitive Coelom
The first occurrence of a coelom is in the extra-embryonic mesoderm (Fig. 40 C). vShortly after, numerous clefts appear in the embryonic mesoderm of each side and split it into somatic and splanchnic layers (cf. Fig. 325).
These clefts coalesce in the cardiac region and form two elongated pericardial cavities, lateral to the paired heart tubes (Fig 165 A, B). Similarly, right and left pleuro-peritoneal cavities are formed between the mesoderm layers caudal to the heart. The paired pericardial cavities extend toward the midplane cranial to the heart and presently communicate with each other (Figs. 117 and 165 C).
Laterally, they are not continuous with the extra-embryonic coelom, for in this region the head of the embryo has already separated from the underlying blastoderm.
The pericardial cavities, nevertheless, are prolonged caudally until they open into the pleuro-peritoneal cavities where these in turn communicate laterally with the extra-embryonic coelom. In an embryo of 2 mm., the coelom thus consists of a U-shaped pericardial cavity, the right and left limbs of which are continued caudally into the paired pleuro-peritoneal cavities; these extend out into the extraembryonic coelom (Fig. 117).
The primitive coelom lies in the horizontal plane (Fig. 117). Coincident with the caudal regression of the primitive diaphragm, the pericardial cavity is bent ventrad and enlarged (Fig. 118). The ventral mesocardium, attaching the heart to the ventral body wall, disappears, and the right and left limbs of the U-shaped cavity become confluent, ventral to the heart. The result is a single, large pericardial chamber, the long axis of which now lies in a dorso-ventral plane, nearly at right angles to the plane of the pleuro-peritoneal cavities, and connected with them dorsally by the right and left pleuro-pericardial canals. On account of the more rapid growth of the embryo, there is an apparent constriction at the yolk stalk, and, with the development of the umbilical cord, the peritoneal cavity is separated definitely from the extra-embryonic coelom (Fig. 45). Dorsally, the pleural and peritoneal cavities are permanently partitioned lengthwise by the dorsal mesentery.
Pleuro-peritoneal canal Entoderm of gut Peritoneal cavity Extra-embryonic coelom Wall of yolk sac
Fig. 117. Diagrammatic model of the fore-gut and coelom in an early human embryo, viewed from above and behind (Robinson-Prentiss)
the cavities of the mesodermal segments are regarded as portions of the coelom, but in man they disappear early. The development of the vaginal sacs, which grow out from the inguinal region of the peritoneal cavity into the scrotum, will be described in Chapter VuI the division of the primitive coelom into separate cavities is accomplished by the development of three types of membrane that join on each side in a Y-shaped fashion (Figs. 122 and 123); (i) the unpaired septum transversum, which separates partially the pericardial and pleural cavities from the peritoneal cavity; (2) the paired pleuro-pericardial membranes, which complete the division between pericardial and pleural cavities; (3) the paired pleuro- peritoneal membranes, which complete the partition between each pleural cavity and the peritoneal cavity.
Fig. 118. Reconstruction, cut at the left of the median sagittal plane of a 3 mm. human emhryo, showing the body cavities and septum transversum (Kollmann).
The Septum Transversum
The vitelline veins, on their way to the heart, course in the splanchnic mesoderm lateral to the fore-gut (Fig. 183). In embryos of 2 to 3 mm., these large vessels bulge into the coelom until they meet and fuse with the somatic mesoderm (Fig. 381). Thus, there is formed caudal to the heart a transverse partition, filling the space between the sinus venosus of the heart, the gut, and the ventral body wall, and separating the pericardial and peritoneal cavities from each other ventral to the gut (Fig. i66). This mesodermal partition was termed by His the septum transversum. In Fig. 118 it comprises both a cranial portion (designated - septum transversum - that is the anlage of a large part of the diaphragm, and a caudal portion, the ventral mesentery, into which the liver is growing.
At first the septum transversum does not extend dorsal to the gut, but leaves on either side a pleura- peritoneal canal through which the pericardial and pleuro-peritoneal cavities communicate (Fig. 118). In embryos of 4 to 5 mm., the lungs develop in the median walls of these canals and bulge laterally into them (Fig. 120). Thus the canals become the pleural cavities, and will be so termed hereafter.
The septum transversum of 2 mm. embryos occupies a transverse position in the middle cervical region (Fig. 119, 2). It then migrates caudally, the ventral portion at first moving more rapidly so that its position becomes oblique. In 5 mm. embryos (Fig. 119, 5) the septum is opposite the fifth cervical segment, at which level it receives the phrenic nerve. During a second period of migration the dorsal attachment travels faster than the ventral portion, and as a result the septum rotates to a position nearly at right angles to its plane at 7 mm. The final location, opposite the first lumbar segments, is attained in an embryo of two months.
The Pleuro-pericardial and Pleuro-peritoneal Membranes. - The common cardinal veins (ducts of Cuvier), on their way to the heart, curve around the pleural cavities laterally in the somatic body wall (Fig. 118). In embryos of 7 mm., each vein, with the overlying mesoderm, forms a ridge that projects from the body wall mesially into the adjacent pleural canal. This ridge, the pulmonary ridge (of Mall), is the anlage of both the pleuro-pericardial and pleuro-peritoneal membrane (Figs. 118 and 120). Later, it broadens and thickens cranio-caudally (Fig. 121), forming a triangular structure whose apex is continuous with the septum transversum (Fig. 122). Its cranial side constitutes the pleuro-pericardial membrane, and, in g to 10 mm. embryos, reduces the opening between the pleural and pericardial cavities to a mere slit. Its caudal side becomes the pleuro-peritoneal membrane, which later completes the partition dorsally between the pleural and peritoneal cavities (Fig. 123).
Fig. 119. Diagram showing the migration of the septum transversum (MallPrentiss). Numerals indicate the length of the embryo at each position of the septum. The letters and numbers at the right represent the occipital, cervical, thoracic and lumbar segments.
Fig. 120. Reconstruction of a 7.5 mm. human embryo, cut across and viewed caudally to show the body cavities and pulmonary ridge (Kollman).
Fig. 121. Reconstruction of a 7 mm. human embryo, showing from the left side the pleuro-pericardial membrane, the pleuro-peritoneal membrane and the septum transversum (Mall in Prentiss). X 20. The phrenic nerve courses in the pleuro-pericardial membrane. An arrow jiasses from pericardial to peritoneal cavity through the pleuro-pericardial canal.
Fig. 122. Reconstruction of an 1 1 mm. human embryo, to show the structures of Fig. 121 at a later stage (Mall in Prentiss). X 14.
The two sets of membranes at first lie nearly in the sagittal plane, and a portion of each lung is caudal to the corresponding pleuro-peritoneal membrane (Fig. 121). Between the stages of 7 and 11 mm. the dorsal attachment of the septum transversum shifts caudad more rapidly than its ventral portion, and carries the pleuro-peritoneal membrane with it until the latter lies caudad to the lung (Figs, 119, 121 and 122). Each lung then occupies a spherical triangle between pleuro-pericardial and pleuro-peritoneal membranes (Fig 122). During this rotation the dorsal end of the pleuro-pericardial membrane lags behind, anchored by the phrenic nerve which courses through it, and so takes up a position in a coronal plane nearly at right angles to the septum transversum (Figs. 122 and 123). In 11 mm. embryos, the pleuro-pericar dial membranes have fused completely on each side with the median walls of the pleural cavities (Fig. 123).
The pleuro-peritoneal membranes are continuous dorsally and caudally with the mesonephric folds; ventrally and caudally, they fuse later with the dorsal pillars of the diaphragm, or coronary appendages of the liver (Figs. 113 and 124). Between the free margins of the membranes and the mesentery a temporary opening is left on each side, through which the pleural and peritoneal cavities communicate (Figs. 108, 113 and 122). Owing to the caudal migration of the septum transversum and the growth of the lungs and liver, the pleuro-peritoneal membrane, at first lying in a nearly sagittal plane (Figs. 120 and 12 1), is shifted to a horizontal position (Fig. 122), and gradually its free margin unites with the dorsal pillars of the diaphragm and with the dorsal mesentery. The opening between the pleural and peritoneal cavities is thus narrowed and finally closed in embryos of 19 to 20 mm.
Fig. 123. Transverse section through a 10 mm. human embryo, showing the pleuro-pericardial and pleuro-peritoneal membranes (Prentiss). X 33.
The Pericardium and Diaphragm
The lungs grow and expand, not only cranially and caudally but also laterally and ventrally (Fig. 125). Room is made for them by the obliteration of the very loose, spongy mesenchyme of the adjacent body wall (Fig. 124). As the lungs burrow laterally and ventrally into the body wall around the pericardial cavity, the pleuro-pericardial membranes enlarge at the expense of this tissue and more and more the heart comes to lie in a mesial position.
Fig. 124. Transverse section through a 10 mm. human embryo, showing the fusion of the pleuro-peritoneal membranes with the coronary appendages (Prentiss). X 16.
Between the lungs, but separated from them by the pericardium (Fig. 125 B). The pleural cavities thus increase rapidly in size at the same time the liver grows enormously, and on either side a portion of the body wall is taken up into the septum transversum and pleuro-peritoneal membranes. The diaphragm, according to Broman, is thus derived from four sources (Fig. 126): (1) its ventral pericardial portion from the septum transversum; its lateral portions from (2) the pleuro-peritoneal membranes, plus (3) derivatives from the body wall; (4) lastly, a median dorsal portion is formed from the dorsal mesentery. In addition to these, the striated muscle of the diaphragm takes its origin from a pair of premuscle masses, which, in 9 mm. embryos, lie one on each side opposite the fifth cervical segment (Bardeen). This is the level at which the phrenic nerve enters the septum transversum (Fig. 124). The exact origin of these muscle anlages is in doubt, but they probably represent portions of the cervical myotomes of this region. The muscle masses migrate caudally with the septum transversum and develop chiefly in the dorsal portion of the diaphragm.
Fig, 125. Diagrams showing the development of the lungs and the formation of the pericardium (Robinson-Prentiss). A, Coronal section; B, transverse section.
Fig. 126. Diagram showing the contributions to the diaphragm (Broman). i. Septum transversum; 2, 3, dorsal mesentery; 4, 4, pleuro-peritoneal membranes; 5, 5, bodj' wall; A, aorta; Oe, esophagus; VC, inferior vena cava.
The persistence of a dorsal opening in the diaphragm, more commonly on the left side, finds its explanation in the imperfect development of the pleuro-peritoneal membrane. Such a defect may lead to diaphragmatic hernia, the abdominal viscera projecting to a greater or less extent into the pleural cavity. Similarly, faulty development of the left pleuro-pericardial membrane sometimes causes the heart and left lung to occupy a common cavity. An intact diaphragm, locally deficient in muscle, may herniate.
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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.
Cite this page: Hill, M.A. (2021, June 15) Embryology Book - Developmental Anatomy 1924-7. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Developmental_Anatomy_1924-7
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