Paper - A stage in the development of the serous cavities
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McGaw WH. A stage in the development of the serous cavities. (1924) Anat. Rec. 28(2): 105-129.
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A Stage in the Development of the Serous Cavities
W. H. McGaw
Anatomical Laboratory, Western Reserve University
The present article is the outgrowth of some elective work carried on in the anatomical laboratory some time ago with Doctor Ingalls. The intention at that time was to make a plastic reconstruction of a small human embryo in such a way as to bring out the important features in the development and gradual transformation of the body cavity. The ﬁnished model proved so clear and instructive that it has seemed worth while to put the results on record. We have chosen the method of stereoscopic photography, ﬁrst, because it has become a part of the daily routine of the anatomical laboratory and, in the second place, because it is the only method of giving an adequate conception of the complicated conditions with which we are dealing.
The embryo used for reconstruction was no. 13 of the Collection of the Department of Anatomy. Its dimensions were G. L. 9.6 mm. and C. R. 8.5 mm.; these were reduced in parafﬁn to 8.6 mm. and 7.6 mm., respectively. The menstrual age of the specimen was about forty-four days; the real age is estimated at from thirty-seven to thirty-eight days. The reconstruction was carried out in the usual method, with wax plates, at a magniﬁcation of 50 diameters. As regards the use of color on the model we need only say that all cut surfaces are white and that all serous surfaces are gray. Details of the cut surface appear against a white background. For present purposes the important distinction is between the white, cut surfaces and edges, and the gray, natural serous surfaces. The lines of reﬂection of the serous ‘membranes have been accentuated for the sake of clearness. It should also be noted that at the upper end of the model there is a slight skewing, so that certain structures on the right are a little lower than they should be. Naturally this is least in evidence close to the median line.
Two peculiarities of this embryo may be noted here; no yolk-sac or stalk was found, and the two umbilical arteries unite within the cord.
It has not seemed necessary to make any speciﬁc references to the literature, and in the descriptions of the Various Views we shall concern ourselves primarily with what can be seen on the View in question, rather than attempt a detailed and connected account of the Various structures and organs. Throughout these descriptions, right and left in the text refer to the right and left sides of the embryo.
A few words regarding the accompanying stereophotographs may not be out of place here. They are so arranged that the bottom of the photograph is close to the outside edge of the page. In viewing them, the card holder should be removed from the hand stereoscope and the page held in its place, level, properly centered, a11d at the right distance from the eyes. A smaller stereoscope, consisting simply of a mounting for a pair of prismatic lenses, Would probably be somewhat easier to manage. If necessary, the page may be folded along its outer border to bring the photograph into proper position.
Fig. 1. This shows the entire model in left vcntrolateral view, head not included. .The plane of tl1e top section passes just above the pericardial cavity, through the larynx, and between the ﬁfth and sixth cervical ganglia behind.
The anterior limb buds are more advanced than the posterior and are ﬂattened distally, they lie closer to the body and are directed caudally. Segmentation, in evidence from the thoracic region backward, is most conspicuous for the lower lumbar and upper sacral somites. The tail is well developed and straight, only the tip deviating slightly to the left. The genital eminence occupies the entire interval between tail and cord, it is somewhat compressed from above downward, the caudal slope showing a wel1—deﬁned median groove. Near the tip of the genital eminence a small, sagittally placed median plate rises from the ﬂoor of the groove, epithelial in character and due to proliferation in the anterior part of the thick eloacal membrane. The cord is straight and directed slightly to the right. Its cut surface shows the large, excentrically placed extra—embryonic coelom. Caudal to the coelom is the allantoic duct between the two umbilical veins, below the duct lies the umbilical artery. In this specimen, at this point, there is a single umbilical artery, the paired arteries joining in the substance of the cord.
The model of the embryonic body is made up of three pieces as shown in ﬁgure 1, an upper, middle, and lower piece. The line of separation between the upper and middle pieces is somewhat irregular. It begins ventrally in the plane of the sections just above the attachment of the cord about opposite the lower part of the seventh thoracic segment. Passing directly dorsal, under the lowest part of the pericardial cavity, it runs obliquely upward just dorsal to the lateral attachments of the septum transversum, to a point a little beyond the cephalic limit of the peritoneal cavity. Here the line of separation again becomes the plane of the sections, cutting the eighth cervical segment behind. The line between the middle and lower pieces of the model is, throughout, in the plane of section. Ventrally its relations are seen in the ﬁgure, dorsally it cuts through the lower part of the tenth thoracic segment. The Window in the upper piece affords a view of the interior of the pericardial cavity. The liver is separate, ﬁtting onto both the upper and middle pieces.
View of the roof and upper part of tlie posterior wall of the pericardial cavity as seen through the window in the upper piece of the model. The dorsal mesocardium consists of two portions, an anterior part or arterial mesocardium and a posterior or venous mesocardium. The former, much smaller, is high up in the roof and encloses the aorta and pulmonary artery; the aorta is on the right. The irregular cut surface below is the venous mesocardium with the cut ends of four Veins. Above and on the right is the right superior cava (right duct of Cuvier), below, nearer the midline and much larger, is the inferior cava, the terminal, hepatic portion; on the left is the small left duct of Cuvier, while the single pulmonary vein is in the median line. The space between the arterial and venous mesocardia is the transverse sinus of the pericardium. The venous mesocardium occupies a little more than the upper third of the posterior pericardial wall, its high, cephalic position being due to the marked obliquity of the septum transversum (fig. 1). In the middorsal line between the mesocardia is a broad ridge which becomes more sharply demarcated below; deeply embedded within this ridge is the trachea. Immediately above the venous mesocardium and on either side of the median ridge are two small, roughly elliptical openings—the pleuropericardial passages. Between these openings appears that part of the ventral mesentery which encloses the lower end of the trachea and whose free, ventral margin looks into the pericardial cavity. Caudal to this point the ventral mesentery is attached to the dorsal border of the septum transvcrsum; it meets the septum at a right angle and through the area of union the pulmonary vein gains the venous mesocardium and thence the left atrium. Below the septum transversum the ventral mesentery (ventral mesogastrium) is in relation with the liver (ﬁgs. 3 to 6).
The pleuropericardial openings are well up on the posterior pericardial wall, close to the roof; dorsally they open into the most anterior part of the pleural cavity close to the ventral mesentery. The left opening ha.s a vertical diameter of 10 mm. and a transverse diameter of 8 mm. The right is slightly smaller and gives the impression of being somewhat constricted. Both openings are bounded laterally by the free, media] margins of the pleuropericardial folds; these differ somewhat on the two sides. The right fold is thicker, due to the larger size of the right cuvierian duct. The mesocardium is also drawn up on its ventral, pericardial surface. On account of the contained vein, the contribution of its dorsal surface in limiting the pleural cavity in front is rather less than on the left side. The left fold is more typical, thinner, its free edge is enlarged to enclose the small left cuverian duct, both its ventral, pericardial and dorsal, pleural surfaces are more extensive than on the opposite side. Both pleuropericardial openings are limited internally by the lateral borders of the ventral mesentery and below by the dorsal border of the septum transversum. The dorsal surfaces of the folds are shown in ﬁgures 7 and 8.
Middle segment of model, from above and in front. The liver has been removed. The upper, ﬂat surface lies in the plane of section and passes through the eighth cervical ganglion behind. There also appear here the ventral rami of the seventh cervical nerves, notochord, the dorsal aortae just above their point of union, and sections of midoesophagus, lower trachea, and vagi. The large veins, opened by both the horizontal and vertical cuts, are the conﬂuent anterior and posterior cardinals, the beginnings of the cuvierian ducts. Outside these veins, on the top section, are the phrenic nerves, they are out again, more obliquely, in the vertical section near the base of the pleuroperitoneal folds.
The pleural cavities are large as compared with the lung buds and are in open communication with the peritoneal cavity below. Their lateral boundaries are formed above, by the body wall with the contained ducts of Cuvier, and below this by the inner surfaces of the pleuroperitoneal membranes which at this stage reach half-way down the lung buds. The pleuroperitoneal folds have been cut by the vertical section at the point where their ventral ends join the posterior edge of the septum transversum and the upper surfaces of the liver (ﬁgs. 6, 7, and 8). Lateral to the folds there is a small, blind pocket of peritoneum on either side. In the midline is the cut edge of the ventral mesentery throughout its entire extent. At its upper end, in the region of the pulmonary vein and to a point just below the tip of the recessus pneumatoentericus, it was connected with the dorsal border of the septum transversum (figs. 6, 7, 8). Below this level the ventral mesentery is split into two portions by the mesenteric recess or omental bursa, both of these parts are attached to the dorsal surface of the liver (ﬁgs. 8 and 9). On the left is the mesentery proper, ventral mesogastrium or lesser onientum, enclosing in its free margin below the portal vein, hepatic artery, and common bile duct. On the right of the omental bursa is the caval mesentery or mesohepar, plica venae cavae.
On the ventral body wall, above the attachment of the cord, is seen the large area over which the liver was fused with the body wall. Through this connection the two umbilical veins enter the liver from the body wall. Caudal to the liver there is a rapid constriction in the extent of body cavity.
Further details of this part of the model are shown in ﬁgures 4 and 5.
Detail of ﬁgure 3 (q.v.) from left side.
The left lung is bilobed, smaller and shorter than the right one. The free, inferior edge of the left pleuroperitoneal membrane is just above the notch between the upper and lower lobes. The lower lobe is very close to the upper end of the stomach. The left pleuroperitoneal fold has its dorsal attachment on the left side of the dorsal mesentery between the lung in front and the cephalic end of the mesonephric ridge overlying the posterior cardinal vein, behind. It describes an arc of about 90° around the lung, coming off the dorsal mesentery almost in a frontal plane, while its anterior, or ventral, attachments meet the liver and septum transversum (fig. 6) farther lateral in a sagittal plane. The free margin is concave, but with a greater radius and the dorsal end extends a little lower down than the ventral end. The phrenic nerve which appears near the base of the pleuroperitoneal membrane does nut enter this fold, but makes its way into the pleuroperieardial membrane (ﬁg. 7).
The stomach has rotated a little more than 45° and is displaced to the left; its epithelial wall shows a deﬁnite spindle shaped enlargement. A somewhat indeﬁnite, vertical ridge on the posterior mesogastrium close to the stomach represents the spleen. A part of this elevation is just visible behind the convexity of the greater curvature. A second more sharply marked ridge (not shown), close to and parallel with root of the mesogastrium, overlies the pancreas.
Partly obscured by the stomach, deeply placed on the dorsal wall near the median line, is the mesonephros. At its extreme anterior end dorsal to the origin of the pleuroperitoneal fold, the ridge is due to the underlying posterior cardinal vein rather than to the mesonephric tissue. The latter is deeply buried here, mesial and ventral to the vein, between it and the aorta. The ﬁne linear elevation formed by the rnesonephrie duct lies at first over the center of the mesonephros, but it soon gains the lateral border, which position it maintains until it reaches the terminal part of its course.
Details of ﬁgure 3 (q.v.) from the right side.
The right lung is three-lobed, larger and longer than the left. The right pleuroperitoneal membrane presents essentially the same features as the one on the left. It is somewhat lower in position, its pleural face is rather less concave and its inferior free margin is nearly straight. Its attachment to the liver appears much broader than on the left, due to the inclusion of some hepatic tissue, being cut off a. little farther ventral than on the left side.
The relations of the ventral mesentery and the omental bursa are well shown. On the right, close to the mesonephros, is the caval mesentery, with a longitudinal section of the primitive vena cava inferior, i.e., the small hepatic vein which grows backward from the liver through the eaval mesentery to connect with the right subcardinal vein in the posterior body wall. The recessus pneumato-entericus dexter (bursa infracardiacus) appears as a narrow slit between the oesophagus and the inferior lobe of the right lung. Below it opens out into the omental bursa, into what Will later be the recessus superior omentalis. Farther back, behind the caudal half of the stomach is a second recess, the recessus 1nesenterico—entericus (pancreatico-entericus), which reaches to the left as far as the attachment of the dorsal mesogastrium to the greater curvature of the stomach. It forms also a deep, narrow pocket dorsal to the pyloric end of the stomach and the upper part of the duodenum, the recessus inferior omentalis. The dorsal wall of the recessus n1esenterico—entericus is formed by the pos« terior mesogastrium in which lie the pancreas and spleen. The common opening of these various recesses, through the omental bursa into the general peritoneal cavity, is the foramen of Winslow (hiatus communis), which is seen between the inferior end of the eaval mesentery and the enlarged end of the ventral mesentery (mesogastrium). In the model a metal bridge extends across the opening into the mesenterico—enteric recess.
The terminal part of the ventral mesogastrium, the ligamcntum hepatoduodenale, bounding the foramen epiploicum below and on the left, contains the large portal vein with the hepatic artery on the left and the common bile duct below.
The opposite, cephalic end of the ventral mesentery, presents a free, rounded, serous surface; it contains the lower part of the trachea and looks into the pericardial cavity immediately above the septum transversum and venous mesocardium (ﬁg. 2).
Right phrenic nerve and mesonephros, as on the left side.
Same view as ﬁgure 3, but more from above and with liver in place.
The extensive white surface on the liver is the area of its attachment to the septum transversum (ﬁg. 7). The large vein is the terminal, hepatic part of the inferior cava. In the midline, immediately above tl1e liver, is the cut edge of the ventral mesentery where it joins the septum transversum, above this the ventral mescntery is free and helps form the pos« terior wall of the pericardial cavity (fig. 2). The tip of the recessus pneumato—entericus appears near the right border of the ventral mesentery above the liver and below the pulmonary vein.
The ventral ends of the pleuroperitoneal folds have two attachments which are naturally continuous. The frontal sections of the folds, seen here and in the preceding figures, represent approximately the connection, in a frontal plane, with the dorsal border of the septum transversum (ﬁg. 7). From the septum the folds continue onto the superior surfaces of the lobes of the liver with which they unite in what may be described as a horizontal plane. The cuts which separate the folds from their attachment to the liver are in a horizontal plane, so that, as see11 in place, the liver is under the ventral ends of the folds. Each fold possesses a triangular cut surface, 11ot shown, which is at right angles to its cut surface which can be seen and which represents its area of fusion with the liver. Both liver lobes project dorsally some distance beyond where they are joined by the pleuroperitoneal folds. In this way they help limit the pleural cavities below and on the outside as well as ventrally.
Both pleuroperitoneal membranes are well shown in ﬁgure 6, especially the left one, the left lobe of the liver is also seen close to the lung bud. The pleural cavities as seen here are entirely open ventrally, the closing structures are shown in ﬁgures 7 and 8.
View of the upper sections of the model from behind and below, same piece as shown in ﬁgure 2. Note that the cut surfaces represented here ﬁt onto the cut surfaces of ﬁgure 6 and also ﬁgures 3 to 5.
In the angle where the horizontal and frontal cut surfaces meet are the ducts of Cuvier, where they are formed by the union of the anterior and posterior cardinals. Outside and below are the phrenic nerves, leaving the body wall to enter the pleuroperieardial membranes. In the midline, pierced by the single pulmonary vein, is the area of fusion between the dorsal border of the septum transversum and the ventral mesentery. Below and on the right, in this area, is the tip of the recessus pneumato~entericus. On either side of this median cut surface, and extending farther cephalad, are seen the ventral Walls of the pleural cavities, the pleuroperitoneal passages. The caudal half, or more, of each lies in a frontal plane and is formed by the free, dorsal margin of the septum transversum. The cephalic portion of the ventral pleural wall has a more oblique position and is formed by the pleuropericardial folds, which laterally are continuous with the body wall in the region of the phrenic nerve; caudally they are fused with the septum transversum, while their internal margins are free and form the lateral boundaries of the pleuropericardial openings. The large, right duct of Cuvier can be seen entering the right fold. The large opening in the center, between the upper ends of the pleural cavities, receives the free edge of the mesentery in front of the lower end of the trachea (ﬁgs. 2 and 3). All that remain normally, then, with the mesentery in position, are the paired pleuropericardial openings, high up under the roof of the pleural cavities. The ventral surfaces of the pleuropericardial folds form a part of the dorsal wall of the pericardial cavity (ﬁg. 2).
The entire caudal, or caudodorsal, surface of the septum transversum, except for two narrow serous strips on either side, is represented as a cut surface, since it is fused with the liver over this entire area (ﬁg. 6). Above and on the right in this area is the inferior cava with its opening into the right atrium (fig. 2). For the purpose of description and topography, one may say that the lower, oblique sections of the phrenic nerves are located approximately at the point where the pleuropericardial and pleuroperitoneal membranes and the dorsolateral angles of the septum transversum join the latera.l body wall. Just caudal to the nerves, between the upper ends of the narrow serous st1'ips on the septum and the extreme lateral limits of the pleural cavity, there may be seen two small areas. These represent the part of the dorsal border of the septum with which the ventral ends of the pleuroperitoneal folds unite (ﬁgs. 3 to 6).
Same view as ﬁgure 7, but with the liver in place (see text to ﬁg. 7).
The ventral mesentery is continued from the dorsal margin of the septum transversum onto the dorsal surface of the liver (fig. 3). Note the continuation of the ventral pleural Wall on both sides onto the dorsal and dorsomesial surfaces of the right and left lobes of the liver. This is especially well seen on the right side where the lobe of the liver is much larger and the pulmonary impression much more distinct. These serous surfaces and also the ventral mesentery, with its two parts between them, were, in the embryo, in uninterrupted continuity. The cutting away of the liver from the septum occasions the transverse interrupting line in the model.
The pleuroperitoneal membranes, from their origins dorsal to the lungs, encircle the lungs dorsally and laterally (ﬁg. 3) ;
their ventral ends reach the septum transversum below the phrenie nerve and are continued beyond this onto the adjacent lobes of the liver.
For further details of liver consult text to ﬁgures 9 and 10.
Dorsal surface of liver (ﬁgs. 8, 5, and 10).
Note the deep concavity between the right and left lobes, the former is broader and much bulkier. In the midliiie is the attachment of the ventra.l mesogastrium, which terminates below, just beyond the section of the common bile duct. On the inner surface of the right lobe is the caval mesentery and the inferior cava, the beginning of the extrahepatic portion. Running forward from caval mesentery is the thin serous fold which forms the right boundary of the recessus pneumatoentcricus. Immediately above the caval mesentery is the pulmonary impression, that portion of the right lobe which helps limit the right pleural cavity, while laterally is an elongated, indistinct mesonephric impression. On the left lobe the deep gastric impression is conﬂuent above with the poorly marked pulmonary impression. On the caudal surface of the liver the two umbilical veins enter.
Liver from below and behind (ﬁgs. 9 and 3). The gastric and mesonephric impression are seen better in this view. The caudate lobe already bulges into the omental bursa. Over the white area below, the liver is fused with the anterior abdominal wall just above the attachment of the umbilical cord and below the ventral limit of the septum transversum. Through this area the two umbilical veins enter the liver from the body wall.
Lower segment of model, looking downward and toward the right. 'I‘he plane of section passes a little above the center of the cord and through the tenth thoracic segment. On the cut surface appear the aorta, posterior cardinal veins, mesoncphric tubules, and duct. In the lateral body wall, close to the cord, are the two umbilical veins. At this level they are running directly upward to reach the liver (fig. 3).
The dorsal mesentery is thin at its attachment and encloses near its free margin the small, epithelial, intestinal tube. The descending limb of the intestinal loop, the remainder of which is shown in ﬁgure 3, is on the right; on the left is the enlarged caecal region of the distal ascending limb. The summit of the loop is within the cord, the cavity of which is in wide communication with the intra-embryonic coelom. The large vessel iii the mesentery is the superior mesenteric artery. N0 yolk stalk was present.
The wolfﬁan bodies taper off into the lowest part of the coelom, the ducts leave their lateral positions and come to lie ventral. As may be seen on the cut surface, only a small part of this longitudinal elevation is due to the presence of mesoncphric tissue. The lowest part of the peritoneal cavity is in the form of a narrow, crescentic pouch between the intestine behind and the bladder in front. The left mesonephric duct can be seen crossing the left end of the crescent to reach the horn of the bladder.
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