Paper - The development of the supra-umbilical portion of the anterior abdominal wall (1938)

From Embryology

Wyburn GM. The development of the supra-umbilical portion of the anterior abdominal wall. (1938) J Anat. 72: 365-373. PMID 17104705

The Development of the Supra-umbilical Portion of the Anterior Abdominal Wall

By George M. Wyburn, M.B., Cu.B., F.R.F.P.S.G.

Senior Demonstrator in Anatomy in the University of Glasgow


This work is a continuation of the study of the development of the ventral body wall, the first part of which, “The development of the infra-umbilical portion of the abdominal wall, with remarks on the aetiology of ectopia vesicae”, was published in the Journal of Anatomy of January 1987.

The supra-umbilical portion of the anterior abdominal wall develops pari passu with the ventral growth of the liver, although, as is apparent from cases of eventration, a normally developing liver is not the controlling factor in the completion of this part of the parietes.

Up to and including a 4-5 mm. embryo the umbilical opening, invested by the amnial reflexions, has, as its superior, limit, the mesodermal mass at the caudal end of the pericardium which more dorsally separates the pericardial from the abdominal coelom. In an embryo measuring 7-5 mm. and more emphatically in one of 12:5 mm. there is a definite interval between the pericardium and the upper border of the umbilical opening, a supra-umbilical wall. The mesodermal basis of this portion of the ventral body wall would seem to be a derivative of the general mass of mesoderm forming a partition between heart and liver, and therefore an endeavour to assess its developmental significance includes an account of the origin of the formations known collectively as the septum transversum.

The Glasgow collection of embryos has again been utilized. The appropriate history of each will be found in the first part of this work. A recent addition to the collection, an embryo 2-6 mm. in length, is incorporated in the series.

Embryo Mcintyre I

In embryo McIntyre I, the youngest embryo of the series (see Bryce, 1924), the head process is continued into the prochordal plate (Text-fig. 1), and cranial to this the ectoderm and endoderm are in contact, a buccopharyngeal membrane lateral to which are the mesodermal sheets (PI. I, fig. 1).

In front of the head fold (Section 50, Pl. I, fig. 2) the ectoderm and endoderm are separated by mesoderm forming a continuous sheet across the middle line and exhibiting cleavage with the formation of a number of small spaces, precursors of the pericardial coelom. This mesoderm at the anterior end of the disc and in the region of the cranial reflexion of the amnion retains its relation to the amnial reflexion during subsequent rotation and development. Its position is therefore constant at the anterior boundary of the umbilical opening where it forms the “anterior mesodermal field”. It is composed of secondary and primary mesoderm. The secondary element is the fusion, in front of the buccopharyngeal membrane, of the mesodermal wings situate on both sides of the axial structures. The primary element is the blending of the amnial and yolk-sac mesoderm at the anterior pole of the blastoderm. In its mode of formation the anterior mesodermal field bears comparison with the posterior mesodermal field bounding the umbilical opening caudally where one also find’ a fusion of the primary and secondary mesoderm distal to the cloacal membrane.

In this embryo the pericardial plate is placed ventral to the head fold and cranial to the buccopharyngeal membrane. Dorsally it is covered by ectoderm, ventrally by endoderm; and its anterior and lateral boundaries are mesodermal, the anterior mesodermal field. In the cranial portion of the disc there are small mesodermal cavities. In the human embryo it seems more probable that the pericardial cavity is the result of confluence of a number of small mesodermal spaces rather than the fusion of two lateral cavities, a bilateral origin which would predestinate a ventral mesocardium, of the existence of which there is little evidence in the human embryo.


Text-fig. 1. Embryo McIntyre I. Reconstruction of a median sagittal section, x 100. The numbers refer to the sections. Description in text. Interrupted horizontal lines =ectoderm. Interrupted vertical lines=head process and prochordal area. Crosses=buccopharyngeal membrane. Black = mesoderm.


Embryo 2.6 mm

At a later stage (Text-fig. 2) inversion of the pericardium is in progress, and it has rotated round a transverse axis to slightly over 90°. The ectodermal covering now occupies the anterior and antero-ventral aspects while the endodermal boundary—the floor of the foregut—is dorso-caudal but separated Supra-umbilical Portion of the Anterior Abdominal Wall 367

from the dorsal wall of the pericardium in the median plane by a considerable thickness of embryonal mesenchyme. There has been proliferation of this tissue, i.e. the splanchnopleural mesoderm on the floor of the gut (PI. I, fig. 3). The mesodermal field, at an early stage forming the anterior pericardial wall, is situated ventro-caudally so that in this embryo the pericardium is covered on the dorso-caudal and caudal half of the ventral aspect by mesoderm forming a continuous sheet in the midline. Laterally this mesoderm forms the ventral wall of the coelomic tubes—communication between pericardial and abdominal coeloms. Compared with the earlier stage the area of yolk-sac connexion is reduced to the relatively small gut portal which cranially is separated from the amnial attachment by the tissue of the mesodermal field— a superior boundary of the umbilical opening—and has on each side the communication of intra and extra embryonic coeloms.


Text-fig. 2. Embryo 2-6mm. Reconstruction of a median sagittal section. x50. Numbers refer to the sections. Description in text. The yolk-sac is not shown. Interrupted horizontal lines = pericardial cavity. Interrupted vertical lines=gut. Large dots=liver. Black = mesoderm. A=amnion.


Into the proliferated mesoderm of the floor of the foregut projects the developing liver, not as a grooved plate or gutter but as a definite hollow outgrowth of the foregut in relation to which the more deeply staining liver cells can be distinguished (Pl. I, fig. 4). A very similar hepatic diverticulum is shown in the reconstruction of the Thompson embryo of twenty-three paired somites (Keibel and Mall, Fig. 236). With the growth of the ectoderm the lateral mesodermal walls of the pericardium are taken into the body of the embryo.

Embryo 4.5 mm

In an embryo of 4.5 mm. (Text-fig. 3) there has been a further rotation of the pericardium; and the cranial attachment of the amnion, carried round from the ventral to the caudal aspect, brings with it the region of the anterior mesodermal field. This now definitely forms the caudal boundary of the pericardium and in the midline is continuous with the splanchnopleure on the floor of the foregut, the two together forming a mass of mesoderm out of which is differentiated a portion of the diaphragm and the supra-umbilical abdominal wall. Following the completion of pericardial rotation the amnion becomes the cranial boundary of the umbilical coelom (PI. II, fig. 5). Cords of liver cells invade the dorsal portion of the mesodermal mass (PI. I, fig. 6), and these are broken up by the vitelline veins. At this stage the liver is confined to the dorsal and dorso-caudal mesoderm and has not yet penetrated the ventral half of the mesoderm on the caudal pericardial wall.


Text-fig. 3. Embryo 4-5 mm. Reconstruction of a median sagittal section. x25. The numbers refer to the sections. Description in text. The yolk-sac is not shown. Interrupted horizontal lines = pericardial cavity. Interrupted vertical lines=gut. Large dots=liver. Black =mesoderm. A=amnion.



The umbilical veins traverse the mesodermal mass laterally on each side to reach the sinus venosus. They are ventral to the pleuro-pericardial openings and are not broken up by the liver cells.

In the interests of convenience the familiar term septum transversum will be used here as denoting the general mass of mesoderm, caudal and dorsocaudal to the pericardium, i.e. including the anterior mesodermal field and the proliferated splanchnopleure of the floor of the foregut, but not confined to the unsplit cranial pericardial rim which according to Frazer forms the septum transversum after rotation of the heart.

As this “‘septum transversum” is but part of the general body mesoderm, merging laterally with the somatopleure and in the midline with the splanchnopleure, it is in many ways unfortunate that it has been singled out by a distinctive name. This tends to isolate it as an anatomical entity and causes considerable confusion when any endeavour is made to trace its ultimate fate as such.

Embryo 7 mm

In a 7mm. embryo (Text-fig. 4A) the liver cells, which in the 4-5 mm. embryo are confined to the more dorsal part of the septum transversum, have now extended ventrally and invaded the region of the septum on the caudal pericardial wall formed from the anterior mesodermal field. Pl. II, fig. 7 is a section through the cranial attachment of what is now the umbilical cord, and it is evident that this is lying some distance caudal to the pericardium, i.e. there is in this embryo a small interval between the umbilical cord and the pericardium. This specimen is the first of the series in which an umbilical “cord” as such can be said to exist, and with its completed formation there appears a supra-umbilical abdominal wall whose mesodermal basis is the ventral-caudal part of the septum transversum, i.e. the anterior mesodermal field pushed caudally and ventrally by the developing liver.

Dorsal to the cranial attachment of the cord the mesoderm of the “septum” blends with the more myxomatous tissue of the cord, the two together forming a partition between the intra and extra umbilical coeloms (Text-fig. 4A and Pl. II, fig. 8).

At this stage the mesodermal mass of the septum transversum is divided by the invading liver into a cranial portion, separating the liver from the pericardium, and a ventro-caudal portion—the mesodermal basis of the supraumbilical wall. In the midline dorsally is the ventral mesentery of the foregut, and on each side of the ventral mesentery the lung buds occupy the pericardioperitoneal channels or coelomic tubes whose ventral walls—the dorsal part of the septum transversum—form a pleuro-peritoneal membrane (PI. III, fig. 9 and Text-fig. 4B).


Text-fig. 4. Embryo 7 mm. The numbers refer to the sections. Description in text. A. Reconstruction of a median sagittal section. x25. B. Reconstruction of a parasagittal section. x25. Interrupted horizontal lines = pericardial cavity and coelomic passage. Interrupted vertical lines=gut. Large dots=liver. Black=mesoderm. Oblique interrupted lines =trachea. Arrow indicates the junction of the ductus venosus with the sinus venosus.

Embryo 12.5 mm

The interval between the caudal margin of the pericardium and the cranial attachment of the umbilical cord increases until in an embryo of 12-5 mm. it measures 0-6 mm. in length (Text-fig. 5A). Its mesodermal basis consists for the most part of a narrow band of tissue of loose texture whose dorso-ventral depth increases caudally. In the midline the mesoderm is denser compared to the more open mesenchymal network of the cord tissue (Pl. ITI, fig. 10).


Text-fig. 5. Embryo 12-5 mm. The numbers refer to the sections. Description in text. A. Reconstruction of a median sagittal section. x25. B. Reconstruction of a parasagittal section. x 12. Interrupted horizontal lines=pericardial cavity. Interrupted vertical lines=liver. Large dots=gut and trachea. Black=mesoderm. C=cranial attachment of umbilical cord. PR=pleural cavity. Arrow indicates the pleuro-peritoneal opening.

Along with the increased supra-umbilical interval is the marked caudal and ventral growth of the liver carrying with it the tissue of the septum transversum, the mesodermal basis of the supra-umbilical wall. In this embryo the “septum transversum” is, in the midline, represented by the mesoderm of the supra-umbilical abdominal wall, the narrow strip of mesoderm (Textfig. 5A) between the liver and pericardium which blends with the ventral mesentery of the foregut, and in the more cranial sections extends the entire width of the body cavity (Pl. ITI, fig. 11), the pericardio-peritoneal membrane.

Caudally the thoraco-abdominal partition is completed on each side by the pleuro-peritoneal membranes which with the expansion of the lungs have extended at the expense of the somatopleure. The somatopleure also furnishes the pericardio-pleural membrane (Pl. III, fig. 12). Text-fig. 5B shows the continuity of these three mesodermal septa with one another and with the supra-umbilical portion of the anterior abdominal wall.

The liver has commenced to separate from its mesodermal bed, the process of “‘ peritonealization”’ (Frazer), but still maintains a broad connexion ventrally where the vitello-umbilical vein enters the liver lateral to the midline, and also dorsally where the ductus venosus enters the sinus venosus.

Conclusions

The older embryologists postulated the formation of skin plates in connexion with the somites, these passing ventrally with the paraxial muscular downgrowths to complete the ectodermal covering of the ventral body wall. The more modern conception of the expansion of the embryonic body round a relatively fixed rim or circumference—the future umbilical orifice—not only affords an explanation of the formation of head and tail folds, pericardial inversion, and the prevailing dorsal convexity, but is also supported by the constant position of the umbilical vein along the “rim”, and which in the later stages encroaches on the midline ventrally.

The mesodermal basis of the lateral portions of the ventral wall is of course, in the first instance, the somatopleure, reinforced later by the muscular sheets of paraxial origin. Above and below the umbilicus are areas of fusion of primary and secondary mesoderm—the anterior and posterior mesodermal fields.

The mesoderm of the supra-umbilical abdominal wall is the ventral portion of the general mass forming a partition between the thoracic and abdominal cavities, and its splanchnopleure becomes the falciform ligament and serous coat of the liver. Into the dorsal part of the mesodermal partition projects the early hepatic bud, while later the liver not only encroaches on the ventral part, but in its further growth appears actually to push before it the abdominal parietes.

The appearance of a supra-umbilical interval synchronizes with the ventral and caudal growth of the liver tissue into the mesoderm of the body wall, and one is tempted to assume the interdependence of cause and effect. Certainly the growth of this part of the abdominal wall follows on the enlargement of the liver but only provided other conditions are normal—the cause is not always followed by the effect. In congenital abnormalities involving deficiency, partial or complete, of the supra-umbilical wall, e.g. abdominal fissure or eventration, there may be a liver normal in size and shape.

The development of the abdominal coverings would therefore seem to some extent to be independent of the viscera and influenced by factors mechanical or otherwise which do not necessarily affect visceral growth.


From the study of this series of embryos one is of the opinion that the name septum transversum, if it must be utilized, should have a much wider and more generous interpretation than it enjoys at present. In the 7 mm embryo the pleuro-peritoneal membrane is the mesoderm of the ventral wall of the coelomic tubes, and with thoracic expansion mesoderm from the somatopleure enters into its formation, but it would clearly be impossible at any given stage to delimit the tissue of an original septum transversum. This would indicate that the only justifiable embryological division of the diaphragm is on the basis of its muscular morphology.

Summary

  1. The development of the supra-umbilical abdominal wall has been studied in a series of embryos ranging from 1-4 to 12-5 mm.
  2. The ventral portion of the mesodermal partition between the thoracoabdominal cavities furnishes the basis of the supra-umbilical wall.
  3. This ventral part of the thoraco-abdominal partition is itself derived from the anterior mesodermal field subsequent to pericardial inversion.
  4. The term “septum transversum” is criticized.as being misleading and the cause of much confusion in the embryological interpretation of the diaphragm.


I have to record my indebtedness to my chief, Prof. Blair, for his supervision of the work.


References

Bryce, T. H. (1924). ‘Observations on the early development of the human embryo.” Trans. Roy. Soc. Edinb. vol. tm, pt. iii, No. 26.

Frazer, J. E. (1931). Manual of Embryology. London: Bailliére, Tindall and Cox.

Kaurss1, F. & Matt, F. P. (1912). Manual of Human Embryology, vol. 1.

PotrrzEr, C. & STERNBERG, H. (1930). ‘Uber die Entwicklung der ventralen Korperwand und des Nabelstranges beim Menschen.” Z. Anat. EntwGesch. Bd. xcu, Heft 4.

Scxuwa.sez, E. (1909). Die Morphologie des Missbildungen des Menschen und der Tiere.


Explanation of Plates

Plate I

Figs. 1, 2. Photomicrographs of embryo McIntyre I. xc. 114. Description in text,

Figs. 3, 4. Photomicrographs of 2-4 mm. embryo. xc. 54. Description in text. Y = cavity of yolksac; B.P.= buccopharyngeal membrane; M = mesoderm; C = coelomic passage; L = liver cells.

Plate II

Figs. 5, 6. Photomicrographs of section of embryo 4-5 mm. xc. 28. Description in text.

Figs. 7,8. Photomicrographs ofsectionofembryo7mm. xc.20. Description in text. A = amnion; M.L. = mesoderm with liver cells; C = coelomic passage; M = mesoderm; P = pleuro-peritoneal opening. pe

Plate III

Fig. 9. Photomicrograph of embryo 7mm. x c. 20. Description in text.

Figs. 10-12. Photomicrographs of embryo 125mm. xc. 21. C=coelomic passage; P.P. = pleuroperitoneal membrane; P.D. = pleuro-pericardial membrane.


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