Paper - The early stages of the development of the pericardium

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Robinson A. The early stages of the development of the pericardium. (1902) J. Anat and Physiol. 35: 1-15.

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This historic 1902 paper by Robinson describes development heart pericardium in several species.



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Robinson A. The early stages of the development of the pericardium. (1902) J. Anat and Physiol. 35: 1-15.

The Early Stages of the Development of the Pericardium

By Arthur Robinson, M.D., M.R.C.S.,

Professor of Anatomy, King’s College, London. (PLATES 1., II.)

1902


The development of the pericardium has usually been considered in association with the development of the great veins and with the development of the diaphragm. In consequence of this much stress has been laid upon phenomena which have no direct bearing upon the formation of the pericardium itself, and the essential simplicity of the process has been obscured, to a. certain extent, by the use of terms which are not in all cases appropriate, and in many are‘ misleading. As a result a large amount of misconception exists with regard to the details of the formation of this important serous sac in mammals, and this misconception is mainly due to inaccurate descriptions of the mode in which the embryo is evolved from the surface of the ovum.


The incorrect descriptions most responsible for the misunderstanding are those which ascribe the demarcation of the embryo and its separation from the non-embryonic part of the ovum as due to the formation of sulci, which appear round the margins of the embryonic area and gradually converge beneath it to form the boundary of a continually narrowing orifice—the umbilical orifice; in other words, to descriptions which lead their readers to infer that the embryo is demarcated by a process of tucking off from the ovum, Whilst at the same time it is moulded into its proper form.


In association with this misconception of the manner in which the embryo is separated from the remainder of the ovum, a description of the early stages of the development of the pericardium is given which, at first sight, is apparently in accordance with the appearances observed in transverse sections of developing ova; but when the examination of such sections is controlled by the observation of longitudinal sections in similar stages of development, then it is found that the appearances seen in transverse sections are deceptive, and that the phenomena observed are capable of a different interpretation to that previously placed upon them.


It has been shown by the examination of transverse sections that, whilst the embryonic area is still outspread upon the upper part of the ovum, the mesoderm within its margins becomes cleft into an outer or somatic and an inner or splanchnic layer, but that in the antero-lateral parts of the area this cleavage of the mesoderm does not extend either to the margins of the area or to the sides of the neural groove, and that, consequently, on each side a mesodermal tube is formed, and in the lower or splanchnic layer of each of these tubes a longitudinal vessel appears which has been described as a rudiment of the heart (fig. 1).

FIG. 1.


The examination of sections of this kind, and the preconceived idea that the folds of the embryo are formed by a process of tucking in of the margins of the embryonic area, have led to the statement that, as the lateral folds pass inwards beneath the growing embryo and approach the mid-ventral line, the two mesodermal tubes meet together and divide the anterior portion of the blastodermic cavity into two parts, an upper (the foregut), and a lower (the anterior part of the yolk sac), and that the socalled rudiments of the heart, which lie in the inner walls of the mesodermal tubes, also fuse at the same time to form a single median heart (fig. 2). Obviously, at the period under consideration, a heart developed in such a manner would be attached for a time by a fold of mesoderm to the foregut dorsally, and to the yolk sac ventrally, but the idea that the lateral folds of the embryo are converging ventrally has been held so tenaciously as to establish the belief that the superficial ectodermal covering of the folds also reaches the middle line; otherwise, it is impossible to account for the statement which constantly reappears, that after the lateral folds have converged and the pericardial tubes have met beneath the ventral wall of the foregut, the heart is not only attached by a dorsal mesentery to the foregut, but also by a ventral mesentery to the ventral Wall of the body. In later stages, it is obvious that no ventral mesentery exists, for the pericardial sac is not divided, and it is asserted that this undivided condition of the pericardium is due to the disappearance of the ventral mesentery.


FIG. 2.


It may be stated at once, after the above account of the very general belief which exists regarding the early stages of the development, that the phenomena described do not occur, for they would necessitate that the ventral wall of the body from the umbilicus to the mouth is, at some period, cleft in the middle line, and that the cleft only disappears as the two lateral folds converge and separate the foregut from the remainder of the blastodermic cavity. In no vertebrate is any such condition found. The ectoderm and entoderm of the anterior part of the ventral Wall of the body, the part in front of the umbilicus, are never cleft in the middle line; but the mesoderm, which is a later formation, enters this section of the body in a different manner and at different periods of development in various groups of animals. In this respect, the anterior part of the ventral wall of the body, the part in front of the umbilious, differs essentially from the posterior part, the part behind the umbilicus, for the former is developed from the portion of the ovum which lies in front of and below the blastoporic region, and the latter is formed from the primitive streak which represents the closed blastopore. The anterior part, therefore, was never cleft mesia1ly,whi1st the posterior was ; and the latter may, and, not uncommonly, does revert to its open condition, such reopening resulting, in certain cases, in extroversion of the bladder. It is the want of appreciation of these facts, combined with a false idea regarding the mode of formation of the folds of the embryo, which has led to the incorrect descriptions of the formation of the pericardium in mammals, for in these vertebrates the pericardium is never separated into right and left halves by a ventral mesocardium; indeed, in them a ventral mesocardium does not exist; and although it is present in other groups, it is not formed in the manner usually described.


Before the early stages of the development of the pericardium in mammals can be properly appreciated, its formation in other vertebrates must be investigated, and at the outset of this investigation it is necessary to obtain a clear idea of the relationship of the embryo to the ovum, and of the origin, extension and differentiation of the mesoderm.


As regards the relationship of the embryo to the ovum, vertebrates are divided into two great groups, those in which the ovum becomes the embryo, as in Amphioxus and amphibians, and those in which only part of the ovum becomes the embryo, the remainder being utilised in the formation of nutritive and protective membranes and appendages, as in reptiles, birds and mammals.


In the simpler forms of vertebrates in which the ovum becomes the embryo, the formation of the pericardial region takes place in the least complicated manner.


IN AMPHIOXUS the unilaminar organism which results from the segmentation of the ovum is converted into a bilaminar in-dividual by invagination, and the invagination cavity becomes the alimentary or enteric cavity of the embryo. The aperture which leads into the enteric cavity is the primitive mouth om’ blastopore; it becomes elongated from before backwards, and? immediately below its margins a series of pocket-like diverticula are projected outwards into the remains of the segmentation cavity. These diverticula are the rudiments of the mesoderm (fig. A), and the cavities contained within them are the rudiments of the coelom or body cavity. The diverticula gradually grow towards the ventral aspect of the embryo, and finally they meet beneath the ventral wall of the alimentary canal, where the adjacent walls of the coelomic rudiments of opposite side fuse together and form, for a time, a mesentery. In the dorsal part of this mesentery a subintestinal blood-vessel is developed, which represents, in the anterior part of its extent at least, the heart of the higher forms. Therefore in Amphioxus the representative of the heart of the higher vertebrates is connected with the ventral wall of the body by a ventral mesentery, which exists for .a considerable time.


IN AMPHIBIA the process of invagination and the formation of the blastopore and enteron are more complicated than in Amphioxus, and the mesoderm is formed simultaneously with the entoderm. Ultimately, however, it is delaminated from the entoderm, and cleft in to outer and inner layers, by the appearance within it of the coelomic space. As in Amphioxus the mesodermal sheets of opposite sides meet together beneath the ventral wall of the alimentary canal, where they form for a time a mesentery (figs. B and C). Beneath that part of the alimentary canal which afterwards becomes the pharynx the heart appears in the dorsal border of the mesentery, and the coelomic spaces in its immediate neighbourhood become converted into the pericardium. In the early stages, therefore, in the tadpole, the two halves of the pericardial section of the coelom are separated from each other by the ventral mesentery which connects the heart with -the ventral wall of the body.


In Reptiles, Birds, and Mammals, the processes of development are much more complicated, for the ovum is no longer utilised for the formation of the embryo alone, but also for the formation of a series of membranes and appendages, which are endowed with protective and nutritive functions, and the two parts, though they are merely segments of one continuous surface, are soon differentiated from each other, so that at a very early period of development it is possible to recognise an embryonic and a non-embryonic section of the ovular surface. The margin of the embryonic section is clearly defined, and from the time of its appearance to the end of life it can always be easily recognised, for it is the margin of the umbilical orifice, which increases until birth, and then is only reduced by a process of cicatrisation, for it is obvious that the diameter of the embryonic area of the ovum is much smaller than the diameter of the umbilical orifice of the adult body. Therefore the orifice is not reduced in size during the early stages of development by the convergence of its margins towards a central point. This being the case, no tucking off of the embryo from the surface of the ovum can occur; on the contrary, what does occur is almost the exact opposite of such a process, for the margin of the area remains as a relatively slow-growing region, whilst the embryonic and the extra-embryonic portions of the wall of the ovum rapidly increase in extent. Under these circumstances, it follows that the margin of the embryonic area will soon appear as a ring between the upper or embryonic and the lower or extra-embryonic parts of the ovum, both of which have expanded beyond it in all directions.


In the consideration of the development of the pericardium, the embryonic region alone requires further study, and it becomes necessary to inquire into its mode of growth, but before this can be appreciated, the positions of certain sections of the embryonic region must be defined.


The embryonic area is at first circular, then ovoid, and finally pear—shaped (fig. D). The narrow end of the pear-shaped area is posterior, and upon it appears a linear thickening, the primitive streak. In front of the primitive streak the neural folds are formed; their posterior ends embrace the anterior part of the primitive streak; and their anterior ends unite together some distance behind the anterior end of the embryonic area. In the meantime the mesoderm has extended from the sides and posterior extremity of the primitive streak, which represents the fused lips of the blastopore of Amphioxus, throughout the embryonic and extra-embryonic areas, except in certain regions which vary in extent in different classes of vertebrates and also in different groups of each class. For purposes of the present study, it is permissible to take an imaginary form in which it may be supposed that the mesoderm has extended over the whole area of the ovum between the two primitive layers, except in the middle line of the embryonic area below the mesial axis of the neural groove, and in a small area, for which I have proposed the term bucco-pharg/ngeal membrane, which is situated in front of the neural groove, and which afterwards becomes the septum between the primitive mouth and the primitive pharynx. It may also be supposed that the mesoderm has become cleft into an outer and an inner layer, except along the margins of the neural groove on each side, where the outer and inner layers are continuous with each other (figs. J, K, L, M, N). Turning again to the embryonic area, it must be noted that the anterior portion of the region which is situated between the bucco-pharyngeal region and the anterior margin of the area is the part which will ultimately lie immediately in the anterior boundary of the umbilical orifice; it is therefore the region in which the pericardium will be developed, and may be termed the pericardial area.


In the early stages the pericardial region occupies the most anterior part of the embryonic area, and in the imaginary specimen under consideration it consists of the four usual layers of the germ—two outer layers, the ectoderm and the somatic mesoderm ; two inner, the entoderm and the splanchnic mesoderm; a11d between the somatic and splanchnic mesoderm there is a portion of the coelom or body cavity (figs. K, L). The main blood-vessels of the embryo have in the meantime commenced their development, and they form two longitudinal trunks, one on each side, which pass backwards in the splanchnic mesoderm from the vascular area on the wall of the yolk sac in front of the embryonic area to the vascular area behind the embryonic region; and the vascular circle on each side is completed by the capillary vessels of the vascular area, through which the blood is returned from the posterior to the anterior end of the germ. At this period, therefore, in this imaginary embryo, the main blood-vessel on each side would pass backwards in the splanchnic mesoderm which forms the lower or ventral boundary of the pericardial section of the coelom (figs. J, K, L).


The first change of importance which takes place in the further development of the pericardial section of the embryo is its reversal, or the alteration of the positions of its extremities and surfaces, the primitive anterior boundary becoming the posterior, and the original ventral surface becoming the dorsal surface (figs X, Y, Z, Z1). As this change of position is due entirely to the manner in which the embryonic area grows, it becomes necessary to consider that growth more carefully; and when embryonic areas of gradually increasing size are carefully examined, it is found that the rate of growth varies considerably in different parts of the area, and that the antero-posterior increase is more rapid than the transverse. It is in the region of the anterior end of the primitive streak that the most rapid antero-posterior growth occurs, and this section of the embryonic area constitutes a kind of nodal point. It follows, therefore, that as the margins of the embryonic area are comparatively stationary, whilst the region within the margins is rapidly growing, that the area must fold, and folding, it might project inwards into the interior of the ovum, forming a hollow invaginated tube; or upwards from the ovular surface; or forwards, backwards, and laterally over its own margins.


The folding inwards or invagination does actually occur in the very early stages of development of rats, mice, squirrels, guinea-pigs, and apparently, to a slight extent, in the human ovum, producing what is known as the inversion of the layers. The upward folding does not occur, being prevented by the Walls of the cavity in which the ovum lies, but the peripheral folding occurs in all animals as the embryo is gradually moulded into form, a11d the preliminary inversion, if it has occurred, disappears. Thus it becomes evident that there is no tucking inwards of the margin of the embryonic area; on the contrary, the head, tail, and lateral folds, which appear as the embryo gradually assumes its form, are due to the relatively stationary condition of the margin of the area and the rapid increase of its surface extent, the two factors combined necessarily giving rise to a folding of the area. Further, as the embryonic area increases in size it also increases in weight, and the increased weight causes it to sink into the interior of the ovum, thus producing around it a second series of folds of the extra-embryonic portion of the ovular surface which constitute the amnion folds. As the amnion folds have no bearing upon the development of the pericardium they need no further consideration, but the folds of the embryonic area have a direct bearing upon the subject of this study, and they require, therefore, more detailed investigation.


It has already been stated that the antero-posterior increase of the embryonic region is more rapid than the lateral increase, and this is soon rendered evident by the preponderance of the head fold over the lateral body folds (figs. X, Y). During the development of the head fold the anterior margin of the embryonic area remains relatively stationary, and the anterior end of the neural groove passes rapidly forwards till it projects well above and beyond the anterior end of the area. As this projection occurs, the posterior ends of the bucco-pharyngeal and pericardial regions are pushed forwards, and their surfaces are necessarily reversed. When the alteration of position is completed, what was originally the ventral surface of the pericardial region has become the dorsal surface, and it forms the ventral wall of the foregut, which is the portion of the blastodermic cavity carried forward into the embryo during the formation of the head fold (fig. X). The foregut, therefore, lies entirely within the head fold, and consequently the formation of its ventral wall cannot in any way be due to the convergence and fusion of lateral folds. In the imaginary embryo at present under consideration it is obvious that, after the formation of the head fold, the two main blood-vessels which passed backwards into the embryo from the anterior part of the vascular area must now ascend in the anterior boundary of the umbilical orifice and turn forwards in the splanchnic mesoderm of the dorsal boundary of the pericardial section of the coelom to the lower end of the bucco-pharyngeal region, and that they will ascend from the pericardial region in the rudimentary mandibular arches which are developing along the lateral margins of the bucco-pharyngeal area to the under surface of the head (fig. X), where they will turn backwards, towards the caudal region of the body of the embryo, under the paraxial mesoderm at the sides of the neural tube, which has been formed in the meantime by the incurvation and fusion of the margins of the neural groove (fig. Z). Subsequently, the two primitive vessels fuse together in the splanchnic mesoderm of the dorsal wall of the pericardial portion of the coelom and project downwards into the cavity, being suspended from the dorsal wall by a mesocardium posterius, but it is perfectly obvious that under the conditions defined no ventral or anterior mesocardium can be produced, and that the pericardium is not formed by the convergence of coelomic spaces which lie in the lateral folds and their fusion in the ventral middle line, but that it is produced by the inclusion of the anterior portion of the coelomic space in the body as the head fold is projected forwards in association with the rapid antero~posterior increase of the embryonic area.


So far, however, only a11 imaginary ovum has been considered, in which the primitive blood-vessels occupy what may be considered to be their original positions and form two vascular circles, one immediately to either side of the middle line, both in the embryonic and the extra-embryonic portions of the ovum. This simple condition no longer exists, if indeed it was present at any time, either in reptiles, birds, or mammals, and the modifications of it which appear in all three groups are associated with differences in the rate of extension of the mesoderm between the two primitive layers, and to differences in its rate of cleavage into somatic and splanchnic portions. These differences are, in their turn, associated with modifications of pericardial development which are of considerable interest, and which have been very imperfectly described.


Reptiles and birds do not differ essentially from each other so far as the peculiarities of mesoderm extension are associated with the early stages of the formation of the pericardium, and birds, therefore, may be used to illustrate the phenomena which are of importance, inasmuch as they are more easily obtainable, and their development is better known than that of reptiles.

Birds

In birds, as in mammals, the mesoderm extends from the sides and posterior end of the primitive streak, and not only does it leave the notochordal and bucco-pharyngeal port-ions of the embryonic area uninvaded, but also, for a considerable time, a large area which extends forwards and outwards from the bucco-pharyngeal region well into the extra-embryonic part of the ovum in front of the embryonic area (fig. D). The embryonic portion of this mesoderm-free region is that in which the pericardium will afterwards be developed, and the extraembryonic portion subsequently becomes bent over the head of the embryo as the anterior amnion fold, which is known as the proamnion because it consists only of ectoderm and entoderm, whilst a true amnion fold is formed by a fold of the somatopleure, that is by a layer of ectoderm lined internally by a layer of somatic mesoderm. Clearly, therefore, as the primitive blood-vessels are developed in the splanchnic mesoderm, they can only pass from the anterior part of the vascular area round the margins of the proamnion and along the sides of the bucco-pharyngeal area into the embryo, and in the early stages no blood-vessels are present in the pericardial region of the ovum. At a later period the mesoderm extends both into the pericardial region and into the proamnion, a11d cleaves as it extends, but before this occurs the head fold is formed by the rapid antero-posterior increase of the embryonic area, and thus for a time, in birds, the whole of the ventral wall of the foregut consists of ectoderm and entoderm alone (fig. E). Subsequently, the two layers of the mesoderm, inclosing between them the coelomic space, sweep into it from the sides, carrying with them the rudiments of the heart, which thus eventually lies in the ventral wall of the foregut, and is connected dorsally with the wall of the gut by a dorsal mesocardium, and ventrally with the Ventral wall of the body by a ventral mesocardium (figs. G, H). In birds, therefore, as in amphibians, a ventral mesocardium is present; nevertheless, only the mesodermal portion of the ventral wall of the foregut is formed by the ingrowth and fusion of lateral folds. The ectoderm and entoderm belong entirely to the primitive head fold, and they are at no time cleft or separated into lateral halves in the ventral middle line. The mesoderm alone, on account of its peculiar mode of extension between the primitive layers, is cleft mesially, and it does not enter the region of the ovum which becomes the ventral wall of the foregut until the head fold has been formed.

Mammals

In mammals, either on account of the smaller size of the ovum or on account of the more rapid extension of the mesoderm, the latter la.yer has extended through the pericardial section of the embryonic area, and is cleft into somatic and splanchnic layers before the head fold is formed, and in the earlier stages the pericardial mesoderm of these animals forms a crescentic mass, bounded in front by the anterior margin of the embryonic area and the posterior margin of the proamnion, if the latter is present, and behind by the bucco-pharyngeal membrane, whilst postero-laterally it is continuous with the general mass of the mesoderm, both of the body of the embryo and of the extraembryonic area, at the level of the posterior part of the bucco— pharyngeal membrane (figs. O, P). In man, where apparently no proamnion is formed, and iii some rodents where the proamnion is a very Small and transitory structure, the mesoderm of the pericardial region is in close association, at the anterior border of the embryonic area, with the more anteriorly Situated extraembryonic mesoderm, but the two portions are always definitely separated by a cleft, and in these mammals, as in those which are provided with a comparatively large proamnion, the only connection of the pericardial with the non-pericardial portion of the mesoderm is situated posteriorly, on each side, at the level of the posterior part of the bucco-pharyngeal membrane. Why this separation of the anterior border of the pericardial mesoderm from the adjacent extra-embryonic mesoderm persists at the anterior border of the embryonic area in those animals in which no proamnion is present is not clear, and it can only be looked upon as an indication of the descent of the animals in question from proamniotic ancestors.


In mammals provided with a proamnion it is Obvious that, as in birds, the blood-vessels passing from the anterior part of the vascular area to the embryo must run along the margins of the proamniotic membrane, and then turn forwards in the splanchnic mesoderm of the pericardial tube towards the anterior end of the embryonic region, where they turn backwards along the sides of the bucco-pharyngeal membrane, and pass to the posterior end of the embryo beneath the paraxial mesoderm of the head and body (figs. O, B). At the posterior end of the embryonic area they enter the posterior part of the vascular area and join the capillaries, by means of which the vascular circle on each side of the ovum is completed, and they run a corresponding course in mammals in which the proamnion is rudimentary.


As the head fold develops, the posterior end of the pericardial region is swung forward, the surfaces of the region are reversed, a11d it is carried into the ventral wall of the developing foregut, consequently the two primitive blood-vessels of the embryo, which previously ran backwards from the anterior border of the pericardial region to the sides of the bucco— pharyngeal membrane, now run forwards from the posterior part of the reversed pericardial region to the lower border of the reversed bucco-pharyngeal membrane, and they lie close together in the dorsal wall of the pericardial cavity, attached to the ventral wall of the foregut by a dorsal mesentery, but their ventral surfaces are free, and quite devoid of a ventral mesocardium (figs. S, T).


At this period the fused somatic and splanchnic mesoderm of the anterior margin of the umbilicus, the original anterior border of the pericardial region, is undergoing rapid proliferation, and it forms the mass of mesodermal tissue, the septum transversum, from which the posterior wall of the pericardium and the central part of the diaphragm are afterwards differentiated. At the lateral margins of this mesoderm lie the lateral cornua of the pericardial tube, which form two small canals situated at the sides of the foregut in the anterior boundary of the umbilicus (fig. V), and in the inner and lower boundaries of these tubes, in the septum transversum, lie what were the anterior extremities of the primitive blood-vessels of the embryo, which have now become the vitelline veins. They ascend from the yolk sac into the lateral part of the anterior boundary of the umbilicus, and then pass forwards through the septum transversum at the sides of the posterior part of the foregut to the posterior part of the heart, for by this time those parts of the primitive blood-vessels which lie in the dorsal wall of the pericardium are fusing to form the single tubular heart (fioz W). It is the relation of the lateral cornua of the pericardial tubes to the wall of the foregut at this period which have given rise to the erroneous descriptions of the completion of the ventral wall of the foregut by the fusion of the ventral ends of the lateral folds of the body wall, and of the formation of the pericardium by the union in the ventral middle line of the portions of the coelom which lie in each lateral fold, such fusion necessarily producing, as in birds, a ventral mesocardium, which disappears when the two halves of the peri» cardial cavity become continuous. The figures which are supposed to substantiate this interpretation of the formation of the pericardium are simply figures of sections which pass through the anterior boundary of the umbilical orifice, and therefore through the primitive septum transversum and the lateral cornua of the pericardial space, which may be called the pleuro-pericardial canals (fig. V), as they afterwards communicate for a time with the pleural sacs which are developed at the sides of the foregut and dorsal to the pericardium, coincidently with the development of the neck and the forward migration of the visceral arches; but as these processes are associated with the later stages of the development of the pericardium, they cannot be considered in the present communication. Finally, it must be noted that the pericardial region of the embryonic area is limited anteriorly, in the early stages, by the anterior border of the embryonic area, which becomes the anterior boundary of the umbilicus, and that, as the pericardial region is reversed during the formation of the head fold, the pericardium expands so rapidly that for a time it projects backwards beyond the ectoderm at the anterior margin of the umbilicus, and lies between the foregut and the yolk sac, as in the ferret (figs. AA, BB). As development proceeds, it gradually assumes a more anterior position, and eventually lies between the foregut and the ventral wall of the body, in front of the umbilical orifice. It assumes its permanent position coincidently with the formation of the neck and the ascent of the auricles to the dorsal wall of the aortic bulb, but even under these circumstances there is no ventral mesocardium.

Summary

  1. In AMPHIBIANS the pericardium is formed by the fusion of the anterior parts of the lateral halves of the coelom in the ventral middle line beneath the anterior part of the foregut, and a ventral mesocardium is present for a time.
  2. In BIRDS the pericardium is formed after the development of the head fold by the ingrowth of the lateral parts of the coelom into the ventral wall of the foregut and their fusion in the middle line. The rudiments of the heart lie along the dorsal part of the line of fusion, and for a time a ventral mesocardium is present.
  3. In MAMMALS the pericardial mesoderm is present in the pericardial portion of the embryonic area, and it is completely separated into somatic and splanchnic layers before the head fold appears; there is therefore a single pericardial cavity which extends from side to side along the anterior boundary of the embryonic area.


As the head fold forms, the pericardial region is reversed, and it is carried into the ventral wall of the foregut, Where it forms a U-shaped tube, which communicates at each end with the general coelom.


The rudiments of the heart are formed in the splanchnic layer of the pericardial mesoderm; therefore, after the reversal of the area, they lie in the dorsal wall of the pericardial space, attached bya dorsal mesentery to the ventral wall of the foregut, but they are never, at any time, connected with the ventral wall of the pericardium by a Ventral mesocardium.


Figures

(A)——diagram of the blastodermic layers in Amphioxus, showing the relation of the mesoderm to the primitive subintestinal bloodvessel. '

(B)—transverse section of a tadpole, showing the formation of the

two halves of the pericardial portion of the coelom and the ‘ventral cardiac mesentery.

(C)—transverse section of an older tadpole, showing that the 16 PROFESSOR ARTHUR ROBINSON.

ventral cardiac mesentery disappears, and that the two halves of the pericardial portion of the coelom fuse together.

(D)—diagram of a portion of theupper part of the ovum of a bird, showing the areas which are devoid of mesoderm and the positions in which the primitive blood-vessels will develop.

(E) — diagram of a lateral longitudinal section of I the ovum of a bird, immediately after the formation of the head fold, and before the mesoderm has entered. the pericardial area.

(F) — diagram of a transverse section through the embryonic portion of the ovum of a bird, along the line a in fig. E.

(G) — diagram of a lateral longitudinal section of the embryonic portion of the ovum of a bird, after the pericardial portion of the coelom has extended into. the embryonic area.

(H) — diagram of a transverse section of the embryonic portion of a bird’s ovum, along the line a in fig. G.

(J) — diagram of an imaginary ovum, in which the mesoderm has extended through all parts except the floor of the neural groove and the bucco-pharyngeal region.

(K)—diagram of a lateral longitudinal section through the imaginary ovum represented in fig. J.

(L), (M), and (N )—diagrams of transverse sections of an imaginary ovum, along the lines a, b, and c, fig. J.

(O) — diagram of a portion of the upper part of a mammalian ovum, showing the mesoderm-free areas and the courses of the primitive blood-vessels.

(P) — diagram of a lateral longitudinal section through the ovum shown in fig O.

(Q) —diagram of a transverse section along the line a in fig. O.

(R) — diagram of a side view of a mammalian ovum, showing the positions of the various areas and the course of the primitive blood vessel on the left side.

(S) — diagram of a side view of an older mammalian ovum, showing the alterations in the positions of the pericardial and bucco-pharyngeal regions, and the corresponding portions of the primitive vessels. The formation of‘ the amnion folds on the lower or extra-embryonic part of the ovum has been omitted.

(T)—diagram of a longitudinal section of a mammalian ovum after the formation of the head and tail folds of the embryo and the formation of the amnion folds.

(U), (V), and (W)—diagrams of transverse sections of a mammalian ovum along the lines c, b, and a respectively in fig. T.

(X)—diagram of a lateral longitudinal section of an imaginary ovum, similar to that shown in fig. K, but after the formation of the head and tail folds, showing a continuity which never actually exists between the pericardial mesoderm and the pericardial coelom, and the extra-embryonic coelom immediately in front.

(Y,), (Z), and (Z)1—diagrams of transverse sections through the imaginary ovum shown in fig. X, along the lines 0, b, and a respec tively in fig. X.

(AA)—-diagram of a lateral longitudinal section of a ferret’s ovum, showing the position of the pericardium after the formation of the head fold; it also shows that for a time it is bounded below by the yolk sac.

(BB)--diagram of a transverse section of a ferret’s ovum along the line a in fig. AA.

EXPLANATION OF FIGURES.

A, aorta. N, AF, amnion fold. N G, AN, anal membrane. NT, BC, blastodermic cavity. MG, BP, bucco-pharyngeal membrane. P,

C, coelom. PA, CM, cardiac mesentery. PB,

E, ectoderm. PPC, EA, embryonic area. PPS, EC, enteric cavity. PR, EE, embryo. PS, EN, entoderm. SC,

FCAA, first cephalic aortic arch. SOM, FG, foregut. : SPM,

H, heart. ST,

HC, hindgut. YS,

notochord.

neural groove.

neural tube.

midgut.

pericardium. proamniotic area. primitive blood-vessel.

pleuro-pericardial canal.

pleuro-peritoneal space. pericardial region. primitive streak. segmentation cavity. somatic mesoderm. splanchnic mesoderm. stomatodeum.

yolk sac.

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Cite this page: Hill, M.A. (2019, August 25) Embryology Paper - The early stages of the development of the pericardium. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_early_stages_of_the_development_of_the_pericardium

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