Paper - The earliest stages of development of the blood-vessels and of the heart in ferret embryos
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Wang CC. The earliest stages of development of the blood-vessels and of the heart in ferret embryos. (1917) J Anat. 52(1): 107-36. PMID 17103829
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The Earliest Stages of Development of the Blood-vessels and of the Heart in Ferret Embryos
By Chung-Ching Wang, M.D., Ch.B. (Edin.),
Acting Lecturer and Demonstrator in Anatomy, University College, London; tate Carnegie Research Fellow in Embryology, and Assistant in the Depariment of Anatomy, Edinburgh University.
Before the details of the reconstructions which were made are given and the conclusions to which they tend are discussed, it seems advisable to state briefly the main results of the observations which have previously been made by other investigators regarding the formation of the intraembryonic vessels and blood-cells, and the earliest stages of development of the heart in mammals generally. It is to be noted that the phenomena observed are, of necessity, intimately associated with the early stages of development of the pleuro-pericardial cavity, and are closely connected with the mode of formation of the pre-umbilical portion of the body of the embryo.
To facilitate references to be made hereafter in this communication, the ferret embryos are classified into stages. Thus in Stage I. the description deals principally with the blood-cells and vascular endothelium; in Stage II. the account relates chiefly to the first appearance of the heart rudiment as a single transverse vascular channel situated caudo-ventral to the pleuro-pericardial cavity; in Stage III. it is shown how the single heart tube is converted into two lateral heart tubes; in Stage IV. the statement refers mainly to the conditions of the two endothelial tubes and their relationships to the muscular wall of the heart; and in Stage V. the site of fusion of the two heart rudiments to form an unpaired heart tube is indicated.
As far as the technique and the histological conditions of the ferret: embryos, to be immediately described, are concerned, the description given in Stage IV. may be applied equally well to all the other specimens selected for the purpose of this investigation. The only differences to be found in this respect are chiefly in the matter of staining and in a few other minor points, such as, whether the embryo was detached or not from the uterus before being sectioned.
- 1 An abstract from a thesis for the degree of M.D., for which a gold medal was awarded by the University of Edinburgh.
Blood-Vessels and Blood-Cells
It may be mentioned at once that the question of the origin of the vascular rudiment is one of the most obscure in the realms of comparative embryology. Even the lowest vertebrates, which present the greatest simplicity in their structures and whose development is most easily understood, have failed to throw satisfactory light on this question.
There is a great diversity of opinion regarding the germ layer from which the vascular endothelium and blood-cells arise. In the literature it may be found that certain competent investigators have in each vertebrate class claimed the vascular endothelium and blood-cells to be derived from entoderm, while other workers of equal authority have found the vessels and blood corpuscles to arise from the mesoderm. It is to be noted, however, that in no case has an author stated that the blood-cells and vascular endothelium are derived from different germ layers.
Ziegler (’87) expresses the view that “the system of blood-vessels and that of the lymphatic vessels are produced in their first fundaments from remnants of the primary body cavity (blastoccel), which at the general distribution of the formative tissue (mesoderm) remain behind as vessels, lacunee, or interstices, and are closed by that tissue and incorporated in it.” This author therefore agrees with Biitschli (’82) that in all metazoa the blood vascular system has its origin from the blastocel. On the other hand, Felix (’97) inclines to the belief that the circulatory system is, from a developmental point of view, closely related with the ccelom.
In reptiles, Strahl (’83), in birds, Kélliker (’84), in Selachians, Ziegler (’92), and in mammals, Kélliker (’84), all claim that the first vessel rudiments are found in the mesoderm and not between the mesoderm and entoderm. The current view which is held by a great number of investigators is that in embryos of the higher vertebrates, the first vascular rudiments which can be identified as the forerunners of the blood-vessels and blood-cells, appear, at first, in the form of localised cell cords lyjng upon the yolk-sac between the mesodermic tissue and the entoderm. His (’00) gives the name of “ angioblast ” to these cell cords.
Upon the question as to whether the angioblast is to be looked upon as a derivative of the mesoderm, or as an offshoot from the entoderm, opinions again diverge. In support of the mesodermic origin, the names of Maximow (’09), Weidenreich (’10), Evans (12), Riickert and Mollier (’06) are of prominence, while the advocates of the entodermic theory are to be found in the persons of Kélliker (’82), Robinson (’92), Keibel (’88), and Van der Stricht (99). Miss Parker (’15), in her recent investigation into the development of the heart in Marsupials, admits the possibility of the entodermal origin of the endothelium. In Fundulus, Stockard (715) finds that vascular endothelium does arise in situ in many parts of the. embryonic body in which blood-cell rudiments are not present, and that independent blood islands are found on the yolk-sae.
The persistent claims that vascular endothelium has the power to change into various types of blood corpuscles have been disproved by the recent experimental work of Stockard (15). After considerable study and -eareful observations, Stockard observed nothing that would indicate that the vascular endothelial cells possess the power to change into the bloodcell type, nor could he find dny evidence to indicate that cells having once assumed even the earliest blood-cell type are capable of metamorphosis to form endothelial cells. He firmly believes that, in Fundulus at least, endothelium is incapable of giving rise to any type of blood-cell.
Whatever its origin, the angioblast, according to current views, is, in the majority of amniota, chick for example, found lying between the mesoderm and entoderm in the form of cell cords in the area vasculosa immediately surrounding the embryonic shield. It is believed that the peripheral part of each angioblastic strand soon resolves itself into an uninterrupted network of endothelium, and the central part into clusters of blood-cells. The endothelial cells which enclose the blood-cells continue to divide and produce vascular sprouts which appear, at first, as solid cords but later become hollow (Hertwig (’92), Minot (’12)).
Around the periphery of the area vasculosa, in the majority of animals, the vitelline plexus resolves itself into a broad circular vessel—the sinus terminalis, which is continuous round the margin of the area except at the: cranial end where it terminates on each side in a vessel which enters the embryo. The vascularisation of the splanchnic layer of the mesoderm gradually extends through the extra-embryonic region of the zygote until it covers the whole extra-embryonic region, where it forms an intermediate layer between the entoderm and mesoderm.
Some of the larger channels of the area vasculosa, even in the early stage of its formation, converge to form a single vessel on each side, which enters the embryonic body through the splanchnopleure and ultimately joins the venous end of the heart rudiments when they are developed. These are the two vessels previously mentioned as connected with the cranial end of the sinus terminalis, and are known as the omphalo-mesenteric (vitelline) veins. It is believed that other channels of the area vasculosa on each side grow towards the median plane of the embryo, and as they approach the region of the notochord their extremities fuse to form a longitudinal vessel which is the dorsal aorta of that side. The dorsal aorta ultimately unites: with the arterial end of the heart. In this way the vitelline circulation of the embryo is completed and plays an important role in all vertebrates in supplying the growing embryo with nutritive materials from the yolk-sac, which is comparatively large.
In reptiles and birds, a second circulation, as it were, develops in connection with the allantois, and persists during incubation; in mammals the allantois is incorporated with the placenta which establishes the communication between the embryo and the mother, and the vessels which correspond to the allantoic vessels in reptiles and birds become associated with the placental circulation (vide infra).
Consideration has been. confined so far mainly to the part played by vessels which in origin are extra-embryonic. The next question to be considered is whether the early blood-vessels in the body of the embryo itself are formed by an ingrowth of the vitelline plexus which is, by origin, composed of extra-embryonic blood-vessels, or whether, on the other hand, the intra-embryonic vascular system, or at least a part of it, arises in situ from the germ layers of the body. The problem is difficult, and it is not surprising that various conflicting views have been formulated regarding the precise mode of origin of the intra-embryonic blood-vessels. Thus, the names of. Hertwig (’92) and His (00) have been identified with the idea that the early blood-vessels in the body of the embryo itself are formed by a budding or ingrowth of the endothelial wall of the vessels from the extra-embryonic vascular area, and the name of Sobotta (’02) is associated with the belief that there is an outgrowth of the vessels from the body of the embryo to the wall of the yolk-sac. Riickert and Mollier (06), on the other hand, maintain that the embryonic vascular stems, or at least a part of them, arise in situ from the mesoderm of the embryo. Other investigators, basing their opinion on the results of a series of experiments. on growing chick embryos, adhere to the conviction that, even after the destruction of the yolk-sac vessels of one side, the heart, the aorta, and the other vessels are found to develop on both sides in the embryo.
On the other hand, Vialleton ('92), His (00), and Evans (09), who have investigated the intra-embryonic blood-vessels in birds, and to whom we are indebted for a comprehensive knowledge. of the formation of the caudal portion of the dorsal aorta, come to the conclusion that the greater part of the dorsal aorta in the bird is formed from the medial margin of the vitelline plexus which has grown into the embryo in the manner already referred to. Tiirstig (’84) also has noticed the frequent early connection of the primitive dorsal aorta with the vitelline plexus in mammals.
With regard to the development of the cranial portion of the aorta, on the other hand, various opposing views are held; thus, His (’00) attributes it to the result of a further growth of the same extra-embryonic vitelline piexus which forms the caudal part of the aorta, but which is reduced to a capillary chain growing headwards, eventually turning ventrally over the blind end of the fore-gut and fusing with the cranial portion of the heart tube. Lewis (’04) and Bremer (’12) both arrive at a somewhat similar conclusion, namely, that in the rabbit embryo, the dorsal aorta, the aortic arch, the conus arteriosus, and the lateral heart are all parts of an original network of angioblastic cords derived from the extra-embryonic plexus of blood-vessels. On the other hand, Mollier (06) believes that the notion of His (75) and Vialleton (’92) is not nearly so probable as that the individual intra-embryonic vessel cells arise in loco and thus form the vascular nets.
Recently the local origin of vascular endothelium received additional support from the experimental results recorded by Miller and M‘Whorter (14) on the origin of blood-vessels in the chick embryo. Such a view is further strengthened by the still more recent experimental evidence pre ‘sented by Reagan (’15), which shows the origin 7 loco of vessels in isolated parts of chick embryos, and by Stockard (’15), which claims that in Teleost embryos there can be no doubt that the heart endothelium and aorte arise im situ within the embryo.
It is to be noted that all these experiments just quoted confirm the earlier results of Hahn (’09) on the origin of vessels in the chick, and that the formation of intra-embryonic blood-vessels is much more extensive and important than has formerly been supposed.
Though the development of blood-cells lies without the scope of the present communication, a few remarks may be made regarding the close relationship of these cells to the vascular endothelium. Nearly all investigators on this subject assert that blood-cells and vascular endothelium arise from either the mesoderm or entoderm. In reviewing the literature on this point no definite statement has been found which might suggest that blood-cells and endothelium develop from different germ layers. . The possibility of these structures having an independent and separate origin cannot, however, be overlooked, as will be seen later.
Maximow (09) believes that the endothelial cells and blood-cells are closely related, and arise from a common stem cell in the blood island, and may continue to arise from such a cell during later development. Stockard (15), on the other hand, states “that vascular endothelium forms in perfectly normal fashion within the heart and head regions of embryos without circulating blood, but in no case in early or late stages was the endothelial lining of the aorta or other vessels capable of giving rise to any type of corpuscles. Yet the power to form blood corpuscles was abundantly present in the same embryos as shown by the huge numbers of blood-cells within the blood-forming regions—the intermediate cell mass and yolk islands.”
Maximow (’09) states further that the intra-vascular primitive bloodcells are not only increased by mitosis but are added to also by the proliferation of the same kind of cells from the fixed endothelial cell of the primitive vessels. The assumption is based on the fact that clusters of blood-cells are often seen adherent to the endothelial wall of the bloodvessels. Maximow thinks that these clusters of cells may arise by the proliferation of the endothelium. Minot (12), however, disagrees with Maximow, because he finds that there is no continuity of the protoplasm of the cells either in the rabbit or in man; also, because mitosis of the endothelium in the neighbourhood of the clusters is almost invariably wanting; and, finally, because the endothelial nuclei are differentiated, while the nuclei of the cells of the clusters are not differentiated. Minot regards the cells composing the clusters as solely primary wandering cells, Stockard (15) concludes that endothelial lining is utterly incapable of giving rise to any form of blood-cell.
As in mammals, before referred to, so in man, in connection with the _vascularisation of the yolk-sac, or, according to Fetzer (’10), even at a period before any vascular rudiment on the yolk-sac proper can be distinguished, there develop, in the belly-stalk and chorion of the embryo, highly characteristic strands of spindle cells, which repeatedly exhibit the nature of having the appearance of a double row of nuclei and of possessing a distinct lumen. This has been observed by Graf Spee. ('96) in the embryo von Herff of 37 mm. The strands of spindle cells have been claimed to form endothelial cells eventually.
In young human embryos, without any vessels or blood islands on the yolk-sac, Jung (07) and Herzog (’09) have called attention to the aggregation of cells, sometimes arranged round a lumen, situated at the periphery of the mesoderm of the yolk-sac and belly-stalk in the neighbourhood of the extra-embryonic area. In slightly older specimens with. recognisable yolk-sac vessels, irregular spaces in the mesoderm, some lined with endothelium, some without any definite lining, have been observed by many authors, and recently Grosser (13) and Debeyre (’12) have independently described, beside the irregular s spaces, true blood islands i in the belly-stalk near the allantois. In human embryo 1:17 mm. described by Frassi (’08) there is an abundance of well-formed vascular rudiments on the ventral surface of the yolk-sac, and Frassi states that with little or no difficulty vessels can be detected also in the belly-stalk and chorion.
The next phase of development of the human vascular system is illustrated by the well-known embryo Glaevecke 1:54 mm. of Graf Spee (’89, ’96). Here again, as in the preceding stage, vascular rudiments are seen on the yolk-sac and in the chorion, but, in addition to these, it is possible to note the first intra-embryonic vascular rudiments.
In embryos of 5 somites and upwards it is observed that the vitelline plexus has established its communications cranially with the heart by means of two channels, the vitelline veins, reaching as far as the first intersegmental cleft, as Dandy (10) first showed, while caudally, in the region of the unsegmental mesoderm, the branches of the vitelline arteries form a plexus of capillary-like vessels from which, as shown by Felix (’10) and confirmed by Evans (12), the umbilical artery takes its origin.
The Heart and Pericardium
In mammals much has been done to throw light upon the develop- ment of the heart, notably by His (’81, ’85, ’86) and Bischoff (’42, ’52), in rabbits, Born (’89), in dogs, Bonnet (’91, ’01, 07), in pigs, Keibel (’88), and in ferrets, Yeates (’11), but many gaps must be filled up before it is possible to obtain a clear conception of the details of its formation.
Tandler (12) says “the earliest developmental processes of the heart, especially in so far as they concern the formation of the endothelium of the heart and vessels, are unknown in the human embryos, but probably one will not be far astray in assuming that the earliest rudiment of the human heart is essentially similar to that of the mammalia.” The earliest stages of development of the mammalian heart are undoubtedly intimately associated with the development of the blood-vessels, but concerning the latter various opposing views have been formulated and already been dwelt upon in the beginning of this communication.
It is clear that the precise mode of development of the blood-vessels is not yet definitely established, and a short survey of the early stages of the development of the heart will show that our knowledge of that subject also is deficient. In mammals, according to Mollier (06), the first rudiment of the heart is the appearance of a number of cells, which are discernible in embryos of 2-3 primitive somites. The vascular cells appear between the entoderm and mesoderm on both sides not far from the median plane of the embryo, at first in the distal portion of the head. They are responsible for the formation of the endothelium of the heart tubes only, the remaining constituents of the wall of the heart— that is, the myocardium and the epicardium—being derived from that part of the visceral coelomic. wall which has been designated by Mollier ('06) the heart-plate or cardiogenic plate.
It is generally held that the first aggregation of the vascular cells of the mammalian heart is paired and is situated ventral to the ccelomic cavity. By a process not yet satisfactorily explained, spaces soon make their appearance in the vascular cell mass, and when these spaces coalesce, two endothelial tubes are thus formed, one on either side of the median plane of the embryo. A fusion of the two endothelial tubes next takes place, and the unpaired heart tube is formed from the paired heart rudiments, but exactly how this fusion is brought about, opinions differ.
It was for long believed (Balfour (’81), Hensen (’76), Hertwig (’92), Kolliker (61), and it is still held by some (Tandler (12), Bryce (’08), Bailey (12), Wilson (14), H. von W. Schulte (’15), and others) that in mammals, as in birds, the two endothelial tubes, out of which the heart is formed, appear at a time when the lateral folds, which are said to form the ventral wall of the throat, are only just visible, that, as the lateral folds of the splanchnic walls increase, the two halves of the heart, enclosed within the hitherto symmetrical and laterally placed pleuropericardial cavities, become carried medially and ventrally until they fuse on the ventral side of the fore-gut, and that the heart is therefore provided, at least for a time, with a ventral and a dorsal mesocardium.
In the chick, a ventral mesocardium is recognisable, but this is due, as Robinson (02) points out, to the relatively late penetration of the mesoderm in the cranial region. In amphibians lateral folds have been described, but it is erroneous to presume that such folds, which, by virtue of their fusion ventrally, form the ventral wall of the fore-gut, really occur in mammals. In the latter, the pericardial mesoderm appears in the pericardial portion of the embryonic area, and it is there completely differentiated into somatic and splanchnic layers before the head bend is developed; there is therefore a single pericardial cavity to begin with, which extends from side to side along the cranial boundary of the embryonic area. As the head bend develops, the single pericardial cavity is reversed, and it is carried into the ventral wall of the fore-gut, where it forms a U-shaped tube which communicates at each end with the general ccelom. The heart rudiments 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 cavity attached only by a dorsal mesocardium to the ventral wall of the fore-gut, but they are never, at any time, connected with the ventral wall of the pericardium by a ventral mesocardium.
Rouviére (04), on the other hand, while he agrees with Robinson as to the’ absence of the ventral mesocardium in mammals, gives a different account of the process which leads to the closure of the fore-gut. He describes the formation of the lateral pleuro-pericardial canals, which grow cranially round the cranial end of the brain-plate and fuse to form a continuous channel. The splanchnopleure forming the caudal wall of the pleuro-pericardial cavity now forms a continuous fold, which Rouviére calls the cardiac fold and which he describes as growing actively backwards as a whole. Grdaper (’12), in a description of the growing processes in the developing chick, asserts that there is considerable evidence in support of the view that the margin of the fore-gut (umbilical orifice) moves caudally concurrently with the growth of the head fold cranially.
Miss Parker (’15), in her studies of the early stages in the development of Marsupials, summarises her statement by saying “that while the initiation of head-fold formation is in all probability due to the forward growth of the brain-plate, there occurs also an active backward growth of the anterior intestinal portal (umbilical orifice).”
“Shore (’89) describes that the head fold of the chick embryo results from a growth of the head forwards over the diblastic part of the blastoderm, and that a “folding off” does not occur, at any rate at first.
Recently Watt (15), in his investigation into two young twin human embryos with 17-19 paired somites, states: “From the atrial canal the ventricle continues on at the left and runs far forward in the pericardial cavity, when it is strongly flexed ventrally and turns caudad as it reaches the median line. It is here attached to the pericardial wall by a short stretch of ventral mesocardium, the only portion of this structure which is still present.” Though a small piece of dorsal mesocardium is depicted in his paper (plate 3, fig. 4, and plate 4, fig. 2), there is nothing to indicate the existence of the ventral mesocardium which Watt describes.
The material for this stage consists of three embryos. Selected sections of each of these specimens have been photographed to show the conditions and relationships of the blood-cells and vascular endothelium.
(a) Ferret Embryo, 1:15 mm. (F.C.Q.Z.(2)). General Description.
The germinal area of this embryo measures 1°4 mm. No mesodermic somites can be detected, and there is no indication of a heart rudiment: The head fold has not yet begun to develop, and no intra-embryonic blood-vessels can be found in the specimen.
There is, however, a shallow neural groove which terminates at the primitive caudal and subsequently dorsal end of the bucco-pharyngeal membrane. The groove broadens out caudally. Beneath the caudal end of the neural plate it becomes the chordal canal. The chordal canal terminates caudally in a mass of cells which fuse with the ectoderm at the cranial end of the primitive streak.
Fig. 1. — Showing blood-cells. x 500.
Though no intra-embryonic blood-vessels can yet be found in this specimen, extra-embryonically there aré solid clusters of blood-cells which are not surrounded by any endothelium (fig. 1). In none of the clusters of blood-cells is it possible to detect that the peripheral layer of the cells resolves into endothelium. The cells forming the clusters are spheroidal in shape, lying between the mesoderm and the entoderm. They are provided with large well-stained nuclei, and are very often adherent to the entodermal cells, which exhibit characters similar to those of the bloodcells. No lumen can be found in any of the blood clusters.
The mesodermal cells spread out in a thin layer to cover the adjacent yolk-sac. They are spindle-shaped, and are attached to each other by long protoplasmic processes.
Fig. 1 shows'a portion of the extra-embryonic area in which a cluster of blood-cells is seen lying free between the mesoderm and the entoderm. The cells forming the mesoderm are, as noted, spindle-shaped, whilst the cells constituting the entoderm are spheroidal and exhibit other characters which are similar to those of the blood-cells. Mitotic division occurring in one of the enfodermal cells can be detected in the neighbourhood of the blood clusters (fig. 1). It is proved that this is not a singular occurrence by the fact that a similar phenomenon is again seen in the entoderm of another embryo (Stage II. (6), figs. 156, 17a and b).
(6) Ferret Embryo, 1°6 mm. (F. 1904, QZ., U2). General Description.
This embryo measures 1°6 mm. after it has been cut. There is, as yet, no mesodermic somites. The heart rudiment is absent, and no intraembryonic blood-vessels can be detected. There is, of course, no head fold. A primitive streak is, however, present, and there is a primitive groove.
The notochord is tubular at its caudal end. In parts its ventral wall opens into the yolk-sac. Its caudal extremity is fused with the ectoderm at the cranial end of the primitive streak, as in the previous specimen. Cranial to the primitive streak a faint neural groove is present on the surface of the embryo. It terminates, as in the previous case, at the buccopharyngeal membrane cranially.
In this specimen blood-cells are found abundantly on the wall of the yolk-sac between the mesoderm and the entoderm. The blood-cells are spheroidal in shape, provided with large well-stained nuclei, as in the preceding specimen. They are arranged in solid clusters, without any lumina in them, and are devoid of any endothelial coverings (figs. 2a, 2b, and 3). The majority of the cell clusters are found to be adherent to the entodermal cells, which exhibit characters similar to those of the bloodcells (fig. 26).
The mesodermal cells, covering the adjacent yolk-sac and in the neighbourhood of the blood-cells, are arranged in a thin layer which is not in contact with the entoderm. They are spindle-shaped, and are connected with one another by long protoplasmic processes, as previously noted (fig. 2a).
Fig. 2a. — Showing blood-cells. x 500.
Fig. 2b. — Showing blood-cells. x 500.
Fig. 3. — Showing blood-cells. x 500. Development of Blood-vessels and Heart in Ferret Embryos 119
(c) Ferret Embryo, 1:74 mm. (F. 1904, Q.A.A. U1) with 3 Somites.
The primitive streak is well marked, and is notched at. the caudal part of its extent by the primitive groove. The mesoderm, covering the caudal part of the embryonic area, is thickened, and indicates the position of the allantoic mesoderm. Cranially the mesoderm of the primitive streak fuses with the caudal end of the chorda.
Fig. 4. — Showing blood-cells. 500.
There is a broad, shallow neural groove which narrows cranially, and its walls become much thickened in the position which is occupied by the trigeminal ganglion. The groove gradually disappears, and becomes continuous cranially with the bucco-pharyngeal membrane.
There is no heart rudiment. The two pleuro-pericardial canals are present one on each side of the embryo, but these have not grown across the median plane cranially ; consequently there is no pleuro-pericardial cavity. No intra-embryonic blood-vessels can be detected.
Blood-Cells and Endothelium. Extra-embryonically clusters of blood-cells are found scattered over the greater part of the yolk-sac, to which they are often adherent (fig. 4). The characters of the blood-cells, the entoderm and the mesoderm are similar to those already seen in Stage I.,(a) and (b) specimens. The mesodermal cells (fig. 5) are, however, more flattened, and are connected with each other with longer protoplasmic processes than those observed in the previous specimens. In addition to the blood-cells, endothelial cells can be detected here and there in the extra-embryonic region lying between the mesoderm and entoderm. These endothelial cells, unlike the blood-cells, are spindleshaped (fig. 4), and can be traced in some cases to their mesodermal origin.
Fig. 5.—Showing blood-cells. x 500.
The material for this stage consists of two embryos, one measuring 1:97 mm. in length with 5 somites, and the other 2°3 mm. with 6 somites.
(a) Description of the Graphic Reconstruction of the Heart and Cranial Portion of a Ferret Embryo 1:97 mm. in Length with 5 Somites. (F.B.A.A., G.A.)
This is the youngest specimen of the series of ferret embryos selected for the purpose of reconstruction in this investigation. Its total length measures 1:97 mm. after preparation and embedding. It may be mentioned that the sections, each of which is 10 in thickness, are perfect, and that the histological condition is excellent.
No plastic reconstruction of the embryo was made, for it appeared that a graphic reconstruction of the heart would be sufficient for the purpose in hand.
The neural groove and the primitive streak are both present. The neural tube is deepest at the brain region, where it shows thickenings which correspond to the positions of the trigeminal and otic ganglia. The cranial end of the neural groove gradually shallows until it disappears at the primitive caudal end of the bucco-pharyngeal membrane.
Tn this specimen the heart is represented merely by a transverse blood channel which lies across the median plane and unites the cranial ends of the two vitelline veins (figs. 6a and 6). It is bounded caudally by the bucco-pharyngeal membrane, and cranially by the pleuro-pericardial cavity (fig. 6a). The cranio-caudal diameter of the heart rudiment is 20u, its breadth, 120u. The rudiment of the pleuro-pericardial cavity is present. It is that portion of the ccelomic space which crosses the median plane of the embryonic area cranial to the rudiment of the heart and is closed cranially and caudally, and on each side it is connected with the pleuropericardial canal (figs. 6a and 7). Together with the pleuro-pericardial canals it forms an inverted U-shaped canal (fig. 6a), which lies dorso-cranial to the vitelline veins and the heart rudiment (fig. 8). Each vitelline vein is placed ventral to the corresponding pleuro pericardial cana] and lateral to the dorsal aorta of the same side (figs. 6a and 9). The two veins converge cranially, and each terminates in the corresponding end of the heart rudiment ‘cranial to the bucco-pharyngeal membrane (fig. 6a).
Graphic reconstruction. x 200.
P., pericardium ; H., heart rudiment (primary union); P.p.c., pleuro-pericardial canal ; V.v., vitelline vein; D.Ao., dorsal aorte; Pl., plexus; Buc., bucco-pharyngeal membrane.
Fig. 6b. — Transverse section through primary heart tube. x 500,
Fig. 7,. — Transverse section through pericardium. x 500,
Fig. 8. — Transverse section through pericardium and heart. x 500.
There are two rudiments of the dorsal aorta (fig. 6a). “They t run caudo cranially one on each side of the medullary groove. They are still more or less plexiform in character, and they terminate blindly at their cranial extremities some distance caudal to the heart rudiment. Communications between the dorsal aorte and the corresponding _vitelline veins are described by Bremer (’12) in a 34 mm. rabbit embryo. Such communications (fig. 6a) can be traced in the caudal portion of the ferret embryo-at this stage, which, so far as its general development is concerned, is considerably younger than Bremer’s embryo. Caudally the ‘dorsal aorta breaks up into a plexus spreading over the wall of the yolk-sac. There is, of course, no ventral mesocardium, and the head fold and foregut are not yet developed. As far as the pericardial cavity and the pleuropericardial canals are concerned, this specimen does not differ, to any great extent from what has been described in Dasywrus viverrinus.(7'5 mm. vesicle) by Miss Parker (’15). The heart of the ferret embryo is, however, in a more. advanced stage of development than the 7:5 mm. Dasyurus, in which the heart rudiment is represented merely by some scattered angioblast cells and strands of -cells, and in which the vitelline veins terminate cranially at the level ‘of the caudal limit of the trigeminal rudiment. In the ferret specimen under consideration the walls of the vessels of the vitelline plexus are in direct continuity with the adjacent splanchnic mesoderm (figs. 10a, 6, and c), a fact whieh is of interest in association with the mesoderm origin of the walls of the vessels.
Fig. 9. — Transverse section through embryo, x 200.
(6) Description of a Ferret Embryo 2°3 mm. with 6 Pairs of Somites. (PF. 1904, B.G.a.)
The primitive streak is present. Its cranial extremity is continuous with the notochord, which shows indications of the notochordal canal, and ‘ the notochord has begun to dovetail with. the entoderm. More cranially the notochord is entirely fused with the entoderm, and for some ten sections cranial to the primitive streak it can scarcely be distinguished except by ' the height of the cells from the entodermal cells.
Fig. 10a,—Transverse section through embryo, showing endothelium. x 375. Ectoderm. Colom. Mesoderm Vitelline plexus. Entoderm Fig.
10b.—Transverse section through embryo, showing endothelium. x 375. eo. “ Ectoderm. —— Colom. - : Mesoderm. Vitelline plexus. — Entoderm.
Fig. 10c. — Transverse section through embryo, showing endothelium. x 375.
The neural groove extends caudally to the primitive streak. It is wide and shallow at the caudal end, but deepens and narrows as it passes cranialward between the laterally placed mesodermal somites, and cranial to the somatic region the walls of the neural groove show thickenings which correspond in positions to the auditory and trigeminal areas. The neural groove then gradually shallows until it disappears at the caudal end of the bucco-pharyngeal membrane.
This embryo exhibits several features which were not present in the preceding specimens. No reconstruction was made at the time, but study of the sections shows that the embryo as regards general development is slightly in advance of the previous specimen. The heart and pericardium (fig. 11a), have practically not changed, the former lying ventral to the latter. Figs. lla and 6 illustrate the union of the two heart tubes across the médian plane, and immediately cranial to this the pericardium is seen stretching transversely to communicate on either side with the pleuro-pericardial canal (figs. 12a, b, and c). In the 1:97 mm. ferret embryo (Stage IT. (a)), it has been noted that there is a communication between the vitelline vein and the dorsal aorta. Such a communication is seen also in this specimen (fig. 18). Some of the intra-embryonic vascular endothelium can be traced to its origin from the mesoderm (figs. 14a and b). Blood-cells.
Fig. 11a. — Transverse section through pericardium and heart. x 200,
Fig. 11b. — Transverse section through pericardium and heart. x 500.
Fig. 12b. — Transverse section through pericardium. — x 500.
Fig. 12c, — Transverse section through pleuro- pericardial cavity. x 500.
Fig, 13. — Transverse section through embryo. x 200.
Fig. 14a. — Transverse section showing endothelium. x 500.
Fig. 14b,—Transverse section showing endothelium. x 500. 128 Dr C. C. Wang
Fig. 15a. — Showing blood-cells. 500.
Fig. 15b, — Showing mitosis in entodermal cells to form blood-cells. x 500.
Fig. 16. — Showing endothelium and blood-cells, x 500.
Fig. 17a. — Showing mitosis in entoderm to form blood-cells, x 500.
Fig. 176.—Showing mitosis in entoderm to form blood-cells. x 500.
In this specimen it is possible to detect that some of the intraembryonic blood-cells are not surrounded by endothelium (figs. 15a and b), whilst others are partially engulfed by flattened vascular endothelium (fig. 16). Figs. 156, 17a and 6, taken from sections of the caudal end of this specimen, show distinctly mitotic divisions in the entodermal cells in the neighbeurhood of some blood-cells.
There is no ventral mesocardium, and the fore-gut has not yet begun to develop.
The material for this stage consists of one embryo which is 23 mm. in length and with 9 somites.
Description of the Graphic Reconstruction of the Heart and the Cranial Portion: of a Ferret Embryo 2:3 mm. in Length with 9 Somites. (F. Ap. 16/28/08.) General Description.
The amnion is closed caudally. The allantoic diverticulum and the allantoic mesoderm are both present. The neural groove extends to the caudal end of the embryo, but terminates, however, at some distance cranial to the tailamnion-fold. It narrows and deepens as it passes cranially.
The rudiment of the otic ganglion is distinct, and the rudiment of the trigeminal ganglion is likewise recognisable. The head fold has begun to form, and a portion of the fore-gut is also defined. There is no indication of the primary optic vesicle.
The heart of this embryo has not yet been reconstructed in wax, but the graphic reconstruction (fig. 18) which has been made shows that, in length, this specimen is identical with the Stage II. (b) embryo, but in its general development it is decidedly in a more advanced stage than the preceding one, as the central part of the transverse rudiment of the heart seen in Stage II. (fig. 6a) has, in this specimen, begun to break up, for two non-vascular loculi have divided it incompletely into a cranial and a caudal portion (figs. 18, 19a-e). Apparently the division of the central part of the rudimentary transverse heart proceeds still further as development goes on, until it is completely separated into right and left halves, for in Stage IV. (fig. 27) the heart rudiment is represented by two separate longitudinal endothelial tubes which lie side by side and are in contact in their middle third.
Cranially two vessels, one on each side of the median plane, run cranialAortic arches, Heart rudiment breaking up.
Cut edge of yp:
Cut edge of pari pleuro-peri i
Pleuro-pericardial canal Z ee —— Pleuro-pericardi
layer of canal. canal. Vitelline vein seen through pericardium.
ne vein seen through pericardium.
. : Pericardium. Pericardium. 32a
Cut edge of pleuro- | Cut edge of pleuropericardial canal. pericardial canal. Vitelline vein. Vitelline vein. Pleuro-pericardial canal. Pleuro-pericardial canal. Intermediate cell-mass. Intermediate cell-mass.
Fig. 18. — Showing primary heart tube breaking up to form two lateral heart tubes. x 100. (Semi-schematic.)
Fig. 19a,— x 100.
Fig. 19b.— x 100.
Fig. 19c,—x 100.
Fig. 19d.— x 100. Development of Blood-vessels and Heart in Fetreéfimbryos 133
ward from the heart rudiment. They arch round the cranial end of the fore-gut and form the first cephalo-aortic arches, which terminate dorsally in the corresponding dorsal aorte (figs. 18 and 20). Caudally the heart rudiment receives the two vitelline veins. It is obvious that the fore-gut has begun to develop pari passu with the head fold.
The pericardial cavity is much wider and longer than it is in the preceding specimen, measuring 870. in the transverse diameter and 140, in the antero-posterior direction. The two dorsal aorte are well developed and run parallel to each other, one on each side of the median plane of the embryo, not far from the medullary groove (figs. 18, 19a-e). Here again there is nothing to indicate the presence of a ventral mesocardium.
This stage of development, as far as the heart is concerned, appears to fall in between the Stage II. Dasyurus viverrinus (8°5 mm.) and the Stage III. Perameles nasuta (7'5 mm., 11 somites) of Miss Parker(’15). In the Dasyurus the endothelial tubes have come actually in contact with each other at their extreme cranial ends, and presumably have united across the median plane of the embryo. It should be noted that the significance of this connection between the two vitelline veins across the median plane was not dwelt upon, and was considered only as being remarkable by Miss Parker, who states also “that lateral and caudal to the median union, each endothelial tube gives rise to the first aortic arch, which follows the antero-lateral margin of the gut almost to the median plane, and there becomes continuous with the corresponding dofsal aorta, the two aorte being well developed at this stage.”
Fig. 19c.— x 100.
It is quite possible that what has been taken for the first aortic arch in Miss Parker's Dasyurus specimen, as in the case of Eternod’s human embryo (’95, ’99), may, after all, prove to be the plexus which lies between the dorsal aorta and the vitelline vein, and which has been described by Bremer (’12) to be present in the rabbit embryo of 5 somites. If this is the case, the Stage II. Dasyurus may be regarded as being similar with the Stage II. (a) ferret embryo, in so far as the development of the heart, the vitelline veins, and the dorsal aorte is concerned.
Fig. 19a. — Showing primary heart tube. x 6500. (Enlarged from fig. 19a.)
Fig. 19b, — Showing primary heart tube breaking up. x6500, (Enlarged from fig. 190.)
In the Perameles (7°5 mm.),the two heart tubes lie separate from each other ventral to the closed fore-gut. At the level of the umbilical orifice they diverge and lie on each side of the open gut in the dorso-medial wall of the pericardium. From the cranial extremity of each endothelial tube there arise two vessels, one of which runs cranially and laterally towards the lateral margin of the gut and then parallel with this margin. It loops round the cranial end of the gut,joins the dorsal aorta, and thus constitutes the first aortic arch. The other vessel is small, and runs caudally and laterally, lateral to and almost parallel with the heart tube. This, according to Miss Parker, is the ventral portion | of the. future second aortic arch. In the median space between the cranial ends of the endothelial heart tubes are a number of scattered angioblast cells lying between the splanchnic mesoderm and the entoderm.
Fig. 19c. — Showing primary heart tube breaking up. 500. (Enlarged from fig. 19c.)
Fig. 19d. — Showing primary heart tube breaking up. 500. (Enlarged from fig. 19d.)
Fig. 19e. — Showing primary heart tube. 500. (Enlarged from fig. 19.)
Fig. 20. — Showing first aortic arch. x 100.
Cite this page: Hill, M.A. (2021, April 10) Embryology Paper - The earliest stages of development of the blood-vessels and of the heart in ferret embryos. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_earliest_stages_of_development_of_the_blood-vessels_and_of_the_heart_in_ferret_embryos
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