Book - Embryology of the Pig 11
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 11. The Development of the Circulatory System
I. The Interpretation of the Embryonic Circulation
The embryonic circulation is difficult to understand only when the meaning of its arrangement is overlooked. If one bears in mind certain fundamental conceptions as to the significance of the circulatory system in organic economics, and the basic morphological principle that any embryo must go through certain ancestral phases of organization before it can arrive at its adult structure, the changes in the arrangement of vascular channels during the course of development form a coherent and logical story.
In the embryo as in the adult the main vascular channels lead to and from the centers of metabolic activity. The circulating blood carries food from the organs concerned with its absorption to parts of the body remote from the source of supplies; oxygen to all the tissues of the body from organs which are especially adapted to facilitate the taking of oxygen into the blood ; and waste materials from the places of their liberation to the organs through which they are eliminated. One of the primary reasons the arrangement of the vessels in an embryonic mammal differs so much from that in the adult, is the fact that the emjbryo lives under conditions totally unlike those which surround its parents. Its centers of metabolic activity are, therefore, different; and, since the course of its main blood vessels is determined by these centers, the vascular plan is different. No such profound changes occur between the embryonic and the adult stages in the circulation of a fish where embryo and adult are both living under similar conditions.
The organs which in the adult mammal carry out such functions as digestion and absorption, respiration, and excretion are extremely complex and highly differentiated structures. They are for this reason slow to attain their definitive condition and are not ready to become functional until toward the close of the embryonic period. Moreover the conditions which surround certain of the developing organs during intra-uterine life absolutely prevent their becoming functional even were they suflSciently developed so to do. Suppose^ the lungs, for example, were functionally competent at an early stage of development. The fact that the embryo is reliving ancestral conditions in its private amniotic aquarium renders its lungs as incapable of functioning as those of a man under water. Likewise the developing digestive organs of the embryo are inaccessible to raw food materials. Further examples are not necessary to make it obvious that were the embryo dependent on the same organs which carry on metabolism in the adult, development would be at an impasse.
An embryo must, nevertheless, solve the problem of existence during the protracted time in which it is building a set of organs similar to those of its parents. In the absence of a dowry of stored food in the form of yolk, the mammalian embryo draws upon the uterine circulation of the mother. Utilization of this source of supplies depends on the development of a special organ which serves through fetal life and is then discarded. The embryo takes food not into its slowly developing gastro-intestinal tract but into its chorion, a membrane projected outside its own body and applied to the uterine wall to form, together with it, the placenta. The nutritive materials there absorbed from the maternal blood must be transported to the body of the growing embryo by its own blood stream.
The use of food materials to produce the energy expressed in growth depends on the presence of oxygen. For growth there must be a means of securing oxygen and carrying it, as well as food, to all parts of the body. Nor can continued growth go on unless the waste products liberated by the developing tissues are eliminated. The blood of the embryo cannot be relieved of its carbon dioxide and acquire a fresh supply of oxygen in the primordial cell clusters which will later become its lungs. It cannot excrete its nitrogenous waste products through undeveloped kidneys. Its respiration and excretion, like its absorption of food, are carried out in the rich plexus of small blood vessels in the chorion. Here the fetal blood is separated from the maternal, by tissues so thin that it can readily give up its waste materials to, and receive food and oxygen from, the maternal blood stream, just as the mother’s own tissues constantly carry on this interchange with the circulating blood. The placenta is thus the temporary alimentary system, lung, and kidney of the mammalian embryo. The large size of the umbilical blood vessels to the placenta is not a surprising thing — it is the entirely logical, the inevitable, expression of the conditions under which the embryo develops.
The enormous chorionic blood supply during fetal life, with the entire disappearance of this special arc of the circulation when the organism assumes adult methods of living, is a striking example of the determination of vascular channels by the location of functional centers. We must not, however, overlook the fact that there are many other centers of activity in the growing embryo less conspicuous but equally important for its continued existence. Each developing organ in the embryonic body is a center of intense metabolic activity. During fetal life it must be supplied by vascular channels adequate to care for its growth. But that is not all. Up to the time of birth each organ has been drawing on blood furnished with food and freed of waste materials by the activities of the maternal organism. At birth all this must change. Each organ essential to metabolism must be ready to assume its own active share in the process. Their vessels must be adequate to take care not only of the needs of these organs themselves but also of the functions these organs must now take over in maintaining the metabolism of the organism as a whole.
While the functional significance of the arrangement of the blood vessels is always of importance, especially in understanding the progressive changes in vascular plan, there is another factor which we cannot overlook. This factor is conservative, having to do with the things we inherit from our forebears. The goal of the embryonic period is the attainment of a bodily structure similar to that of the parents. Because it is so familiar, we accept with complaisance the remarkable fact that this goal is attained with absolute regularity. Accidents there may be, leading to defective development or malformation — but the fertilized ovum of a pig never gives rise to a cow. The new individual will show detailed differences from its parents, differences which are capitalized in the slow march of evolution; but in a single generation these differences are never radical. We say that the offspring has inherited the structure of its parents. It does more. It inherits the tendency to arrive at its adult condition by passing through the same sort of changes which its ancestors underwent in the countless millions of years it took their present structure to evolve.
Applied to the development of the circulatory system of mammals this means that the earliest form in which it appears will not be a miniature of the adult circulation. The simple tubular heart pumping blood out over aortic arches to be distributed over the body and returned to the posterior part of the heart by a bilaterally symmetrical venous system, in short the vascular plan which we see in young mammalian embryos (Fig. 45), is essentially the plan of the circulation in fishes. When we realize this, we are not puzzled either by the appearance of a full complement of aortic arches, or by their subsequent disappearance to make way for a new respiratory circulation in the lungs. We see the march of progress from a logical beginning in ancestral conditions toward the consummation of fetal life with an organization like that of the parent.
In addition to the fundamental ground plan of the circulation of the mammalian embryo, recapitulations account for many transitory peculiarities. The formation of a conspicuous though empty yolk-sac with a complement of blood vessels almost as well developed as the vitelline vessels of animals well endowed with yolk, is clearly a recapitulation of ancestral conditions. So also is the highly developed system of venous channels in the mesonephros. If the organ itself appears it brings with it its quota of vessels, no matter whether or not the organ is destined to degenerate later in development.
Whatever peculiarities may be impressed on the course of the circulation by the appearance of ancestral structures or by the development of special fetal organs such as the yolk-sac and the placenta, the main blood currents will at any time be found concentrated at the centers of activity. Changes of these main currents as one center retrogresses and another becomes dominant, must take place gradually. Large vessels become smaller, what was formerly an irregular series of small vessels becomes excavated to form a new main channel, but the circulation of blood to all parts of the body never ceases. Even slight curtailment of the normal blood supply to any region would stop its growth ; any marked local decrease in the circulation would result in local atrophy or malformation ; complete interruption of any important circulatory channel, even for a short time, would inevitably mean the death of the embryo.
II. The Arteries
The Derivatives of the Aortic Arches
In vertebrate embryos six pairs of aortic arches are formed connecting the ventral with the dorsal aorta. The portions of the primitive paired aortae which bend around the anterior part of the pharynx constitute the first (i.e., the most anterior) of these aortic arches. In its course around the pharynx the first aortic arch is embedded in the tissues of the mandibular arch (Fig. 136, A). The other aortic arches develop later, in sequence, one aortic arch in each branchial (gill) arch posterior to the mandibular arches, leaves only the ventral and dorsal aortic roots and the third, fourth, and sixth arches to play an important r61e in the formation of adult vessels.
Fig. 134. Ventral aspect of vessels in the branchial region of pig embryos of various ages. (After Heuser.) The drawings in this and the three following figures were made directly from injected specimens rendered transparent by treatment with wintergreen oil. A, 24 somites; B, 4.3 mm.; C, 6 mm.; D, 8 mm.; E, 12 mm.
In dealing with embryos of 9 to 12 mm. we have already seen how the portions of the ventral aortic roots which formerly acted as feeders to the first two arches were retained as the external carotid arteries. These vessels, in part through the small channels left by the disintegration of the aortic arches with which they were originally associated, and in part through the formation of new branches to subsequently formed structures, nourish the oral and cervical regions (Figs. 67, 133 and 137, A, B).
The internal carotid arteries also, are, familiar as vessels which arise as prolongations of the dorsal aortic roots and extend to the brain (Fig. 67). When the portion of the dorsal aortic root which lies between arch 3 and arch 4 dwindles and drops out, the third arch is left constituting the curved proximal part of the internal carotid artery (Figs. 133, B, C, and 137, A~C). The part of the ventral aortic root which, from the first, has fed the third aortic arch becomes somewhat elongated and persists as the common carotid artery (Figs. 133, 137).
The fourth aortic arch has a different fate on opposite sides of the body. On the left it is greatly enlarged and persists as the arch of the adult aorta (Figs. 133 and 134-137). On the right the fourth arch forms the root of the subclavian artery. The short section of the right ventral aortic root proximal to the fourth arch persists as the innominate (brachiocephalic) artery from which both the right subclavian and the right common carotid artery arise (Fig. 133, C).
The sixth aortic arch changes its original relationships somewhat more than the others. At an early stage of development branches extend from its right and left limbs toward the lungs (Fig. 134, D, E). After these pulmonary vessels have been established^ the right side of the sixth aortic arch loses communication with the dorsal aortic root and disappears (Fig. 135, A, B). On the left, however, the sixth arch retains its communication with the dorsal aortic root. The portion of it between the point where the pulmonary trunk is given off and the dorsal aorta is called the ductus arteriosus (Figs. 133, C, and 138). During the fetal period when the lungs are not inflated the ductus arteriosus shunts the excess blood from the pulmonary circulation directly into the aorta. The functional importance of this channel will be more fully appreciated when we have given it further consideration in connection with the development of the heart and the changes which take place in the circulation at the time of birth.
- The details of the formation of the pulmonary arteries differ somewhat in different mammals. In most of the forms which have been carefully studied (man, cat, dog, sheep, cow, opossum) the pulmonary arteries maintain their original paired condition throughout their entire length. In these forms part of the right sixth arch is retained as the proximal portion of the adult right pulmonary artery (Fig, 133). The pig is unusual in having its pulmonary branches fuse with each other proximally, forming a median vessel ventral to the trachea (cf. Figs. 134, E, and 135, A). Distal to this short median trunk the pulmonary vessels retain their original paired condition, each running to the lung on its own side of the body. Proximally the median trunk becomes associated with the left sixth arch and the right sixth arch drops out altogether.
While these changes have been taking place in the more peripheral part of the vascular channels which lead to the lungs, a fundamental alteration has occurred in the main ventral aortic stem. Formerly a single channel leading away from the undivided ventricle of the primitive tubular heart, the ventral aorta now becomes divided lengthwise into two separate channels. This division begins in the aortic root just where the sixth arches come off, and progresses thence toward the heart. Meanwhile, as wc shall see when we take up the development of the heart, the ventricle has become divided into right and left chambers. The final result of these two synchronous partitionings is the establishment of a channel leading from the right ventricle to the lungs by way of the sixth aortic arches, and another separate channel leading from the left ventricle to the dorsal aorta by way of the left fourth aortic arch.
The Derivatives of the Intersegmental Branches of the Aorta
In dealing with the structure of 9-12 mm. embryos comment was made on the importance of the small intersegmental branches from the dorsal adrta (Figs. 67 and 136, F). At that time, too, we became familiar with some of the vessels which are derived from these branches. The anterior appendage bud first appears at the level of the seventh cervical intersegmental, and it is this artery which becomes enlarged to form the subclavian. With the enlargement of the left fourth aortic arch to form the main channel leading from the heart to the dorsal aorta, the dorsal aortic root on the right side becomes much reduced (Fig. 135, A-D). Caudal to the level of the subclavian it drops out entirely. It will be recalled that the sixth aortic arch also drops out on this side. This leaves the right subclavian communicating with the dorsal aorta by way of a considerable section of the old dorsal aortic root and the fourth aortic arch. In the adult, both the distal part of this vessel (formed by the intersegmental artery) and its proximal portion (appropriated from the old aortic arch system) pass under the name subclavian. This accounts for the striking dissimilarities of origin between the right and left subclavian arteries in the adult.
Fig. 136. Lateral aspect of vessels in the branchial region of pig embryos of various ages. (After Heuser.) A, 10 somites; B, 19 somites; C, 26 somites; D, 28 somites; E, 30 somites; F, 36 somites (6 mm.).
Fig. 137. Continuation of the same series of lateral views of injected embryos. A, 14 mm.; B, 17 mrn.; C, 19.3 mm.; D; 20.7 mm.
Cephalic to the subclavian arteries, a series of longitudinal anastomoses appear connecting the cervical intersegmentals to form the vertebral arteries (Fig. 137). When the vertebral arteries are thus established, all the intersegmental roots back to the subclavian drop out, leaving the vertebral as a branch of the subclavian (Figs. 1 33, C, and 137). The manner in which the vertebrals swing in to the mid-line rostrally and become confluent with each other to form the basilar artery, and the anastomosis between the internal carotids and the basilar artery in the region of the hypophysis, are already familiar.
Caudal to the subclavian, the internal mammary artery is formed by longitudinal anastomosing of the more cephalic of the thoracic intersegmental arteries. Subsequent dropping out of the proximal parts of the other intersegmentals leaves it arising from the subclavian. Thus the steps in its origin are strikingly similar to the processes by which the vertebral artery was established cephalic to the subclavian (Fig. 133). Still farther caudally in the body, the intersegmental arteries retain their original independent condition as paired branches extending from the aorta dorsad on either side of the neural tube and the developing spinal column (Fig. 67). Even in the adult these vessels appear with little change in their original relations.
The Enteric Arteries
The first of the three enteric arteries to appear is the anterior {superior) mesenteric. We have already followed its origin as a pair of arteries originally called the omphalomesentcrics which, in young embryos, extended to the surface of the yolk-sac (Fig. 45). When the yolk-sac degenerates and the ventral part of the body closes in, these paired channels fuse with each other to form a median vessel situated in the mesentery and extending to the gut loop in the belly-stalk (Fig. 66). This is now called the anterior mesenteric artery. With the elongation and coiling of the intestine in the region fed by it, the anterior mesenteric artery acquires many radiating terminal branches. Its primary relations, however, remain unchanged.
The celiac artery arises from the aorta in a manner basically similar to the origin of the anterior mesenteric artery, but at a slightly later stage of development. The embryonic body has, therefore, become more nearly closed ventrally and the primary dorsal mesentery has been established. As a result the primary paired condition we expect to see in the early stages of the formation of all of the main enteric arteries is greatly abbreviated in the case of the celiac artery, and almost from its first appearance it is a median vessel extending in the mesentery toward the gastric region of the gut (Fig. 67). As development progresses it becomes extensively branched, being the main artery which feeds the gastro-hepato-pancreatic region of the digestive system and also the spleen which arises in its territory (Fig. 111).
The inferior mesenteric artery has an origin similar to that of the celiac. It is established caudal to the anterior mesenteric artery slightly later in development than the time at which the celiac appears and is the main vessel to the posterior part of the intestinal tract (Fig. 111).
Fig. 138. Drawing (X 10) of parasagittal section of 15 mm. pig embryo. The section is in a plane slightly to the left of the mid-line and passes through the ductus arteriosus, lung bud, stomach, gonad, and metanephros.
The Renal Arteries
The mesonephros is suppliea by many small arteries which arise ventro-laterally from the aorta. While the metanephroi or permanent kidneys are still very small, they lie in close proximity to the mesonephroi and are fed by small arteries which arise from the aorta along with the mesonephric vessels (Fig. 127). The local vessels associated with the kidneys are progressively enlarged as the kidneys themselves grow in bulk and become the renal arteries of the adult (Fig. 128).
The Arteries Arising from the Caudal End of the Aorta
Themain aortic trunk decreases abruptly in size where the large umbilical (allantoic) arteries (Fig. 138) turn off into the belly-stalk. Beyond this point the aorta is continued toward the tail as a slender median vessel called the caudal artery (Fig. 67).
The posterior appendage buds arise some time after the placental circulation has been established. The umbilical arteries are consequently of considerable size, and the small vessels which branch off from them to feed the appendage buds are by comparison quite insignificant. As the appendage buds grow, these small vessels grow with them to become the external iliac arteries. When, at birth, the placental circulation stops, the umbilical arteries are reduced to small vessels nourishing the local tissues between their point of origin and the umbilicus. We then know their proximal portions as the internal iliac^ or hypogastric arteries^ and the fibrous cords which still mark their course along the wall of the bladder (the old allantoic stalk) as the obliterated branches of the hypogastric arteries. Thus the tables are turned between fetal and adult life. In the fetus the external iliac artery to the leg appears as a branch of the dominating umbilical artery. After birth the reduced umbilical arteries under their new name of internal iliacs, or hypogastrics, appear as branches of the now larger external iliacs. The original umbilical root proximal to the origin of the external iliac is called the common iliac (Fig. 150).
III. The Veins
There is a natural grouping of the veins according to their relationships which it is convenient to follow in discussing their development. Under the term systemic veins we can include all the vessels which collect the blood distributed to various parts of the body in the routine of local metabolism. In young embryos these would be the cardinal veins and their tributaries, that is, the return channels of the primitive intra-embryonic circulatory arc. In older embryos and adults the systemic veins would include the anterior (superior) caval system which is evolved from the anterior cardinals, and the posterior (inferior) caval system which takes the place of the postcardinals and their tributaries.
We can set apart from the general systemic circulation three special venous arcs: the umbilical, returning the blood from the })lacenta; the pulmonary, returning the blood from the lungs; and the hepatic portal, carrying blood from the intestinal tract to the liver. I'he specialized nature of the placental and pulmonary circulations is obvious. The peculiarities of the hepatic portal system call, perhaps, for a word of explanation. Ordinarily veins^ collect blood from local capillaries and pass it on directly to the heart. Their blood stream is away from the organ with which they arc associated ; once collected within a vein the blood is not redistributed in capillaries until it has again jiassed through the heart. The portal vein arises in typical fashion by collecting the blood from capillaries in the digestive tube. But then, contrary to the usual procedure, its blood flows, not directly to the heart, but to the liver where it enters a second capillary bed and is returned by a second set of collecting vessels to the heart. With reference to the plexus of capillaries in the liver this vein is aflferent. Hence its designation as a portal (translated = carrying to) vein, setting it apart from other veins which carry blood only away from the organ with which they are associated.
There is a tendency among those who have done but little work on the circulation to regard any vessel which carries oxygenated blood as an artery, and any vessel which carries blood poor in oxygen and high in carbon dioxide content as a vein. This is not entirely correct even for the circulation of adult mammals on which the ( onception is based. In comparative anatomy and especially in embryology it is far from being the .case. It is necessary, therefore, in dealing with the circulation of the embryo, to eradicate this not uncommon misconception.
The differentiation between arteries and veins which holds good for all forms, both embryonic and adult, is based on the structure of their walls, and on the direction of their blood flow with reference to the heart. An artery is a vessel carrying blood away from the heart under a relatively high, fluctuating pressure due to the pumping of the heart. Correlated with the pressure conditions in it, its walls are heavily reinforced by elastic tissue and smooth muscle. A vein is a vessel carrying blood toward the heart under relatively low and constant pressure from the blood welling into it from capillaries. Correlated with the pressure conditions characteristic for it, the walls of a vein have much less elastic and muscle tissue than artery walls, and more non-elastic connective-tissue fibers.
Fig. 139. Diagrams illustrating stages in the development of the systemic veins of the pig. (After Butler.) The cardinal and omphalomesenteric veins are shown in black, the subcardinal system is stippled, the supracardinals are horizontally hatched, and vessels arising independently of these three systems are indicated by small crosses,
A, Ground plan of the veins of a young mammalian embryo (cf. Fig. 45).
B, Cross-section (at level of arrow in A) showing dorso-ventral relations of the various veins.
C, Diagrammatic plot of veins of mm. pig embryos.
D, Arrangement of veins in 6“-7 ram. pig embryos.
E, Cross-section (at level of arrow in D) showing dorso- ventral relations of vessels.
F, Veins in 12—13 mm. embryos.
G, Veins in 16-19 mm. embryos.
H, Veins in 22-24 mm. embryos.
I, Veins in 30-35 mm. embryos.
J, Cross-section of 17 mm, embryo at level of arrow in G.
K, Plan of veins in adult pig^
Changes in the Anterior Systemic Veins Resulting in the Establishment of the Anterior (Superior) Vena Cava.
The main tributaries draining the anterior parts of the adult body are the external and internal jugulars and the subclavians. In 12 mm. embryos we saw all these vessels laid down. The internal jugular is merely the original anterior cardinal under a new name. The external jugular develops from the small branch draining the mandibular region, and the subclavian as an enlargement of one of the segmental tributaries at the level of the anterior appendage bud (Fig. 68).
When the appendage buds first appear, the heart lies far forward in the body. As development progresses it is carried caudad. With this change in the position of the heart, the common cardinal veins (ducts of Cuvier) change their relative position in the body and come to lie caudal to the anterior appendage buds. As a result of this altered relation the subclavian veins from the anterior appendages, which early in development drain into the posterior cardinals (Fig. 139, C) eventually empty into the anterior cardinals (Fig. 139, G).
The outstanding characteristic of the systemic venous plan of a young embryo is its bilateral symmetry. Paired vessels from the anterior and from the posterior parts of the body become confluent to enter the sinus region of a simple tubular heart (Figs. 1 39, A and 144, A-C). The rerouting of the blood to enter the right side of the heart is the all-important end toward which the mammalian venous system is progressing throughout its development. In dealing with local changes this basic trend should never for a moment be forgotten.
In the anterior systemic channels this shift to the right is accomplished very simply and directly. A new vessel forms between the right and left anterior cardinals and shunts the left anterior cardinal blood stream across to the right (Fig. 139, H). With the establishment of this new channel, the part of the left anterior cardinal toward the heart drops out (Fig. 139, I). We have now but to apply the familiar adult names (Fig. 139, K): the new connecting vessel is the left innominate; the old anterior cardinal between the union of the subclavian with the jugulars and the new transverse connection is the right innominate ; from the confluence of the innominates to the heart is the anterior vena cava. The anterior vena cava is thus composed of the most proximal part of the right anterior cardinal and the right common cardinal vein (duct of Cuvier). The small azygos (cervicothoracic) vein, which is the reduced posterior cardinal, indicates the old point of transition from anterior cardinal to common cardinal.
Changes in the Posterior Systemic Veins Resulting in the Establishment of the Posterior (Inferior) Vena Cava. The changes in the systemic veins of the posterior part of the body are much more radical than they are anteriorly. The posterior cardinal veins which are the primitive systemic drainage channels are associated primarily with the mesonephroi. When the mesonephroi degenerate, it is but natural that the posterior cardinals should degenerate with them. Hie posterior vena cava which replaces the cardinals is a composite vessel which gradually takes shape by the enlargement and straightening of small local channels which are, as it were, pressed into service as the posterior cardinals degenerate.
The subcardinal veins initiate the diversion of the postcardinal blood stream. The subcardinals arc established as vessels lying along the ventro-mesial border of the mesonephros parallel with, and ventral to, the postcardinals. Taking origin from an irregular plexus of small vessels emptying from the mesonephroi into the posterior cardinals, the subcardinals from their first appearance have many channels connecting them with the postcardinals (Fig. 139, A-C). As the mesonephroi increase in size and bulge toward the mid -line the subcardinals are brought very close together. In the mid -mesonephric region they anastomose with each other to form a large median vessel, the subcardinal sinus, or intersubcardinal anastomosis (Figs. 60, 77, and 139, D-F). When this sinus is established the small vessels connecting sub- and postcardinals drain into the capacious sinus rather than toward the posterior cardinals. The result of this change very soon becomes apparent in the disappearance of the posterior cardinals at the level of this sinus. The blood from the posterior part of the body is still collcctecl by the distal ends of the postcardinals but it returns to the heart by way of the subcardinal sinus. Consequently the anterior portions of the posterior cardinals, although they persist, are much reduced in size (Fig. 139, F).
Meanwhile the increased volume of blood entering the subcardinal sinus is finding a new and more direct route to the heart. The cephalic pole of the right mesonephros lies close to the liver. A fold of dorsal body-wall tissue, just to the right of the primary dorsal mesentery, early makes a sort of bridge between these two organs. This fold is known as the caval plica {caval mesentery) (Fig. 140). In it, as everywhere in the growing body, are numerous small vessels. Connection of these small vessels with the plexus of channels in the liver cephalically (Fig. 140, A), and the mesonephros caudaliy (Fig. 140, B), provides the entering wedge. Once a current of blood finds its way from the mesonephros to the liver through these small vessels, enlargement of the channel proceeds with great rapidity. This new channel becomes the mesenteric part of the inferior vena cava, (See relations of posterior vena cava in figures 75 and 76, and portion of cava indicated with small crosses in figure 139, D.)
Fig. 140. Drawings of transverse sections of 5 mm. pig embryos showing the relations of the caval fold of the mesentery (caval plica). A, Section at level where cephalic end of caval plica merges with liver. Here the small vessel which is the primordium of the mesenteric portion of the posterior vena cava enters the sinusoidal circulation of the liver. B, Section showing merging of caudal end of caval plica with right mesonephros. At this level the developing mesenteric portion of the posterior vena cava anastomoses with the right subcardinal vein (cf. Fig. 139, D).
Within the liver this new blood stream at first finds its way by devious small channels eventually entering the sinus venosus along with the omphalomesenteric circulation. As its volume of blood increases it excavates through the liver a main channel which gradually becomes walled in. As this new vessel becomes more and more definitely organized it gradually crowds toward the surface and eventually appears as a great vein lying in a notch along the dorsal side of the liver. This is the hepatic part of the inferior vena cava.
From the subcardinal sinus the most direct route to this new outlet is by way of the right subcardinal vein. Thus in embryos as young as 9 to 12 mm., the formation of the posterior vena cava is well started with its proximal portion consisting of subcardinal sinus, a portion of the right subcardinal vein, and the new channels through the mesentery and through the liver. (Follow this part of the cava through figure 139 from D to K.)
Posterior to the level of the subcardinal sinus still amother set of veins enters into the formation of the post-cava. These are the supracardinal veins which appear, relatively late in development, as paired channels draining the dorsal body-wall (Fig. 139, G, J).
At the mid-mesonephric level, the supracardinals are diverted into the subcardinal sinus just as happened with the postcardinals earlier in development. Cephalic to the sinus, parts of the supracardinals persist as the azygos group draining in a somewhat variable manner into the reduced proximal part of the postcardinals. Caudal to the anastomosis with the subcardinal sinus, the right supracardinal becomes the principal drainage channel of the region. Its appropriation of the tributary vessels from the posterior appendages establishes it as the postrenal portion of the inferior vena cava and is the last step in the formation of that composite vessel (Fig. 139, G—K).
The Coronary Sinus
The ultimate fate of the left common cardinal vein (duct of Cuvier) is a result of the shift in the course of the systemic blood so that it all enters the right side of the heart. Formerly returning a full half of the systemic blood stream to the heart, the left common cardinal vein is finally left almost without a tributary from the body. In the pig a small amount of blood usually does continue to enter it over the left azygos (Fig. 139, K). Occasionally in the pig, and normally in most other mammals, even this is cut off and the azygos drainage is by way of the right side to the superior cava (dotted line in Fig. 139, K). Nevertheless the proximal part of the old left cardinal channel is utilized. Pulled around the heart in the course of the migration of the sinus venosus toward the right, the left common cardinal vein lies close against the heart wall for a considerable distance (Fig. 144, D). As the heart muscle grows in bulk it demands a greater blood supply for its metabolism. The small returning veins of this circulation find their way into this conveniently located main vessel (Fig. 144, E). Thus even when its peripheral circulation is cut off the left common cardinal vein still persists as the coronary sinus into which the vessels of the cardiac wall drain (Fig. 144, F).
Fig. 141. Diagrams showing the development of the hepatic portal circulation from the omphalomesenteric veins, and the relations of the umbilical veins to the liver. (Adapted from several sources.)
A is based on conditions in pig embryos of 3-4 mm.; B, on embryos of about 6 mm.; C, on embryos of 8-9 mm.; D, on embryos of 20 mm. and older.
The Pulmonary V eins
Phylogenetically the lungs are relatively new structures. It is not surprising, therefore, that we find the pulmonary veins arising independently and not by the conversion of old vascular channels. They originate as vessels which drain the various branches of the lung buds and converge into a common trunk (Fig. 107), entering the left atrium dorsally. In the growth of the heart this trunk vessel is gradually absorbed into the atrial wall and two or more of its original branches open directly into the left atrium as the main pulmonary veins of the adult (Fig. 144, D-F).
The Portal Vein
The blood supply to the intestines is first established through the omphalomesenteric arteries which later become modified to form the anterior mesenteric artery. Likewise the drainage of the intestinal tract is provided for by vessels which were originally the return channels of the primitive omphalomesenteric circulatory arc (Fig. 45). In dealing with 9 to 12 mm. embryos we have already seen the primary changes which occur in these vessels. The growing cords of hepatic tissue break up the proximal portion of the omphalomesenteric veins into a maze of small channels ramifying through the substance of the liver (Fig. 141, A, B). But the stubs of the omphalomesenterics persist and drain this plexus. Distal to the liver, the original veins are for a time retained, bringing blood from the yolk-sac and intestines to the liver. With the disappearance of the yolk-sac and the growth of the intestines, the omphalic (yolk-sac) portions of these veins necessarily disappear, but the mesenteric branches persist and become more extensive concomitantly with the increased length and complexity of the intestinal tract.
The original omphalomesenteric trunks into which these tributaries converge become the unpaired portal vein by forming transverse anastomoses and then abandoning one of the original channels. The curious spiral course of the portal vein is due to the dropping out of the original left channel cephalic to the middle anastomosis and the original right channel caudal to the anastomosis (Fig. 141, D).
The Umbilical Veins
When they are first established the umbilical (allantoic) veins are embedded in the lateral body-walls throughout their course from the belly-stalk to the sinus venosus (Fig. 45). As the liver grows in bulk, it fuses with the lateral body-wall. Where this fusion occurs vessels develop connecting the umbilical veins with the plexus of vessels in the liver (Fig. 141, B). Once these connections are established the umbilical stream tends more and more to pass by way of them to the liver, and the old channels to the sinus venosus gradually degenerate (Fig. 141, C).
Meanwhile the umbilical veins distal to their entrance into the body become fused with each other so that there comes to be but a single vein in the umbilical cord (Fig. 141, C). Following this fusion in the cord, the intra-embryonic part of the umbilical channel also loses its original paired condition. The right umbilical vein is abandoned as a route to the liver and all the placental blood is returned over the left umbilical vein. It is interesting to note that in spite of its ceasing to be a through channel, part of the right umbilical vein persists, draining the body-wall. The small blood stream it then carries is reversed in direction, flowing back into the left umbilical (Fig. 141, D).
When first diverted into the liver, the umbilical blood stream passes through by way of a mesh work of small anastomosing sinusoids. As its volume increases it excavates a main channel through the substance of the liver which is known as the ductus venosus (Figs. 138 and 141, B~D). Leaving the liver, the ductus venosus becomes confluent with the hepatic veins (omphalomesenteric stubs) which drain the maze of small sinusoids in the liver. At this point, also, the vena cava joins the others. Thus the blood streams from the posterior systemic circulation, from the portal circulation, and from the placental circulation all enter the heart together. Embryologically this great trunk vessel represents the fused proximal parts of the old omphalomesenteric veins enlarged by the placental blood from the ductus venosus and by the systemic blood from the vena cava (Fig. 141). During its early developmental phases it is often called the common revehent hepatic vein. In the adult or in older fetuses it is more convenient to regard it as a part of the vena cava because, with the cessation of the placental circulation at the time of birth, the caval blood stream becomes the dominant one.
IV. The Lymphatic System
Important and interesting as the subject is, it has seemed expedient to omit any account of the development of the lymphatic vessels. Those interested in this field will find that an unusual amount of careful work has been done on the development of the lymphatics in pig embryos. References to some of the more important recent papers have been included in the bibliography appended at the end of the book.
V. Blood Corpuscles
The first blood corpuscles which appear in the circulation are produced extra-embryonically in the blood islands of the yolk-sac (Fig. 48). Later in development there are many blood-forming centers within the embryo. Concerning the establishment of these centers we find the same controversy that was commented on in connection with the origin of blood vessels. It is maintained by some authorities that these centers always arise from cells originally produced in the yolksac blood islands. According to this interpretation some of these blood cells are believed to remain sufficiently undifferentiated to retain their power of active proliferation. As such cells are carried by the circulation to various parts of the body they settle in favorable locations and raise new families of blood corpuscles.
According to the local origin idea it is not necessary to account for all the centers of blood corpuscle formation on the basis of bloodmother-cells wandering in from the yolk-sac and settling down in new locations within the embryo. It is maintained that mesodermal cells arising in the body have the same capacity of becoming differentiated into blood-mother-cells as mesodermal cells which arise in the yolksac. On this interpretation blood-forming centers arise in various parts of the body from blood-mother-cells differentiated in situ from local mesoderm. The recent experimental work tends to indicate that such “local origin†does occur. That this same evidence proves that the origin of blood-forming centers from migrating cells never occurs is by no means so clear.
Whatever the source of the original blood-mother-cells may be we always find them establishing their centers of proliferation in places where the current of the circulation is sluggish. In very young embryos the centers of corpuscle formation are located in the maze of small channels in the yolk-sac and the allantois. When the yolk-sac degenerates and the allantois becomes highly specialized as part of the chorion, these centers cease to be active and new ones are established in connection with such rich vascular plexuses as those in the mesonephros and the liver. Still later other centers appear in the lymphoid organs, and last of all in the bone marrow (Fig. 152).
The histological details of the processes involved in the production of red blood corpuscles and the various types of white blood corpuscles are exceedingly complex. Moreover there is by no means agreement as to the exact manner in which these processes occur, nor as to the genetic relations of one type of blood corpuscle to another. This whole subject, other than the recognition of the multiplicity of blood-forming centers and their shifting locations in the embryo at various phases of development, is a special field of histogenesis entirely beyond the scope of an elementary text.
VI. The Heart
To appreciate the significance of the changes which occur in the growing heart one must have in mind the exigencies under which it develops. Starting as a simple tube with the blood passing through
Fig. 142. Ventral aspect of the heart of the pig at various stages showing the formation of the cardiac loop and the establishment of the primary regional divisions of the heart. Drawn (A-E, X 30; F, X 20) from reconstructions made from series in the Carnegie Collection (A-D) and in the Western Reserve University Collection (E, F).
A, 7 somites; B, 13 somites; C, 17 somites; D, 25 somites; E, 3.7 mm. (after flexion); F, 6 mm.
Abbreviations: At., atrium (t., right; 1., left); Bui., aortic bulb (bulbus arteriosus); Endc., endocardial tubes; Myc., cut edge of epi-myocardium; S.V., sinus venosus; Tr. Art., truncus arteriosus; V. ao. r., ventral aortic roots; Vent., ventricle (r., right; 1., left); V.O.M., omphalomesenteric veins.
it in an undivided stream, it must become converted into an elaborately valved, four-chambered organ, partitioned in the mid-line and pumping from its right side a pulmonary stream which is returned to the left side and pumped out again as the systemic blood stream. And the heart cannot cease work for alteration ; there can be no interruption in the current of blood it pumps to the growing embryo. This is but one phase of the matter. At the end of gestation the vascular mechanism must be prepared to function under conditions radically different from those surrounding the embryo. In spite of the impossibility of the developing lungs being effectively exercised under air-breathing conditions, they themselves, their blood vessels, and the right ventricle which pumps blood to them, must at the moment of birth be ready to take over the entire r^esponsibility of oxygenating the blood. And the systemic part of the circulation as well as the pulmonary must be prepared. Throughout intra-uterine development the left side of the heart receives less blood from the pulmonary veins than the right side of the heart receives from the venae cavae. Yet after birth the left ventricle is destined to carry a greater load than the right ventricle. It must pump through the myriad peripheral vessels of the systemic circulation,, sufficient blood to care for the active metabolism and continued growth of the entire body. These are some of the situations which must be faced before the heart can arrive at its adult condition. The manner in which they are met is doubly interesting because they seem at first sight so difficult.
Fig. 143. Sinistral aspect of reconstructions of the pig heart.
A, 13 somite embryo (cf. Fig. 142, B); B, 17 somite embryo (cf. Fig. 142, C); C, 25 somite embryo (cf. Fig. 142, D); D, 3.7 mm. embryo (cf. Fig. 142, E); E, 6 mm. embryo (cf. Fig. 142, F).
Abbreviations: A.C.V., anterior cardinal veins; Al.V., allantoic (umbilical) vein; Ao., aorta; At., atrium; Bui., aortic bulb; Cav. P., posterior vena cava; Cuv. d., duct of Cuvier (common cardinal vein); Myc., cut edge of epi-myocardium; P.C.V.; posterior cardinal vein; S.V., sinus venosus; Sin-at., sino-atrial region of heart; Tr. Art., truncus arteriosus; V. ao. r., ventral aortic roots; Vent., ventricle; V.O.M., omphalomesenteric veins.
The Formation of the Cardiac Loop and the Establishment of the Regional Division of the Heart. In dealing with the establishment of the circulatory system in very young embryos we saw how the tubular heart was formed by the fusion of paired primordia (see Chap. 5 and especially Figs. 43 and 44). The primary factor which brings about its regional differentiation is the rapid elongation of this primitive cardiac tube. The heart increases in length so much faster than the chamber in which it lies that it is first bent to the right and then twisted into a loop. Since the anterior end of the heart is anchored in the body by the aortic roots, and the posterior end by the great veins, it is the mid-portion of the heart-tube which, in this process, undergoes the most extensive changes in position. This is facilitated by the early disappearance of the dorsal mesocardium which leaves the heart entirely free in its mid-region.
During the period in which the cardiac loop is being formed, the primary regional divisions of the heart become clearly differentiated. The sinus venosus is the thin-walled chamber in which the great veins become confluent to enter the heart at its primary posterior end (Fig. 144). The atrial region is established by transverse dilation of the heart-tube just cephalic to the sinus venosus (Fig. 144).
The ventricle is formed by the bent mid-portion of the original cardiac tube. As this ventricular loop becomes progressively more extensive, it at first projects ventrally beneath the attached aortic and sinus ends of the heart (Fig. 143, A-C). Later it is bent caudally so that the ventricle, formerly situated cephalic to the atrium, is brought into its characteristic adult position caudal to the atrium (Fig. 143, D, E). Between the atrium and ventricle the heart remains relatively undilated. This narrow connecting portion is the atrioventricular canal.
Fig. 144. Six stages in the development of the heart, drawn in dorsal aspect to show the changing relations of the sinus venosus and great veins entering heart. (Patten: ‘‘Human Embryology,†The Blakiston Company.)
The most cephalic part of the cardiac tube undergoes least change in appearance, persisting as the truncus arteriosus connecting the ventricle with the ventral aortic roots (Fig. 142). In very young embryos there is a conspicuous bulge where the truncus arteriosus swings toward the mid-line to break up in the aortic arches. This sharply bent and somewhat dilated region is called the aortic bulb (Fig. 142, B, C). Its location is of interest as being the place at which the paired endocardial primordia first fused with each other (Figs. 43 and 44) and the place at which, later in development, the division of the truncus arteriosus into separate aortic and pulmonary roots will first become apparent. The bulge itself soon merges into the rest of the truncus arteriosus without giving rise to any special structure.
Almost from their earliest appearance the atrium and the ventricle show external indications of the impending division of the heart into right and left sides. A distinct median furrow appears at the apex of the ventricular loop (Figs. 142, E, F, and 144, U, E). The atrium meanwhile has undergone rapid dilation and bulges out on either side of the mid-line (Fig. 144). Its bilobed configuration is emphasized by the manner in which the truncus arteriosus compresses it mid-ventrally (Fig. 142, E, F).
The Partitioning of the Heart
These superficial features suggest the more important changes going on internally. As the wall of the ventricle increases in thickness it develops on its internal face a meshwork of interlacing muscular bands, the trabeculae carneae. Opposite the external furrow in the ventricle these muscular bands become consolidated as a partition which appears to grow from the apex of the ventricle toward the atrium. This is the interventricular septum (Fig. 147).
Meanwhile two conspicuous masses of peculiar, loosely organized mesenchyme (called endocardial-cushion tissue) develop in the walls of the narrowed portion of the heart between the atrium and the ventricle. One of these so-called endocardial cushions of the atrio-ventricular canal is formed in its dorsal wall (Fig. 147, A) and the other is formed opposite, on the ventral wall. These two masses nearly occlude the central part of the canal and thus initiate its separation into right and left channels.
At the same time a median partition appears in the cephalic wall of the atrium. Because another closely related partition is destined to form here later, this one is called the first interatrial septum or septum primum. In shape it is crescentic with its concavity directed toward the ventricle and the apices of the crescent extending, one along the dorsal wall, and one along the ventral wall of the atrium, all the way to the atrio-ventricular canal where they merge, respectively, with the dorsal and ventral endocardial cushions (Fig. 147). This leaves the atria separated from each other except for an opening called the interatrial foramen primum.
While these changes have been occurring, the sinus venosus has been shifted out of the mid-line so that it opens into the atrium to the right of the interatrial septum (Fig. 147). The heart is now in a critical stage of development. Its simple tubular form has been altered so that the four chambers characteristic of the adult heart are clearly recognizable. Partitioning of the heart into right and left sides is well under way. But there is as yet no division of the blood stream because there are still open communications from the right to the left side in both atrium and ventricle. A little further progress in the growth of the partitions, however, and the two si^cs of the heart would be completely separated. Were this to occur now, the left side of the heart would become almost literally dry. For the sinus venosus, into which systemic, portal, and placental currents all enter, opens on the right of the interatrial septum, and not until much later do the lungs, and their vessels develop sufficiently to return any considerable volume of blood to the left atrium. The partitions in the ventricle and in the atrio-ventricular canal do progress rapidly to completion (Figs. 147, 148, and 149), but an interesting series of events takes place at the interatrial partition which assures an adequate supply of blood reaching the left atrium and thence the left ventricle.
Fio. 145. Reconstruction of the heart of a 9.4 mm. pig embryo cut open somewhat to the right of the mid-line to show its internal structure (cf. Fig. 146).
Fig. 146. Section (X 45) through the heart of a 9.4 mm. pig embryo, at the level of the atrio-ventricular canals. (From the series used in making the reconstruction appearing as figure 145. This section may be oriented as passing horizontally through Fig. 145 at the level of the most caudal portion of the interventricular foramen.)
Just when it appears that the septum primum is going to fuse with the endocardial cushions of the atrio-ventricular canals, closing the interatrial foramen primum and isolating the left atrium, a new opening is established. The more cephalic part of the septum primum ruptures to form the interatrial for amen secundum, thus keeping a route open from the right to the left atrium (Fig. 148).
At about this time the second interatrial partition makes its appearance just to the right of the first. Also crescentic in form, this septum secundum extends its apices along the dorsal and ventral walls of the atrium to fuse with the septum primum near the endocardial cushion mass which now completely divides the atrio-ventricular canals. But the septum secundum never becomes a complete partition. An oval opening of considerable size persists in its center. This is the foramen ovale (Fig. 149).
Fig. 147. Drawings showing the initial steps in the partitioning of the heart.
A, Slightly schematized drawing from reconstruction of heart of 3.7 mm. pig embryo. The heart has been opened by a diagonally frontal cut in the plane which can be indicated by drawing a line through the labels At. and Vent, in figure 143, D. The dorsal portion of the heart is shown viewed from the ventral side.
B, Slightly schematized drawing from a reconstruction of the heart of a 6 mm. pig embryo. The heart has been opened in a plane which can be indicated by drawing a line through the labels At, and Vent, in figure 143, E. The dorsal portion of the heart is shown viewed from the ventral side.
Fig, 148. Slightly schematized drawing from a reconstruction of the heart of a 9.4 mm. pig embryo. Dorsal part of heart, interior view. The plane in which the heart is opened can be indicated by drawing a line through the center of the interatrial and the interventricular foramina in figure 145.
The newly established septum secundum and the flap-like remains of the septum primum constitute an efficient valvular mechanism between the two atria. When the atria are filling, some of the blood discharged from the venae cavae into the right atrium can pass freely through the foramen ovale by merely pushing aside the flap of the septum primum. The inferior caval entrance lies adjacent to, and is directed straight into, the orifice of the foramen ovale (Fig. 149). Consequently it is primarily — some think exclusively — blood from the inferior vena cava which passes through the foramen ovale into the left atrium. When the atria start to contract, pressure of the blood within the left atrium forces the flap of the septum primum against the foramen ovale, effectively closing it against return flow into the right atrium. Without some such mechanism affording a supply of blood to its left side, the developing heart could not be partitioned in the mid-line ready to assume its adult function of pumping two separate blood streams.^
Fig. 149. Schematic drawing based on dissected heart of pig fetus shortly before birth. Interior aspect of dorsal part of heart to show valvular mechanism at foramen ovale.
While alf these changes have been going on in the main part of the heart, the truncus arteriosus has been divided into two separate channels. Reference has already been made to the start of this process in the aortic root between the fourth and sixth arches (Fig. 133). Continuing toward the ventricle, the division is effected by the formation of longitudinal ridges of plastic young connective tissue of the same type as that making up the endocardial cushions of the atrioventricular canal. These ridges, called truncus ridges^ bulge progressively farther into the lumen of the truncus arteriosus and finally meet to separate it into aortic and pulmonary channels. ^ (Note shape of lumen in Figs. 142, F, and 145.) The semilunar valves of the aorta, and of the main pulmonary trunk (Fig. 138), develop as local specializations of these truncus ridges. Toward the ventricles from the site of formation of the semilunar valves the same ridges are continued into the conus of the ventricles. The fact that proximal to the valves these ridges are, for descriptive purposes, called conus ridges should not be allowed to obscure their developmental and functional continuity with the truncus ridges. The truncus and conus ridges follow a spiral course such that where the conus ridges extend down into the ventricles they meet and become continuous with the interventricular septum. The right ventricle then leads into the pulmonary channel and the left into the aorta. With this condition established the heart is completely divided into right and left sides except for the interatrial valve which must remain open throughout fetal life, until, after birth, the lungs attain their full functional capacity and the full volume of the pulmonary stream passes through them to be returned to the left atrium.
- There are on record a few cases of a peculiar cardiac malformation in which this valvular communication was prematurely closed and the left atrium thereby shut off from the right. In all these cases the left side of the heart has not been sufficiently developed to support life for any length of time after birth.
This leaves but one of the exigencies of heart development still to be accounted for. If, during early fetal life, before the lungs were well developed, the pulmonary channel were the only exit from the right side of the heart, the right ventricle would have an outlet inadequate to develop its pumping power. For it is only late in fetal life that the lungs and their vessels develop to a degree which prepares them for assuming their postnatal activity, and the power of the heart muscle must be built up gradually by continued functional activity. This situation is met by the ductus arteriosus leading from the pulmonary trunk to the aorta (Fig. 138). The right ventricle is not unprepared for its adult function because it pumps its full share of the blood throughout fetal life. Instead of all going to the lungs, however, part of the blood pumped by the right ventricle passes by way of the ductus arteriosus into the aorta (Fig. 150). As the lungs increase in size, relatively more blood goes to them and relatively less goes through the ductus arteriosus. By the time of birth enough blood is passing through the lungs to support life, and within a short time after birth, under the stimulus of functional activity, the lungs are able to take all the blood from the right side of the heart and the ductus arteriosus is gradually obliterated. The ductus arteriosus, therefore, serves during intra-uterine life as what might be called a ‘‘compensated exercising channel†for the right ventricle.
Changes in the Sinus Region
In following the story of the development of the heart from the functional standpoint, many things of less striking significance have been passed over. Some of these should have a word of comment. Cephalic to the sinus orifice the valvulae venosae which guard it against return flow fuse and are prolonged onto the dorsal wall of the atrium in the form of a ridge called the septum spurium (false septum). This septum is, for a time, too conspicuous to ignore, but it is of little importance in the partitioning of the heart and soon undergoes retrogression.
As the heart grows it absorbs the sinus venosus into its walls so that eventually the anterior and posterior venae cavae and the coronary sinus all open separately into the right atrium (cf. Figs. 147, 148, and 149). Portions of the right valvula venosa are retained as the valves of the caval and coronary orifices. In the adult heart a small external sulcus can usually be found between the entrance of the anterior and the posterior vena cava which records the old line of demarcation between sinus venosus and atrium.
The Atrio-ventricular Valves and the Papillary Muscles. At the point where the atrio-ventricular canals open into the ventricles there are early indications of the establishment of valves. From the partition in the atrio-ventricular canal and from the outer walls on either side, masses of tissue in the shape of thick, blunt flaps project toward the ventricle (Figs. 146 and 148). It is these masses of a primitive type of connective tissue, similar to that in the endocardial cushions of the canal, which later become differentiated into the flaps of the adult valves (Fig. 149). The papillary muscles and tendinous cords which, in the adult, act as stays to these valves, arise by modification of soine of the related trabeculae carneae (cf. Figs. 146, 148, and 149).
The Aortic and Pulmonary Valves
The valves which guard the orifices of the aorta and of the pulmonary artery arise from mesenchymal pads (Fig. 138) already mentioned as developing in connection with the truncus septum. In early histological appearance and in the manner in which this loose tissue gradually becomes organized into the exceedingly dense fibrous tissue characteristic of adult valves, they are similar to the atrio-ventricular valves. They do not, however, develop any supporting strands comparable to the papillary muscles and tendinous cords.
Course and Balance of Blood Flow in the Fetal Heart
All the steps in the partitioning of the embryonic heart lead gradually toward the final adult condition in which the heart is completely divided into right and left sides. Yet from the nature of its living conditions it is not possible for the tetus in utero fully to attain the adult type of circulation. The plan of the completely divided circulation is predicated on lung breathing. In the adult the right side of the heart receives the blood returning from a circuit of the body and pumps it to the lungs where it is relieved of carbon dioxide and acquires a fresh supply of oxygen. The left side of the heart receives the blood that has just passed through the lungs and pumps it again through ramifying channels to all the tissues of the body. In the fetus the function of respiration is carried out in the placenta by interchange with the maternal blood circulating through the uterus. The lungs, although in the last part of fetal life they are fully formed and ready to function, cannot actually begin their work until after birth. The radical change which must inevitably take place immediately following birth in the manner in which the blood is oxygenated has led to a widespread belief that there must be revolutionary changes in the routing of blood through the heart and great vessels. However, as the embryology of the circulatory system has been studied more closely from a functional angle it is becoming increasingly clear that the heart and the major vascular channels develop in such a manner that the pumping load on the different parts of the heart remains balanced at all times during fetal life. Moreover, the very mechanisms which maintain this cardiac balance during intra-uterine life are perfectly adapted to rebalance the circulatory load on the new postnatal basis without involving any sudden overloading of previously inactive parts of the vascular system.
To understand the changes in circulation which are so smoothly accomplished at the time of birth it is necessary to have clearly in mind the manner in which the way for them has been prepared during intra-uterine life. In the foregoing account of the development of the interatrial septal complex, emphasis was laid upon the fact that at no time were the atria completely separated from each other. This permits the left atrium, throughout prenatal life, to receive a contribution of blood from the inferior cava and the right atrium by a transseptal flow which compensates for the relatively small amount of blood entering the left atrium of the young embryo by way of the pulmonary circuit, and maintains an approximate balance of intake into the right and left sides of the heart.
The precise manner in which this transseptal flow occurs, and where, and to what extent the various blood streams of the fetal circulation are mixed has long been a controversial subject. The recent brilliant work of Barcroft and Barron and their co-workers has gone far toward putting some of these old controversies into proper perspective. Their first approach was through the quantitative analysis of blood samples drawn from various critical parts of the fetal circulation. The oxygen content of such samples has given important evidence as to what mixing of the currents is actually taking place in the living fetus. Later work involving the collaboration of Barclay and Franklin utilized serial x-ray photography following the injection of opaque material into the blood stream at various points. This method has given further direct evidence as to the course followed by some of the important blood currents. Synthesizing the most significant of the anatomical evidence with the newer experimental evidence, the course followed by the blood in passing through the fetal heart may be summarized somewhat as follows. The inferior caval entrance is so directed with reference to the foramen ovale that a considerable portion of its stream passes directly into the left atrium (Figs. 149 and 150). Under fluctuating pressure conditions — say following uterine contractions which send a surge of placental blood through the umbilical vein — the placental flow may temporarily hold back any blood from entering the circuit by way of either the portal vein or the inferior caval tributaries (Fig. 150). For a time, under these conditions, the left atrium would be charged almost completely with fully oxygenated blood. Such conditions, however, would be but temporary and would be counterbalanced 6y periods when the portal and systemic veins poured enough blood into the common channels to load the heart for a time with mixed or depleted blood. The ipiportant thing physiologically is not the fluctuations, but the maintenance of the average oxygen content of the blood at adequate levels.
Compared with conditions in adult mammals, the mixing of oxygenated blood freshly returned from the placenta with depleted blood returning from a circuit of the body may seem inefficient. But this is a one-sided comparison. The fetus is an organism in transition. Starting with a simple ancestral plan of structure and living an aquatic life, it attains its full heritage but slowly. It must be viewed as much in the light of the primitive conditions from which it is emerging as in comparison with the definitive conditions toward which it is progressing. Below the bird-mammal level circulatory mechanisms with partially divided and undivided hearts and correspondingly unseparated blood streams meet all the needs of metabolism and growth. Maintenance of food, oxygen, and waste products at an average level which successfully supports life does not depend on pure currents,†although such separated currents undoubtedly make for higher efficiency in the rate of interchange of materials. From a comparative viewpoint, the fact that the mammalian fetus is supported by a mixed circulation seems but natural.
Another significant fact is that careful measurements have shown that the interatrial communication in the heart of the fetus at term is considerably smaller than the inferior caval inlet. This would mean that the portion of the inferior caval stream which could not pass through this opening into the left atrium would eddy back and mix with the rest of the blood in the right atrium.
One of the most important inferences as to the fetal circulation based on vessel size is that the circulation through the lungs in a fetus which is sufficiently mature to be viable is of considerable volume. This too has now been supported by experimental work. From the standpoint of smooth postnatal circulatory readjustments, the larger the fetal pulmonary return becomes the less will be the balancing transatrial flow, and the less will be the change entailed by the assumption of lung breathing. Very early in development, before the lungs have been formed, the pulmonary return is negligible and the flow from the right atrium through the interatrial ostium primum constitutes practically the entire intake of the left atrium. After the ostium primum is closed and while the lungs are but little developed, flow through the interatrial ostium secundum must still be the major part of the blood entering the left atrium. During the latter part of fetal life the foramen ovale in septum secundum becomes the transseptal route. As the lungs grow and the pulmonary circulation increases in volume, a progressively smaller proportion of the left atrial intake comes by way of the foramen ovale and a progressively larger amount from the vessels of the growing lungs.
The balanced atrial intake thus maintained implies a balanced ventricular intake, and this in turn implies a balanced ventricular output. Although not in the heart itself, we have seen that there is in the closely associated great vessels a mechanism which affords an adequate outlet from the right ventricle during the period when the pulmonary circuit is developing. When the pulmonary arteries are formed from the sixth pair of aortic arches, the right sixth arch soon loses its original connection with the dorsal aorta. On the left, however, a portion of the sixth arch persists as a large vessel connecting the pulmonary artery with the dorsal aorta (Fig^. 138 and 150). This vessel, already familiar to us as the ductus arteriosus, remains open throughout fetal life and acts as a .shunt, carrying over to the aorta whatever excess of blood the pulmonary vessels at any particular phase of their development are not prepared to receive from the right ventricle. As has already been pointed out, the ductus arteriosus can be called the “exercising channel†of the right ventricle because it makes it possible for the right ventricle to carry its full share of work throughout development and thus to be prepared for pumping all the blood through the lungs at the time of birth.
VII. The Changes in the Circulation Following Birth
The two most obvious changes which occur in the circulation at the time of birth are the abrupt cutting off of the placental blood stream and the immediate assumption by the pulmonary circulation of the function of oxygenating blood. One of the most impressive things in embryology is the perfect preparedness for this event which has been built into the very architecture of the circulatory system during its development. The shunt at the ductus arteriosus, whic ii has been one of the factors in balancing ventricular loads throughout development, and the valvular mechanism at the foramen ovale, which has at the same time been balancing atrial intakes, are perfectly adapted to effect the postnatal rebalancing of the circulation. The closure of the ductus arteriosus is the primary event and the closure of the foramen ovale follows as a logical sequel.
It has long been known that the lumen of the ductus arteriosus is gradually occluded postnatally by an overgrowth of its intimal tissue. This process in the wall of the ductus is as characteristic and regular a feature of the development of the circulatory system as the formation of the cardiac septa. Its earliest phases begin to be recognizable in the fetus as the time of birth approaches, and postnatally the process continues at an accelerated rate to terminate in complete anatomical occlusion of the lumen of the ductus about six to eight weeks after birth, Barcroft, Barclay, and Barron have conducted an extensive series of experiments on animals delivered by Cesarean section which, indicate that the ductus arteriosus closes functionally far sooner than it does anatomically. Following birth there appears to be a contraction of the circularly disposed smooth muscle in the wall of the ductus which promptly reduces the flow of blood through it. This reduction in the shunt from the pulmonary circuit to the aorta, acting together with the newly assumed respiratory activity of the lungs themselves, aids in raising the pulmonary circulation promptly to full functional level. At the same time the functional closure of the ductus by muscular contraction paves the way for the ultimate anatomical obliteration of its lumen by overgrowth of intimal connective tissue. This concept of the immediate functional closure of the ductus is so appealing on theoretical grounds that a little extra caution in evaluating the evidence is indicated. It should be borne in mind that an initial tendency on the part of the circular smooth muscle of the ductus to contract does not necessarily imply a contraction sufficiently strongly and steadily maintained to shut off all blood flow during the six to eight weeks occupied by morphological closure. The dramatic quality of an immediate muscular response should not cause us to forget the importance of the slower but more positive structural closure.
The results of increased pulmonary circulation with the concomitant increase in the direct intake of the left atrium are manifested secondarily at the foramen ovale. Following birth, as the pulmonary return increases, compensatory blood flow from the right atrium to the left decreases correspondingly, and finally ceases altogether. This is indicated anatomically by a progressive reduction in the looseness of the valvula foraminis ovalis and the consequent diminution of the interatrial communication to a progressively narrower slit between the valvula and the septum. When equalization of atrial intakes has occurred, the compensating one-way valve at the foramen ovale falls into disuse, and the foramen ovale may be regarded as functionally closed.
Anatomical obliteration of the foramen ovale follows leisurely in the wake of its functional abandonment. There is a considerable interval following birth before the septum primum fuses with the septum secundum to seal the foramen ovale. This delay is, however, of no import because as long as the pulmonary circuit is normal and pressure in the left atrium does not fall below that in the right, the orifice between them is functionally inoperative. It is not uncommon to find the fusion of these two septa incomplete in the hearts of individuals who have, as far as circulatory disturbances are concerned, lived uneventfully to maturity. Such a condition can be characterized as ‘‘probe potency’’ of the foramen ovale. When, in such hearts, one inserts a probe under the margin of the fossa ovalis and pushes it toward the left atrium one is, so to speak, prying behind the no longer used, but still unfastened, interatrial door.
With birth and the interruption of the placental circuit there follows the gradual fibrous involution of the umbilical vein and the umbilical arteries. The flow of blood in these vessels, of course, ceases immediately with the severing of the umbilical cord, but obliteration of the lumen is likejy to take from three to , five weeks, and isolated portions of these vessels may retain a vestigial lumen for much longer. Ultimately these vessels are reduced to fibrous cords. The old course of the umbilical vein is represented in the adult by the round ligament of the liver extending from the umbilicus through the falciform ligament, and by the ligamentum venosus within the substance of the liver. The proximal portions of the umbilical arteries are retained in reduced relative size as the hypogastric or internal iliac arteries. The fibrous cords extending from these arteries on either side of the urachus toward the umbilicus represent the remains of the more distal portions of the old umbilical arteries. They are known in the adult as the obliterated branches of the hypogastric arteries, or as the lateral umbilical ligaments.
Much yet remains to be learned as to the more precise physiology of the fetal circulation and as to the interaction of various factors during the transition from intra-uterine to postnatal conditions. Nevertheless, with our present knowledge it is quite apparent that the changes in the circulation which occur following birth involve no revolutionary disturbances of the load carried by different parts of the heart. The fact that the pulmonary circulation is already so well developed before birth means that the changes which must occur following birth are far less profound than was formerly believed ; and the compensatory mechanisms at the foramen ovale and the ductus arteriosus which have been functioning all during fetal life are entirely competent to effect the final postnatal rebalancing of the circulation with a minimum of functional disturbance. It is still true that as individuals we crowd into a few crucial moments the change from water living to air living that in phylogeny must have been spread over eons of transitional amphibious existence. But as we learn more about this change in manner of living, it becomes apparent that we should marvel more at the completeness and the perfection of the preparations for its smooth accomplishment, and dwell less on the old theme of the revolutionary character of the changes involved.
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Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Embryology of the Pig 11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_11
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