Book - Aids to Embryology (1948) 10
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter X The Circulatory System
The developing blood vessels that are first seen are extra -embryonic in position ; they are the vessels of the yolk sac, chorion, and body stalk. Isolated masses and cords of mesodermal angioblastic tissue, known as blood islands, become hollowed out. The peripheral cells become transformed into endothelium while the central ones are the primitive blood cells. By growth and extension these primitive angioblastic foci become linked up thus giving rise to a plexus of extra-embryonic blood vessels. This process occurs during the latter part of the third week of development and at the beginning of the fourth week connections are established with the intra-embryomc vessels. It was formerly thought (His, 1900) that the latter were formed from extensions of the extra-embryonic vessels into the embryo, but it is now generally held that the vessels in the body of the embryo originate from local mesodermal foci an.d secondarily link up with each other.
The heart arises in the cardiogenic area which lies in the splanchnopleuric mesoderm at the anterior end of the embryonic disc. This mesoderm forms the floor of the anterior end of the horseshoe-shaped coelom and a pair of endothelial tubes arise in it ; these then fuse to form a single heart tube. The formation of the head fold of the embryo (see Fig. 9) brings this heart tube to a position just ventral to the developing fore-gut and slung from the roof (formerly floor) of the anterior part of t e coelom which may now be termed the primitive pericardial cavity. The vitelline veins from the yolk sac pass into the septum transversum where tney become connected with the caudal ends of the fusing heart tubes, and the terminal parts of these soon receive the umbilical veins and the common cardinal veins so forming the sinus venosus. Cramally the heart tubes are continued into vessels lying in t e first branchial arch and so pass dorsally to the dorsal aortse lying ventro-lateral to the neural tube.
When the endothelial heart tubes fuse, the splanchnopleuric mesoderm around them is called the myo-epicardial mantle. This is connected for a short time with the dorsal wall of the pericardium y tne dorsal mesocardium, but this soon breaks down an the passage thus formed, dorsal to the heart tube, is the rudiment of the transverse sinus of the pericardium.
Differential growth of the heart tube now takes place and four regions, separated by grooves, may be distinguished. From the caudal end forwards they are, in order, the sinus venosus with its right and left horns into which the veins of the corresponding side open, the primitive atrium, the primitive ventricle and lastly the bulbus cordis. The bulbus cordis leads into a dilatation (not part of the heart tube) called the aortic sac, from which the branchial arch arteries pass dorsally around the pharynx to the dorsal aorta (p. 109).
Fig. 24. - The Development of the Bulbo-ventricular Loop.
1, Bulbus cordis ; 2, ventricle ; 3, atrium ; 4, sinus venosus.
The arrows indicate the direction of rotation of the loop.
Growth of the heart tube causes it to form a loop, convex ventrally, in the pericardial cavity, and as this growth continues the bulbo-ventricular portion of the tube, which is the most mobile portion, swings caudally and to the left, ventral to the primitive atrium (see Fig. 24). During this .process the sinus venosus is to some extent pulled cranially out of the substance of the septum transversum. The primitive atrium, now grows rapidly. As it is limited dorsally by the developing pharynx and ventrally by the bulbus cordis, it balloons out on either side of the bulbus into right and left atria. These, of course, communicate freely with each other. The sinus venosus, which extends in a transverse direction in the tubular heart, is pulled cranially, as was mentioned above, during the formation of the bulbo-ventricular loop. At the same time changes (p. 124) in the venous channels returning blood to the sinus venosus result in most of it being diverted to enter the right horn which enlarges. The left horn becomes correspondingly reduced in size and the sinus now opens into the right part of the common atrial chamber upon its dorsal wall. The opening is vertical and guarded by the right and left venous valves. A single pulmonary vein has developed and it opens into the left part of the common atrium. The left bulbo-ventricular groove becomes much reduced and so some of the cavity of the bulbus is incorporated in the ventricle. This chamber (ventricle) is separated from the atrium by a constriction, the atrio-ventricular canal, in which ventral and dorsal swellings, the endocardial cushions, still further narrow the lumen.
Formation of the Interatrial Septa
The right and left venous valves guard the opening of the sinus venosus into the right part of the common atrium at the fifth week. Their cranial extremities are fused to form the septum spurium. Their caudal ends likewise fuse into a less pronounced ridge running caudally towards the A.-V. canal. During the sixth week, a fold of the endocardium grows caudally in the sagittal plane from the roof of the atrial chamber towards the A.-V. canal. This fold is called the septum primum and its caudal free border eventually meets and fuses with the two endocardial cushions of the canal which have united with each other to divide the canal into right and left halves. In this manner the primitive atrium is separated into two chambers, but even before it has been completed the cranial part of the septum primum breaks down, and a new communication, the foramen secundum, puts the right and left atria into communication once more. This happens towards the end of the seventh week. Next, another septum grows caudally from the atrial wall. This is the septum secundum and it lies to the right of the septum primum, in the space (interseptovalvular) between it and the left venous valve. Growth caudally takes place until the free concave border of the septum secundum overlaps the cranial border of the septum primum. The cleft between the two free edges of these septa is now known as the foramen ovale, and the two flaps of tissue are obviously arranged so that oxygenated blood reaching the right atrium from the inferior vena cava during foetal life readily passes through it to the left atrium. After birth the pulmonary circulation is established, pressure rises in the left atrium and the two septa are pressed together and eventually adhere. The annulus ovalis of the adult heart represents, then, the free edge of the septum secundum while the septum primum forms the membranous fossa ovalis.
Fig. 25. - The Formation of the Four Chambers of the Heart.
1, Sinus venosus with its valves ; 2, right atrium ; 3, septum primum ; 4, endocardial cushion ; 5, right ventricle ; 6, interventricular septum ; 7, left ventricle ; 8, left atrium ; 9, foramen secundum.
The left venous valve eventually fuses with the right aspect of the interatrial septum. The upper part of the right venous valve, together with the septum spurium, is converted into the crista terminalis while the lower part of it is represented in the adult heart by the valves of the inferior vena cava and of the coronary sinus.
Further Development of the Atria
Reference was made earlier to changes which resulted in the transfer of much of the venous return to the right horn of the sinus venosus, and as a consequence, how the left horn of the sinus became diminished in size. It persists in the adult as the coronary sinus. The right horn of the sinus has opening into it the right common cardinal vein and a new formation, the inferior vena cava (p. 122). With further development of the right atrium the sinus venosus and its right horn are taken up and incorporated in it. The right common cardinal vein (the proximal part of the superior vena cava) and the inferior vena cava acquire thus separate openings into the chamber, but these are, however, posterior to the right venous valve remnants, the crista terminalis, and the valve of the inferior vena cava. That part of the chamber derived from the primitive atrium is characterized by the presence of musculi pectinati and forms mainly the auricular appendage.
A single pulmonary vein opens into the left atrium in early development, but later the proximal portion of this and its first two tributaries become taken up into the wall of the expanding atrium and help to form its cavity. Normally four pulmonary openings are found in the left atrium.
The Interventricular Septum
The interventricular septum begins as a ridge which grows upwards from the floor of the bulbo-ventricular loop to meet and fuse with the dorsal endocardial cushions of the A.-V. canal near its right side. The ventral extremity of the septum extends to the ventral endocardial cushion near its left end. A foramen (interventricular) persists for some time between the two ventricles cranial to the free border of the interventricular septum and this is eventually closed in the eighth week by the fusion of the proliferations from the right and left bulbar ridges (see below) and the fused endocardial cushions of the A.-V. canal with the free border of the interventricular septum. The pars membranaceae septi marks this place in the adult heart.
Formation of the Aortic and Pulmonary Trunks
During the fifth week longitudinal endocardial thickenings appear in the bulbus cordis. Proximally these are situated on the right and left walls of the bulbus and when traced distally towards the aortic sac they pursue a spiral anti-clockwise course and then fade out. These two ridges fuse during the eighth week so giving rise to a spiral aorticopulmonary septum dividing this portion of the heart tube into aorta and pulmonary trunks. Closure of the interventricular foramen puts each of these vessels into communication with the appropriate ventricle.
The Valves of the Heart
The atrio-ventricular valves arise from sub-endocardial proliferations of tissue at the A.-V. canals. These become excavated or hollowed out on the ventricular aspect in such a manner that strands of muscle tissue, which are later transformed into chordae tendineae and papillary muscles, connect them with the ventricular wall. There are three proliferations developed in the right A.-V. canal, and two in the left, and these form the cusps of the tricuspid and mitral valves.
The semilunar valves of the aorta and pulmonary artery are found in the distal part of the bulbus cordis. Two accessory thickenings form here between the main bulbar swellings. When the main swellings fuse and the aortico-pulmonary septum is formed, each resultant channel contains three swellings which become excavated on their distal aspect to form the semilunar valve cusps.
The Atrio-ventricular Bundle
The myo-epicardial mantle gives origin to the heart muscle which is at first continuous throughout the heart. A ring of fibrous tissue is developed at the A.-V. canal and so the continuity of the primitive cardiac muscle is interrupted except at one place just dorsal to the dorsal endocardial cushion. This persisting band of tissue forms the atrio-ventricular bundle of His.
Summary of the Development of the Heart
- The heart is formed by the fusion of two endothelial tubes which arise in the ^ splanchnic meso derm of the cardpo g enic ar ea at the anterior end of the embryonic disc. Folding of the head end of the embryo brings this heart tube ventral to the fore-gut in the roof of the pericardial coelom.
- The primitive veins (vitelline, umbilical, and common cardinal) open into a caudally placed sinus venosus segment of the heart tube ; the branchial arcETarteries lead away from the cephalic end to the dorsal aorta.
- The tube is fixed at it s cepha lic and cajidal ends, and between these points it "bulges ventrally to form an S-shaped loop.
- The ventricular portion of the tube at first grows more rapidly than the atrial, and as a result it falls caudally ventral to that segment.
- The atrial portion grows laterally and ventrally forming a dilatation on each sid^ of the bulbus cordis, the right and left atrium.
- The primitive atrium is partitioned into right and left chambers by the se ptum primu m and the septum secundum. The A.-V. canal is subdivided into two by the fusion of dorsal and ventral endocardial cushions. The septum primum fuses with the cranial aspect of these.
- The proximal part of the bulbus cordis is absorbed into the primitive ventricle. The remainder becomes split into aorta and pulmonary artery by the union of spirally running bulbar ridges.
- The ventricle is subdivided into two by a primitive interventricular septum aided by growth of tissue from the caudal aspect of the endocardial cushions and the proximal ends of the bulbar ridges.
- During foetal life the right and left atria communicate by way of the foramen ovale, which lies between the free edges of septum primum and septum secundum.
Anomalies of Development of the Heart
- "Double heart" is a rare condition due to incomplete fusion of the paired primordia of the heart.
- Ectopia cordis is another rare condition where the heart protrudes through a cleft in the ventral thoracic wall. It is probably due to failure of the lateral body folds to meet and fuse in this region.
- Dextrocardia may occur alone or as part of general situs inversus. The arteries and veins are found on the opposite side to the normal, and the heart apex is directed to the right. The aortic and pulmonary trunks may be transposed if the aorticopulmonary septum rotates in the reverse direction to the normal.
- Persistent foramen ovale is due to defective development of the interatrial septum.
- Persistent interventricular foramen is due to failure of development of the upper part of the interventricular septum. If the interventricular septum is not developed at all, a cor triloculare results.
- Stenosis or atresia of the aorta or pulmonary artery result from excessive development of the aortico-pulmonary septum. They are often associated with other defects such as patent interventricular foramen.
The Vascular System
The primitive vascular system has already been described (p. 99) as arising in the extra-embryonic mesoderm from blood islands which coalesce and form a plexus of vessels from which, main, channels develop, and these become continuous with the caudal end of the primitive tubular heart. Within the embryo a fine plexus develops in the embryonic mesoderm and this plexus becomes continuous with the cephalic end of the heart. The main blood vessels of the embryonic body differentiate from this plexus in response to various factors. At first these vessels are simple tubes of endothelium ; differentiation into definitive arteries and veins occurs later but the vessels may be named according to their relationships. The first vessels to appear in mammals are the aortic arches (branchial arch arteries) which arise from a bulbous aortic sac lying ventral to the pharyngeal floor cranial to, and connected with, the bulbus cordis. These arches, of which six pairs appear in succession, run dorsally around the lateral aspect of the pharynx and are connected with the dorsal aortae. These pass obliquely caudo-medially to fuse at an early date, forming a single dorsal aorta. The dorsal aorta; give off various branches in their course caudalwards, prominent among which are vitelline arteries to the yolk sac and a large umbilical branch on each side to the body stalk and thence to the chorion. A series of modifications of the aortic arches gives rise to the definitive vessels of the head end of the embryo and these will now be described.
The Branchial Arch Arteries
Six pairs of branchial arch arteries are normally developed in the human embryo, and of these, the fifth pair is rudimentary. As for the others, they are never all present at the same time. The first two pairs regress at an early stage leaving only unimportant mandibular and stapedial vessels to represent them. The distal portion of each third arch artery together with the dorsal aorta extending cranially becomes the internal carotid artery. The cephalic extremity of each grows alongside the developing brain where an ophthalmic branch is given off to the optic vesicle, and anterior and middle cerebral branches to the brain. It terminates as the posterior communicating artery turning caudally to link up with the developing basilar. The external carotid arteries arise as new vessels which bud out part of the way along the third arch vessel. The proximal part of the third arch artery between this bud and the aortic sac becomes the common carotid artery.
By this time the current of blood in the third arch is directed towards the head end of the embryo, and the blood of the fourth arch caudally, so the segments of the dorsal aortae between the third and fourth arches disappear. The fourth pair of arch vessels have a different fate on the two sides. That of the right side forms the proximal part of the right subclavian artery. The distal part of the adult subclavian represents the seventh right cervical intersegmental artery (p. 113) which has migrated along the dorsal aorta to the place where the fourth arch artery connects with it. The right dorsal aorta between the subclavian and the common dorsal aorta disappears. The aortic sac becomes drawn out into right and left limbs, the right one elongating to form the innominate artery. The left one, together with the aortic sac and the left fourth arch artery, forms the arch of the adult aorta.
From the sixth arches a pair of capillary plexuses arise which extend into the developing lung buds. On the right side, distal to the capillary outgrowth, the arch degenerates and disappears so that the proximal part, and the outgrowth, form the right pulmonary artery and its branches. On the left side, that part of the arch distal to the capillary outgrowth persists as the ductus arteriosus. The inner part of the arch forms the proximal part of the left pulmonary artery, which remains therefore in communication with the left dorsal aorta through the ductus. When the pulmonary circulation becomes established after birth the ductus arteriosus contracts down and is obliterated. It is represented in the adult by the fibrous ligamentum arteriosum.
Fig. 26. - Scheme to show the Vessels Derived from the: Branchial Arch Arteries. (Adapted from Congdon.)
1, Internal carotid artery ; 2, external carotid artery ; 3, common carotid artery ; 4, arch of the aorta ; 5, ductus arteriosus ; 6, left subclavian artery ; 7, pulmonary artery. The definitive arteries are printed in solid black, the transitory portions in outline.
It may now readily be understood why the right and left recurrent laryngeal nerves differ in their course and relations. Initially these nerves arise from their respective vagal trunks immediately caudal to the sixth branchial arch vessel. The degeneration of the lateral part of this vessel on the right side, and the elongation of the structures of the neck cause this nerve eventually to loop around the caudal aspect of the persisting right fourth arch vessel, that is, the subclavian artery, The persistence of the distal part of the left sixth arch artery as the ductus arteriosus means that the left recurrent laryngeal nerve must retain its primitive relationship in hooking around the ligamentum arteriosum of the adult.
The Carotid Sinus and Carotid Body
The third branchial arch artery has associated with it a pressorreceptor mechanism, the carotid sinus, which is supplied with sensory fibres from the nerve of the arch, the glosso-pharyngeal. Similar mechanisms are developed in relation to the fourth arch vessels, the arch of the aorta and the right subclavian artery. These are innervated by the fourth arch nerve, the superior laryngeal branch of the vagus. There is reason to believe that the sixth arch vessels (pulmonary arteries and ductus arteriosus) may also develop a mechanism of this nature. All these are influenced by changes in the blood pressure.
Closely related to these pressor-receptors there develop chemo-receptors such as the carotid body and the aortic arch bodies. The carotid body develops primarily as a condensation of mesoderm around the third branchial arch artery (Boyd, 1937). This condensation is later invaded by neuroblasts which are transformed into chemo-receptor cells, and the structure is supplied by the glosso-pharyngeal.
Branches of the Dorsal Aorta
From the entire length of the definitive dorsal aorta, branches arise which may be arranged in three groups : (a) intersegmental dorso-lateral branches to the body wall and neural tube ; (b) lateral branches to the derivatives of the intermediate cell mass of the mesoderm (p. 19) ; (c) ventral branches to the gut and its associated structures.
The Intersegmental Arteries
These vessels are branches of the dorsal aorta which lie between the somites. Running laterally each divides into a dorsal and a lateral ramus. The dorsal rami give rise to spinal branches supplying the meninges and the cord. The lateral rami behave differently in the various regions of the body. In the thoracic region they give rise to the intercostal arteries ; in the lumbar region the upper four represent the four lumbar arteries, but the fifth of the series becomes much enlarged and develops into the common iliac artery. Longitudinal anastomoses between the successive intersegmental arteries are formed in pre-costal, post-costal, preneural, and post-neural situations. The position of these is indicated in Fig. 27, and they are of importance in forming certain vessels in the neck, such as the vertebral artery. This vessel has a composite origin as follows :
(a) The dorsal branch of the seventh cervical intersegmental artery forms that part of it from the origin to the foramen in the tranverse process of the sixth cervical vertebra.
(b) Enlargement of the post-costal anastomoses from the sixth to the first cervical segments, with regression of the stems of the upper six cervical vessels, gives rise to the second part of the vertebral.
(c) The spinal branch of the first cervical intersegmental artery enlarges and forms that portion of the vertebral artery which lies on the arch of the atlas.
(d) The neural division of this spinal branch with the pre-neural anastomosis then forms that portion of the vertebral artery which lies in the cranial cavity and fuses with its fellow of the opposite side to form the basilar.
Fig. 27. - Scheme to show the Arrangement of the Intersegmental Arteries.
1, Post-neural anastomosis ; 2, pre-neural anastomosis ; 3, dorsal aorta ; 4, post-costal anastomosis ; 5, pre-costal anastomosis ; 6, ventral anastomosis.
The main ventral continuations of the segmental arteries follow the course of the anterior primary divisions of the corresponding nerves, and form ventral anastomotic chains on each side of the middle line in the trunk region. From these chains there are derived the internal mammary and the superior and inferior epigastric arteries.
The Lateral Branches of the Aorta
These vessels supply the derivatives of the intermediate cell mass and at first these structures extend along a great length of the body. The vast majority of the vessels disappear, but some persist as the renal, spermatic or ovarian, suprarenal, and inferior phrenic arteries.
The Ventral Branches of the Aorta
The primitive ventral branches of the dorsal aorta are paired and pass to the yolk sac and the mesoderm of the body stalk surrounding the allantois. With fusion of the dorsal aortse the yolk sac (vitelline) vessels become median in position and are eventually reduced to three in number. These represent the cceliac axis, the superior mesenteric, and the inferior mesenteric arteries of adult anatomy. They are the original branches of the seventh cervical, third thoracic, and fifth thoracic segments respectively, which have migrated from their primitive to their definitive adult positions by a progressive subaortic anastomosis with successive new caudal stems. By similar migration the umbilical arteries move caudally from their original position, and when in the lower lumbar region they establish connections with the fifth lumbar intersegmental vessels. The portion of the umbilical proximal to this anastomosis now disappears and the vessel appears to take origin from the fifth lumbar artery which is converted into the common iliac. The external and internal iliac arteries then bud off from the umbilical, and by a process of unequal growth the umbilical later appears as if a branch from the internal vessel. When, at birth, the placental circulation is interrupted, the umbilical arteries become fibrosed and remain as the obliterated umbilical arteries. The proximal portion however remains pervious as the stem of the superior vesical artery.
Arteries of the Upper Limb
The seventh cervical intersegmental artery grows outwards into the limb bud and terminates in a capillary plexus. Later on, digital branches form from this plexus. The primitive axial vessel is divided, into two portions, a proximal, or brachial artery, and a distal, or interosseous artery. A median branch growing distally from the brachial segment connects up with the palmar vessels, and the interosseous now loses its communication with them. Ulnar and radial arteries also grow out from the brachial and connect distally with an arch formed at the proximal parts of the digital branches. With this the distal part of the median vessel regresses. The origin of the radial is at first high up in the arm. Later, a connection forms between the brachial and this superficial radial in the elbow region, and the proximal part of the latter vessel disappears. The original interosseous artery is represented by the anterior interosseous of the adult.
Arteries of the Lower Extremity
The original axial artery of the lower limb is an outgrowth from the fifth lumbar inter-segmental artery. It runs along with the sciatic nerve and hence is termed the sciatic artery. Passing distally it ends in a plexus from which digital branches arise (Fig. 28 a). A retiform plexus appears in the ventral part of the thigh and becomes connected with the sciatic vessel by a branch which represents the adult femoral artery (stage b in Fig. 28). The middle portion of the sciatic now disappears. The distal portion, lying deep to the tibialis posterior muscle, forms a connection with the new popliteal artery, which has grown distally from the femoral and lies posterior to the popliteus muscle, and so the peroneal artery is formed. A prolongation of the ventral plexus has grown distally to form the anterior tibial artery which secondarily links up with the popliteal vessel at the lower border of the popliteus, the proximal portion of the downgrowth disappearing. A superficial -descending artery growing from the popliteal now forms the posterior tibial vessel. In the adult the proximal portion of the original sciatic artery is represented by the arteria comes nervi ischiadici.
Summary of the Development of the Arteries
- Six pairs of aortic or branchial arch vessels are normally developed in the human embryo ; the fifth pair are rudimentary and soon disappear.
- The first two pairs of arches degenerate and disappear.
- Each third arch and the dorsal aorta leading cranially forms the common and internal carotid arteries of that side.
- The dorsal aorta between the third and fourth arches disappears on each side.
- The right fourth arch persists as the proximal portion of the right subclavian artery.
- The fourth arch of the left side forms the arch of the adult aorta.
- The proximal portion of the sixth right arch forms the right pulmonary artery. The distal part disappears.
- The proximal portion of the sixth left arch forms the left pulmonary artery, and the distal portion, as the ductus arteriosus connects it with the left dorsal aorta.
- The right dorsal aorta disappears between the level of the seventh right intersegmental artery and the place of fusion with the left one.
- The dorso-lateral intersegmental arteries supply the structures of the body wall and spinal cord ; the lateral branches of the aorta supply the kidneys and gonads ; the ventral branches become the arteries to the gut and the umbilical arteries to the placenta.
- The middle sacral artery represents the caudal portion of the fused primitive dorsal aortae.
- Each limb has a primitive axial artery. In the upper limb this is represented by the brachial and anterior interosseous arteries of the adult ; in the lower limb the arteria comes nervi ischiadici and the distal part of the peroneal artery are all of the primitive axial vessel that persist. The other vessels of the limbs a.re all later formations.
Fig. 28. - Diagrams to illustrate the Development of the Arteries of the Lower Extremity. (Modified from Senior.)
F., Femoral artery ; Sc., sciatic artery ; T., tibia ; P.M., popliteus muscle ; T.P.M., tibialis posterior muscle ; I., interosseous artery ; Po., popliteal artery ; A.T.A., anterior tibial artery ; P.T.A., posterior tibial artery.
Anomalies of Development of the Arteries
Abnormalities of the arterial system are of common occurrence and only some of the most important are mentioned here.
- Coarctation of the aorta occurs as a stenosis of the vessel where the arch joins the descending portion. The internal mammary and the epigastric arteries are greatly enlarged to form a collateral circulation.
- Double and right-sided aortic arches sometimes occur instead of the normal left-sided arch.
- Persistence of a lumen in the ductus arteriosus occurs in some instances. After birth blood will pass from the aorta along the ductus to the pulmonary artery giving an â€œarterio-venous shuntâ€. In uncomplicated cases there is no cyanosis.
- It is not uncommon to find that the right subclavian artery takes origin from the descending thoracic aorta and passes upwards and to the right behind the oesophagus. Here the fourth right arch has degenerated and that part of the right dorsal aorta caudal to the seventh cervical intersegmental artery has persisted.
The Venous System
The venous system arises from capillary plexuses in the mesoderm just as does the arterial system and from these certain channels differentiate which, in early embryos, form the following primitive veins : (a) right and left vitelline veins pass from the yolk sac wall into the septum transversum and there join the corresponding horn of the sinus venosus ; ( b ) right and left umbilical veins drain blood from the chorionic villi by way of the body stalk to the sinus venosus ; (c) two pairs of veins drain the cephalic and caudal portions of the embryo and are termed the anterior and posterior cardinal veins respectively. These join on each side to form a short transverse channel, the common cardinal vein (duct of Cuvier) which terminates in the sinus venosus.
The Anterior Cardinal Veins
Each anterior cardinal vein may be divided into two parts, one lying cranially in the region of the developing brain, and one concerned with the venous drainage of the body segments above the level of the sinus venosus. The first of these two segments is situated medial to the trigeminal ganglion and is termed the primary head vein. It receives tributaries from three plexuses, anterior, middle, and posterior, situated in the loose mesoderm around the three developing brain vesicles. From these plexuses are formed the venous sinuses of the dura mater and the cerebral veins (Streeter, 1918). The three plexuses establish connections with each other, the posterior with the middle, and the middle with the anterior. This is shown in Fig. 30, A. Those parts of the primary head vein in front of, and behind the trigeminal ganglion disappear. The persisting portion medial to the ganglion forms the cavernous sinus, and this has already established a connection with the anastomosis between the middle and posterior plexuses. This connection is transformed into the superior petrosal sinus of the adult (Fig. 30, B) .
The superior sagittal sinus is formed from the dorsal parts of the two anterior plexuses, while the straight and inferior sagittal sinuses are differentiated from parts of all the plexuses which extend downwards in the mid -line between the two cerebral hemispheres. The superior sagittal and straight sinuses open into the trunk connecting the middle and posterior plexuses, which trunk ultimately becomes the transverse sinus. The inferior petrosal sinus is formed by a reconstitution of the posterior part of the primary head vein, and this, together with the termination of the developing transverse sinus, is continuous with the second part of the anterior cardinal vein. This latter portion of the anterior cardinal undergoes a series of changes which result in the formation of the internal jugular and innominate veins. The anterior cardinal veins are at first short, but they elongate as the neck develops and the heart descends in the thorax. During this elongation the segmental veins which drain into the cephalic end of the posterior cardinal become transferred to them. During the eighth week an obliquely placed cross anastomosis is formed, which passes between the left anterior cardinal close to the termination in it of the left subclavian vein, and the right anterior cardinal. This is the left innominate vein of the adult. The left anterior cardinal lying caudal to this anastomosis is partly transformed into the proximal portion of the left superior intercostal vein, and partly into a fibrous cord, the ligament of Marshall. The left common cardinal vein with the left horn of the sinus venosus persists as the oblique vein of Marshall and the coronary sinus. The portion of the left anterior cardinal vein cranial to the obliquely placed left innominate forms the left internal jugular, while on the right side that part cephalad to the entry of the subclavian forms the other internal jugular. The right innominate and part of the superior vena cava represent the most proximal part of the right anterior cardinal, while the remainder of the superior vena cava is derived from the right common cardinal. The fate of the right horn of the sinus venosus has already been described to be that it is absorbed into the wall of the right atrium of the developing heart.
Fig. 29. - The Fate of the Anterior Cardinal Veins.
1, Superior vena cava; 2, right innominate vein; 3, subclavian vein ; 4, internal jugular vein ; 5, left innominate vein ; 6, left superior intercostal vein ; 7> ligament of Marshall ; 8, oblique vein of Marshall ; 9, coronary sinus.
Fig. 30. - Diagrams to show the Development of the Intracranial Venous Sinuses from Anterior, Middle and Posterior Plexuses. (Modified from Streeter.)
i, Secondary communication between anterior and middle plexuses ; 2, secondary communication between middle and posterior plexuses ; 3, portion of the primary head vein which disappears, and is later reconstituted as the inferior petrosal sinus.
The external jugular vein develops from a capillary plexus in the region of the face. It secondarily forms a connection with the vein draining the upper limb, the subclavian vein.
The Posterior Cardinal Veins
These paired veins are primarily concerned with the venous drainage of the body wall and mesonephros. Each runs cranially in the urogenital folds (p. 137) to reach the septum transversum where, as already described, it unites with the anterior cardinal vein. The posterior cardinal veins are transitory structures and are soon supplemented and eventually replaced in large part by other longitudinal venous plexuses. The most important are the subcardinal, supracardinal and azygos line veins. The subsequent changes in these veins have been described differently by various writers and there is still confusion with regard to their fate.
The Inferior Vena Cava
The development of the inferior vena cava in man has been described by McClure and Butler (1925). The embryonic veins concerned in its formation are the posterior cardinals, the subcardinals and the supracardinals. The position of the posterior cardinal vein dorso-lateral to the mesonephros has already been described. The subcardinal vein is to be found along the medial aspect of the mesonephros and it is connected cranially and caudally with the corresponding posterior cardinal.
Fig. 31. - Scheme to show the Development of the Inferior Vena Cava. (Adapted from Huntingdon and McClure.)
1, Hepatic segment ; 2, pre-renal segment of subcardinal vein ; 3, renal segment of subcardinal vein ; 4, connection between sub- and supracardinal veins ; 5, posterior cardinal vein ; 6, supracardinal vein ; 7, subcardinal vein.
The supracardinal develops just lateral to the sympathetic trunk. The subcardinal veins anastomose freely with the corresponding posterior cardinals, and, during the sixth week are joined, to each other by an intersubcardinal anastomosis. The right subcardinal effects a junction with the hepatic sinusoids about the same time, and this channel becomes the hepatic portion of the inferior vena cava. The supracardinals are draining cranially into the posterior cardinals and when they are fully established, the posterior cardinals regress except in their caudal parts where a transverse anastomosis indicates the future left common iliac vein. When a connection between the right sub- and supracardinal veins now occurs just caudal to the intersubcardinal anastomosis, the tendency is for the blood to pass from the left to the right side of the body. The composite channel on the right side enlarges to form the inferior vena cava and the components of this vein are then :
- The caudal portion of the right supracardinal.
- The communication between the right supracardinal and the subcardinal.
- That portion of the right subcardinal opposite the intersubcardinal anastomosis.
- The right subcardinal cranial to the connection between it and its fellow of the opposite side.
- The communication between the right subcardinal and the hepatic sinusoids.
- The terminal part of the right vitelline vein which connects the hepatic sinusoids with the sinus venosus.
The left renal vein is formed from the intersubcardinal anastomosis. Since the subcardinals drain the genital glands their caudal parts form the testicular or ovarian veins. The azygos and hemiazygos veins are variously described. According to McClure and Butler (1925) they arise fiom the cranial portions of the supracardinals, but Reagan, in a series of papers, contends that they take origin from a pair of longitudinal venous channels (the azygos line veins) which lie medial to the sympathetic trunks. In this view, the cranial portions of the supracardinals are not represented in the adult.
Development of the Portal System
The vitelline and umbilical veins pass through the septum transversum on their way to the sinus venosus, and consequently they are brought into close association with the developing liver tissue. The vitelline veins become broken up in this part of their course into sinusoids lying between the developing columns of liver cells, and there remain caudal portions of the original veins between the liver and yolk sac, and cranial portions between the liver and the sinus venosus. Two transverse communications, cephalic and caudal, passing ventral to the primitive gut, develop between the caudal portions of the two veins. A third communication arises dorsal to the gut and midway between the other two. The right vitelline vein degenerates between the caudal and middle anastomoses, and the left one between the middle and cranial anastomoses. This results in the formation of an S-shaped spiral vitelline venous channel around this segment of the gut, the primitive duodenum. This is the portal vein. From the dorsal aspect of the middle communication a new vein is growing out and it passes in the mesentery to the gut wall. It is the superior mesenteric vein. The distal portions of the vitelline veins disappear along with the atrophy of the yolk sac duct.
Fig 32 - Three Stages in the Development of the Portal â€˜ 0 ' Vein. Right umbilical vein ; 2, right vein ; 4, left umbilical vein venosus ; 8, portal vein.
vitelline vein ; 3, left vitelline
5, gut ; 6 , liver ; 7, ductus
During the time of transformation of the vitelline veins, changes are also occurring in the umbilical veins. The segments of them that lie in the septum transversum become involved in the growing hepatic tissue and are split up into sinusoids. The proximal parts of both veins (â– i.e ., those cranial to the septum) along with the distal part of the right one, atrophy during the sixth week, and so all the blood coming from the placenta via the left vein now passes through the liver sinusoids. To cope with this relatively large volume of blood a short circuiting channel forms between the left umbilical vein where it enters the liver and the right vitelline vein. This new structure (Fig. 32, C) is the ductus venosus and it appears as the direct continuation of the left umbilical, most of the blood in which now passes direct to the inferior vena cava without entering the hepatic sinusoids.
After birth, when the placental circulation ceases, the left umbilical vein becomes impervious forming the ligamentum teres of the falciform ligament. The usefulness of the ductus venosus being ended, it, like the umbilical vein, becomes fibrosed and persists in the adult as the ligamentum venosum of the liver.
Summary of Development of Venous System
- The main intra-embryonic veins are the anterior and posterior cardinals, the subcardinals, the supracardinals and the azygos line veins. These are all paired structures.
- The posterior cardinal veins disappear except for their extreme caudal parts which are connected by a transverse channel, the primordium of the left common iliac vein.
- The subcardinal veins enter into the formation of the genital gland veins of the adult. The right one also contributes to the inferior vena cava.
- The inferior vena cava is a composite vessel formed by anastomosing portions of the right supracardinal and subcardinal veins, the latter becoming secondarily connected by an hepatic prolongation with the right vitelline vein and thus with the sinus venosus.
- A cross connection between the two anterior cardinals forms the left innominate vein.
- Each internal jugular is formed from a part of the corresponding anterior cardinal. On the right side the remainder of the anterior cardinal is represented by the right innominate and part of the superior vena cava. On the left side this segment forms part of the left superior intercostal vein and a fibrous cord, the ligament of Marshall.
- The vitelline veins form three transverse communications, one behind and two in front of the duodenal segment of the gut. An S-shaped portal vein arises by the disappearance of the left vitelline between the upper and middle communications, and the right vitelline between the middle and lower communications.
- The umbilical veins in the septum transversum become broken up by the liver tissue into sinusoids. Distal to this the right one atrophies, and a short circuit, the ductus venosus, between the left one and the inferior caval segment of the right vitelline allows the placental blood easy access to the right atrium of the heart.
Anomalies of Development of the Veins
As in the case of the arterial system abnormal veins are of relatively common occurrence.
- Two superior venae cavae may be present; they are due to the persistence of both right and left anterior cardinal veins.
- Two inferior venae cavae may be found ; they are due to persistence of the sub-supracardinal systems on the left as well as on the right side.
- The great veins may be transposed giving rise to left superior and inferior venae cavae.
The Foetal Circulation
Oxygenated blood from the placenta is returned to the foetus by way of the left umbilical vein. Most of it passes directly into the inferior vena cava through the ductus venosus, a little entering the liver sinusoids. In the inferior vena cava the placental blood is diluted somewhat by impure blood coming from the caudal region of the foetus. It then passes into the right atrium where the remains of the right sinus valve direct it towards the foramen ovale. The sharp concave margin of the upper border of the foramen splits the blood stream into two parts. Most of it passes into the left atrium through the foramen ovale, thence into the left ventricle and out into the systemic circulation. This blood is chiefly distributed to the arteries of the head, neck, and upper limbs which have therefore priority as regards the oxygen content of their blood supply. The lesser amount of blood from the inferior caval stream is mixed in the right atrium with venous blood entering by way of the superior vena cava. The mixed blood passes into the right ventricle, and emerging from this chamber, is mostly carried through the ductus arteriosus to the descending thoracic aorta. Here it has added to it the oxygenated blood not required for the supply of the head and neck and is distributed in small part to the organs of the foetus, the rest passing along the umbilical arteries to the placenta where it is once more purified and recommences the circulation.
Changes in the Circulation at Birth
The changes that occur in the circulation after birth are of two kinds ; there are the* functional changes which occur in a matter of minutes, and the structural changes which are spread over a period of months. Very shortly after birth the umbilical arteries in the cord contract down and cease pulsating. Somewhat later the umbilical vein and the ductus venosus also contract so that if the cord be not tied some of the placental blood is drained back in the interval into the infant's body. The ductus arteriosus is very muscular and very soon after birth it contracts like a sphincter causing the right ventricular blood to pass now into the pulmonary circulation. The expansion of these pulmonary vessels with the commencement of respiration results in an increased flow of blood to the left atrium. At the same time there is diminished venous return to the right atrium because the placental circulation is interrupted. These changes mean that the pressure in the left atrium is now relatively greater than in the right and the septum primum is pressed against the septum secundum thus functionally obliterating the foramen ovale.
Thrombosis and fibrosis of the occluded channels results in their anatomical closure, but this is a slow process requiring several months for its completion. Fusion of the apposed tissues at the foramen ovale is also a slow process, and even in adults may not be quite complete. In such cases, however, there is no functional disability.
The Lymphatic Vessels and Glands
The lymphatic vessels arise in close relationship with the venous system in five regions of the embryo. There are lymph sacs in the region of the internal jugular veins on either side, similar lymph sacs associated with the sciatic veins, and an unpaired retroperitoneal sac. There has been considerable discussion with regard to the origin of these primordia. One view states that the lymphatic vessels develop as endothelial sprouts from already formed Venous channels. A second view claims that they arise primarily by the running together of spaces in the mesenchyme to form lymphatic vessels which secondarily become connected with the venous system. The cells bounding such spaces become transformed into endothelium. The work of Huntingdon (i 9 I 4 ) and of McClure (1915) is strong evidence in favour of the second view. The lymph sacs appear during the sixth and seventh weeks and numerous anastomosing vessels grow outwards from them tending in their course to be perivenous. The jugular sacs obtain connections with the internal jugular veins which persist as the terminal part of the thoracic duct and the right lymphatic duct. lymphoblasts become aggregated in various places around these capillary meshworks from the eighth week onward and produce lymphocytes. These embryonic lymph nodes are later encapsuled by condensation of the surrounding mesenchymal cells.
The Blood Cells
The red and white cells of the body are all derived from a common ancestral cell, the haemocytoblast of mesodermal origin (Maximow, 1927). The first blood cells arise in blood islands in the mesoderm covering the yolk sac (p. 99) as spherical cells with a relatively large nucleus and finely granular basophilic cytoplasm. Following one line of differentiation from this cell the erythrocyte arises. The stages in its formation are, first the megaloblast, which is a cell with a large vesicular nucleus and slightly acidophilic cytoplasm, and then the normoblast, which has a small compact nucleus and a considerable content of haemoglobin. From this cell, by loss of the nucleus, the erythrocyte is formed. According to some observers, the nucleus of the normoblast is extruded into the blood plasma ; others claim that it is absorbed.
Haemocytoblasts are also the stem cells for the granular leucocytes. They give rise to myelocytes, which are acidophil, basophil, or neutrophil according to the nature of the specific granules in their cytoplasm, and from which are formed the various types of granular leucocytes. The lymphocytes likewise result from changes in cells known as lymphoblasts which are in turn derived from haemocytoblasts or from undifferentiated mesenchymal cells.
While the earliest formation of blood cells takes place in the wall of the yolk sac, the liver soon becomes an important centre for their formation. Shortly afterwards the spleen also joins in the process. Both the liver and spleen continue to function in erythropoiesis until birth, although from the third foetal month onward the bone marrow becomes increasingly important in this connection. After birth the bone marrow remains as the sole blood forming tissue.
During the sixth week the primordium of the spleen may be recognised as a thickening on the left side of the dorsal mesogastrium. This splenic condensation subdivides this part of the dorsal mesentery into two, and the more dorsal of these becomes partly applied to, and fused with, the peritoneum in front of the developing left kidney. The attachment of the spleen to the dorsal abdominal wall is thus moved to the left and the attaching part of the mesentery may now be called the lienorenal ligament. The remainder of the mesentery between the splenic condensation and the greater curvature of the stomach is the gastro-splenic ligament.
Commencing about the ninth week the cells of the mesodermal condensation become arranged in anastomosing columns with sinusoidal spaces between them. These spaces secondarily become connected with branches of the splenic artery and vein. The cells of the trabeculae resemble lymphoblasts, and for some time red cells, lymphocytes and leucocytes are produced in the spleen. During the later months of foetal life the production of granulocytes ceases, and red cells are not formed after birth.
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