In the human, an embryo may be defined as the developing organism from fertilization until the end of the second month of gestation, that is, from 0 to 60 days of life.
The First Week of Life
The salient events of the first week of life ( Fig. 2.1 ) are (1) ovulation, (2) fertilization, (3) segmentation, (4) blastocyst formation, and (5) the beginning of implantation.
A living human ovum surrounded by its corona radiata is shown in Fig. 2.2 . The single-celled ovum stage is Streeter’s horizon 1. This ovum is thought to be 1.25 days old or less. One cannot tell by inspection whether this ovum has been fertilized. Fertilization normally occurs in the distal fallopian tube (see Fig. 2.1 ).
When fertilization has occurred, the next stage is known as cleavage . The large single-celled ovum undergoes mitotic division forming two cells ( Fig. 2.3 ). A rapid succession of mitotic divisions produces a progressively larger number of smaller cells known as blastomeres ( blastos = offspring or germ, and meros = part, Greek). A morula is shown in Fig. 2.4 . A morula consists of 16 cells with no central cavity. Morula means “little mulberry” ( morus = mulberry, Latin). This solid mass of blastomeres, formed by the cleavage of a fertilized ovum, fills all the space occupied by the ovum before cleavage. The stage of cleavage ( Figs. 2.3 and 2.4 ) is Streeter’s horizon 2 . Cleavage occurs during the voyage of the zygote down the fallopian tube and into the uterine cavity. It is thought to take 3 to 4 days to reach the morula stage.
Then the morula develops a cavity, forming a blastocyst ( Fig. 2.5 ). Blastocyst literally means “offspring” or “germ” ( blastos , Greek) plus “bladder” ( kystis , Greek). The formation of a cavity (bladder) separates the thick inner cell mass (future individual) from the thin-walled trophoblast (future placenta). Trophoblast means “nourishment” ( trophe , Greek) plus “offspring” or “germ” ( blastos , Greek). The blastocyst stage is reached by 4½ to 6 days of age and constitutes Streeter’s horizon 3 .
Parenthetically, etymologies are included to help the reader to remember these terms. If one understands what a designation really means (its etymology), then it is much easier to remember.
The blastocyst begins to implant in the uterine mucosa at about 7 days of age, that is, 7 days after ovulation ( Figs. 2.1 and 2.6 ). Implantation is Streeter’s horizon 4 .
The Second Week of Life
The principal developments during the second week of life are summarized diagrammatically in Fig. 2.7 :
Implantation is completed.
A bilaminar disc of ectoderm and endoderm develops out of the inner cell mass.
The amniotic cavity appears.
The yolk sac develops.
Primitive villi of the developing placenta make their appearance.
At the beginning of the second week, that is, at about 7½ days of age, the zygote normally is implanted, but the trophoblast still has no villi. This stage is horizon 5 ( Fig. 2.8 ).
About 2 days later, that is, at 9 days of age, primitive villi are seen ( Fig. 2.9 ). The embryonic disc is now bilaminar, consisting of columnar ectodermal cells and cuboidal endodermal cells. The amniotic cavity and the yolk sac can now be seen. This is Streeter’s horizon 6 . The embryo shown in Fig. 2.9 closely resembles the diagram of Fig. 2.7 .
Although the cardiovascular system is the first organ system to reach functional maturity, during the first two weeks of life, humans have no heart and no vascular system; that is, the cardiovascular system does not yet exist.
What germ layer does the cardiovascular system come from? From the mesoderm. But where does the mesoderm come from? As will soon be seen, from the ectoderm.
Ectoderm means “outside skin” ( ektos = outside + derma = skin, Greek). Endoderm means “inside skin” ( endon = within or inside + derma = skin, Greek). Mesoderm means “middle skin” ( mesos = middle + derma = skin, Greek).
The Third Week of Life
The main events during the third week of embryonic life from the cardiovascular standpoint normally are:
the development of the mesoderm from the ectoderm on the 15th day of life,
the appearance of the cardiogenic crescent of precardiac mesoderm on the 18th day of life,
the development of the intra-embryonic celom on the 18th day of life,
the development of the straight heart tube at 20 days of age,
the beginning of D-loop formation in normal development, or the beginning of L-loop formation in abnormal development, at 21 days of age, and
the initiation of the heartbeat at the straight tube stage or at the early D-loop stage.
In somewhat greater detail, the main events in the development of the cardiovascular system during the third week of embryonic life are as follows:
The mesoderm develops from the ectoderm, appearing in the normal human embryo on the 15th day of life ( Fig. 2.10 ). Note that the villi are branching and that the primitive streak has appeared, these being the features that typify horizon 7.
The primitive streak is a depression that marks the long axis of the embryo when viewed from the dorsal aspect ( Fig. 2.11 ). As the mesoderm buds off from the ectoderm, the right-sided mesoderm migrates rightward and then cephalically, while the left-sided mesoderm migrates leftward and then cephalically. Since the mesoderm remains ipsilateral (right remains right sided and left remains left sided), rather than crossing the midline, the result is a depression between the right-sided and left-sided mesoderm—the primitive streak—that marks the long axis of the embryo when viewed from its dorsal or amniotic sac aspect ( Figs. 2.11 and 2.12 ). This lateral and then cephalic migration of the mesoderm bilaterally can be well documented in explanted chick embryos by cinephotomicrography. I have made many movies of this process.
The cardiogenic crescent of precardiac mesoderm appears on day 18 in the normal human embryo. The left-sided and right-sided precardiac mesoderm unite in front of the developing brain, forming a horseshoe-shaped crescent of precardiac mesoderm, as in Carnegie embryo 5080 of Davis ( Fig. 2.13 ). The reconstruction of this embryo is shown in Fig. 2.14 . This embryo was 1.5 mm in length. The first pair of somites was just forming. This stage corresponds to Streeter’s late horizon 8 (no somites) and early horizon 9 (one to three pairs of somites), that is, at the junction of horizons 8 and 9 (horizon 8/9). A late horizon 9 embryo with three pairs of somites is shown in Fig. 2.15 .
The notochord gives our phylum its name: Phylum Chordates . This phylum includes all animals with a notochord and is essentially synonymous with the craniates and the vertebrates.
The prochordal plate , as its name indicates, lies anterior to (in front of) the notochord. The prochordal plate consists of ectoderm and endoderm, is never normally invaded by mesoderm, and subsequently breaks down, contributing to the formation of the mouth.
The cloacal membrane caudally also is normally not invaded by mesoderm. This membrane subsequently breaks down to help create the cloacal opening.
The intraembryonic celom appears on the 18th day of life, in horizon 9, because the mesoderm cavitates (see Fig. 2.15 ). The mesoderm splits into dorsal and ventral layers, which are separated by the intraembryonic celom (or space). The dorsal layer of the mesoderm is called the somatopleure because this layer is adjacent to the body wall and forms, for example, the pericardial sac. ( Soma = body + pleura = side, Greek.) The ventral layer of the mesoderm is known as the splanchnopleure because this layer is on the inside, that is, on the visceral side. ( Splanchnos = viscus + pleura = side, Greek.) The splanchnopleure forms, for example, the myocardium.
The intraembryonic celom communicates with the extraembryonic celom ( Fig. 2.15 , arrows). The intraembryonic celom forms all of the body cavities, which at this stage are not divided from each other. The intraembryonic celom includes the future pericardial, pleural, and peritoneal cavities. Note that even the somites, which form the future skeletal muscles, contain small central cavities (see Fig. 2.15 ). The ability to form cavities is one of the more important characteristics of mesoderm.
In Fig. 2.15 , buccopharyngeal membrane is another name for the prochordal plate. The intermediate cell mass is early kidney (see Fig. 2.15 ). The brain is still a neural plate , not having formed a tubular structure as yet. The notochord indicates the long axis of the embryo. The somites ( soma = body, Greek) form from the paraxial mesoderm , the mesoderm that is beside ( para = beside, Greek) the long axis of the body, indicated by the notochord. By contrast, the heart forms from the lateral plate mesoderm , so called because it is lateral to the paraxial mesoderm (see Fig. 2.15 ). The precardiac mesoderm of the cardiogenic crescent then continues to migrate cephalically on the foregut endoderm to form a straight heart tube ( Fig. 2.16 ).
The straight heart tube or preloop stage normally occurs in the human embryo at 20 days of age ( Fig. 2.17 ). The straight heart tube stage can be achieved in the human embryo by horizon 9 , in Carnegie embryo 1878 of Davis and Ingalls that had two pairs of somites and was 1.38 mm in length ( Fig. 2.18 ). However, the straight tube stage often is not reached until horizon 10 , as in Carnegie embryo 3709 ( Fig. 2.19 ), with four pairs of somites, 2.5 mm in length, estimated age 20 to 22 days, and as in Carnegie embryo Klb ( Fig. 2.20 ), with six pairs of somites and a length of 1.8 mm.
At the straight tube stage, note that the endocardial lumina of the left and right “half hearts” may be largely unfused ( Fig. 2.20 ) or incompletely fused (see Fig. 2.19 ). The space between the myocardium and the endocardium is filled with cardiac jelly. As the precardiac mesoderm migrates cephalically and medially onto the foregut endoderm to form a straight heart tube, the foregut endoderm is growing caudally or posteriorly, as is well shown by my time-lapse movies in the chick embryo.
D-loop formation normally begins at the end of the third week of embryonic life in humans (see Figs. 2.16 and 2.17 ). By analogy with other vertebrates, it seems very likely that this is when the heart in human embryos starts to beat: Carnegie embryo 4216 ( Fig. 2.21 ), seven pairs of somites, 2.2 mm in length, horizon 10 (20–22 days of age); Carnegie embryo 391 ( Fig. 2.22 ), eight pairs of somites, 2 mm in length, horizon 10 (day 20–22); and Carnegie embryo 3707 ( Fig. 2.23 ), 12 pairs of somites, 2.08 mm in length, horizon 10 (20–22 days of age). When D-loop formation begins—the heart bending convexly to the right—the endocardial tubes have fused forming a single endocardial lumen. ( Dexter , dextra , dextrum are the masculine, feminine, and neuter adjectives, respectively, meaning “right sided,” Latin.)
At the horizon 10 stage (see Figs. 2.21–2.23 ), the future morphologically right ventricle (RV), which develops from the proximal bulbus cordis, is superior to the future morphologically left ventricle (LV), which develops from the ventricle of the bulboventricular loop. The future interventricular septum—between the bulbus cordis and the ventricle of the straight bulboventricular tube—lies in an approximately horizontal position. If an arrest in development were to occur at the horizon 10 stage (20–22 days of age), superoinferior ventricles would result, with the RV superior to the LV and the ventricular septum approximately horizontal. The atrioventricular canal, which is in common (not divided into mitral and tricuspid valves) at this stage, opens superiorly only into the ventricle—future LV. Hence, common-inlet LV is potentially present during horizon 10 (see Figs. 2.21–2.23 ). Both future great arteries originate only from the bulbus cordis (future RV). Thus, double-outlet RV would result from an arrest of development during horizon 10 (20–22 days of age).
To put it another way, common-inlet LV, superoinferior ventricles, and double-outlet RV are all normal findings at the horizon 10 (20–22 day) stage. This understanding illustrates why a knowledge of normal cardiovascular embryology appears to be so relevant to the understanding of the pathologic anatomy of complex congenital heart disease.
However, it must also be borne in mind that much remains to be learned concerning the etiology and morphogenesis of congenital heart disease. For example, if developmental arrest really is a pathogenetic mechanism leading to congenital heart disease, as is widely assumed, it remains to be proved when and why such developmental arrests occur in the human embryo. We think we know when —but this is only an extrapolation based on normal cardiovascular development—and we often have no idea why . Hence, in this chapter, I am not endeavoring to make implications concerning the causation of congenital heart disease. Instead, I am presenting factual data concerning normal cardiovascular development. The precise relevance of this understanding to the etiology and morphogenesis of human congenital heart disease remains to be proved. Nonetheless, when obvious correlations appear to exist, I will point them out, with the aforementioned mental reservations being understood.
The Fourth Week of Life
The main features of normal cardiovascular development during the fourth week of embryonic life are:
the completion of D-loop formation,
the beginning of the development of the morphologically LV and of the morphologically RV,
the beginning of the circulation, and
the initiation of cardiovascular septation.
In somewhat greater detail, the salient features of normal human cardiovascular development during the fourth week of embryonic life, that is, from day 22 to day 28 inclusive, include the changes that occur during Streeter’s horizons 11 to 13, inclusive ( Fig. 2.24 ). It is noteworthy that each of Streeter’s horizons covers an approximately 2-day time interval. One doubles the horizon number to find the embryonic age in days at the beginning of the horizon. For example, horizon 11 indicates the stage beginning at an age of 22 days (11 × 2), which lasts for 2 days—from day 22 to day 24 inclusive (22 + 2) (see Fig. 2.24 ).
The diagram (see Fig. 2.24 ) also makes it possible to estimate the approximate age of an embryo (on the horizontal axis) from its length in millimeters (on the vertical axis). For example, an embryo with a crown–rump length of 5 mm falls into the middle of horizon 13, which corresponds to an embryonic age of 26 to 28 days (see Fig. 2.24 ).
Streeter’s horizons are an aging and staging system not just for the heart but for all organ systems (see Fig. 2.24 ).
D-loop formation normally is completed in horizon 11 (22–24 days), as is seen in Carnegie embryo 470, which is 3.3 mm long and has 16 pairs of somites ( Figs. 2.25 and 2.26 ).
Fig. 2.26 shows a reconstruction of the lumen of this embryo, like a perfect angiocardiogram. Note that the future ventricular apex points rightward following the completion of D-loop formation; that is, dextrocardia is present. The blood flows from the right atrium (RA) to the left atrium, as in tricuspid atresia . The blood then flows from the left atrium only into the future LV, similar to common-inlet or double-inlet LV . The development of the LV sinus is somewhat more advanced than is that of the RV sinus. The developing LV is anterior (ventral) relative to the RV, as the right lateral and left lateral views demonstrate. Hence, the anterior (ventral) ventricle is not necessarily the RV, contrary to what works on angiocardiography often say. Following D-looping, the ventral ventricle is the LV, and the dorsal ventricle is the RV, because dextrocardia with a rightward pointing apex is present.
Note also that the right atrial appendage lies to the left of the vascular pedicle; that is, left-sided juxtaposition of the atrial appendages is present.
A reconstruction of the atria of the same embryo (Carnegie embryo 470 of Davis) is shown in Fig. 2.27 . When the dorsal walls of the atria are removed, revealing the interior, the floor of the morphologically RA strongly resembles that seen in typical tricuspid atresia (see Fig. 2.27 , right panel). The atrioventricular canal opens from the left atrium into the LV.
In Fig. 2.25 , it will be seen that the vitelline veins are adjacent to the yolk sac ( vitellus = yolk, Latin). The right and left umbilical veins are lateral to the right and left vitelline veins, respectively. The umbilical veins plus the vitelline veins are together known as the omphalomesenteric veins. The septum transversum is the embryonic diaphragm. The anterior intestinal portal leads into the foregut behind the heart. Note the pericardial sac, the first pair of aortic arches, and the pharyngeal membrane.
By 26 to 28 days of age (horizon 13), as illustrated by Carnegie embryo 836 ( Figs. 2.28 and 2.29 ), the ventricular D-loop has descended relative to the atria. Development of the LV is more advanced than that of the RV. The ventricular apex is still pointing to the right. The LV remains ventral to the RV. The RA opens only into the left atrium, as in tricuspid atresia. The left atrium opens only into the LV. The LV ejects into the RV. And both future great arteries—still undivided—originate only from the RV.
A true circulation, as opposed to ebb and flow, is thought to begin at this stage. This is the ancient in-series circulation , as in aquatic vertebrates such as sharks. It is a single, as opposed to a double, circulation. It is called an in-series, as opposed to an in-parallel, circulation because the blood passes in series from RA to LA to LV to RV to the undivided great artery. Note that aortic arches 2 and 3 have appeared, whereas the first pair of aortic arches have undergone involution (see Fig. 2.29 ).
In Figs. 2.26, 2.28, and 2.29 , note that there is a smooth or nontrabeculated area between the LV and the RV that will become the smooth crest of the muscular interventricular septum. The trabeculated portions of the greater curvature of the D-loop evaginate (pouch outward), forming the ventricular sinuses. The smooth or nontrabeculated portion of the bulboventricular D-loop do not evaginate.
The development of the aortic arches during the fourth week of embryonic life is presented in Figs. 2.30 to 2.32 , inclusive. At the beginning of the fourth week (horizon 11), the first pair of aortic arches appears, as in Carnegie embryo 2053, 3 mm long, 20 pairs of somites. Each first aortic arch passes above (cephalad to) the first pharyngeal pouch on either side (see Fig. 2.30 ). The second pair of aortic arches is beginning to form.
At this stage, the heart is a cervical organ. The aortic arches are related to the gill arches of our aquatic vertebrate ancestors. Arrest of development at the fourth week stage appears to result in cervical ectopia cordis . Subsequently, the heart descends into the thorax.
Later in the fourth week of life, the second, third, and an early fourth pair of aortic arches appear, as in Carnegie embryo 836 (early horizon 13, 26 days of age, 4 mm in length, 30 pairs of somites present) (see Fig. 2.31 ). Each aortic arch passes cephalad to its pharyngeal pouch. Note the thoracic location of the stomach and the large tracheo-esophageal communication (“fistula”) that are normal at this stage.
By late in the fourth week, as in Carnegie embryo 1380 (see Fig. 2.32 ), the third and fourth pairs of aortic arches have developed, aortic arches 1 and 2 have involuted, and the sixth aortic arches are developing. In this embryo 1380 (5 mm in length, horizon 13), the right ductus arteriosus (sixth arch) is completely formed, but the left ductus arteriosus still has not formed.
Note that both the left and the right pulmonary artery branches have formed, despite the fact that a complete left sixth aortic arch is not present (see Fig. 2.32 , left panel). This appears to explain why it is possible to have pulmonary artery branches present in association with congenital absence of the ductus arteriosus. The pulmonary artery branches initially arise as outpouching from the aortic sac (i.e., from the aorta), not from the sixth arches (as has often been said). Later, as will be seen, the pulmonary artery branches appear to originate from the sixth aortic arches. But the point I seek to make is that this is not where the pulmonary artery branches start from (see Fig. 2.32 ). Note also that the pulmonary vein has appeared. Hence, in the fourth week of life, all of the foregoing are normal findings: dextrocardia, tricuspid atresia, an undivided atrioventricular canal from the left atrium to the LV, an in-series circulation, and an undivided great artery that arises only above a poorly developed RV sinus.
The Fifth Week of Life
The salient cardiovascular developments during the fifth week of embryonic life (from day 29 to day 35 inclusive, i.e., from horizon 14 to horizon 17 inclusive, see Fig. 2.24 ) are:
continuation of the development of the LV, RV, and ventricular septum;
approximation of the aorta to the interventricular foramen, the mitral valve, and the LV;
separation of the aorta and pulmonary artery;
separation of the mitral and tricuspid valves;
enlargement of the RV sinus;
movement of the muscular ventricular septum to the left, beneath the atrioventricular canal;
opening of the tricuspid valve into the RV; and
closure of the ostium primum by the endocardial cushions of the atrioventricular canal, thereby separating the atria; and leftward movement of the ventricles and ventricular apex, thus “curing” dextrocardia and resulting in mesocardia (a ventrally or anteriorly pointing ventricular apex).
Fig. 2.33 is a diagram of the heart of Carnegie embryo 3385 in horizon 15 (day 30–32), 8.3 mm in length. Fig. 2.15 presents the reconstruction of this embryo photographically. Note that the development of the RV sinus is starting to catch up with that of the LV sinus. The ventricles and the ventricular apex are swinging leftward. The atrioventricular canal is still opening into the larger LV. Left-sided juxtaposition of the atrial appendages is no longer present, the large RA now lying to the right of the great arteries that are undergoing septation longitudinally. Aortic arches 3, 4, and 6 are now present, as are small, downward dangling pulmonary artery branches ( Fig. 2.34 ).
Fig. 2.35 shows the heart of Carnegie embryo 6510 diagrammatically, this embryo being in horizon 16 (32–34 days of age), with a length of 10.1 mm. The reconstruction of this embryo ( Fig. 2.36 ) shows that RV growth is catching up with that of the LV. The ventricular septum now lies in an approximately anteroposterior plane; that is, mesocardia is now present. In the right and left lateral views of this reconstruction, one can see that the ostium primum is now very small and is being closed by the endocardial cushions of the atrioventricular canal. Fig. 2.37 is a reconstruction of the ventricular lumina of this embryo, which confirms the presence of mesocardia and shows that the right and left ventricular sinuses are now approximately the same size.