The intraembryonic coelom (body cavity) first appears as small, isolated, coelomic spaces in the lateral intraembry- onic mesoderm and cardiogenic (heart-forming) meso- derm (see Fig. 5-7A to D). These spaces coalesce to form a single, horseshoe-shaped cavity—the intraembryonic coelom (see Fig. 5-7E and F). The coelom divides the lateral mesoderm into two layers (see Fig. 5-7F):
•
A somatic, or parietal (somatopleure), layer that is continuous with the extraembryonic mesoderm cover- ing the amnionFigure 5–5 Illustrations of the development of the notochordal process. The small sketch at the upper left is for orientation. A, Dorsal view of the embryonic disc (at approximately 16 days), exposed by removal of the amnion. The notochordal process is shown as if it were visible through the embryonic ectoderm. B, C, and D, Median sections, at the same plane as shown in A, illustrating successive stages in the development of the notochordal process and canal.
The stages shown in C and D occur at approximately 18 days.
Amnion
Embryonic disc
Umbilical vesicle
Prechordal plate Primitive pit in primitive node
Cloacal membrane Notochordal process
under ectoderm Plane of sections B, C, and D
A
Cranial end
Connecting stalk
B
Embryonic ectoderm
Cardiogenic
(heart-forming) area Embryonic
endoderm Allantois Notochordal
process
Primitive pit
Connecting stalk
Prechordal plate
C Notochordal
canal
Allantois Intraembryonic mesoderm
D Notochordal canal
Primitive pit Primitive
streak Cloacal membrane
Embryonic endoderm
Allantois Notochordal
process Neural
plate Primitive pit Primitive streak
4
C H A P T E R 5 THIRD WEEk OF HuMAN DEvELOPMENT 41
Figure 5–6 Development of the notochord by transformation of the notochordal process.
A, Dorsal view of the embryonic disc (at approximately 18 days), exposed by removing the amnion. B, Three-dimensional median section of the embryo. C and E, Similar sections of slightly older embryos. D, F, and G, Transverse sections of the trilaminar embryonic disc shown in C and E.
Cut edge of amnion Amniotic cavity
Primitive pit
Allantois
Umbilical vesicle Notochordal process
Plane of sections B, C, and E
Neural plate
Oropharyngeal membrane
Neurenteric canal (arrow)
Level of section D, F, G
A B
C
E
D
F
G
Prechordal plate
Primitive streak
Primitive streak
Embryonic endoderm Primitive pit
Connecting stalk
Connecting stalk
Notochord Neurenteric canal
Cloacal membrane
Embryonic
ectoderm Neural groove Intraembryonic mesoderm
Notochordal plate intercalated in the embryonic endoderm Neural groove
Neural groove Neural fold
Umbilical vesicle
Paraxial mesoderm Notochordal plate infolding
Notochord Embryonic endoderm Lateral
mesoderm Intermediate
mesoderm
42 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Figure 5–7 Illustrations of embryos 19 to 21 days old, illustrating the development of the somites and the intraembryonic coelom. A, C, and E, Dorsal view of the embryo, exposed by removal of the amnion. B, D, and F, Transverse sections through the embryonic disc at the levels shown. A, A presomite embryo of approximately 18 days. C, An embryo of approximately 20 days, showing the first pair of somites. A portion of the somatopleure on the right has been removed to show the isolated coelomic spaces in the lateral mesoderm. E, A three-somite embryo (approximately 21 days old), showing the horseshoe-shaped intraembryonic coelom, exposed on the right by removal of part of the somatopleure.
A
Cardiogenic area Neural plate
Neural groove Intermediate mesoderm Paraxial
mesodermNeural groove
Embryonic ectoderm
Amnion
Lateral mesoderm
Coelomic spaces Neural fold
Coelomic spaces Cut edge of amnion
Umbilical vesicle covered with extraembryonic mesoderm Coelomic
spaces
First somite Connecting stalk
Pericardial coelom Pericardio- peritoneal canal Peritoneal coelom (cavity)
Neural folds
Somite
Intraembryonic splanchnic mesoderm
Neural folds about to fuse to form neural tube
Intraembryonic coelom
Splanchnopleure
Somite Somatopleure
Intraembryonic coelom
Intraembryonic somatic mesoderm Level of
section B
B
C D
E F
Level of section D
Level of section F Neural fold
C H A P T E R 5 THIRD WEEk OF HuMAN DEvELOPMENT 43
Vasculogenesis and Angiogenesis
Blood vessel formation in the embryo and the extraem- bryonic membranes during the third week may be sum- marized as follows (see Fig. 5-9C to F):
Vasculogenesis:
•
Mesenchymal cells differentiate into endothelial cell precursors, or angioblasts (vessel-forming cells), that aggregate to form isolated angiogenic cell clusters known as blood islands (see Fig. 5-9B and C).early formation of the cardiovascular system correlates with the urgent need for transportation of oxygen and nourishment to the embryo from the maternal circula- tion through the chorion. At the beginning of the third week, blood vessel formation, or vasculogenesis, begins in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion. Vasculogenesis begins in the chorion (Fig. 5-9A and B). Blood vessels develop approximately 2 days later. At the end of the third week, a primordial uteroplacental circulation has developed (Fig. 5-10).
Figure 5–8 A to F, Diagrammatic transverse sections through progressively older embryos, illustrating the formation of the neural groove, neural tube, and neural crest up to the end of the fourth week.
B
Neural fold
Neural folds approaching each other
Neural groove Notochord Neural crest
Neural groove
C
Surface ectoderm
D
Neural groove Neural crest
E
Neural crest Neural canal Neural tube
F
Developing epidermis
Developing spinal ganglion Neural tube
A
Neural fold Neural groove
Somite
Primitive node
Level of section B
Primitive streak Cut edge of amnion
44 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
grow on the umbilical vesicle and allantois at the end of the third week (see Fig. 5-9E and F). Blood formation (hematogenesis) does not begin within the embryo until the fifth week. This process occurs first in various parts of the embryonic mesenchyme, chiefly the liver, and later in the spleen, bone marrow, and lymph nodes. Fetal and adult erythrocytes are also derived from hematopoietic progenitor cells (hemangioblasts). The mesenchymal cells that surround the primordial endothelial blood vessels differentiate into muscular and connective tissue elements of the vessels.
The heart and great vessels form from mesenchymal cells in the heart primordium, or cardiogenic area (see Fig. 5-7A and Fig. 5-9B). Paired, endothelium-lined channels—endocardial heart tubes—develop during the
•
Small cavities appear within the blood islands by the confluence of intercellular clefts.•
Angioblasts flatten to form endothelial cells that arrange themselves around the cavities in the blood islands to form the primordial endothelium.•
The endothelium-lined cavities soon fuse to form net- works of endothelial channels.Angiogenesis:
•
Vessels sprout by endothelial budding into adjacent nonvascularized areas and fuse with other vessels.Blood cells develop from hematopoietic stem cells or from hemangiogenic endothelium or blood vessels as they
Figure 5–9 Successive stages in the development of blood and blood vessels. A, The umbilical vesicle (yolk sac) and a portion of the chorionic sac (at approximately 18 days).
B, Dorsal view of the embryo exposed by removing the amnion. C to F, Sections of blood islands, showing progressive stages in the development of blood and blood vessels.
A
Cut edge of amnion
Umbilical vesicle with blood islands
Heart primordium Neural plate
Cut edge of amnion
Primordial blood vessel
Blood island
Blood island
Precursor blood cell arising from endothelium
Progenitor
blood cells (hemangioblast) Fusion of
adjacent vessels Wall of umbilical vesicle Lumen of
primordial blood vessel
Primordial blood vessel
Endoderm of umbilical vesicle Connecting
stalk
Wall of chorionic sac Embryonic disc
Developing blood vessels
B
C D
E F
C H A P T E R 5 THIRD WEEk OF HuMAN DEvELOPMENT 45
(anchoring villi). The villi that grow from the sides of the stem villi are branch chorionic villi (terminal villi). It is through the walls of the branch villi that the main exchange of material between the blood of the mother and the embryo takes place. The branch villi are bathed in continually changing maternal blood in the intervillous space (see Fig. 5-11C).
third week and fuse to form a primordial heart tube. The tubular heart joins with blood vessels in the embryo, con- necting stalk, chorion, and umbilical vesicle to form a primordial cardiovascular system (Fig. 5-11C). By the end of the third week, blood is flowing and the heart begins to beat on day 21 or 22. The cardiovascular system is the first organ system to reach a primitive functional state.
The embryonic heartbeat can be detected by Doppler ultrasonography (detects motion by monitoring the change in frequency or phase of the returning ultrasound waves) during the fourth week, approximately 6 weeks after the last normal menstrual period (see Fig. 5-10).