35
C H A P T E R
Third Week of Human Development
5
36 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
The primitive streak actively forms mesoderm until the early part of the fourth week; thereafter, its produc- tion slows down. The streak diminishes in relative size and becomes an insignificant structure in the sacrococ- cygeal region of the embryo (Fig. 5-4A to D).
Notochordal Process and Notochord
Some mesenchymal cells migrate cranially from the prim- itive node and pit, forming a median cellular cord, the notochordal process (see Fig. 5-2G, Fig. 5-4B to D, and Fig. 5-5A to C). This process soon acquires a lumen, the notochordal canal (see Fig. 5-5C and D). The notochordal process grows cranially between the ectoderm and endo- derm until it reaches the prechordal plate, a small, circu- lar area of cells that is an important organizer of the head region (see Fig. 5-2C). The rod-like notochordal process can extend no farther because the prechordal plate is firmly attached to the overlying ectoderm. Fused layers of ectoderm and endoderm form the oropharyngeal mem- brane (Fig. 5-6C) located at the future site of the oral cavity (mouth).
Mesenchymal cells from the primitive streak and the notochordal process migrate laterally and cranially between the ectoderm and endoderm until they reach the margins of the embryonic disc. These mesenchymal cells are continuous with the extraembryonic mesoderm that covers the amnion and the umbilical vesicle (see Fig. 5-2D and F). Some cells from the primitive streak migrate cra- nially on each side of the notochordal process and around the prechordal plate. They meet cranially to form the cardiogenic mesoderm in the cardiogenic area, where the heart primordium begins to develop at the end of the third week (see Fig. 5-9B). Caudal to the primitive streak, there is a circular area—the cloacal membrane—that indicates the future site of the anus (see Fig. 5-5A and D).
The notochord is a cellular rod that:
•
Defines the axis of the embryo and gives it some rigidity•
Serves as the basis for the development of the axial skeleton (such as the bones of the head and vertebral column)•
Indicates the future site of the vertebral bodies The vertebral column forms around the notochord, which extends from the oropharyngeal membrane to the primitive node. The notochord degenerates and disap- pears as the bodies of the vertebrae form, but parts of it persist as the nucleus pulposus of each intervertebral disc.The notochord functions as the primary inductor in the early embryo. It induces the overlying embryonic ecto- derm to thicken and form the neural plate (see Fig. 5-4B and C and Fig. 5-6A to C), the primordium of the central nervous system.
Allantois
The allantois appears on approximately day 16 as a small, sausage-shaped diverticulum (outpouching) from the caudal wall of the umbilical vesicle into the connecting stalk (Fig. 5-5B, C, and D and Fig. 5-6B). The allantois is involved with early blood formation and is associated Gastrulation is the beginning of morphogenesis—
development of body form and structure of various organs and parts of the body. It begins with the formation of the primitive streak (see Fig. 5-2B and C).
Primitive Streak
At the beginning of the third week, the primitive streak appears on the dorsal aspect of the embryonic disc (see Fig. 5-2B). This thickened linear band results from pro- liferation and migration of cells of the epiblast to the median plane of the embryonic disc (see Fig. 5-2D). As soon as the primitive streak appears, it is possible to identify the embryo’s craniocaudal axis (cranial and caudal ends), dorsal and ventral surfaces, and right and left sides. As the primitive streak elongates by the addi- tion of cells to its caudal end, its cranial end proliferates to form the primitive node (see Fig. 5-2E and F). Concur- rently, a narrow primitive groove develops in the primi- tive streak that ends in a small depression in the primitive node, the primitive pit (see Fig. 5-2F). Shortly after the primitive streak appears, cells leave its deep surface and form mesoderm, a loose network of embryonic connec- tive tissue known as mesenchyme (see Figs. 5-2H and Fig.
5-3B and C) that forms the supporting tissues of the embryo.
Under the influence of various embryonic growth factors, including bone morphogenetic protein signaling, epiblast cells migrate through the primitive groove to become endoderm and mesoderm (see Fig. 5-3B). Mes- enchymal cells have the potential to proliferate and dif- ferentiate into diverse types of cells, such as fibroblasts, chondroblasts, and osteoblasts. Recent studies indicate that signaling molecules (nodal factors) of the transform- ing growth factor-β superfamily induce the formation of mesoderm.
Figure 5–1 Endovaginal ultrasonogram of a conceptus 3 weeks after conception implanted in the posterior endome- trium, showing the umbilical vesicle. The endometrium com- pletely surrounds the conceptus. A, Amnion; UV, umbilical sac;
E, endometrium.
UV
E
A
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C H A P T E R 5 THIRD WEEk OF HuMAN DEvELOPMENT 36.e1
(Courtesy E. A. Lyons, MD, Professor of Radiology, and Obstetrics and Gynecology, and Anatomy, Health Sciences Centre and Uni- versity of Manitoba, Winnipeg, Manitoba, Canada.)
C H A P T E R 5 THIRD WEEk OF HuMAN DEvELOPMENT 37
Figure 5–2 Formation of the trilaminar embryonic disc (days 15 to 16). The arrows indicate invagination and migration of the mesenchymal cells between the ectoderm and the endoderm.
C, E, and G, Dorsal views of the embryonic disc early in the third week, exposed by removal of the amnion. A, B, D, F, and H, Transverse sections through the embryonic disc at the levels indicated.
A
Bilaminar embryonic disc Amnion
Umbilical vesicle (yolk sac)
Caudal end Cranial end
Umbilical vesicle Primitive pit
in primitive node Embryonic
ectoderm Prechordal plate
Prechordal plate
Notochordal process
Notochordal process
Prechordal plate Amnion
Umbilical vesicle
Amnion
Amniotic cavity
Primitive streak Connecting stalk
B
C D
E F
G H
Extraembryonic mesoderm covering umbilical vesicle Cut edge of amnion Embryonic ectoderm
Primitive streak
Primitive streak Primitive groove
Primitive groove
Embryonic ectoderm
Embryonic endoderm Primitive groove
Primitive node
Primitive streak
Intraembryonic mesoderm
Extraembryonic somatic mesoderm
Intraembryonic mesoderm
Extraembryonic splanchnic mesoderm
Trilaminar embryonic disc Embryonic endoderm Embryonic
ectoderm
Level of section D
Level of section F
Level of section H
38 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
(neuroectoderm) gives rise to the central nervous system (CNS)—the brain and spinal cord and other structures such as the retina. At first, the neural plate corresponds in length to the underlying notochord. It appears cranial to the primitive node and dorsal to the notochord and the mesoderm adjacent to it (see Fig. 5-4B). As the noto- chord elongates, the neural plate broadens and eventually extends cranially as far as the oropharyngeal membrane (see Fig. 5-4C). Eventually the neural plate extends beyond the notochord.
On approximately day 18, the neural plate invaginates along its central axis to form a median longitudinal neural groove that has neural folds on each side (see Fig.
5-6F and G). The neural folds are particularly prominent at the cranial end of the embryo and are the first signs of brain development (Fig. 5-7C). By the end of the third week, the neural folds have begun to move together and fuse, converting the neural plate into the neural tube, the primordium of the brain vesicles and spinal cord (see with the urinary bladder as well. The blood vessels of the
allantois become the umbilical arteries and veins.