Embryonic development results from genetic plans in the chromosomes. Knowledge of the genes that control human development is increasing (see Chapter 20). Most developmental processes depend on a precisely coordi- nated interaction of genetic and environmental factors.
Several control mechanisms guide differentiation and ensure synchronized development, such as tissue interac- tions, regulated migration of cells and cell colonies, con- trolled proliferation, and apoptosis (programmed cell death). Each system of the body has its own developmen- tal pattern, and most processes of morphogenesis are regulated by complex molecular mechanisms.
Embryonic development is essentially a process of growth and increasing complexity of structure and func- tion. Growth is achieved by mitosis, together with the production of extracellular matrices, whereas complexity is achieved through morphogenesis and differentiation.
The cells that make up the tissues of very early embryos are pluripotential; that is, depending on the circum- stances, they are able to follow more than one pathway of development. This broad developmental potential becomes progressively restricted as tissues acquire the specialized features necessary for increased sophistication of structure and function. Such restriction presumes that choices must be made to achieve tissue diversification.
Most evidence indicates that these choices are deter- mined not as a consequence of cell lineage, but rather in response to cues from the immediate surroundings, includ- ing the adjacent tissues. As a result, the architectural preci- sion and coordination that are often required for normal function of an organ appear to be achieved by the interac- tion of its constituent parts during development.
The interaction of tissues during development is a recurring theme in embryology. The interactions that lead to a change in the course of development of at least one of the interactants are called inductions. Numerous examples of such inductive interactions can be found in the literature; for example, during the development of the eye, the optic vesicle induces the development of the lens from the surface ectoderm of the head. When the optic vesicle is absent, the eye does not develop. Moreover, if the optic vesicle is removed and placed in association with surface ectoderm that is not usually involved in eye development, lens formation can be induced. Clearly then, the development of a lens depends on the ectoderm acquiring an association with a second tissue. In the pres- ence of the neuroectoderm of the optic vesicle, the surface ectoderm of the head follows a pathway of development that it would not otherwise have taken. Similarly, many of the morphogenetic tissue movements that play such important roles in shaping the embryo also provide for the changing tissue associations that are fundamental to inductive tissue interactions.
The fact that one tissue can influence the developmen- tal pathway adopted by another tissue presumes that a signal passes between the two interactants. Analysis of the molecular defects in mutant strains that show abnor- mal tissue interactions during embryonic development and studies of the development of embryos with targeted
52 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Figure 6–2 Folding of cranial end of the embryo. A, Dorsal view of an embryo at 21 days. B, Sagittal section of the cranial part of the embryo at the plane in A, showing the ventral move- ment of the heart. C, Sagittal section of an embryo at 26 days.
Note that the septum transversum, heart, pericardial coelom, and oropharyngeal membrane have moved to the ventral surface of the embryo.
A
B
C
Developing brain Cardiogenic mesoderm
Cut edge of amnion
Amnion
Developing brain
Forebrain
Notochord
Foregut Primordial heart
Stomodeum
Septum transversum Pericardial coelom Neural tube (future spinal cord) Notochord
Oropharyngeal membrane
Oropharyngeal membrane Pericardial coelom Primordial heart Septum transversum
Level of section B
Figure 6–3 Folding of caudal end of the embryo. A, Lateral view of a 4-week embryo. B, Sagittal section of the caudal part of the embryo at the beginning of the fourth week. C, Similar section at the end of the fourth week. Note that part of the umbili- cal vesicle is incorporated into the embryo as the hindgut and that the terminal part of the hindgut has dilated to form the cloaca. Observe also the change in position of the primitive streak, allantois, cloacal membrane, and connecting stalk.
A
B
C
Primitive streak Neural
tube
Primitive streak Connecting stalk
Allantois
Allantois Notochord Notochord
Cloaca
Amniotic cavity
Umbilical cord Developing spinal cord Cloacal membrane
Cloacal membrane Hindgut
C H A P T E R 6 FOuRTH TO EIGHTH WEEkS OF HuMAN DEvELOPMENT 53
Figure 6–4 Illustration of derivatives of the three germ layers: ectoderm, endoderm, and mesoderm. Cells from these layers contribute to the formation of different tissues and organs;
for example, the endoderm forms the epithelial lining of the gastrointestinal tract and the mesoderm gives rise to connective tissues and muscles.
Muscles of head, striated skeletal muscle (trunk, limbs), skeleton except cranium, dermis of skin, and connective tissue
Cranium
Connective tissue of head Dentin
ENDODERM ECTODERM
NEUROECTODERM SURFACE ECTODERM
MESODERM
Trilaminar embryonic disc
Embryoblast Epiblast
Neural crest
Cranial and sensory ganglia and nerves Medulla of suprarenal gland Pigment cells Pharyngeal arch cartilages
Head mesenchyme and connective tissue Bulbar and conal ridges in heart
Neural tube
Central nervous system Retina Pineal body Posterior part of pituitary gland Connective tissue and muscle of viscera
Serous membranes of pleura, pericardium, and peritoneum Primordial heart
Epidermis, hair, nails, and cutaneous and mammary glands
Anterior part of pituitary gland Enamel of teeth
Internal ear Lens of eye
Blood and lymphatic cells Spleen
Suprarenal (adrenal) cortex
Epithelium of gastrointestinal tract, liver, pancreas,
urinary bladder, and urachus
Epithelial parts of Trachea Bronchi Lungs
Epithelial parts of Pharynx Thyroid gland Tympanic cavity Pharyngotympanic tube Tonsils
Parathyroid glands
Urogenital system, including gonads, ducts, and accessory glands
HEAD
PARAXIAL MESODERM
INTERMEDIA TE MESODERM
LATERAL MESODERM
54 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
fourth weeks (Fig. 6-5A), their measurements indicate the greatest length. The sitting height, or crown–rump length, is used to estimate the age of older embryos (see Fig. 6-5B and C). Standing height, or crown–heel length, is some- times measured during weeks 14 to 18 (see Fig. 6-5D).
The Carnegie Embryonic Staging System is used interna- tionally for comparison (see Table 6-1).