The embryonic period, or period of organogenesis, occurs from the third to the eighth weeks of development and is the time when each of the three germ layers, ectoderm, mesoderm, and endoderm, gives rise to a number of specific tissues and organs. By the end of the embryonic period, the main organ systems have been established, rendering the major features of the external body form recognizable by the end of the second month.
Derivatives of the Ectodermal Germ Layer
At the beginning of the third week of development, the ectodermal germ layer has the shape of a disc that is broader in the cephalic than in the caudal region (Fig. 6.1). Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate (Fig. 6.2A,B). Cells of the plate make up the neuroectoderm, and their induction represents the initial event in the process of neurulation.
A. Dorsal view of a 16-day presomite embryo. The primitive streak and primitive node are visible. B. Dorsal view of an 18-day presomite embryo. The embryo is pear-shaped, with its cephalic region somewhat broader than its caudal end. C. Dorsal view of an 18-day human embryo. Note the primitive node and, extending forward from it, the notochord. The yolk sac has a somewhat mottled appearance. The length of the embryo is 1.25 mm, and the greatest width is 0.68 mm.
A. Dorsal view of a late presomite embryo (approximately 19 days). The amnion has been removed, and the neural plate is clearly visible. B. Dorsal view of a human embryo at 19 days. C. Dorsal view of an embryo at approximately 20 days showing somites and formation of the neural groove and neural folds. D. Dorsal view of a human embryo at 20 days.
Molecular Regulation of Neural Induction
Upregulation of fibroblast growth factor (FGF) signaling together with inhibition of the activity of bone morphogenetic protein 4 (BMP4), a transforming growth factor-? (TGF-?) family member responsible for ventralizing ectoderm and mesoderm, causes induction of the neural plate. FGF signaling probably promotes a neural pathway by an unknown mechanism while it represses BMP transcription and upregulates expression of chordin and noggin, which inhibit BMP activity. In the presence of BMP4, which permeates the mesoderm and ectoderm of the gastrulating embryo, ectoderm is induced to form epidermis, and mesoderm forms intermediate and lateral plate mesoderm. If ectoderm is protected from exposure to BMPs, its “default state” is to become neural tissue. Secretion of three other molecules, noggin, chordin, and follistatin, inactivates BMP. These three proteins are present in the organizer (primitive node), notochord, and prechordal mesoderm. They neuralize ectoderm by inhibiting BMP and cause mesoderm to become notochord and paraxial mesoderm (dorsalizes mesoderm); however, these neural inducers induce only forebrain and midbrain types of tissues. Induction of caudal neural plate structures (hindbrain and spinal cord) depends on two secreted proteins, WNT3a and FGF. In addition, retinoic acid (RA) appears to play a role in organizing the cranial-to-caudal axis because it can cause respecification of cranial segments into more caudal ones by regulating expression of homeobox genes (see p. 78).
Neurulation is the process whereby the neural plate forms the neural tube. By the end of the third week, the lateral edges of the neural plate become elevated to form neural folds, and the depressed midregion forms the neural groove (Fig. 6.2). Gradually, the neural folds approach each other in the midline, where they fuse (Fig. 6.3A,B). Fusion begins in the cervical region (fifth somite) and proceeds cranially and caudally (Fig. 6.3C,D). As a result, the neural tube is formed. Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by way of the anterior (cranial) and posterior (caudal) neuropores, respectively (Figs. 6.3C,D and 6.4A). Closure of the cranial neuropore occurs at approximately day 25 (18-to 20-somite stage), whereas the posterior neuropore closes at day 28 (25-somite stage) (Fig. 6.4B). Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the spinal cord, and a much broader cephalic portion characterized by a number of dilations, the brain vesicles (see Chapter 18).
A. Dorsal view of an embryo at approximately day 22. Seven distinct somites are visible on each side of the neural tube. B. Dorsal view of a human embryo at 21 days. C. Dorsal view of an embryo at approximately day 23. Note the pericardial bulge on each side of the midline in the cephalic part of the embryo. D. Dorsal view of a human embryo at 23 days.
A. Lateral view of a 14-somite embryo (approximately 25 days). Note the bulging pericardial area and the first and second pharyngeal arches. B. The left side of a 25-somite embryo approximately 28 days old. The first three pharyngeal arches and lens and otic placodes are visible.
Neural Crest Cells
As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbors. This cell population, the neural crest (Figs. 6.5 and 6.6), will undergo an epithelial-to-mesenchymal transition as it leaves the neuroectoderm by active migration and displacement to enter the underlying mesoderm. (Mesoderm refers to cells derived from the epiblast and extraembryonic tissues. Mesenchyme refers to loosely organized embryonic connective tissue regardless of origin.) Crest cells from the trunk region leave the neuroectoderm after closure of the neural tube and migrate along one of two pathways: (1) a dorsal pathway through the dermis, where they will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles, and (2) a ventral pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons, Schwann’s cells, and cells of the adrenal medulla (Fig. 6.5). Neural crest cells also form and migrate from cranial neural folds, leaving the neural tube before closure in this region (Fig. 6.6). These cells contribute to the craniofacial skeleton, as well as neurons for cranial ganglia, glial cells, melanocytes, and other cell types (Table 6.1). Neural crest cells are so fundamentally important and contribute to so many organs and tissues that they are sometimes referred to as the fourth germ layer. Evolutionarily, these cells appeared at the dawn of vertebrate development and expanded this group extensively by perfecting a predatory lifestyle.
Figure 6.5. Formation and migration of neural crest cells in the spinal cord.
A, B. Crest cells form at the tips of neural folds and do not migrate away from this region until neural tube closure is complete. C. After migration, crest cells contribute to a heterogeneous array of structures, including dorsal root ganglia, sympathetic chain ganglia, adrenal medulla, and other tissues (Table 6.1). D. In a scanning electron micrograph, crest cells at the top of the closed neural tube can be seen migrating away from this area.
Figure 6.6.Drawing shows the migratory paths of neural crest cells in the head region.
These cells leave the crests of the neural folds prior to neural tube closure and migrate to form structures in the face and neck (blue area). 1 to 6, pharyngeal arches; V, VII, IX, and X, epibranchial placodes.
Table 6.1.Neural Crest Derivatives
|Connective tissue and bones of the face and skull|
|Cranial nerve ganglia (see Table 17.2)|
|C cells of the thyroid gland|
|Conotruncal septum in the heart|
|Dermis in face and neck|
|Spinal (dorsal root) ganglia|
|Sympathetic chain and preaortic ganglia|
|Parasympathetic ganglia of the gastrointestinal tract|
|Smooth muscle cells to blood vessels of the face and forebrain|
Molecular Regulation of Neural Crest Induction
Induction of neural crest cells requires an interaction at the junctional border of the neural plate and surface ectoderm (epidermis) (Fig. 6.5A). Intermediate concentrations of BMPs are established at this boundary compared to neural plate cells that are exposed to very low levels of BMPs and surface ectoderm cells that are exposed to very high levels. The proteins noggin and chordin regulate these concentrations by acting as BMP inhibitors. The intermediate concentrations of BMPs, together with FGF and WNT proteins, induce PAX3 and other transcription factors that “specify” the neural plate border (Fig. 6.5A). In turn, these transcription factors induce a second wave of transcription factors, including SNAIL and FOXD3, which specify cells as neural crest, and SLUG, which promotes crest cell migration from the neuroectoderm. Thus, the fate of the entire ectodermal germ layer depends on BMP concentrations: High levels induce epidermis formation; intermediate levels, at the border of the neural plate and surface ectoderm, induce the neural crest; and very low concentrations cause formation of neural ectoderm. BMPs, other members of the TGF-? family, and FGFs regulate neural crest cell migration, proliferation, and differentiation, and abnormal concentrations of these proteins have been associated with neural crest defects in the craniofacial region of laboratory animals (see Chapter 17).
By the time the neural tube is closed, two bilateral ectodermal thickenings, the otic placodes and the lens placodes, become visible in the cephalic region of the embryo (Fig. 6.4B). During further development, the otic placodes invaginate and form the otic vesicles, which will develop into structures needed for hearing and maintenance of equilibrium (see Chapter 19). At approximately the same time, the lens placodes appear. These placodes also invaginate and, during the fifth week, form the lenses of the eyes (see Chapter 20).
In general terms, the ectodermal germ layer gives rise to organs and structures that maintain contact with the outside world:
- The central nervous system;
- The peripheral nervous system;
- The sensory epithelium of the ear, nose, and eye; and
- The epidermis, including the hair and nails.
In addition, it gives rise to:
- Subcutaneous glands,
- The mammary glands,
- The pituitary gland,
- And enamel of the teeth.
Neural Tube Defects
Neural tube defects (NTDs) result when neural tube closure fails to occur. If the neural tube fails to close in the cranial region, then most of the brain fails to form, and the defect is called anencephaly (Fig. 6.7A). If closure fails anywhere from the cervical region caudally, then the defect is called spina bifida (Fig. 6.7B,C). The most common site for spina bifida to occur is in the lumbosacral region (Fig. 6.7C), suggesting that the closure process in this area may be more susceptible to genetic and/or environmental factors. Anencephaly is a lethal defect, and most of these cases are diagnosed prenatally and the pregnancies terminated. Children with spina bifida lose a degree of neurological function based on the spinal cord level of the lesion and its severity.
Figure 6.7.Examples of NTDs, which occur when closure of the neural tube fails.
A. Anencephaly. B, C.Infants with spina bifida. Most cases occur in the lumbosacral region. Fifty to seventy percent of all NTDs can be prevented by the vitamin folic acid.
Occurrence of these types of defects is common and varies by different regions. For example, prior to fortification of enriched flour with folic acid in the United States, the overall rate was 1 in 1,000 births, but in North and South Carolina, the rate was 1 in 500 births. In parts of China, rates were as high as 1 in 200 births. Various genetic and environmental factors account for the variability, but regardless of the region rates have been reduced significantly following folic acid administration. For example, rates throughout the United States are now approximately 1 in 1,500 births. It is estimated that 50% to 70% of NTDs can be prevented if women take 400 ?g of folic acid daily (the dose present in most multivitamins) beginning 3 months prior to conception and continuing throughout pregnancy. Because 50% of pregnancies are unplanned, it is recommended that all women of childbearing age take a multivitamin containing 400 ?g of folic acid daily. If a woman has had a child with an NTD or if there is a history of such defects in her family, it is recommended that she take 400 ?g of folic acid daily and then 4,000 ?g per day starting 1 month before she tries to become pregnant and continuing through the first 3 months of pregnancy.