This article will provide a brief overview of the ectoderm, which is one of the three layers of the early trilaminar embryo formed by gastrulation during early development. Normal embryonic development requires the proper formation of all three layers and the complex signaling between them. The development, pathophysiology and clinical significance of the ectodermal layer will be discussed in the following paragraphs.
Gastrulation is a phase that occurs during the third week of early embryonic development, occurring immediately after the blastula phase in most animals. Here, the single-layer hollow blastula reorganizes and differentiates into the multilayer gastrula with three distinct layers: the ectoderm, mesoderm, and endoderm. This differentiation is a change from homogeneous single-layered cells to distinct cell lineages, and it also establishes the axis along which the embryo will continue to develop.
Soon after the gastrulation phase, the process of neurulation will occur. During this phase, mesodermal cells (middle layer of gastrulation) will form the notochord and signal the overlying ectodermal cells in the neural plate to fold inward to form the neural tube, with the ends coming together to form the neural crest. The neural tube and neural crest are separated from the rest of the overlying ectoderm. The neural tube will become the central nervous system. The neural crest will give rise to the peripheral nervous system, enteric nervous system, melanocytes, facial cartilage, odontoblasts, enterochromaffin cells, spiral membrane, and much more. Ectoderm that does not involute will form the epidermis of the skin, hair, exocrine glands, and the anterior pituitary. Furthermore, proper development requires communication between the three layers.
The neuroectoderm will form the neural tube and neural crest. The neural tube will form the central nervous system (CNS: brain and spinal cord) and will control most body and mind functions, including controlling body movement, thoughts, and homeostasis. The peripheral nervous system (PNS) includes the nerves and ganglia outside the CNS and is divided into the somatic and autonomic nervous systems. The main function of the PNS is to connect the organs to the CNS and it is formed from the neural crest. The neural crest also gives rise to Schwann cells, chromaffin cells of the adrenal medulla, inner ear, cornea, odontoblasts, melanoblasts, pharyngeal arches, and meninges of the brain and spinal cord. The superficial ectoderm will give rise to the epidermis, external glands, hair, nails, anterior pituitary, apical ectodermal crest, among others. Functions of the superficial ectoderm include hormone regulation by the anterior pituitary gland, acting as a barrier against external influences and homeostasis.
Ectodermal differentiation towards the neural crest and neural tube pathway is correlated with member proteins of fibroblast growth factors (Fgf) that act to modulate Bmp proteins (bone morphogenetic proteins) simultaneously in a negative manner.At the same time, expression of Bmp and Wnt signals blocks FGF signals in ectodermal cells and allows it to continue into non-neural ectodermal lineage, including the epidermis.
Although most cases of ectodermal dysplasia are diagnosed clinically after birth, there are other testing options available. A sweat test may be done to identify women who have the X-linked form of hypohidrotic-dermal dysplasia. These carriers may have mild symptoms, such as defective dentition or an abnormally irregular distribution of sweat glands.
Genetic testing is available for several ectodermal dysplasia syndromes, although patients with a clinical diagnosis do not change their medical treatment because of additional genetic testing. Therefore, genetic testing is most useful for patients with mild or moderate symptoms for whom a genetic test will provide a definitive diagnosis.
Radioimmunoassay may also be performed in some cases, such as to measure EDA levels with receptor binding assays to help diagnose EDA deficiencies.
The pathophysiology of ectodermal dysplasias involves a complex set of genes and gene products that result in disruption of pathways necessary for proper development of the ectoderm. One such gene is the EDA gene, which is part of a group of genes that provide instructions for making proteins such as ectodysplasin A. Ectodysplasin A is part of a group of proteins important in signaling interactions between ectoderm and mesoderm. interactions are essential for the formation of various structures arising from the ectoderm, including skin, hair, nails, teeth, and sweat glands. Mutations in the EDA, EDAR, or EDARADD genes result in defective formation of ectodysplasin A, preventing normal interactions between these layers and thus impairing the normal development of ectodermal and mesodermal tissue. Hypohidrotic ectodermal dysplasia is a disorder that is associated with a mutation in the EDA-ectodyplastin A gene pathway.
Ectodermal dysplasias are a rare group of conditions (more than 200) arising from the abnormal development of ectodermal-derived tissues, which may include hair, teeth, nails, and glands. The conditions can have different genetic causes, but share similar characteristics and have a combination of symptoms. The diagnosis is made clinically. Ectodermal dysplasias are due to genetic defects in different chromosomes, and cases of spontaneous mutations have also been reported. One example is hypohidrotic ectodermal dysplasia (HED), which is usually transmitted as an X-linked recessive trait that can result from mutations in the EDA gene (as discussed above). The reported prevalence of ectodermal dysplasias is approximately 7 per 100,000 newborns.
Ectodermal dysplasias can be organized based on their molecular pathways; some of them are listed below:
Presents with hypohidrosis, hypotrichosis, hypodontia, smooth and dry skin, craniofacial dysmorphology, periorbital pigmentation
Presents with short stature, cataracts, microphthalmia, hypodontia, chest abnormalities, phased skin involvement, nail dystrophy, atrophic hair
Ectodermal dysplasia and immunodeficiency 1 (EDAID1)—IKBKG
Presents with hypohidrosis, hypotrichosis and immunodeficiency
Presents short stature, hypoacusis, oral papillomas, hypodontia, syndactyly
Hypodontia, eyelid cysts, keratoderma, hypoplastic nails, hypotrichosis(Video) Embryology - Neurulation
Lacrimal obstruction, hypodontia, dysplastic teeth, mammary hypoplasia, ectrodactyly, thin skin, dysplastic nails
Lacrimal duct atresia, hypodontal fissure, hypoplastic breasts, syndactyly, ectrodactyly, nail dysplasia
Current treatment of ectodermal dysplasia syndromes focuses on the multidisciplinary efforts of several specialists, including dermatologists, otolaryngologists, pediatricians, psychologists, and dentists. It is essential to diagnose the syndrome early in order to initiate early interventions to prevent new disorders and restore aesthetic features.
- (Video) Ectoderm, Endoderm, & Mesoderm
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