C2 Oral Histo (Development of face and oral cavity)

• The enamel-forming cells are derived from ectoderm lining the oral cavity.
• Besides inducing the neural plate from overlying ectoderm, the chorda mesoderm organizes the positional relationships of various neural plate components, such as the initial primordium of the eye.
• Neural crest cells give rise to a variety of different cells that form components of many tissues.
• Of the skeletal or connective tissue of the facial region, it appears that tooth enamel is the only one not formed by crest cells.
• In the trunk, all skeletal and connective tissues are formed by mesoderm.
• Later these structures form myoblasts and some of the skeletal and connective tissues of the head.
• A unique population of cells develops from the ectoderm along the lateral margins of the neural plate.
• These are the neural crest cells.
• Neural crest cells undergo extensive migrations, usually beginning at about the time of tube closure.
• The mesodermal portion differentiates into well-organized blocks of cells, called somites, caudal to the developing ear and less organized somitomeres rostral to the ear.
• Pharyngeal clefts and pouches have a specific fate.
• The nerve supply and development of the tongue can be correlated.
• The derivatives of the neural crest cells include muscular, skeletal, nerve, and arterial elements formed in each pharyngeal arch.
• After fertilization of the ovum, a series of cell divisions gives rise to an egg cell mass known as the morula in mammals.
• In most vertebrates, including humans, the major portion of the egg cell mass forms the extraembryonic membranes and other supportive structures, such as the placenta.
• The inner cell mass separates into two layers, the epiblast and hypoblast.
• Cell marking studies in chick and mouse embryos have shown that only the epiblast forms the embryo, with the hypoblast and other cells forming supporting tissues, such as the placenta.
• The anterior (rostral) end of the primitive streak forms the lower germ layer, the endoderm, in which are embedded the midline notochordal (and prechordal) plates.
• The heart and adjacent portions of the visceral arches have been removed in Figure 2.7A, and most of the heart has been removed in Figure 2.7B and Figure 2.7C.
• On the completion of the initial crest cell migration and the vascularization of the derived mesenchyme, a series of outgrowths or swellings termed “facial prominences” initiates the next stages of facial development.
• After the crest cells arrive in the future location of the upper face and midface, this area is referred to as the frontonasal region.
• Experimental evidence indicates that the lateral edges of the placodes actively curl forward, which enhances the initial development of the lateral nasal prominence (LNP, sometimes called the nasal wing).
• Facial prominence development begins with the curling forward, lateral portion of the nasal placode and is completed after fusion of prominences with each other or with other structures.
• The growth and fusion of upper facial prominences produce the primary and secondary palates.
• Mesenchymal cell process meshwork (CPM) is exposed after removal of epithelium in Figure 2.7C and is illustrated to the right side of Figure 2.7C.
• The combined right and left MxPs are sometimes called the intermaxillary segment.
• As will be described below, other prominences developing from the first two visceral arches considerably alter the nature of these arches.
• A single mesenchymal cell body is outlined by a broken line in Figure 2.7C.
• The first structures to become evident in the frontonasal region are the olfactory placodes, which are thickenings of the ectoderm that appear to be derived at least partly from the anterior rim of the neural plate.
• With development of the LNP–MNP contact, all the three prominences contribute to the initial separation of the developing oral cavity and nasal pit, which is usually called the primary palate.
• In the scheme of development of facial prominences, the outgrowths or swellings are termed “facial prominences”.
• The maxillary prominence (MxP) has already grown forward from its origin at the proximal end of the first visceral arch to merge with the LNP and make early contact with the medial nasal prominence (MNP).
• Prospective mesodermal cells migrate from the epiblast through the primitive streak to form the middle germ layer, the mesoderm.
• Accumulation of fluid within the egg cell mass (morula) leads to the development of a blastula.
• The inner cell mass (heavily stippled cells in the blastula) will form a two-layered embryonic disk in the blastula.
• It appears that only the epiblast will form the embryo, with the hypoblast and other cell populations forming supporting tissues of the embryo.
• In the blastula, the notochord and its rostral extension, the prechordal plate, as well as associated pharyngeal endoderm, form as a single layer.
• Prospective mesodermal cells migrate through the primitive streak and insert themselves between the epiblast and endoderm.
• Cells of the notochord and prechordal plate, and adjacent mesoderm, are termed chorda mesoderm.
• Cleft palate can also be produced in experimental animals with a wide variety of chemical agents or other manipulations affecting the embryo.