Origin Of facial Tissuesfertilization ovum egg cell mass (morula) epiblast, hypoblast inner cell mass the anterior end of the primitive streak form endoderm which embedded the midline notochondral, prospective mesodermal cell migrate from the epiblast through the primitive streak to form mesoderm, cell remaining in the epiblast form ectoderm, completing formation of the three germ layer
• Figure 2-11 A, The mesoderm, situated between the ectoderm and endoderm in the trilaminar disk. B, Differentiation of the mesoderm into three masses: the paraxial, intermediate, and lateral plate mesoderm. C through E, With lateral folding of the embryo, the amniotic cavity encompasses the embryo, and the ectoderm constituting its floor forms the surface epithelium. Paraxial mesoderm remains adjacent to the neural tube. Intermediate mesoderm is relocated and forms urogenital tissue. Lateral plate mesoderm cavitates, the cavity forming the coelom and its lining the serous membranes of the gut and abdominal cavity.
Development of Facial Prominences• Development of nasal placodes, frontonasal region, primary palate, and nose• Development of maxillary prominences and secondary palate• Development of visceral arches and tongue
Development of nasal placodes, frontonasal region, primary palate, and nose• Thickening of the surface ectoderm on either side of the frontal prominence just above the stomodeum is the first indication of the nasal cavity called the nasal (olfactory) placodes• Nasal placodes are ectoderm induced by ventral forebrain• At this time there are 5 prominence and 2 nasal placodes
• Nasal (olfactory) pits are located on either side of the frontonasal prominence and are surrounded by horseshoe-shaped eminences. The medial portion of these eminences is called the medial nasal process (MNP). The lateral portion of which is called the lateral nasal process (LNP). The lateral nasal process is separated from the maxillary process (the more rostral portion of the first branchial arch) by a furrow which reaches the medial aspect of the developing eye.
• Figure 3-17 Human facial development from 24 days through 38 days. Left-column photographs shows actual embryos; the middle and right columns are diagrams of frontal and lateral views. A, Boundaries of the stomatodeum in a 26-day embryo. B, A 27-day embryo. The nasal placode is about to develop, and the odontogenic epithelium (white bars) can be identified. C, A 34- day embryo. The nasal pit, surrounded by lateral and medial nasal processes, is easily recognizable. D, A 36-day embryo shows the fusion of various facial processes that are completed by 38 days (E)
• The anterior aspect of this partition is derived from the area of the upper jaw formed by the medial nasal processes (intermaxillary segment) and is called the primary palate (median palatine process).
Development of maxillary prominences and secondary palate• Most of the palatine partition, is derived from the medial growth of shelf-like processes originating from the maxillary process called the palatine shelves (lateral palatine processes). This segment of the palate is called the secondary palate. As the secondary palate is formed, the nasal septum grows inferiorly toward it. The nasal septum and the two palatine shelves unite to form separate right and left nasal chambers
• Most of the hard palate and all of the soft palate form from the secondary palate
Development of visceral arches and tongue• In humans there are six visceral arches, which the fifth is rudimentary.• The proximal portion of the first arch becomes maxillary prominence, the mandibular and hyoid arches develop at their distal portion to become consolidated in the ventral midline• Nerve fibers from 5, 7, 9, and 10 cranial nerves extend to the mesoderm of the first four visceral arches.
• Figure 3-22 Development of the tongue. A, The floor of the primitive stomatodeum, viewed from above, is formed by the branchial arches. Three swellings, the tuberculum impar and the paired lingual swellings, appear in the mesenchyme of the first arch beneath the epithelium. A midline swelling (the hypobranchial eminence) appears in the third arch; the sagittal section through the arches is shown in the lower drawing. B, The increased swelling of the lingual swellings, together with the tuberculum impar, will form the anterior two thirds of the tongue. The hypobranchial eminence overgrows the second arch (depicted in the sagittal section in the lower drawing). C, Final disposition of the tongue and the relative contributions of the first and third arches. The sagittal section is shown in the lower drawing. The arrow depicts the route of incoming occipital myotomes that form the tongue muscle.
• Know that the anterior two thirds of the tongue is covered by ectoderm and derived from first arch mesenchyme• And posterior one third of tongue is covered by endoderm and be primarily derived from the third arch mesenchyme