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EMBRYOLOGY7 Dr.Ban A.Ghani EMBRYOLOGY7 Dr.Ban A.Ghani Developmdent of external /Sequence of developmental During the third week of development an oropharyngeal membrane buccopharyngeal , or oral membrane) is first seen at the site of the future face, between the primordium of the heart and the rapidly enlarging primordium of the brain. It is composed of ectoderm externally and endoderm internally. It lies at the beginning of the digestive tract and breaks down during the 4th week in order to form the opening between the future oral cavity (primitive mouth or stomodeum) and the foregut. The oropharyngeal membrane breaks down when it stops growing. While tissues around it expand very rapidly, the oropharyngeal membrane’s non-proliferating cells are gradually pulled apart because they cannot fill the expanding area. The human face begins to form during the 4th week of embryonic development. By the 6th week the external face is completed. The development of the palate subdivides nasal and oral cavities. This development continues into the 12th week with completion of the soft palate. Human face during the 4th prenatal week. Around the centrally located oral pit are grouped the frontal and maxillary processes and the mandibular arch. 1 EMBRYOLOGY7 Dr.Ban A.Ghani Human face during the 5th prenatal week. The nasal pits develop and appear on the sides of the face. The frontal process now becomes the frontonasal process Human face during the 6th prenatal week. Nasal pits appear more centrally located in the medial nasal process. This is the result of growth of the lateral face, which also causes the eyes to approach the front of the face. The enlarged maxillary processes are near contact with the medial nasal process. Development of the face At the end of the 4th week, the center of the face is formed by the stomodeum , surrounded by the first pair of pharyngeal arches. Face initially formed by 5 mesenchymal swellings ( prominences): Two mandibular prominences (right and left, from 1st arch neural crest mesenchyme) . Two maxillary prominences (right and left, from 1st arch neural crest mesenchyme). The maxillary prominence is initially located superior/lateral to the stomodeum while the mandibular prominence is located inferior to it. 2 EMBRYOLOGY7 Dr.Ban A.Ghani Frontonasal prominence (midline structure, from cranial neural crest mesenchyme) is a single structure that is ventral to the forebrain. It is derived from neural crest cells, which have an ectodermal origin. Nasal placodes originate on the frontonasal prominence from the ectoderm. They thicken and sink in to form nasal pits. At the same time, mesodermal cells proliferate around the placodes, and the sides of these swellings form the medial and lateral nasal prominences. As the maxillary prominences continue growing they merge laterally with the mandibular prominences to form the cheeks. The cheeks :after formation of the upper and lower lips,the stomodium is very broad .In it’s lateral part,it is bounded above by the maxillary process & below by the mandibular process.These processes undergo progressive fusion with each other to form the cheeks. Macrostomia (incomplete lateral merging of maxillary and mandibular processes). The growth of maxillary prominences compresses the medial nasal prominences and causes them to fuse around the 10th week of development. This establishes a- the bridge of the nose and b-the intermaxillary segment, The intermaxillary segment gives : a- the portion of the upper lip containing the philtrum, b- the upper jaw with 4 incisors, and c- the primary palate. 3 EMBRYOLOGY7 Dr.Ban A.Ghani Origin of the human face and mouth. The face develops from five primordia that appear in the fourth week: the frontonasal prominence, the two maxillary swellings, and the two mandibular swellings. The buccopharyngeal membrane breaks down to form the opening to the oral cavity. The medial prominence fuses with the maxillary prominence, giving rise to a smooth upper lip while fusing the primary and secondary palate. The lateral nasal prominence gives rise to the alae of the nose and fuses with the maxillary prominence, forming the nasolacrimal duct. This duct is formed when the ectoderm thickens into a cord and sinks into the underlying mesenchyme. 4 EMBRYOLOGY7 Dr.Ban A.Ghani Development of the nasal cavity The formation of the lateral and medial nasal prominences makes the nasal placodes lie in the floor of the depression, called nasal pits. The nasal pits deepen and develop the nasal sacs in the 6th week. These new structures grow dorsocaudally in front of the forming brain. In the beginning, the nasal sacs are separated from the oral cavity by the oronasal membrane. This membrane disappears in the 7th week leaving a connection between the nasal cavities and the oral cavity, called the primitive choanae. Later, when the development of the secondary palate occurs, the choanae changes its position and locates at the junction of the nasal cavity and the pharynx. The nasal septum grows as a downgrowth from the merged nasal prominences and fuses with the palatine process between the 9th and 11th week. Finally, the superior, middle and inferior conchae develop the lateral wall of each nasal cavity. 5 EMBRYOLOGY7 Dr.Ban A.Ghani The epithelial covering of the medial nasal and maxillary processes normally contact and create a zone of fusion named nasal fin. This epithelial fin is soon presented by connective tissue growth, which binds together the two maxillary and medial nasal parts of the lip. If this penetration were not to occur, the lip could pull apart. 6 EMBRYOLOGY7 Dr.Ban A.Ghani Schematic depiction of breakdown of nasal fin and formation of nostrils. Arrows indicate disintegration of the nasal fin between the medial nasal and maxillary prominences External ear External ear is formed around the dorsal part of the 1st ectodermal cleft.A series of mesoderal thickenings appear on the mandibular and hyoid arches where they adjoin this cleft.The pinna is formed by the fusion of these thickenings,when first formed the pinna lies caudal to the developing jaw. It is pushed upward and backward to it’s definitve position due to the great enlargement of the mandibular process.If the mandibular process fails to enlarge ,the ears remain low down. 7 EMBRYOLOGY7 Dr.Ban A.Ghani 8 EMBRYOLOGY7 Dr.Ban A.Ghani Occurrence of abnormalities throughout a complicated developmental process is inevitable. The incidence of congenital malformations of the face is approximately 1 in 700 births. This number includes major defects incompatible with life and minor defects that are surgically correctable. The most severe congenital problems are those that develop early in facial development (4th –8th weeks); relatively minor problems develop later (8th –12th weeks). Clinically, it is important to realize that the tissues surrounding the forebrain: frontonasal process develop separately from the tissues of the first pharyngeal arch( mandibular and maxillary processes) ,therefore a developmental problem may be seen in one or the other but usually not in both places. 9 .
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