Lec. 2 Dr.Ali H.Murad

Development of the face &oral cavity

When the stomatodeum 1st forms, it bounded above by neural plate, below by heart, & laterally by 1st branchial arch. The 1st ,2nd, & 3rd branchial arches play an important role in the development of the face, mouth, & tongue. With spread of the arches midventrally, the cardiac plate is eliminated from the stomatodeum, & the floor of the mouth is formed by the epithelium covering the mesenchyme of the 1st, 2nd, & 3rd branchial arches.

At about 24 days the 1st arch establishes an another process (maxillary process). So that the stomatodeum is now limited cranially by the frontal prominence, laterally by the newly formed maxillary process, & ventrally by the 1st arch (mandibular arch). The buccopharyngyeal membrane at the first separates the stomatodeum from the foregut, but this soon breaks down so that the stomatodeum (primitive mouth) communicates directly with the foregut.

Formation of the face At about 28 days localized thickenings develop within the ectoderm of the frontal prominence, just above the opening of the stomatodeum. These thickenings are the (olfactory placodes)

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By rapid proliferation of underlying mesenchyme around the placodes bulges the frontal eminence foreward & also produce a horse shoe shaped ridge that converts the placode into nasal pit.

The lateral arm of the horseshoe is called lateral nasal process, & the medial arm called medial nasal process. Between the two nasal process a depressed area formed called the frontonasal process. The medial nasal process of both sides together with frontonasal process give the middle portion of nose, middle portion of upper lip, anterior portion of maxilla & the primary palate

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The maxillary process grows medially & approaches both the lateral & the medial nasal processes but remains separated from them by distinct grooves. The medial growth of maxillary process pushes the medial nasal process toward the midline, where it merges with opposite side, eliminating the frontonasal process. In this way the upper lip is formed from the maxillary processes of each side & the medial nasal process, with fusion occurring between the foreword extent of the maxillary process & the lateral face of the medial nasal process. The lower lip is formed by merging of the two ectomesenchyme of mandibular process

Tongue formation The tongue begins to develop at about 4 weeks. By local proliferation of the mesenchyme of the 1st 3 branchial archs gives rise to a number of swellings in the floor of the mouth. 1st ,a swelling (tuberculum impar) arises in the midline in the mandibular process & is flanked by 2 other bulges (lingual swelling) Very quickly these lateral lingual swelling enlarged & merge with each other & the tuberculum impar to form a large mass, from which the mucous membrane of the anterior 2\3 of the tongue is formed.

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The posterior 1\3 of the tongue arises from the hypobranchial eminence, which is a large midline swelling developed from mesenchyme of 3rd arch. The mesenchyme of the 3rd arch rapidly overgrows that of the second arch, which is thereby excluded from further involvement in the development of the tongue. The hypobranchial eminence gives rise to the mucosa covering the posterior 1/3 of the tongue. The tongue separates from the floor of the mouth by a down growth of ectoderm around it’s periphery, which subsequently degenerates to form the lingual sulcus & give the tongue mobility

Formation of the palate  Initially there is a common oronasal cavity bounded anteriorly by the primary palate & occupied mainly by the developing tongue.  The formation of the secondary palate occurs between 7 & 8 weeks of development, & results from the fusion of shelves formed from each maxillary process  The palatine shelves, are first directed downward on each side of the tongue.  After 7th week of development, the tongue withdrawn from between the shelves, which now elevate & fuse with each other above the tongue & with primary palate.  The fusion of the shelves occurs by eliminating their epithelium, this achieve 4

by cessation of DNA synthesis within the epithelial some 24-36 h. before epith. Contact  Surface epithelial cells are sloughed off as they undergo physiologic cell death to expose the basal cells.  These cells have a carbohydrate-rich surface coat that permits adhesion & the formation of junction to achieve fusion of the processes.

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