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embryology 2 l Oral histology and biology MCQs for dental students

Embryology of  Head, Face , nasal cavity Oral Cavity l Oral histology & biology MCQs for dental students

Embryology of  Head, Face , nasal cavity Oral Cavity l Oral histology & biology MCQs for dental students 


 for first part click here

 

Embryology of  Head , Face , nasal cavity , palate and oral Cavity 

 

Development of the primitive oral cavity (stomodeum)

  • At the 3rd. WIUL, the primitive oral cavity (stomodeum) lies between the forebrain cranially and the developing heart.
  • At the end of the stomodeum, the oral ectoderm separated from foregut endoderm by buccopharyngeal (orophanyngeal) membrane.
  • During the 4th WIUL, the oropharyngeal membrane ruptures to get continuity between the two cavities (oral and pharynx).

Branchial arches

  • They are 6 cylindrical pairs of parallel bars of mesodermal tissue covered externally by ectoderm and internally by endoderm.
  • They are separated externally by groove and internally by pouches.
  • During the 4th WIUL, neural crest cells migrate into the branchial.

Each branchial arch has:

  1. Cartilage.
  2. Mesoderm (giving the muscles)
  3. Covered by ectoderm
  4. Lined with endoderm (except 1st arch lined with ectoderm)
  5. Nerve & artery

The first branchial arch (The mandibular arch):

Cartilage:

  • Meckel’s cartilage which has a key role in the development of the mandible.

Mesoderm:

  • gives the Muscles of mastication

Blood Supply:

  • The external and internal carotid arteries.

Nerve Supply:

  • Mandibular nerve (branch of Trigeminal nerve V) and chorda tempani branch of the Facial nerve (VII).

Derivatives of the 1st arch:

  1. mandible and maxilla
  2. lower lip and part of upper lip and cheek
  3. all the teeth
  4. all the salivary glands
  5. all the muscle of mastication
  6. Anterior 2/3 of the tongue.

 

Development of the early face

During the 5th to 8th WIUL, it develops from 5 primordia:

  1. The frontonasal process.
  2. Two maxillary processes.
  3. Two mandibular process.

  • Both maxillary and mandibular processes are derived from the first branchial arch.
  • At the end of the 5th WIUL, the mandibular processes give rise to:

  1. mandible
  2. lower part of the face
  3. body of the tongue.
A 4-week-old embryo showing the facial processes.


Embryo showing the median nasal process, lateral nasal process and olfactory pit.
Embryo showing the naso-optic furrow and auricular hillocks.



Development of the nose:

  1. At the 5th WIUL, the 2 nasal placodes develop which are thickening of the surface ectoderm on the lower part of the frontonasal processes.
  2. The nasal placodes produce the medial and lateral nasal processes, which transform nasal placodes into nasal pits (nostrils) at the 6th. WIUL.
  3. The two medial nasal processes form the center, tip of the nose and the nasal septum.
  4. The two lateral nasal processes form the lateral sides of the nose.
Development of the nose


Development of the upper lip:

  • During the 6th WIUL, the two medial nasal processes merge together in the middle to form the intermaxillary segment, which give

  1. The center of the upper lip (philtrum).
  2. The primary palate.
  3. Parts of the alveolar bone carrying the upper incisors.

  • The maxillary processes will form the lateral parts of the upper lip.
  • Thus, the upper lip is formed from:

  1. The middle third from 2 medial nasal processes.
  2. The two lateral thirds form the maxillary processes.

  • Improper fusion of one or both of maxillary processes with the medial nasal processes, or between the 2 medial nasal processes results in cleft of the upper lip.
Development of the upper lip


Development of the palate

  • It develops from two parts:

Primary palate:

  • developed at the beginning of 6 WIUL from merging of the 2 medial nasal processes. It’s called pre-maxillary bone which supports the 4 incisors.

Secondary palate:

  • started from 7 - 8 WIUL and completes at 3rd month. It developed from maxillary processes which give the two lateral palatine processes on either side of the tongue. These lateral palatine processes (shelves) elevate over the tongue.

Mechanism of palatal shelves elevation:

  1. Due to tongue depression by the growth of the mandible.
  2. Due to the presence of contractile force of the myofibroblasts in the palatal processes.
  3. High mucopolysacharides content of the shelves which attract water makes the shelves more turgid.
Formation of the secondary palate and its fusion with the primary palate and nasal septum.


Development of the tongue

 

  • The tongue is composed of 2 parts:

  1. Antelior 2/3 or Papillary portion (Ectodermal in origin).
  2. Posterior 1/3 or lymphoid portion (Endodermal in origin).

  • The two parts connected with V-shape sulcus “sulcus terminals”.
  • The tongue begins to develop at about 4 WIUL from the 1st., 2nd, 3rd and part of the 4th branchial arches.

Anterior 2/3 (Body of the Tongue):

1. It is developed from two growth centers:

  • Two lateral lingual swellings and inner swilling called tuberculum impar from the first branchial arch.

2. These lateral lingual swellings rapidly enlarge and merge with each other and overgrow the tuberculum impar.

Posterior 1/3 (Base of the Tongue):

  1. It develops from the copula of the 2nd branchial arches.
  2. The hypobranchial eminence from the 3rd. and 4th arches.
  3. The hypobranchial eminence rapidly overgrows the copula which disappears.

  • The posterior one third become fused with the anterior two thirds of the tongue, by the V shaped sulcus terminalis.
  • The lingual sulcus develops and gives the tongue mobility except in the posterior part where lingual frenum still attaches the tongue to the floor of the mouth 

Developmental anomalies:

  • May occur to Thyroid gland , tongue , lip , palate and mandible

1. cleft lip

  1. Unilatelal cleft lip
  2. Bilatelal cleft lip
  3. Median cleft lip (rare and called hare lip).

  • varies from a notch to a cleft extending into the floor of the nostrils.

2. Median cleft of the mandible

  • It is rare, results from failure of the 2 mandibular processes to merge with each other. A dimple of the chin is the simplest form of incomplete merging of the 2 mandibular processes.

3. cleft palate

  • Cleft of primary palate

  1. Results from failure of the lateral palatine processes to fuse with the primary palate and may results in missing upper lateral and canine.

  • Cleft of the secondary palate

  1. Results from failure of the lateral palatine processes to merge with each other and with the nasal septum. The degree of the cleft may vary from the simplest form or “bifid uvula” to complete cleft involving both hard and soft palate

4. Malformations of the tongue

  1. Ankyloglossia : (tongue - tie) The tip of the tongue remains tied to the floor of the mouth.
  2. Bifid tongue : failure of merging of the 2 lateral lingual swellings.
  3. Median rhomboidal glossitis :  central papillary atrophy due to failure of the lateral lingual swellings to overgrow the tuberculum impar.
  4. Macroglossia : (large tongue) in mongolism.
  5. Microglossia : small tongue.

5. Mandibulofacial dysostosis (Treacher Collin’s Syndrome)

  • It results from failure or incomplete migration of neural crest cells to the facial region.
  • Characterized by:

  1. Underdeveloped face and mandible.
  2. Improper development of all tissues of teeth (expect for Enamel).
  3. Cleft palate may be occasionally seen.
  4. Micrognathism (bird-like face).

 

 



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