2. Development of face & oral cavity

3. Introduction

4. It's important to study the development of the face:
The knowledge about growth and development is very important for the orthodontist since during this period any disturbances may give rise to certain congenital
malformations,
malocclusion and
facial deformities … etc.
It's important to study the development of the face:
1-We must understand the variability that can occur in the facial form.
2-We must have awareness that the human face and oral cavity among the area that most susceptible to oral formation.

5. We know that developing human pass through three period:
1-Proliferative period (0-2 weeks) (ovum, morula & blastocyte)
2-Embryonic period (2-8 weeks)
3-Fetus period (8weeks-9months)

7. Primitive oral mouth cavity:
In 3-4th weeks old primitive mouth cavity stomodeum is a narrow slit like space lined by ectoderm. It is bounded above by the under surface of brain capsule, and below by the upper surface of preicardial sac. The developing mandibular processes form a rim-like elevation bounding the cavity at the sides. At the back of the cavity there is a thin oral membrane forming a septum, which at this stage separates the primitive mouth from cavity from the pharynx. Later, this membrane will breakup so the stomodeum becomes directly continuous with the foregut (at end of third week of I.U.L)

8. The head and neck are developed from the branchial apparatus, which consists of branchial arches, pharyngeal pouches, branchial grooves, and branchial membranes. Branchial arches are derived from the neural crest cells and begin to develop early in the 4th week, as the neural crest cells migrate into the future head and neck region. By the end of the 4th week, four pairs of branchial arches are visible. The fifth and sixth pairs are too small to be seen on the surface of the embryo. The branchial arches are separated by the branchial grooves, which are seen as prominent clefts in the embryo.
The branchial arches contribute to formation of the face, neck, nasal cavities, mouth, larynx, and pharynx, with the muscular components forming striated muscles in the head and neck.

10. The face develops from the tissues surround the oral pit during the second period & between 3-7 weeks, so:
-At the 3rd week:
Pros-encephalon overhangs the developing oral groove to form the stomodeum, the oral groove deepens and oral plate & buccopharyngeal membrane which consists of ectodermal floor of stomodium & endodermal lining & forgout ruptures to form oral opening.

11. The face develops from the tissues surround the oral pit during the second period & between 3-7 weeks, so:
-At the 4th week:
Above the oral pit is the covering of brain termed frontal process from which forehead develop, lateral to oral pit are 2 maxillary processes and below is the mandibular arch which form the lower jaw and inferior to it second branchial arch (hyoid bone) which contribute in face development..

12. The face develops from the tissues surround the oral pit during the second period & between 3-7 weeks, so:
-At the 5th week:
Fusion of the medial end of mandible & in the midline to form chin, lower lip. Any failure in fusion leads to cleft (uncommon) the epithelium covering the brain capsule has thickening in 2 places on each side of head, the more ventral thickening is the nasal placode that develop into nostrils & the more dorsal is the lens placode from which eye lens develops.
At this stage the frontal becomes frontonasal process, as the the tissue around the nasal placode grow resulting in 2 opening in the oral pit and nastrial becomes deeper & the tissue lateral to it termed lateral nasal process & tissue medial to it termed medial nasal process.

13. The face develops from the tissues surround the oral pit during the second period & between 3-7 weeks, so:
-At the 6th/7th week:
The lateral part of the face expand, this will change the eye from the lateral of the head toward the medial location.
The maxillary process move to each other & fuse in the mid-line to form the intermaxillary segment which will give rise to :
1-Philtrum (middle portion of upper lip)
2-prelabium.
3-Collumella.
4-Nasal septum.
5-Primary palate & anterior part of palate.
6-4 incisors, alveolar bone & gingiva.

14. The face develops from the tissues surround the oral pit during the second period & between 3-7 weeks, so:
-At the 6th/7th week:

15. The union between the medial nasal & maxillary process give rise:
2-Palate
3-upper lip
4-lower central part nose.
Failure of fusion of medial nasal process & maxillary process leads in cleft lip (unilateral or bilateral).
At the line of fusion of lateral nasal process & maxillary process is the naso-lacrimal duct which connect the lacrimal sac to the nasal cavity & there will be a modification of 1st branchial groove into ear canal.
The nasal septum consists of 2 parts :
1-Anterior part form frontonasal process.
2- Posterior part form maxillary process.
Here nasal septum is free with the dorsal of tongue.

16. Development of the palate (6-12 weeks)
Human palate is the tissue that separate the oral & nasal cavites .
Entire palate consists from 2structures :
1-Primary palate (premaxilla which derive from intermaxillary segment )
2-Secondary palate (hard & soft palate ) which derive from lateral palatine shelves of the maxilla.
These shelves oriented in the vertical plane & tongue between them; later the palatine shelves elongated & the tongue becomes small & move inferiorly, this allows the shelves to become horizontally & fused in midline (median palatine raphe), incisive foramen at the junction between primary palate and lateral palatine shelves. Failure of fusion of the lateral palatine shelves with each other & with the nasal septum & with primary palate result in cleft palate (uni-, bilateral or with cleft lip)

17. Development of the tongue :
The mucosa of body of tongue (ant. 2/3 develop from 1st branchial arch (Mand.) & mucosa of base (Post. 1/3) of the tongue develop from 3rd branchial arch.
During tongue development there is enlargement called tuberculum impar & 2 lateral lingual swellings adjacent to tuberculum impar, these 3 structures arise from 1st branchail arch & give rise to body of tongue, the base of tongue develop from hypobranchial eminence over grow & fused with tuberaculum impar & lateral lingual swelling. The line between body and base of tongue called sulcus terminalis at which foramen cecum. Tongue may show developmental abnormality.

18. Cheek and parotid gland:
Cheek consists of tissue derived from mandibular & maxillary processes. At the back laterally on each side of oral cavity. The parotid salivary gland develop as an out growth of oral epithelium & open the opening of its duct related to upper second molars.

19. Facial skeleton
Face is formed by cartilage. Meckel's cartilage develop within mandibular arch & cartilage of nasal capsule develops in the maxillary process & extend forward into frontonasal process & behind , it is continues with cartilage of cranial base (beneath the brain )
Nasal septum form septal cartilage. At later stage in the development, the maxilla, premaxilla & lacrimal bones develop on the outer side of cartilage of the nasal capsule, the palatine bone develop on its inner aspect the mandibule develops on the outer side of Meckel's cartilage & vomer developes from septal cartilage.

20. The development of the jaws :
The jaws begins to form during the period of early development of deciduous teeth, the first indication of bone formation occur with the appearance of dental lamina in the maxillary and mandibular processes & by the time that the deciduous tooth germs lie in cavities, the alveoli, formed by bone of the mandible & maxilla, they have come in to close relationship with jaws.

21. Development of the mandible
The mandible is formed in the lower or deeper part of the first visceral (mandibular) arch, & it's preceeded the Meckel's cartilage :
Meckel's cartilage : At 6th week of the fetal life (15mm length) , It's attains it's fill form & then stretches downward & forward as a continuous rod from the cartilaginous otic capsule to the midline, where its ventral end turns upward in contact with the cartilage of the opposite side, to which it's joined by mesenchyme. The dorsal end of the cartilage gives rise to the malleus of the middle ear. The remaining part of the cartilage is largely associated with the development of the mandible which form the replacing skeletal structure.

22. The mandibular nerve :
It is the nerve of first visceral arch, at this stage of development the Meckel's cartrilage in close relation to it & act as a skeletal support to it.
The main nerve comes out from the skull medial & ventral to the dorsal end of the cartilage & at the junction of the dorsal & middle 1/3 of the cartilage the nerve into direct relationship with it & at this region & after giving its other branches it is divided into lingual & inferior dental nerve. The lingual nerve pass forward on its medial side, the inferior dental nerve lies lateral to its upper margin & running forward parallel to its terminals by dividing into mental and incisive branches, the incisive branch continues its course parallel to the cartilage.

23. The body of the mandible:
The mandible firstly appear as a band of dense fibrocellular tissue lies on the lateral side of the inferior dental nerve & incisive nerve.
At the 7th week of the fetal life (17-18mm length) ossification occurs in the angle formed between the mental & incisive nerve which is the site of future mental foramen.
From this center bone formation spread rapidly backward below the mental nerve & lie in a notch on the lateral side of the inferior dental nerve. In this notch the bone grow medially below the incisive nerve & soon afterward it goes upward between the incisive nerve & meckel’s car. so contained in a trough channel of bone formed by medial & lateral plates which united below the nerve.
At the same stage & by the extension of bone over the nerve from the anterior to posterior edge of the notch, the notch contining the mental nerve converted into mental foramen.
The bony trough grows rapidly forward toward the midline where it become in close relationship with similar bone formation on the opposite side but firstly these 2bones separated from each other by connective tissue & the union between them occur before the end of the first year of life.
By the growth of bone over the incisive nerve from the lateral &
medial plates the trough of bone is converted into incisive canal.

24. In the backward direction a similar spread of ossification which produce first a plate of bone in relation to the lateral aspect of inferior dental nerve, then a bony trough in which the nerve lies & very much later the canal for it.
So by these processes of growth the original primary center of ossification produce the body proper of the mandible as for back as the mandibular foramen & as forward as the symphysis, which is the part of mandible surround the inferior dental nerve and the incisive nerve.
At this stage the developing tooth grows lie some little distance superficial to the mandible & are not contained by it.
The cavities continue to increase in size with growth of the condylar & of the articular fossa. The articular eminence is hardly apparent at birth & only begins to attain its typical form afte the establishment of the deciduous, small areas of secondary cartilage appear in the temporal region, they appear ;later the condylar cartilage & disappear before birth.

25. The alveolar bone :
When the deciduous enamel organs reach the early bell stage. The bone of the mandibular begins to come in close relationship with them & this happen by the upward growth, on each side of the tooth germ of the tooth germ of the medial and lateral plates above the level of the roof which is formed for the canal of the incisive & inferior dental nerve.
So the developing tooth come to lie in the trough of bone, which is later divided into separate small alveoli & this is done by the formation of septa between its walls.

26. Fate of Meckels' cartilage :
The anterior part of the mandible, except in midline, in front of the mental foramen, include cartilage in its substance which is firstly surrounded by bone extend from the medial plate & then gradually resorbed & replaced by ossification extend from the membrane bone around it.
During the later fetal period & at least until the time of birth one or two nodules of cartilage are seen in the fibrous tissue of the symphysis. Which are the remnants of the ventral end of the cartilage.
The rest disappear completely except for a part of its fibrous covering which persists as the sphenomandibular & sphenomalleolar ligaments.
The most dorsal part of the cartilage ossifies to form the malleus, which is attached to the sphenoidal spine by the sphenomalleolar ligament.

27. The ramus:
The ramus is produced by a spread of ossification from the body of mandible, behind & above the mandibular foramen.
From this region the mandible diverge laterally from the the line of the meckel's cartilage.
Just like the body of the mandible, the ramus & its processes are firstly mapped out by an extension of the fibrocellular condensation.
The formation of bone in this tissue occur rapidly so that the coronoid & condylar processes are to large extent ossified at 40mm CR (10th week). The further growth of these processes is modified by the appearance of secondary cartilage.

28. The secondary (Accessory) cartilages:
The occur at various sites in the region of membrane bone formation since they are not part of, & have no connection with primary cartilage (meckel's cartilage), so they called secondary.
They differ from the primary cartilages which are formed by Hyaline cartilage In their behaviour & histological appearance.
Which have less inner cellular matrix than hyaline cartilage.
Secondary cartilage increase in size by the proliferation & trans formation of the cells of the thick layer of the fibrocellular tissue which covers them.
In the mandible there are 3 main sites of secondary cartilage formation which are:
1-Condylar.
2-Coronoid.
3-Symphyseal.

29. 1-The condylar cartilage:
cartilage in anterior older part of the tissue begins to show endochondral ossification & this continue until the only area of the cartilage left un resorbed in a zone immediately beneath the proliferating tissue of the condylar articular surface. So in the 5th months of fetal life, the original cone of cartilage is largely replaced by bony tubercular which run through the ramus & contrast with the membrane bone of the ramus. (The zone of the cartilage left behind the articular surface of the condyle persists until at least the end of the 2nd decade)
During this period as proliferative activity of cells of its covering fibrocellular layer grows less, the thickness of zone of cartilage disappear & replacing bone forms whole of condylar.
So the cartilage will increase the length of the mandible during this period.
By 5th month a large vascular canals have appeared in the condylar cartilage, these are still present at birth & they are related to the nutritive requirements if the rapidly growing cartilage.
Largest,1st secondary cartilage & it is of great importance for the growth of mandible.
1st appearance at the 12th week (50mm length) at which its seen as a fringe of cartilage on the superior & lateral aspects of the bone in the condylar process which will merges on oneside into this bone & on the other into the fibrocellular layers which limits the condylar region.
By additions from the cells of the covering fibrocellular tissue, the cartilage from a cone shaped mass which not only occupy the whole of the condylar process but reaches forward & downward into the ramus as far as level of mandibular foramen.

30. 2-the coronoid cartilage: disappeared long before birth.
It appear in 80 mm CR.
It forms a strip along anterior border & submit of the coroniod process. It is covered superficially by thick fibrocellular layer & rests on the membrane bone below. All traces of it has
disappeared long before birth.

31. 3-the Symphyseal cartilage:
In 100 mm CR.
At each end of two halves of bony mandible , these enable mandible to grow in width, while they persist it is entirely in dependent of Meckel's cartilage & it's perichondrium.
Shortly after birth the 2 halves united & this union obliterate these cartilage so that they take no further part in growth of the mandible. Small areas of secondary cartilage, irregular in their location & appearance may supply for a time the growth of the alveolar bone at its margin .
At birth the mandible differs in several respects from the adult bone. The chief differences are
1-the absence of any definite chin
2-wide mandibular angle.
3-small size of ramus compare with the body.

32. Development of Mandibular joint:
It first induced by growth of tissue condensation of the developing mandible, which every where precedes ossification, towards the corresponding condensation for the temporal region.
The mandibular condensation maps out the shape of the condylar.
At this stage the mandibular & temporal elements of the joint are still separated by a wide interval. The closure approximation of the mandibular to the temporal region is brought about by the development of secondary cartilage in the condylar process. By the rapid growth of the cartilage of the previously wide interarticular interval is largely obliterated. The only intervening tissue left is a strip of dense tissue immediately above the upper surface of the condylar. This tissue appears at the same time of as the condensation for the condylar & it's connected to the lateral pterygoid muscle from its first appearance. The strip of tissue becomes the articular disc. The formation of the joint cavities above & below this strip of tissue occurs as the condyle becomes approximated to the temporal element of the joint. Joint cavity development is virtually complete between the 65-70mm C.R. stages)

33. The development of maxilla:
The upper jaw (maxilla proper) excluding the premaxilla is developed in the maxillary process of mandibular arch.
The first appearance is a membraneous ossification, just like the mandible, but unlike the mandible , its further development & growth are little affected by the appearance of secondary cartilage.
Ossification of maxilla start slightly later than in the mandible & center of ossification first appears in a band of fibrocellular tissue, which lies to the outer side of the cartilage of the nasal capsule & immediately lateral to & slightly below the infra orbital nerve where its gives of its anterior superior dental branch. Ossification center lies above that part of the lamina from which develops the enamel organ of the canines tooth germ.

34. The ossification spreads from this center backward towards the developing zygomatic bone below the orbit & forward infront of anterior – superior dental nerve below the terminal part of the infra orbital nerve toward the developing premaxilla.
At this stage the forming bone takes the shape of a curved strip, arranged vertically with the convex side directed medially.
From anterior extension there develops the upward directed frontal process which, with a corresponding process of premaxilla, forms the frontal process of the adult bone.
Early in development the developing maxilla forms a bony trough for the infra orbital nerve & by downward growth, an outer alveolar plate in relation to the canine & deciduous molar tooth germs.
The growth of maxilla continues mainly upward, downward & backward & with the development of a palatal process also spreads towards the midline in the substance of the anterior part of the united palatal folds.
By the appearance of secondary cartilage in the zygomatic process & by its proliferation for a time adds considerably to the bulk of this part of maxilla, during this period the palatal process extends backward, at the union of the palatal process & the main part of the developing maxilla a large mass of bone is produced.

35. From this region, on the inner side of dental lamina & tooth germs, the medial alveolar plate develops some what later than the lateral alveolar plate. The trough of bone formed is later than the lateral alveolar plate. The trough of bone formed is later divided by septa into alveoli.
Also some areas of secondary cartilage may develop along the growing margins of the alveolar plates & in the midline of the developing hard palate between the 2 palatal processes

36. The maxillary sinus:
At the 4th month of the fetal life its first appearancre as a small out pocketing of the mucosa from the lateral wall of the nasal cavity. It is firstly separated from the developing maxilla by the cartilage of the nasal capsule & only comes into direct relationship with the bone after the cartilage has atrophied.
In its gradual extension the sinus comes into relation with the maxilla above the level of the palatal process & hollows out the interior of the bone, so separating its upper or orbital surface from its lower dental region.
At birth, however, these parts are still close together & it is only very gradually that the maxilla increases in height with an associated increase in the size of the sinus.
Near the time of the completed eruption of the permanent teeth, the maxilla reaches its final height.
The sinus may increase in size however during the adult life by extension into alveolar process.

37. Comparison of mandibular & maxillary development
both have neural and alveolar elements.
Both develop secondary cartilages in their back ward extension.
Both commence as a centers of ossification in close relation to a nerve at a place of bifurcation & in close relation to elements of cartilaginous facial skeleton.
The condylar cartilage, however, remains active as a growth center for along period in relation to the mandibular joint, the zygomatic (Malar) cartilage of the maxilla is restricted in its appearance & activity to a limited period of fetal life & in related to the suture between it & the zygomatic bone.
The maxilla has no muscular processes & the mandible has no palatal process.
In the growth, the maxilla depends upon surface deposition & growth at the suture at which it articulate with the adjacent bones, the mandible depends for its growth on the surface growth on the surface deposition & the replacement of the cartilage by bone.