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Embryology

Lec7 Dr.Ban

Development of face and nasal cavity The external human face develops between the 4th and 6th weeks of embryonic

development. Facialswellings arise on the frontonasal process (2 medial nasal and

2 lateral nasal processes) and the firstpharyngeal arch (2 mandibular and 2

maxillary processes). By a process of merging and some localizedfusion these

processes come together to form the continuous surfaces of the external face. The

primary palate is formed in this period by fusion/merging of the medial nasal and

maxillary processes.

Sequence of developmental events :

During the 3rdweek of development an oropharyngeal membranebuccopharyngeal

,or oralmembrane) is first seen at the site of the future face, between the

primordium of the heart and the rapidlyenlarging primordium of the brain. It is

composed of ectoderm externally and endoderminternally. It lies at the beginning

of the digestive tract and breaks down during the 4th week in orderto form the

opening between the future oral cavity (primitive mouth or stomodeum) and the

foregut. Theoropharyngeal membrane breaks down when it stops growing. While

tissues around it expand veryrapidly, the oropharyngeal membrane’s non-

proliferating cells are gradually pulled apart because theycannot fill the expanding

area.

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Origin of the human face and mouth. The facedevelops from five primordia that appear in the fourth week:

thefrontonasal prominence, the two maxillary swellings, and thetwo mandibular swellings. The buccopharyngeal membranebreaks down to form the opening to the oral cavity.

Subsequently, between 6th and 12th embryonic/fetal weeks, the secondary palate is

formed as the result of fusion between palatal processes growing from the oral

surfaces of the maxillary processes. Each merging and fusion site is also the site of

a potential facial or palatal cleft.

The human face begins to form during the 4thweek of embryonic development. By

the 6th weekthe external face is completed. Between the 6th and 8thweeks the

development of the palate subdividesnasal and oral cavities. This development

continues into the 12th week with completion of the soft palate.

Occurrence of abnormalities throughout a complicated developmental process is

inevitable. Theincidence of congenital malformations of the face is approximately

1 in 700 births. This numberincludes major defects incompatible with life and

minor defects that are surgically correctable. The mostsevere 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 tissuessurrounding the forebrain: frontonasal process develop separately

from the tissues of thefirst pharyngeal arch: mandibular and maxillary processes .

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Therefore you may find adevelopmental problem in one or the other but usually

not in both places.

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.

• 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,thestomodium 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. Macrostoma (incomplete lateral merging of

maxillary and mandibular processes).

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• The growthofmaxillary 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,

which is the region of the medial nasal prominence located inferior to the

bridge of the nose and superior to the mandibular prominence.

• The intermaxillary segment yieldsa- the portion of the upper lip containing

the philtrum, b-the upper jaw with 4 incisors, and c-the primary palate.

• The medial prominence fuses with the maxillary prominence, giving rise to

a smooth upper lip while fusing the primary and secondary palate.

• Meanwhile, 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.

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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 9thand 11thweek. Finally, the superior, middle and inferior

conchae develop the lateral wall of each nasal cavity.

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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.

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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’sdefinitve position due to the great enlargement of the mandibular

process.If the mandibular process fails to enlarge ,the ears remain low down.

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

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