development of face and oral cavity

what is the stomodeum?

the primitive oral cavity that is formed around 3-4 weeks I.U.L. it is about 1-3 mm in length, located below the foregut and above the developing heart

what forms the stomodeum?

the forebrain, midbrain and hindbrain, which are covered externally by ectoderm with a small underlying layer of mesoderm

what separates the foregut and stomodeum?

the oropharyngeal membrane, or the buccopharyngeal membrane, which is a double epithelial membrane formed of ectoderm externally and endoderm internally

what happens to the buccopharyngeal membrane 4 weeks I.U.L?

it breaks down, allowing for communication between the foregut and stomodeum, causing the formation of the primitive oral cavity and pharynx connection, as well as being the 1st connection between the oral cavity and digestive tract via pharynx

when does development of the face begin?

4 weeks I.U.L from two sources, the frontal prominence (frontonasal) and the mandibular arch (1st pharyngeal arch)

the frontal prominence and first pharyngeal arch (mandibular arch) form what?

they form 5 swellings that surround the stomodeum, which are as follows:

• 1 frontonasal prominence

• 2 mandibular prominences

• 2 maxillary prominences

which all contribute to the formation of the upper face, lower face and jaws

the mandibular process gives rise to what?

• lower part of cheek

• lower lip

• lower jaw

the maxillary process gives rise to what?

• upper part of cheek

• upper lip EXCEPT philtrum

• upper jaw EXCEPT the part carrying the incisors

• palate EXCEPT 1ry (primary) palate

what gives rise to the philtrum, the part carrying incisors and 1ry palate?

medial nasal prominences (part of the frontonasal prominence), more specifically, the intermaxillary segment

what is the most common facial malformation / congenital anomaly of the face?

cleft lip, which happen due to absence, failure of fusion or incomplete fusion of embryonic processes

what are the types of cleft lip?

1. cleft lower lip

2. cleft upper lip

cleft lower lip

• very rare

• occurs due to a failure of fusion of the mandibular processes at the midline

• it may only involve the lip or both the lip and the jaw

cleft upper lip

also known as a hare lip, with two types:

1. median cleft upper lip, which is more rare and is due to complete or partial fusion of medial nasal processes to merge

2. uni or bilateral cleft upper lip, which is more common and is due to a failure in meeting and fusion of the medial nasal processes with the maxillary processes

cleft lips can be what?

• unilateral (one side only)

• bilateral (on both sides)

• complete (extends fully)

• incomplete (partial cleft)

what is the path of a unilateral/bilateral cleft lip?

it extends just below the nostril, lateral to the midline, passing downward through the upper lip and continuing into the maxillary alveolar process. in more severe cases, it extends further back to the incisive foramen

oblique facial cleft

a rare facial defect that is the result of the failure of fusion between the maxillary process and the medial and lateral nasal processes. it extends from the inner canthus of the eye down to the upper lip near the philtrum. in some cases, it may pass laterally near the infraorbital foramen

macrostomia

also known as a transverse facial cleft, it is a rare congenital anomaly caused by failure fusion between the maxillary and mandibular processes. as a result, the mouth is abnormally large, with a cleft extending from the angle of the mouth towards the ear, and it may occur on one side (unilateral) or both sides (bilateral)

microstomia

a very rare congenital anomaly that occurs due to an over fusion of the maxillary and mandibular processes

how can a cleft in the jaw affect the teeth?

it usually occurs between maxilla and premaxilla, which is the area around the lateral incisor. because of this, the lateral incisor may be missing or fail to develop, or sometimes, the opposite happens and supernumerary tooth may develop

when does the development of the nasal cavity and palate begin?

5th week I.U.L. they start initially as a shared common cavity, the oro-nasal cavity, which is occupied by the developing tongue

what causes the separation of the oro-nasal cavity?

by palate development:

• 1ry palate (primary palate)

• 2ry palate (secondary palate)

what does the common oro-nasal cavity divide into?

1 oral cavity and 2 nasal cavities, which happens due to a formation of the primary (1ry) palate and primitive nasal septum, both which originate from the frontonasal process

what occurs after the separation of the oro-nasal cavity?

the deepening of the nasal sacs, with the direction of the growth mainly towards the stomodeum (primitive mouth)

what temporarily separates the nasal sacs and stomodeum?

the nasobuccal (or oronasal / bucconasal) membrane, which is made of double ectoderm. at this point the mouth and nose are still not connected internally

what happens to the nasobuccal membrane?

it ruptures and breaks down, with communication forming between the stomodeum and nasal sacs as a result

the opening between the stomodeum and nasal sacs is called what?

primitive posterior nasal opening (primitive choana)

the ectoderm of the primitive nasal cavity differentiates into what?

olfactory epithelium

when does the 1ry (primary) palate begin to form?

5th-6th week I.U.L

why is the primary palate referred to as 1ry / primary?

it is the first structure that separates the oral and nasal cavities, though it is only a partial / initial separation, not complete

what forms the 1ry palate?

the posterior triangular part of the intermaxillary segment, which is located between the incisors and the incisive foramen (fossa), which forms the anterior part of the palate (premaxilla)

the roof of the oral cavity has what shape prior to 2ry palate development?

a horse shoe shaped, formed by:

1. primary palate anteriorly

2. maxillary processes laterally

how does development of the 2ry (secondary) palate occur?

spanning from the 6th-9th weeks I.U.L. the stomodeum increases in height, and the tongue becomes depressed (moves downward) and the nasal septum grows downward and backward, which will later fuse with the palate. this is crucial becomes it creates space for palatal development

what is the position of the tongue during 2ry development?

high and narrow, reaching towards the lower end of the nasal septum, and this point, still interfere with shelf elevation

what forms the 2 palatine shelves?

the medial edges of the maxillary processes, with the right and left palatine shelves developing. they are vertical (on the sides of the tongue)

what direction do the palatine shelves grow?

vertically downwards on each side of the tongue

why does the mandible enlarge rapidly?

to make space for the tongue, making it move downward and forward. the tongue is withdrawn from between the palatine shelves, allowing them to elevate

what occurs 9 weeks I.U.L?

the palatine shelves that are initially vertical on both sides of the tongue elevate and become horizontal, and are now positioned above the tongue

what is the force behind the elevation of the palatine shelves?

it is not known but there are 3 main theories, which are:

• contractile fibroblasts

• differential growth

• tongue movement

how can contractile fibroblasts possibly cause the elevation of the palatine shelves?

the cells inside the shelves may contract, helping lift them upward

how can differential growth / differential proliferation of the mesoderm of the shelves possibly cause the elevation of the palatine shelves?

the oral (outer / lateral) surface of the palatal shelf grows faster than the nasal (inner / medial) surface, especially in the angle between the palatine shelf and the maxillary process. this uneven growth pushes the shelf upward into a horizontal position

how can displacement of the tongue possibly cause the elevation of the palatine shelves?

the tongue moves downwards because the mandible grows, creating space which allows the shelves to elevate

once horizontal, what do the palatine shelves do?

they grow towards each other and fuse with 3 structures, which are the primary (1ry) palate, with each other, forming the 2ry (secondary) palate, and with the lower border of the nasal septum. this step results in the complete separation of the oral and nasal cavities

the fusion of the palatine processes with each other and the 1ry palate form what shape?

Y-shape

what direction does the fusion start of the Y-shape?

it starts from the center going into two directions:

• in a (anterior) forward direction, which is the fusion between the 1ry palate and the palatine shelves, which forms the anterior palate

• and in a (posterior) backwards direction, which is the fusion between the right and left palatine shelves, which forms the secondary palate

what occurs after the palatine shelves fuse fully?

the 2ry palate differentiates into 2 regions

what is formed in the anterior region of the 2ry palate during differentiation?

the hard palate. this region is also where the nasal septum joins from above from its cephalic end. it ossifies and becomes bony, thus forming the hard palate

what is formed in the posterior region of the 2ry palate during differentiation?

it does not ossify, remaining muscular, giving rise to the soft palate and the uvula

what is cleft palate?

a development anomaly that is the result due to a failure of fusion of the palatal processes. it can involve the 1ry palate, 2ry palate or both

what causes cleft of the 1ry (primary) palate?

it is caused due to a failure of fusion between the lateral palatine process (shelf) and the primary palate, and it is located in the anterior part of the palate

what causes cleft of the 2ry (primary) palate?

it is caused due to a failure of fusion between right and left palatine shelves. the defect can occur with OR without involvement of the nasal septum.

what are the types of cleft of the 2ry palate?

1. mild → bifid uvula

2. moderate → cleft of the soft palate

3. severe → cleft involving both the soft and hard palate

what causes cleft of both the primary and secondary palate?

it is caused by a failure of fusion between the primary palate, right palatine shelf, left palatine shelf and the nasal septum, making it the most severe cleft of the palate

what problems are associated with bifid uvula?

usually, it has no pain or discomfort associated with it, making it the most mild type of cleft palate

what problems are associated with cleft in the soft palate?

speech problems and difficulty in swallowing

what problems are associated with cleft in the hard palate?

causes severe feeding problems. food and liquids may pass into the nasal cavity and can be aspirated into the lungs