(Transcribed from Dr. Martin’s lecture by Brian Buschman)
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This set of notes is more a transcription the lecture handout and not necessarily of the material presented in lecture as Dr. Martin said the exam questions would all be able to be answered from the information in the handout (and the handout is very complete).
During the development of the head and neck the three developmental layers come together to form the pharyngeal apparatus (also called branchial apparatus) in the embryo on either side of the neck from the area of the future ear on down. They begin to form from tissue migrating from the neural crest cells (ectoderm). They end up with three layers, the outer layer of ectoderm and the inner layer of endoderm have varying amounts of mesoderm between them. The pockets of mesoderm are called arches. The indentations into this structure, where the mesoderm has migrated away, on the external surface form the clefts and the spaces on the inner surface are the pouches. The membranes are the thin layer of endoderm and ectoderm that are right against each other because of the mesoderm migrating away. They are the membrane between a pouch and a cleft.
Six arches develop but we study four arches because the fifth arch degenerates and the fourth and sixth arches fuse to form one. For orientation sake the first pair of arches surround the stomodeum (to be discussed later), which will become the mouth.
The arches all contain their own cranial nerve, vascular supply and a cartilage bar.
In anatomy we studied which muscles are innervated by which cranial nerve and here we will see that the muscles innervated by one of the four SVE nerves all develop from the same arch.
When all is said and done the first arch gives:
I) The cartilage becomes the incus and malleus.
II) The nerve is the trigeminal nerve (CN V).
III) The muscles are:
a) Muscles of mastication (temporalis, masseter, lateral and medial pterygoids).
b) Mylohyoid.
c) Anterior belly of the digastric.
d) Tensor veli palatine.
e) Tensor tympani.
There is also mesenchyme of the first arch which gives rise to the premaxilla, maxilla, zygomatic bone and part of the temporal bone (via intramembranous ossification because they are cartilage becoming bone).
This arch is called the hyoid arch and becomes:
I) The cartilage becomes the stapes, styloid process and the superior part of the hyoid bone.
II) The nerve is the facial nerve (CN VII).
III) The muscles are:
a) Posterior belly of the digastric.
b) Muscles of facial expression.
c) Stapedius.
d) Stylohyoid.
I) The cartilage becomes the inferior part of the hyoid bone.
II) The nerve is the glossopharyngeal nerve (CN IX).
III) The muscle is the stylopharyngeus.
I) The cartilage of the fourth is the thyroid cartilage and the laryngeal cartilages come from the sixth.
II) The nerve is the vagus nerve. To be technical the nerve of the fourth arch is the superior laryngeal branch and the sixth is the recurrent branch.
III) The muscles are the:
a) All muscles of the palate.
b) Constrictors of the pharynx.
c) Cricothyroid.
d) Muscles of the larynx (6th arch).
e) Smooth muscle of the esophagus (6th arch).
The first pouch gives tubotympanic recess which will become the middle ear and auditory tube.
The second pouch gives the palatine tonsils.
The third pouch gives the ventral part of the thymus and the inferior part of the thymus.
The fourth pouch gives the superior part of the parathyroid gland. It also gives the ultimobranchial body that will become part of the thyroid gland. This part will become the part which gives the principle (oxyphil) cells which secrete calcitonin which inhibits osteoclasts.
Of the clefts only one develops because the second arch grows inferiorly over all of the other clefts filling them all in. The first cleft does exist as the external acoustic meatus.
Because of the filling in of all of the clefts other then the first only the first membranes is able to develop into anything. Since it lies between the external acoustic meatus and the tubotympanic recess it should be easy to figure out that it will develop into the tympanic membrane.
Pharyngeal fistula is a hole that can be found between the outer and inner parts of the neck located near the anterior border of the sternocleidomastoid muscle. It develops because of the second arch failing to properly fill in all of the clefts other then the first.
Pharyngeal cysts occur when parts of the pharyngeal grooves persist rather then digressing.
First arch syndromes is characterized by abnormal development of the face due to a shortage of neural crest cells migrating into the pharyngeal arch.
The thyroid gland develops as an endodermal thickening between the first two arches at the foramen cecum of the tongue. It descends as the thyroid diverticulum leaving the thyroglossal duct (connected to the foramen cecum) as it goes.
Anomalies include:
1) Cretinism which is a shortage of thyroid hormone resulting in dwarfism and retardation.
2) Thyroglossal cysts which are parts of the thyroglossal duct left behind in the migration path.
The tongue grows in multiple segments. The anterior 2/3 develop from two lingual swellings and one medial swelling from the region of the first pair of arches. The lingual swellings will overgrow the medial swelling. This section of the tongue ends at the median sulcus. This line will be called the terminal sulcus when development is complete.
The posterior 1/3 of the tongue develops from the copula, from the 2nd arch, and the hypobranchial eminence which develops from the 3rd arch. The hypobranchial eminence overgrows the copula.
The posterior part of the 4th arch is the site of development of the root of the tongue and the epiglottis.
Occipital somites migrate to form the muscles of the tongue and bring the hypoglossal nerve with them as they come.
Anomalies include ankyloglossia which is a fusion of the tongue to the floor of the mouth because the frenuluma is too short.
The face develops from five swellings:
1) One frontonasal process located above the stomodeum (future mouth opening) that come from neural crest cells.
2) Two maxillary processes located lateral to the stomodeum (separated from each other by the stomodeum and the frontonasal process).
3) Two mandibular processes located inferior to the stomodeum which come together at the midline.
The frontal nasal process gives optic, otic and nasal placodes to form sensory tissues of the eye, ear and nose.
The placode development produces two swellings in the frontonasal process called the lateral and medial nasal prominences. The medial nasal prominences come together to form the nasal septum and the lateral nasal prominences give the lateral parts of the nose. They are separated from the maxillary prominences by nasolacrimal grove (which will become the nasolacrimal duct).
The upper lip is formed from the maxillary prominences and the lateral and medial swellings.
The maxillary areas are formed from the maxillary processes and the mandibular areas from the mandibular processes.
The primary palate comes from the intermaxillary segment which is made from the merging of the medial nasal swellings.
The secondary palate develops from the palatine shelves which grow from the maxillary processes. The palatine shelves fuse with the primary palate at a point called the incisive foramen. Bone gradually develops around the palate to give the hard palate.
Cleft lip and palate are a concept you just need to understand rather then memorize. They are a deformity leaving an opening between the outside and inside of the face. What they leave the cleft between is dependant of what they pass through. If you know how everything forms you can determine what the deformity is based on what you know.
Oblique facial cleft is a cleft that is caused by failure of fusion of the maxillary swelling to the medial nasal swelling resulting in exposure of the nasolacrimal duct.
As the medial and lateral nasal swellings for the nostrils and nasal cavity the primitive choanae (opening at posterior end of nasal cavity) is formed right behind the palate. As development continues the definitive choanae is formed at it’s adult location between the nasal cavity and nasopharynx.
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