Development of the Head and Neck
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Introduction
Contributing Tissues
The head is first determined in the precordal plate of the trilaminar embryonic disc, at days 15 or 16 (check). The precordal plate is a small circular area of columnar endermal cells where both the ectoderm and endoderm are tightly in contact, devoid of mesoderm. This therefore represents the end of the primitive streak's advance, and is the likely formation of the mouth, or stomodeum. The mouth first opens ~26 days, resulting in amniotic fluid flowing into the developing gastrointestinal tract.
The pharyngeal apparatus develops beginning of the 4th week from neural crest cells. The entire head of the covered by a layer of ectoderm, with endoderm forming the rest. Mesoderm is prominent around the vessels.
Neural crest cells also give rise to the peripeheral nervous system and melanocytes and ectomesenchyme, which contributes to the development of:
- bone and cartilage of skeleton, primarily the face
- dermis
- eye
- teeth
- parafollicular cells of the thyroid
Stages of Development
Neurulation, or formation of the neural tube, is completed by the end of the fourth week. The neural tube differentiates into the CNS, while the neural crest gives rise to most of the peripheral nervous system and autonomic nervous system.
Pharyngeal Arches
By the 4th week, the head of the embryo develops 6 pharygeal arches from thickening of neural crest cells that wrap around the soon-to-be pharynx. Arches contain all three tissue types, and each contains an aortic arch artery, a cartilagenous rod, skeletal muscle, and a nerve.
Behind each arch, on each side of the embryo are ectoderm-lined grooves. Inside endoderm-composed pouches develop.
The first arch plays a major role in facial development by becoming the bones, muscles, and other constituents of the jaw. The first groove and pouch form the external and internal ear.
The third pouch forms the inferior parathyroid and thymus.
The fourth and sixth arches contribute to the thyroid and cricoid cartilages.
Bones
Most bones in the head appear in weeks 6-8 and begin to ossify by 12 weeks. The skull is formed from neurocranium and the lower face is composed of viscerocranium, with most of the latter formed by the first pharyngeal arch. In the 5th week, five swellings emerge: two mandibular prominences, two maxillary prominences, and a frontonasal prominence. The frontonasal prominence contributes to the nasal septum.
Palate
At about 5 weeks, nasal pits deepen and become separated from the mouth by the primary palate, which emerges from the 1st arch.
At 8 weeks the secondary palate, which forms most of the hard and soft palate, begins to fuse. It is formed from the lateral palatine processes of the maxillary prominences with the nasal septum , derived from the medial nasal prominences.
Congenital Head and Neck Problems
Nervous system defects
Craniofacial defects
up to 1/3 of all congenital anomalies
holoprosencephaly
- midline face and forebrain anomalies
- very common, but most abort; 1:15,000 live births
- problem in precordal plate
- can have cyclopia
- if there are problems in the face, expect problems in the brain too
craniosynostosis
- premature closure of the cranial sutures can lead to various deformations of the cranium, depending on which suture is affected.
- 1:3000
- growth restriction leads to compensatory growth in opposite direction
- 2e craniosynostosis
- metabolic: hyperthyroidism
- mucopolysaccharidosis
- hematologic disorders: thalassemia, sickle cell disease
- many teratogens
cleft lip and palate
- common - 1:1000; 60-80% male
- most result from multifactorial inheritance ( recurrence risk to siblings is ~4%) though some teratogens (ie anticonvulsants) may also be involved
unilateral cleft lip
- failure of maxillary prominence to unite with merged medial nasal prominences; defect is in the mesenchyme
bilateral cleft lip
- failure of mesenchymal masses in both maxillary prominences to meet and unite with the medial nasal prominences
- especially deforming due to loss of continuity in the obicularis oris muscle
cleft anterior palate
- failure of mesenchymal masses in lateral palatine processes to meet and fuse with mesenchyme in primary palate
cleft posterior palate
- failure of mesenchymal masses in lateral palatine processes to meet and fuse with each other and the nasal septum
pharyhgeal arch syndromes
Pierre Robin sequence
- hypoplasia of mandible (micrognathia), cleft palate, defects in eye and ear, glossoptosis and absence of gag reflex
DiGeorge syndrome
- absence or hypoplasia of thymus and parathyroid, resulting from failure of differentiation of 3rd and 4th pharyngeal pouches
- also associated with cardiac abnormalities because of role of neural crest cells in heart development
Branchial cysts, sinuses, and fistulas (lateral cervical cysts)
- Remnants of part of the cervical sinus and/or second pharyngeal groove can result in cysts or fistulas. Fistulas open internally into the tonsillar sinus or externally. Cysts often do not become evident until later in childhood.
Thyroglossal duct remnants (midline cervical cysts)
- occurs following downward migration of thyroid; duct normally atrophies, but can remain
- ducts or sinuses can form anywhere along the path, including with communication to outside
- cyst can contain thyroid tissue
Ectopic Thyroid Tissue
- infrequent; lingual thyroid glandular tissue is most common type
- ectopic sublingual thyroud gland can also replace normal position of thyroid; bad news to mistake this for thyroglossal duct cyst, remove it, and leave the person with no thyroid
Resources and References
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