Development of the Head and Neck

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Introduction

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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:

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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.

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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

craniosynostosis

cleft lip and palate

unilateral cleft lip

bilateral cleft lip

cleft anterior palate

cleft posterior palate

 

pharyhgeal arch syndromes

Pierre Robin sequence

 

DiGeorge syndrome

 

 

Branchial cysts, sinuses, and fistulas (lateral cervical cysts)

 

Thyroglossal duct remnants (midline cervical cysts)

Ectopic Thyroid Tissue

 

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Resources and References

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