Development of Head and Neck

The development of the head and neck shows a number of special features not found in the development of the rest of the body (trunk and limbs). The cranial part of the central nervous system (the brain) is quite large and is associated with a bony case more complex than the vertebral column. The head contains the apertures for the alimentary and respiratory systems, and all of the special sense organs (eyes, ears, olfactory epithelium and taste buds).

Special Tissues and Structures Involved in Head and Neck Development

Prechordal plate

The prechordal (prochordal) plate is a midline area anterior to the notochord and neural plate. The term "prechordal plate" has been used to designate two adjacent regions of the trilaminar embryo.

• Some use the term to refer to the area immediately anterior to the notochord that is an important "head organizer". The prechordal mesoderm in this area is an inducer of forebrain development and plays other roles in the development of the head.
• Others use the term to refer to the area, anterior to the "head organizer", that is devoid of mesoderm so that ectoderm and endoderm are in contact. This is the presumptive oropharyngeal membrane, the site of the future mouth.

Head mesenchyme

Head mesenchyme is derived from mesoderm and cranial neural crest. Mesenchyme derived from cranial neural crest is called ectomesenchyme (or mesectoderm). Like the trunk neural crest, cranial neural crest derivatives include parts of the peripheral nervous system and melanocytes. In addition, cranial neural crest gives rise to parafollicular (C cells) of the thyroid, carotid body and ectomesenchyme.

• Mesodermal contributions to head and neck development
  • Except for four pairs of "occipital" somites superior to the cervical somites, most of the rostral paraxial mesoderm forms less-organized somitomeres rather than somites. In the head and neck, paraxial mesoderm gives rise to most of the muscles, most of the brain case, some other connective tissues, and the endothelium of the aortic arch arteries.
  • Lateral plate mesoderm, which is also located anterior to the prechordal plate in the trilaminar embryonic disc, makes some contributions to the pharyngeal arches, e.g., the laryngeal cartilages.
• Ectomesenchyme contributes to the development of:
  • Bone and cartilage of face and visceral skeleton
  • Anterior part of the cranial vault
  • Leptomeninges of midbrain and forebrain
  • Dermis of face and ventral neck
  • Wall of aortic arches and aorticopulmonary septum
  • Corneosclera and uvea of the eye
  • Dentin and cementum of teeth

Ectodermal placodes

Ectodermal placodes are focal thickenings of the cranial surface ectoderm of the embryo proper. They appear at about 4 weeks of age and contribute to the development of special sense organs and sensory components of the cranial nerves V, VII, IX and C.

• Lens (or optic) placode - forms the lens and is required for formation of the eyes.
• Nasal (or olfactory) placode - forms the nasal pit, which gives rise to much of the nasal cavity.
• Otic (or auditory) placode - forms the otic (auditory) vesicles, which develop into the membranous labyrinth of the inner ear.
• Neural (neurogenic or epipharyngeal) placodes - contribute (with cranial neural crest) to the formation of the trigeminal, geniculate, inferior glossopharyngeal, and inferior vagal ganglia.

Development of Posterior Cervical Region

The posterior region of the neck contains the cranial end of the spinal cord and vertebral column.

Neurulation

The neural tube begins to fuse at 4 weeks at the cervical-thoracic junction and proceeds zipper-like cranially and caudally, leaving anterior and posterior neuropores that close a day or two later.

Formation of the vertebral column

The cervical somites and neural tube form at about the same time. During the 5th week the cervical sclerotomes appear and form a perinotochordal sheath, which becomes the centra and neural processes of the vertebrae. The vertebrae are cartilaginous at about 6 weeks and begin ossifying by 12 weeks. Early in the fetal period the neural processes fuse to form the neural arch.

Dorsal view of human embryo with 8 somites (about 4 weeks old). Gray's Anatomy (1918) figure 20 (originally from Eternod). Source: Wikimedia Commons

Transverse section of embryo during the 5th week showing differentiation of a somite into sclerotome (sc) and dermatomyotome (s.p. and m.p.). Gray's Anatomy (1918) figure 64 (originally from Kollmann). Source: Wikimedia Commons

Diagrams showing the formation of the vertebral bodies (centra) during the 5th and 6th weeks from a sclerotome-derived perinotochordal sheath. Gray's Anatomy (1918) figure 65. Source: Wikimedia Commons

Diagrams showing the portions of adult vertebae derived from the bodies (yellow), neural processes (vertebral arches, red), and costal processes (blue) of embryonic vertebrae. Gray's Anatomy (1918) figure 67. Source: Wikimedia Commons

Development of Pharyngeal Apparatus

Pharyngeal arches

At the beginning of the 5th week, pharyngeal ("branchial") arches appear ventral and lateral to the cranial end of the foregut (the pharyngeal gut). In humans, four arches are visible on the surface of the embryo in the 5th and 6th weeks. Caudal to these arches, rudimentary 5th and 6th arches are described, and structural derivatives from this area are ascribed to the 6th arch.

Embryo during the 5th week showing pharyngeal arches and clefts. Gray's Anatomy (1918) figure 40 (originally from His). Source: Wikimedia Commons

Floor of the pharynx of embryo during the 5th week showing pharyngeal arches and pouches. Gray's Anatomy (1918) figure 42 (originally from His). Source: Wikimedia Commons

Components of the pharyngeal apparatus. Pharyngeal arches are numbered I to IV and VI (not shown). Each contains a nerve, artery, cartilage and muscle. Pharyngeal pouches and clefts are numbered 1 to 4. Other identified parts of the pharyngeal apparatus include tuberculum laterale (a), tuberculum impar (b), foramen cecum (c), thyroglossal duct (d), and cervical sinus (e). Source: Wikimedia Commons

Pharyngeal pouches (I, II, III, and IV)and their derivatives, and the thyroid primordium. Gray's Anatomy (1918) figure 1175 (originally from Koh). Source: Wikimedia Commons

Each arch is a thickening of the tissue between the endoderm and ectoderm by a band of mesoderm that is supplemented by ectomesenchyme of neural crest origin. Each arch contains an aortic arch artery, a cartilaginous rod, skeletal muscle and a nerve. In general, the endothelium of the arteries, cartilage and muscles derive from mesoderm, whereas the other connective tissues and nerves derive from neural crest. The major derivatives of the components of the pharyngeal arches are listed in a table below.

Pharyngeal clefts and pouches

Caudal to each of the visible arches, the lateral walls ("membranes") of the pharyngeal gut are thin, so that four ectoderm-covered clefts (or grooves) are seen externally, and four endoderm-lined pouches, internally. The major derivatives of the pharyngeal clefts, membranes and pouches are listed in a table below.

The ultimobranchial (post-branchial or telopharyngeal) body migrates to and fuses with the thyroid gland. Neural crest cells that migrate into the ultimobranchial body differentiate into parafollicular (C or calcitonin-producing) cells of the thyroid.

Ventral diverticuli

The pharyngeal gut also has two ventral diverticuli: the thyroid primordium and the respiratory diverticulum.
The thyroid primordium appears as an outpouching of endoderm within the 2nd pharyngeal arch. The developing thyroid gland descends into the neck, connected to the developing tongue by the thyroglossal duct. After the thyroglossal duct disappears, the foramen cecum, between the anterior and posterior parts of the tongue, continues to mark the site of origin of the thyroid gland.
The respiratory diverticulum (lung bud) appears at the caudal end of the pharyngeal gut and develops into the lower respiratory tract (trachea, bronchi and lungs) by branching morphogenesis.

Development of the Skull

The skull is composed of the brain case (neurocranium) and the bones of the lower face (viscerocranium). Most skull bones appear in weeks 6-8, and begin to ossify by 12 weeks.

Neurocranium

The neurocranium forms from paraxial mesoderm around the base of the developing brain, and ectomesenchyme anterior to the developing forebrain.
The base of the brain case (cartilaginous neurocranium or chondrocranium) is formed by endochondral ossification, chiefly from paraxial mesoderm (somitomeres and occipital somites). The early chondrocranium consists of midline cartilages and capsules of special sense organs (nasal, optic and otic capsules).
The membranous neurocranium, which is formed by intramembranous ossification, makes up the cranial vault (or calvarium) and the bones around the eyes and nose. The anterior membranous neurocranium is derived from ectomesenchyme, whereas the posterior membranous neurocranium is derived from paraxial mesoderm. Fibrous sutures and fontanelles allow intrapartum molding and postnatal growth of the cranial vault.

Viscerocranium

The viscerocranium forms from mesenchyme in the pharyngeal apparatus.
The membranous viscerocranium forms by intramembranous ossification of (neural crest-derived) ectomesenchyme from the 1st pharyngeal arch. These bones include the mandible, maxilla, zygomatic bones, and the squamous part of the temporal bones.
The cartilaginous viscerocranium, consisting of the auditory ossicles, hyoid bone and laryngeal cartilages, is derived from the pharyngeal arch cartilages.

Diagrams of the development of the cartilaginous neurocranium from midline cartilages and capsules of special sense organs. Gray's Anatomy (1918) figure 69 (originally from Wiedersheim). Source: Wikimedia Commons

Dorsal view of the chondrocranium of a fetus (about 12-weeks-old) Gray's Anatomy (1918) figure 70 (originally from Hertwig). Source: Wikimedia Commons

Lateral view of the chondrocranium of a fetus (about 12-weeks-old). The cartilaginous parts of the neurocranium and viscerocranium are blue. Parts of the membranous viscerocranium are shown in yellow. Gray's Anatomy (1918) figure 71 (originally from Hertwig). Source: Wikimedia Commons

Lateral view of skull of newborn showing fibrous sutures and fontanelles. Gray's Anatomy (1918) figure 198. Source: Wikimedia Commons

Development of the Face

In the 5th week, development of the face begins with development of five swellings (prominences or processes) around the stomodeum: two mandibular prominences, two maxillary prominences and a frontonasal prominence.

• The mandibular and maxillary prominences develop on the 1st pharyngeal arch. The mandibular processes quickly merge in the midline forming the lower lip and jaw.
  
• The frontonasal prominence forms rostral and ventral to the forebrain. Nasal placodes (disks) appear on the frontonasal prominence.
  

In the 6th week, the nasal placodes become nasal pits associated with medial and lateral nasal prominences on each side of the frontonasal prominence.
By 7 weeks, the six prominences superior to the stomodeum merge and the face becomes recognizably human, although the ears (i.e., the pinnae or auricles, which are formed by fusion of swellings on the 1st and 2nd pharyngeal arches) are still low-set.

• The maxillary processes form the lateral parts of the upper lip and jaw;
• The medial nasal prominences form the philtrum of the upper lip, anterior part of the upper jaw and nasal septum; and
• The lateral nasal prominences form the alae (sides) of the nose.

Face at about 5 weeks of age. Gray's Anatomy (1918) figure 44 (originally from His). Source: Wikimedia Commons

Face at about 6 weeks of age. Gray's Anatomy (1918) figure 45 (originally from Peters). Source: Wikimedia Commons

Face at about 7 weeks of age. Gray's Anatomy (1918) figure 47 (originally from His). Source: Wikimedia Commons

Adult face showing derivatives of the embryonic structures, and their lines of merger. Gray's Anatomy (1918) figure 48. Source: Wikimedia Commons

Development of Oral and Nasal Cavities, and Palate

Oral cavity

The presumptive oropharyngeal membrane can be identified in the anterior end of the trilaminar embryonic disc. The presumptive oropharyngeal membrane, the site of the future mouth, is an area that is devoid of mesoderm so that ectoderm and endoderm are in contact.
At the end of the 4th week, after embryo folding, the cranial end of the embryo has an invagination in the ectoderm, the stomodeum (the primordial oral cavity). The oropharyngeal (buccopharyngeal) membrane separates the stomodeum (and amniotic cavity) from the foregut, until the membrane ruptures at about 4 ½ weeks.
The stomodeum is initially a shallow primary oral cavity. The ectoderm-endoderm junction shifts caudally and becomes obscured, but most of the definitive oral cavity derives from the stomodeum.

Tongue

The anterior 2/3 of the tongue develops from the 1st pharyngeal arch, and the posterior 1/3 develops from the 3rd pharyngeal arch. Therefore, sensory innervation is by cranial nerve V anteriorly, and cranial nerve IX posteriorly.
Musculature of the tongue derives from occipital somites, rather than more anterior somitomeres, thus motor innervation is also different: cranial nerve XII.

Sagittal section of embryo at about 4 weeks showing the oropharyngeal membrane positioned between the stomodeum and the foregut. Gray's Anatomy (1918) figure 977 (originally from His). Source: Wikimedia Commons

Floor of the pharynx of embryo during the 5th week showing pharyngeal arches and pouches, and the anterior tongue precursors (the tuberculum impar and the lateral swellings) and the posterior tongue precursor (the copula). Gray's Anatomy (1918) figure 979 (originally from Peters). Source: Wikimedia Commons

Floor of the pharynx of embryo during the 5th week showing development of the anterior tongue precursors (the tuberculum impar and the lateral swellings) and the posterior tongue precursor (the copula). Gray's Anatomy (1918) figure 980 (originally from Peters). Source: Wikimedia Commons

Floor of the pharnyx at about 6 weeks showing the primordial anterior tongue ("tongue") and posterior tongue (copula). Gray's Anatomy (1918) figure 981 (originally from Kollmann). Source: Wikimedia Commons

Nasal Cavity

At about 5 weeks, the nasal pits deepen, forming nasal sacs. The nasal sacs are separated from the oral cavity by the primary palate (the "premaxillary" or "intermaxillary" part of the hard palate), which is formed from the medial nasal processes and mesenchyme from the maxillary prominences. The frontonasal prominence also contributes to the nasal septum.
At about 6 weeks, the oronasal (bucconasal) membranes rupture, creating the primary choanae, the passages between the nasal cavities and the oropharyngeal cavity.

Nasal cavities and primitive palate during the 6th week. Gray's Anatomy (1918) figure 49 (originally from Peters). Source: Wikimedia Commons

Roof of the mouth at about 7 weeks showing the palatal processes extended inferiorly. Gray's Anatomy (1918) figure 50 (originally from His). Source: Wikimedia Commons

Frontal section of nasal cavities of an 8-week-old embryo showing approximation of the palatal shelves and nasal septum prior to fusion. Gray's Anatomy (1918) figure 51 (originally from Kollmann). Source: Wikimedia Commons

Palate

The secondary palate develops from palatal processes that appear on the maxillary processes in the 5th week. The palatal processes grow into "palatal shelves".
The palatal shelves shift into a horizontal position in the 8th week and begin to fuse at about 8 weeks. The secondary palate forms most of the hard and soft palate.

Development of the Brain

See Development of the Nervous System

Development of Special Senses Organs

See sections on development in the Eye and Ear articles.

Selected Anomalies of Head and Neck Development

Neural tube defects

• Spina bifida occulta
• Spina bifida cystica
• Anencephaly
• Encephalocele

Craniofacial anomalies

Craniofacial anomalies account of about 1/3 of all congenital anomalies. Disorders of dentition (not included below) are particularly common.

• Holoprosencephaly
• Craniosynostosis
• Orofacial clefts (e. g., Cleft lip and cleft palate)
• Pharyngeal arch anomalies

Cervical cysts

• Branchial cysts, sinuses and fistulas
• Thyroglossal duct remnants

External Links

Human Developmental Anatomy Center (HDAC) of the National Museum of Health & Medicine

• O'Rahilly R & Müller F. Developmental Stages in Humans. Carnegie Institution of Washington, 1987.
  

Embryology.ch - an online course in embryology for medical students

Other Resources

O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.