The face and neck development of the human embryo refers to the development of the structures from the third to eighth week that give rise to the future

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may ...

and

neck The neck is the part of the body on many vertebrates that connects the head with the torso. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In ...

. They consist of three layers, the

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...

and

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...

, which form the

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...

(derived form the

lateral plate mesoderm The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow ...

and paraxial mesoderm), neural crest and

neural placode In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

s (from the ectoderm). The paraxial mesoderm forms structures named

somites The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide in ...

and

somitomeres In the developing vertebrate embryo, the somitomeres (or somatomeres) are collections of cells that are derived from the loose masses of paraxial mesoderm that are found alongside the developing neural tube. In human embryogenesis they appear towar ...

that contribute to the development of the floor of the brain and voluntary muscles of the craniofacial region. The

consists of the laryngeal cartilages ( arytenoid and

cricoid The cricoid cartilage , or simply cricoid (from the Greek ''krikoeides'' meaning "ring-shaped") or cricoid ring, is the only complete ring of cartilage around the trachea. It forms the back part of the voice box and functions as an attachment sit ...

). The three tissue layers give rise to the pharyngeal apparatus, formed by six pairs of pharyngeal arches, a set of pharyngeal pouches and

pharyngeal groove A pharyngeal groove (or branchial groove, or pharyngeal cleft) is made up of ectoderm unlike its counterpart the pharyngeal pouch on the endodermal side. The first pharyngeal groove produces the external auditory meatus (ear canal). The rest (2, ...

s, which are the most typical feature in development of the head and neck. The formation of each region of the face and neck is due to the migration of the neural crest cells which come from the ectoderm. These cells determine the future structure to develop in each pharyngeal arch. Eventually, they also form the neurectoderm, which forms the forebrain,

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

and hindbrain, cartilage, bone,

dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...

, tendon,

dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...

, pia mater and

arachnoid mater The arachnoid mater (or simply arachnoid) is one of the three meninges, the protective membranes that cover the brain and spinal cord. It is so named because of its resemblance to a spider web. The arachnoid mater is a derivative of the neural c ...

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...

s, and glandular stroma.

Pharyngeal arches

Pharyngeal arches are formed during the fourth week. Each arch consists of a mesenchymal tissue covered on the outside by ectoderm and on the inside by epithelium of endodermal origin. In human embryology, there are six arches which are separated by pharyngeal grooves externally and pharyngeal pouches internally. These arches contribute to the physical appearance of the embryo because they are the main components that build the face and neck. In addition, the muscular components of each arch have their own cranial nerve, and wherever the muscle cells migrate, they carry their nerve component with them. Plus, each arch has its own arterial component. When neural cells migrate to the arches and surround them, they begin to increase in size. The six pharyngeal arches give rise to much of the skeletal and muscular tissue in the head and neck region. When the embryo is 42 days old, the mesenchymal arches can be recognized with its corresponding cranial nerve. The first pharyngeal arch forms

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

ry and mandibular processes. It is innervated by the

trigeminal nerve In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewin ...

and molds muscles related to mastication such as temporal, masseter, medial, lateral, pterygoid bones, tensor palati, and tensor tympani. This arch originates maxillar and mandibular prominences, part of the temporal bone and Meckel's cartilage (malleus and incus) as skeletal structures. The second pharyngeal arch is innervated by the facial cranial nerve. Muscles that arise from the arch are those involved with facial expression and the posterior

digastric muscle The digastric muscle (also digastricus) (named ''digastric'' as it has two 'bellies') is a small muscle located under the jaw. The term "digastric muscle" refers to this specific muscle. However, other muscles that have two separate muscle bellie ...

. Skeletal structures that originate here are the cervical sinus, Reichert cartilage (stape) the styloid process of the temporal bone, the lesser cornu and the

hyoid bone The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical vertebr ...

. The third pharyngeal arch is innervated by

glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper Medulla oblongata, medulla, just anterior (closer to the nose) to t ...

. It molds the stylopharyngeus muscle and forms the skeletal structures of the greater horn and lower portion of body hyoid bone. The fourth and sixth arches are innervated by the vagus cranial nerve. Both arches will fuse to form the laryngeal cartilages. The fifth cartilage does not appear to have any contribution to adult anatomy and disappears.

Pharyngeal pouches

Pharyngeal pouches develop into future parts in face and head. The pouches penetrate the surrounding mesenchyme but do not establish communication with the pharyngeal grooves. They appear simultaneously with the development of the arches. The first pharyngeal pouch is characterized by narrowing at its final segment. However, it does not disappear and eventually forms the

eustachian tube In anatomy, the Eustachian tube, also known as the auditory tube or pharyngotympanic tube, is a tube that links the nasopharynx to the middle ear, of which it is also a part. In adult humans, the Eustachian tube is approximately long and in d ...

. Second pharyngeal pouch develops differently from the first one mainly because most of it disappears, leaving the

tonsillar fossa The tonsillar fossa (or tonsillar sinus) is a space delineated by the triangular fold (plica triangularis) of the palatoglossal and palatopharyngeal arch The palatopharyngeal arch (pharyngopalatine arch, posterior pillar of fauces) is larger and ...

(Rohen). At the end of the fifth month, the palatine tonsil as a protective structure is completed. This structure covers the tonsillar fossa. The third pharyngeal pouch will give rise to the inferior parathyroid gland and thymus. The fourth and fifth pouches develop as a unique structure that molds the superior parathyroid and parafolicular cells of thyroid gland.

Pharyngeal grooves

Initially, pharyngeal grooves consist of four bars of mesenchymal tissue that separate pharyngeal nerves. Most of these structures obliterate, only the

ear canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about in length and in diameter. Struc ...

remains.

Development of the tongue

In the fourth week of the pregnancy the structures that develop the tongue appear. These structures develop from the first pharyngeal arch are two lingual lateral prominences and one in the middle that does not develop and disappears. A second prominence, the hypobranchial eminence, comes from the second, third and fourth pharyngeal arches. A third prominence that comes from the fourth arch develops the epiglottis. The laryngeal orifice is behind the third prominence, which is surrounded by the arytenoid prominences. Later, the lateral and middle prominences join forming the first of the three parts of the tongue. The surcus terminalis linguae is a V shaped line that separates the body of the tongue from the posterior part. The corresponding nerve for the three prominences of the anterior tongue is the

. The posterior tongue is innervated by the glosopharyngeal nerve. The muscles of the tongue are innervated by the hypoglossal nerve.

Development of the thyroid

The

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...

appears as an epithelial proliferation in the pharynx floor between the

copula linguae The copula linguae or copula, is a swelling that forms from the second pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors ...

and the

tuberculum impar The median tongue bud (also tuberculum impar) marks the beginning of the development of the tongue. It appears as a midline swelling from the first pharyngeal arch late in the fourth week of embryogenesis. In the fifth week, a pair of lateral li ...

. This point later will be the foramen cecum. Later, the thyroid descends in front of the pharyngeal gut when it already has a belobed diverticulum shape. The

thyroglossal duct The thyroglossal duct is an embryology, embryological anatomical structure forming an open connection between the initial area of development of the thyroid gland and its final position. It is located exactly mid-line, between the anterior 2/3 and ...

keeps the thyroid joined to the tongue until it disappears. The thyroid keeps descending in front of the

until finally it affixes to the front of the

trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...

in the seventh week. The thyroid starts working in the third month when the first follicles are visible and start producing

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

. The parafollicular cells come from the ultimobranchial body and produce

calcitonin Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid (or endostyle) in humans and other chordates. in the ultimopharyngeal body. It acts to reduce blood calcium (Ca2+), opposing the ...

Development of the face

The facial prominences are five swellings that appear in the fourth week and come from the first and second pharyngeal arch. They are basically made of mesenchyme that comes from the neural crest. The

frontonasal prominence The frontonasal process, or frontonasal prominence is one of the five swellings that develop to form the face. The frontonasal process is unpaired, and the others are the paired maxillary prominences, and the paired mandibular prominences. During t ...

is a single structure that is ventral to the forebrain. It is derived from neural crest cells, which have an ectodermal origin. These neural crest cells migrate from the ectoderm as the forebrain closes, invading the space that will form the frontonasal prominence. The maxillary and mandibular prominences are derived from the first arch. The maxillary prominence is initially located superior/lateral to the

stomodeum The stomodeum, also called stomatodeum or stomatodaeum, is a depression between the brain and the pericardium in an embryo, and is the precursor to the mouth and the anterior lobe of the pituitary gland. Development The mouth is developed partly ...

while the mandibular prominence is located inferior to it and will fuse early on. Nasal placodes originate on the frontonasal prominence from the ectoderm. They thicken and sink in to form Nasal Pits, which deepen to form the Nasal Sacs. At the same time, mesodermal cells proliferate around the placodes, and the sides of these swellings form the medial and lateral nasal prominences. The lateral nasal prominence is separated from the maxillary prominence by the nasolacrimal groove. As the maxillary prominences continue growing they merge laterally with the mandibular prominences to form the cheeks. Their growth compresses the medial nasal prominences and causes them to fuse around the 10th week of development. This establishes the bridge of the nose and the intermaxillary segment, which is the region of the medial nasal prominence located inferior to the bridge of the nose and superior to the mandibular prominence. The intermaxillary segment yields the portion of the upper lip containing the

philtrum The philtrum ( la, philtrum from Ancient Greek ''phíltron,'' lit. "love charm"), or medial cleft, is a vertical indentation in the middle area of the upper lip, common to therian mammals, extending in humans from the nasal septum to the tubercl ...

, the upper jaw with 4 incisors, and the primary palate. The medial prominence fuses with the maxillary prominence, giving rise to a smooth upper lip while fusing the primary and secondary palate. Meanwhile, the lateral nasal prominence gives rise to the alae of the nose and fuses with the maxillary prominence, forming the Nasolacrimal duct. This duct is formed when the ectoderm thickens into a cord and sinks into the underlying mesenchyme.

Development of the nasal cavity

The formation of the lateral and medial nasal prominences makes the nasal placodes lie in the floor of the depression, called

nasal pit The nasal placode (or olfactory placode) gives rise to the olfactory epithelium of the nose. Two nasal placodes arise as thickened ectoderm from the frontonasal process. They give rise to the nose, the philtrum of the upper lip, and the primary pa ...

s. The nasal pits deepen and develop the nasal sacs in the sixth week. These new structures grow dorsocaudally in front of the forming brain. In the beginning, the nasal sacs are separated from the oral cavity by the oronasal membrane. This membrane disappears in the seventh week leaving a connection between the nasal cavities and the oral cavity, called the primitive

choanae The choanae (singular choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the throat in tetrapods, including humans and other mammals (as well as crocodilia ...

. Later, when the development of the secondary palate occurs, the choanae changes its position and locates at the junction of the nasal cavity and the pharynx. The nasal septum grows as a downgrowth from the merged nasal prominences and fuses with the palatine process between the ninth and eleventh week. Finally, the superior, middle and inferior conchae develop the lateral wall of each nasal cavity.

References

{{Embryology of head and neck Embryology