Development of the Pharyngeal Apparatus and Face

List and identify cartilage derivaties of each pharngeal arch.

1.The dorsal end of the first pharyngeal arch cartilage (Meckel cartilage) is closely related to the developing ear. Early in development, small nodules break away from the proximal part of this cartilage and form two of the middle ear bones, the malleus and incus. 2.An independent cartilage anlage near the dorsal end of the second pharyngeal arch cartilage (Reichert cartilage), also closely related to the developing ear, ossifies to form the stapes of the middle ear and the styloid process of the temporal bone (see Fig. 9-5B). 3.The third pharyngeal arch cartilage, located in the ventral part of the arch, ossifies to form the greater cornu and the inferior part of the body of the hyoid bone. 4.The fourth and sixth pharyngeal arch cartilages fuse to form the laryngeal cartilages (see Fig. 9-5B, Table 9-1), except for the epiglottis.

Describe the innervation of the tongue.

1.The sensory supply to the mucosa of almost the entire anterior two thirds of the tongue is from the lingual branch of the mandibular division of the trigeminal nerve (CN V), the nerve of the first pharyngeal arch. 2.Although the facial nerve is the nerve of the second pharyngeal arch, its chorda tympani branch supplies the taste buds in the anterior two thirds of the tongue, except for the vallate papillae. 3.The vallate papillae in the anterior part of the tongue are innervated by the glossopharyngeal nerve (CN IX) of the third pharyngeal arch (see Fig. 9-24C). 4.The posterior third of the tongue is innervated mainly by the glossopharyngeal nerve of the third pharyngeal arch. 5.The superior laryngeal branch of the vagus nerve (CN X) of the fourth arch supplies a small area of the tongue anterior to the epiglottis (see Fig. 9-24C). 6.All muscles of the tongue are supplied by the hypoglossal nerve (CN XII), except for the palatoglossus, which is supplied from the pharyngeal plexus by fibers arising from the vagus nerve (CN X).

Determine innervation of each pharyngeal arch and its derivatives.

Each arch is supplied by its own cranial nerve (CN). The special visceral efferent (branchial) components of the CNs supply muscles derived from the pharyngeal arches (Fig. 9-7, Table 9-1). Because mesenchyme from the pharyngeal arches contributes to the dermis and mucous membranes of the head and neck, these areas are supplied with special visceral afferent nerves. 1.The facial skin is supplied by the CN V-the trigeminal nerve. However, only its caudal two branches (maxillary and mandibular) supply derivatives of the first pharyngeal arch (see Fig. 9-7B). CN V is the principal sensory nerve of the head and neck and is the motor nerve for the muscles of mastication (see Table 9-1). Its sensory branches innervate the face, teeth, and mucous membranes of the nasal cavities, palate, mouth, and tongue (see Fig. 9-7C). 2.The Facial Nerve (CN VII) supplies the second pharyngeal arch. 3.The Glossopharyngeal nerve (CN IX) supplies the third pharyngeal arch. 4.The Superior laryngeal branch of the Vagus nerve (CN X) supplies the fourth pharyngeal branch. 5.The Recurrent Laryngeal branch of the Vagus nerve (CN X) supplies the sixth pharyngeal branch. The nerves of the second to sixth pharyngeal arches have little cutaneous distribution (see Fig. 9-7C); however, they innervate the mucous membranes of the tongue, pharynx, and larynx.

What are the nasal conchae? How & where do they develop?

After the secondary palate develops, the choanae are located at the junction of the nasal cavity and pharynx (see Figs. 9-34D and 9-37). While these changes are occurring, the superior, middle, and inferior nasal conchae develop as elevations of the lateral walls of the nasal cavities (see Fig. 9-34D). Concurrently, the ectodermal epithelium in the roof of each nasal cavity becomes specialized to form the olfactory epithelium. Some epithelial cells differentiate into olfactory receptor cells (neurons). The axons of these cells constitute the olfactory nerves, which grow into the olfactory bulbs of the brain (see Fig. 9-34C and D).

What is the developmental basis for ectopic thyroid gland?

An aberrant or ectopic thyroid gland may occur anywhere along the path of initial descent of the thyroid, although it is most common at the base of the tongue, just posterior to the foramen cecum. In this location, an aberrant or ectopic thyroid gland is known as a lingual thyroid and represents a failure of the thyroid to descend. This failure to descend contrasts with the incomplete descent of the thyroid, in which case the resulting final resting point of the gland may be high in the neck or just below the hyoid bone. Incomplete movement of the thyroid gland results int the sublingual thryroid gland appearing high in the neck, at or just inferior to the hyoid bone.

How do nasal placodes transform as the nasal prominences develop?

As the face develops, the nasal placodes become depressed, forming nasal pits (see Figs. 9-28, 9-29, and 9-32). Proliferation of the surrounding mesenchyme forms the medial and lateral nasal prominences, which results in deepening of the nasal pits and formation of primordial nasal sacs.

How does the nasal cavity become continuous with the oral cavity? What is the primary choanae?

As the face develops, the nasal placodes become depressed, forming nasal pits (see Figs. 9-28, 9-29, and 9-32). Proliferation of the surrounding mesenchyme forms the medial and lateral nasal prominences, which results in deepening of the nasal pits and formation of primordial nasal sacs. Each nasal sac grows dorsally, ventral to the developing forebrain (Fig. 9-34A). At first, the nasal sacs are separated from the oral cavity by the oronasal membrane. This membrane ruptures by the end of the sixth week, bringing the nasal and oral cavities into communication (see Fig. 9-34C). Temporary epithelial plugs are formed in the nasal cavities from proliferation of the cells lining them. Between 13 to 15 weeks, the nasal plugs disappear. The regions of continuity between the nasal and oral cavities are the primordial choanae, which lie posterior to the primary palate.

What are Nasal Placodes and their embryonic origin?

By the end of the fourth week, bilateral oval thickenings of the surface ectoderm-nasal placodes-the primordia of the nasal epithelium, have developed on the inferolateral parts of the Frontal Nasal Prominence. (Figs. 9-28 and 9-29A and B).

Evaluate developmental basis of cleft lip and cleft palate.

Clefts of the lip and palate are the most common craniofacial anomalies. The defects are usually classified according to developmental criteria, with the incisive fossa as a reference landmark. These clefts are especially conspicuous because they result in an abnormal facial appearance and defective speech. There are two major groups of cleft lip and cleft palate. Anterior cleft anomalies include cleft lip, with or without cleft of the alveolar part of the maxilla. A complete anterior cleft anomaly is one in which the cleft extends through the lip and alveolar part of the maxilla to the incisive fossa, separating the anterior and posterior parts of the palate (see Fig. 9-40). Anterior cleft anomalies result from a deficiency of mesenchyme in the maxillary prominence(s) and the median palatal process. Posterior cleft anomalies include clefts of the secondary palate that extend through the soft and hard regions of the palate to the incisive fossa, separating the anterior and posterior parts of the palate (see Fig. 9-40G and H). Posterior cleft anomalies result from defective development of the secondary palate and growth distortions of the lateral palatal processes, which prevent their fusion. In addition, other factors such as width of the stomodeum, mobility of the shelves, and altered focal degeneration sites of the palatal epithelium may also contribute to these anomalies. A cleft lip, with or without a cleft palate, occurs approximately once in 1000 births; however, the frequency varies widely among ethnic groups; 60% to 80% of affected infants are males. The clefts vary from small notches of the vermilion border of the lip to larger ones that extend into the floor of the nostril and through the alveolar part of the maxilla. Cleft lip can be unilateral or bilateral.

Identify persistent thyroglossal duct.

Cysts may form anywhere along the course of the thyroglossal duct (Fig. 9-19). Normally, the thyroglossal duct atrophies and disappears, but a remnant of it may persist and form a cyst in the tongue or in the anterior part of the neck, usually just inferior to the hyoid bone (Fig. 9-20). Most thyroglossal duct cysts are observed by the age of 5 years. Unless the lesions become infected, most of them are asymptomatic. The swelling produced by a thyroglossal duct cyst usually develops as a painless, progressively enlarging, movable mass (Fig. 9-21). The cyst may contain some thyroid tissue. After infection of a cyst, a perforation of the skin occurs, forming a thyroglossal duct sinus that usually opens in the median plane of the neck, anterior to the laryngeal cartilages (see Fig. 9-19A).

What are the tissue components of each pharyngeal arch and what are the embryonic origins of these tissues?

Each pharyngeal arch consists of a core of mesenchyme (embryonic connective tissue) and is covered externally by ectoderm and internally by endoderm (see Fig. 9-1H and I). Originally, this mesenchyme is derived from mesoderm in the third week. During the fourth week, most of the mesenchyme is derived from neural crest cells that migrate into the pharyngeal arches. Endothelial cells in the arches are derived from both the lateral mesoderm and the invasive angioblasts that move into the arches. A typical pharyngeal arch contains: •A pharyngeal arch artery that arises from the truncus arteriosus of the primordial heart (Fig. 9-3B) and passes around the primordial pharynx to enter the dorsal aorta •A cartilaginous rod that forms the skeleton of the arch •A muscular component that differentiates into muscles in the head and neck •Sensory and motor nerves that supply the mucosa and muscles derived from the arch The nerves that grow into the arches are derived from neuroectoderm of the primordial brain.

Describe the Copula and Hypopharyngeal Eminence and their transformation.

Formation of the posterior third (pharyngeal part) of the tongue is indicated in the fetus by two elevations that develop caudal to the foramen cecum (see Fig. 9-24A): •The copula (Latin, bond, tie) forms by fusion of the ventromedial parts of the second pair of pharyngeal arches. •The hypopharyngeal eminence develops caudal to the copula from mesenchyme in the ventromedial parts of the third and fourth pairs of arches. As the tongue develops, the copula is gradually overgrown by the hypopharyngeal eminence and disappears (see Fig. 9-24B and C). As a result, the posterior third of the tongue develops from the rostral part of the hypopharyngeal eminence.

Describe the Median and Lateral lingual swellings and their transformation.

Near the end of the fourth week, a median triangular elevation appears in the floor of the primordial pharynx, just rostral to the foramen cecum (Fig. 9-24A). This swelling-the median lingual swelling (tongue bud) (Latin, tuberculum impar)-is the first indication of tongue development. Soon, two oval lateral lingual swellings (distal tongue buds) develop on each side of the median tongue bud. The three lingual swellings result from the proliferation of mesenchyme in ventromedial parts of the first pair of pharyngeal arches. The lateral lingual swellings rapidly increase in size, merge with each other, and overgrow the median lingual swelling. The merged lateral lingual swellings form the anterior two thirds (oral part) of the tongue (see Fig. 9-24C). The fusion site of the lateral lingual swellings is indicated by the midline groove of the tongue and internally by the fibrous lingual septum. The median lingual swellings do not form a recognizable part of the adult tongue.

Idenitfy the parafollicular cells.

Parafollicular cells are also known as C cells. These cells are a special subset of cells within the thyroid gland that secrete calcitonin, a hormone necessary for the regulation of calcium. The parafollicular cells arise from the ultimopharyngeal body. This body represents the last structure derived from the pharyngeal pouches, hence its name. The ultimobranchial body arises from the ventral portion of the fourth pharyngeal pouch. Migrating cells from the neural crest region infiltrate the ultimobranchial body. This structure is then incorporated into the thyroid gland, as the ultimobranchial body fuses with the thyroid gland and disseminates its cells into it. The C cells of the thyroid, therefore, are of neural crest origin. .

Given an adult organ, tissue or structure of the face, neck and pharynx, determine its embryonic origin & vice versa.

Refer to pharyngeal arch derivates chart.

How many pairs of pharyngeal arches develop?

Six pairs of pharyngeal arches develop. The fifth and sixth arches are rudimentary and are not visible on the surface of the embryo.

What does the taste buds of the tongue arise from?

Taste buds develop during weeks 11 to 13 by inductive interaction between the epithelial cells of the tongue and invading gustatory nerve cells from the chorda tympani, glossopharyngeal, and vagus nerves. Most taste buds form on the dorsal surface of the tongue, and some develop on the palatoglossal arches, palate, posterior surface of the epiglottis, and the posterior wall of the oropharynx

Identify the Isthmus of the Thyroid Gland.

The Isthmus of the Thyroid Gland connects the left and right lobes differentiated from the divided solid thyroid primordium. It lies anterior to the developing second and third tracheal rings.

Identify the Thyroglossal Duct.

The Thyroglossal duct is a narrow tube that connects the the developing thyroid to the tongue.

What are the pharyngeal arches involved in tongue development?

The first arch contributes to the lingual swellings. The second arch contributes to the copula The third arch contributes to the hypopharyngeal eminence. The fourth arch contributes to the hypopharyngeal eminence.

What are the five facial prominences that give rise to the facial structures?

The five facial primordia that appear as prominences around the stomodeum (see Fig. 9-26A) are •The single frontonasal prominence •The paired maxillary prominences •The paired mandibular prominences The five facial prominences are active centers of growth in the underlying mesenchyme. This embryonic connective tissue is continuous from one prominence to the other.

Identify the remnants associated with the foramen cecum.

The foramen cecum represents the opening of the thyroglossal duct into the tongue; its remains may be observed as a small blind pit in the midline between the anterior two thirds and the posterior third of the tongue.

Describe and identify the Pharyngeal Grooves.

The head and neck regions of the human embryo exhibit four pharyngeal grooves (clefts) on each side during the fourth and fifth weeks (see Figs. 9-1B to D and 9-2). These grooves separate the pharyngeal arches externally. Only one pair of grooves contributes to postnatal structures; the first pair persists as the external acoustic meatus or ear canals (see Fig. 9-8C). The other grooves lie in a slitlike depression-the cervical sinus-and are normally obliterated along with the sinus as the neck develops (see Fig. 9-4B, D, and F).

Describe the terminal sulcus.

The line of fusion of the anterior and posterior parts of the tongue is roughly indicated by a V-shaped groove-the terminal sulcus (see Fig. 9-24C).

What does the muscosa of the tongue arise from?

The mucosa of the anterior 2/3 of the tongue arises from the first pharyngeal arch. The mucosa of the posterior 1/3 of the tongue arises from the third arch. The fourth arch contributes to the mucosa of much of the posterior tongue and epiglottis.

List and identify muscle derivatives of each pharyngeal arch.

The muscular components of the arches form various muscles in the head and neck. 1.The musculature of the first pharyngeal arch forms the muscles of mastication (temporalis, masseter, medial, and lateral) and other muscles (mylohyoid and anterior belly of digastric, tensor tympani, and tensor veli palatini) (Fig. 9-6, Table 9-1). 2.The musculature of the second pharyngeal arch forms the stapedius, stylohyoid, posterior belly of digastric, auricular, and muscles of facial expression. 3.The musculature of the third pharyngeal arch forms the stylopharyngeus. 4.The musculature of the fourth pharyngeal arch forms the cricothyroid, levator veli palatini, and constrictors of the pharynx. 5.The musculature of the sixth pharyngeal arch forms the intrinsic muscles of the larynx.

Describe formation of the nasal septum.

The nasal septum develops as a downgrowth from internal parts of the merged medial nasal prominences (see Figs. 9-37). The fusion between the nasal septum and the palatal processes begins anteriorly during the ninth week and is completed posteriorly by the 12th week, superior to the primordium of the hard palate.

Describe the formation of the lacrimal ducts.

The nasolacrimal duct develops from a rodlike thickening of ectoderm in the floor of the nasolacrimal groove. This thickening gives rise to a solid epithelial cord that separates from the ectoderm and sinks into the mesenchyme. Later, as a result of apoptosis, this epithelial cord canalizes to form a duct. The superior end of this duct expands to form the lacrimal sac. By the late fetal period, the nasolacrimal duct drains into the inferior meatus in the lateral wall of the nasal cavity. The duct usually becomes completely patent only after birth.

Describe the development of the frontonasal prominence.

The neural crest cells that settle to form the frontonasal mass first migrate from the prosencephalic region (forebrain) and are later joined by other migrating cells, mainly from the anterior mesencephalic region (midbrain).

Describe the development of the paired Maxillary and paird Mandibular promineneces.

The paired maxillary and mandibular prominences are derivatives of the first pair of pharyngeal arches. The prominences are produced mainly by the expansion of neural crest populations that originate from the mesencephalic and rostral rhombencephalic neural folds during the fourth week.

What are the structures of the Pharyngeal Apparatus?

The pharyngeal apparatus (Fig. 9-1) consists of: 1.pharyngeal arches 2.pouches 3.grooves 4.membranes These embryonic structures contribute to the formation of the lateral and ventral regions of the head and neck. Most congenital anomalies, often characterized as branchial anomalies, in these regions originate during transformation of the pharyngeal apparatus into its adult derivatives.

When where and how do the pharyngeal arches develop?

The pharyngeal arches begin to develop early in the fourth week as neural crest cells migrate into the future head and neck regions

Describe and identify the pharyngeal membranes and derivatives of each.

The pharyngeal membranes appear in the floors of the pharyngeal grooves (see Figs. 9-1H and 9-3C). These membranes form where the epithelia of the grooves and pouches approach each other. The endoderm of the pouches and the ectoderm of the grooves are soon separated by mesenchyme. Only one pair of membranes contributes to the formation of adult structures; the first pharyngeal membrane, along with the intervening layer of mesenchyme, becomes the tympanic membrane (see Fig. 9-8C).

List and identify derivatives of each pharnygeal pouch.

The primordial pharynx, derived from the foregut, widens cranially where it joins the stomodeum (see Figs. 9-3A and B and 9-4B), and narrows caudally where it joins the esophagus. The endoderm of the pharynx lines the internal aspects of the pharyngeal arches and passes into diverticula-the pharyngeal pouches (see Figs. 9-1H to J and 9-3B and C). The pouches develop in a craniocaudal sequence between the arches. The first pair of pouches, for example, lies between the first and second pharyngeal arches. There are four well-defined pairs of pharyngeal pouches; the fifth pair is rudimentary or absent. The endoderm of the pouches contacts the ectoderm of the pharyngeal grooves and together they form the double-layered pharyngeal membranes that separate the pharyngeal pouches from the pharyngeal grooves (see Figs. 9-1H and 9-3C). Derivatives of the Pharyngeal Pouches The endodermal epithelial lining of the pharyngeal pouches gives rise to important organs in the head and neck. The First Pharyngeal Pouch The first pharyngeal pouch expands into an elongate tubotympanic recess (Fig. 9-8B). The expanded distal part of this recess contacts the first pharyngeal groove, where it later contributes to the formation of the tympanic membrane (eardrum). The cavity of the tubotympanic recess becomes the tympanic cavity and mastoid antrum. The connection of the tubotympanic recess with the pharynx gradually elongates to form the pharyngotympanic tube (auditory tube). The Second Pharyngeal Pouch Although the second pharyngeal pouch is largely obliterated as the palatine tonsil develops, part of the cavity of this pouch remains as the tonsillar sinus or fossa (Figs. 9-8C and 9-9). The endoderm of the second pouch proliferates and grows into the underlying mesenchyme. The central parts of these buds break down, forming tonsillar crypts (pitlike depressions). The pouch endoderm forms the surface epithelium and the lining of the tonsillar crypts. At approximately 20 weeks, the mesenchyme around the crypts differentiates into lymphoid tissue, which soon organizes into the lymphatic nodules of the palatine tonsil. The Third Pharyngeal Pouch The third pharyngeal pouch expands and develops a solid, dorsal bulbar part and a hollow, elongate ventral part (see Fig. 9-8B). Its connection with the pharynx is reduced to a narrow duct that soon degenerates. By the sixth week, the epithelium of each dorsal bulbar part of the pouch begins to differentiate into an inferior parathyroid gland. The epithelium of the elongate ventral parts of the pouch proliferates, obliterating their cavities. These come together in the median plane to form the thymus. The bilobed form of this lymphatic organ remains throughout life, discretely encapsulated; each lobe has its own blood supply, lymphatic drainage, and nerve supply. The developing thymus and parathyroid glands lose their connections with the pharynx. The brain and associated structures expand rostrally while the pharynx and cardiac structures generally expand caudally. This causes the derivatives of pharyngeal pouches two to four to become displaced caudally. Later, the parathyroid glands separate from the thymus and lie on the dorsal surface of the thyroid gland (see Figs. 9-8C and 9-9). The Fourth Pharyngeal Pouch The fourth pharyngeal pouch also expands into dorsal bulbar and elongate ventral parts (see Figs. 9-8 and 9-9). Its connection with the pharynx is reduced to a narrow duct that soon degenerates. By the sixth week, each dorsal part develops into a superior parathyroid gland, which lies on the dorsal surface of the thyroid gland. Because the parathyroid glands derived from the third pouches accompany the thymus, they are in a more inferior position than the parathyroid glands derived from the fourth pouches (see Fig. 9-9).

Identify the right, left, and pyramidal lobes.

The right and left lobes differentiate from the solid throid primordium. The pyramidal lobe differentiates from the distal end of the thyroglossal duct and attaches to the hyoid bone by fibrous tissue and/or smooth muscle-the levator muscle of thyroid gland.

Identify embryonic origin of the secondary palate.

The secondary palate is the primordium of the hard and soft parts of the palate. The secondary palate begins to develop early in the sixth week from two mesenchymal projections that extend from the internal aspects of the maxillary prominences.

What are all the structures involved in the development of the thyroid?

The structues involved in the development of they thymus are: 1.thyroid primordium 2.thyroglossal duct 3.isthmus of the thyroid 4.right, left & pyramidal lobes 5.follicles vs. parafollicular (C-) cells

Identify tissue & site of origin of the thyroid.

The thyroid gland is the first endocrine gland to develop in the embryo. It begins to form approximately 24 days after fertilization from a median endodermal thickening in the floor of the primordial pharynx (Fig. 9-17).

Identify the Thyroid primordium.

The thyroid primordium is an outpouching of the median endodermal thickening in the floor of the primoridal pharynx.

Identify the follicle cells.

Thyroid follicle cells are colloid structures that begin to appear in cell clusters of thyroid primordium surrounding lumens during the eleventh week. Thyroid follicle cells allow for the concentration of iodine and the synthesis of thyroid hormones.

Describe transformation of the lateral palatal processes. What gives rise to the hard palate? What gives rise to the soft palate?

Initially the lateral palatal processes (shelves) project inferomedially on each side of the tongue (Figs. 9-37B and 9-38A and B). As the jaws elongate, they pull the tongue away from its root, and, as a result, it is brought lower in the mouth. During the seventh and eighth weeks, the lateral palatal processes assume a horizontal position above the tongue (see Figs. 9-37E to H and 9-38C). This change in orientation occurs by a flowing process facilitated in part by the release of hyaluronic acid by the mesenchyme of the palatal processes. Bone gradually develops in the primary palate, forming the premaxillary part of the maxilla, which lodges the incisor teeth. Concurrently, bone extends from the maxillae and palatine bones into palatal processes to form the hard palate (see Fig. 9-37E and G). The posterior parts of these processes do not become ossified. They extend posteriorly beyond the nasal septum and fuse to form the soft palate, including its soft conical projection-the uvula (see Fig. 9-37D, F, and H). The median palatine raphe indicates the line of fusion of the palatal processes.

What gives rise to the intermaxillary segment and what are the derivatives of the intermaxillary segment?

Merging of the medial nasal and maxillary prominences results in continuity of the upper jaw and lip and separation of the nasal pits from the stomodeum. As the medial nasal prominences merge, they form an intermaxillary segment. The intermaxillary segment gives rise to: •The middle part (philtrum) of the upper lip •The premaxillary part of the maxilla and its associated gingiva (gum) •The primary palate

What embryonic tissue does the musculature of the tongue arise from?

Most of the tongue muscles are derived from myoblasts that migrate from the occipital myotomes (see Fig. 9-6A).

How do nasal prominences become the nose?

•The frontal nasal prominence forms the forehead and dorsum and apex of the nose (see Fig. 9-26). •The lateral nasal prominences form the alae (sides) of the nose. •The medial nasal prominences form the nasal septum, ethmoid, and cribriform plate. By the end of the fourth week, bilateral oval thickenings of the surface ectoderm-nasal placodes-the primordia of the nasal epithelium, have developed on the inferolateral parts of the FNP. Initially these placodes are convex, but later they are stretched to produce a flat depression in each placode. Mesenchyme in the margins of the placodes proliferates, producing horseshoe-shaped elevations-the medial and lateral nasal prominences. As a result, the nasal placodes lie in depressions-the nasal pits. These pits are the primordia of the anterior nares (nostrils) and nasal cavities. Proliferation of mesenchyme in the maxillary prominences causes them to enlarge and grow medially toward each other and the nasal prominences (see Figs. 9-26D to G). This proliferation-driven expansion results in movement of the medial nasal prominences toward the median plane and each other. Each lateral nasal prominence is separated from the maxillary prominence by a cleft called the nasolacrimal groove (see Figs. 9-26C and D). By the end of the sixth week, each maxillary prominence has begun to merge with the lateral nasal prominence along the line of the nasolacrimal groove (Figs. 9-32). This establishes continuity between the side of the nose, formed by the lateral nasal prominence, and the cheek region formed by the maxillary prominence.