development_of_face.pdf

DEVELOPMENT OF THE FACE Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 1 Objectives • To learn the development of face • To explain the development of palate 2 • The facial primordia appear early in the fourth week around the stomodeum (primordium of t...

DEVELOPMENT OF THE FACE Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 1 Objectives • To learn the development of face • To explain the development of palate 2 • The facial primordia appear early in the fourth week around the stomodeum (primordium of the mouth) • Facial development depends on the inductive influence of the forebrain (through sonic hedgehog morphogenic gradients), frontonasal ectodermal zone, and developing eye. 3 • Five facial primordia appear as prominences around the stomodeum 1. frontonasal prominence (single) 2. maxillary prominences (paired) 3. mandibular prominences (paired) 4 The 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. These cells are the major source of connective tissue components, including cartilage, bone, and ligaments in the facial and oral regions. 5 The frontonasal prominence surrounds the ventrolateral part of the forebrain, which gives rise to the optic vesicles that form the eyes . The frontal part of the frontonasal prominence forms the forehead; the nasal part forms the rostral boundary of the stomodeum and nose. The maxillary prominences form the lateral boundaries of the stomodeum, and mandibular prominences constitute the caudal boundary of the stomodeum. The facial prominences are active centers of growth in the underlying mesenchyme. This embryonic connective tissue is continuous from one 6 prominence to the other. 7 • Facial development occurs mainly between the fourth and eighth weeks. By the end of the embryonic period, the face has human appearance. Facial proportions develop during the fetal period. • The lower jaw and lower lip are the first parts of the face to form. They result from merging of the medial ends of the mandibular prominences in the median plane. • The common chin dimple results from incomplete fusion of the prominences. 8 • 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 frontonasal prominence. 9 • These placodes are initially convex, but they are later 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 and the lateral nasal prominences form the alae (sides) of the nose. 10 11 • Proliferation of mesenchyme in the maxillary prominences makes them enlarge and grow medially toward each other and the nasal prominences. This proliferation-driven expansion results in movement of the medial nasal prominences toward the median plane and each other; a process regulated by platelet-derived growth factor receptor α-polypeptide (PDGFRA) signaling. Each lateral nasal prominence is separated from the maxillary prominence by a cleft, the nasolacrimal groove 12 • By the end of the fifth week, the primordia of the auricles (external part of ears) have begun to develop. Six auricular hillocks (three mesenchymal swellings on each side) form around the first pharyngeal groove, the primordia of the auricle, and the external acoustic meatus, respectively. • Initially, the external ears are located in the neck region; however, as the mandible develops, they become located on the side of the head at the level of the eyes. 13 By the end of the fifth week, six auricular hillocks (mesenchymal swellings) form around the first pharyngeal groove (three on each side), the primordia of the auricle, and the external acoustic meatus (canal). 14 • 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. This establishes continuity between the side of the nose, which is formed by the lateral nasal prominence, and the cheek region formed by the maxillary prominence. 15 16 • The nasolacrimal duct develops from a rod-like thickening of ectoderm in the floor of the nasolacrimal groove. This thickening forms a solid epithelial cord that separates from the ectoderm and sinks into the mesenchyme. Later, as a result of apoptosis (programmed cell death), the epithelial cord canalizes to form a duct. The superior end of the 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 becomes completely patent after birth. 17 • Between the 7th and 10th weeks, the medial nasal prominences merge with the maxillary and lateral nasal prominences. • Merging 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. This segment forms the middle part (philtrum) of the upper lip, the premaxillary part of the maxilla and its associated gingiva (gum), and the 18 primary palate. Intermaxillary segment It is composed of (1) a labial component, which forms the philtrum ofthe upper lip; (2) an upper jaw component, which carries the four incisor teeth; (3) a palatal component, which forms the triangular primary palate The intermaxillary segment is continuous with the rostral portion of the nasal septum, which is formed by the frontal prominence. 19 • Upper lip is formed from the maxillary prominences. • The lower parts of the medial nasal prominences appear to have become deeply positioned and covered by medial extensions of the maxillary prominences to form the philtrum. • In addition to connective tissue and muscular derivatives, various bones are derived from mesenchyme in the facial prominences. 20 • Until the end of the sixth week, the primordial jaws are composed of masses of mesenchymal tissue. The lips and gingivae begin to develop when a linear thickening of the ectoderm, the labiogingival lamina, grows into the underlying mesenchyme. Gradually, most of the lamina degenerates, leaving a labiogingival groove between the lips and gingivae. A small area of the labiogingival lamina persists in the median plane to form the frenulum of the upper lip, which attaches the lip to the gum. 21 • Further development of the face occurs slowly during the fetal period and results mainly from changes in the proportion and relative positions of the facial components. During the early fetal period, the nose is flat, and the mandible is underdeveloped. • At 14 weeks, the nose and mandible have their characteristic form as facial development is completed. 22 Facial development requires all of the following components: ● The frontal nasal prominence forms the forehead and dorsum and apex of the nose ● The lateral nasal prominences form the alae (sides) of the nose. ● The medial nasal prominences form the nasal septum, ethmoid bone, and cribriform plate (openings for passage of olfactory nerves). ● The maxillary prominences form the upper cheek regions and lip. ● The mandibular prominences form the chin, lower lip, and cheek regions. 23 DEVELOPMENT OF NASAL CAVITIES • As the face develops, the nasal placodes become depressed, forming nasal pits. 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. 24 • Each sac grows dorsally and ventral to the developing forebrain. At first, the 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. Temporary epithelial plugs are formed in the nasal cavities from proliferation of the cells lining them. By the middle of the 16th week, the nasal plugs disappear. 25 • The regions of continuity between the nasal and oral cavities are the primordial choanae (openings from the nasal cavity into the nasal pharynx). After the secondary palate develops, the choanae are located at the junction of the nasal cavity and pharynx. While these changes are occurring, the superior, middle, and inferior nasal conchae develop as elevations of the lateral walls of the nasal cavities. • 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 neuronal axons constitute the olfactory nerves, which grow into the olfactory bulbs of brain 26 • Most of the upper lip, maxilla, and secondary palate forms from the maxillary prominences. • These prominences merge laterally with the mandibular prominences. The primordial lips and cheeks are invaded by mesenchyme from the second pair of pharyngeal arches, which differentiates into the facial muscles. The muscles of facial expression are supplied by the facial nerve (CN VII), the nerve of the second arch. The mesenchyme in the first pair of arches differentiates into the muscles of mastication (chewing) and a few others, all of which are innervated by the trigeminal nerves (CN V), which supply the first pair of arches. 27 Paranasal Sinuses • Some paranasal sinuses, such as maxillary sinuses, begin to develop during late fetal life; the remaining sinuses develop after birth. They form from diverticula (outgrowths) of the walls of the nasal cavities and become pneumatic (air-filled) extensions of the nasal cavities in the adjacent bones, such as the maxillary sinuses in the maxillae, and the frontal sinuses in the frontal bones. The original openings of the diverticula persist as the orifices of the adult sinuses. 28 POSTNATAL DEVELOPMENT OF PARANASAL SINUSES • Most of paranasal sinuses are rudimentary or absent in neonates. • The maxillary sinuses are small at birth. They grow slowly until puberty and are not fully developed until all the permanent teeth have erupted in early adulthood. • No frontal or sphenoidal sinuses are present at birth. The ethmoidal cells (sinuses) are small before the age of 2 years, and they do not begin to grow rapidly until 6 to BW 8 years of age. 29 • At approximately 2 years of age, the two most anterior ethmoidal cells grow into the frontal bone, forming a frontal sinus on each side. Usually, the frontal sinuses are visible in radiographs by the seventh year. • The two most posterior ethmoidal cells grow into the sphenoid bone at approximately 2 years of age, forming two sphenoidal sinuses. • Growth of the paranasal sinuses is important in altering the size and shape of the face during infancy and childhood and in adding resonance to the voice during adolescence. 30 DEVELOPMENT OF THE PALATE The palate develops from two primordia: • the primary palate • the secondary palate. • Palatogenesis begins in the sixth week, but it is not completed until the 12th week. • Molecular pathways, including WNT and PRICKLE1, are involved in this process. • The critical period of palatogenesis is from the end of the sixth week until the beginning of the ninth week. 31 Primary Palate • Early in the sixth week, the primary palate (median process) begins to develop. Formed by merging the medial nasal prominences, this segment is initially a wedge-shaped mass of mesenchyme between the internal surfaces of the maxillary prominences of the developing maxillae. The primary palate forms the anterior and midline aspect of the maxilla, the premaxillary part of the maxilla. It represents only a small part of the adult hard palate (anterior to the incisive fossa). 32 Secondary Palate • The secondary palate is the primordium of the hard and soft parts of the palate • The palate begins to develop early in the sixth week from two mesenchymal projections that extend from the internal aspects of the maxillary prominences. These lateral palatine processes (palatal shelves) initially project inferomedially on each side of the tongue. As the jaws elongate, they pull the tongue away from its root, and it is brought lower in the mouth. 33 • During the seventh and eighth weeks, the lateral palatine processes assume a horizontal position above the tongue. • Bone gradually develops in the primary palate, forming the premaxillary part of maxilla, which lodges the incisor teeth • bone extends from the maxillae and palatine bones into the lateral palatine processes to form the hard palate. • The posterior parts of these processes do not ossify. They extend posteriorly beyond the nasal septum and fuse to form the soft palate, including its soft conical projection, the uvula. • The palatine raphe indicates the line of fusion of the palatine processes 34 • A small nasopalatine canal persists in the median plane of the palate between the anterior part of the maxilla and the palatine processes of the maxillae. This canal is represented in the adult hard palate by the incisive fossa, which is the common opening for the small right and left incisive canals. An irregular suture runs on each side from the fossa to the alveolar process of the maxilla between the lateral incisor and canine teeth on each side. It is visible in the anterior region of the palates of young persons. • This suture indicates where the embryonic primary and secondary palates fused.35 • The nasal septum develops as a down growth from internal parts of the merged medial nasal prominences. • The fusion between the nasal septum and palatine processes begins anteriorly during the ninth week, and it is completed posteriorly by the 12th week superior to the primordium of the hard palate 36 TEETH • The shape of the face is determined not only by expansion of the paranasal sinuses but also by growth of the mandible and maxilla to accommodate the teeth. Teeth themselves arise from an epithelial— mesenchymal interaction between overlying oral epithelium and underlying mesenchyme derived from neural crest cells. • By the sixth week of development, the basal layer of the epithelial lining of the oral cavity forms a C-shaped structure, the dental lamina, along the length of the upper and lower jaws. This lamina subsequently gives rise to a number of dental buds 37 • 10 in each jaw, which form the primordia of the ectodermal components of the teeth. Soon, the deep surface of the buds invaginates, resulting in the cap stage of tooth development • Such a cap consists of an outer layer, the outer dental epithelium; an inner layer, the inner dental epithelium; and a central core of loosely woven tissue, the stellate reticulum. The mesenchyme, which originates in the neural crest in the indentation, forms the dental papilla 38 • As the dental cap grows and the indentation deepens, the tooth takes on the appearance of a bell (bell stage). • Mesenchyme cells of the papilla adjacent to the inner dental layer differentiate into odontoblasts, which later produce dentin. With thickening of the dentin layer, odontoblasts retreat into the dental papilla, leaving a thin cytoplasmic process (dental process) behind in the dentin • The odontoblast layer persists throughout the life of the tooth and continuously provides predentin. The remaining cells of the dental papilla form the pulp of the tooth 39 • In the meantime, epithelial cells of the inner dental epithelium differentiate into ameloblasts (enamel formers). • These cells produce long enamel prisms that are deposited over the dentin. • Furthermore, a cluster of these cells in the inner dental epithelium forms the enamel knot that regulates early tooth development. 40 • Enamel is first laid down at the apex of the tooth and from here spreads toward the neck. When the enamel thickens, the ameloblasts retreat into the stellate reticulum. Here they regress, temporarily leaving a thin membrane (dental cuticle) on the surface of the enamel. • After eruption of the tooth, this membrane gradually sloughs off 41 • Formation of the root of the tooth begins when the dental epithelial layers penetrate into the underlying mesenchyme and form the epithelial root sheath. 42 • Cells of the dental papilla lay down a layer of dentin continuous with that of the crown. As more and more dentin is deposited, the pulp chamber narrows and finally forms a canal containing blood vessels and nerves of the tooth. 43 • Mesenchymal cells on the outside of the tooth and in contact with dentin of the root differentiate into cementoblasts. • These cells produce a thin layer of specialized bone, the cementum. Outside of the cement layer, mesenchyme gives rise to the periodontal ligament, which holds the tooth firmly in position and functions as a shock absorber. 44 • With further lengthening of the root, the crown is gradually pushed through the overlying tissue layers into the oral cavity. • The eruption of deciduous or milk teeth occurs 6 to 24 months after birth. 45 • Buds for the permanent teeth, which lie on the lingual aspect of the milk teeth, are formed during the third month of development. These buds remain dormant until approximately the sixth year of postnatal life. • Then they begin to grow, pushing against the underside of the milk teeth and aiding in the shedding of them. As a permanent tooth grows, the root of the overlying deciduous tooth is resorbed by osteoclasts. 46 References 1. Sadler TW, Langman's Medical Embryology 13E 2. Moore KL, Persaud TVN, Torchia MG Developing Human 9e 3. Carlson BM Human Embryology & Developmental Biology, 5E 47