Development of Pharyngeal Apparatus, Head and Face PDF

Development of Phayngeal Apparatus, Head and Face The pharyngeal apparatus consists of: Pharyngeal arches Pouches Grooves and Membranes These early embryonic structures contribute to the formation of the face and neck  Pharyngeal Arches The pharyngeal arches begin to develop early in...

Development of Phayngeal Apparatus, Head and Face The pharyngeal apparatus consists of: Pharyngeal arches Pouches Grooves and Membranes These early embryonic structures contribute to the formation of the face and neck  Pharyngeal Arches The pharyngeal arches begin to develop early in the fourth week as neural crest cells migrate into the future head and neck region The first pair of arches, the primordial jaws, appears as surface elevations lateral to the developing pharynx Other arches soon appear as ridges on each side of the future head and neck regions  By the end of the fourth week, four pairs of arches are visible externally The fifth and sixth arches are rudimentary and are not visible on the surface of the embryo  The pharyngeal arches are separated by pharyngeal grooves (clefts) Like the arches, the grooves are numbered in a craniocaudal sequence The first arch separates into the maxillary and mandibular prominences The maxillary prominence forms the maxilla, and a portion of vomer bone  The mandibular prominence forms the mandible, and squamous temporal bone Along with the third arch, the second arch (hyoid arch) contributes to the formation of the hyoid bone  The arches support the lateral walls of the primordial pharynx, which is derived from the cranial part of the foregut The stomodeum (primordial mouth) initially appears as a slight depression of the surface ectoderm It is separated from the cavity of the primordial pharynx by a bilaminar membrane, the oropharyngeal membrane, which is composed of ectoderm externally and endoderm internally  This membrane ruptures at approximately 26th days, bringing the pharynx and foregut in communication with the amniotic cavity Persistency of the oropharyngeal membrane may result in orofacial defects The ectodermal lining of the first arch forms the oral epithelium  Pharyngeal Arch Components Each arch consists of a core of mesenchyme (emb. conn. tissue) and is covered externally by ectoderm and internally by endoderm Originally the mesenchyme is derived during the third week from mesoderm During the fourth week, most of the mesenchyme is derived from neural crest cells that migrate into the arches  Migration of the multipotent neural crest stem cells into the arches and their differentiation into mesenchyme produce maxillary and mandibular prominences in addition to all connective tissue, including the dermis (layer of skin) and smooth muscle  Coincident with the immigration of neural crest cells, myogenic mesoderm from paraxial regions moves into each arch, forming a central core of muscle primordium Endothelial cells in the arches are derived from the lateral mesoderm and invasive angioblasts (cells that differentiate into blood vessel endothelium) that move into the arches The endothelium of the pharyngeal arches 3 to 6 is derived from endothelial progenitors of the second heart field The pharyngeal endoderm plays an essential role in regulating the development of the arches  A typical pharyngeal arch contains several structures An artery from the truncus arteriosus of the primordial heart and passes around the primordial pharynx to enter the dorsal aorta A cartilaginous rod forms the skeleton of the arch A muscular component differentiates into muscles in the head and neck Sensory and motor nerves supply the mucosa and muscles derived from each arch. The nerves that grow into the arches are derived from neuroectoderm of the primordial brain  Fate of Pharyngeal Arches The arches contribute extensively to the formation of the face, nasal cavities, mouth, larynx, pharynx and neck During the fifth week, the second arch enlarges and overgrows the third and fourth arches forming an ectodermal depressions, the cervical sinus By the end of the seventh week, the second to https://swallowedeasy.com/hi52st2crr?key=0f22c1fd609f13cb7947c8cabfe1a90d&su fourth grooves and cervical sinus have bmetric=14961614 disappeared giving the neck a smooth contour  Derivatives of Pharyngeal Arch Cartilages The dorsal end of the first arch cartilage (Meckel cartilage) is closely related to the developing ear Early in the development small nodules break away from the proximal part of the cartilage and form two of the middle ear bones, the malleus and incus The middle part of the cartilage regresses but its perichondrium forms the anterior ligament of malleus and sphenomandibular ligament  Ventral part of the first arch cartilages form the horseshoe-shaped primordium of the mandible By keeping pace with its growth, they guide its early morphogenesis Each half of the mandible forms lateral to and in close association with its cartilage The first arch cartilage disappears as the mandible develops around it by intramembranous ossification  An independent cartilage, the anlage (primordium) near the dorsal end of the second arch cartilage (Reichert cartilage) participates in ear development It contributes to the formation of the stapes, of the middle ear and the styloid process of the temporal bone The cartilage between the styloid process and hyoid bone regress; its perichondrium forms the stylohyoid ligament The ventral end of the second arch cartilage ossifies to form the hyoid lesser cornu (lesser horn)  The third arch cartilage located in the ventral part of the arch, ossifies to form the greater cornu of the hyoid bone and the superior cornu of the thyroid cartilage The body of the hyoid bone is formed by the hypobranchial eminence The fourth and sixth arch cartilages fuse to form the laryngeal cartilages except for the epiglottis  The cartilage of the epiglottis develops from mesenchyme in the hypopharyngeal eminence, a prominence in the floor of the embryonic pharynx that is derived from the third and fourth arches The fifth arch, if present, is rudimentary and has no derivatives  Derivatives of Pharyngeal Arch Muscles The muscular components of the arches derived from unsegmented paraxial mesoderm and prechordal plate from various muscles in the heart and neck The musculature of the first arch forms the muscles of mastication and other muscles The musculature of the second arch forms the stapedius, styloid, posterior belly of digastric, auricular, and muscles of facial expressions The musculature of the third arch forms the stylopharyngeus The musculature of the fourth arch forms cricothyroid, levator veli palatini and constictors of pharynx The musculature of the sixth arch forms the intrinsic muscles of the larynx  Derivatives of the Pharyngeal Arch Nerves Each arch is supplied by its own cranial nerve (CN). The special visceral efferent (branchial) components of these nerves supply muscles derived from the arches Because mesenchyme from the arches contributes to the dermis and mucous membranes of the head and neck, these areas are supplied with special visceral afferent nerves The facial skin is supplied by the trigeminal nerve (CN V) however, only its caudal two branches (maxillary and mandibular) supply derivatives of the first arch  CN V is the principal sensory nerve of the head and neck and its motor nerve for the muscles of the mastication Its sensory branches innervate the face, teeth, and mucous membranes of nasal cavities, palate, and tongue  The facial nerve (CN VII), glossopharyngeal nerve (CN IX) and vagus nerve (CN X) supply the second, third and fourth to six (caudal) arches, respectively The fourth arch is supplied by the superior laryngeal branch of CN X and by its recurrent laryngeal branch The nerves of the second to sixth arches have little cutaneous distribution, but they innervate the mucous membranes of the tongue, pharynx and larynx  Pharyngeal Pouches The primordial pharynx which is derived from the foregut, widens cranially as it joins the stomodeum and narrows as it joins the esophagus The endoderm of the pharynx lines the internal aspects of the arches and the pharyngeal pouches The pouches develop as outpocketing of the endoderm in a craniocaudal sequence between the arches  For example, first pair of pouches lies between the first and second arches Four pairs of pouches are well defined, the fifth pair (if present) is rudimentary The endoderm of the pouches contacts the ectoderm of the pharyngeal grooves, and they form the double-layered pharyngeal membranes that separate the pouches from the grooves  Derivatives of the Pharyngeal Pouches The endodermal epithelial lining of the pouches forms important organs of the head and neck First pharyngeal pouch expands into an elongated tubotympanic recess The expanded distal part of this recess contacts the first 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) Second pharyngeal pouch is largely obliterated as the palatine tonsil develops, part of the cavity of this pouch remains as the tonsillar sinus (fossa), the depression between the palatoglossal and palatopharyngeal arches  The endoderm of the second pouch proliferates and grows into the underlying mesenchyme The central part of these buds break down, forming tonsillar crypts (pit-like depressions) The pouch endoderm forms the surface epithelium and 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 Initial lymphoid cell infiltration occurs at approximately the seventh month with germinal centers forming in the neonatal period and active germinal centers within the first year of life  The third pharyngeal pouch expands and forms a solid dorsal, bulbar part and a hollow, elongated ventral part 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 elongated ventral parts of the pouch proliferates, obliterating their cavities These parts come together in the median plane to form the thymus, which is a primary lymphoid organ  The bilobed structure 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 inferior parathyroid glands lose their connection with the pharynx when the brain and associated structures expand rostrally, and the pharynx and cardiac structures expand caudally Later, the parathyroid glands separate from the thymus and lie on the dorsal surface of the thyroid gland  Fourth pharyngeal pouch expands into dorsal bulbar and elongated ventral parts Its connection with the pharynx is reduced to a narrow duct that soon degenerates By the sixth week each dorsal parts 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  Pharyngeal Grooves The head and neck regions of the embryo exhibit four grooves (branchial clefts) on each side during the fourth and fifth weeks These grooves separate the arches externally Only one pair of grooves contributes to postnatal structures, the first pair persists as the external acoustic meatus The other grooves lie in a slit-like depression (cervical sinus) and are normally obliterated along with the sinus as the neck develops Birth defects of the second groove are relatively common  Pharyngeal Membranes The pharyngeal membranes appear in the floor of the pharyngeal grooves These membranes form where the epithelia of the grooves and pouches approach each other The endoderm of the pouches and ectoderm of the grooves are soon infiltrated and separated by both neural crest cells and mesenchyme Only one pair of membranes contributes to the formation of adult structures the first membrane becomes the tympanic membrane Congenital Malformations  Cervical (Branchial) Sinuses External cervical sinuses are uncommon, and most result from failure of the second groove and cervical sinus to obliterate The sinus typically opens along the anterior border of the sternocleidomastoid muscle in the inferior third of the neck Internal cervical sinuses open into the tonsillar sinus and near the palatopharyngeal arch These sinuses are rare Most result from persistence of the proximal part of the second pouch  Cervical (Branchial) Fistula A cervical fistula is an abnormal canal that typically opens internally into the tonsillar sinus and externally in the side of the neck The canal results from persistence of parts of the second pouch The fistula ascends from its opening in the neck through the subcutaneous tissue and platysma muscle to reach the carotid sheath The fistula then passes between the internal and external carotid arteries and opens into the tonsillar sinus  Piriform Sinus Fistula The piriform sinus fistula is thought to result from persistence of remnants of the ultimopharyngeal body along its path to the thyroid gland  Cervical (Branchial Cysts) Remnants of parts of the cervical sinus and/or the second groove may persist and form a spherical or elongated cyst Although they may be associated with cervical sinuses and drain through them the cyst often lie free in the neck just inferior to the angle of the mandible However they can develop anywhere along the anterior border of the sternocleidomastoid muscle or preauricular region Cervical cysts do not usually become apparent until late childhood or early adulthood when they produce a slowly enlarging, painless swelling in the neck The cyst enlarges because of the accumulation of fluid and cellular debris derived from desquamation of their epithelial linings  First Pharyngeal Arch Syndrome Abnormal component of the first arch results in various birth defects of the eyes, ears, mandible and palate, which together constitute to the first arch syndrome This birth defect is thought to result from insufficient migration of neural crest cells into the first and arch during the fourth week Treacher Collins syndrome and Pierre Robin sequence are two main types of this anomaly  DiGeorge Syndrome Infants with DiGeorge syndrome (also known as 22q11.2 deletion syndrome) are born without a thymus and parathyroid glands and have defects in the cardiac outflow tracts In some cases ectopic glandular tissue has been found The disease is characterized by congenital hyperparathyroidism, increased susceptibility to infections, birth defects of the mouth, low set and notched ears, nasal clefts, thyroid hypoplasia and cardiac anomalies Only 1,5% of infants have the complete form of T-cell deficiency and approximately 30% have only partial deficiency  Ectopic Parathyroid Glands This anomaly may be found in anywhere near or within the thyroid gland or thymus The superior glands are more constant in position than the inferior ones Abnormal numbers of parathyroid Glands Uncommonly there are more than four parathyroid glands Supernumerary parathyroid glands probably result from the division of the primordia of the original glands  Development of Face The facial primordia appear early in the fourth week around the stomodeum Facial development depends on the inductive influence of the forebrain, frontonasal ectodermal zone and developing eye Five facial primordia appear as prominences around the stomodeum A frontonasal prominence Paired maxillary prominences Paired mandibular prominences  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 rhombocephalic 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  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 prominence to the other  Facial development occurs mainly between the fourth and eight weeks By the end of the embryonic period, the face has an unquestinonably 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 Results from incomplete fusion of the prominences the common chin dimple  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 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 the 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  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 and as the mandible develops, they become located on the side of the head at the level of the eyes  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, the epithelial cord canalizes to form a duct The superior end of the duct expands to form the lacrimal sac  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 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 primary palate Clinical and embryologic studies indicate that the upper lip is formed entirely from the maxillary processes 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 philtrum  In addition to connective tissue and muscular derivatives, various bones are derived from mesenchyme in the facial prominences 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 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 As the brain enlarges, the cranial cavity expands bilaterally This causes the orbits which were oriented laterally, to assume a forward-facing orientation The opening of the external acoustic meatus appears elevate but it remains stationary, elongation of the lower jaw creates the false impression  Facial development requires all of the following components: The frontal nasal prominence forms the forehead and dorsum 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 The maxillary prominences form the upper cheek regions and lip The mandibular prominences form the chin, lower lip and cheek regions  Atresia of the Nasolacrimal Duct Part of the nasolacrimal duct occasionally fails to canalize, resulting in congenital atresia of the nasolacrimal duct Obstruction of this duct with clinical symptoms occurs in approximately 6% of neonates  Congenital Auricular Sinuses and Cysts Small auricular sinuses and cysts are usually located in triangular area of skin anterior the auricle of the external ear Although some sinuses and cysts are remnants of the first pharyngeal groove, others represents ectodermal folds sequestered during formation of the auricle from six auricular hillocks  Cleft Lip and Cleft Palate Clefts of upper lip and palate are common craniofacial birth defects A 2014 report from USA indicated that approximately 7000 neonates have orofacial clefts each year At the beginning of the second trimester features of the fetal face can be identified using sonography There are two major groups of cleft lip and palate  Anterior cleft defects include cleft lip with or without a cleft of the alveolar part of the maxilla In a complete anterior cleft defect, the cleft extends through the upper lip and alveolar part of the maxilla to the incisive fossa, separating the anterior and posterior parts of the palate Anterior cleft defects result from deficiency of mesenchyme in the maxillary prominences and the median palatine process  Posterior Cleft Defects 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 Posterior cleft defects result from defective development of the secondary palate and growth distortions of the lateral palatine processes that prevent their fusion Other factors such as the width of the stomodeum, mobility of the lateral palatine processes and altered focal degeneration sites of the palatal epithelium may contribute to these birth defects  A cleft lip with or without a cleft palate occurs approximately once in 1000 births, but the frequency varies among the ethnic groups Between 60% and 80% of affected neonates are male The clefts vary from incomplete cleft lip to those that extend into the nose and through the alveolar part of the maxilla Cleft lip may be unilateral or bilateral Thank you