SEM_14_Musculoskeletal system_PARTE2.docx

Development of the head The head is structurally quite different in origin from the rest of the body. The head is one of the most complicated structures to form in the mammalian embryo, which development reuses much of the ancient branchial structures developed in aquatic predecessors and embraces...

Development of the head The head is structurally quite different in origin from the rest of the body. The head is one of the most complicated structures to form in the mammalian embryo, which development reuses much of the ancient branchial structures developed in aquatic predecessors and embraces a significant contribution from cells migrated from cranial somites and neural crests. Skull The bones of the head or skull are assembled in two components: the neurocranium, which contains the brain within a cranial cavity, and the splanchnocranium which forms the visceral skeleton of the face that supports the oral cavity, pharynx and upper respiratory system. The neurocranium or braincase consists of two parts: The base of the cranium and the roof of the cranium or calvaria. The base of the cranium develops by endochondral ossification from the cranial paraxial mesoderm and the first four somites (occipital somites). The roof of the cranium is made of relatively flat bones that ossify by intramembranous ossification from the ectomesenchyme migrated from the branchial arches. The visceral skeleton of the face differentiates from mesenchyme migrated from the first pharyngeal arches, which grows rostrally to the cranium to form the frontonasal prominence. Individual bones of the face develop subsequently by intramembranous ossification from the branchial ectomesenchyme. - The skull consists of two parts of different embryological origin: the neurocranium is a protective cranial vault that surrounds the brain and brainstem; the viscerocranium (also splanchnocranium or facial skeleton) is formed by the bones supporting the face. Nasal cavity The frontonasal prominence expands and divides into the frontal prominence, which forms the frontal bone of the forehead, and the medial and lateral nasal processes, which contribute to the formation of the nose, nasal cavity and primary palate. Initially, two bilateral ectoderm thickening called nasal placodes, start to grow at the rostral end of the frontonasal prominence. The growth of the medial and lateral nasal processes deepens the placodes into the bilateral nasal pits, which penetrate caudally into the underlying mesenchyme to form the primitive nasal cavity, while the rostral openings differentiate into the nostril. Inside, a longitudinal fold, called nasal septum, grows in the midline to divide the primitive nasal cavity into the two nasal passages at the same time that the initially broadly communicated nasal and oral cavities become separated by the development of the palate. - The shape of the head is the result of the growth of the frontal, nasal, maxillary and mandibular prominences. The growth of the nasal prominences leads to the formation of the nasal cavities. The growth of the maxillary prominences and their lateral fusion with the mandibular prominences gives rise to the cheeks that laterally close the oral cavity. Development of the palate Rostrally, the nasal and oral cavities are quickly separated by the development of a primary palate which is formed when the adjacent right and left medial nasal processes fused in the midline. The primary palate will develop into the rostral upper lip, incisive bone and upper incisor teeth. Caudally to the primary plate, the nasal and oral cavities remain broadly communicated until a secondary palate is formed and the nasal-oral communication is shifted caudally into the pharynx. The secondary palate, or hard palate, starts to grow bilaterally as two transversal plates from the maxillary processes. These plates, called palatine processes, meet at the midline, merging dorsally with the nasal septum, and rostrally with the primary palate. Caudal to the hard palate, the mesenchyme extends into the pharynx to create the soft palate which separates the dorsal nasopharynx from the ventral oropharynx. - The palate, and therefore the separation of the oral and nasal cavities, occurs when the developing palate components (primary and secondary palate) come together and become continuous in the midline. The primary palate grows as a wedge-shaped mesodermal mass between the maxillary prominences of the developing upper jaw. The secondary palate develops from two horizontal projections, called lateral palatine processes, formed on the inner surfaces of the maxillary prominences. Palate congenital abnormalities include: Cleft palate results from failure of the palate to close along the midline, leaving a gap or cleft. The secondary palate is affected more commonly than the primary palate. The condition may be inherited or be the result of exposure to a teratogen. Cleft palate is often fatal in animals due to inability to suckle or because of aspiration of milk into the lungs (aspiration pneumonia). Cheiloschisis or hare lip is referred to a defect in the formation upper lip which is usually associated with a failure in the fusion of the primary palate. Dogs and particularly brachiocephalic breeds are considered more prone to this congenital condition because of their peculiar skull development. Nonetheless, hare lip and cleft palate have also been described in a great variety of domestic species. - Types of oral clefts. In each group, complete unilateral or bilateral clefts are shown. However, a cleft from Group I can be left- or right-sided and affect only the lip, the lip and the alveolar process, or include the entire extension of the primary palate, as shown in the illustration. Likewise, a cleft from Group II may affect only the soft palate or the soft palate and the hard palate. Oral cavity The oral cavity or mouth develops as a consequence of the upper and lower jaw formation. The first evidence of the oral cavity is the formation of an ectodermal invagination in the head, the stomodeum. The deep boundary of the stomodeum is the oropharyngeal membrane, where the ectoderm of the stomodeum apposes to the pharyngeal endoderm. Soon after its formation, the oropharyngeal membrane becomes fenestrated and disintegrates (the palatoglossal fold marks its location in adults). Lips and gingivae The ectoderm lining the oral cavity thickens in the upper and lower jaws to form two thickened ectodermal arches called labiogingival laminae. Each lamina invaginates into underlying ectomesenchyme, forming a labiogingival groove which will be transformed into the oral vestibule. The tissue located external to the upper and lower grooves will form the lips while the internal tissue will differentiate into the gingivae. Caudally to the mouth, the upper and lower lips fuse to form the cheeks. Tongue The tongue develops from several swellings situated on the floor of the pharynx. The rostral two- thirds of the tongue (the body and apex) arise from a pair of lateral swellings located at the level of the first branchial arch; these swellings fuse along the midline with a median swelling, or tuberculum impar, and grow forward into the oral cavity; thereby, the tongue acquires an ectodermal covering. The body and apex of the tongue innervation arise predominantly from the first pharyngeal arch (trigeminal nerve, V) which supplies the general sensation. The second pharyngeal arch (facial nerve, VII) also contributes to the taste sensation. The caudal third of the tongue (root) has an endodermal covering and is formed by two swellings, the copula and the hypobranchial eminence. Its nerve sensation is supplied by the nerve of the third pharyngeal arch (glossopharyngeal nerve, IX).On the other hand, the muscles of the tongue originate from occipital somites and are innervated by the hypoglossal nerve. https://sway.office.com/yWTQKQ6u2mrsRcAT#content=V7ombCnlCurr4W - The anterior 2/3 of the tongue (apex and body) is derived from lateral and median swellings that arise from the floor of the 1st pharyngeal arch and then grow rostrally. The posterior 1/3 of the tongue (root) is derived from swellings from the floor of the 3rd and 4th pharyngeal arches that fuse with the anterior 2/3 of the tongue. Teeth The dental lamina is an arch of thickened ectoderm situated inside of the labiogingival lamina. Periodic thickenings of the lamina produce the dental buds, which give rise to the individual teeth. If a bud is destined to form a deciduous tooth, then an additional permanent bud develops superficially and medially to the deciduous bud. In summary, each dental bud develops into a tooth by means of the process described below: The dental bud assumes a cup-shaped configuration, becoming an enamel organ. Cells of the inner enamel epithelium differentiate into ameloblasts, which produce enamel in the form of prisms or rods over the dentin layer, thus help form the outer layer of the tooth or the crown. As enamel increases, the ameloblasts regress. Condensation of mesenchymal cells within the concavity of the cup forms a dental papilla. The mesenchymal cells in the dental papilla, adjacent to the inner enamel epithelium, differentiate into odontoblasts, which produce predentin which is deposited adjacent to the inner enamel epithelium. The predentin later calcifies to form dentin. Mesenchymal cells surrounding the enamel organ and dental papilla condense into a dental sac that gives rise to the alveolar bone (tooth socket), the periodontal ligament (which anchors the tooth within the alveolus) and the cement (modified bone) which adheres to the surface of the tooth. https://sway.office.com/yWTQKQ6u2mrsRcAT#content=pIElgidxzm1ooJ - Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. The tooth germ is an aggregation of cells that eventually forms a tooth. These cells are derived from the ectoderm of the first pharyngeal arch (dental laminae) and the subjacent mesenchyme. The tooth germ is organized into three parts: the enamel organ, the dental papilla and the dental sac or follicle. Salivary glands Salivary glands are derived from the ectoderm (parotid, zygomatic, and labial and buccal accessory salivary glands) or from endoderm (mandibular and mono- and polystomatic sublingual salivary glands). The process of salivary gland formation is typical of exocrine gland development in general: surface epithelial cells undergo localised proliferation, forming a cellular cord that invades underlying mesenchymal cells; the initial site of penetration ultimately becomes the duct opening at the surface; the invading cord of cells begins to branch, ultimately becoming the main duct and branched ducts of the gland; masses of epithelial cells accumulate at the ends of each branch, forming secretory acini of the gland. - A schematic view of the salivary gland development stages