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Face and neck development of the embryo
Face and neck development of the embryo refers to the development of the structures from the third to eighth week that give rise to the future head and neck. They consist of three layers, the ectoderm, mesoderm and endoderm, which form the mesenchyme(derived form the lateral plate and paraxial mesoderm), neural crest and neural placodes (from the ectoderm). The paraxial mesoderm forms structures named somites and somitomeres that contribute to the development of the floor of the brain and voluntary muscles of the craniofacial region. The lateral plate mesoderm consists of the laryngeal cartilages (arytenoid and cricoid). The three tissue layers give rise to the pharyngeal apparatus., formed by 6 pairs of branchia/pharyngeal arches, a set of pharyngeal pouches and pharyngeal grooves, 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. This cells determine the future structure to develop in each pharyngeal arch. Eventually, they also form the neurectoderm, which forms the forebrain, midbrain and hindbrain, cartilage, bone, dentin, tendon, dermis, pia mater and arachnoid mater, sensory neurons 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 weeks old, the mesenchymal arches can be recognized with its corresponding cranial nerve.〔
The first pharyngeal arch forms maxillary and mandibular processes. It is innervated by the trigeminal nerve and molds muscles related to mastication such as temporal, masseter, medial, lateral, pterygoid bones, tensor palatine, 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. 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 third pharyngeal arch is innervated by glossopharyngeal nerve. 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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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