<P> The face and neck development of the human 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 from the mesenchyme (derived form the lateral plate mesoderm 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 . </P> <P> 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 days old, the mesenchymal arches can be recognized with its corresponding cranial nerve . </P>

When does development of the human face occur