Embryonic head development is the term used to summarize skull development, the differentiation of the pharyngeal arch anlagen, and the development of the craniofacial system. Cranial development primarily forms the bony skull base, whereas organs form from the pharyngeal arches. Developmental abnormalities cause dysplasias (visible malformations).
What is embryonic head development
Embryonic head development is a multiphasic process during which the embryonic neck develops in addition to the head and its structures. Embryonic head development is a multiphasic process during which the embryonic neck develops in addition to the head and its structures. The developmental phases correspond to skull development, differentiation of pharyngeal arches, and differentiation of the craniofacial system. The basic element for the embryonic development of the head and neck are the pharyngeal arches and the para- and prechordal cartilages, which are attached to the uppermost somites. The realization of the developmental steps occurs on a genetic basis. The responsible genes are linked to the homeobox genes. For the skull itself, the neural crest, the paraxial mesoderm, the occipital somites and the two upper pharyngeal arches are relevant as starting materials. From the pharyngeal arches, the masticatory apparatus, the ossicles, the mimic musculature, the hyoid bone, the larynx and parts of the arteries differentiate. The development of the craniofacial system corresponds to a facial structural development from the previously laid out facial bulges.
Function and task
There is a close relationship between cranial capsule development and the development of the meninges. By the sixth week of embryonic development, the anlage for the brain is surrounded by compacted mesenchymal cells. The outer sheet is compacted to form the dura mater encephali. The inner sheet gives rise to the leptomeninx. In the section of the brain base, the meninx primitiva becomes the chondrocranium precartilaginous cells. Desmocranium osteoblasts are also formed. The preformed portion of the skull is cartilaginous and is called the chondrocranium. After ossification, this section corresponds to the base of the skull. A portion of the skull is mesenchymal. This so-called desmocranium ossifies into the cranial vault and forms a major part of bones located in the viscerocranium. Chondral and desmal origin have the squama occipitalis and the pars squamosa ossis temporalis. The base of the skull is formed during embryonic development primarily by processes of chondral ossification that occur at the chondrocranium. The calvaria of the skull originates from desmal ossification based on the desmocranium. The cartilaginous base of the skull forms from investment material of the chorda dorsalis. The base of it is formed by prechordal paired median cartilages and their lateral pairs of cartilages from alae temporales and orbitales. The basal plate of the skull arises at the anterior end of the chorda dorsalis. On the opposite side, the paired ear capsule arises, which later receives the inner ear. The basal plate connects to occipital somites, which are involved in the development of the foramen magnum. Remnants of cartilage from the ossification centers remain in the clivus until puberty. Some parts of the skull remain cartilaginous throughout life, such as the nasal septum. In the desmocranium, an opposing interaction of bone-forming osteoblasts and bone-degrading osteoclasts builds up, allowing for broad ossification. This allows the complicated shape relationships and length ratios of the individual cranial bones to develop. Sutures are the contact points of the skull plates growing towards each other, which give rise to bone sutures. The sutures usually ossify postnatally. The cranial roof can therefore expand to conform to the shape. At the points of contact, large covering bone plates and connective tissue gaps can be seen in newborns, which are called fontanelles. These skull formation processes are followed by pharyngeal arch differentiation. Development begins at four or five weeks of age. Four ectodermal depressions, called gill furrows, are located in the ventrolateral region of the head by the fifth week. Four gullet pockets of the entoderm grow internally toward these gill furrows. These processes divide the mesodermal tissue into four gullet arches. The caudal, fifth pharyngeal arch is less well differentiated and soon regresses.All pharyngeal arches become cartilaginous elements or muscular systems, each of which is associated with a pharyngeal nerve and a pharyngeal artery. The entodermal inner pharyngeal furrows form individual organs of the head and neck region. Among the ectodermal outer pharyngeal furrows, only the first one gives rise to an organ that becomes the external auditory canal and a tympanic membrane portion. The pharyngeal bay undergoes a caudal migratory movement toward the second pharyngeal arch, forming a cavity with reclosure at the lateral neck. The subsequent development of the craniofacial system focuses on the creation of the facial bulges. The forebrain vesicles expand and, together with the first pharyngeal arch and the heart bulge, delimit the head area and the oral bay of the child. The oral bay is closed by the oral pharyngeal membrane, which later tears and connects the foregut to the amniotic cavity. By the fourth week, an ectodermally covered cushion of mesenchyme forms, from which the medio-cranial frontal nasal bulges and the maxillary and mandibular bulges originate. The first differentiation of the facial bulges occurs through ectodermal thickening, which gives rise to the olfactory placodes at the frontal-nasal bulge ends. Proliferation of the mesoderm turns them into the olfactory pits and the olfactory sacs and also separates a median from a lateral nasal bulge on both sides. Then, the lacrimal nasal furrow separates the lateral nasal bulge from the maxillary bulge. The sprouting of the surface epithelium supports lacrimal sac and nasolacrimal duct development. The nasal wings form from the lateral nasal bulges. The intermaxillary segment is formed by the converging median nasal bulges and joins the paired maxillary palatal systems. After the elements grow together, a nasal bridge is formed. The canalis incisivus remains open as a suture. The ocular anlagen undergo frontalization. From the cervical region, the external ear anlagen move in a cranial direction. At the same time, the maxillary bulge pushes past the lateral nasal bulge and merges with the middle nasal bulge. The maxillary bulge gives rise to the lateral upper lip, the maxilla, and the paired secondary palatal anlage. The medially fused maxillary bulges give rise to the base of the lower lip and desmal mandible. The lateral maxillary and maxillary ridges fuse, narrowing the wide stomatodeum opening to form a defined mouth.
Diseases and complaints
Embryonic developmental disorders beginning in the fourth week of embryonic development can cause various malformation syndromes by interfering with head development. Some of these disorders have genetic causes and are associated with mutations. Others are promoted by external factors such as exposure to toxins or malnutrition during pregnancy. Disorders in desmocranium development, for example, may correspond to premature ossification of the individual sutures. This phenomenon is known as craniosynostosis and gives rise to deformed skull shapes, such as the tower skull, the pointed skull, the toothed skull, the triangular skull, or the crooked skull. Some skull dysplasias are associated with mental developmental delays or mental retardation, such as premature ossification of all sutures, which constricts the patient’s brain and prevents it from expanding. If the developmental disorder corresponds not to a disorder of cranial development but to a disorder of the development of the pharyngeal arches, severe symptoms may also develop. For example, remnants of the lateral cervical sinus may develop into cervical fistulas that extend down into the pharynx or terminate blindly. Other symptoms are present in actual malformation syndromes such as Goldenhar syndrome, which causes oculo-auriculo-vertebral dysplasia. This syndrome is caused by combined anomalies of the first and second pharyngeal arches and is conductively symptomatic with an underdeveloped jaw and hypoplastic ear region. These malformations are associated with dysplasia of the cervical spine. Disturbed development of the craniofacial system may also result in obvious malformations. For example, if the middle nasal bulges fuse incompletely with the maxillary bulge, a cleft lip and palate will result. Cleft formation disorders may result in anomalies such as transverse facial cleft or mandibular cleft lip.The clinical picture of disorders during embryonic head development is accordingly multifaceted.
