Body processes

Embryonic head development – function, task and diseases

Embryonic head development

The development of the skull, the differentiation of the pharyngeal arch systems and the development of the craniofacial system are summarized as embryonic head development . The cranial development mainly forms the bony base of the skull, while organs form from the pharyngeal arches. Developmental disorders cause dysplasia (visible abnormalities).

What is the embryonic head development

The head development of the embryo is a multi-phase process during which the embryonic neck develops in addition to the head and its structures. The developmental phases correspond to skull development, the differentiation of pharyngeal arches and the differentiation of the craniofacial system. The basic elements for the embryonic formation of the head and neck are the pharyngeal arches and the parachordal and prechordal cartilages, which are attached to the uppermost somites.

The realization of the development steps takes place on a genetic basis. The responsible genes are linked to the homeobox genes. The relevant starting materials for the skull itself are the neural crest, the paraxial mesoderm , the occipital somites and the two upper pharyngeal arches. The chewing apparatus, the auditory ossicles , the facial muscles , the hyoid bone , the larynx and parts of the arteries differentiate from the pharyngeal arches . The development of the craniofacial system corresponds to a facial structural development from the previously created facial ridges.

Function & task

There is a close connection between the development of the cranium and the development of the meninges . By the sixth week of embryonic development , the brain lay-out is surrounded by condensed mesenchymal cells . The outer sheet is compressed into the dura mater encephali. The leptomeninges arise from the inner leaf. In the section of the base of the brain, the procartilage cells of the chondrocranium emerge from the meninx primitive. Desmocranium osteoblasts also form . The protruding part of the skull is cartilaginous and is called the chondrocranium. After ossification, this section corresponds to the base of the skull.

Part of the skull is mesenchymal. This so-called desmocranium is ossified to form the skullcap and forms a large part of the bones that lie in the viscerocranium. The squama occipitalis and the pars squamosa ossis temporalis have chondral and desmal origin.

During embryonic development, the base of the skull develops primarily through processes of chondral ossification that take place at the chondrocranium. The calotte of the skull has its origin in demented ossification based on the desmocranium. The cartilaginous base of the skull is formed from the attachment material of the notochord. The basis of this is formed by prechordally paired central 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 notochord. The paired ear capsule, which later accommodates the inner ear, develops on the opposite side . The basal plate is connected to occipital somites 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 area of ​​the desmocranium, an opposing interaction of the bone-forming osteoblasts and bone-degrading osteoclasts develops , which enables a wide-ranging ossification. This is how the complicated shape and length ratios of the individual skull bones can arise.

The contact points of the skull plates growing towards one another, which create bone sutures, are called sutures. The sutures usually ossify postnatally. The cranium can therefore expand properly. Large covering bone plates and connective tissue gaps, which are called fontanelles, can be seen in newborns at the points of contact.

The pharyngeal arch differentiation follows these skull formation processes. Development begins at four or five weeks of age. In the fifth week there are four ectodermal depressions in the ventrolateral area of ​​the head, which are called gill sulci. Four pharyngeal pouches of endoderm grow towards these gill sulci. These processes divide the mesodermal tissue into four pharyngeal arches. The caudal, fifth pharyngeal arch is poorly differentiated and soon recedes. All pharyngeal arches become cartilage elements or muscle systems, each of which is assigned a pharyngeal nerve and a pharyngeal artery.

The endodermal inner pharyngeal bags form individual organs of the head and neck region. Below the ectodermal external pharyngeal fissures, only the first produces an organ that becomes the external auditory canal and a portion of the tympanic membrane . The cervical cavity migrates caudally and migrates toward the second pharyngeal arch to form a reclosing cavity at the lateral neck.

The subsequent development of the craniofacial system focuses on the formation of the facial ridges. The forebrain vesicles expand and, together with the first pharyngeal arch and the cardiac bulge, surround the child’s head and mouth. The oral cavity is closed by the oropharyngeal membrane, which later tears and connects the foregut to the amniotic cavity. In the fourth week, an ectodermally covered cushion of mesenchyme forms, from which the medio-cranial frontal nasal ridges and the maxillary and mandibular ridges emanate.

The first differentiation of the facial ridges occurs through an ectodermal thickening, which gives rise to the olfactory placode at the ends of the forehead and nasal ridges. The proliferation of the mesoderm makes the olfactory pits and sacs out of it and also separates a central nasal ridge from a lateral nasal ridge on both sides. The lacrimal nasal furrow then separates the lateral nasal bulge from the upper jaw bulge. The ingrowth of the surface epithelium supports lacrimal sac and nasolacrimal duct development. The nostrils form from the lateral nostrils.

The premaxillary segment is formed by the middle nasal ridges growing towards each other and fits into the paired upper jaw and palate rudiments. After the elements grow together, a bridge of the nose is formed. The canalis incisivus remains open as a suture.

The eye systems experience a frontalization. The appendages for the outer ear migrate from the neck area in a cranial direction. At the same time, the maxillary ridge pushes past the lateral nasal ridge and merges with the central nasal ridge. The lateral upper lip, the upper jaw and the paired secondary palate develop from the upper jaw bulge. The medially fused maxillary ridges give rise to the base of the lower lip and the desmal mandible . The lateral maxillary and maxillary ridges fuse so that the broad stomatodeal opening narrows to form a defined mouth .

Diseases & Ailments

Embryonic development disorders from the fourth week of embryonic development can cause various malformation syndromes by disrupting head development. Some of these disorders have genetic causes and are associated with mutations . Others are favored by external factors such as exposure to toxins or malnutrition during pregnancy .Disorders in the development of the desmocranium can, for example, correspond to early ossification of the individual sutures. This phenomenon is known as craniosynostosis and causes deformed skull shapes to develop, such as the tower skull, the pointed skull, the kahn skull, the triangular skull or the wry skull. Some cranial dysplasias are associated with intellectual developmental delays or mental retardation , such as the premature ossification of all sutures, which constricts the patient’s brain and prevents it from expanding.

If the developmental disorder does not correspond to a disorder in the development of the skull, but rather to a disorder in the development of the pharyngeal arches, serious symptoms can also occur. Remnants of the lateral cervical sinus, for example, can develop into cervical fistulas that reach down into the pharynx or end 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 arch and is associated with an underdeveloped jaw and hypoplastic ear region.

These malformations are associated with dysplasia of the cervical spine . Impaired development of the craniofacial system can also result in obvious malformations. For example, if the central nasal prominences only incompletely fuse with the maxillary prominence, a cleft lip and palate develops . The cleft formation disorders can result in anomalies such as the transverse facial cleft or the mandibular cleft lip. The clinical picture of disorders during embryonic head development is correspondingly diverse.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.