The external human face develops between the 4th and 6th week of embryonic development. This development is completed by the 6th week, and between the 6th and 8th week, the palate begins to develop, leading to the distinction between the nasal and oral cavities. This development is completed by the 12th week, and in this article, we will outline the processes involved in the development of the face and palate as well as discuss the conditions associated with the complex development of these structures.
At the beginning of the third week of embryonic development, an oropharyngeal membrane appears at the site of the future face. It is comprised of ectoderm and endoderm - externally and internally, respectively. During the forth week, the oropharyngeal membrane begins to break down in order to become the future oral cavity, and sits at the beginning of the digestive tract.
The structures of the external face are derived from two sources - Frontonasal prominence, formed by the proliferation of mesenchymal neural crest cells ventral to the forebrain, and mandibular and maxillary prominences - parts of the 1st pharyngeal arch. A space lies between the maxillary prominences, covered by the oropharyngeal membrane; this is known as the stomatodeum, the precursor to the mouth and pituitary gland.
Nasal development is instigated by the appearance of raised bumps called nasal placodes on both sides of the frontonasal prominence. These then invaginate to form nasal pits, with medial and lateral nasal prominences on either side.
In understanding the development of the face and palate, the two important tissue structures involved are the pharyngeal arches and neural crest cells. In the developing embryo, there are six pharyngeal arches. They arise in the fourth week of development as out-pocketings of mesoderm on both sides of the pharynx. Each pharyngeal arch has a branch of the aorta, a cranial nerve and a cartilage bar associated with it. Neural crest cells are a specialised cell lineage which originate from neuroectoderm. As the neural tube forms, cells from the lateral border of the neuroectoderm are displaced into mesoderm, and from there they migrate throughout the body to form various structures. Of relevance to the head and neck, these cells enter the pharyngeal arches to help contribute to their derivatives.
Initially, the nasal cavity is continuous with the oral cavity. A series of steps lead to their separation, and the establishment of the palate. As the nose forms, the fusion of the medial nasal prominence with its contralateral counterpart creates the intermaxillary segment - which forms the primary palate (becomes the anterior 1/3 of the definitive palate). The intermaxillary segment also contributes to the labial component of the philtrum and the upper four incisors. The maxillary prominences expand medially to give rise to the palatal shelves. These continue to advance medially, fusing superior to the tongue. Simultaneously, the developing mandible expands to increase the size of the oral cavity; this allows the tongue to drop out of the way of the growing palatal shelves.
Understanding the complex developmental process of the face and palate begins with understanding the two important tissue structures involved - the pharyngeal arches and neural crest cells. The pharyngeal arches form out-pocketings of mesoderm and are associated with a branch of the aorta, a cranial nerve and a cartilage bar. Neural crest cells are a specialised cell lineage derived from neuroectoderm, which migrate throughout the body to form various structures.
The face begins to form with the oropharyngeal membrane, which is composed of ectoderm and endoderm. During the fourth week, the oropharyngeal membrane breaks down to become the future oral cavity. The structures of the external face are derived from the frontonasal prominence, formed by neural crest cells, and the mandibular and maxillary prominences, which are parts of the 1st pharyngeal arch. Nasal development is instigated by the appearance of nasal placodes on both sides of the frontonasal prominence.
The nasal cavity is initially continuous with the oral cavity, but a series of steps lead to their separation, and the establishment of the palate. The fusion of the medial nasal prominence with its contralateral counterpart creates the intermaxillary segment - which forms the primary palate. The maxillary prominences expand medially to give rise to the palatal shelves, which fuse superior to the tongue. At the same time, the mandible expands to increase the size of the oral cavity, allowing the tongue to move out of the way of the growing palatal shelves.
The development of the face and palate is a complex process involving the cooperation of a number of different tissue structures. Understanding these structures and how they interact with each other is essential for a full appreciation of the development of the face and palate.
The formation of the nasal cavity and palate begins with the invagination of the nasal pits, with medial and lateral nasal prominences on either side. The maxillary prominences then expand medially, 'pushing' the nasal prominences towards the midline. Those prominences eventually fuse with each other to form a single, continuous structure.
At the outset, the nasal cavity is contiguous with the oral cavity. A series of steps lead to their separation, resulting in the establishment of the palate. The fusing of the medial nasal prominences with each other yields the intermaxillary segment that forms the primary palate (this becomes the anterior one-third of the definitive palate). The intermaxillary segment also creates the upper four incisors and the labial component of the philtrum. The maxillary prominences extend further medially to build the palatal shelves. As the palatal shelves advance towards the midline, the mandible develops and widens to enlarge the oral cavity, enabling the tongue to fall out of the way of the palatal shelves. The palatal shelves then fuse with one another in the horizontal plane and the nasal septum in the vertical plane, creating the secondary palate.
A cleft is a gap or split in the upper lip or the palate. It occurs when there is a defect in the development of the face and palate.
Cleft lip and cleft palate are common, occurring in around one in 1,000 births. In Native Americans, the rate is roughly four times that. In addition to the psychological and cosmetic impacts of severe cleft lip/palate, infants can be incapable of feeding and can even die due to the abnormality. Other complications include recurrent ear infections and speech impediment.
