2. Development

2.2. Formation of the Face

Vertebrate Axis Formation

Even as the germ layers form, the ball of cells still retains its spherical shape. However, animal bodies have lateral-medial (left-right), dorsal-ventral (back-belly), and anterior-posterior (head-feet) axes, as illustrated in the figure below.

Figure 1. Animal bodies have three axes for symmetry. More details.

In the phylum Chordata, which includes all vertebrates, one of the first structures to develop reflecting the axis formation is the notocord. Spemann and Mangold took dorsal cells from one embryo and transplanted them into the belly region of another embryo. They found that the transplanted embryo now had two notochords: one at the dorsal site from the original cells and another at the transplanted site. This suggested that the dorsal cells were genetically programmed to form the notochord and define the axis. Since then, researchers have identified many genes that are responsible for axis formation. Mutations in these genes lead to the loss of the symmetry required for organism development.

These genes regulate not only the formation of symmetry in the body but also of asymmetry. Animal bodies have externally visible symmetry but the internal organs are not fully symmetric. For example, the heart is on the left side and the liver on the right. This internal asymmetry is established very early during development and involves many genes. Research is still ongoing to fully understand the developmental implications of these genes.

Pharyngeal arches

Unique to the development of vertebrates, a series of pharyngeal arches forms on each side of the pharynx. These arches contain all three embryonic tissues and they are precursors for numerous structures in the developing animal. Each arch contains an artery, a cartilage, a cranial nerve and muscle tissue.

Figure 2. Schematic representation of a frontal section across the pharyngeal arches of a human embryo, showing the main components of each arch. More details.

The first, most anterior pharyngeal arch gives rise to the lower jaw. The second arch becomes part of the hyoid and most muscles of the facial expression. In fishes, the posterior pharyngeal arches develop into the branchial arches or gill arches that support the gills for respiratory gas exchanges with the medium.

Figure 3. Ventral view of a fish (pike) with the operculums held open to show the gill arches bearing gills. More details.

In tetrapods, the anterior arches also develop into components of the ear, larynx, tonsils, and thymus, whereas the posterior arches form the larynx, its muscles and part of the hyoid. The genetic and developmental basis of pharyngeal arch development is well characterized. It has been shown that Hox genes and other developmental genes such as DLX are important for patterning the anterior/posterior and dorsal/ventral axes of the branchial arches. Some fish species have a second set of jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation.

In the human embryo, the arches are first seen during the fourth week of development. They appear as a series of outpouchings of mesoderm on both sides of the developing pharynx. The arches are numbered from 1 to 6, with 1 being the arch closest to the head of the embryo, and arch 5 existing only transiently. The vasculature of the pharyngeal arches is also known as the aortic arches.

The development of the pharyngeal arches provides a useful landmark with which to establish the precise stage of embryonic development. Their formation and development corresponds to Carnegie stages 10 to 16 in mammals, and Hamburger-Hamilton stages 14 to 28 in the chicken. Although there are six pharyngeal arches, in humans the fifth arch exists only transiently during embryogenesis.

Formation of the palate

The palate is the roof of the mouth and it is exclusive to mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior bony hard palate, and the posterior fleshy soft palate (or velum).

Figure 4. Sagittal section of the head showing the palate as a physical separation between the oral and nasal cavities. More details.

Around the 5th week of human development, the intermaxillary segment arises as a result of fusion of the two medial nasal processes and the frontonasal process within the embryo. The intermaxillary segment give rise to the primary palate. The primary palate will form the premaxillary portion of the maxilla (anterior one-third of the final palate). This small portion is anterior to the incisive foramen and will contain the maxillary incisors.

Figure 5. Primitive (primary) palate of a human embryo of thirty-seven to thirty-eight days. More details.

The development of the secondary palate commences in the sixth week of human embryological development. It is characterized by the formation of two palatal shelves on the maxillary prominences, the elevation of these shelves to a horizontal position, and then a process of palatal fusion between the horizontal shelves. The shelves will also fuse anteriorly upon the primary palate, with the incisive foramen being the landmark between the primary palate and secondary palate. This forms what is known as the roof of the mouth, or the hard palate.

Figure 6. Formation of the secondary palate in a human embryo about twenty-nine days old. The palatal shelves are yet to form from the maxillary processes. More details.

The formation and development of the secondary palate occurs through signaling molecules SHH, BMP-2, FGF-8 among others. Failure of the secondary palate to develop correctly may result in a cleft palate disorder.

Figure 7. Formation of the secondary palate. The superior central structure is the nasal septum and inferior to it is the tongue. Palatal shelves are developing on each side of the tongue. They will elevate and fuse to each other, separating the nasal and oral cavities. More details.

Cleft palate

Cleft lip and cleft palate, also known as orofacial cleft, is a group of conditions that includes cleft lip, cleft palate, and both together. A cleft lip contains an opening in the upper lip that may extend into the nose. The opening may be on one side, both sides, or in the middle.

Figure 8. Types of cleft lip. More details (left, middle, right).

A cleft palate is when the roof of the mouth contains an opening into the nose. The two plates of the skull that form the hard palate (roof of the mouth) are not completely joined. The soft palate is in these cases cleft as well. In most cases, cleft lip is also present. Cleft palate occurs in about one in 700 live births worldwide.

Cleft palate can occur as complete (soft and hard palate, possibly including a gap in the jaw) or incomplete (a 'hole' in the roof of the mouth, usually as a cleft soft palate). When cleft palate occurs, the uvula is usually split. It occurs due to the failure of fusion of the lateral palatine processes, the nasal septum, and/or the median palatine processes.

These disorders can result in feeding problems, speech problems, hearing problems, and frequent ear infections. Less than half the time the condition is associated with other disorders.

Figure 9. Types of cleft palate. More details (left, middle, right).

The development of the face involves the fusion of elements that grow from each side and meet in the middle. If these tissues fail to meet, a gap appears where the tissues should have joined (fused). This may happen in any single joining site, or simultaneously in several or all of them. The resulting birth defect reflects the locations and severity of individual fusion failures (e.g., from a small lip or palate fissure up to a completely malformed face).

The upper lip is formed earlier than the palate. Formation of the palate is the last step in joining five embryonic facial lobes, and involves the most posterior of them. This process is vulnerable to multiple toxic substances, environmental pollutants, and nutritional imbalance, in addition to being influenced by genetic predisposition.

A cleft lip or palate can be successfully treated with surgery. This is often done in the first few months of life for cleft lip and before eighteen months for cleft palate. Speech therapy and dental care may also be needed. The surgeon approximates the sides of the lips that should have fused and tries to line up the cut with the natural lines in the upper lip to conceal any scar. Stitches are positioned far up the nose to make them less visible. Incomplete cleft provides more tissue for the reconstruction work and tends to result in a more supple and natural-looking upper lip.

A cleft palate can be temporarily covered with a palatal obturator (a prosthetic device made to fit the roof of the mouth covering the gap). It can be corrected by surgery, usually performed between 6 and 12 months. Approximately 20–25% require a single palatal surgery to achieve normal, non-hypernasal speech. Combinations of surgical methods and repeated surgeries are often necessary, however, as the child grows. If the cleft extends into the maxillary alveolar ridge, the gap is usually corrected by filling the gap with bone tissue. The bone tissue can be acquired from the patients own chin, rib or hip.

Cleft palate in animals

Cleft lips and palates are occasionally seen in cattle and dogs, and rarely in goats, sheep, cats, horses, pandas and ferrets. Most commonly, the defect involves the lip, rhinarium, and premaxilla. Clefts of the hard and soft palate are sometimes seen with a cleft lip. Difficulty with nursing is the most common problem associated with clefts, but aspiration pneumonia, regurgitation, and malnutrition are often seen with cleft palate.

Figure 10. Cleft lip in a Boxer. More details.

Summary

The activation of controller genes coordinates the secretion of chemical signals that direct the differentiation of cells. A series of pharyngeal arches forms on each side of the pharynx and provides the precursors to most cranial and facial structures. The hard palate is formed through the early development of its medial anterior portion, followed by growth of the secondary portion from lateral to medial. Incomplete growth or fusion of the secondary portion results in cleft palate.

Key terms

Pharyngeal arch, body axis, palate, cleft palate

Figure Credits

Figure 1 by Loki austanfell - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27844883

Figure 2 by User:Uwe Gille - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1184299

Figure 3 by Arcadian - http://training.seer.cancer.gov/head-neck/anatomy/overview.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1678037

Figure 4 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 49, Public Domain, https://commons.wikimedia.org/w/index.php?curid=792248

Figure 5 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 44, Public Domain, https://commons.wikimedia.org/w/index.php?curid=792243

Figure 6 by DeepDrillDentist - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55243719

Figure 7 by DnetSvg at English Wikipedia - Transferred from en.wikipedia to Commons by ILA-boy., Public Domain, https://commons.wikimedia.org/w/index.php?curid=4412485

Figure 8 by Felsir (talk) - Own work (Original text: self-made), Public Domain, https://commons.wikimedia.org/w/index.php?curid=4412489

Figure 9 by Felsir at English Wikipedia - Transferred from en.wikipedia to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1519781

Figure 10 by Felsir at English Wikipedia - Transferred from en.wikipedia to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1519784

Figure 11 by Felsir at English Wikipedia - Transferred from en.wikipedia to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1519785

Figure 12 by Felsir at English Wikipedia - Transferred from en.wikipedia to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1519784

Figure 13 by Joel Mills - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=927282

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