**2.2. Facial development**

At Carnegie stage 12, three pharyngeal arches appear. The first pharyngeal arch emerges from the maxillary and mandibular prominences (stage 13, **Figure 1**), which will later constitute the lateral and caudal boundaries of the stomodeum (i.e., primitive oral cavity), respectively.

The sides and front of the neck arise from the second pharyngeal arch, also known as the hyoid arch. Meanwhile, the frontonasal prominence (FNP) grows and covers the ventral part of the forebrain (stage 13), which will eventually form the forehead (frontal part of the FNP) and the primordial mouth and nose (nasal part of the FNP).

During the fifth developmental week, primordial ear auricles form around the first pharyngeal groove, at the interface between the mandibular prominences and the hyoid arches (stage 16). While the auricle emerges from the auricular hillocks, the external acoustic meatus arises from the first pharyngeal groove. At an early stage of ear development, the external ears are located in the neck region, which then start to ascend toward the level of the eyes on either

Congenital Anomalies in Human Embryos http://dx.doi.org/10.5772/intechopen.72628 25

The maxillary and lateral nasal prominences fuse with the nasolacrimal groove during the sixth developmental week, which enables the nose and cheek to be continuous (stage 18).

The seventh developmental week is marked by the fusion of the medial nasal prominence and the maxillary and lateral nasal prominences (stage 19~). Merging of the maxillary and medial nasal prominences creates continuity between the upper jaw and lip, leading to the segrega-

side of the head, simultaneously with the development of the mandible.

tion of the nasal cavity and oral cavity.

**Figure 1.** Development of human embryo, Carnegie stages 12–23.

By the end of the fourth developmental week, nasal placodes (thickening of surface ectoderm that later becomes peripheral neural tissue) develop on the frontolateral aspects of the FNP (stage 13). The mesenchyme swells around the nasal placodes, which leads to the formation of medial and lateral nasal prominences (stage 16). The maxillary prominence merges with the medial nasal prominence, leading to its fusion. The fused medial nasal prominence will not only form the primary palate (stage 16–18), but also the midline of the nose and that of the upper lip.

The nasolacrimal groove divides the lateral nasal prominence from the maxillary prominence (observed in stages 16, 17).

**Figure 1.** Development of human embryo, Carnegie stages 12–23.

The head becomes round and the superficial vascular plexus spreads to surround the head. Meanwhile, the tail becomes rudimentary. At this time, the hands are slightly flexed at the

The vascular plexus of the head becomes more distinct at this stage. The eyelids start to thicken and encroach into the eyes. In the meantime, the tragus and antitragus of the external ear assume a more definite form, as the external ear repositions higher on the head. The tail

At this stage, the head would be observed with a more rounded appearance and the trunk with a more mature shape. The eyelids and ear auricles have become conspicuous, the limbs have increased in length, and the forearms have ascended toward the level or higher than that of the shoulders. Meanwhile, the vascular plexus approaches the vertex of the head. Although external sex differences are not yet apparent, the external genitalia have developed relatively

At Carnegie stage 12, three pharyngeal arches appear. The first pharyngeal arch emerges from the maxillary and mandibular prominences (stage 13, **Figure 1**), which will later constitute the lateral and caudal boundaries of the stomodeum (i.e., primitive oral cavity), respectively. The sides and front of the neck arise from the second pharyngeal arch, also known as the hyoid arch. Meanwhile, the frontonasal prominence (FNP) grows and covers the ventral part of the forebrain (stage 13), which will eventually form the forehead (frontal part of the FNP)

By the end of the fourth developmental week, nasal placodes (thickening of surface ectoderm that later becomes peripheral neural tissue) develop on the frontolateral aspects of the FNP (stage 13). The mesenchyme swells around the nasal placodes, which leads to the formation of medial and lateral nasal prominences (stage 16). The maxillary prominence merges with the medial nasal prominence, leading to its fusion. The fused medial nasal prominence will not only form the primary palate (stage 16–18), but also the midline of the nose and that of the

The nasolacrimal groove divides the lateral nasal prominence from the maxillary prominence

wrists and are placed closely over the cardiac prominence. Carnegie stage 22: Eyelids and external ears develop

Carnegie stage 23: The end of embryonic period

well by this time. The tail is no longer observed at this stage.

and the primordial mouth and nose (nasal part of the FNP).

50 days after fertilization

24 Congenital Anomalies - From the Embryo to the Neonate

52 days after fertilization

CRL: 23 mm

is about to disappear.

CRL: 30 mm

**2.2. Facial development**

upper lip.

(observed in stages 16, 17).

During the fifth developmental week, primordial ear auricles form around the first pharyngeal groove, at the interface between the mandibular prominences and the hyoid arches (stage 16). While the auricle emerges from the auricular hillocks, the external acoustic meatus arises from the first pharyngeal groove. At an early stage of ear development, the external ears are located in the neck region, which then start to ascend toward the level of the eyes on either side of the head, simultaneously with the development of the mandible.

The maxillary and lateral nasal prominences fuse with the nasolacrimal groove during the sixth developmental week, which enables the nose and cheek to be continuous (stage 18).

The seventh developmental week is marked by the fusion of the medial nasal prominence and the maxillary and lateral nasal prominences (stage 19~). Merging of the maxillary and medial nasal prominences creates continuity between the upper jaw and lip, leading to the segregation of the nasal cavity and oral cavity.
