C H A P T E R 10 PHARYNGEAL APPARATuS, FACE, AND NECk 115
These buds branch and canalize to form 10 to 12 ducts that open independently into the floor of the mouth.
116 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
stomodeum and the mandibular prominences constitute the caudal boundary of the stomodeum (see Fig. 10-23A and Fig. 10-24). The lower jaw and the lower lip are the first parts of the face to form. They result from merging of the medial ends of the mandibular prominences. The Figure 10–21 A and B, Schematic horizontal sections through the pharynx showing successive stages in the development of the tongue during the fourth and fifth weeks. C, Drawing of an adult tongue, showing the pharyngeal arch derivation of the nerve supply of its mucosa (mucous membrane). CN, Cranial nerve.
A
Pharyngeal
arches: Lateral lingual
swelling Median lingual swelling Foramen cecum of tongue Copula Hypopharyngeal eminence
Hypopharyngeal eminence Rima glottidis (opening to vocal apparatus)
Midline groove Circumvallate papillae
Terminal sulcus
Foramen cecum
Epiglottis Pharyngeal
part of tongue Oral part of tongue
Migration of third arch mesoderm 1
2 3
4
Esophagus Laryngotracheal groove
B
C
Pharyngeal Arch Derivatives of Tongue
1st pharyngeal arch
(CN V—mandibular division) 2nd pharyngeal arch (CN VII—chorda tympani) 3rd pharyngeal arch
(CN IX—glossopharyngeal) 4th pharyngeal arch (CN X—vagus)
Figure 10–22 An infant with ankyloglossia (tongue-tie). Note the short frenulum, which extends to the tip of the tongue. Anky- loglossia interferes with protrusion of the tongue and it may make breast-feeding difficult.
common “chin dimple” results from incomplete fusion of the prominences.
By the end of the fourth week, bilateral oval thicken- ings of the surface ectoderm—nasal placodes—have developed on the inferolateral parts of the frontonasal prominence (see Fig. 10-24 and Fig. 10-25A and B). Ini- tially, these placodes are convex, but later, they are stretched to produce a flat depression in each placode.
The mesenchyme in the margins of the placodes prolifer- ates, producing horseshoe-shaped elevations—the medial and lateral nasal prominences (see Fig. 10-23B and Fig.
10-25D and E). As a result, the nasal placodes lie in depressions—nasal pits (see Figs. 10-23B and Fig. 10-25C and D). These pits are the primordia of the anterior nares (nostrils) and nasal cavities (see Fig. 10-25E).
Proliferation of mesenchyme in the maxillary promi- nences causes them to enlarge and grow medially toward each other and the nasal prominences (see Fig. 10-23B and C and Fig. 10-24). The medial migration of the max- illary prominences moves the medial nasal prominences toward the median plane and each other. This process is regulated by transcription factor PDGFRa signaling.
Each lateral nasal prominence is separated from the max- illary prominence by a cleft called the nasolacrimal groove (see Fig. 10-23B).
By the end of the fifth week, six auricular hillocks—
primordia of the auricles (mesenchymal swellings) form around the first pharyngeal groove (three on each side), the primordium of the external acoustic meatus (ear canal). Initially, the external ears are positioned in the neck region; however, as the mandible develops, they ascend to the side of the head at the level of the eyes (see Fig. 10-23B and C).
By the end of the sixth week, each maxillary promi- nence has begun to merge with the lateral nasal promi- nence along the line of the nasolacrimal groove (Fig.
10-26A and B). This establishes continuity between the side of the nose, formed by the lateral nasal promi- nence, and the cheek region, formed by the maxillary prominence.
C H A P T E R 10 PHARYNGEAL APPARATuS, FACE, AND NECk 116.e1 (Courtesy Dr. Evelyn Jain, Lakeview Breastfeeding Clinic, Calgary,
Alberta, Canada.)
C H A P T E R 10 PHARYNGEAL APPARATuS, FACE, AND NECk 117
Figure 10–23 A to D1, Diagrams illustrating progressive stages in the development of the human face.
A
B
A1
B1
Frontonasal prominence Maxillary prominence Mandibular prominence
Stomodeum 2nd pharyngeal arch
Lens placode
Nasal pit
Nasal prominences Median
Lateral
Nasolacrimal groove
External acoustic meatus (first pharyngeal groove)
Nasal placode
28 days
33 days
C C1
D D1
Eyelid
Philtrum of lip Medial nasal prominences merging with each other and maxillary prominences 48 days
14 weeks
The nasolacrimal duct develops from a rod-like thick- ening of ectoderm in the floor of the nasolacrimal groove.
This thickening gives rise to a solid epithelial cord that separates from the ectoderm and sinks into the mesen- chyme. Later, as a result of apoptosis (programmed cell death), this cord canalizes to form the nasolacrimal duct.
The cranial end of this duct expands to form the lacrimal
sac. In the late fetal period, the nasolacrimal duct drains into the inferior meatus in the lateral wall of the nasal cavity. The duct usually becomes completely patent (open) after birth.
Between weeks 7 and 10, the medial nasal promi- nences merge with each other and with the maxillary and lateral nasal prominences (see Fig. 10-23C), resulting in
118 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
disintegration of their contacting surface epithelia. This causes intermingling of the underlying mesenchyme.
Merging of the medial nasal and maxillary prominences results in continuity of the upper jaw and lip and separa- tion of the nasal pits from the stomodeum. As the medial nasal prominences merge, they form an intermaxillary segment (see Fig. 10-26C to F). The segment gives rise to the:
•
Median part (philtrum) of the upper lip•
Premaxillary part of the maxilla and its associated gingiva (gum)•
Primary palateThe lateral parts of the upper lip, most of the maxilla, and the secondary palate form from the maxillary promi- nences (see Fig. 10-23D). These prominences merge later- ally with the mandibular prominences. Recent studies indicate that the lower part of the medial nasal promi- nences appears to have become deeply positioned and covered by medial extensions of the maxillary promi- nences to form the philtrum.
Figure 10–24 Scanning electron micrograph of a ventral view of a human embryo at approximately 33 days (Carnegie stage 15; crown–rump length, 8 mm). Observe the prominent frontonasal prominence (FNP) surrounding the telencephalon (forebrain). Also observe the nasal pits (NP) located in the ven- trolateral regions of the frontonasal prominence. Medial and lateral nasal prominences surround these pits. The wedge- shaped maxillary prominences (MXP) form the lateral boundaries of the stomodeum. The fusing mandibular prominences (MDP) are located just caudal to the stomodeum. The second pharyn- geal arch (BA2) is clearly visible and shows overhanging margins.
The third pharyngeal arch (BA3) is also clearly visible. (From Hinrichsen K: The early development of morphology and patterns of the face in the human embryo. Adv Anat Embryol Cell Biol 98:1, 1985.)
FNP
MXP MDP
BA2 BA3 NP
Figure 10–25 Progressive stages in the development of a human nasal sac (primordial nasal cavity). A, Ventral view of an embryo at approximately 28 days. B to E, Transverse sections through the left side of the developing nasal sac.
Level of sections B to E
Stomodeum Nasal placode
Forebrain
Nasal placode
Lateral nasal prominence
Nasal sac (primordium of nasal cavity)
Naris (nostril)
Lateral nasal prominence Nasal pit
Mesenchyme (derived from mesoderm) Frontonasal prominence
A
B
C
D
E
Nasal pit Medial nasal prominence
Nasal placode (derived from surface ectoderm)
C H A P T E R 10 PHARYNGEAL APPARATuS, FACE, AND NECk 119