Development Of Nasal Cavities | Embryology скачать в хорошем качестве

Development Of Nasal Cavities | Embryology

the first 28 day of gestational life the face develops from five swellings: The paired maxillary and mandibular processes and the unpaired frontonasal process. During the fifth week, the nasal placodes (i.e., nasal discs and nasal plates) develop as a result of ectodermal thickenings and can be observed on the frontonasal process. In the sixth week, infolding of the ectoderm at the epicenter of these nasal placodes initiates the formation of an oval pit (see below description of nasal pits) resulting in the division of the raised edge of each placode into medial and lateral nasal processesDuring the sixth week, the medial nasal processes fuse to form the intermaxillary process (Figure 1B), which is the primordia of the septum and bridge of the nose (Figure 1C). By the end of the seventh week, there is a lateral and inferior expansion of the medial nasal processes at their inferior tips before fusing to form the anterior roof of the oral cavity (Figure 1B). As the poles of the maxillary swellings continue to develop they come into contact with the intermaxillary process where they fuse with each other. On the superior labial region, the intermaxillary process develops into the philtrum (Figure 1C). Formation of the nasal passages result as the nasal pits deepen penetrating its underlying mesenchyme during week 6 of development. Mesenchyme is loosely organized cells derived from mesodermal embryonic tissue that develops into connective and skeletal tissues. An ectodermally enlarged nasal sac is formed during the last days of the sixth week by the fusion of the deep endings of the nasal pits, which are topographically located superoposterior to the intermaxillary process. During the last days of the sixth week and the first few days of the seventh week, a proliferation of cells occurs at the posterior wall and floor of the nasal sac forming a thickened plate-like fin of ectoderm origin essentially isolating the oral cavity from the nasal sac but still maintaining an epithelial continuity between the regions. This “keel” structure is now referred to as the nasal fin. The nasal sac enlarges as a result of vacuoles developing within the nasal fin and then it fuses with the sac. The nasal fin begins to attenuate to a thin membrane named the oronasal membrane, which demarcates the oral cavity from the nasal sac. Towards the end of the seventh week, the oronasal membrane obliterates creating the opening of the primitive choana. Formation of the nasal cavity floor, or primary palate, occurs by the backward growing of the intermaxillary process. Throughout the eighth and ninth week, the development of the definitive and secondary palate occurs. The main portion of the definitive palate develops by two shelve-like outgrowths from the maxillary processes. These two thin medial extension outgrowths are called the palatine shelves, which appear during the sixth week of development. While these shelves are directed in a downward manner on either side of the tongue, it is during the ninth week where these shelves rotate and ascend rapidly attaining a horizontal position above the tongue. Fusion of the primary palate and the palatine shelves (along the midline) assists in the formation of the secondary palate (Figure 1D). The order of fusion first begins at the ventral region of the palatine shelves before proceeding dorsally. Mesenchymal condensations occurs when previously dispersed mesenchymal cells come together to differentiate into a single tissue type and is considered the critical transitional stage that precedes cartilage formation during embryonic development[6]. When these mesenchymal cell condensations occur in the ventral region of the secondary palate endochondral ossification ensues to achieve the formation the hard palate. At the dorsal region of the secondary palate, myogenic mesenchymal cells come together to form the muscular layer of the soft palate. During the formation of the secondary palate, there is a proliferation of cells from the mesoderm and ectoderm region of the medial nasal and frontonasal processes that help form the nasal septum along its midline. As a result, the two nasal passages of the nasal cavity have now been established, communicating with the pharynx located posterior to the secondary palate. This communicating portal is now termed the definitive choana. According to classical concept, the philtrum of upper lip, the nasal dorsum, septum, and primary palate originate from the development of the intermaxillary process, whereas the lateral walls of the nasal pyramid develop from the lateral nasal processes