Histological and Radiological Observations of
Ayako Okuda Japan
Noden and deLahunta (1985) described the comparative embryology of domestic animals including cattle, swine, sheep, horses, rodents, rabbits, carnivores, primates, other mammals, and avians. The craniofacial development of all mammals is similar, but the tooth development is varied because of different tooth types. Radiological observation of canine tooth development has been reported, but it is difficult to generalize stages of development because of variation in head and body sizes. Compared to the canine, it is easy to generalize about the body and head sizes of domestic cats, which vary less, but there were fewer reports of feline tooth development. Major developmental events in craniofacial structures and development of the deciduous and permanent dentition were observed histologically in the prenatal cat, and radiologically, in the kitten.
Japanese mixed breed prenatal cats were donated from abortions or necropsy in small animal clinics. As the days of gestation were not clear, all samples were measured for crown-rump (CR) length to estimate gestation days
Orofacial formation begins with the breakdown of the oropharyngeal membrane following cranial flexure. Externally, the frontal-nasal process, the right and left maxillary processes and the mandibular processes surround the stomodeum. The oropharyngeal membrane is ruptured following fusion of the left and right mandibular arches at day 18. Each nasal placode invaginates to form a nasal pit, which deepens and contacts the roof of the oral cavity. Medial and lateral nasal processes appear around the nasal pits. The maxillary processes expand rostrally to contact the lateral nasal processes, forming the nasolacrimal groove between them at day 20. The maxillary processes continue to grow rostrally and subsequently contact the medial nasal processes, which completes the upper lip.
The primary palate appeared in the 23�24 day embryo. At this stage, the muscles of the tongue are not clearly visible. The secondary palate developed after 24 days by lifting and fusing the right and left palatal processes. During this process and during differentiation of the tongue muscles, the tongue itself was sinking down to the floor of the oral cavity. The fusion of palatal processes was completed at 28 days. The palatal processes fused together starting centrally. This fusion advanced forward to contact the primary palate, and backward to complete the hard palate and continued on to form the soft palate. Mesenchyme of the palatal processes extended to the nasal septum at day 28. Ossification of the maxilla around the zygomatic bone was observed at day 26, prior to the ossification of the hard palate after the 28th day.
The formation of the mandible started with the accumulation of mesenchymal cells to form Meckel's cartilage at day 20. The matrix formation of cartilage was completed and the beginning of mandibular bone formation was clearly visible at day 26, and advanced mandibular bone formation could be seen on day 33. Bone formation cannot be observed at the center of the mentalis even in a late stage. Mandibular bone is forming beside Meckel's cartilage with the formation of the mental foramen, and the mental nerve and vessels passing through. Degeneration of Meckel's cartilage was found around the major mental foramen at day 39 while anterior and posterior parts of the cartilage were still visible.
The deciduous dental lamina appeared at day 24 with the 1st, 2nd, and 3rd incisors, canines, and posterior teeth. Tooth germs of the deciduous canines and molars were in the bud stage in the day 26 embryo. All of the deciduous tooth germs were developed to the bell stage by day 33. Dentin and enamel formation were found in the deciduous molar of the 39-day embryo. Permanent tooth germs were found in the 39 or 45-day embryo. Tooth germs of the permanent upper and lower canines, upper fourth premolars and lower molars appeared, and were followed by the upper and lower incisors and other premolars and molars. At 45 days calcification was not yet started.
Clinical and radiological eruption and maturation of the deciduous and permanent dentitions were observed in five kittens. Eruption of the deciduous dentition started before day 25 with the lower and upper first and second incisors, and then all third incisors and lower canines starting to erupt around day 25. On day 39, all of the deciduous teeth were erupted while root apices of the deciduous teeth were still widely open. Calcification of the lower canines, molars, and upper canines were observed in radiographs. Radiologically most of the root apices of the deciduous teeth were closed and all crowns of the permanent teeth started to be calcified by day 53. On day 67, the coronal shapes of all permanent teeth were very clear. The incisors of the maxilla and mandible were ready for shedding by day 81. Most permanent teeth erupted during, or just after, shedding of the deciduous teeth, but the lower molar erupted on day 112. On day 151, all the permanent incisors except the thirds were erupted, and these, as well as the rest of the permanent dentition, were ready to erupt. All permanent teeth did erupt into the oral cavities. Closure of the root apices was found in all incisors radiologically on day 202. Apices of the lower molars and all canines were still open on day 297.
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