ONTOGENETIC TRANSFORMATIONS OF THE FACIAL RUDIMENTS OF THE SKULL BONES IN THE FIRST TRIMESTER OF PRENATAL HUMAN ONTOGENESIS

Abstract

The purpose of the research – to clarify the sources and terms of the rudiments, to findout the age-specific dynamics of changes and features of the morphogenesis of the bonesof the facial part of the skull in the first trimester of intrauterine development (IUD).Material and methods. Serial microspecimens of 20 embryos (4.0-13.5 mm parietalcoccygeal length (PCL)) and 25 prefetuses (14.0-80 mm PCL) covering the period fromthe 4th to the 12th weeks of the IUD were studied. A complex of methods of morphologicalresearch is applied: creation of series of consecutive histological sections, microscopy,morphometry, three- dimensional reconstruction.The results. The rudiments of the bones of the facial part of the skull are the mesenchyme ofthe five facial outgrowths of 4-week-old human embryos. The rudiment of the anterior partof the ectomeningeal capsule has a neuroectodermal origin, and its outer layer forms thesplanchnocranium – the rudiments of the frontal, zygomatic, nasal, lacrimal bones, vomer,and jaws. In 5-week-old embryos, the process of fusion of the frontal process, the medialand lateral nasal processes of maxillar process and mandibular process of the I branchialarch and the hyoid branchial arch begins, resulting in the formation of nasal pits and the primary nasal cavity. In 6-week-old embryos, the rudiments of the mandible are formed.During the 7-8th week of IUD, the cartilaginous rudiment of the skull becomes a continuousstructure. The mesenchyme of the mandibular branchial arch is the source of the developmentof the frontal, lacrimal, nasal bones, vomer, and premaxillary part of the maxilla. The sourcesof development of the maxilla and zygomatic bone are the mesenchyme of the maxillaryprocess of the I branchial arch, while the mandible and the tympanic part of the temporal boneare formed from the mesenchyme of the mandibular process of this branchial arch.Conclusions. Violation of the proliferation of processes of the I branchial arch, theirconsolidation and transformation of the branchial apparatus at the end of the embryonic period(5-6th week of IUD) can lead to the appearance of severe congenital defects of the face. Also,the 7th and 10th weeks of IUD are critical periods of the morphogenesis of the rudiments of thehuman skull, the time of the possible appearance of variants of the structure or birth defects.