PECULIARITIES OF DEVELOPMENT OF THE HUMAN FACIAL REGION OF SKULL

Abstract

Introduction. Elucidation of the peculiarities of the morphogenesis of the structures of the human maxillofacial region does not lose its relevance among scientists-anatomists, embryologists, maxillofacial and pediatric surgeons, which is explained by a rather large congenital pathology of the structures of the human face, which rank third among all congenital malformations (CMF). Morphological studies of the sources of rudiments and the chronological sequence of the appearance of the bones of the facial part of the human head, clarifying the critical periods of their development will contribute development of new and improvement of existing methods of early diagnosis and effective surgical correction of CMF of the face. The aim of the study. The aim of this paper was to clarify the sources, to find out the chronological sequence of the appearance of rudiments and the peculiarities of the morphogenesis of the bones of the facial part of the human skull. Material and methods. Specimens of 20 embryos and 25 human pre-fetuses aged from 4 to 12 weeks of intrauterine development (IUD) (4.0-80.0 mm parietal-coccygeal length (PCL)) were studied. A complex of classical and modern methods of morphological research is applied: making and studying a series of consecutive histological sections, microscopy, morphometry, three-dimensional reconstruction. The investigations were performed keeping to the major regulations of the Resolution of the First National Congress on Bioethics «General Ethic Principles of Experiments on Animals» (2001), ICH GCP (1996), the European Union Convention on Human Rights and Biomedicine (04.04.1997), and the European Convention for the Protection of Vertebrate Animals Used for Experimental and Other Scientific Purposes (18.03.1986), the Declaration of Helsinki on Ethical Principles for Medical Research Involving Human Subjects (1964-2008), EU Directives №609 (24.11.1986), the Orders of the Ministry of Health of Ukraine № 690 dated 23.09.2009, №944 dated 14.12.2009, № 616 dated 03.08.2012. The work is carried out within the framework of the initiative research work of the Department of Histology, Cytology and Embryology of Institution of higher education “Bukovinian State Medical University" "Structural and functional peculiarities of tissues and organs in ontogenesis, regularities of variant, constitutional, sex-, age-related and comparative human morphology". State registration number: 0121U110121. Terms of execution: 01.2021-12.2025. Results. Sources of facial structures – mesenchyme of five facial evaginations (frontal process, paired mandibular and hyoid branchial arches) are determined on specimens of 4-week aged human embryos. At the end of the 4th week of IUD, two processes of the mandibular branchial arch are visible – maxillary and mandibular, and the brain is surrounded by an ectomeningeal capsule, the source of which is mesenchyme of neuroectodermal origin. Its outer layer (ectomeninx) forms the splanchnocranium – the rudiment of the bones of the facial part of the skull, which ossifies in both a membranous and cartilaginous way. In the 5th week of IUD, the process of fusion of the derivatives of facial evaginations begins. In the 6th week of IUD, the mandibular processes of the I branchial arch merge along the midline, forming the rudiment of the mandible. At the 7th week of IUD, the ectomeningeal capsule differentiates into a cartilaginous structure, which at the 8th week of IUD becomes continuous around the brain and gives the sources of the bones of the base of the skull and the cartilaginous nasal capsule. The nasal capsule is the source of development of the ethmoid bone, nasal septum, and lower concha. At the 7th week of IUD the maxillary, medial, and lateral nasal processes connect with each other, which leads to the completion of the morphogenesis of the maxilla. In the 8th week of IUD, the center of ossification is first detected in the suprabrow region of the frontal bone rudiment. In the 9th week of IUD, active processes of osteogenesis occur in the mandible, as a result of which its base is formed, while the coronal and condylar processes are ossified by cartilaginous osteogenesis from secondary centers that appear after the 10th week of IUD. Each nasal bone ossifies from one cartilaginous center at the beginning of the 9th week of IUD, and the lacrimal bones ossify from one membranous center during the 12th week of IUD. Conclusions. 1. Disruption in the processes of proliferation, fusion and transformation of the branchial apparatus at the 5-6th weeks of intrauterine development leads to the appearance of severe defects, in particular, cleft upper lip, alveolar process and palate. 2. The condensed mesenchyme of the front part of the ectomeningeal capsule (in front of the pituitary gland) has a neuroectodorsal origin, and its outer layer (ectomeninx) forms the splanchnocranium – the source of the bones of the facial part of the skull (frontal, lacrimal, zygomatic, nasal bones, vomer, maxilla and mandible), and ossifies in both a membranous and cartilaginous way. 3. The frontal, lacrimal, nasal bones, vomer, as well as the premaxillary part of the maxilla (incisive bone) originate from the mesenchyme of the mandibular branchial arch. The maxilla and the zygomatic bone originate from the mesenchyme of the maxillary process, while the mandible and the tympanic part of the temporal bone originate from the mesenchyme of the mandibular process of the 1st branchial arch. 4. Time intervals during which active proliferative changes and differentiation of embryos occur (7 and 10 weeks of human prenatal development) can be classified as critical periods of development of bone rudiments of the human skull with the possible appearance of congenital malformations.

Keywords

• Embryonic Development; Facial Part of the Skull; Membranous and Cartilaginous Osteogenesis; Human.