Peculiarities of Development and Embryotopography of the Thyroid and Parathyroid Glands in the Fetal Period of Human Development

Abstract

Active implementation of perinatal prevention and treatment of congenital malformations requires modern approaches and methods of research of intrauterine development, the use of which is impossible without a comprehensive morphological study of development of the structure and topography of organs and structures of various systems in the early period of human ontogenesis. A priority task of the modern anatomy is the study of patterns of prenatal morphogenesis and syntopia of human endocrine glands. The purpose of the research was to study the peculiarities of the development and formation of the topography of the thyroid and parathyroid glands during the human embryonic period. Material and methods. The study was performed using microscopy of a series of consecutive histological sections of 18 human embryos of 4.0-13.5 mm parietal-coccygeal length and graphical reconstruction. Results and discussion. The thyroid gland forms as the epithelium cell’s protrusion along the midline between the I and II pharyngeal pockets, and appears on the 4th week of embryonic development. The rudiments of parathyroid glands in the form of epithelial protrusions of the dorsal part of the III and IV pharyngeal pockets appear during the 5th week of the development of fetus. In 6-week-old embryos, the thyroid gland enters into a complex syntopic relationships with adjacent organs and structures; herewith the topographic boundaries of the organ are not clearly defined. The growth of the thyroid gland goes along the common carotid arteries and gradually loses contact with the aortic arch. A diffuse vascular network is formed around the thyroid gland, which penetrates into it. It is clearly traced that the upper thyroid arteries originate from the external carotid arteries. At the end of the embryonic period the tempo of growth of thyroid gland acutely increases, the main variants of the thyroid gland's shape (with an isthmus and without an isthmus) can be defined. Conclusion. In the embryos of 4.0-4.5 mm parietal-coccygeal length, the thyroid gland's rudiment is detected as a protrusion of the epithelium along the midline between the I and II pharyngeal pockets, which begins to lose contact with the oropharyngeal cavity in embryos of 5.5-6.0 mm parietal-coccygeal length. Subsequently, appear close interrelations between the thyroid gland's rudiment and the arterial trunk. As a result of the formation of new syntopic connections of the thyroid gland’s rudiment with the IV pharyngeal arteries and the lower nodes of the vagus nerves, the thyroid gland's rudiment takes the form of a grooved plate that grows and models along the primitive aortic arch and localizes between the right and left common carotid arteries. The rudiments of the thyroid glands appear in embryos of 6.0-8.0 mm parietal-coccygeal length in the form of epithelial protrusions of the dorsal part of the III and IV pharyngeal pockets, from which in the future the lower and upper parathyroid glands will be formed, respectively. The critical periods of the thyroid and parathyroid glands development on the early stages of human ontogenesis are: 5th week – a period of intensive formation of the rudiments of the thyroid and parathyroid glands, and 6th week – the formation of laryngeal cartilages and cellular structures of the neck