PECULIARITIES OF THE ORBIT MORPHOGENESIS AT AN EARLY PERIOD OF HUMAN ONTOGENESIS

The aim: To clarify the sources and determine the chronological sequence of the germs of anatomical structures of the orbit at the early period of human ontogenesis.

Materials and methods: Using a complex of methods of morphological examination (morphometry, microscopy, three-dimensional computer reconstruction and statistical analysis) 30 series of consecutive histological sections of human embryos and prefetuses aged from 3 till 8 weeks od IUD (3.0-30.0 mm parietal-coccygeal length (PCL)) were studied.

Results: At the 4rd week of IUD the orbital region is represented by the place of close contact of the neuroectoderm of the optic vesicle with the adjacent integumentary ectoderm,as well as the mesenchyme surrounding this place of contact. In embryos of 4.0 mm PCL, the rudiment of the optic stalk is observed as a result of the transformation of the junction of the area of the eye rudiment with the brain.

Conclusions: 1. The rudiments of the organ of vision (lens placodes) appear in the 4rd week of IUD. At the 5th week of IUD as a result of gradual intussusception of crystalline placodes into the adjacent mesenchyme, lens pit are formed, and then - lens vesicles. 2. Rudiments of the extraocular muscles (except for the inferior oblique muscle) was detected at the end of the 5th week of IUD. The rudiment of the inferior oblique muscle develops from a single mesodermal islet located in the mesenchyme medially and below the eyeball. Simultaneously with the rudiments of the extraokular muscles, the trochlear and abductor nerves are develop and ingrown into the orbit. 3. The development of blood vessels of the orbit occurs from two sources - from the islands of local angiogenesis, which begins in the 5th week of IUD, and from extraorganic vessels, which can be traced in the form of a vascular network at the end of the 6th week of IUD. The combination of both sources is observed at the end of the 7th week of IUD. 4. The embryonic period of ontogenesis is the first critical period in the development of the human orbit, due to the formation of the muscles, nerve and vascular structures.