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Development and age specific features of lymphatic vessels
Last reviewed: 19.10.2021
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The lymphatic system was first found in bony fishes in the form of intestinal mesenteric lymphatic vessels and their extensions - lymphatic sinuses between internal organs, between the pericardium and gill sacks, near the fins. Amphibians and reptiles develop contractile organs - lymphatic hearts, connected on the one hand with lymphatic sinuses and vessels, and on the other - with veins. In birds, lymphatic hearts are present only in the embryonic period; in waterfowl for the first time there are lymph nodes (lumbar and cervical). The number of lymph nodes increases in mammals, they have valves in the lymphatic vessels.
In humans, during the 6th week of intrauterine development, the mesoderm is separated from the circulatory system, but near the large veins that form, slitlike spaces are formed, limited by mesenchymal cells, which later turn into endothelial cells. By merging the slit-like spaces, a system of canals is formed, expanding and converting into lymphatic sacs. The first appear left and right lymphatic sacs, a little later - subclavian lymphatic sacs. A chain of bags located near the dorsal wall of the embryo's body gives rise to the main lymphatic duct - the thoracic duct, which opens at the 9th week of development into the left jugular sac. Located on the right and left of the jugular, subclavian lymphatic sacs connect to the veins in the neck. Of the paired iliac lymphatic vessels develop lymphatic vessels of the pelvis and lower limbs.
Age features of lymphatic vessels.
Lymphatic capillaries in newborn children, as well as in adolescence and adolescence, have a relatively larger diameter than those of mature age; the contours of the capillaries are even. Lymphatic capillaries form dense, fine-meshed networks. In an adult human lymphatic capillaries have a smaller diameter, they become narrower, part of the capillaries becomes lymphatic vessels. In lymphatic nets appear open loops, as well as protrusions, swelling of the capillary walls. In the elderly and senile age, the phenomenon of reduction of lymphatic capillaries is more pronounced.
Lymphatic vessels in newborns and children of the first years of life have a characteristic clear pattern due to the presence of constrictions (constrictions) in the area of valves that are not yet fully formed. Valvular apparatus of lymphatic vessels reaches its maturity by 13-15 years of life.
In childhood and adolescence, adjacent lymphatic vessels are connected to each other by numerous transverse and obliquely oriented anastomoses, as a result of which lymphatic plexuses form around the arteries, veins, and ducts of the glands. In people 40-50 years, there are signs of reduction of lymphatic vessels. The contours of the vessels become uneven, in places appear protrusions of the walls, the number of anastomoses between the lymphatic vessels decreases, especially between the surface and deep ones. Some vessels empty. The walls of the lymph vessels in elderly and senile people thicken, their lumen decreases.
The thoracic duct in newborns and older children is correspondingly smaller in size than in an adult, its walls are thin. The maximum development of the thoracic duct reaches in adulthood. In elderly people and the elderly in the walls of the thoracic duct with some atrophy of smooth (involuntary) muscles, connective tissue grows.
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