The features of the human mesenteric lymph node histogenesis in the first half of the prenatal period of ontogenesis

Abstract

In recent years the important meanings are given to the role of mesenteric lymph node as a peripheral immune organ in the formation of local and general immune reactions. Features of the mesenteric lymph nodes morphogenesis are not well understood due to the different functions of these organs [1, 2]. There is still no consensus on the initial stages of the mesenteric lymph nodes. Many authors attribute the histogenesis of lymph nodes with the development of the mesenteric lymph vessels [3, 8]. In other studies determinant role of these is given to blood vessels [5, 6,7]. The discrepancy of views on the mesenteric lymph nodes early morphogenesis as well as a certain deficiency of works devoted to this problem require a comprehensive research of the mesenteric lymph nodes development with using of modern methods.

The purpose: To study the features of human mesenteric lymph node development in the first half of the prenatal period of ontogeny.

Materials and Methods: 19 mesentery of the human embryos and fetuses in 8-20 weeks of fetal development were investigated. Total film preparations and histological sections were stained with hematoxylin and eosin, resorcin-fuchsin by Weigert. Morphometric studies were carried out by S.B. Stefanov (1988).

The research results: It was established that cells of the fibroblastic differon and cleft-like elements of vascular network are revealed in 8-9 weeks of fetal development in the loose connective tissue of the human small intestine, collagen and elastic fibers.

Since the end of the 10th week clearly visible blood vessels appear in the loose connective tissue of the mesentery, among them the units of the arterial and venous channels are found. Under the upper sheet of the mesothelium the large gaps are determined, which are the germs of the mesentery lymphatic network. In 10-11 week of the human embryogenesis the first zones of the hematopoietic cells emigration from blood vessels are registered in the points of blood and lymph vessels contacts eviction. The clusters without any borders are formed in these places of migration.

In the 13th week most clusters grow due to the ongoing process of hematopoietic cells emigration. Increasing clusters grow into the lumen of the primary lymphatic vessel. A closed system of the lymphatic circulation is formed. The amount of myelopoetic cells decreases gradually and the number of lymphoid cells, mainly medium and small lymphocytes, increases. Stromal cells are differentiated into reticulocytes. Lymph protocapillaries transform into the sinuses system of the future medulla. In the 14-15 weeks the quantity of lymph node germs in the mesentery continues to increase, there are sporadic macrophages. The formation of a capsule from collagen fibers of the lymphatic vessels adventitia and mesentery connective tissue is observed in the majority of lymph node germs.

Endothelial cells of post-capillary venules thicken in the 16-17 week of fetal development. The amount of hemopoietic cells decreases. Dense clusters of lymphoid cells around the blood capillary glomeruli form the beginnings of the lymphoid follicles. To the 20th week the content of lymphoid cells continues to increase, the majority of lymphocytes are found in the lymph node parenchyma and in the venules lumen. Cortex and medulla are visualized clearly.

CONCLUSIONS:

1. The development of the human mesenteric lymph node is directly associated primarily with the development of the mesentery blood vessels.

2. We may identify the following stages in the histogenesis of the mesenteric lymph node in the first half of the prenatal period of the ontogeny: formation and differentiation of the structural units of the mesentery vascular network (8-10 week of fetal development); forming of hemo- and lymphopoietic cells clusters – lymph node germs (11-12 weeks), differentiation of the lymph node germs structures and reduce of the myelopoiesis (13-14 weeks), differentiation of structural zones of the mesenteric lymph node germs lymph (from the 15 weeks of fetal development).