Comparative Studies On The Histological Structure And Dynamic Changes Of Immunity-associated Cells In Important Immune Organs Of The Yak

Thymus is the central immune organ of livestock as well as the place of differentiation and maturation for immune cells. As the biggest secondary immune organ, spleen plays an important role in provide the place for the the immune cells to grow, proliferate and produce immune responses. During the development of the body,there is a series of changes occurred in the structure of immune organs and the related immune cells, which reflect the differences exist not only among inter-species, but also among the inner-species for different ages and individual development status. Growing in the environment of high altitude, hypoxia, cold and strong ultraviolet radiation, yaks have tenacious ability to adapt. Yaks’ important immune organs play a obvious role for their adaptation and disease resistance. However,so far,there are few basic researches on structural changes of yaks’ main immune organs and dynamic changes of immunityassociated cells,it brought obvious obstacles of breeding and disease prevention of yaks. Use the theory of the anatomy, histology, histochemistry and electron microscopy, this research mainly studied on the structural changes of important immune organs and dynamic changes of immunity-associated cells of yaks in different ages.The results show that Yaks’ thymus is flat and long, including leaves cervical, thoracic leaves and intermediate link leaves. Each leaf consists of the cortex and the medulla,it is outer packaged by connective tissue capsule. Young Yaks’ thymus often have thin capsule which is gradually thickened with age growth and achieves the biggest thickness(8.7651±2.2061μm)at age 3. Later, the capsule thickness gradually thinning and stops changing at age 6. Young Yaks’ thymic medullary has small percentage and the boundaries between cortex and medulla are clear.As yaks older, the area of thymic medulla gradually increases. At the age of 6, the thymic cortex has been replaced by fat and connective tissue,the area of the medulla increase obviously and the boundaries between cortex and medulla become fuzzy. At the age of 10, it is unable to distinguish between the thymic cortex and medulla. Caspase-3 positive cells(apoptotic cells) are distributed in the yaks’ thymus of different ages and the percentage of the number of apoptotic cells in the cortex of is higher. During the age of 1-2 and 6-8, the number changes of Caspase-3 positive cells is not obvious and the other ages is opposite. Mast cells are mainly distributed around the medulla and the thymus corpuscles. During the age of 3-4 and 10-12, the number changes of Mast cells is not obvious and the other ages is opposite. At age 4, the number of Mast cells reached to a maximum(19.4286±4.0356). Macrophages almost distributed in the medulla. During the age of 3-4 and 8-10, the number changes of Mast cells is not obvious and the other ages is opposite.Yaks’ spleen is flat and long with thick center and thin margin. Histologically it is constituted with white pulp, red pulp and marginal zone and its structure is similar to other ruminants with some certain differences. Young Yaks’ spleen capsule consists of a layer of loose connective tissue. With age growth, yaks’ spleen capsule gradually thickened, the difference of capsule thickness between 1-year-old to 2-year-old is obvious. After 2-year-old, yaks’ spleen capsule grows slowly. Caspase-3 positive cells distributed in the every ages of yaks’ spleen, which are mainly distributed under the capsule, around the splenic corpuscle and trabecular. The overall trend shows that the number of Caspase-3 positive cells in old yak’s spleen is higher than young ones. Compared with younger yaks, the number change difference of caspase-3 positive cells of old yaks is obvious, however the difference among the similar ages is non-obviously. Mast cellss are mainly distributed in the splenic corpuscle and trabecular. During the age of 5-6 and 10-12, the numbers changes non-obviously and the other ages is opposite. At 4-year-old, the number of Mast cellss achieves a maximum(12.8571±2.4103). Macrophages are mainly distributed in splenic cord of red pulp and marginal zone, especially around peripheral arterial lymphatic sheath. With age growth, the number of macrophages in the spleen gradually increases.This research shows that yaks’ important immune organs have its own structural and functional characteristics and the dynamic changes of the relevant immunocompetent cells. These characteristics play an important role on yak adaptability and disease resistance.