What type of structures do lymphatic system vessels begin with?

Lymphatic system vessels begin with lymphatic capillaries, which are small, thin-walled vessels that are responsible for collecting excess interstitial fluid from tissues. These capillaries have a unique structure that allows them to absorb the large proteins and other substances that cannot be taken up by blood capillaries. They are more permeable than blood capillaries due to the presence of overlapping endothelial cells, which function like one-way valves, enabling fluid to enter but preventing it from exiting. This is a crucial mechanism in maintaining fluid balance in the body and in initiating the immune response by transporting lymph, which contains immune cells, to larger lymphatic vessels and eventually to the lymph nodes.

The other structures listed, like lymphatic ducts and lymphatic trunks, are formed from the convergence of lymphatic capillaries and play different roles in the larger lymphatic system. Microvessels, which generally refer to very small blood vessels in the circulatory system, do not specifically pertain to lymphatic vessels. Thus, lymphatic capillaries represent the foundational element from which the lymphatic system functions effectively.