Intrinsic innervation and vasculature of pre- and post-natal human thymus

Human thyro-thymic venous connections were more numerous in pre-natal life. The thymus was vascular since the 10th week. At 11 weeks, nerves invaded the capsule and a few trabeculae. At 16 weeks, nerve bundles accompanied arteries and veins. Their branches ran close to intralobular arterioles and venules. Parenchymal nerves appeared in the cortex and medulla. Radial cortical capillaries drained into capsular venules at 20 weeks. From 28 weeks pre-natally to 3 years post-natally, nerves in the medulla ended in knobs or fine tapers. Some encircled or relayed on Hassall's corpuscles. Others were traceable to blood vessel walls. The pattern of innervation and vascularization was not altered up to 9 years. The vasculature began to be irregular at 11--13 years. Parenchymal and perivascular nerves existed up to 15 years. At 25 years, nerves were detected in the capsule and proximal parts of the trabeculae.