The neonatal thymus contains an immature, dense vasculature. (A) In vivo tomato lectin staining acquired by confocal microscopy as described in “In vivo labeling of the thymic vasculature” showing the vasculature of a neonatal (left panel) and adult (right panel) mouse thymus. (B) Graph shows the average percentage increase (± SD) in fluorescent intensity above the control (no lectin staining) threshold. *P < .001. (C) Immunofluorescent staining of sections of neonatal (left panel) and adult (right panel) thymus using anti-CD31 antibody. (D) Quantitation of average numbers of CD31⁺ vessels per section in adult and neonate. **P = .05; n = 7 sections for neonate and adult. (E) Average number of SMA-positive areas per field examined in adults (n = 12 fields) and neonate (n = 6 fields). ***P < .001. Error bars represent standard deviation.