Prenatal development of the post-capillary venules in human lymphatic tissue

Lymph nodes, Peyer's patches and tonsils from 19 human fetuses or stillborn babies between 13 and 40 weeks of pregnancy were collected, and their post-capillary venules (PCV) were subjected to direct morphometric measurements. The aims were to gain information about their development as well as about the circumstances of lymphocyte recirculation during the fetal period. First PCV:s detectable in fetuses aged 15 weeks had a low endothelium (2.3 to 3.2 micrograms), which increases in height thereafter up to 10.1 migrograms in full-term pregnancies. This increase was not directly related to gestational age, however, but correlated most closely with the state of supposed lymphocyte traffic, as defined on morphological grounds. The results suggest that active lymphocyte recirculation is established in in fetal period, and because of absence of external antigenic stimuli it must be an inherent property of lymphocytes acquired by them during the second trimester.