Objective To investigate the histogenesis of lymphoid tissue and the development of T and B lymphocytes in human fetal appendix. Methods Totally 26 samples of human fetal appendix from the fetus at the 9th to 28th gestational week were collected with informed consent signed by the fetus’ parents. The morphology of the tissues was observed by hematoxylin and eosin (HE)staining. T and B lymphocytes were identified by immunohistochemistry using the streptavidin - peroxidase and SABC methods. The immunoreactive positive - cells were counted using BioMiaspro image analysis software. Results HE

- staining showed that the 4 layers of the appendix were not formed at gestational week 9. The epithelium of the appendix was stratified columnar epithelium. There were rare and diffused lymphocytes among mesenchymal cells. At week 11

the lymphocytes were appeared in the epithelium. The amounts of lymphocytes were increasing slightly in the mesenchyme with the elapse of gestational time. The lymphocytes were obviously infiltrated into the epithelium from week 13 to 15. The primary nodules were consisted of a group of lymphocytes. From 16 to 20 week

the primary lymphoid nodules and diffuse lymphoid tissue were seen obviously. At gestational week 21 to 28

the number of primary nodules gradually increased with fetal age

reaching the peak at week 28. Immunohistochemistry indicated that there were few IgM - positive cells were observed in the mesenchyme. At gestational week 11

CD3 - positive cells were seen. At week 15

the primary lymphoid nodule were appeared which were gathered by many IgM - positive cells and a small quantity of CD3 - positive cells. From 17 to 28 gestational weeks

the amounts of the primary lymphoid nodules and the 2 types of positive - cells were increased gradually with fetal age (

P

0 . 01

P

0 . 05). Conclusion  B cells present at the gestational week 9 and prior to T cells by 2 weeks in the appendix. The number of 2 types of positive cells are gradually increased with fetal age.