PP are first visible at ∼15.5 wk gestation after which there is a rapid spurt in the development
and maturation of lymphoid follicles so that at any given point of time new foci of PP development
are continuously formed at a rapid rate. Addition of rows of follicles results in the
formation of a PP. Immature PP of younger fetuses have a spongy structure in contrast with
the compact lymphoid follicles of mature PP of older fetuses. Immunocytochemical studies
reveal that there is a subtle gradation in the expression of lymphocyte surface markers with
increasing fetal age. Expression of antigenic markers occurs in an ordered sequence viz. HLA – DR, CD19 (B cell population), CD9 (pre-B cells), CD3 T lymphocytes, CD4 helper / inducer lymphocytes, the CD8 suppressor / cytotoxic cells and lastly, the CD57 Natural Killer cells. The antigens are expressed first on lymphocytes of PP and thereafter in those of the appendix. Our findings clearly demonstrate that the ∼5 wk fetal period from 17.5 wk to 22 wk
represents a major growth phase in the development of surface markers of lymphocytes in the
mucosal immune system of the gut.
