DEVELOPMENT AND CELL PHENOTYPES IN PRIMARY FOLLICLES OF FETAL SHEEP LYMPH-NODES

Lymph nodes from sheep foetuses and postnatal lambs were examined to d etermine the participation of different leucocyte populations in prima ry follicle formation, with special emphasis on the emergence and subs equent development of follicular dendritic cells during late gestation and early postnatal life. A series of immune and enzyme histochemical markers was used. The first 5'-nucleotidase-positive primary follicle s were found at 80 days gestational age (gestation in sheep is 150 day s) in superficial cervical lymph nodes. In the last month of gestation the primary follicles possessed follicular dendritic cells, macrophag es, dendritic cells, and CDS-positive lymphocytes, in addition to IgM- positive cells. Follicular dendritic cells in primary follicles were f ound to be ultrastructurally immature. These follicular dendritic cell s were characterised by a few, coarse surface projections and many rib osomes attached to the endoplasmic reticulum. A final differentiation to mature follicular dendritic cells was coincident with the postnatal germinal centre reaction. Computer-assisted morphometric analysis dem onstrated that the size of 5'-nucleotidase-positive primary follicles in the distal jejunal lymph node, but not in the superficial cervical lymph node, increased significantly during late gestation. It was conc luded that stromal cells in primary follicles of foetal sheep lymph no des were a continuously developing population but that ultrastructural maturity was only achieved in the germinal centres of postnatal lambs .