A cross section through a femur from a control, non-transgenic mouse. Compare this background fluorescence to the green fluorescence expressed by B lineage progenitors expressing rag2-GFP in slide 4.

A cross section through a murine femur reveals B lineage progenitors nestled within the bone cavity.

Lymphocytes within the bone marrow (upper right) are outlined in faint green; greenish line at lower left is edge of bone.

Green fluorescence marks T cell progenitors undergoing V(D)J recombination to assemble a T cell receptor. Slide 6 shows this same section under transmitted light. Note that virtually all T cells express rag2.

Compare to rag2 expression in bone marrow B cells, slide 4.

Dim green fluorescence marks B lineage progenitors (middle and upper right) undergoing V(D)J recombination to make antibodies. Slide 2 show this B cell section under transmitted light, and slide 1 shows a comparable image from a control mouse that does not express rag2-GFP.

Note that rag2+ B lymphocytes in bone marrow are rare as compared to rag2+ T cells in the thymus (slide 3). This is because B cell precursors are not the only hematopoietic subset in the bone marrow while T cell precursors form the bulk of thymus cells.

B lineage progenitors (small, round) receive critical developmental signals from bone marrow stromal cells (larger, adherent).

The thymus is packed full of developing T lineage progenitors. Visualize which cells are in the process of making a T cell receptor by seeing this same section with fluorescent imaging in slide 3.