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58 Part II: The Organization of the Lymphohematopoietic Tissues Chapter 5: Structure of the Marrow and the Hematopoietic Microenvironment 59

Figure 5–4. Scanning electron micrograph of
rat marrow sinus. The floor of the lumen (L) is
indicated. The arrow on the left indicates the cell
body of an adventitial reticular cell, which is just
beneath the endothelial cell layer. Reticular cell
processes can be seen coursing between the
sinus wall and the hematopoietic compartment
(small arrows).(Reproduced with permission from
Lichtman MA: The ultrastructure of the hemopoi-
etic environment of the marrow: A review. Exp
Hematol 9:391, 1981.)

L

The reticular cells synthesize reticular (argentophilic) fibers that, with vessels and extend the venous sinuses so that release of precursors is
their cytoplasmic processes, extend into the hematopoietic compart- restrained while accommodating an increased entry of mature cells into
ments and form a meshwork on which hematopoietic cells rest (Figs. the circulation. 131
5–4 and 5–5). The cell bodies, their broad processes, and their fibers Studies in both mice and humans have identified subsets of adven-
constitute the reticulum of the marrow. titial reticular cells as MSCs with adipocytic-osteogenic potential that
Adventitial reticular cells can differentiate along the smooth mus- in mice appear to have significant overlap: (1) CXCL12–abundant
cle pathway and contain α smooth-muscle actin, vimentin, laminin, FN, reticular (CAR) cells, (2) adventitial reticular cells expressing the
132
and collagens I, III, and IV. 129,130 More specialized contractile reticular intermediate filament protein Nestin and displaying the surface pro-
“barrier cells” have been described in mouse marrow after hematopoi- teins platelet-derived growth factor (PDGF) receptor-α and CD51
etic stress. Barrier cells increase in number and seem to enclose small (Nestin+, PDGFRα+, CD51+), and (3) adventitial reticular cells
133
131

Figure 5–5. Scanning electron micrograph
L of rat femoral marrow sinus. The lumen (L)
of an exposed sinus that has been cut open
is indicated. The single asterisk indicates the
process of an adventitial reticular cell and the
intimate contact it makes with a hematopoi-
etic cell. To the left of this process are adven-
titial reticular cell fibers, which form a scaffold
∗ for hematopoietic cells. The double asterisk
identifies a portion of a reticular cell. The hole
in the sinus floor is an artifact of preparation
or a migration channel bereft of the emigrat-
ing cell. Empty spaces between cells and
fibers are artifacts of preparation. The arrow
to the left points to thin-walled fenestrae in
the endothelial cytoplasm. The arrow to the
right identifies the portion of a reticulocyte
∗ ∗ that may be penetrating the sinus wall, early
in egress (see Fig. 5–8A). (Used Lichtman MA,
University of Rochester.)

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