Skeletal Stem Cells - Engineered Bone: Tissue Engineering Intelligence Unit - page 13

C HAPTER 2

Skeletal Stem Cells

Sergei A. Kuznetsov and Pamela Gehron Robey

Evolution of Skeletal Stem Cell Biology

More than 30 years ago, Friedenstein demonstrated that in low-density marrow cell

cultures, discrete colonies of adherent fibroblast-like cells are formed. 20,22 Each colony
was generated by proliferation of a single precursor cell 12,20,47 referred to as a colony
forming unit-fibroblast (CFU-F). 19 The colonies could be easily detached with trypsin and
expanded many fold by consecutive passages. Strains thus generated consisted of virtually ho-
mogenous cells (with the exception of mouse cultures, which also contain hematopoietic cells)
displaying fibroblastic morphology, called marrow stromal fibroblasts (MSFs), by Friedenstein
and coworkers. 21,24 MSFs lacked basic characteristics of macrophages, shared some properties
of fibroblastic cells of other tissues 11,17,57,65 and, additionally, demonstrated some characteris-
tics of smooth muscle and endothelial cells (Table 1). Consequently, the more general term
“bone marrow stromal cell” (BMSC) has evolved in view of their properties that distinguish
them from fibroblasts of other connective tissues.
The identity of BMSCs in situ has long remained elusive. Morphological data point to cells
with ill-defined morphology and multiple names: adventitial cells, reticular cells, stromal fibro-
blasts, preadipocytes, undifferentiated mesenchymal cells, Westen-Bainton cells. In the post-natal
marrow, these cells form both an adventitial (outer) coating of the sinusoid wall and branching
extravascular meshwork that provides the physical substrate for myelopoiesis. Westen-Bainton
cells are characterized by extensive, elongated and attenuated cell processes with
membrane-bound alkaline phosphatase activity. 61 They express low levels of collagen types I
and III and osteonectin, indicative of their partial fibroblastic character. 8,51 In human post-natal
marrow, Weston-Bainton cells also demonstrate characteristics of preadipocytes. 5 In different
animal species, varying proportions of these cells survive mechanical dissociation and plating
in vitro. Of those that survive and attach to the culture vessels, the great majority begin to
proliferate to form BMSC colonies, and are likely CFU-Fs, at least in part.
It is now well known that when animal and human BMSCs are transplanted in vivo, bone,
cartilage, and fibrous tissue, 1,2,13,33,37,50 or bone accompanied by hematopoiesis-supporting
stroma, including adipocytes 24,28,35,38,43-45,49 are formed, depending on the technique that is
utilized. The ability of BMSCs to proliferate extensively and to differentiate into diverse tissues
led to the hypothesis that the BMSC population is composed of multi-potential progenitors,
akin to the hematopoietic stem cells. 51 The term “skeletal stem cells” (SSCs) was introduced
based on the fact that they could differentiate into all major tissues of the skeleton.
In recent years, SSCs have been found to have unexpected and “nonconventional” differen-
tiation capacities. Recently, myogenic cells, 59 to skeletal muscle, 18 and tendocytes 3,64 were added
to the list of phenotypes that can be generated by SSCs, which represents differentiation into
other tissues that arise from mesoderm. It has also been suggested that SSCs are able to differ-
entiate into neurons, 62 astrocytes, 42 and cardiomyocytes, 30,60 and finally, to endothelium 53
which derive from different embryonic tissues. However, some of these conclusions of
trans-germal differentiation were made based solely on in vitro experiments and the use of