Skeletal Stem Cells - Engineered Bone: Tissue Engineering Intelligence Unit - page 17

transplantation, rabbit SSCs derived from red or yellow marrow form heterotopic ossicles
containing stroma with abundant hematopoiesis, or hypocellular stroma with mainly fat cells,
respectively, thus mirroring the composition of the original tissues. 52 SSCs derived from hu-
man cementum and dental pulp form unique hard tissues reminiscent of cementum and den-
tin, respectively, and yet distinct from typical bone. 34,36 SSCs from transgenic mice deficient
for membrane-bound matrix metalloproteinase reproduce impairment of both osteogenic ca-
pacity and collagenolytic activity. 39 SSCs from patients with fibrous dysplasia of bone and the
McCune-Albright syndrome recapitulate abnormal features of the fibrous dysplastic lesion. 6
SSCs from a patient with a rare, newly described syndrome, gnathodiaphyseal dysplasia, closely
imitate pathological characteristics of the native lesion. 7 The two latter findings open a novel
direction in developing small animal models of human skeletal disorders. Taken together, these
data demonstrate that skeletal stem cells “remember” the exact features of the part of skeleton
they originated from, preserve this “memory” during prolonged ex vivo expansion, and reveal it
upon in vivo transplantation.
Use of in Vivo Transplantation Assay for Identification of SSCs
from Nonskeletal Tissues
As early as the 1960s, Friedenstein developed a concept of determined and inducible osteo-
genic precursor cells. The former, DOPC, included BMSCs and other spontaneously osteo-
genic cell populations of skeletal origin. The latter, IOPC, comprised a number of extraskeletal
populations requiring inductive stimuli to initiate osteogenic differentiation. The analysis of
osteogenic potential of these populations was performed entirely by means of in vivo trans-
plantation. When cell suspensions of thymic, splenic, and peritoneal origin, as well as blood
leukocytes, were transplanted in diffusion chambers together with an “inducer” (epithelium of
the urinary bladder, or decalcified bone matrix), bone was formed inside the chambers. 22,27,29
These findings directly proved nonbone related tissues include precursor cells capable of bone
formation under the influence of inductive stimuli. 19-21 Transplantation of these cells into an
open system shed some light on the potential origin of IOPCs. Epithelium of the urinary
bladder was implanted autologously into a guinea pig's tibia locally irradiated with 1000 to
5000 rad. Despite pronounced radiation injury, the course of bone induction was unaffected
suggesting that IOPC were not local but migrated from nonirradiated areas, 19 possibly via the
blood stream.
Evidence that spontaneously osteogenic SSCs can also be found outside the skeleton came
from studies of pericytes, cells embedded within the basement membrane of microvessels
(venules, arterioles, and capillaries). Pericytes grown from retinal and brain microvessels formed
mineralized nodules in vitro and express multiple osteogenic markers, such as alkaline phos-
phatase, bone sialoprotein, osteonectin, osteopontin, and osteocalcin. 9,10,15,55,56 Pericytes in-
oculated into diffusion chambers and implanted into athymic mice, and recovered 4 to 8 weeks
later, the chambers reproducibly contained bone, cartilage, soft fibrous tissue, and adipocytes. 15
These findings demonstrate that pericytes represent precursor cells able to differentiate along
osteogenic, chondrogenic, and adipogenic pathways and are true SSCs. Moreover, based on
morphological and histochemical evidence, it can be speculated that bone marrow
Weston-Bainton (reticular, adventitial) cells, the very cells that give rise to BMSC colonies
upon in vitro explantation, represent a subtype of pericytes. The existence of ubiquitous,
microvessel-associated system of primitive multi-potential SSCs has been postulated. 7
Recent studies have also identified cells in adipose tissue that have osteogenic properties.
Preadipocytes, or stromal cells, were isolated from human subcutaneous adipose tissue by col-
lagenase digestion, centrifugation, and plastic adherence. In vitro, these cells were able to sup-
port hematopoiesis. 32 In medium supplemented with calcitriol, the adipose tissue-derived cells
demonstrated synthesis of osteoblastic mRNAs (Cbfa1, alkaline phosphatase, osteocalcin, and
osteopontin) and they also developed a mineralized matrix after ascorbic acid had been added. 41
When these cells were grown in three-dimensional alginate cultures in the presence of
chondrogenic supplements (ITS+, pyruvate, ascorbate 2-phosphate, dexamethasone, and