Histology, Osteoprogenitor Cells

Article Author:
Ahmed Nahian
Article Editor:
Donald Davis
6/4/2020 8:41:04 AM
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Histology, Osteoprogenitor Cells


Osteoprogenitor cells, also known as osteogenic cells, are stem cells located in the bone that play a prodigal role in bone repair and growth. These cells are the precursors to the more specialized bone cells (osteocytes and osteoblasts) and reside in the bone marrow. Developed from infant mesenchymal cells, osteoprogenitor cells turn into spindle cells at the surface of matured bones. In developing bones, they appear more frequently and activate multifunctional stages to remodel the bones. The body loses the ability to synthesize or utilize more osteoprogenitor cells with age. Dysfunction of osteoprogenitor cells may delay ossification and lead to a spectrum of diseases such as dwarfism and Kashin-Beck disease.[1]


Osteoprogenitor cells are often referred to as preosteoblasts. They can be present within the endosteum, the cellular layer of the periosteum, and the lining of the osteogenic cells. In matured bones that no longer display active bone remodeling or formation, osteoprogenitor cells exist as flattened spindle-shaped structures. They attach to the bone surface and are referred to as “inactive osteoblasts” during this period. In maturing bones, however, these cells appear in their largest form. During fetal development or high turnover periods in adult osteogenesis, numerous osteoprogenitor cells function to give rise to osteoblasts. At this stage, these structures display plump oval nuclei and emboldened abundant spindle-shaped cytoplasm, converting later to characteristic cuboidal active osteoblasts.[2][3]


Osteoprogenitors can self-proliferate and self-renew. They participate in osteogenic differentiation and play a role in regulating angiogenesis. Osteoprogenitor cells can differentiate into osteoblasts through mitotic division, or by dividing into two stem cells via a high level of the regulation mechanism, which remains static during the proliferation process. After the completion of DNA synthesis and cell expansion, the cell retains its original genetic information. The endocrine system and local factors (growth factors and cytokines) mainly regulate osteogenesis. Recent studies have confirmed that several osteoprogenitor cells, originating from hypothalamic Y2-/-, enhance osteogenic activities. "Proline-rich tyrosine kinase 2" supposedly regulates differentiation of early osteoprogenitors, and "proline-rich tyrosine kinase 2" inhibitors promote osteogenesis and could act as treatment of osteoporosis. Osteoprogenitor cells also reside in the perichondrium. These osteoprogenitors hyper regulate bone morphogenetic proteins during differentiation into mature osteoblasts responsible for the production of bone matrix.[4][5][6]

Tissue Preparation

Bone marrow stromal cells (BMSCs) yield testable osteoprogenitors. Researchers dilute the cell sample with phosphate-buffered saline (PBS) at a ratio of 1:3. They isolate nucleated cells with a density gradient solution. Preparatory phase complete medium (CM) consists of  0.1 mmol/L nonessential amino acids, alpha-MEM with 10% fetal bovine serum, 4.5 mg/mL d-glucose, 1 mmol/L sodium pyruvate, 100 U/mL penicillin, 100 mmol/L HEPES buffer, 100 μg/mL streptomycin, and 0.29 mg/mL l-glutamine.[7] The scientists plate the collected nucleated cells at a density of 100,000 cells/cm^2 in supplemented CM. CM supplement requires an additional 5 ng/mL fibroblast growth factor-2 (GF2) and 10 nmol/L dexamethasone; both elements proliferate osteogenic commitment of bone marrow stromal cells. The supplemental process undergoes humidification at 37 degrees C in a 5% CO2 incubator.

Histochemistry and Cytochemistry

Osteoprogenitor cell development is under active research. A study suggests that the perichondrial Thy-1–positive cells demonstrate potential osteogenic activity and participate in osteoblast formation during endochondral ossification. Another study shows intense alkaline phosphatase activity two weeks after osteogenic induction, as well as the presence of mineralized nodules. The expression of bone sialoprotein, dentin matrix protein-1, and osteocalcin increases due to ALP proliferation. 55 to 65% of cells, as confirmed by flow cytometry, display the cell-surface markers Sca1+ and Thy1+ in vitro expansion for an alpha-SMA-GFP positive population. The alpha-SMA–GFP-positive population also exhibits high proliferative and osteogenic probabilities when compared to an alpha-SMA–GFP-negative population.[8][9]

Microscopy Light

Final visualization under the microscope involves a delicate preparation phase: the scientists fix the explants in 4% buffered formalin for one full day and decalcify the sample with 0.5 mol/L ethylenediaminetetraacetic acid (pH 8) for 7 to 10 days. They ensure to deposit the sample in paraffin and cross-section the samples to sub-samples of 5 um thickness at three different strata.

The scientists stain the samples with hematoxylin/eosin and Masson/Trichrome; they conduct both a qualitative exam for the presence of bone tissue and a quantitative exam by computerized bone histomorphometry. For each hematoxylin/eosin-stained cross-section, the scientists acquire 3 or 4 images (adequate to fill the construct cross-sections) and use them to calculate the bone tissue area and available area for tissue ingrowth (net implant area - undegraded scaffold area) by digital imaging analysis. Under the visuals of scanning electron microscopy (SEM), red color in the slides that are stained by Masson/Trichrome indicates lamellar and remodeled bone, while blue color shows freshly deposited and immature bone.[7]


Often overlooked in clinical investigation, bone tumors formed within osteoprogenitor or stromal cell lineage during Paget disease (PD) are likely deriving from genetic alterations related to those of familial Paget’s disease. During Paget’s disease, the endosteal surface goes through active remodeling, and abnormal osteoclasts bearing nuclear inclusions take place. The “fibrotic” tissues in bone biopsies from PD patients have revealed to be made of surplus elongated, sophisticatedly branched stromal cells that display high levels of alkaline phosphatase; these cells are similar to the pre-osteogenic stromal cells located within the normal bone marrow.

Early and full-blown pagetic lesions show dynamic transformations in the arrangement, number, and function of stromal cells within endosteal/medullary tissue. An excessive amount of bone marrow osteoprogenitor cells in pagetic lesion areas verify earlier data on both static and dynamic histomorphometry in patients with Paget’s disease. Previous data revealed that the rate of osteogenesis increases during Paget disease, but active research confirms that there is also a surge in the “birthrate” of osteoblasts.

The quality of the new bones depends on nature and hypermineralization taking place. Occasional characteristic patterns of Schmorl’s mosaic may form because of peculiarly high amounts of turnover events outlined by cement or reversal cement lines. Most stromal/osteoblastic abnormalities result from bone mass increment, characterized by denser and thicker trabecular structures. Compromised mechanical integrity caused by poor architectural organization and transformations within the mineralized matrix material are additional characteristics.[10][11][12]

Clinical Significance

Mesenchymal stem cells (MSCs) collected from adipose and bone marrow tissue hold therapeutic value for various bone disease treatments. Current studies demonstrate the benefit of bone grafts based on combinations of MSC, biomimetic scaffolds, and growth factor delivery, which showed an increased osteogenic regeneration rate with minimal side effects. The specific mechanisms of cellular signaling in bone remodeling are important in understanding the incorporation of newer effective treatment methods for numerous bone diseases. 

Various transplant therapies involving osteogenic autologous bone grafts are currently less-frequently used; patient-specific cell therapies involving autologous BM-MNCs (bone marrow mononuclear cells) composed of stem cells, monocytes, lymphocytes, and dendritic cells are rising in popularity for bone pathology treatments. All patients suffering from long-bone pseudoarthrosis attained full bone consolidation when treated with autologous BM-MNCs and allogeneic cancellous bone grafts. Bone marrow aspirate utilized during osteonecrosis treatment via minimally invasive decompression of the femoral head decreased disease progress and yielded overall pain and symptom relief. Bone marrow concentrated injections lead to better osteogenic unions, resulting in complete recovery in post-operative achondroplastic dwarf patients within 2 to 10 months after femoral lightening surgeries. Intravenous BMC injections, mixed with iloprost, proliferate fracture healing in patients with avascular necrosis. Patients receiving Intra-articular BMNc injected patients displayed better chewing and maximum interincisal opening with integral pain relief.[13][14][15]


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