Table Of ContentM E T H O D S I N M O L E C U L A R M E D I C I N E TM
Bone
Research
Protocols
Edited by
Miep H. Helfrich, PhD
Stuart H. Ralston, MD
Bone
Research
Protocols
Edited by
Miep H. Helfrich, PhD
Stuart H. Ralston, MD
Human Osteoblast Culture
3
3
From: Methods in Molecular Medicine, Vol. 80: Bone Research Protocols
Edited by: M. H. Helfrich and S. H. Ralston © Humana Press Inc., Totowa, NJ
1
Human Osteoblast Culture
James A. Gallagher
1. Introduction
Osteoblasts are the cells responsible for the formation of bone; they
synthesize almost all of the constituents of the bone matrix and direct its
subsequent mineralization. Once a phase of active bone formation is com-
pleted the osteoblasts do not become senescent but instead redifferentiate
into one of two other cell types: osteocytes and bone lining cells, both of
which play a major role in the regulation of calcium homeostasis and bone
remodeling.
Researchers have endeavored to culture osteoblasts from human bone for
several reasons:
1. To investigate the biochemistry and physiology of bone formation.
2. To investigate the molecular and cellular basis of human bone disease.
3. To investigate the role of cells of the osteoblastic lineage in regulating bone
resorption.
4. To screen for potential therapeutic agents.
5. To develop and test new biomaterials.
6. To use cell therapy in tissue engineering and bone transplantation.
The structure of bone tissue, the heterogeneity of cell types, the cross-
linked extracellular matrix, and the mineral phase combine to make bone
a difficult tissue from which to extract cells. Consequently, early attempts
to culture osteoblasts avoided human tissue and instead relied on enzy-
matic digestion of poorly mineralized fetal or neonatal tissue from
experimental animals. The first attempt to isolate cells from adult human
bone, using demineralization and collagenase digestion, was reported by
Bard and co-workers (1). The cultured cells were low in alkaline phos-
phatase and collagen synthesis, which were then regarded as the best
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markers of the osteoblastic phenotype. Although the cells remained viable
for up to 2 wk they did not proliferate, and it was concluded that osteocytes
were the predominant cell type present. Mills et al. used the alternative
approach of explant culture and were successful in culturing cell populations
that included parathyroid hormone (PTH) responsive and alkaline phosphatase
positive cells (2).
The first successful attempts to isolate large numbers of cells that expressed
an osteoblastic phenotype from human bone were undertaken in Graham
Russell’s laboratory at the University of Sheffield in the early 1980s. The
defining characteristics of these studies were (1) the use of explant cultures,
which avoided the need for digestion of the tissue and (2) the availability of an
appropriate phenotypic marker. Successful culture of any cell type can be
achieved only if there is a specific marker of the phenotype that can be used to
confirm the identity of the cells in vitro. In this case, the marker was the then
recently discovered bone gla protein as measured by a radioimmunoassay
developed by Jim Poser (3,4). Nearly 20 yr later, bone gla protein, now known
as osteocalcin, undoubtedly remains the most specific marker of the osteoblas-
tic phenotype.
Although this culture system has been extensively modified by several
groups of researchers (see Note 1), the vast majority of published reports on
isolation of human osteoblasts still essentially use this simple but highly repro-
ducible explant technique. This technique and its modifications have been
described, compared, and reviewed elsewhere (5,6). The aim of this chapter is
to describe the basic methodology that is used in the author’s laboratory. This
is shown schematically in Fig 1. The nomenclature used by various research
groups to describe the isolated cells includes “human bone cells,” “human
osteoblasts in vitro,” “human osteoblastic cells,” and “HOBS.” We have pre-
ferred the conservative term “human bone derived cells” (HBDCs), and this is
used throughout this chapter.
HBDCs have been widely used to investigate the biology of the human
osteoblast, and their use has facilitated several major developments in our
understanding of the hormonal regulation of human bone remodeling. These
cells have also been used to investigate the cellular and molecular pathology of
bone disease. The major milestones in the culture of human osteoblasts are
summarized in Table 1. Figure 2 shows the increase in the application of
human osteoblast cultures since the initial reports in 1984.
Human bone cell culture is now becoming an important tool in tissue engi-
neering to test the biocompatibility and osteogenicity of novel biomaterials
and also for autologous transplantation of osteoblastic populations expanded
in vitro.
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Human Osteoblast Culture
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2. Materials
2.1. Tissue-Culture Media and Supplements
1. Phosphate-buffered saline (PBS) without calcium and magnesium, pH 7.4
(Invitrogen).
2. Dulbecco’s modification of minimum essential medium (DMEM) (Invitrogen)
supplemented to a final concentration of 10% with fetal calf serum (FCS), 2 mM
L-glutamine, 50 U/mL of penicillin, 50 µg/mL streptomycin. Freshly prepared 50
µg/mL of L-ascorbic acid should be added to cultures in which matrix synthesis
or mineralization is being investigated (see Note 2).
3. Serum-free DMEM (SFM).
4. FCS (see Note 3).
5. Tissue culture flasks (75 cm2) or Petri dishes (100-mm diameter) (see Note 4).
2.2. Preparation of Explants
1. Bone rongeurs from any surgical instrument supplier.
2. Solid stainless steel scalpels with integral handles (BDH Merck).
Fig. 1. Technique used to isolate cells expressing osteoblastic characteristics
(HBDCs) from explanted cancellous bone. E1, explant 1; E2, explant 2.
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Gallagher
2.3. Passaging and Secondary Culture
1. Trypsin–EDTA solution: 0.05% Trypsin and 0.02% EDTA in Ca2+- and Mg2+-
free Hanks’ balanced salt solution, pH 7.4 (Invitrogen).
2. 0.4% Trypan blue in 0.85% NaC1 (Sigma Aldrich).
3. 70-µm “Cell Strainer” (Becton Dickinson).
4. Neubauer hemocytometer (BDH Merck).
5. Collagenase (Sigma type VII from Clostridium histolyticum).
6. DNase I (Sigma Aldrich).
Table 1
Phenotypic Milestones in the Culture and Characterization
of Osteoblastic Cells (HBCDs) from Human Bone
Isolation of viable cells from human bone
Bard et al., 1972 (1)
Introduction of explant culture
Mills et al., 1979 (2)
Production of osteocalcin
Gallagher et al., 1984 (3)
Beresford et al., 1984a (4)
High alkaline phosphatase activity
Gallagher et al., 1984 (3)
Beresford et al., 1984 (4)
Gehron-Robey and Termine 1985 (7)
Auf’molk et al., 1985 (8)
Responsiveness to PTH
Beresford et al., 1984 (4)
MacDonald et al., 1984 (9)
Gehron-Robey and Termine 1985 (7)
Auf’mkolk et al., 1985 (8)
MacDonald et al., 1986 (10)
Synthesis of type I but not type III collagen
Beresford et al., 1986 (11)
Synthesis of other bone matrix proteins
Gehron-Robey and Termine 1985 (7)
Fedarko et al., 1992 (12)
Response to cytokines
Beresford et al., 1984 (13)
Gowen et al., 1985 (14)
Response to oestrogen
Vaishnav et al., 1984 (15)
Eriksen et al 1988 (16)
Expression of purinoceptors
Schoefl et al., 1992 (17)
Bowler et al., 1995 (18)
Production of nitric oxide
Ralston et al., 1994 (19)
Investigation of specific pathologies
Marie et al., 1988 (20)
Walsh et al., 1995 (21)
Formation of mineralised nodules
Beresford et al., 1993 (22)
Formation of bone in vitro and in vivo
Gundle et al., 1995 (23)
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Human Osteoblast Culture
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2.4. Phenotypic Characterization
1. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] (Leo Pharmaceuticals or Sigma
Aldrich).
2. Menadione (vitamin K3) (Sigma Aldrich).
3. Alkaline phosphatase assay kit (Sigma Aldrich).
4. Staining Kit 86-R for alkaline phosphatase (Sigma Aldrich).
5. Osteocalcin radioimmunassay (IDS Ltd., Boldon, UK) (see Note 5).
6. Polymerase chain reaction (PCR) primers and reagents for a panel of osteoblastic
markers including osteocalcin (IDS Ltd., Boldon, UK).
2.5. In Vitro Mineralization
1. Dexamethasone (Sigma Aldrich).
2. Hematoxylin (BDH Merck).
3.
L-Ascorbic acid (see Note 2).
4. Inorganic phosphate solution: Mix 500 mM solutions of Na2HPO4 and NaH2PO4
in a 4:1 (v/v) ratio. Sterile filter and store at 4°C prior to use.
2.6. Cryopreservation of Cells
1. Dimethyl sulfoxide (DMSO) (Sigma Aldrich).
2. Cryoampules.
3. Cell freezing container.
Fig. 2. Graph showing the increase in the application of human osteoblast cultures
since the initial reports in 1984.
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3. Methods
3.1. Establishing Primary Explant Cultures
A scheme outlining the culture technique is shown in Fig. 1.
1. Transfer tissue, removed at surgery or biopsy, into a sterile container with PBS or
serum-free medium (SFM) for transport to the laboratory with minimal delay,
preferably on the same day (see Note 6). An excellent source is the upper femur
of patients undergoing total hip replacement surgery for osteoarthritis. Cancel-
lous bone that would otherwise be discarded is removed from this site prior to the
insertion of the femoral prosthesis. The tissue obtained is remote from the hip
joint itself, and thus from the site of pathology, and is free of contaminating soft
tissue (see Note 7).
2. Remove soft connective tissue from the outer surfaces of the bone by scraping
with a sterile scalpel blade.
3
Rinse the tissue in sterile PBS and transfer to a sterile Petri dish containing a
small volume of PBS (5–20 mL, depending on the size of the specimen). If the
bone sample is a femoral head, remove cancellous bone directly from the open
end using sterile bone rongeurs or a solid stainless steel blade with integral
handle. Disposable scalpel blades may shatter during this process. With some
bone samples (e.g., rib), it may be necessary to gain access to the cancellous bone
by breaking through the cortex with the aid of the sterile surgical bone rongeurs.
4. Transfer the cancellous bone fragments to a clean Petri dish containing 2–3 mL
of PBS and dice into pieces 3–5 mm in diameter. This can be achieved in two
stages using a scalpel blade first, and then fine scissors.
5. Decant the PBS and transfer the bone chips to a sterile 30-mL “universal con-
tainer” with 15–20 mL of PBS.
6. Vortex-mix the tube vigorously three times for 10 sec and then leave to stand for
30 sec to allow the bone fragments to settle. Carefully decant off the supernatant
containing hematopoietic tissue and dislodged cells, add an additional 15–20 mL
of PBS, and vortex-mix the bone fragments as before. Repeat this process a mini-
mum of three times, or until no remaining hematopoietic marrow is visible and
the bone fragments have assumed a white, ivory-like appearance.
7. Culture the washed bone fragments as explants at a density of 0.2–0.6 g of tissue/
100-mm diameter Petri dish or 75-cm2 flask (see Note 4) in 10 mL of medium at
37° in a humidified atmosphere of 95% air, 5% CO2.
8. Leave the cultures undisturbed for 7 d, after which time replace the medium with
an equal volume of fresh medium taking care not to dislodge the explants.
9. Check for outgrowth of cells at 7–10 d (see Note 8).
10. Replace the medium at 14 d and twice weekly thereafter until the desired cell
density has been attained.
3.2. Passaging Cells and Establishing Secondary Cultures
1. Remove and discard the spent medium.
2. Gently wash the cell layers three times with 10 mL of PBS without Ca2+ and Mg2+.
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3. To each flask add 5 mL of freshly thawed trypsin–EDTA solution at room tem-
perature (20°C) and incubate for 5 min at room temperature with gentle rocking
every 30 sec to ensure that the entire surface area of the flask and explants is
exposed to the trypsin–EDTA solution.
4. Remove and discard all but 2 mL of the trypsin–EDTA solution, and then incu-
bate the cells for an additional 5 min at 37°C.
5. Remove the flasks from the incubator and examine under the microscope. Look
for the presence of rounded, highly refractile cell bodies floating in the trypsin–
EDTA solution. If none, or only a few, are visible tap the base of the flask sharply
on the bench top in an effort to dislodge the cells. If this is without effect, incu-
bate the cells for a further 5 min at 37°C.
6. When most of the cells have become detached from the culture substratum, trans-
fer to a “universal container” with 5 mL of DMEM with 10% FCS to inhibit
tryptic activity.
7. Wash the flask two to three times with 10 mL of SFM and pool the washings with
the original cell isolate.
8. Centrifuge at 250g for 5 min to pellet the cells.
9. Remove and discard the supernatant, invert the tube, and allow the medium to
drain briefly.
10. Resuspend the cell in 2 mL of SFM. If the cells are clumping see Note 9. If
required, the cell suspension can be filtered through a 70-µm “Cell Strainer”
(Becton Dickinson) to remove any bone spicules or remaining cell aggregates.
For convenience and ease of handling the filters have been designed to fit into the
neck of a 50-mL polypropylene tube. Wash the filter with 2–3 mL of SFM and
add the filtrate to the cells.
11. Take 20 µL of the mixed cell suspension and dilute to 80 µL with SFM. Add5 µL
of trypan blue solution, mix, and leave for 1 min before counting viable (round
and refractile) and nonviable (blue) cells in a Neubauer Hemocytometer. Using
this procedure, typically 1–1.5 × 106 cells are harvested per 75-cm2 flask, of
which ≥75% are viable.
12. Plate the harvested cells at a cell density suitable for the intended analysis. We
routinely subculture at 5 × 103–104 cells/cm2 and achieve plating efficiencies
measured after 24 h of ≥70% (see Note 10).
3.3. Phenotypic Characterization
The phenotypic characterization of HBDCs is described in detail in ref. 5.
The simplest phenotypic marker to investigate is the enzyme alkaline phos-
phatase, a widely accepted marker of early osteogenic differentiation. Alkaline
phosphatase can be measured by simple enzyme assay or by histochemical
staining. Basal activity is initially low, but increases with increasing cell den-
sity. Treatment with 1,25-(OH)2D3 increases alkaline phosphatase activity. The
most specific phenotypic marker is osteocalcin. This is a protein of Mr 5800
containing residues of the vitamin K-dependent amino acid γ-carboxyglutamic
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acid. In humans its synthesis is restricted to mature cells of the osteoblast lin-
eage. It is an excellent late stage markers for cells of this series despite the fact
that its precise function in bone has yet to be established. Osteocalcin can be
measured by one of the many commercially available kits. 1,25-(OH)2D3
increases the production of osteocalcin in cultures of HBDCs, but not fibro-
blasts obtained from the same donors. More recently, researchers have adopted
the use of reverse transcription (RT)-PCR to look at the expression of osteo-
blastic markers in HBDCs. PCR primers for a panel of osteoblastic markers
including osteocalcin are shown in Table 2.
3.4. Phenotypic Stability in Culture
As a matter of routine we perform all of our studies on cells at first passage.
Other investigators have studied the effects of repeated subculture on the phe-
notypic stability of HBDCs and found that they lose their osteoblast-like char-
acteristics. In practical terms this presents real difficulties, as it is often
desirable to obtain large numbers of HBDCs from a single donor. As an alter-
native to repeated subculture, trabecular explants can be replated at the end of
primary culture into a new flask (see Fig. 1). Using this technique, it is possible
to obtain additional cell populations that continue to express osteoblast-like
characteristics, including the ability to mineralize their extracellular matrix,
and maintain their cytokine expression profile (6). Presumably, these cultures
are seeded by cells that are situated close to the bone surfaces, and that retain
Table 2
PCR Primers for a Panel of Osteoblastic Markers
Osteoblastic
phenotype
marker
Primer pairs
Tm (°C)
Product size (bp)
Osteocalcin
5'-ccc tca cac tcc tcg ccc tat-3'
5'-tca gcc aac tcg tca cag tcc -3'
65
246
PTH receptor
5'-agg aac aga tct tcc tgc tgc a-3'
5'-tgc atg tgg atg tag ttg cgc gt-3'
55
571
Alkaline
5'-aag agc ttc aaa ccg aga tac aag-3'
phosphatase
5'-ccg agg ttg gcc ccg at-3'
68
715
CBFA1
5'-ccc cac gac aac cgc acc-3'
5'-cac tcc ggc cca caa atc tc-3'
60
388
Osteoprotegerin
5'-ggg cgc tac ctt gag ata gag tt-3'
5'-gag tga cag ttt tgg gaa agt gg-3'
60
760
RANKL
5'-act att aat gcc acc gac atc-3'
5'-aaa aac tgg ggc tca atc ta-3'
54
462
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the capacity for extensive proliferation and differentiation. The continued sur-
vival of these cells may be related to the gradual release over time in culture of
the cytokines and growth factors that are known to be present in the extracellu-
lar bone matrix, many of which are known to be produced by mature cells of
the osteoblast lineage. The addition of 25 µM L-ascorbic acid (50 µg/mL) (see
Note 2) to HBDCs in secondary culture (E1P1) produces a sustained increase
in the deposition of matrix due to an increase in the synthesis of collagen and
noncollagenous protein and bone sialoprotein and osteocalcin.
3.5. Passaging Cells Cultured in the Continuous Presence
of Ascorbate
Because of their synthesis and secretion of an extensive collagen-rich extra-
cellular matrix, HBDCs cultured in the continuous presence of ascorbate can-
not be subcultured using trypsin–EDTA alone. They can, however, be
subcultured if first treated with purified collagenase. The basic procedure is as
follows:
1. Rinse the cell layers twice with SFM (10 mL/75-cm2 flask).
2. Incubate the cells for 2 h at 37°C in 10 mL of SFM containing 25 U/mL of puri-
fied collagenase (Sigma type VII) and 2 mM additional calcium (1:500 dilution
of a filter-sterilized stock solution of 1 M CaCl2).
3. Gently agitate the flask for 10–15 sec every 30 min.
4. Terminate the collagenase digestion by discarding the medium (check that there
is no evidence of cell detachment at this stage).
5. Gently rinse the cell layer twice with 10 mL of Ca2+- and Mg2+-free PBS. To
each flask add 5 mL of freshly thawed trypsin–EDTA solution, pH 7.4, at room
temperature (20°C).
6. Typically this procedure yields ~3.5–4 × 106 cells/75-cm2 flask after 28 d in pri-
mary culture. Cell viability is generally ≥90%.
3.6. Setting Up Mineralizing HBDC Cultures
The function of the mature osteoblast is to form bone. Despite the over-
whelming evidence that cultures of HBDCs contain cells of the osteoblast lin-
eage, initial attempts to demonstrate the presence of osteogenic (i.e., bone
forming) cells proved unsuccessful. Subsequently, several authors reported that
culture of HBDCs in the presence of ascorbate and millimolar concentrations
of the organic phosphate ester β-glycerol phosphate (β-GP) led to the forma-
tion of mineralized structures resembling the nodules that form in cultures of
fetal or embryonic animal bone derived cells (reviewed in ref. 22). These have
been extensively characterized and shown by a variety of morphological, bio-
chemical, and immunochemical criteria to resemble embryonic/woven bone
formed in vivo. An alternative to the use of β-GP is to provide levels of inor-
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