Isolation and culture of primary adult skin fibroblasts from the Asian elephant (Elephas maximus)

Tissue sample collection

Tissue samples were obtained from six Asian elephant carcasses housed at different facilities/field settings (Licence number U1006312558). Details of the elephants are given in Table 1, including age, sex, cause of death, location, and length of time after death until samples were brought to the laboratory.

Reagents used in this study are listed in Table 2, including the main components of reagents and medium formulations and their vendor and concentration.

Ear samples were excised after the mahout had conducted a death ritual or funeral ceremony according to Thai belief (Keyes, 1977), which occurred around 2 to 14 h after death. Ear skins were washed with water and scrubbed with povidone–iodine or chlorhexidine solution, using an impregnated brush to remove any remaining dirt, then rinsed with phosphate-buffered saline (PBS) or normal saline and dried with a sterile gauze pad. Ear samples were cut with scissors into 3 × 3 cm² pieces; these were transferred to a sterile 50 mL test tube containing 25 mL of transportation medium with ten times the normal concentration of a routine antibiotic dose of antibiotic/antimycotic. Note that the solution volume should be enough to entirely cover the ear samples. The collection tube with tissue samples was then transported to the laboratory in a chilled carrier at 0−4 °C (on ice or in the presence of ice packs, but not in direct contact with the coolant to avoid sample freezing) within 3 h after collection.

Explant preparation

For the isolation of skin fibroblasts, ear tissues were prepared and cultured under a modified protocol, as described in previous reports (Mestre-Citrinovitz et al., 2016; Reiisi, Esmaeili & Shirazi, 2010; Rittié & Fisher, 2005; Vangipuram et al., 2013). In brief, the ear skin biopsies were dried on cellulose filter paper (Fig. 1A) (Whatman^®; Sigma-Aldrich, St. Louis MO, USA), soaked once in 70% ethanol for 1 min, allowed to dry, and then washed three times with washing PBS containing ten times the routine antibiotic/antimycotic dose plus gentamicin. Tissue fragments were transferred into a 100 mm tissue culture dish using a sterile scalpel. A 5 mm perimeter was trimmed from the edges of the excised skin (Fig. 1B); the front and back of excised skin samples were then separated (Fig. 1C). Subcutaneous tissue (loose connective tissue and lobules of fat) was removed and rinsed with washing PBS (Figs. 1D and 1E). Excised samples were first cut with a scalpel into ∼1-cm-long strips, then chopped into pieces approximately 1 to 2 mm² in size (explants) (Figs. 1F and 1G) and placed in washing DMEM containing ten times the routine antibiotic/antimycotic dose plus gentamicin. These skin explants were minced with iris scissors (Fig. 1H). Samples were centrifuged at 200 g for 10 min to remove the supernatant, then digested in collagenase solution (DMEM + 10% v/v collagenase type II + 10× antibiotic/antimycotic) in a 60-mm-diameter culture dish and incubated at 37 °C and 5% CO₂ for 21 h (Fig. 1I). The next day, the explant samples were further washed twice with PBS. After the first 1 to 3 days of culture, explant samples were incubated in a 60-mm-diameter culture dish with explant medium containing 20% heat-inactivated FCS together with a routine antibiotic dose to protect against microbial contamination.

Monitoring for outgrowth of primary cell cultures and contamination

Once collagenase was removed, the cultures were monitored daily under an inverted microscope at low magnification to observe explant dislodging and the overall radial migration of primary cells around the explants (see Fig. 2), together with monitoring for any microbial or fungal contamination by checking the size, shape and movement pattern of the particles in the culture at high magnification. If microbial contamination was detected in any dish or flask, the entire contents were immediately discarded (Ryan, 2012). Initially, if no contamination was found at around day 4 of culture or if migration of primary cells was observed, the explants were moved from the dish to a culture flask. Tissue fragments were split into ∼20 smaller fragments in a T25 culture flask or ∼50–60 fragments in a T75 culture flask (making sure fragments did not touch each other). Tissue fragments were covered with a thin film of explant medium; culture continued until the primary outgrowing cells reached confluence around the explants or reached ∼80% confluence in culture flasks (Figs. 3A and 3B).

Establishing secondary cultures

Once reaching confluence (Fig. 4A), fibroblast cultures surrounding the pieces of skin were further expanded. The explant medium was poured out of the culture flasks and the cell surfaces washed with PBS three times. In these second and third passages, keratinocytes were removed from the cultures by short trypsinization with 0.1% trypsin/EDTA, as previously described (Hentzer & Kobayasi, 1975). The volume was adjusted as appropriate for different-sized vessels (for T75 culture flasks we added 1.0 mL of 0.1% trypsin/EDTA), followed by incubation at 37 °C. After a few minutes (2–3 min), when observed under a microscope, fibroblasts became rounded and began to detach from the plastic surface of the culture flask, due to the different attachment characteristics of keratinocytes and fibroblasts to the plastic culture surface, thus enabling separation of the two cell populations. To dislocate the remaining loosely attached cells, the flask was tilted to distribute the trypsin evenly over the culture surface. Separated fibroblasts were transferred to a sterile 15 mL centrifuge tube, followed by immediately adding at least five volumes of fresh growth medium containing 10% FBS and re-cultivating to expand cell numbers in a new T75 culture flask. Cultures were grown at 37 °C in a humidified atmosphere containing 5% CO₂. During this time, keratinocytes were still attached to the plastic bottom and preserved their membranous shapes, as seen before trypsin treatment. To culture keratinocytes, cells were incubated at 37 °C for ∼10 min longer to disperse the cells; then the skin explants were removed and fresh growth medium added to re-cultivate keratinocytes. Moreover, to obtain a larger amount of fibroblasts, explants were left in the first flask for a second round of cell growth and incubated as before, while keratinocytes were separated and moved to a new T75 culture flask instead. The culture medium was changed once every 48 h. The 2nd to 4th passages of cultures of these cells were frozen for long-term storage stock. Fibroblasts at the 4th to 7th passages were used for further experiments.

Routine trypsinization, cryopreservation and resuscitation of fibroblasts

Cryogenic preservation of elephant skin fibroblasts is generally the same as cryopreservation of most continuous cell lines: by using common dimethyl sulfoxide (DMSO), which permits long-term storage of cells in liquid nitrogen. After routine trypsinization of cells with 0.25% trypsin/EDTA and incubation for 5 min, cell suspensions were diluted with fresh growth medium and the sample thoroughly mixed. Cells were centrifuged at 200 g for 10 min. The pellets were re-suspended in freezing medium consisting of 50% DMEM, 10% DMSO and 40% FBS (v/v). Cell suspensions were aliquoted into cryogenic storage vials (at approximately 1.5–2.0 ×10⁶ cells/vial) and frozen at −80 °C in a deep freezer overnight. The next day the vials were transferred to a liquid nitrogen tank for long-term storage until used for further experiments.

To resuscitate fibroblasts or determine post-cryopreservation cell viability, the cells in the frozen vials were quickly thawed at 37 °C and promptly mixed with 6.0 mL of growth medium in a sterile centrifugal tube, then cultured in T75 culture flasks without centrifugation. Viable cells were counted manually using a hemocytometer and expressed as the % of live cells from the total cell count. The next day, attached cells were washed twice with PBS and new growth medium added.

Measuring cell viability and generating a growth curve

Cells were counted to assess viability using a traditional cell-counting method, with a bright-line hemocytometer and trypan blue dye (Jauregui et al., 1981; Louis & Siegel, 2011). Briefly, cells were first trypsinized with a routine subculture. Cell suspension was mixed 1:1 with 0.4% trypan blue solution and let stand for 5 min at room temperature. Then 20 µL of the cell suspension was applied between the cover slip and the edge of the hemocytometer chamber and examined immediately under a microscope following the method of Louis & Siegel (2011). For measuring cell growth, seed cells at passage 6 (P6) of approximately 5,000 cells per well in 0.5 mL of growth medium in a 24-well plate were cultured under normal culture conditions. Viable cells in each well were counted over a 12-day period using a hemocytometer. The total count was calculated to obtain the growth curve and population-doubling time.

Tissue sample collection

To our knowledge this is the first report on the successful isolation and culture of skin fibroblasts from post-mortem ear tissues of a mature Asian elephant. In addition to providing convenience of tissue sampling, the ear serves as ideal tissue for fibroblast isolation, with an excellent number and quality of cells (Mestre-Citrinovitz et al., 2016). We collected ear samples after the elephant had died and the traditional requiem ritual had been performed by the mahout (Keyes, 1977). Including post-mortem examination (necropsy), ear cleaning and transportation time, the ear samples arrived at the lab around 10 to 24 h after the elephant’s death. This was still considered good quality remains for primary cell culture (Seluanov, Vaidya & Gorbunova, 2010). Previous studies have shown effective methods of preserving tissues and protocols to isolate outgrowth of fibroblast-like cells several days after death, taking into account different factors such as the storage temperature and animal species, e.g., 12 days after animal death from ears stored at 4 °C in goats and sheep (Silvestre, Sánchez & Gómez, 2004), 10 days post-mortem for sheep ear skin when skin tissues were exposed to around 25 to 26 °C (Singh & Ma, 2014), and recently published data showing a recovery method for fibroblast cells from goat skin up to 160 days post-mortem at 4 °C storage (Aoued & Singh, 2015). Of these protocols mentioned, the important point is that tissues were processed for good preservation within “an hour” of collection, which is almost impossible in the case of an elephant carcass in Thailand due to Thai cultural beliefs concerning the death ceremony. In this study, we failed to isolate any outgrowth of fibroblast cells from post-mortem tissues harvested 20 h after the animal’s death (elephant E6), even after culturing the explants for 1 month and finding no presence of contamination. It is possible that elephant E6 had been left in unsuitable conditions, i.e., at a high environmental temperature (∼33–35 °C in June) for too long a time, before ear samples were collected and transferred to the laboratory. Another important thing that should not be overlooked is the awareness of zoonoses. Wild animals may contain pathogens, such as anthrax tuberculosis and leptospirosis, that can be transmitted from animals to humans; and most samples were collected under field conditions, so taking care to avoid sharp surfaces that can pierce the skin and awareness of sterile technique are essential at all steps to minimizes any risks from infectious diseases (Kruse, Kirkemo & Handeland, 2004).

Outgrowth of primary cell cultures

For our study we therefore adapted a collagenase digestion technique together with an explant technique to establish elephant fibroblasts (Mestre-Citrinovitz et al., 2016; Pruniéras, Delescluse & Regnier, 1976; Reiisi, Esmaeili & Shirazi, 2010; Rittié & Fisher, 2005; Vangipuram et al., 2013). Because trypsin digestion technique (Orazizadeh et al., 2015) was not completed to separate the dermis and epidermis layers in our study, outgrowth of epidermis (keratinocytes) and dermis (fibroblasts) could therefore be seen (Pruniéras, Delescluse & Regnier, 1976). Explant dislodging and overall radial migration around explants could be observed, as shown in Fig. 2 at low magnification. Both fibroblast-like and epithelial-like cells could be seen migrating from the tissue pieces 4–12 days after explanting. The earliest observed outgrowth around explants that dislodged was at day 4 of culture in E1 and E5 (Figs. 2A and 2B), compared with E2, E3 and E4 on the same day (Fig. 2C for comparison) in which primary cells were observed around day 7–10 (Figs. 2D–2F). We found outgrowths of epithelial keratinocytes around some explants in our experiments. They grew in squamous epithelia shapes until confluent and produced stratified multilayer sheets around the explants beginning after 4–7 days, while fibroblasts started later at around 7–10 days. At the time of fibroblast appearance during the following weeks the fibroblasts overgrew the keratinocytes and the cultures mainly consisted of fibroblasts with only small islands of epidermal cells around the explants (Fig. 3). Cells continued to proliferate and were subcultured when they reached 80% confluence (Figs. 3A and 3B).

Establishing secondary cultures

During the first few minutes (2–3 min) of observation under a microscope, fibroblasts became rounded and began to dislodge from the plastic surface of the culture flasks, while the epidermal cells stuck to the bottom and preserved their membranous shapes (Figs. 4B and 4C). After 15 min of trypsinization, both fibroblasts and keratinocytes became detached from the culture flasks (Fig. 4D). After subculture, the fibroblast cells grew rapidly, gradually outgrowing.

Measuring cell viability and generating a growth curve

The viabilities of elephant skin fibroblasts before freezing and recovery (percent cell viability was calculated immediately post-thaw) were around 97.3 ± 4.3%, and 95.5 ± 7.3%, respectively, when their post-freezing recovery and survival was improved with freezing medium consisting of 50% DMEM + 10% DMSO + 40% FCS (v/v). This indicates that the cells were grown in good culture conditions and that the freezing medium was appropriate. Note that at 10–20% FBS freezing medium the cells had very low recovery rates and cell morphology was different from cells observed prior to cryopreservation, which may be caused by apoptosis (Xu et al., 2010). In this study after 12 days continuous count the elephant skin fibroblast growth curve was as shown in Fig. 5. The curve was sigmoidal with a population doubling time (at the 6th passage) of about 25 h.