The overall goal of this procedure is to expand the cells on a high available surface area and to further deliver these cells to sites of defect or demand for clinical applications. This method can help answer key questions in the tissue engineering field, such as cell expansion, cell delivery, and vascularization. The main advantage of this technique is that we can expand the cells and deliver them, without any harvesting step, into defect sites, in a biocompatible way.
Visual demonstration of this method is critical, as working microcarriers and bioreactors can be complicated, at first, due to the many steps involved. Helping with the procedure will be Jihyoung Choi, a PhD student from our laboratory. One day before the experiments, set up the spinner flasks and prepare them for sterilization by autoclave.
Wrap the spinner flasks with aluminum foil around the side caps separately to avoid contamination during unwrapping. Wrap the flasks with one to two layers of aluminum foil for autoclaving to minimize contamination risks after sterilization. Leave the caps of the spinner flasks loose and when possible, sterilize them in an upright position.
Next, weigh the required amount of RCP macroporous microcarriers in 20 milliliters of Dulbecco's phosphate buffered saline. Let them hydrate for at least one hour before heat sterilization by autoclave. Use the bioreactor system referenced in text protocol to produce vascularized tissue equivalents.
This bioreactor consists of a vessel in which the biomatrix is placed, a medium reservoir, and a pressure bottle. Connect the vessel with the medium reservoir and the pressure bottle through silicone tubes. Leave the caps loose and place the system in an autoclave plastic bag.
Close the bag well and place this in a second autoclave plastic bag before sterilizing by autoclave. Let the sterilized RCP macroporous microcarriers settle to the bottom and wash them with 10 milliliters of culture medium. Repeat this process three times.
Also wash the spinner flasks with 10 milliliters of culture medium. Next, add 30 milligrams of RCP macroporous microcarriers per spinner flask in eight milliliters of culture medium. Equilibrate the spinner flasks RCP macroporous microcarriers at 37 degrees Celsius and 5%carbon dioxide for 30 minutes before seeding the cells.
Then, seed half a million human bone marrow derived mesenchymal stromal cells, or human dermal microvascular endothelial cells per spinner flask in two milliliters of culture medium. Place the spinner flasks on a stirrer plate at 37 degrees Celsius and 5%carbon dioxide and start stirring at 90rpm for five minutes. Then, rest for 55 minutes for a total of one hour static dynamic incubation.
Repeat this incubation cycle four times. Now, add 10 milliliters of cell culture medium per spinner flask. Then start continuous stirring at 95rpm while incubating at 37 degrees Celsius and 5%carbon dioxide.
One day before starting the bioreactor cultures, cut open the biomatrix longitudinally on the antimesenteric side and fix it in a previously disinfected polycarbonate frame. After detaching and counting the cells, use a two milliliter syringe to inject 10 million HDMECs in one thousand microliters of culture medium through the preserved vasculature of the biomatrix. Inject approximately 700 microliters through the arterial inlet, and approximately 300 microliters through the vein outlet.
Incubate the biomatrix with injected cells at 37 degrees Celsius and 5%carbon dioxide for three hours to allow cell attachment. Following incubation, the connection of the biomatrix to the bioreactor has to be done by two individuals, taking care that the arterial and the venous pedicles are connected to the medium flow correctly. Fill the bioreactor system with 350 milliliters of the vascular endothelial growth factor, 1%penicillin streptomycin, and incubate at 37 degrees Celsius and 5%carbon dioxide.
Place the frame inside the vessel of the bioreactor. Then, carefully connect the arterial and venous pedicles to the vessel. Start perfusion by connecting the bioreactor system to a peristaltic pump.
Set the pressure at 10 millimeters of mercury amplitude plus minus one. Increase the pressure stepwise every hour until reaching a pressure of 100 millimeters of mercury amplitude plus minus 20. Mix 330 microliters of 0.4%Collagen R solution, 170 microliters of 0.1%acetic acid, and 50 microliters of medium M199 in a 15 milliliter Falcon tube.
Keep the mixture on ice. Then, take the RCP microcarriers from the spinner flasks. Remove the culture medium completely.
Next, add the volume of sodium hydroxide required to neutralize the collagen mixture. Once neutralized, immediately mix the mixture with the RCP microcarriers. It is important to allow gelation of the collagen gel before starting medium flow.
Otherwise, the gel will detach and the microcarriers will be washed away from the lumen of the biomatrix. Add the collagen gel RCP microcarrier mixture on the lumen of the biomatrix. Allow gelation for 30 minutes.
Then, connect the bioreactor to the peristaltic pump and set up the pressure at 100 millimeters of mercury amplitude plus minus 20. Finally, perform analysis and read outs of bioreactor cultures as described in the text protocol. Representative results of cell proliferation on microcarriers is shown by live dead staining in which many living cells can be observed on the microcarriers after seven days of dynamic cultures.
These results were confirmed by scanning electron microscopy analysis showing completely colonized microcarriers after seven days of dynamic cultures. HDMECs kept their functionality after seven days of dynamic cultures on RCP microcarriers. Here, bright field microscopy reveals sprouts growing from the colonized microcarriers when cultured on a collagen gel.
RFP labeled HDMECs were observed in the vascular structures of the biomatrix after 21 days of bioreactor cultures, indicating their migration from the RCP microcarriers to the biomatrix. Furthermore, after 21 days of bioreactor cultures, the scanning electron microscopy image shows that the microcarriers were in close proximity to the biomatrix. Interestingly, they are not colonized by cells anymore.
After watching this video, you should have a good understanding on how to expand the cells on microcarriers to prepare them in an injectable cell delivery system for therapeutic application. Once mastered, this technique can be done in four hours. This procedure represents a promising strategy in regenerative medicine to obtain and maximize cell expansion and to deliver cells to implantation sites where they could improve vascularization for tissue engineered grafts.
The implications of this technique extend toward vascularization because it can be used on any cell type for a tissue specific application in a clinical scenario. This method can also provide understanding into the potential employment of microcarriers as an injectible cell delivery system for therapeutic purposes. Don't forget that working with primary cells can be extremely hazardous, and precautions such as wearing gloves and lab coat, and working in a biosafety cabinet should always be taken when performing this procedure.
After its development, this technique paved the way for researchers in the field of tissue engineering to explore cell migration in vitro which could help to understand or predict in vivo situations.
