Module 2: Cell Sources
This is the second module in a four part series titled “Zero to One on the basic science behind cultivated meat”. You can click on these links to access: Course Overview, Module 1 (Cell Culture Basics), Module 3 (Culture Medium), Module 4 (Process monitoring).
By the end of this module, you should understand:
- Overview of stem cells: embryonic, adult and induced pluripotent stem cells vs. differentiated cells;
- Muscle tissue cell types;
- Comparison between mammalian, avian, fish and seafood cells;
- Cell sourcing.
Stem cells vs. differentiated cells
When choosing your cell source, a major consideration is choosing whether to use stem cells or differentiated cells, and if using stem cells, choosing which type. Read this chapter on Cellular differentiation (Oregon State University). This is a good general description of what stem cells are, as compared to differentiated cells.
Another good resource is this article on Stem cells (Library of Congress), which also helps to explain the difference between totipotent and pluripotent stem cells.
In your own tissues, cells are constantly dying and being replaced by new cells. Read this article on cell differentiation and tissue (Nature education), in order to gain an understanding of how cells are replaced in the body, and which cell types are responsible for this maintenance.
If you choose to culture stem cells, there are several different types to choose from, including embryonic stem cells, adult stem cells, and induced pluripotent stem cells. Read this article on embryonic stem cells (NIH). Although it is focused on human embryonic stem cells, the science is relevant to animals as well.
There are several types of adult stem cells (University of Nebraska Medical Center), but the type from this list that is most relevant to generating muscle tissue is the mesenchymal stem cell (DVCSTEM). Mesenchymal stem cells are multipotent, and are also called MSCs. Although some sources suggest that MSCs can become a wide range of different cell types, their precise multipotency is still under investigation. MSCs are mostly known for their ability to differentiate into bone, muscle, fat, and cartilage. This article on Mesenchymal stem cells (Wikipedia) provides a good additional resource on the topic.
A new stem cell source was developed which does not occur naturally, but uses cellular reprogramming to return differentiated cells to a stem cell-like state. These are called induced pluripotent stem cells (NIH), and they present an exciting new cell source which several companies are already exploring for cultivated meat products.
1. What is the difference between totipotent, pluripotent and multipotent stem cells?
2. Why would you choose to culture stem cells instead of differentiated cells
3. Which types of stem cells can be used to produce muscle cells?
Muscle tissue cell types
This website provides a straightforward overview of the different types of muscle cells (Ken Hub). In this article, read the section on skeletal muscle, and skim over cardiac and smooth muscle, but read the short section under cardiac muscle on satellite cells.
Skeletal muscle cells are referred to by many names, including myocytes, muscle fibers, and myofibers.
Watch this video to get a broader overview of the anatomy and structure of skeletal muscle (Youtube).
Another important muscle cell type is the muscle satellite cell (Wikipedia), also referred to as myosatellite cells, and muscle stem cells. While these have not historically been considered stem cells and more of a precursor cell type, they can be considered another type of adult stem cell because of their ability to produce more satellite cells and new muscle cells.
The main types of cells that make up skeletal muscle tissue are the muscle cells and satellite cells, but other cell types include fibroblasts (News Medical Life Sciences), adipocytes (News Medical Life Sciences), neurons (Queensland Brain Institute), and endothelial (vascular) cells (PromoCell).
These cell types are also described in greater detail in this article on Tissue Engineering for Clean Meat Production (Frontiers in Sustainable Food Systems). Within this article, specifically read the section on ‘Cells.’
4. What is the difference between a myofiber and a myofibril?
5. Which cell type is responsible for muscle regeneration?
6. Which cell type was used to generate the first clean meat prototype in 2014?
Inter-species cell culture differences
Mammalian cell sources have been the first choice for creating cultivated meat, largely driven by the fact that the majority of cell culture research has been performed either on human cells or mammalian model systems that serve as adequate substitutes for human cell culture. Mark Post chose to culture beef (Annals of the New York Academy of Sciences) for the first-ever demonstration of a cultivated hamburger in 2014.
Avian (bird) cells can be cultured using methods that are very similar to those used for mammalian cell culture. This article on chicken cell-based lab-grown meat (Vice) describes the work done by Prof. Paul Mozdziak at NC State, who works with chicken and turkey satellite cells. The whole article is worth reading, but the following quotes point out some key differences between culturing mammalian and avian cells:
Part of his message to these companies is that the focus on growing cattle and pig muscle cells to produce beef and pork is noble, but just from a technological standpoint, Mozdziak argues that chicken and turkey cells are much easier to work with. “First things first, they grow a lot better in culture than mammalian cells do. They have better plasticity — you can get them to do what you want much more easily.” Interestingly, he’s not sure why, but Mozdziak points out that with mammals, it’s easier to work with cells biopsied from younger animals, whereas with birds, a more mature animal has better satellite cells with which to start.
Fish and seafood cell sources for cultivated meat have recently gained a lot of interest. This is a great review article on Cell-based fish (Frontiers in Sustainable Food Systems).
You can skim over most of the article, but be sure to read the following sections (including their sub-sections):
- Basic methodology for cell-based seafood production
- Oxygen requirements, pH considerations, and Temperature requirements
- Cell Lines, and Cell isolation methods
7. What are some of the advantages of working with avian cells, compared to mammalian cells?
8. What are some of the advantages of working with fish or seafood cells, compared to mammalian or avian cells?
9. Which type of cells (mammalian, avian, or fish/seafood) are best understood by the scientific community? Which are understood the least?
Typical commercial suppliers of cells include companies such as ATCC and Kerafast. However, their cell sources tend to be weighted towards humans and mice, and there are few cell types available from species that are relevant for food production.
Because of the lack of commercially-available cell sources, there is a new focus on developing cell lines in both academic settings and start-ups.
Start-up companies such as Vow Foods (Australia) and Cell Farm Food Tech (Argentina) are positioning themselves as cell line suppliers. Vow Foods is banking cells from unusual species such as kangaroo and zebra, whereas Cell Farm Food Tech is banking stem cells from high quality Argentinian cattle. While these are exciting, it is unclear when these cell lines will be available for purchase.
Several academic researchers have become interested in developing and banking relevant cell types. This is a great New Harvest blog post about a supported research project aimed at producing an avian cell line (New Harvest), specifically turkey. The work by Paul Mozdziak’s lab at NC state was further publicized in this article on making your own meat with open-source cells (New Scientist).
A great resource to learn about which academic researchers are working in cultivated meat is GFI’s scientific research database. You can use the filters to identify which research labs are working on cell lines.
The other way to source cells is directly from a tissue biopsy. Biopsies usually need to be performed by a trained veterinarian. The website for Mosa Meat describes this process generally, where their aim is to isolate the myosatellite cells. From the biopsy, isolating different cell types is typically performed using a flow cytometry technique called FACS (Fluorescence-Activated Cell Sorting) (Sino Biological). Another useful resource is the Cellular Agriculture Course at Tufts University, which has a fairly detailed practical section on Primary bovine cell isolations.
10. What is the preferred type of cell to use to generate cultivated meat?
11. Are these cell types usually immortalized or non-immortalized? What are the pros and cons of each approach?