The Human Body is a complex interplay of different organs with specialized functions. These organs are very well integrated, working together towards one goal- the sustenance of life. Scientists have been working for ages trying to understand the normal functioning of the human body, yet there is so little that we know.
The liver is a vital organ of the animal body. Among its varied physiologic roles; detoxification, metabolism, and the secretion of plasma proteins are some of the important functions. Most of the liver is made up of cells called the hepatocytes and these cells are responsible for majority of its functions. Thus, to further understand the physiological functions, the diseased state or response of liver to drugs/toxins, scientists need to study at the level of the functional unit-the hepatocyte. It has always been a quest for researchers to understand the human biology in systems as close as possible to themselves. Before engaging animal models, such studies are done in a 2-dimensional culture system (2-D) of hepatocytes either derived from humans or immortalized cell lines. Both the systems have their own benefits and drawbacks that need to be taken care of.
Among the many, one limitation of 2-D culturing of cell lines is that the immortalized cells are present as monolayers like sheets, which is very different from their organization inside our body. In the body (or organ) they are like balls in a bag, they touch, talk and efficiently work with each other. Thus the 2-D system fails to represent the human organ closely.
The other approach is the use of 3-dimensional culture systems. Scientists have tried different substrates or surfaces like extracellular matrix (ECM) sandwiching, ECM hydrogels, alginate sponges, self-assembly peptide fibers, electrospun fibers, 3D printed scaffolds and spheroid cultures for 3-D culturing of cells. For varied reasons, like organization or poor mechanical properties or cost, to name a few, most of these systems cannot be routinely used except for spheroid cultures.
S K Onteru and coworkers at the National Dairy Institute, Karnal, (Onteru 2017) have tried to establish a 3-D spheroids culture system of hepatocytes that mimics the human liver more closely as compared to the available 2-D arrangement. They took cells from sheep and buffalo livers from slaughterhouses and cultured them in varying media and culture conditions and showed that in one of the conditions called Hanging Drop method with a special medium called Williams medium, the cells were very similar in their organization and gene make up as the freshly derived liver cells. In this method, there were no substrates onto which the cells adhered, the cells formed micro tissues under the gravitational force in the absence of any synthetic materials.
Cells in the hanging drop are more like the cells in our body. The DNA and protein analysis of human and cattle studies revealed a close resemblance between the two. The liver of humans, cattle, sheep and goat have also been shown to be very similar in structure, which makes it relevant to study human liver functions and response to drugs in these systems apart from small rodents like mice. Onteru’s group shows that the sheep and buffalo cells can be maintained in such spherical hanging drops for 12 and 6 days respectively before they start losing the 3-D structure.
Although the results seem to be promising, a lot of hurdles need to be crossed. The overall viability of these cells was only 33% (sheep cells) and 20% (buffalo cells). They suggest that this can be improved by a reduction in the contamination and more defined sample collection procedures.
In Dr. Onteru’s opinion, “Particularly, selecting the liver samples from healthy young animals of less than two years old without any pathologies and infections would be beneficial for the success of this culture system”.
A new culturing system that is less expensive and closer to the human organ is proposed but a lot of obstacles need to be overcome before this system can be routinely used in the laboratory as a substitute to human derived primary cells or small animal models.
Dr. Dheer Singh (team leader and co-author) states, “Currently, the culture systems are being used for toxicology studies. If these studies are successful, we can use this system routinely in laboratories within 4-5 years”.
These results are published in April 2017 issue of Scientific Reports, a Nature research journal and can be accessed at Onteru 2017.