Florence J. Wu

Efficient assembly of rat hepatocyte spheroids for tissue engineering applications.

Freshly harvested primary rat hepatocytes cultivated as multicellular aggregates, or spheroids, have been observed to exhibit enhanced liver‐specific function and differentiated morphology compared to cells cultured as monolayers. An efficient method of forming spheroids in spinner vessels is described. Within 24 h after inoculation, greater than 80% of inoculated cells formed spheroids. This efficiency was significantly greater than that reported previously for formation in stationary petri dishes. With a high specific oxygen uptake rate of 2.0 × 10−9 mmol O2/cell/h, the oxygen supply is critical and should be monitored for successful formation. Throughout a 6‐day culture period, spheroids assembled in spinner cultures maintained a high viability and produced albumin and urea at constant rates. Transmission electron microscopy indicated extensive cell‐cell contacts and tight junctions between cells within spheroids. Microvilli‐lined bile canaliculus‐like channels were observed in the interior of spheroids and appeared to access the exterior through pores at the outer surface. Spheroids from spinner cultures exhibited at least the level of liver‐specific activity as well as similar morphology and ultrastructure compared to spheroids formed in stationary petri dishes. Hepatocytes cultured as spheroids are potentially useful three‐dimensional cell systems for application in a bioartificial liver device and for studying xenobiotic drug metabolism.

Formation of porcine hepatocyte spheroids for use in a bioartificial liver

Xenogeneic hepatocytes have recently been used in a bioartificial liver device as a potential short-term extracorporeal support of acute liver failure. Scaling up the system requires large quantities of viable and highly active cells. Hepatocytes grown as spheroids manifest higher metabolic activities for longer time periods as compared to those in monolayer cultures. Use of hepatocyte spheroids for application in a bioartificial liver can possibly alleviate the need of scaling up. Porcine hepatocytes when cultured under stirred conditions, from multicellular spheroids in a defined culture medium. Spheroids were formed 24 h after cell inoculation with an efficiency of 80-90% and a mean diameter of about 135 microns. Scanning electron microscopy revealed numerous microvilli projecting from the entire surface of the spheroids. Transmission electron microscopy revealed differentiated hepatocytes which displayed well-developed cytoplasmic structures separated by bile canaliculus-like structures. The morphological studies show a resemblance between cells in the spheroids and in the liver in vivo. Urea-genesis by spheroids was twice as active and was sustained for a longer culture period than that by hepatocytes cultured as monolayers. Preparation of porcine hepatocyte spheroids in an agitated vessel is simple efficient and reproducible. It will allow for preparation of large quantities of spheroids to be employed in a bioartificial liver device as well as in liver metabolism studies.

Mechanistics of formation and ultrastructural evaluation of hepatocyte spheroids

SummaryFreshly harvested rat hepatocytes form spheroids on uncoated positively charged polystyrene surfaces. Time lapse microscopy revealed that cell movement and reorganization were involved in spheroid formation. Ultrastructural evaluation using scanning and transmission electron microscopy indicated polarized cellular morphology and extensive cell-cell communication within spheroids. Bile canalicular structures were observed to surround each individual hepatocyte, forming an intricate three-dimensional continuous network of channels that appeared to end as pores/holes on the surface of the spheroid. The maintenance of differentiated cellular morphology coincided with preservation of hepatocyte viability and enhanced levels of tissue specific functions in spheroids.

Enhanced cytochrome P450 IA1 activity of self-assembled rat hepatocyte spheroids.

Primary rat hepatocytes can self-assemble to form multicellular spheroids when plated onto Primaria petri dishes. Spheroids have been observed to exhibit enhanced liver-specific functions and differentiated ultrastructure compared to monolayer cultures on dry collagen. With confocal scanning laser microscopy, CYP1A1 activity was evaluated in situ by detecting resorufin. This highly fluorescent molecule is the P450-mediated product of 7-ethoxyresorufin O-dealkylation (EROD). Significantly higher P450 activity was observed in spheroids compared to monolayers on collagen upon induction with 50 μM β-naphthoflavone (BNF), a CYP1A inducer. This was confirmed by measuring microsomal EROD activity. The distribution of CYP1A1 activity within spheroids was heterogeneous, with higher activity localized to the hepatocytes in the interior. During the process of spheroid formation, cells were initially seen to attach and spread out as a monolayer. This stage was associated with relatively low CYP1A1 activity. As cells formed multicellular structures and aggregated into spheroids, the level of CYP1A1 activity increased over time. At least a fivefold higher fluorescence intensity was observed in spheroids compared to that of monolayers maintained on collagen. The higher P450 activity within spheroids may be associated with their ability to maintain a greater degree of differentiation compared to monolayers. These studies demonstrate the potential of hepatocyte spheroids as a model system for investigating drug metabolism, tissue engineering, and tissue self-assembly.

Entrapment of hepatocyte spheroids in a hollow fiber bioreactor as a potential bioartificial liver.

A bioartificial liver (BAL) employing xenogeneic hepatocytes has been developed as a potential interim support for patients in hepatic failure. For application in human therapy, the BAL requires a substantial increase in liver-specific functions. Cultivation of hepatocytes as spheroids leads to enhanced liver specific functions. We explored the possibility of entrapping spheroids into the BAL in order to improve device performance. Rat hepatocyte spheroids were entrapped in collagen gel within the lumen fibers of the BAL. The morphology and ultrastructure of collagen-entrapped spheroids resembled those of suspended spheroids formed on petri dishes. Albumin synthesis and P-450 enzyme activity were measured as markers of liver specific functions of spheroids entrapped in the BAL. At least a 4-fold improvement in these functions was observed compared to BAL devices entrapped with dispersed hepatocytes in collagen gels.

Development of a bioartificial liver employing xenogeneic hepatocytes.

Liver failure is a major cause of mortality. A bioartificial liver (BAL) employing isolated hepatocytes can potentially provide temporary support for liver failure patients. We have developed a bioartificial liver by entrapping hepatocytes in collagen loaded in the luminal side of a hollow fiber bioreactor. In the first phase of development, liver-specific metabolic activities of biosynthesis, biotransformation and conjugation were demonstrated. Subsequently anhepatic rabbits were used to show that rat hepatocytes continued to function after the BAL was linked to the test animal. For scale-up studies, a canine liver failure model was developed using D-galactosamine overdose. In order to secure a sufficient number of hepatocytes for large animal treatment, a collagenase perfusion protocol was established for harvesting porcine hepatocytes at high yield and viability. An instrumented bioreactor system, which included dissolved oxygen measurement, pH control, flow rate control, an oxygenator and two hollow fiber bioreactors in series, was used for these studies. An improved survival of dogs treated with the BAL was shown over the controls. In anticipated clinical applications, it is desirable to have the liver-specific activities in the BAL as high as possible. To that end, the possibility of employing hepatocyte spheroids was explored. These self-assembled spheroids formed from monolayer culture exhibited higher liver-specific functions and remained viable longer than hepatocytes in a monolayer. To ease the surface requirement for large-scale preparation of hepatocyte spheroids, we succeeded in inducing spheroid formation in stirred tank bioreactors for both rat and porcine hepatocytes. These spheroids formed in stirred tanks were shown to be morphologically and functionally indistinguishable from those formed from a monolayer. Collagen entrapment of these spheroids resulted in sustaining their liver-specific functions at higher levels even longer than those of spheroids maintained in suspension. For use in the BAL, a mixture of spheroids and dispersed hepatocytes was used to ensure a proper degree of collagen gel contraction. This mixture of spheroids and dispersed cells entrapped in the BAL was shown to sustain the high level of liver-specific functions. The possibility of employing such a BAL for improved clinical performance warrants further investigations.

Evaluation of a microcarrier process for large-scale cultivation of attenuated hepatitis A

Microcarrier culture was investigated for the propagation of attenuated hepatitis A vaccine in the anchorage-dependent human fibroblast cell line, MRC-5. Cells were cultivated at 37°C for one to two weeks, while virus accumulation was performed at 32°C over 21 to 28 days. The major development focus for the microcarrier process was the difference between the cell and virus growth phases. Virus antigen yields, growth kinetics, and cell layer/bead morphology were each examined and compared for both the microcarrier and stationary T-flask cultures. Overall, cell densities of 4–5×106 cells/ml at 5–10 g/l beads were readily attained and could be maintained in the absence of infection at either 37°C or 32°C. Upon virus inoculation, however, substantial cell density decreases were observed as well as 2.5 to 10-fold lower per cell and per unit surface area antigen yields as compared to stationary cultures. The advantages as well as the problems presented by the microcarrier approach will be discussed.

Formation and characterization of hepatocyte spheroids.

Several investigators have demonstrated that freshly harvested hepatocytes self-assemble into three-dimensional, compacted, freely suspended aggregates known as spheroids (1-3). These aggregates have smooth, undulating surfaces and average approx 120 µm in diameter. Hepatocyte spheroids exhibit enhanced liver-specific activities and prolonged viability, compared to cells maintained as a monolayer (4,5). Extensive cell-cell contacts, tight junctions, and microvilli-lined channels that resemble bile canaliculi have been observed between hepatocytes in spheroids (6,7). Thus, these cells appear to mimic the morphology and ultrastructure of an in vivo liver lobule. Reorganization of hepatocytes into these three-dimensional structures is hypothesized to contribute to their enhanced liver-specific functions. Because of their enhanced function and tissue-like ultrastructure, hepatocyte spheroids show great promise for use in tissue-engineering applications and drug metabolism studies.