Biomedical Engineering Reference
In-Depth Information
assess safety and efficacy of this process, although this report indicates the feasibility of tissue
engineering as a new option for patients who suffer from airway diseases.
While tissue engineering of hollow organs is making exciting progress in this field, the
development of methods to generate larger, solid organs with a complicated histological
structure remains challenging. To build fully functional, engineered organs such as liver and
kidney in the laboratory, several issues must be addressed. First, as these organs contain
numerous cell types with complicated structures, simple cell-seeding techniques may not be
sufficient. In addition, as size of the target organ increases, the delivery of oxygen and nutri-
ents becomes an obstacle unless a method for engineering a functional vascular network
within the organ can be found. Despite these challenges, several studies showed promising
results. Kidney contains multiple cell types and a complex functional anatomy; therefore, it
is one of the most difficult organs to reconstruct [118, 119].
Our laboratory has been able create a basic form of kidney. In this study, the principles of
both tissue engineering and therapeutic cloning were applied with the aim of producing
genetically identical renal tissue in a bovine model [120]. Skin fibroblasts from adult Holstein
bovines were obtained by ear notch to isolate single cells, and then single donor cells were
transferred into the perivitelline space of the enucleated oocytes. Nuclear transfer embryos
were activated and the blastocysts were implanted into progestin-synchronized recipients to
allow for further
in vivo
growth. After 7-8 weeks, cloned renal cells from fetuses were
harvested and expanded
in vitro
. Propagated cloned renal cells were seeded onto scaffolds
consisting of three cylindrical polycarbonate membranes. The ends of the membranes of
each scaffold were connected to 16G silastic catheters that terminated into a collecting res-
ervoir. These cell-polymer constructs served as a renal neo-organ and were implanted into
the flank subcutaneous tissue of the same animal from which the cells were cloned. Twelve
weeks after implantation, the constructs were retrieved and the yellow urine-like fluid was
observed within the reservoir of the tissue engineered device. Chemical analysis of this fluid,
including urea nitrogen and creatinine levels, electrolyte levels, specific gravity, and glucose
concentration, revealed that the implanted renal cells possessed filtration, reabsorption, and
secretory capabilities. Histological examination of the retrieved implants showed extensive
vascularization and had self-assembled into glomeruli and tubule-like structures, and a clear
continuity between the glomeruli, the tubules, and the polycarbonate membrane was found.
Thus, the passage of urine into the collecting reservoir was confirmed. This study demon-
strated that cells derived from nuclear transfer can be successfully harvested, expanded
in vitro
, and transplanted
in vivo
with the use of biodegradable scaffolds. In this process,
single suspended cells, which are genetically identical to the host, can be organized into
tissue structures with kidney function. This was the first demonstration of therapeutic
application of cloning for regenerating a tissue
in vivo
. While the size of this tissue-engineered
device was small, the creation of a larger device with functioning vasculature and innervations,
which can work as a fully functional kidney, will be necessary for successful translation
to the clinic.
Challenges and Future Directions in Regenerative Medicine:
Translating Regenerative Therapies to the Clinic
Tissue engineering of organs such as urethra, bladder, blood vessel, and trachea shows
promise toward developing the ability to engineer more complicated organs in the laboratory
in the future. Important concerns still need to be addressed, for example innervation of tis-
sues and organs to achieve full functionality. To this end, experiments to engineer a bladder
in the canine have revealed positive S-100 staining that was consistent with growth of neural
structures into the neo-bladders, and restoration of bladder function following implantation
