What term is used to describe the process that moves fluid and small particles out of the blood through the glomerulus and into the nephron tubule?

Abstract

Transporter-mediated organic solute flux across the renal proximal tubule epithelium is an essential process for the elimination of metabolic waste products and xenobiotics. During chronic kidney disease or renal failure clearance of these compounds is reduced or lost and many elevate to toxic levels, triggering new pathologies. The solute carrier 22 (SLC22) family of transporters represents one pathway mediating the active renal secretion of such organic solutes and evidence is growing that dysfunction of these transporters may be a key factor in the initiation and/or progression of some disease states. Recent insights to SLC22 transporter function and chronic kidney disease, uremia, diabetes, creatinine clearance and blood pressure are discussed.

Slurry stabilization

Mixing exposes microorganisms to the maximum amount of food, lessens temperature stratification in the digester, reduces the volume occupied by settled inorganic material (such as grit), evenly distributes metabolic waste products during the digestion process, and prevents the formation of a floating crust layer (which can slow the percolation of biogas out of the slurry). Mixing creates a homogeneous environment throughout the digester that enables the digester volume to be fully utilized (Schlicht, 1999). The benefits of mixing are as follows:

To speed up the process of volatile solids breakdown and

To increase the amount of biogas production.

Mixing can be accomplished by bubbling biogas through the slurry column in the digester or by mechanical means. An explanation of each method follows.

8.4 Future considerations

A major challenge to the field of biofabrication is the in vitro fabrication of large, solid organs with high densities of living cells. Diffusion of oxygen and nutrients and the removal of metabolic waste products limit current engineered tissues to thicknesses of approximately 100–200 µm in order to maintain cell viability. Natural organs and tissues are much larger and contain a branching vascular network that perfuses the entire organ and ensures all cells are close to blood vessels with adequate nutrient and oxygen supply. As the field of biofabrication and tissue engineering struggles with this major limitation, the field of induced pluripotent (iPS) stem cells is well on its way to providing a plentiful source of immune-matched cells of a variety of tissues and organs. Although we do not yet have a means for the in vitro fabrication of large 3D organs and tissues from this source of cells, multicellular building blocks may someday be useful for this problem.

In their present form, microtissue building blocks are not able to fabricate large vascularized organs in vitro, but they do approximate the size and complexity of pancreatic islets, with islet diameters of approximately 200 microns. Given the correct source of differentiated cells, technologies such as micro-molds that can be scaled up would be able to produce sufficient numbers of islets for transplantation to achieve insulin independence (approximately 10,000 islets per kg body weight). With regard to the effort to fabricate large vascularized organs in vitro, building blocks may be helpful if they are combined with other technologies used in biofabrication. Bioprinting, a well-advanced technology described in another chapter in this book, is based on the principle of inkjet printing and uses cells and ECM materials to build 3D constructs layer by layer[48,49]. Building blocks produced using micro-molds may be useful as a material for bioprinting. Cell sheets are another technology that might be combined with microtissue building blocks. Cell sheets are produced by culturing cells attached to a thermo-responsive polymer. When the cells reach confluence, the temperature is decreased, causing the release of an intact cell sheet [50,51]. Building blocks of defined geometries might be combined with cell sheets to form larger structures. Finally, building blocks might be combined with scaffolds, a large and very active area of investigation. Building blocks will readily attach to and spread on the many natural and synthetic scaffolds that have been engineered for cell attachment.

If the goal is to use solely building blocks to fabricate a large vascularized organ in vitro, there are significant challenges yet to be overcome, and it is informative to discuss these challenges in the context of the current progress in that direction. Regardless of their shape, building blocks are rapidly self-assembled from mono-dispersed cells within 24 to 48 hours. Micro-molds can be scaled up to produce sufficient numbers of building blocks, and building blocks can be made as very large structures (e.g., honeycombs) so fewer parts are needed for construction. These building parts can be melded within 48 and 72 hours, so the assembly of a large construct from individual parts could theoretically be accomplished in a relatively short period of time (1 to 2 weeks). This, of course, depends on the time required for maturation and does not count the time necessary to culture the large number of cells needed to form building parts. Building parts with lumens (toroids) have begun to approximate a crude vascular network, and the fusion of overlapping toroids generates a size range of lumens, all smaller than the lumen of the building unit. However, these neo-lumens approximate neither the density nor the diameter of capillaries (~10 µm). They may be useful for recreating the range of vessels that connect capillaries to small-diameter arteries and veins (~0.1–5.0 mm).

It may be possible to form building blocks that have a preformed capillary network. As presented here, endothelial cells will self-organize and form a prevascular network as a mixed microtissue self-assembles. Also needed are strategies to endothelialize the lumens of the building blocks to determine if these lumens anastomose with the capillary network and if the bioengineered vascular tree can be perfused as it is being built and as it matures. Since the critical diffusion distance for cells to receive necessary nutrients and to offload waste products is approximately 150 microns, perfusion is critical to maintain cell viability in large bioengineered structures with a high density of cells. A robust and functional vascular supply is also critical from the surgical perspective if a biofabricated tissue is to be translated to the operating room. A tissue with high cell density will require integration into the blood supply through arterial and venous anastomosis with the host. For tissues that are body site–specific, such as a renal tissue that requires a urinary drainage system or a hepatic tissue that requires a biliary drainage system, the scale of the engineered tissue must be adequate to properly and safely anastomose the drainage portion to the corresponding host organ. In summary, tissue engineering, and specifically scaffold-free tissue engineering, has come a long way, but there is still much progress to be made, and keeping the ultimate application of that work in mind should guide methods and designs of engineered tissues. These are just a few of the many challenges and clearly much work needs to be done to make advances toward this goal.

13.5 Kidney

The kidney is a complex solid organ with important roles in endocrine, metabolic, and immunologic homeostasis. Specifically, the kidney functions to filter the blood and to concentrate urine with toxic metabolic waste products, which highlights the crucial role of the kidney. Currently, dialysis and transplantation represent the reference standard treatment modalities for chronic kidney disease. However, dialysis does not cure kidney damage; it assumes a portion of the kidney's functions, particularly filtration. As such, dialysis leaves much to be desired. With the growing worldwide burden of hypertension and diabetes, the prevalence of chronic kidney disease is reaching epidemic proportions [69,70] and the need for functional kidneys is rising correspondingly.

The translational success of TERM strategies for the replacement of renal function depends on a better understanding of the native repair and regenerative processes occurring in vivo at the cellular and molecular levels [71,72]. Because of the proportionally increased metabolic demand of the kidney and its waste and toxin filtration function, renal tubular cells are constantly under the threat of acute injury and oxidative stress. For this reason, these cells contain unique regenerative abilities when damaged. In fact, researchers have observed surviving renal tubular cells giving rise to a new population of the cells after physiologic kidney damage [73–75]. Nagaike et al. observed that unilateral nephrectomy induces mitogenesis and hypertrophy in the contralateral kidney [76]. Cochrane et al. induced cortical tubular cell atrophy, tubular dilation, and interstitial macrophage infiltration via ureteral obstruction in a murine model of renal injury. They showed that reversal of the obstruction induced a rapid process of reconstruction and interstitial matrix expansion that ultimately restored the glomerular filtration rate [77]. However, continuous and supraphysiologic damage characteristic of chronic kidney disease overpowers the regenerative properties of these cells. Furthermore, the growth of new nephrons, i.e., frank nephrogenesis, has not been shown to occur [78].

Researchers have thus explored the potential of cell therapy to restore kidney function in the face of widespread damage. Cell-based approaches seek to achieve kidney repair and regeneration in situ upon therapeutic administration. They are based on the observation that exogenously supplied cells stimulate the repair and proliferative process [79]. Progenitor cells harvested from the proximal tubules, glomerulus, peritubules, and papillae have all demonstrated some level of therapeutic capacity [80,81]. Stem cells obtained from urine have also shown some potential to reverse kidney damage and aid in the repair process [82].

Recruitment of bone marrow–derived MSC, a type of adult stem cell, to renal tubular damage sites has enhanced regenerative outcomes [83]. It is thought that MSCs stimulate the release of more proliferative mediators from native cells to enhance their own regenerative process. MSCs can also secrete specific chemokines, cytokines, and growth factors to promote growth and cellular protection from further damage. Using the nephrotoxic drug cisplatin to induce acute kidney damage in mice models, researchers have looked at MSC-induced proliferation and regeneration of epithelial cells in damaged renal tubules [84]. Although the use of MSCs is promising for acute kidney injury, there is a paucity of current literature regarding their use in models of chronic kidney disease, which hosts a different set of challenges and conditions. A study using rat models found that chronic kidney disease leads to premature senescence of MSCs and inhibits their typical regenerative potential, potentially limiting their use [85].

Enhanced understanding of the regenerative properties of renal cells has led to another avenue for the treatment of kidney damage: the use of embryonic kidney tissue. These primordial cells have been shown to integrate within adult organ systems, richly vascularize, and form new, mature nephrons (i.e., frank nephrogenesis) [86,87]. Ureteric bud and metanephric mesenchyme cultures have shown, though the inherent ability of mesonephric duct tissue, the ability to form collecting ducts through tubulogenesis and epithelization. The in vitro tissue was implanted into mice models and survived for over 5 weeks, demonstrating glomerular vascularization in vivo and thereby pointing to the therapeutic potential of these primordial tissues [88]. Imberti et al. implanted renal primordia under the kidney capsule of male rats with kidney injury [89]. The grafts went on to develop glomeruli and tubuli that filtered blood and produced urine in cyst-like structures. Furthermore, they initiated a process of regeneration in host tissue segments indicated by increased cell proliferation and vessel growth.

Researchers have explored the potential to bioengineer kidneys de novo for eventual implantation into patients, harnessing the advantages of surgical transplantation. The most promising strategy under investigation has been the cell-scaffold approach. Native kidneys are stripped of cellular material using detergents while preserving the ECM upon which to seed new cellular material. ECM scaffolds produced from animal and human kidneys have been shown to retain their innate biomolecular and biophysical properties along with their external and internal three-dimensional architecture [90]. Upon seeding with cells, signaling cytokines and growth factors from the retained ECM can guide cell differentiation toward organ-specific phenotypes and promote vasculogenesis necessary for full integration into the host.

Bonandrini et al. used ESCs to recellularize rat whole-kidney scaffolds by perfusing them through the renal artery along with circulating cell medium [91]. The investigators observed the loss of pluripotency and the shift toward mesoendodermal lineage, supporting the idea that decellularized kidneys can undergo rapid recellularization of vascular structures and glomeruli and induce differentiation along appropriate pathways. Nakayama et al. decellularized both the kidneys and lungs of rhesus monkeys and reseeded them with undifferentiated ESCs [92]. Expression levels of tubule markers and other kidney genes were higher in cells cultured on kidney ECM compared with those on lung ECM, which lent evidence to the idea that ECM guides differentiation selectively.

Our group successfully decellularized porcine kidneys and implanted them into porcine hosts with evidence of preserved functionality [93]. Moving to more clinically relevant models, a source of human kidneys available for bioengineering investigations was identified: organs originally intended for transplant purposes but discarded because of functional or anatomical anomalies (approximately 2600 kidneys annually in the United States) [94]. Upon detergent-perfusion, the successful production of viable ECM scaffolds using discarded kidneys was confirmed [95]. In a later study, amniotic fluid–derived stem cells were seeded on discarded kidney scaffolds to assess the potential to recellularize [96]. The cells attached, proliferated, and furthermore synthesized and secreted various chemokines and growth factors involved in angiogenesis and matrix remodeling including vascular endothelial growth factor, transforming growth factor-α, interleukin-8, metalloproteinase-2, and tissue inhibitor of metalloproteinase-2. Furthermore the cells were found to express early kidney developmental markers after 2 weeks of culture on the ECM, suggesting regeneration.

Both cell therapy and cell-scaffold strategies demonstrated potential for the treatment of kidney disease, which is currently limited to dialysis and transplantation. Further studies are needed to determine the best cell type for renal regeneration: adult stem cells, progenitor cells, or frank stem cells.

Which is the process in which there is the passage of fluid through the glomerulus into the nephron tubule?

What is the name for the process by which water and nutrients are moved from the nephron to the blood?

When they move from the tubular fluid back into the blood?

What is the driving force that pulls water out of the nephron?