Hypothesis / aims of study
The urinary bladder undergoes continuous volume changes, demanding a robust support system. Frequently, the perception of bladder support is confined to its muscle-ligament suspension. However, the support function extends beyond this to include force transmission and amplification. Unlike muscles in rigid skeletons that work in pairs to stabilize and amplify leverage around joints, the bladder faces a distinct challenge due to its lack of a conventional skeleton. Evolutionary pressures have led the bladder to develop a unique mechanism to fulfill the role of a traditional skeleton, crucial for maintaining structural integrity and facilitating effective force transmission.
Hydrostatic skeleton (HS) is a structural design principle in which muscles work against a contained volume of fluid to effect movement while maintaining shape [1]. This design principle is found in many soft-bodied animals, including humans. In HS, fibers, comprising both muscle and connective tissue, are arranged in a geodesic manner. Geodesic lines are the shortest paths between two points on a curved surface, and they provide a strong and efficient way to distribute forces. Another component of HS- the enclosed fluid transmits pressure generated by muscle contraction in all directions so that the action of any muscle affects all the others. This literature review explores the possibility that the urinary bladder incorporates features reminiscent of hydrostatic skeletons (HS). Considering this potential analogy, the study aims to gain insights into bladder function and contribute to the development of novel treatment strategies for bladder disorders.
Study design, materials and methods
Utilizing a multidisciplinary approach, we conducted an extensive literature review spanning a 40-year timeframe, incorporating insights from comparative biology, embryology, biomechanics, fluid mechanics, and structural mechanics. The initial search, guided by terms such as "hydrostatic skeleton," "urinary bladder morphology and mechanics," "prestress," and “shape” was refined through searches on Google Scholar, ScienceDirect, and PubMed. Selection criteria focused on biomechanical modeling, morphology, and comparative studies of living fluid-filled structures.
Results
Signs of the HS design principle are found in various animal fluid-filled structures, including those derived from the coelom during embryonic development. They are even observed in excretory organs formed from coelomic compartments [2], hinting at their potential role in the urinary bladder.
Our review revealed that certain features of the urinary bladder exhibit characteristics consistent with the HS design principle:
• Beyond the conventional circular and longitudinal layers, the bladder possesses an intricate network of oblique spiral fibers. This arrangement mirrors the geodesic fiber organization found in HS, offering an efficient mechanism for distributing and amplifying forces across the bladder wall. Morphologically, the inclusion of helical fibers in the design is crucial to prevent potential kinking and buckling of the fibers. Depending on the angle of the contractile fiber to the longitudinal axis, it may act towards either elongation or shortening of the entire structure, effectively balancing its integrity [Fig1].
• Enclosed fluid: The bladder's fluid (urine) plays a pivotal role in its function. Acting as a hydrostatic medium, the urine transmits the pressure generated by muscle contraction in all directions, facilitating optimal bladder filling and emptying control. Moreover, the enclosed fluid helps maintain the bladder's shape and prevents collapse, further contributing to its structural integrity. Therefore the urine may be considered an inherent biomechanical constituent of the bladder.
• Prestress, a key feature of hydrostatic skeletons, arises in the bladder, similarly to other living fluid-filled structures, due to the intricate interplay between enclosed urine and the fibers of the wall. Prestress determines the stiffness of the bladder wall, potentially influencing its response to mechanical loads and its ability to accommodate varying urine volumes while maintaining continence. Notably, the degree of prestress directly correlates with the structural stiffness [3]. Additionally, prestress plays a decisive role in efficient stress distribution, mitigating the risk of localized damage.
Interpretation of results
Our findings suggest a strong parallel between the geodesic-like muscle arrangement, enclosed fluid, and prestress in the urinary bladder and the HS design principle. This alignment potentially adds to a holistic explanation of the bladder's remarkable ability to accommodate significant volume changes. The convergence of these features underscores their collective role in maintaining structural integrity and efficient bladder function. Conceding that other factors play a role, the incorporation of HS-like design emerges as an important element in comprehending bladder mechanics and has the potential to inspire innovative treatment strategies.
Limitations: while studies suggest a link between HS and bladder mechanics, the reviewed literature doesn't conclusively differentiate its role from nervous control in generating and coordinating muscle movements.
Most reviewed studies relied on observations and models, lacking direct experimental confirmation of HS features and their functional significance in the bladder. Dedicated experiments are crucial for more conclusive data. Further research is necessary to explore the full extent of this potential analogy and establish a deeper understanding of the role the HS-like features play in bladder function.