Mechanical and structural plasticity

Membrane adhesion junctions regulate airway smooth muscle phenotype and function

The local environment surrounding airway smooth muscle (ASM) cells has profound effects on the physiological and phenotypic properties of ASM tissues. ASM is continually subjected to the mechanical forces generated during breathing and to the constituents of its surrounding extracellular milieu. The smooth muscle cells within the airways continually modulate their properties to adapt to these changing environmental influences. Smooth muscle cells connect to the extracellular cell matrix (ECM) at membrane adhesion junctions that provide mechanical coupling between smooth muscle cells within the tissue. Membrane adhesion junctions also sense local environmental signals and transduce them to cytoplasmic and nuclear signaling pathways in the ASM cell. Adhesion junctions are composed of clusters of transmembrane integrin proteins that bind to ECM proteins outside the cell and to large multiprotein complexes in the submembranous cytoplasm. Physiological conditions and stimuli from the surrounding ECM are sensed by integrin proteins and transduced by submembranous adhesion complexes to signaling pathways to the cytoskeleton and nucleus. The transmission of information between the local environment of the cells and intracellular processes enables ASM cells to rapidly adapt their physiological properties to modulating influences in their extracellular environment: mechanical and physical forces that impinge on the cell, ECM constituents, local mediators, and metabolites. The structure and molecular organization of adhesion junction complexes and the actin cytoskeleton are dynamic and constantly changing in response to environmental influences. The ability of ASM to rapidly accommodate to the ever-changing conditions and fluctuating physical forces within its local environment is essential for its normal physiological function.

Airway smooth muscle function in asthma

Known to have affected around 340 million people across the world in 2018, asthma is a prevalent chronic inflammatory disease of the airways. The symptoms such as wheezing, dyspnea, chest tightness, and cough reflect episodes of reversible airway obstruction. Asthma is a heterogeneous disease that varies in clinical presentation, severity, and pathobiology, but consistently features airway hyperresponsiveness (AHR)—excessive airway narrowing due to an exaggerated response of the airways to various stimuli. Airway smooth muscle (ASM) is the major effector of exaggerated airway narrowing and AHR and many factors may contribute to its altered function in asthma. These include genetic predispositions, early life exposure to viruses, pollutants and allergens that lead to chronic exposure to inflammatory cells and mediators, altered innervation, airway structural cell remodeling, and airway mechanical stress. Early studies aiming to address the dysfunctional nature of ASM in the etiology and pathogenesis of asthma have been inconclusive due to the methodological limitations in assessing the intrapulmonary airways, the site of asthma. The study of the trachealis, although convenient, has been misleading as it has shown no alterations in asthma and it is not as exposed to inflammatory cells as intrapulmonary ASM. Furthermore, the cartilage rings offer protection against stress and strain of repeated contractions. More recent strategies that allow for the isolation of viable intrapulmonary ASM tissue reveal significant mechanical differences between asthmatic and non-asthmatic tissues. This review will thus summarize the latest techniques used to study ASM mechanics within its environment and in isolation, identify the potential causes of the discrepancy between the ASM of the extra- and intrapulmonary airways, and address future directions that may lead to an improved understanding of ASM hypercontractility in asthma.

Flexibility of microstructural adaptations in airway smooth muscle

The airway smooth muscle undergoes an elastic transition during a sustained contraction, characterized by a gradual decrease in hysteresivity caused by a relatively greater rate of increase in elastance than resistance. We recently demonstrated that these mechanical changes are more likely to persist after a large strain when they are acquired in dynamic versus static conditions; as if the microstructural adaptations liable for the elastic transition are more flexible when they evolve in dynamic conditions. The extent of this flexibility is undefined. Herein, contracted ovine tracheal smooth muscle strips were kept in dynamic conditions simulating tidal breathing (sinusoidal length oscillations at 5% amplitude) and then subjected to simulated deep inspirations (DI). Each DI was straining the muscle by either 10%, 20%, or 30% and was imposed at either 2, 5, 10, or 30 min after the preceding DI. The goal was to assess whether and the extent by which the time-dependent decrease in hysteresivity is preserved following the DI. The results show that the time-dependent decrease in hysteresivity seen pre-DI was preserved after a strain of 10%, but not after a strain of 20% or 30%. This suggests that the microstructural adaptations liable for the elastic transition withstood a strain at least twofold greater than the oscillating strain that pertained during their evolution (10% vs. 5%). We propose that a muscle adapting in dynamic conditions forges microstructures exhibiting a substantial degree of flexibility.NEW & NOTEWORTHY This study confirms that airway smooth muscle undergoes an elastic transition during a sustained contraction even when it operates in dynamic conditions simulating breathing at tidal volume. It also demonstrates that the microstructural adaptations liable for this elastic transition withstand a strain that is at least twice as large as the oscillating strain that pertains during their evolution. This degree of flexibility might be an asset with major significant impact for a tissue such as the airway smooth muscle that displays an everchanging shape due to breathing.

Interactive three-dimensional teaching models of the female and male pelvic floor

Controversies regarding structure and function of the pelvic floor persist because of its poor accessibility and complex anatomical architecture. Most data are based on dissection. This "surgical" approach requires profound prior knowledge, because applying the scalpel precludes a "second look." The "sectional" approach does not entail these limitations, but requires segmentation of structures and three-dimensional reconstruction. This approach has produced several "Visible Human Projects." We dealt with limited spatial resolution and difficult-to-segment structures by proceeding from clear-cut to more fuzzy boundaries and comparing segmentation between investigators. We observed that the bicipital levator ani muscle consisted of pubovisceral and puborectal portions; that the pubovisceral muscle formed, together with rectococcygeal and rectoperineal muscles, a rectal diaphragm; that the external anal sphincter consisted of its subcutaneous portion and the puborectal muscle only; that the striated urethral sphincter had three parts, of which the middle (urethral compressor) was best developed in females and the circular lower ("membranous") best in males; that the rectourethral muscle, an anterior extension of the rectal longitudinal smooth muscle, developed a fibrous node in its center (perineal body); that the perineal body was much better developed in females than males, so that the rectourethral subdivision into posterior rectoperineal and anterior deep perineal muscles was more obvious in females; that the superficial transverse perineal muscle attached to the fibrous septa of the ischioanal fat; and that the uterosacral ligaments and mesorectal fascia colocalized. To facilitate comprehension of the modified topography we provide interactive 3D-PDFs that are freely available for teaching purposes. Clin. Anat. 33:275-285, 2020. © 2019 Wiley Periodicals, Inc.

Architectural differences in the anterior and middle compartments of the pelvic floor of young-adult and postmenopausal females

The pelvic floor guards the passage of the pelvic organs to the exterior. The near-epidemic prevalence of incontinence in women continues to generate interest in the functional anatomy of the pelvic floor. However, due to its complex architecture and poor accessibility, the classical 'dissectional' approach has been unable to come up with a satisfactory description, so that many aspects of its anatomy continue to raise debate. For this reason, we opted for a 'sectional' approach, using the Chinese Visible Human project (four females, 21-35 years) and the Visible Human Project (USA; one female, 59 years) datasets to investigate age-related changes in the architecture of the anterior and middle compartments of the pelvic floor. The puborectal component of the levator ani muscle defined the levator hiatus boundary. The urethral sphincter complex consisted of a circular proximal portion (urethral sphincter proper), a sling that passed on the vaginal wall laterally to attach to the puborectal muscle (urethral compressor), and a circular portion that surrounded the distal urethra and vagina (urethrovaginal sphincter). The exclusive attachment of the urethral sphincter to soft tissues implies dependence on pelvic-floor integrity for optimal function. The vagina was circular at the introitus and gradually flattened between bladder and rectum. Well-developed fibrous tissue connected the inferior vaginal wall with urethra, rectum and pelvic floor. With eight-muscle insertions, the perineal body was a strong, irregular fibrous node that guarded the levator hiatus. Only loose areolar tissue comprising a remarkably well developed venous plexus connecting the middle and superior parts of the vagina with the lateral pelvic wall. The posterolateral boundary of the putative cardinal and sacrouterine ligaments coincided with the adventitia surrounding the mesorectum. The major difference between the young-adult and postmenopausal pelvic floor was the expansion of fat in between the components of the pelvic floor. We hypothesize that accumulation of pelvic fat compromises pelvic-floor cohesion, because the pre-pubertal pelvis contains very little fibrous and adipose tissue, and fat is an excellent lubricant.