Predicting and understanding arterial elasticity from key microstructural features by bidirectional deep learning

Microstructural features and mechanical properties are closely related in all soft biological tissues. Both yet exhibit considerable inter-individual differences and are affected by factors such as aging and disease and its progression. Histological analysis, modern in situ imaging, and biomechanical testing have deepened our understanding of these complex interrelations, yet two key questions remain: (1) Given the specific microstructure, can one predict the macroscopic mechanical properties without mechanical testing? (2) Can one quantify individual contributions of the different microstructural features to the macroscopic mechanical properties in an automated, systematic and largely unbiased way? Here we propose a bidirectional deep learning architecture to address these two questions. Our architecture uses data from standard histological analyses, two-photon microscopy and biaxial biomechanical testing. Its capabilities are demonstrated by predicting with high accuracy (R²=0.92) the evolving mechanical properties of the murine aorta during maturation and aging. Moreover, our architecture reveals that the extracellular matrix composition and organization are the most prominent factors governing the macroscopic mechanical properties of the tissues studied herein. STATEMENT OF SIGNIFICANCE:  .

Recent Advances on Stimuli-Responsive Hydrogels Based on Tissue-Derived ECMs and Their Components: Towards Improving Functionality for Tissue Engineering and Controlled Drug Delivery

Due to their highly hydrophilic nature and compositional versatility, hydrogels have assumed a protagonic role in the development of physiologically relevant tissues for several biomedical applications, such as in vivo tissue replacement or regeneration and in vitro disease modeling. By forming interconnected polymeric networks, hydrogels can be loaded with therapeutic agents, small molecules, or cells to deliver them locally to specific tissues or act as scaffolds for hosting cellular development. Hydrogels derived from decellularized extracellular matrices (dECMs), in particular, have gained significant attention in the fields of tissue engineering and regenerative medicine due to their inherently high biomimetic capabilities and endowment of a wide variety of bioactive cues capable of directing cellular behavior. However, these hydrogels often exhibit poor mechanical stability, and their biological properties alone are not enough to direct the development of tissue constructs with functional phenotypes. This review highlights the different ways in which external stimuli (e.g., light, thermal, mechanical, electric, magnetic, and acoustic) have been employed to improve the performance of dECM-based hydrogels for tissue engineering and regenerative medicine applications. Specifically, we outline how these stimuli have been implemented to improve their mechanical stability, tune their microarchitectural characteristics, facilitate tissue morphogenesis and enable precise control of drug release profiles. The strategic coupling of the bioactive features of dECM-based hydrogels with these stimulation schemes grants considerable advances in the development of functional hydrogels for a wide variety of applications within these fields.

Engineering and Assessing Cardiac Tissue Complexity

Cardiac tissue engineering is very much in a current focus of regenerative medicine research as it represents a promising strategy for cardiac disease modelling, cardiotoxicity testing and cardiovascular repair. Advances in this field over the last two decades have enabled the generation of human engineered cardiac tissue constructs with progressively increased functional capabilities. However, reproducing tissue-like properties is still a pending issue, as constructs generated to date remain immature relative to native adult heart. Moreover, there is a high degree of heterogeneity in the methodologies used to assess the functionality and cardiac maturation state of engineered cardiac tissue constructs, which further complicates the comparison of constructs generated in different ways. Here, we present an overview of the general approaches developed to generate functional cardiac tissues, discussing the different cell sources, biomaterials, and types of engineering strategies utilized to date. Moreover, we discuss the main functional assays used to evaluate the cardiac maturation state of the constructs, both at the cellular and the tissue levels. We trust that researchers interested in developing engineered cardiac tissue constructs will find the information reviewed here useful. Furthermore, we believe that providing a unified framework for comparison will further the development of human engineered cardiac tissue constructs displaying the specific properties best suited for each particular application.

Fundamental Technologies and Recent Advances of Cell-Sheet-Based Tissue Engineering

Tissue engineering has attracted significant attention since the 1980s, and the applications of tissue engineering have been expanding. To produce a cell-dense tissue, cell sheet technology has been studied as a promising strategy. Fundamental techniques involving tissue engineering are mainly introduced in this review. First, the technologies to fabricate a cell sheet were reviewed. Although temperature-responsive polymer-based technique was a trigger to establish and spread cell sheet technology, other methodologies for cell sheet fabrication have also been reported. Second, the methods to improve the function of the cell sheet were investigated. Adding electrical and mechanical stimulation on muscle-type cells, building 3D structures, and co-culturing with other cell species can be possible strategies for imitating the physiological situation under in vitro conditions, resulting in improved functions. Finally, culture methods to promote vasculogenesis in the layered cell sheets were introduced with in vivo, ex vivo, and in vitro bioreactors. We believe the present review that shows and compares the fundamental technologies and recent advances for cell-sheet-based tissue engineering should promote further development of tissue engineering. The development of cell sheet technology should promote many bioengineering applications.

Circadian Entrainment Triggers Maturation of Human In Vitro Islets

Stem-cell-derived tissues could transform disease research and therapy, yet most methods generate functionally immature products. We investigate how human pluripotent stem cells (hPSCs) differentiate into pancreatic islets in vitro by profiling DNA methylation, chromatin accessibility, and histone modification changes. We find that enhancer potential is reset upon lineage commitment and show how pervasive epigenetic priming steers endocrine cell fates. Modeling islet differentiation and maturation regulatory circuits reveals genes critical for generating endocrine cells and identifies circadian control as limiting for in vitro islet function. Entrainment to circadian feeding/fasting cycles triggers islet metabolic maturation by inducing cyclic synthesis of energy metabolism and insulin secretion effectors, including antiphasic insulin and glucagon pulses. Following entrainment, hPSC-derived islets gain persistent chromatin changes and rhythmic insulin responses with a raised glucose threshold, a hallmark of functional maturity, and function within days of transplantation. Thus, hPSC-derived tissues are amenable to functional improvement by circadian modulation.

Production of Tissue-Engineered Small Intestine in Rats with Different Ages of Cell Donors

IMPACT STATEMENT

This study compared side-by-side the impact of donor age on the production of tissue-engineered small intestine (TESI). Each age represents a specific period of life: E18 for fetuses, 5-day-old pups for neonates, 21-day-old rats for weanlings, and 6-week-old rats for adults. The TESI produced was compared macroscopically and microscopically. The mechanism(s) contributing to the differences observed was explored by detecting proliferating cells in the TESI and by analyzing intestinal stem cell gene expression in donor cells. These data may provide valuable information for future application of TESI clinically.

Comparison of Different In Vivo Incubation Sites to Produce Tissue-Engineered Small Intestine

OBJECTIVE

The objective of this study was to compare the impact of different in vivo incubation sites on the production of tissue-engineered small intestine (TESI).

MATERIALS AND METHODS

Green fluorescent protein transgenic rat pups (3-5 days) were used as donors of intestinal organoids. Harvested intestine was exposed to enzymatic digestion to release intestinal stem cell-containing organoids. Organoids were purified, concentrated, and seeded onto tubular polyglycolic acid scaffolds. Seeded scaffolds were implanted in each of five locations in recipient female nude rats: wrapped with omentum, wrapped with intestinal mesentery, wrapped with uterine horn membrane, attached to the abdominal wall, and inserted into the subcutaneous space. After 4 weeks of in vivo incubation, specimens from each site were explanted for evaluation.

RESULTS

Wrapping seeded scaffolds with vascularized membranes produced TESI with variable lengths of vascularized pedicles, with the longest pedicle length from uterine horn membrane, the shortest pedicle length from intestinal mesentery, and intermediate length from omentum. The quantity of TESI, as expressed by volume and neomucosal length, was identical in TESI produced by wrapping with any of the three membranes. The smallest quantity of TESI was found in TESI produced from insertion into the subcutaneous space, with an intermediate quantity of TESI produced from attachment to the abdominal wall. Periodic acid-Schiff and immunofluorescence (IF) staining confirmed the presence of all intestinal epithelial cell lineages in TESI produced at all incubation sites. Additional IF staining demonstrated the presence of enteric nervous system components and blood vessels. Wrapping of seeded scaffolds with vascularized membranes significantly increased the density of blood vessels in the TESI produced.

CONCLUSION

Wrapping of seeded scaffolds in vascularized membranes produced the largest quantity and highest quality of TESI. Attaching seeded scaffolds to the abdominal wall produced an intermediate quantity of TESI, but the quality was still comparable to TESI produced in vascularized membranes. Insertion of seeded scaffolds into the subcutaneous space produced the smallest quantity and lowest quality of TESI. In summary, wrapping seeded scaffolds with vascularized membranes is favorable for the production of TESI, and wrapping with omentum may produce TESI that is most easily anastomosed with host intestine.

Age-related modulation of angiogenesis-regulating factors in the swine meniscus

An in‐depth knowledge of the native meniscus morphology and biomechanics in its different areas is essential to develop an engineered tissue. Meniscus is characterized by a great regional variation in extracellular matrix components and in vascularization. Then, the aim of this work was to characterize the expression of factors involved in angiogenesis in different areas during meniscus maturation in pigs. The menisci were removed from the knee joints of neonatal, young and adult pigs, and they were divided into the inner, intermediate and outer areas. Vascular characterization and meniscal maturation were evaluated by immunohistochemistry and Western blot analysis. In particular, expression of the angiogenic factor Vascular Endothelial Growth Factor (VEGF) and the anti‐angiogenic marker Endostatin (ENDO) was analysed, as well as the vascular endothelial cadherin (Ve‐CAD). In addition, expression of Collagen II (COLL II) and SOX9 was examined, as markers of the fibro‐cartilaginous differentiation. Expression of VEGF and Ve‐CAD had a similar pattern in all animals, with a significant increase from the inner to the outer part of the meniscus. Pooling the zones, expression of both proteins was significantly higher in the neonatal meniscus than in young and adult menisci. Conversely, the young meniscus revealed a significantly higher expression of ENDO compared to the neonatal and adult ones. Analysis of tissue maturation markers showed an increase in COLL II and a decrease in SOX9 expression with age. These preliminary data highlight some of the changes that occur in the swine meniscus during growth, in particular the ensemble of regulatory factors involved in angiogenesis.

Longitudinal Stretching for Maturation of Vascular Tissues Using Magnetic Forces

Cellular spheroids were studied to determine their use as “bioinks” in the biofabrication of tissue engineered constructs. Specifically, magnetic forces were used to mediate the cyclic longitudinal stretching of tissues composed of Janus magnetic cellular spheroids (JMCSs), as part of a post-processing method for enhancing the deposition and mechanical properties of an extracellular matrix (ECM). The purpose was to accelerate the conventional tissue maturation process via novel post-processing techniques that accelerate the functional, structural, and mechanical mimicking of native tissues. The results of a forty-day study of JMCSs indicated an expression of collagen I, collagen IV, elastin, and fibronectin, which are important vascular ECM proteins. Most notably, the subsequent exposure of fused tissue sheets composed of JMCSs to magnetic forces did not hinder the production of these key proteins. Quantitative results demonstrate that cyclic longitudinal stretching of the tissue sheets mediated by these magnetic forces increased the Young’s modulus and induced collagen fiber alignment over a seven day period, when compared to statically conditioned controls. Specifically, the elastin and collagen content of these dynamically-conditioned sheets were 35- and three-fold greater, respectively, at seven days compared to the statically-conditioned controls at three days. These findings indicate the potential of using magnetic forces in tissue maturation, specifically through the cyclic longitudinal stretching of tissues.

Precocious progression of tissue maturation instructs basipetal initiation of leaflets in Chelidonium majus subsp. asiaticum (Papaveraceae)

PREMISE OF THE STUDY

On a compound leaf, leaflet primordia are repetitively formed along the apical-basal axis, with the direction varying among taxa. Why and how the directions vary among species is yet to be solved, although a change in a single factor was proposed to cause the variation. In this study, we compared two species in the Papaveraceae with different directions of leaflet initiation, Chelidonium majus subsp. asiaticum (basipetal) and Eschscholzia californica (acropetal). Because E. californica has been studied in some detail, we focused on C. majus and asked how basipetal pattern is achieved. •

METHODS

Since only immature leaf primordial tissue has leaflet-generating competency, we performed histological and gene expression analyses on markers of the tissue maturation state. In addition, we performed a time-course analysis of leaf primordial growth. •

KEY RESULTS

Quantitative reverse transcription-PCR analysis demonstrated that a putative regulator of tissue maturation in C. majus, the CINCINNATA homolog, had higher expression in apical parts than in basal parts during the organogenetic phase. In contrast, expression of the CIN homolog was not elevated in either the apical or basal parts in E. californica during the organogenetic phase. •

CONCLUSIONS

In C. majus, apical parts of leaf primordia have already lost leaflet-generating competency during the organogenetic phase. We propose that precocious progression of the maturation process instructs basipetal progression of leaflet initiation in C. majus. This is not the mirror image of data on E. californica, which shows the opposite direction in leaflet formation, indicating that variation in direction is not attributable to a change in a single factor.