Hollow pollen grains as scaffolding building blocks in bone tissue engineering

Abstract

Introduction: The current study, for the first time, suggests nature-made pollen grains (PGs) of Pistacia vera L. as a potential candidate for using as scaffolding building blocks with encapsulation capability of bioactive compounds, such as bone morphogenetic protein 4 (BMP4). Methods: A modified method using KOH (5%, 25ºC) was developed to produce nonallergic hollow pollen grains (HPGs), confirmed by energy dispersive X-ray (EDX) analysis, field emission scanning electron microscopy (FESEM), and DNA and protein staining techniques. The in-vitro study was conducted on human adipose-derived mesenchymal stem cells (hAD-MSCs) to investigate the applicability of HPGs as bone scaffolding building blocks. Cytocompability was evaluated by FESEM, MTT assay, and gene expression analysis of apoptotic markers (BAX and BCL2). The osteoconductive potential of HPGs was assessed by alkaline phosphatase (ALP) activity measurement and gene expression analysis of osteogenic markers (RUNX2 and osteocalcin). Results: Findings demonstrated that HPGs can be considered as biocompatible compounds increasing the metabolic activities of the cells. Further, the bioactive nature of HPGs resulted in suitable cellular adhesion properties, required for a potent scaffold. The investigation of apoptotic gene expression indicated a reduced BAX/BCL2 ratio reflecting the protective effect of HPGs on hAD-MSCs. The increased ALP activity and expression of osteogenic genes displayed the osteoconductive property of HPGs. Moreover, the incorporation of BMP4 in HPGs initiated a synergistic effect on osteoblast maturation. Conclusion: Owing to the unique compositional and surface nanotopographical features of the Pistacia vera L. HPG, this microscale architecture provides a favorable microenvironment for the bottom-up remodeling of bone.

Extraordinary microcarriers derived from spores and pollens

“…15,16 Differentiated by the shape of the openings on exine, the pollens of Lotus, Camellia, Poppy and Poinsettia possess a colpus structure with elliptical or rectangular aperture, whose length is at least twice the width; Ricinus communis pollens exhibit a triaperturate structure with an exine ornamentation consisting of numerous depressions or projections of o1 mm in diameter; Carica papaya and Nicotiana sylvestris pollens also have a triaperturate structure and high porosity; Iris germanica pollens display a unique aperture shape with reticulate structure; Matthiola tricuspidata pollens lack pores and their exine layer appears thin and uniform; Ganoderma spores have many small pores while Lycopaceae pollens have extremely distinct open cavities on their surface. 15,30,52 The pollens' unique three-dimensional structure facilitates interaction with living organisms. In detail, microparticles with a rough surface structure can influence the cell behavior on them, such as adhesion, migration, and growth, due to their structural similarities with cells and microorganisms.…”

“…Zakhireh et al used this property of sporopollenin exine shells to design microcarriers utilized in bone tissue engineering. 30 This journal is © The Royal Society of Chemistry 2023…”

“…[9][10][11] Paleontologists have found fossils of pollen shells in 500 million-year-old sedimentary rocks, demonstrating sporopollenin's amazing mechanical stability. 12 The sporopollenin exine shells can serve as an excellent substrate for several types of microcarriers due to their inherent size uniformity, 4,8,13 porosity, 14,15 elasticity, [16][17][18] amphiphilicity, 19 chemical stability, [20][21][22] anti-ultraviolet and anti-oxidation, [23][24][25] photothermal effects, 26 adhesion, [27][28][29] biocompatibility, 30 and stability within body fluids. 31 For example, their inside cavity and surface pores facilitate drug loading and cargo delivery; [30][31][32][33][34] the robust exine structure can achieve long-term preservation; 25 the unique surface decoration increases the specific surface area enhancing the adsorption of probe molecules and target detection; 35 sporopollenin is rich in carboxyl and hydroxyl groups, which can be easily functionalized or formed into composites and further derived into microrobots, 8,36 cell scaffolds, 37,38 etc.…”

“…12 The sporopollenin exine shells can serve as an excellent substrate for several types of microcarriers due to their inherent size uniformity, 4,8,13 porosity, 14,15 elasticity, [16][17][18] amphiphilicity, 19 chemical stability, [20][21][22] anti-ultraviolet and anti-oxidation, [23][24][25] photothermal effects, 26 adhesion, [27][28][29] biocompatibility, 30 and stability within body fluids. 31 For example, their inside cavity and surface pores facilitate drug loading and cargo delivery; [30][31][32][33][34] the robust exine structure can achieve long-term preservation; 25 the unique surface decoration increases the specific surface area enhancing the adsorption of probe molecules and target detection; 35 sporopollenin is rich in carboxyl and hydroxyl groups, which can be easily functionalized or formed into composites and further derived into microrobots, 8,36 cell scaffolds, 37,38 etc. Compared to artificial microcarriers, which are complex, expensive and poorly monodispersed in mass production, the intrinsic superiorities and tailorable properties of spores and pollens make them suitable template-free materials to construct biological microcarriers or synthetic biomimetic particles.…”

Sporopollenin exine capsules modulate the function of microglial cells

“…The original 3D morphological characteristics of pollen grains and the micro-nanopore structure on the surface can still be maintained after treatment. 30 Due to high biocompatibility, simple preparation process, low price, high stability, uniform size, and abundant resources, SEC has been applied in different fields, including drug and living cell encapsulation, 29,31,32 cell printing and tissue engineering, 33,34 biosensing and cancer treatment. 35,36 In addition, SEC has immunomodulatory effects on immune cells such as macrophages and dendritic cells.…”