1. Introduction
The Philippines, ranking 8th globally in fish production, places significant emphasis on its aquaculture industry. In 2018, tilapia, the second-highest produced fish, generated substantial waste, including skin, scales, bones, viscera, and heads [
1]. However, this waste if not properly disposed can lead to environmental concerns [
1]. Current waste utilization focuses on low-value products like fishmeal and fertilizers, but efforts are underway to explore higher-value applications [
2]. Extracting valuable products, especially from tilapia heads, has promising potential for industries like food, pharmaceuticals, and cosmetics [
3].
Fish heads, constituting 29-35% of the fish's mass, serve as a significant source of marine-based extracellular matrix (ECM). Characterized by its richness in diverse proteins (collagens, elastin, fibrillin, fibronectin, laminin), proteoglycans (heparin sulfate, chondroitin sulfate, keratin sulfate, GAGs), and growth factors, the ECM serves as a reservoir for the molecular elements and growth factors present in the native tissue [
4]. The predominant utilization of ECM revolves around its role in constructing tissue scaffolds and presents a significant avenue in tissue engineering and regenerative medicine [
5,
6].
Demineralization and decellularization are essential methods employed to extract ECM from fish heads. A common demineralizing agent is hydrochloric acid, specifically at a concentration of 0.5 N for a duration of 1 h [
7,
8]. Important factors in an effective demineralization method include efficient mineral removal, collagen preservation, and the assessment of micro-porosity [
9]. Concurrently, decellularization involves the removal native cells from tissue while retaining a three-dimensional ECM structure, preserving bioactivity and mechanical properties [
10]. This approach offers a distinctive top-down method for creating natural scaffolds in tissue engineering [
10]. However, careful selection of effective decellularization agents is imperative to minimize disruptions and ensure thorough cell removal while preserving the structural and functional proteins within the three-dimensional structure, referred to as decellularized extracellular matrix (dECM). As each decellularization agent induces distinct effects on ECM proteins, the choice of method should be tailored to the tissue biomechanics essential for optimal functionality.
In tissue decellularization, various chemical detergents are commonly used. A study by Li et al. (2021) introduced an optimized dextrose/sodium lauryl ester sulfate (SLES)/Triton X-100 (TX-100) cocktail method for porcine whole lungs decellularization to generate a clinical-scale bioengineered scaffold [
11]. This investigation, alongside with previous study, highlighted Triton X-100’s potential disruption on lipid–lipid and lipid–protein interactions, despite its effectiveness in solubilizing cell membranes, disengaging cytoskeletal proteins, and detaching DNA and DNA remnants from proteins [
11,
12]. Another study recommended the utilization of SLES over sodium dodecyl sulfate (SDS) due to SDS's prolonged tissue treatment, causing significant ECM degradation [
13]. Other studies also highlighted the capability of SDS to remove cells and DNA components while potentially damaging collagen and glycosaminoglycans during prolonged treatment [
14,
15]. However, Kawasaki et al. (2015) identified SDS as a promising decellularization agent due to its accessibility and efficient cell removal despite the issue of causing pronounced ECM damage [
16]. Nevertheless, TX-100 and SDS are recognized for meeting the rigorous criteria for effective decellularization [
15,
17]. The effectiveness of TX-100 on decellularization is contingent upon the inherent characteristics of the tissue undergoing the procedure and the integration of other decellularization methods [
17]. Prior assessments of SDS’s affirm its capability for removal of cellular components and achieving a minimum 90% reduction in host DNA content [
15]. Moreover, SDS is a viable decellularizing agent which exhibited low likelihood of inducing toxic effects on cells, provided if thoroughly removed [
15].
Several studies, including those conducted by Keshvari et al. (2023) on kidney tissue with SLES and SDS, Yaghoubi et al. (2022) and Hassanpour et al. (2018) employing SLES for rat liver and human ovarian tissue respectively, and Miranda et al. (2021) using TX-100 for murine skeletal muscles, have explored diverse decellularization methodologies [
18,
19,
20,
21]. However, these methods have their limitations, including extended treatment duration, alterations in mechanical properties, and potential residual toxicity [
22]. To address the constraints of existing decellularization protocols, researchers are actively developing new methods with the goal of reducing treatment duration, minimizing exposure to chemical or organic agents, and diminishing tissue damage [
23]. Sonication, recognized as an alternative for decellularization, involves the application of ultrasonic energy to induce cavitation phenomena, leading to the physical dissociation of molecules by causing the implosion of air bubbles in a liquid [
23,
24]. Notably, Shen et al. (2020) introduced a method incorporating freezing-thawing, sectioning, and sonication in deionized water for the decellularization of cartilage [
25]. Azhim et al. (2013) proposed a sonication protocol using SDS for decellularizing aortic tissues [
26]. These studies highlight the potential of sonication in achieving complete decellularization of the ECM, contributing valuable insights to decellularization methods.
At present, a standardized methodology for the decellularization of tilapia heads is yet to be established. This study explores the viability of tilapia fish heads as a promising source for the production of dECM. The primary aim is to evaluate the impact of integrating sonication in decellularization, utilizing 1% TX-100, 1% SDS, and deionized water, into established methods. This integration is designed to reduce treatment duration, limit exposure to decellularizing agents, and minimize tissue damage in the resulting dECM. To assess the effectiveness of sonication-assisted decellularization of tilapia heads, specific criteria established by Kawecki et al. (2018) are employed [
27]. These criteria include the absence of visible nuclear remnants, a DNA content of less than 50 ng/mg of dried tissue, and the preservation of the ECM structure [
28]. These benchmarks serve as critical indicators to assess the success of decellularization while ensuring the integrity of the ECM [
27]. This research represents a significant endeavor to enhance and optimize the decellularization process for tilapia heads, providing valuable insights for the development of robust protocols in the broader context of tissue engineering and regenerative medicine.
4. Discussion
The utilization of fish waste, particularly tilapia heads, presents significant environmental and economic benefits by enabling the production of high-value products. Treated fish waste finds diverse applications, including the extraction of collagen and antioxidants for cosmetics, generating biogas/biodiesel, creating fertilizers, producing chitosan for dietary purposes, utilizing gelatine and chitosan in food packaging, and isolating enzymes like proteases. These applications showcase the versatility and value derived from fish waste processing across multiple industries [
37]. The collagen-rich ECM of tilapia heads makes it an alternative source of collagen which is distinct from traditional sources such as bovine and pig skin [
32,
38]. However, it is important to note that there is still no decellularization method specifically tailored for tilapia heads. Therefore, research is needed to explore and establish effective decellularization methods specifically for tilapia heads. These efforts would significantly contribute to the utilization of tilapia heads in the development of ECM-based biomaterials for various applications.
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The effects of sonication-assisted decellularization of tilapia heads with different reagents and varying contact time were investigated. Histological analysis revealed that sonication enhanced cell removal. Cell removal rate also increased with longer decellularization times. These histological findings were supported by the DNA quantification, which exhibited a similar trend of increased DNA removal with sonication and longer decellularization time. The decellularization was able to achieve the residual DNA levels below 50 ng/mg of dry tissue weight which is the criteria to ensure minimal potential immunogenicity and promote compatibility when utilizing the decellularized ECM for various applications [
6].
The enhanced cell removal achieved through sonication can be attributed to its bioeffects on cell cytoplasm, inducing physical and chemical disruptions such as cavitation and emulsion [
23]. Moreover, Azhim et al. (2013) highlighted that sonication can enhance the efficiency of decellularization agents like SDS by reducing dissolved oxygen levels thus maximizing the cavitation effect during the decellularization process. Additionally, in a separate study by Syazwani et al. (2014), sonication was successfully employed to achieve complete decellularization of an aorta, providing further evidence of the significant enhancement that sonication can bring to the removal of cellular components [
24].
The analysis of protein content in the decellularized samples revealed a significant decrease, particularly in the samples treated with sonication using SDS. This decrease was expected as it is known that SDS is disruptive to ECM causing denaturation and unfolding of the protein structure [
35]. Moreover, the decrease in protein could also be caused by the demineralization process involved in the dcTH samples, which is primarily concerned with the removal of minerals such as calcium that is bound to bind with collagen [
8]. The depletion of non-collagenous proteins, therefore, could be the primary cause of the observed decrease in protein content while the collagen structure remained intact on TX-100 and DEW-treated samples. However, several studies have showed that sonication could be beneficial in dissociating cells from ECMs while preserving the major proteins and biomolecules in the ECM [
23]. The difference on the results could be attributed the power or duration of sonication which could disrupt the main structural fibers of the ECM [
23,
39].
The preservation of essential collagen markers and their structural integrity in the dcTH samples was successfully confirmed through ATR-FTIR and SDS-PAGE analysis. These analytical techniques allowed for the identification and verification of collagen preservation despite the observed decrease in protein content. ATR-FTIR spectra exhibited similar peaks and bands for various functional groups in both dNS and dWS samples, indicating the effective preservation of collagen, the main component of the ECM. However, the dWS samples displayed more intense bands compared to the dNS samples. This finding suggests that the combined demineralization and sonication-assisted decellularization effectively removed hydroxyapatite while retaining important collagen markers such as amide I, II, III, and amides A and B [
8]. Notably, the sonication treatment demonstrated a noticeable decrease in the V
4PO peak compared to the without sonication, indicating the protonation of hydroxyapatite as a possible contributing factor to this observation.
The molecular weight bands on SDS-PAGE were predominantly observed in the range of 50-225 kDa for the dcTH samples. However, a more emphasized molecular weight bands were observed in dWS samples, especially with the use of SDS. Additionally, the analysis of dcTH samples revealed higher quantities of β dimer and molecular cross-linked components [
32]. The method employed pepsin solubilization, which resulted in a prominent proportion of α chains, in accordance with the findings of Iijima et al. (2018) [
30]. Furthermore, it was discovered that these collagens exhibited a chain composition of two α
1 chains and a single α
2 chain, as reported by Barajan et al. (2013) [
32].
Thermal analysis of the dcTH samples revealed significant shifts in temperature peaks. The observed shifts indicate changes in the protein structure and composition due to the decellularization process, as well as the influence of sonication. In the case of the dcTH samples, it was observed that the denaturation peak temperatures were higher in the dWS compared to the dNS. This may suggests that the dWS samples contained proteins with higher thermal stability, possibly due to differences in tissue composition or the preservation of specific protein structures.
The presence of residual materials (detergent) on the ECM can lead to significant and potentially harmful inflammatory reactions [
23]. In order to ensure thorough removal of nuclear materials (detergent), it is necessary to implement rigorous washing procedures. The use of SDS requires extensive washing process to avoid cytotoxic effect on the cell [
35,
40]. Moreover, future research should assess the biocompatibility and cytotoxicity of the generated decellularized extracellular matrix (dECM) to determine its suitability for tissue applications.
Author Contributions
Conceptualization R.B.; methodology, L.B., R.B. and H.B.; software, L.B., K.D.D.V., and J.A.; formal analysis, L.B., M.J.N., F.A., G.L., and J.A.; investigation, L.B. and R.B.; data curation, R.B., L.B., and M.L.; writing—original draft preparation, L.B.; writing—review and editing, R.B., L.B., J.P., K.D.D.V., and M.L.; supervision, R.B. and H.B. All authors have read and agreed to the published version of the manuscript.