## 4 Discussion  
This study analyzed the distinctive proteins of articular cartilage and bone tissue on 3D-printed scaffolds at 3 and 6 months in an ovine experimental model. According to the results, there was a prevailing expression of cartilage markers (SOX9, Aggrecan, and COL2A1) on the implant inside the joint with MSCs that were pre-differentiated to chondrocytes; there was a significant positive difference of 0.8% corresponding to SOX9, 0.8% to Aggrecan, and 0.45% to COL2A1, compared to the positive control of the native tissue. [PERSON] et al. [26] and collaborators demonstrated that mesenchymal stem cells assist with chondrogenic differentiation when the cells are stimulated in vivo, which means that their regeneration can be stimulated by the following mechanisms: (1) MSCs can differentiate directly in native cells. (2) The cells can experience paracrine effects due to the secretion of hormones and growth factors, rescuing the damaged tissue. (3) The cells can transfer mitochondria through the tunnel's nanotubes and secrete specific exosomes and microvesicles that influence the surrounding cells. Our results matched those of the above-mentioned authors because the mechanism of potential regeneration on a scaffold charged with MSCs can directly differentiate if the microenvironment is appropriate. According to the considered points in Figure 3C, in which we can observe the external joint implant with MSCs that were pre-differentiated to chondrocytes, the analysis in Table 2 showed no significant difference between the implants  
Figure 6: Characterization via immunofluorescence of external and internal implants after six months. The nuclei of each cell appear in blue (DAPI), and green markings indicate positivity for different cartilage markers (COL2A1, Aggrecan, and SOX9). The positive markers are labeled in green with fluorescein isothiocyanate (FITC). c. Implant with pre-differentiated MSCs. nc. Implant without pre-differentiated MSCs. for Sox9, Aggrecan, COL2A1, and the positive control (normal tissue), as mentioned in point 2: \"the cells can have paracrine effects due to the secretion of hormones and growth factors, rescuing the damaged tissue.\" This supports our results because, for 6 months, the implant remained external to the articulation, meaning that the surrounding microenvironment might have supplied nutrients to the implant, helping it to differentiate directly in similar tissue to the native cartilage. A similar effect occurred with the section of the implant with MSCs pre-differentiated to bone, 3 months after the lateral implant was placed in the sheep (see Table 1). On the other hand, in the implant inside the joint with MSCs that were pre-differentiated to chondrocytes, we could see the same behavior, because the implant was inside an articulation, with the most suitable environment in comparison to the external implant. We observed a significant difference that benefited the intraarticular implant, supporting the third statement: \"The cells can transfer and secrete specific exosomes and microvesicles that influence surrounding cells\" and the first statement: The MSCs can differentiate directly in native cells if they are stimulated\". In this instance, the MSCs benefitted the injured tissue [26]. The report also described the in vivo stimulation of the charged zones of articular cartilages that are exposed to contact pressures between 0 and 20 megapascals (MPa) in frequencies that run between 0.1 and 10 Hz [27]. According to these authors, the cyclic charging period rarely lasts more than 1 h and alternates with periods of partial recovery and total recovery of at least 14 h per day. Concurrently, [PERSON] et al. [28] mentioned that charges under 7 MPa do not show measurable cell death, and charges over 20 MPA result in prolonged structural tissue damage. Meanwhile, [PERSON] et al. [29] established that low-pressure levels are those that comprise 5 and 15 MPa, while high-pressure levels are between 20 and 50 MPa.