This protocol describes the isolation of satellite cells from branchiomeric head muscles of a 9 week-old rat. The muscles originate from different branchial arches. Subsequently, the satellite cells are cultured on a spot coating of millimeter size to study their differentiation. This approach avoids the expansion and passaging of satellite cells.
Fibrosis and defective muscle regeneration can hamper the functional recovery of the soft palate muscles after cleft palate repair. This causes persistent problems in speech, swallowing, and sucking. In vitro culture systems that allow the study of satellite cells (myogenic stem cells) from head muscles are crucial to develop new therapies based on tissue engineering to promote muscle regeneration after surgery. These systems will offer new perspectives for the treatment of cleft palate patients. A protocol for the isolation, culture and differentiation of satellite cells from head muscles is presented. The isolation is based on enzymatic digestion and trituration to release the satellite cells. In addition, this protocol comprises an innovative method using extracellular matrix gel coatings of millimeter size, which requires only low numbers of satellite cells for differentiation assays.
About 1:500 to 1:1,000 newborns exhibit a cleft involving the lip and/or palate (CLP); thus this is the most common congenital malformation in humans1. The muscles of the soft palate are critical for the functioning of the soft palate during speech, swallowing, and sucking. If a cleft of the soft palate is present, these muscles are abnormally inserted into the posterior end of the palatal bone.
The soft palate moves up and down during speech, preventing air to escape through the nose. Children with a cleft in the palate do not have this control function resulting in a phenomenon known as velopharyngeal dysfunction2,3. Although the treatment protocols are variable, surgical repair of the soft palate takes place in early childhood (6-36 months of age)4. The abnormally inserted muscles of the soft palate can be surgically corrected5-7, however, velopharyngeal dysfunction persists in 7% to 30% of the patients2,3,8-10.
The ability of skeletal muscle to regenerate through the action of satellite cells (SCs) is well established11,12. Upon muscle injury, SCs are activated and migrate to the site of injury. They then proliferate, differentiate, and fuse to form new myofibers or repair damaged ones13. Quiescent SCs express the transcription factor Pax714,15, while their progeny, the proliferating myoblasts, additionally express the myogenic determination factor 1 (MyoD)16. Differentiating myoblasts start to express myogenin (MyoG)17. The terminal differentiation of myoblasts is marked by the formation of myofibers, and the expression of muscle-specific proteins such as myosin heavy chain (MyHC)16,18.
Recently, several strategies have been used in regenerative medicine to improve muscle regeneration of limb muscles19-23. Specific studies on branchiomeric head muscles are also important because it was recently demonstrated that they differ from other muscles in several aspects24. In contrast with limb muscles, it has been suggested that branchiomeric head muscles contain less SCs25, regenerate slower, and more fibrous connective tissue is formed after injury26 In addition, proliferating SCs from branchiomeric head muscles also express other transcription factors. For instance, Tcf21, a transcription factor for craniofacial muscle formation is strongly expressed in regenerating head muscles but hardly in regenerating limb muscles25. The muscles in the soft palate of CLP patients are usually smaller and less well-organized compared to normal palatal muscles27,28. Slow and fast fibers are both present in the soft palate muscles but the slow fibers are more abundant. In contrast, cleft muscles contain a higher proportion of fast fibers and also a reduced capillary supply compared with normal soft palate muscles29-31. Fast fibers are more prone to contraction-induced injury31-33. The accompanying poor capillary supply may also promote fibrosis34,35. All these aspects may contribute to the poor regeneration of soft palate muscles after surgical cleft closure36. In view of this, a protocol for the isolation and characterization of branchiomeric head muscle SCs is crucial. This provides the possibility to study SC biology of branchiomeric head muscles. In addition, new therapies based on tissue engineering can be developed to promote muscle regeneration after surgery in CLP and other conditions compromising the craniofacial area.
In general, SCs can be obtained after dissociation of muscle tissue14. Mincing, enzymatic digestion, and trituration are generally required to release SCs from their niche. SCs can be purified by pre-plating on uncoated dishes14,37,38, fractionation on Percoll39,40, or fluorescent- or magnetic cell sorting41-43. Here we present a new economic and rapid protocol for the isolation of satellite cells from branchiomeric head muscles of young adult rats. This protocol is based on a previous manuscript14 and specifically adapted for small tissue samples. The isolation of SCs from representative muscles originating from the 1st, 2nd, and 4th branchial arches are described. After isolation, low numbers of satellite cells are cultured on extracellular matrix gel spots of millimeter size to study their differentiation. This approach avoids the requirement for the expansion and passaging of SCs.
All experiments described herein were approved by the local Board for Animal Experiments from the Radboud University Nijmegen in accordance with Dutch laws and regulations (RU-DEC 2013-205).
1. Extracellular Matrix Gel Spots
2. Dissection of Head Muscles (Masseter, Digastric, and Levator Veli Palatini)
3. Isolation of Satellite Cells
4. Differentiation of Satellite Cells on Extracellular Matrix Gel Spots
Using this protocol, the masseter muscle (one side) yields 0.8-1 x 106 cells, the digastric muscle (posterior belly) yields 1.5-2 x 105 cells, and levator veli palatini muscle yields 1-1.5 x 105 cells. Cell yields depend on the muscle type, strain, and age of the animal. For comparison between the three muscle groups, freshly isolated SCs were seeded at the same cell density (1.5 x 103/10 µl). Directly after isolation, more than 90% of the freshly isolated cells express Pax7 (Figure 6).
Day 4, 7 and 10 cultures were stained with antibodies against Pax7, MyoD, MyoG and MyHC immunostaining. Five arbitrary fields were counted per culture using a 20X objective. At day 4 Pax 7 and Myo D is expressed in all muscle groups (Figures 6 and 7 and 8), however the progeny of SatCs from the masseter and digastric muscles start expressing myogenin earlier than the levator veli palatini muscle (Figure 9). At day 10, the expression of MyoG is strongly reduced in all groups (Figure 9). A few days after seeding on the extracellular matrix gel spots, the proliferating cells begin to fuse and form multi-nucleated myotubes, which express myosin heavy chain. Small myotubes are clearly visible at day 7 (Figure 10). At day 10, twitching of the myotubes can be observed (Video 1).
Figure 1: Extracellular matrix gel spots in a chamber slide. (A) For easy manipulation, place the 8-well chamber slide into a 100 mm Petri dish. Pipet 10 µl extracellular matrix gel in each chamber and put it on a cold surface (7 min). (B) Chamber slide after the excess extracellular matrix gel is removed.
Figure 2: Dissection of the masseter muscle. (A) Head of the animal in a lateral view. Ear (E), Parotid gland (P) and facial nerve (VII). (B) Tendinous aponeurosis (Te) of the superficial head of the masseter muscle (Ms) and temporal muscle (T). Separate the tendon from its insertion with a forceps. (C) Carefully dissect the muscle until its insertion at the ramus of the mandible. E: ear, P: parotid gland, VII: facial nerve, T: Temporal muscle, Ms: superficial head of the masseter muscle, Te: tendon, Mp: deep head of the masseter muscle.
Figure 3: Dissection of the posterior belly of the digastric muscle. (A) Head of the animal in a supine position. Localize the submandibular gland (Sg), masseter muscle (M), facial nerve (VII) and sternocleidomastoid muscle (SCM). Remove the submandibular gland. (B) Localize the digastric muscle anterior (AD) and posterior belly (PD). With a straight forceps, take the anterior tendon of the posterior belly, cut it and dissect it carefully until its origin in the tympanic bulla (ty). E: ear, Sg: submandibular gland, VII: facial nerve, M: masseter muscle, SMC: sternocleidomastoid muscle, AD: anterior belly digastric muscle, PD: posterior belly digastric muscle, Ty: Tympanic bulla.
Figure 4: Dissection of the levator veli palatini muscle. (A) General view after dissection of the digastric muscle (posterior belly). Stylohyoid muscle (St) and tendon of the levator veli palatini can be localized. Note the trachea (T) and esophagus (Es) running behind it. (B) After lifting the trachea and the esophagus the pharynx (P) is exposed. The levator veli palatini that runs laterally towards the soft palate is now visible. The arrow indicates the dissected superior pharyngeal constrictor muscle; note the levator veli palatini muscles at both sides. E: ear, St: stylohyoid muscle, VII: facial nerve, M: masseter muscle, AD: anterior belly digastric muscle, PD: posterior belly digastric muscle, T: trachea, Es: esophagus, P: Pharynx, *levator veli palatini muscle.
Figure 5: Appearance of the muscle tissue (A) before and (B) after enzymatic digestion with pronase. Note that muscle bundles appear to be loosened after enzymatic digestion.
Figure 6: Pax 7 immunostaining. Freshly isolated SCs, applied to extracellular matrix gel at the end of isolation (about 6 hours after initial tissue digestion). Five arbitrary fields were counted using a 10X objective with an average of 210 cells per field. Approximately 90% of the cells are Pax 7 positive. DAPI: blue, Pax7: red. Scale bar, 100 µm.
Figure 7: Pax 7, MyoD immunostaining. Day 4, 7 and 10 cultures were stained with antibodies against Pax7, and MyoD immunostaining. (A–C) and (D–F) Representative photomicrographs of day 4 and 7 cultures from the masseter muscle. (G and H) The number of Pax7+ and MyoD+ nuclei per microscopic field was counted and expressed as a percentage of the total number of nuclei (DAPI). DAPI: blue, Pax7: red, and MyoD: green. Scales bar, 100 µm. Please click here to view a larger version of this figure.
Figure 8: Distribution of Pax7±/MyoD± in cultures from mononucleated cells in cultures from masseter, digastric and levator veli palatine muscle. (A–C) Day 4, 7 and 10 cultures were stained with antibodies against Pax7, and MyoD immunostaining. The total number of cells is based on of the total number of nuclei (DAPI). (D) Data quantification of Pax7±/MyoD± cells. Please click here to view a larger version of this figure.
Figure 9: Myogenin immunostaining. Day 4, 7 and 10 cultures were stained with antibodies against Myogenin. (A–D) Representative photomicrographs of day 4 and 7 cultures from the levator veli palatine muscle. (E) The number of MyoG+ nuclei per microscopic field was counted and expressed as a percentage of the total number of nuclei (DAPI). (F) Data quantification of MyoG+ cells. DAPI: blue, Myogenin: green. Scales bar, 100 µm. Please click here to view a larger version of this figure.
Figure 10: Myosin Heavy Chain immunostaining. Day 4, 7 and 10 cultures were stained with antibodies against myosin heavy chain (MyHC). Representative photomicrographs of day 4, 7 and 10 cultures from the digastric (DIG) muscle. At day 7, small myotubes are present while at day 10 long and well-organized myotubes are evident. Scales bar, 200 µm. Please click here to view a larger version of this figure.
Video 1: Myotube twitching. Examples of two representative fields with twitching myotubes are shown for day 10 cultures from digastric muscle. Please click here to view this video.
SCs from different branchiomeric head muscles were isolated from one 9-week-old Wistar rat and cultured directly on extracellular matrix gel spots without prior expansion and passaging. After isolation, the cells were counted and seeded at the same cell density. For the parallel isolation of three different muscles, this method takes about 4 hr. To avoid culture contamination, a critical step is the rapid washing in alcohol 70% after dissection of the muscles.
During SC isolation it is important to cut the muscle tissue into small pieces (about 2 mm) but avoid too much mincing as this will result in a small cell yield because of cell damage. Also, the duration of the enzymatic digestion must be checked carefully under the microscope to avoid further damage. The aim of the digestion is to dissociate the myofibers. Since more than 90% of the isolated cells express Pax7, no further purification is required (Figures 6-8). This avoids extra purification steps in other methods such as pre-plating on uncoated dishes14,37,38, fractionation on Percoll39,40, or fluorescent- or magnetic cell sorting41,43. For trituration it is essential to induce shear between the tissue fragments and the opening of the pipette tip as this allows the mechanical release of the SCs. If the trituration with a 10 ml pipette (inside diameter tip: 1 mm) is difficult, a 5 ml (inside diameter tip: 2 mm) pipette can be used first. Alternatively, glass Pasteur pipettes can be cut at the desired diameter and be used. This method is simple, efficient and allows the simultaneous isolation of SC from different muscle samples.
The culture plates for SCs can also be coated with gelatin or collagen, but our previous studies show that extracellular matrix gel is far better for the maintenance of the myogenic potential than collagen38. The extracellular matrix gel spots of millimeter size (10 µl/Ø 2 mm or 20 µl/Ø 4 mm) allows the study of proliferation and differentiation of SCs with limited numbers of cells. For the differentiation assay about 8 to 20 times fewer cells are required compared to a 24-well plate (Ø 15.6 mm), and about 80 to 200 times fewer compared to 35 mm Petri dishes (Ø 35 mm)14,38.
Since extracellular matrix gel is expensive, this method is also more cost-efficient. In addition, the chamber slides can be replaced by plastic cover slips to further reduce the costs. For the preparation of the extracellular matrix gel spots overnight drying of the chamber slides is essential. As the extracellular matrix gel spots are transparent, it is necessary to mark the spots at the bottom side using back lighting. The chambers slides are fixed in a Petri dish for easy manipulation. Further cell culture expansion is not necessary, which offers the possibility to study the SCs of smaller muscles or small muscle samples. Alternatively, e.g. for PCR or muscle constructs if more cells are needed, the freshly isolated SCs can first be expanded in T75 flasks as indicated above.
SCs isolated using this protocol are not suitable for further purification with flow cytometry immediately after isolation. The digestion with pronase causes extensive digestion of the surface antigens14. Horse serum and fetal bovine serum that are used for cell culture must first be properly characterized before isolation, as different lot numbers differentially affected myoblasts proliferation and differentiation.
In recent years, there is a growing interest in the muscles derived from the branchial arches and the head mesoderm (e.g. the extraocular muscles)24. It has been clearly demonstrated that head and limb muscles possess highly different properties. Masseter muscle from old animals seems to retain their regenerative capacity in comparison with limb muscles25,26. SCs from the extraocular muscles possess a robust proliferation and differentiation capacity comparable to SCs from head muscles, and show a larger engraftment potential than limb muscle SCs24.
The fiber type distribution and myosin composition varies among muscle groups and also between species. Muscles originating from the first branchial arch in humans contain both slow and fast fibers (subtypes IIA and IIX), neonatal myosins and myosins typical for developing cardiac muscle. In rodents these muscles contain about 95% fast fibers myosin IIA and IIb)44-46. Studies on avian muscles show that SCs from different muscle fiber types vary in differentiation capacity. SCs from fast fibers only differentiate into fast muscle fibers, while SCs from slow fibers can differentiate into both fiber types47. In addition, the percentage of SCs in fast muscle fibers is lower than in slow muscle fibers48,49. This indicates that the fiber type distribution must be taken into account for studies on muscles in the craniofacial area. Similar to cleft palate muscles, the LVP in rodents contains almost exclusively fast fibers50. For that reason, SCs from the LVP are suitable for pre-clinical studies in the field of cleft palate.
This protocol offers new possibilities to study SCs derived from branchiomeric head muscles or other smaller muscles or smaller muscles samples. This will facilitate the development of new therapies to improve the regeneration of muscles in the maxillofacial area in conditions such as cleft palate but also in other conditions affecting smaller muscles.
The authors have nothing to disclose.
This study was funded by a Mosaic grant (017.009.009) from The Netherlands Organization for Scientific Research (NWO) and a Start grant (S-13-167C) for young investigators from the AO Foundation. Z.Y.R is supported by the National Institutes of Health (grant # AG021566, AG035377, NS090051).
Hypodermic Needle 25G 0,5x25m | BD Microlance | 300400 | |
Dissecting scissors | Braun | BC154R | |
Micro forceps straight | Braun | BD330R | |
Surgical Scalpel Blade No.15 | Swann-Morton | 0205 | |
Alcohol 70% | Denteck | 2,010,005 | |
Permanox Slide, 8 Chamber | Thermo Scientific | 177445 | |
6 well cell culture plate | Greiner bio-one | 657160 | |
Cell Culture Dishes (100 x 20 mm) | Greiner bio-one | 664160 | |
15 ml sterile conical centrifuge tube | BD Biosciences | 352097 | |
50 ml sterile conical centrifuge tube | BD Biosciences | 352098 | |
Cell strainer (40 μm) | Gibco | 431750 | |
10 mL serological pipette | Greiner bio-one | 607180 | |
20µL FT20 | Greiner bio-one | 774288 | |
Matrigel, Phenol-Red Free | BD Biosciences | 356237 | 10 mL |
Pronase | Calbiochem | 53702 | 10KU |
Phosphate Buffered Saline | Gibco | 14190-144 | 500 mL |
Dulbecco's Modified Eagle Medium, high glucose, GlutaMAX Supplement, pyruvate | Gibco | 10569-010 | 500 mL |
Fetal Bovine Serum | Fisher Scientific | 3600511 | 500 mL |
Horse Serum | Gibco | 26050088 | 500 mL |
Penicillin-Streptomycin (10,000 U/mL) | Gibco | 15140-122 | 100 mL |
Chicken Embryo Extract | MP Biomedicals | 2850145 | 20 mL |