Figure 1.
Collagen I is concentrated at the surface of the somites.
(A and B) Frontal sections through the trunk of a 13 dpf trout embryo. (A) Posterior tail. Collagen I immunofluorescence localises to the anterior and posterior edges and at the lateral surface of the dermomyotome. (B) Anterior tail. Collagen I immunofluorescence is present along the space separating adjacent somites. der: dermomyotome. Scale bars in A and B, 15 μm.
Figure 2.
The intermyotomal space is colonized by fibroblast-like cells that do not express myogenic markers.
(A and B) Semithin frontal sections through the trunk of an eyed stage (17 dpf) embryo with final number of somites and pigmented eyes. (A) Posterior tail. Myoseptal cells are visible in the medial half (arrows) of the space separating two somites. (B) Anterior tail. Myoseptal cells are visible throughout the medio-lateral extent of the intermyotomal space. (C) Pax 7 expression is restricted to cells forming the dermomyotome-like epithelium at the surface of the myotome. (C′) Merged image showing Pax7 labeling and Hoechst staining for nuclei visualisation. (D) Myogenin expression is observed in the primary myotome located below the dermomyotome-like (D′) Merged image showing myogenin labeling and Hoechst staining. mc: myoseptal cells, m: myotome, der: dermomyotome. Scale bars in A, B, C and D, 25 μm.
Figure 3.
Myoseptal cells express genes involved in extracellular matrix production and remodelling.
(A–D) Frontal sections of eyed stage (17 dpf) embryos. Expression of (A) col1a1, (B) col5a2, (C) col12a1 and (D) Angiopoietin-7 like. mc: myoseptal cell; sc: skeletogenic cells; m: myotome; n: notochord. Scale bars, 20 μm.
Figure 4.
Apparent movement of col1a1 expressing cells suggests a sclerotomal origin of myoseptal cells.
(A) Transverse section of a 13 dpf embryo. Col1a1 staining is present in the ventrally located sclerotome cells (arrow). (B) Transverse section of a 14 dpf embryon. Labelled cells have migrated dorsally to surround the notochord (arrow). (C–E) Frontal section of trout embryo at the level of the notochord. (C) 14 dpf embryo. Labelled cells surround the notochord. (D) 15 dpf embryo. Some labelled cells occupy the medial aspect of the intermyotomal space. (E) 16 dpf embryo. The medial-lateral extent of the intermyotomal space contains labeled cells. (C′–E′) Merged images showing col1a1 labelling and Hoechst nuclear staining, white arrows indicate the intermyotomal space. n: notochord; m: myotome. Scale bars in A and B, 50 μm; C, C′, D, D′, E and E′, 30 μm.
Figure 5.
Skeletogenic cell specifically express osteoblast-specific factor2/periostin.
Eyed-stage (17dpf) embryo, frontal section. (A) Labelling is restricted to sclerotome-derived skeletogenic cells (sc) surrounding the notochord. (B) Merged images with nuclei stained with Hoechst to visualise myoseptal cells (ms). Scale bars in A and B, 20 μm.
Figure 6.
Expression of Scleraxis in myoseptal cells.
Eyed stage (17dpf) embryo. (A–B) whole mount in situ hybridization. (A) Lateral view. (B) Dorsal view. Scleraxis is localised at the anterior and posterior myotome borders. (C–F) Serial frontal sections through the trunk of an eyed stage trout embryo. (C) A section at the dorsal neural tube level shows few labelled myoseptal cells medially in the intermyotomal space. (D) A section at the ventral neural tube level shows that labelled myoseptal cells are visible within the medial half of the intermyotomal space. (E) Sections at the level of the notochord and (F) ventral to the notochord show that labelled cells are present throughout the medio-lateral extent of the intermyotomal space. (C′–F′) Merged images showing Scleraxis labelling and Hoechst nuclear staining; there is no labelling in skeletogenic cells immediately adjacent to the neural tube and notochord. nt: neural tube; n: notochord, mc: myoseptal cell; sc: skeletogenic cells. Scale bars in A, 300 μm; B, 200 μm Scale bars in C, C′, D and D′, 25 μm; E, E′, F and F′, 40 μm.
Figure 7.
Comparison of the early axial musculoskeletal system between amniote and fish.
Schematic frontal sections through trunk region of amniote (A) and fish embryos (B). (A) Amniote embryo. The sclerotome that derives from an important portion of the somite gives rise to chondroprogenitors surrounding axial structures (notochord and neural tube). Scleraxis expressing tendon progenitors originating from the dorsolateral edge of the early sclerotome lie between adjacent myotomes (adapted from [13]). (B) Fish embryo (17dpf). The sclerotome that represents a reduced somite derivative gives rise to a limited number of chondroprogenitors that express osteoblast-specific factor2/periostin. Adjacent myotomes are separated by Scleraxis-expressing myoseptal cells. A dermomyotome (white and red vertical lines) at the surface of the myotome persists late during fish development and expresses high levels of col1a1, col5a2 and col12a1.