Part 2 book “Muscles of chordates - Development, homologies, and evolution” has contents: Development of muscles of paired and median fins in fishes, pectoral and pelvic appendicular muscle evolution from sarcopterygian fishes to tetrapods, forelimb muscles of tetrapods, including mammals,… and other contnents.
15 Development of Muscles of Paired and Median Fins in Fishes In Chapter 15 we use the zebrafish Danio rerio as a case study to illustrate the development of the muscles of all the five types of fins (pectoral, pelvic, caudal, anal, and dorsal) covered in this book One reason is that D rerio is the only fish for which the development of the muscles of all these types of fins was studied in detail in the same project, namely, by ourselves and our colleagues (see the following text), which allows a better comparison between the ontogeny of all these fins Another reason is that, as noted in the preceding chapters, D rerio is one of the most popular model organisms in various fields of biological research, particularly developmental biology A significant percentage of evolutionary and developmental studies use this fish for evo-devo comparisons with different vertebrate taxa and for general discussions on the evolution of the appendages and even on paired fin–limb transitions that occurred during the origin of the tetrapod lineage (Zhang et al 2010; Yano et al 2012; Leite-Castro et al 2016; Nakamura et al 2016; Saxena and Cooper 2016) However, most of such studies are based on gene expressions and anatomical comparisons of the skeleton, usually not including details about soft tissues such as muscles (Nakamura et al 2016; Saxena and Cooper 2016) Accordingly, despite the common use of the zebrafish as a model organism for developmental works and discussions on both paired and median appendages, almost nothing is known about the development of the fin musculature in these fishes Patterson et al (2008) studied the growth of the pectoral fin and trunk musculature and looked at different fiber types that constitute the abductor and adductor muscles, but the differentiation of these muscles and development of other pectoral muscles were not studied by them in detail Cole et al (2011) provided a general discussion on the development and evolution of the musculature of the pelvic appendage, but their study was mainly focused on developmental mechanisms and migration of muscle precursors and not on specific muscles Thorsen and Hale (2005) did refer to specific muscles in their report on the development of the pectoral fin musculature of zebrafishes, but they omitted some muscles such as the arrector 3 Surprisingly, the development of the musculature of the median fins in the zebrafish has never been studied In order to tackle this scarcity of information on the development of the zebrafish appendicular musculature, we thus studied in detail and briefly describe in the following text, the ontogeny of each muscle in each fin from the time it first becomes visible until it displays a configuration basically similar to that of the adult stage (summarized in Table 15.1 and Figure 15.1) This work was performed and published with our colleagues Fedor Shkil and Elena Voronezhskaya (Siomava et al in press), and the following sections are mainly based on that paper, which should be consulted for more details on the specific methodology used in that work DEVELOPMENT OF THE PAIRED AND MEDIAN MUSCLES OF THE ZEBRAFISH The caudal fin is the first fin to develop in the zebrafish (Table 15.1) It appears as a continuation of the zebrafish postcranial axial skeleton and is surrounded by the caudal fin fold with the mesenchyme condensation ventrally, where the first caudal muscles and bones will later develop (Figure 15.2A) At stage 2.95 mm notochord length (NL), it is already associated with muscles By 3.30 mm NL it includes, two muscle masses—dorsal and ventral caudal muscles— are continuous with the trunk muscles (epaxialis and hypaxialis, respectively) They expand posteriorly to the tip of the tail (Figure 15.2A) Even though there is no clear border between these early caudal muscles and the trunk muscles, they can be distinguished from myotomes by the absence of segmentation At 4.4 mm NL, three new ventral muscles can be seen (Figure 15.2B and C) Myofibrils of the adductor caudalis ventralis and flexor caudalis ventralis, which at this time included both the flexor caudalis ventralis superior and inferior, start bifurcating from the ventral caudal muscle (Figure 15.2B) The flexor caudalis ventralis extends ventrally toward the caudal fin fold The adductor caudalis ventralis mainly follows the direction of the ventral caudal muscle but has shorter fibers that end halfway to the tip of the tail Several short muscle fibers of the lateralis profundus ventralis begin separating from the hypaxialis at this stage The adductor caudalis ventralis becomes more distinguishable but still keeps the direction of the ventral caudal muscle By 5.0 mm standard length (SL; tip of snout to posterior end of last vertebra or to posterior end of midlateral portion of hypural plate), when the notochord starts bending dorsally, both the adductor caudalis ventralis and flexor caudalis ventralis substantially increase in size (Figure 15.3A) The flexor caudalis ventralis is attached to the ventral rays The adductor caudalis ventralis becomes more separated from the ventral caudal muscle and changes the direction towards the dorsal fin rays At this stage, fibers of the lateralis profundus ventralis are relatively short and they not insert onto the caudal rays At stage 5.2 mm SL, the flexor caudalis dorsalis inferioris can be seen for the first time, arising deeply from the dorsal side of the ventral caudal muscle (Figure 15.3B) This flexor runs medial to the adductor caudalis ventralis, which is well developed by this stage, but does not insert onto the fin rays 321 322 Muscles of Chordates Pectoral fin Arrector ventralis Arrector-3 Abductor superficialis Abductor profundus Arrector dorsalis Adductor superficialis Adductor profundus Pelvic fin Abductor superficialis pelvicus Abductor profundus pelvicus Arrector ventralis pelvicus Arrector dorsalis pelvicus Adductor superficialis pelvicus Adductor profundus pelvicus Dorsal fin Inclinatores dorsales Erectores dorsales Depressores dorsales Protractor dorsalis Retractor dorsalis Anal fin Inclinatores anales Erectores anales Depressores anales Protractor anales Retractor anales Caudal fin Lateralis profundus dorsalis Flexor caudalis dorsalis superioris Flexor caudalis ventralis superior Flexor caudalis ventralis inferior Adductor caudalis ventralis Flexor caudalis dorsalis inferioris Ventral caudal muscle Lateralis profundus ventralis Interradialis caudalis Lateralis superficialis dorsalis Lateralis superficialis ventralis Interfilamenti caudalis dorsalis Interfilamenti caudalis ventralis 8.10 mm SL 7.50 mm SL 7.10 mm SL 6.70 mm SL 6.60 mm SL 6.40 mm SL 6.20 mm SL 6.00 mm SL 5.80 mm SL 5.60 mm SL 5.50 mm SL 5.40 mm SL 5.20 mm SL 5.00 mm SL 4.60 mm NL 4.40 mm NL 3.30 mm NL 3.15 mm NL Muscles/Stages 2.65 mm NL TABLE 15.1 Diagram of Development of Appendicular Muscles in the Zebrafish * * * * * ? ? Note: Arrows indicate development from another muscle Stars mark the stage when the adult muscle configuration is achieved “?” refers to the question of whether the ventral caudal muscle and/or the flexor caudalis ventralis inferioris contribute or not fibers to the adductor caudalis ventralis Shaded cells show stages when muscles have no attachment to fin rays as it does in later stages At stage 5.5 mm SL, new muscles appear via rearrangements of the previous ones Thus, the flexor caudalis ventralis splits into the large flexor caudalis ventralis superior and small flexor caudalis ventralis inferior, which inserts onto one ventral ray (Figure 15.3C) The dorsal caudal muscle breaks up into the flexor caudalis dorsalis superioris and lateralis profundus dorsalis overlying the former The lateralis profundus ventralis stretches closer to the fin rays During the growth of the tail, the ventral caudal muscle splits into superficial and deep layers (at 5.6 mm SL), which shift toward the midline Lastly, superficial fibers become reduced, while deep fibers increase in number, and now instead of inserting onto fin rays they insert onto proximal caudal bones and vertebrae (Figure 15.4A and B) At 6.4 mm SL, long and very thin fibers of the lateralis superficialis dorsalis are visible (Figure 15.4A), and the interradialis caudalis already connects the bases of the all three long dorsal rays (Figure 15.4B) The last muscles to develop are the lateralis superficialis ventralis, interfilamenti dorsalis, and interfilamenti ventralis The three muscles can be seen at 6.7 mm SL (Figure 15.4C) At this stage, basically all muscles are present and have a configuration that resembles the adult condition (see Figure 14.9 and Table 14.7) Interestingly, in addition to these muscles, in young specimens the space between the hypural bones was filled with muscle fibers (Figure 15.5) that then disappear before the adult stage, when this space becomes smaller We observed these fibers between hypurals 1–2, 2–3, and 3–4 from 5.0 to 7.1 mm SL, forming very thin muscles that we designate here as interhypurales Concerning the pectoral fins, they are already formed by 2.65 mm NL Our results and previous studies have shown 323 Development of Muscles of Paired and Median Fins in Fishes Total number of apeendicular muscles 30 20 10 Caudal muscles N L m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m SL m m N L L m m 4 m m N N L L m m L N m m m m N L N m m m m N L L N m m 95 m m N L Pectoral muscles Dorsal muscles Anal muscles Pelvic muscles FIGURE 15.1 Danio rerio (Teleostei): total number of appendicular muscles during different developmental stages NL, notochord length; SL, standard length that pectoral fin musculature start developing during early embryogenesis (Figure 15.6) and both abductor and adductor muscle masses differentiate as early as 2.8 mm NL (~46 hours post fertilization) Before (2.65-2.9 mm NL) and after (3.15 mm NL) hatching of larvae, we observe continuous fibers of the abductor and adductor masses (Figure 15.6) By 3.3 mm NL fibers extend to the edge of the endoskeletal disc and attach to actinotrichia (Figure 15.7) Along with the growth of the fin, the abductor and adductor extensively increase in size until 6.4 mm SL, when they split into deep and superficial layers (adductor profundus and superficialis; abductor profundus and superficialis) At 6.6 mm SL, a small bundle attached to the first pectoral ray starts separating from the abductor superficialis (Figure 15.8) This bundle later gives rise to both the arrector ventralis and arrector (Figure 15.9A) On the medial side, only one arrector (arrector dorsalis) develops, apparently from the adductor mass (Figure 15.9D) At 6.7 mm SL (Figure 15.9), all seven pectoral fin muscles are present and display the adult configuration (see Figure 13.11 and Table 13.1) We describe the dorsal and anal fin muscle development together here, because of the striking similarity of both their adult anatomy and ontogenetic development At 5.8 mm SL, muscle fibers appear in the region of several middle rays of the anal fin (Figure 15.10A) By 6.0 mm SL, these fibers elongate proximally and distally towards the body and fin rays respectively and myofibrils appear in more anterior and posterior serial units of the fin (Figure 15.10B) The dorsal fin musculature develops slightly later than the anal fin musculature, and by 6.0 mm SL, no muscle fibers can be seen (Figure 15.10B) By this stage, any structural rearrangements mainly occur in the same proximo-distal axis, but at 6.2 mm SL, muscle differentiation into deep and superficial layers is visible (Figure 15.11) The deep layer soon gives rise to the erectors and depressors of each ray (Figure 15.12) that are covered by the overlying superficial inclinators (Figure 15.12C) The development of muscles corresponding to different rays is asynchronous, resulting in muscle units in the dorsal and anal fins (i.e., including an inclinator, depressor, and erector going to both the left and right sides of each half ray on each side of the body, each ray therefore receiving six muscles in total), which are developed to a different extent at the same stage Thus, muscle units of the rays that are more central antero-posteriorly can have all six muscles differentiated into superficial-deep layers while the outermost rays may have undifferentiated developing muscle fibers or even single myofibrils (Figure 15.12) By 6.7 mm SL, development of each muscle unit is accomplished and the depressors partially overlie the erectors, which are subdivided into two small heads attached to the dorsal fin rays (Figure 15.13A) In addition to these units of six muscles going to each ray, there are also two longitudinal muscles that develop by 6.7 mm SL within each fin to move the first and last rays, which are named the protractor and retractor anales and the protractor and retractor dorsales (Figure 15.13) Therefore, by 6.7 mm SL, all dorsal and anal fin muscles are already present and have a configuration that resembles that seen in adults (see Figure 14.8 and Table 14.7) 324 Muscles of Chordates DOR Dorsal caudal muscle Epaxialis ANT POS VEN Ventral caudal muscle Hypaxialis 100 µm A DOR Epaxialis POS ANT Dorsal caudal muscle VEN Ventral caudal muscle Lateralis profundus ventralis Hypaxialis Flexor caudalis ventralis (superior and inferior) Adductor caudalis ventralis 50 µm B DOR Epaxialis Dorsal caudal muscle POS ANT VEN Ventral caudal muscle Lateralis profundus ventralis Hypaxialis Adductor caudalis ventralis Flexor caudalis ventralis (superior and inferior) 50 µm c (C) FIGURE 15.2 Danio rerio (Teleostei): early development of the caudal fin musculature At 3.3 mm NL, two muscles are present in the caudal fin (A) Ventral caudal muscles develop before the dorsal muscles At 4.4 mm NL, the first fibers of ventral caudal muscles can be seen (B) At 4.6 mm NL, ventral caudal muscles grow toward the caudal fin rays (C) 325 Development of Muscles of Paired and Median Fins in Fishes Epaxialis Ventral caudal muscle Dorsal caudal muscle Adductor caudalis ventralis DOR ANT Hypaxialis POS VEN Lateralis profundus ventralis Flexor caudalis ventralis (superior and inferior) 100 µm A DOR Epaxialis Flexor caudalis dorsalis inferioris Hypaxialis Flexor caudalis ventralis (superior and inferior) ANT Ventral caudal muscle POS VEN Adductor caudalis ventralis 100 µm B Lateralis profundus dorsalis Ventral caudal muscle Flexor caudalis dorsalis superioris Adductor caudalis ventralis Flexor caudalis dorsalis inferioris Flexor caudalis ventralis superior DOR ANT POS VEN Lateralis profundus ventralis Flexor caudalis ventralis inferior 100 µm C FIGURE 15.3 Danio rerio (Teleostei): by 5.0 mm SL, ventral caudal muscles reach the caudal fin rays (A) and development of the deep dorsal fin muscles starts (B and C) The flexor caudalis dorsalis inferioris can be seen at 5.2 mm SL (B), and the flexor caudalis dorsalis superioris appears at 5.5 mm SL (C) The ventral caudal muscle is still attached to the dorsal caudal fin rays, and the flexor caudalis ventralis splits into superior and inferior portions (C) 326 Muscles of Chordates Lateralis profundus dorsalis Lateralis superficialis Flexor caudalis dorsalis dorsalis superioris Flexor caudalis dorsalis inferioris Adductor caudalis ventralis Flexor caudalis ventralis superior DOR ANT Ventral caudal muscle A Lateralis profundus Flexor caudalis ventralis ventralis inferior POS VEN 250 µm Flexor caudalis Lateralis profundus Interradialis dorsalis dorsalis caudalis superioris Flexor caudalis DOR dorsalis inferioris ANT POS VEN Adductor caudalis ventralis Flexor caudalis ventralis superior Ventral caudal Lateralis profundus muscle ventralis Flexor caudalis ventralis inferior 250 µm B Lateralis profundus Flexor dorsalis Interradialis caudalis dorsalis caudalis (inferioris and superioris) Adductor caudalis ventralis Inerfilamenti caudalis dorsalis Inerfilamenti caudalis ventralis DOR ANT POS VEN Lateralis profundus ventralis Flexor caudalis ventralis (inferior and superior) 250 µm C FIGURE 15.4 Danio rerio (Teleostei): at 6.4 mm SL, caudal muscles lateralis superficialis dorsalis (A) and interradialis caudalis (B) are formed The ventral caudal muscle is shifted backward and attaches to the proximal caudal bones and vertebrae At 6.7 mm SL, interfilamenti caudalis dorsalis and ventralis are formed and thus all caudal muscles are present (C) 327 Development of Muscles of Paired and Median Fins in Fishes DOR ANT Interhypurales POS VEN Hypural Hypural Hypural Hypural Hypural Hypural Interhypurales Interhypurales Hypural 50 µm 100 µm A B FIGURE 15.5 Danio rerio (Teleostei): deep interhypural fibers at 6.0 mm SL Interhypurales were present between the hypural bones of the caudal fin from 5.00 to 7.1 mm SL Panel (B) shows a high magnification of the rectangle outlined in the main image (A) DOR ANT POS VEN Endoskeletal disc Abductor Adductor 100 µm FIGURE 15.6 Danio rerio (Teleostei): undifferentiated abductor and adductor muscle masses of the zebrafish pectoral fin at 3.15 mm NL The pelvic fins are the last to develop in the zebrafish (Table 15.1) Their buds become visible after 6.7 mm SL At 7.1 mm SL each pelvic fin already has three differentiated muscle masses (Figure 15.14): the undifferentiated abductor and adductor consist of long thin muscle fibers that stretch proximo-distally along the fin for approximately one-third of its length (Figure 15.14B), and in addition there is also an arrector ventralis pelvicus (Figure 15.14A) Notably, fibers of the arrector dorsalis pelvicus cannot be seen until 7.1 mm SL The growth of the muscles proceeds quickly, and at 7.5 mm SL, both arrectors are well developed and attach to the base of the first ray (Figure 15.15A) The abductor and adductor muscle masses differentiate into the deep and superficial layers (abductor superficialis and profundus pelvicus; adductor superficialis and profundus pelvicus), which are still difficult to recognize at this stage (Figure 15.15B) By 8.1 mm SL (Figure 15.16), all pelvic muscles are clearly present and have an adult configuration (see Figure 13.14 and Table 13.2) 328 Muscles of Chordates DOR Endoskeletal disc Abductor ANT POS VEN Abductor Adductor Adductor DOR ANT POS VEN 50 µm 100 µm A B Adductor MED ANT POS LAT Abductor 50 µm C FIGURE 15.7 Danio rerio (Teleostei): abductor and adductor masses form two muscle layers, each, in the zebrafish pectoral fin Lateral view (A) and dorsoventral (B) and anteroposterior (C) cross sections showing that abductor and adductor muscles extend to the edge of the endoskeletal disk and form two muscle layers at 3.3 mm NL DOR ANT POS VEN Arrectors (ventralis and 3) Abductor superficialis Ray 150 µm FIGURE 15.8 Danio rerio (Teleostei): ventral arrector complex of the pectoral fin in the zebrafish at 6.6 mm SL The ventral arrector complex will later give rise to the arrector ventralis and arrector DEVELOPMENTAL AND EVOLUTIONARY UNIQUENESS OF THE CAUDAL FIN As previously noted, unlike other fins, which are functionally and developmentally distinct structures locally connected to the body, the caudal fin is mainly a posterior continuation of the trunk and of the vertebral column in particular It has been suggested that such a peculiar position and association with the posterior elements of the postcranial axial skeleton make this fin developmentally and evolutionary distinct from other 329 Development of Muscles of Paired and Median Fins in Fishes Arrector ventralis Ray Ray DOR ANT POS VEN Arrector-3 DOR ANT POS Abductor supeficialis Abductor supeficialis Deep fibers of the abductor profundus 100 µm VEN A 100 µm C Ray DOR Ray Ray ANT POS VEN Arrector dorsalis Adductor profundus Abductor supeficialis DOR ANT Deep fibers of the adductor superficialis Abductor supeficialis overlaps with the abductor profundus POS VEN 100 µm B 100 µm D FIGURE 15.9 Danio rerio (Teleostei): all the muscles of the ventral/abductor (A and B) and dorsal/adductor (C and D) masses of the zebrafish pectoral fin are developed by 6.7 mm SL fins (Quint et al 2002; Agathon et al 2003) Our observations and comparisons provide additional evidence supporting this idea From very early development, the caudal fin is supported by musculature, while other fins appear as relatively simple homogeneous structures—fin folds or fin buds—that grow and acquire muscles much later in development (exceptionally, pectoral fins develop muscles during embryogenesis: see above) (see Table 15.1) In contrast to such gradual development, the caudal fin at 2.95 mm NL already has two muscles (the dorsal and ventral caudal muscles) that differ from the trunk muscles by their orientation and composition (i.e., absence of myomeric pattern) (Figure 15.2A) Interestingly, even though the caudal fin is the first fin to appear in the zebrafish, it does not reach the adult configuration before other fins do, probably because of its complexity as it includes more muscles and skeletal elements than any other fin (Table 15.1) That is, the caudal fin develops gradually along with fish growth and the adult muscle configuration of the caudal fin is attained at a similar developmental stage as in the pectoral, dorsal, and anal fins (i.e., by about 6.7 mm SL) (Figure 15.4C) Another distinguishing feature of the caudal fin muscles is the proximal shift during development, that is, away from the fin rays While muscles of all other fins mainly grow toward the rays and insert onto their bases, the position of the ventral caudal muscle changes from more dorsal to more medial and later the connection between this muscle and the caudal fin rays is lost (Figures 15.3C and 15.4A) In adult zebrafishes, the ventral caudal muscle becomes a deep trunk muscle, attached to the caudal vertebrae and proximal caudal bones This particular muscle rearrangement along with the intensive growth of the ventral caudal muscles and notochord bending (i.e., the adductor caudalis ventralis and flexor caudalis ventralis superior and inferior) results into the peculiar marked dorsoventral asymmetry of the caudal fin, which initially was mainly symmetrical (until 4.4 mm NL: Figure 15.2A) The presence of the temporary interhypural muscles on the dorsal side only, development of the lateralis superficialis dorsalis before the lateralis superficialis ventralis, and appearance of the interradialis dorsalis at 6.4 mm SL enhance the difference between the dorsal and ventral sides of this fin Thus, what appears (externally) to be a dorsoventral symmetrical caudal fin, with roughly equal dorsal and ventral lobes and somewhat evenly distributed fin rays, is a fin with muscles that display a marked dorsoventral asymmetry (Schneider and Sulner 2006; see Chapter 14 and Figure 14.9) 330 Muscles of Chordates DOR ANT POS VEN Hypaxialis First myofibrils 100 µm (A) DOR ANT POS VEN Epaxialis Hypaxialis First myofibrils 100 µm (B) FIGURE 15.10 Danio rerio (Teleostei): early development of the dorsal and anal fin musculature The first muscle fibers are seen in the anal fin at 5.8 mm SL (A) Number of fibers and serial units with the muscle fibers increases by 6.0 mm SL (B) ... Fl pol br and dig, + oppo pol and di mi + int palm + part of int dor – (part of fl dig profundus) Pronator quadratus – (part of fl dig profundus) – – (part of fl dig profundus) Part of biceps... phylogeny and ontogeny was also reported in the head muscles of zebrafish (Diogo et al 20 08c) as well in studies of muscles of other vertebrate 335 Development of Muscles of Paired and Median... comparisons of pelvic and pectoral appendages among these and other fishes and in tetrapods, leading to proposal of new names, evolutionary origins, and one-to-one homology hypotheses for all muscles of