viernes, 19 de septiembre de 2014

The growth of fishes

Post-embryonic increase of muscle mass takes place via two distinct mechanisms called hypertrophy and hyperplasia (Johnston, 1999; Rowlerson and Veggetti, 2001). Hypertrophy is the increase in muscle fibre diameter while hyperplasia is the increase in muscle fibre number (recruitment of new muscle fibres). Teleosts recruit muscle fibres throughout the juvenile phase and into adulthood until a certain body size (Weatherley et al., 1988; Kiessling et al, 1991; Zimmerman and Lowery 1999; Johnston et al., 2000c, 2002, 2003a, 2003b), which is different from mammals where fibre numbers are fixed at birth (Rowe and Goldspink, 1969; Stickland, 1981). However, Artic char (Salvelinus alpinus) slow muscle fibre recruitment 
continues after fast fibre number is established (Johnston et al., 2004a).  Fish with a small final body size tend to stop recruiting muscle fibres at an earlier stage compared to fish with a large ultimate body size (Weatherley et al., 1988). Since the majority of the slow and fast muscle fibres seldom achieve diameters of more then 50 and 240 µm respectively (Weatherley at al., 1988; Johnston et al., 2000a), fish with a large body size recruit fast muscle fibres for a prolonged period to achieve a large size. For example, a 0.2 kg halibut has ~320000 muscle fibres, while a halibut on 96 kg has ~1.7 million fibres (> 4.3 fold increase, Hagen et al., 2008a). In contrast, the notothenoids (sub-Antarctic family) recruit a modest number of fast 29 fibres, but compensate by growing unusually large muscle fibres (500-650 µm) allowing them 
to reach a relatively large final size (Johnston et al., 2003b). Hyperplastic growth is divided into two distinct phase’s occurring at different stages of the fish life cycles, referred to as stratified (Fig. 1.8 A) and mosaic hyperplasia (Fig. 1.8 B) (Rowlerson and Veggetti, 2001). Stratified hyperplastic growth is a process where new muscle fibres are continuously recruited through the early phases of ontogeny, in a germinal 
zone located in the periphery of the myotome, just beneath the superficial layer of slow muscle fibres (Fig. 1.8 A). These germinal zones are the primary source for new fibres recruitment throughout the late embryonic and larval stage (Rowlerson and Veggetti, 2001) and has been described in the primary Norwegian aquaculture species, cod (Gadus morhua) (Galloway et al., 1999a), halibut (Hippoglossus hippoglossus) (Galloway et al., 1999b) and salmon (Salmo salar) (Johnston and McLay, 1997) including several other species as well (Brooks and Johnston, 1993; Gibson and Johnson, 1995; Johnston et al., 1998; Johnston et al., 2003a). 
The timing of stratified hyperplasia varies between species and occurs at different developmental stages. For example, during first feeding of cod larvae (Gadus morhua) the number of newly recruited fibres increased significantly in dorsal, ventral and lateral germinal zones of the fast muscle, triggered by endogenous feeding (Galloway et al., 1999a) and was found to be the dominant contributor of muscle growth.
Thus, at 13 mm (230 degree days) the contribution of hyperplasia to muscle growth was only estimated to be 4-6% of total fast muscle cross-section area, indicating that hypertrophy was the dominant contributor of myotomal growth during the larval stage (Galloway et al., 1999b). 
When the germinal zones are depleted in the late larval stage, mosaic hyperplasia takes over as the main mechanism of muscle recruitment (Rowlerson and Veggetti, 2001). In herring (Clupea harengus) the onset of mosaic hyperplasia overlaps with the stratified hyperplasia growth phase (Johnston et al., 1998), an observation made for other species as well (Brooks and Johnston, 1993; Johnston and McLay 1997). During the mosaic hyperplastic growth phase newly recruited muscle fibres are scattered throughout the entire myotome, giving a mosaic appearance with intermingled muscle fibres of different diameters (Fig. 1.8 B). The mosaic hyperplastic contribution is highest during the early juvenile stages and the percentage 
of small muscle fibres gradually decline with size. Weatherley et al. (1988) revealed that the mosaic hyperplastic growth phase came to a halt at 44% of final body length, and further growth is only achieved by hypertrophy (Stickland 1983; Weatherley et al., 1988; Veggetti et al., 1990; Johnston et al., 2000a). In contrast, growth dynamics of the white seabass (Atractoscion nobilis) showed that muscle fibre recruitment did not terminate until 74% of ultimate body size (Zimmerman and Lowery, 1999)
The myogenic progenitor cells are characterised as small circular shaped cells containing a heterochromatic nucleus with few organelles and are located between the sacrolemma and the basal lamina (Koumans and Akster, 1995). When activated (which a wide range of growth factors and transcript factors are identified being involved in) the fate of the myogenic progenitor cell is decided and the myogenic progenitor cells differentiate into a myoblasts. The myoblast further undergoes a proliferation and is then either absorbed into an existing muscle fibre, or fuses with other myoblasts to produce a myotube which matures into a muscle fibre (Koumans and Akster, 1995; Watabe, 1999; Johnston, 2001)

No hay comentarios:

Publicar un comentario