Muscular senescence in cetaceans: adaptation towards a slow muscle fibre phenotype

Abstract

Sarcopenia, or senile muscle atrophy, is the slow and progressive loss of muscle mass with advancing age that constitutes the most prevalent form of muscle atrophy. The effects of ageing on skeletal muscle have been extensively studied in humans and laboratory animals (mice), while the few reports on wild animals are based on short-lived mammals. The present study describes the age-related changes in cetacean muscles regarding the three factors that determine muscle mass: fibre size, fibre number, and fibre type. We show that the skeletal muscle fibres in cetaceans change with advancing age, evolving towards a slower muscle phenotype. We suggest that this physiological evolution constitutes an adaptation that allows these marine mammals to perform prolonged, deep dives.

Figure 1. Gross appearance of senile and cachectic specimens.

(A): Female juvenile specimen of Steno bredanensis that had been undergoing a consumptive process, showing severe emaciation characterised by the protuberance of the axial bones (arrowheads). (B): Female senile specimen of Stenella frontalis (Case N. 10) that had not been undergoing consumptive process, showing a moderate to good external body condition. Older Atlantic spotted dolphins were identified mainly by their spot pattern (as the animals age, spots develop on both the ventral and dorsal surfaces).

Figure 2. Histopathological study.

(A): Cross section of a muscle from a senile spotted dolphin specimen. The muscle exhibits a variety of abnormal features consisted of an increased variations in the fibre sizes and longitudinal splitting (*). Immunostaining for the fast MHC isoform. 20×. (B): Longitudinal section of a muscle from a senile spotted dolphin specimen. The muscle exhibits a large amount of juxtanuclear lipofuscin and significant vascularisation within the fibres. PAS. 20×. 1: Lipofuscin appears as a faint yellow pigment, mostly adjacent to the nucleus. HE. 20×. 2: An EM image showing the lipofuscin accumulation was located mostly in the mitochondria-rich subsarcolemmal area. Bar = 1 μm.

Figure 3. Immunohistochemical stain for the fast MHC antibody showed differences within fibre type involved and alterations in fibre type distribution between adult and senile cetaceans.

10×. (A): Juvenile specimen of striped dolphin. Muscle cross section. The muscle contains two clearly different fibre type populations, forming a mosaic-like pattern, in which type II (red-stained fibres) are much larger and more numerous than are the type I fibres (blue-stained fibres). The shape and size of the fibres appear normal, with almost all of fibres having an angular shape. (B): Senile specimen of striped dolphin. Muscle cross-section. A larger population of type I fibres is observed, which tend to appear in clusters. The muscle contains fibres with different structural irregularities, including the “flattened” fibre. (C): Adult specimen of Atlantic spotted dolphin. Muscle cross section. The muscle exhibits the above-described features for juvenile specimens of the striped dolphin. (D): Adult-senile specimen of Atlantic spotted dolphin. Muscle cross-section. In senile muscles, both fibre types have a similar fibre size, and a higher proportion of type I fibres is present. Fibres tend to form clusters of one type of fibre, which leads to a deviation from the normal mosaic arrangement of fibres within the muscle fascicle than is observed in the younger animals.

Figure 4. Mean cross sectional area (μm²) (±SEM) of Type I and Type II fibres in the adult and senile groups.

Asterisks denote significant differences across age groups of spotted dolphins for type I fibres (P < 0.05, the Mann-Whitney U test).

Figure 5. Lesser diametre (μm) (±SEM) of Type I and Type II fibres in the adult and senile groups.

Asterisks denote significant differences across age groups of spotted dolphins for type I fibres (P < 0.05, the Mann-Whitney U test).

Figure 6. Fiber type proportion (%) (±SEM) of Type I and Type II fibres in the adult and senile groups.

Asterisks and cross denote significant differences across age groups of spotted dolphins for type I fibres (P < 0.05, the Mann-Whitney U test).