Age‐related morphometry, typology and spatial distribution of <i>anterior latissimus dorsi</i> muscle fibre in broiler chicken

The acquisition and processing of muscle tissue images through optical microscopy, along with manual and semi-automatic software techniques, present significant constraints on comprehensive research of a muscle's entire cross-sectional area. To address this limitation, we focused on the anterior latissimus dorsi (ALD) muscle, enabling us to analyse its complete cross-section on a single slide throughout all study stages. This allowed for a detailed assessment, incorporating age-related variations, of histoenzymatic activity across the entire cross-sectional area, along with fibre typology and spatial distribution, and to encourage comparative research across vertebrates to examine species-specific, genetic, ecological and functional influences on histo-enzymomorphometric changes. Leveraging advancements in image acquisition and processing technologies, including slide scanning and automated software, we conducted a comprehensive study on broiler chickens (Gallus gallus Domesticus) at post-hatch ages (D0, D7, D14, D21, D28, D35, D42, D49 and D56), with 10 subjects per age group. The myofibrillar network was visualized using Azorubin staining, while identification of different fibre types in the ALD muscle was achieved through co-revelation of ATPase activity at acidic pH (4.10). Our investigation revealed a progressive decline in the total number of muscle fibres with age. The ALD muscle demonstrated variability in intramuscular distribution, with type IIIa fibres dispersed across the entire muscle surface, showing a consistent increase in percentage with age. Conversely, type IIIb fibres were initially well distributed across the muscle surface during early growth stages but exhibited a gradual decrease with age, particularly in the caudal half of the muscle, reaching minimal values in adulthood. Type IIa fibres were scarce in early ages but appeared in later stages, with percentages not exceeding 5%. The coefficient of variability of type IIa fibres was notably high, indicating the presence of multiple fibre types sharing a common negative reaction to ATPase revelation at acidic pH. This phenomenon suggests a potential conversion of type IIIb fibres into type IIa and IIb fibres. In conclusion, our histoenzymatic study of the entire cross-sectional area of the ALD muscle has provided novel insights into fibre typology and spatial distribution within muscle bundles. Further research to unravel the mechanisms of fibre-type distribution leading to explore the genetic effects of domestication and ecological pressures across species.