Microplastics (MPs; less than 5 mm in size) are becoming increasingly prevalent in both terrestrial and aquatic ecosystems.
As these particles enter the food chain, they have the potential to pose signifcant risks to human health. However, their
efects on vital fsh tissues, such as skeletal muscle, are not yet fully understood. In this study, we examined Nile tilapia
(Oreochromis niloticus) from two distinct sites on the Nile River in Egypt: the Nile branch (Damietta branch) and Riah ElTowfqi. Using Fourier Transform Infrared Spectroscopy (FTIR) and histological study, we confrmed the presence of MPs
in both gastrointestinal and muscle tissues. We focused on understanding how MPs might afect fsh muscle by investigating
the expression of genes involved in muscle atrophy and hypertrophy using Real Time-PCR and histological alterations in
muscle tissues of tilapia collected from the two studied sites in the four seasons. Our results revealed histological alterations
in muscle tissues collected from the two sites studied in the four seasons. The expression levels of atrophy-related genes,
Atrogin-1 (Fbxo32), Capn-1, and the apoptosis marker Caspase3a (Casp3a), showed increased expression, especially during the summer at both sites. On the other hand, the hypertrophy-related gene Igf-1 exhibited a signifcant decrease while,
muscle stem cell genes (Pax3, Pax7) and muscle diferentiation gene markers (Myf5, Mrf6) displayed seasonal upregulation,
with heightened activity during winter and summer, depending on the location. Additionally, immune-related genes (Ccr9,
Irak4, Igl-1, Tlr1) demonstrated notable seasonal changes, with a peak during summer at the Nile branch. These fndings
demonstrate that MPs can disrupt muscle integrity and immune function in fsh, with implications for ecosystem health and
potential risks to human food security