Abstract
Background:
Assisted reproductive technologies (ARTs), including in vitro maturation (IVM), hold promise for improving reproductive efficiency in dromedary camels, especially under challenging environmental conditions.

Aim:
This study aimed to evaluate the potential effects of cysteine supplementation in IVM medium on nuclear maturation, mitochondrial activity, antioxidant status, oocyte ultrastructure, and heat stress marker genes expression using a dromedary camel oocyte model.

Methods:
Two experiments were conducted. In Experiment 1, cumulus-oocyte complexes (COCs) were collected and matured in vitro using varying cysteine concentrations (0.0, 0.8, 1.6, and 2.4 mM) under standard conditions (38.5 °C, 5% CO₂, 40 h) to identify the optimal concentration. Experiment 2 evaluated the protective effect of the selected concentration (1.6 mM) during heat stress, where COCs were exposed to 40.5 °C for the first 24 h of maturation followed by exposure to 38.5 °C for 16 h. Three groups were compared: control, heat-stressed without cysteine, and heat-stressed with cysteine. We evaluated oocyte nuclear maturation, mitochondrial intensity, ATP content, and the activity of antioxidant markers and oocyte ultrastructure. We used RT-qPCR to assess the mRNA expression of cumulus expansion, oxidative stress, and mitochondrial health-related genes.

Results:
Results showed that heat stress conditions significantly reduced nuclear maturation, mitochondrial activity, ATP and antioxidant levels, and the mRNA expression levels of antioxidant, anti-apoptotic, cumulus expansion, and oocyte quality markers, while increased pro-apoptotic CASP3 transcription level (p < 0.05). Conversely, cysteine administration markedly reversed heat stress-induced detrimental effects by restoring oocyte maturation and ultrastructure, mitochondrial intensity, ATP content, antioxidant balance, and gene expression profile modulation.

Conclusion:
Supplementation with 1.6 mM cysteine during IVM of camel oocytes effectively improves oocyte quality and mitigates heat stress-related damage. This research supports the practical application of cysteine as a supplement to IVM media, specifically to enhance oocyte developmental competence and improve ART success rates in camels exposed to thermal stress.

Key words: Cysteine; Heat stress; IVM; Dromedary camel; Antioxidant activity.