Table 1 Developmental competence of cumulus oocyte complexes and denuded oocytes, and the viability of cumulus cells in response to supplementation of maturation medium with the different levels of urea.

Urea concentration	PN-0% 18 hours PI	Cleav. rate% (8–16 cell stage), Day 3 PI		Blast. rate% Day 7 PI	Hatch. rate% Day 9 PI	Viability% 24 hours
Urea concentration	COCs	COCs	DOs	COCs	COCs	CCs
0 mg/dl	12.9 ± 0.46^a	46.2 ± 4.02^a	16.7 ± 1.42^b	29.1 ± 1.71^a	28.4 ± 2.28^a	75.6 ± 2.33^c
20 mg/dl	17.1 ± 3.78^ab	37.2 ± 4.42^ab	4.43 ± 1.07^a	22.0 ± 0.69^b	21.1 ± 1.91^ab	55.3 ± 2.60^b
40 mg/dl	24.9 ± 2.78^b	35.2 ± 2.10^b	3.1 ± 1.88^a	16.7 ± 2.51^c	16.1 ± 2.48^bc	17.7 ± 1.45^a

Respectively, 210, 230, and 235 cumulus oocyte complexes (COCs) and 180, 210, and 200 denuded oocytes (DOs) were randomly allocated to the groups supplemented with 0, 20, or 40 mg/dl urea. Cumulus cells (CCs) had a density of 2 × 10⁵ cells/ml. The cleavage rate (8–16 cell stage, day 3 post-insemination), blastocyst rate (day 7 post-insemination), and hatching rate (day 9 post-insemination) were recorded based on the number of COCs or DOs cultured in the maturation medium. The viability of 24-h cultured CCs under different levels of urea was evaluated using Trypan blue staining. PI: post-insemination; Cleav: cleavage; Blast: blastocyst; Hatch: hatching. Different letters (a, b, c, etc.) within a column indicate significant differences at P < 0.05. Data were presented as mean ± SEM indicated by one-way ANOVA followed by Tukey’s multiple comparisons test.

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