Yolk sac development in dogs: from morphology to functional aspects.

The yolk sac (YS) is a multifunctional and essential structure for early maternal-fetal interactions, playing roles in nutrient supply, organogenesis, immune modulation, and hematopoiesis. Unlike most species, dogs maintain the YS until the end of pregnancy, yet little is known about its functional activity, particularly in later stages. Animal models are crucial for exploring maternal-fetal processes and offering insights into comparative development, including in humans. This study aimed to analyze the YS structure in canine embryos and fetuses at early (up to 20 days, n = 6), middle (21-40 days, n = 6), and late (41-60 days, n = 6) stages. Gestational age was estimated using crown-rump length (CRL) and external anatomy. Samples from the university's Veterinary Hospital and neutering campaigns in Londrina (Paraná, Brazil) were fixed in 10% formaldehyde for morphological analysis (H&E, PAS, and Picrosirius red) or 4% paraformaldehyde for OCT4 immunostaining (CEUA/UEL protocol number 029.2022). Histologically, the YS showed a trilaminar structure with blood islands in the intermediate layer surrounded by mesenchyme. Within these islands, undifferentiated cellular niches (OCT4+) were identified. During gestation, the YS changed from a thin, smooth, pale-red membrane to a wrinkled structure with narrowed ends and a thickened central region, resembling a "whale tail." The red color, indicative of vascularization, intensified with blood island enlargement. Collagen secretion, particularly type I, progressively increased around the blood islands. Glycoprotein deposits persisted until late gestation, as shown by PAS staining. These findings highlight the structural, secretory, and reserve roles of the canine YS throughout pregnancy.