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

Area of Science:

• Veterinary Anatomy
• Developmental Biology
• Comparative Embryology

Background:

• The yolk sac (YS) is crucial for early maternal-fetal interactions, nutrient supply, and organogenesis.
• Dogs uniquely retain the YS throughout gestation, but its late-stage function remains understudied.
• Animal models, like dogs, offer valuable insights into comparative developmental processes, including human development.

Purpose of the Study:

• To investigate the structural and functional characteristics of the canine yolk sac (YS) during early, middle, and late pregnancy stages.
• To analyze morphological changes and cellular activities within the YS across different gestational periods.
• To understand the YS's role in nutrient provision, vascularization, and tissue remodeling in canines.

Main Methods:

• Histological analysis of canine YS samples from early (≤20 days), middle (21-40 days), and late (41-60 days) gestation.
• Morphological assessment using Hematoxylin & Eosin (H&E), Periodic Acid-Schiff (PAS), and Picrosirius red staining.
• Immunostaining for OCT4 to identify undifferentiated cellular niches and evaluation of collagen deposition and vascularization.

Main Results:

• The YS exhibited a trilaminar structure with vascularized blood islands containing OCT4+ cells.
• Significant morphological changes were observed, transitioning from a thin membrane to a thickened, wrinkled structure resembling a 'whale tail'.
• Increased vascularization, progressive collagen type I secretion, and persistent glycoprotein deposits were noted throughout gestation.

Conclusions:

• The canine yolk sac (YS) maintains structural integrity and functional activity throughout pregnancy.
• The YS plays critical roles in structural support, secretion, and nutrient reserve in canine development.
• These findings enhance our understanding of canine embryology and maternal-fetal interactions, with implications for comparative studies.