Changes in density and architecture of microvessels of the rat mammary gland during pregnancy and lactation.

This paper deals with changes in intralobular microvascular density and architecture of the rat mammary gland during the reproductive cycle. Morphometrical analysis of india ink-perfused histological sections showed that the vascular density (the percentage of the total area of the vascular lumen to the whole lobular area), was consistently low before pregnancy and slightly increased by the 5th day of gestation. The vascular density increased twofold from the 5th to 10th days and decreased gradually thereafter. This substantial increase may presumably be associated with rapid glandular development, and the subsequent decrease may reflect the hypertrophy of individual acini. Scanning electron microscopy of corrosion casts showed that blood vessels were sparse before the 5th day of gestation and proliferated by capillary sprouts and globular outgrowths of venules by the 18th day. After the 12th day, the blood vessels were organized into round lumps which appeared to correspond in size to individual lobules. By the 18th day, the vessels were remodeled into highly ordered basket like units which disintegrated by the 20th day. The findings suggest that homogenious acini, formed temporally, eventually undergo uneven growth.