Study on the structure and the development of the canalicular system of the mammary gland during lactation in Manchega and Lacaune dairy sheep

摘要

In order to observe the changes in the glandular system of the udder during lactation, a total of 18 ewes of Manchega (n=9) and Lacaune (n=9) dairy breeds in the 3rd lactation were used. The study was conducted at three stages of lactation: weeks 1(n=6), 5 (n=4) and 13 (n=8). After an injection of 5IU of oxytocin and drainage of the total milk in the udder, ewes were euthanised and immediately injected with a colored epoxy resin in the right mammary gland through the papillary opening. Repletion of the canalicular system (alveoli, ducts and sinus) was obtained using a syringe pump. Udders were removed after hardening of the epoxy during 48h under refrigeration and the surrounding organic tissue was corroded using a 20 percent OHK solution. The resulting casts were washed, dried and processed for scanning electron microscopy. Different stages of development of the canalicular system, and frequency and types of tubulogenic structures were identified in the electron microscopy images according to stage of lactation. At week 1, an intussusceptive growth from preexisting tubules was found, showing an increase in the number of tubules after parturition. A further development occurred at the alveoli level in the 5th lactation week. At this time, structures like angiogenic buds were frequently identified at the end of the tubules. These buds appeared as semispherical enlargements that grow from the ducts and become almost spherical by the narrowing of their connection with the duct. The bud surface then loses its smoothness and develops small sockets with a golf ball-like image identified as alveoli. At week 15, segmentation of the alveoli was the main development observed. Both dairy breeds followed the same pattern in development during lactation despite their differences in milk yield. In conclusion, the mammary gland of dairy sheep showed a relevant development of the canalicular system and proliferation of alveoli during lactation. A parallel was found between the development of the canalicularand vascular systems.