Sinus-like dilatations of the mammary milk ducts, Ki67 expression, and CD3-positive T lymphocyte infiltration, in the mammary gland of wild European rabbits during pregnancy and lactation

Abstract

Sinus-like dilatations of the mammary duct are recognisable in the mammary gland of pregnant and lactating wild European rabbits. These dilatations exhibit a bilaminar epithelial lining, with luminal epithelial cells expressing basal and lateral E-cadherin. Occasional binucleated mammary epithelial cells are present in the luminal layer. Underlying the luminal epithelial cells is a basal layer of cytokeratin 14-positive cells, supported by a thin layer of fibrous tissue. Multi-segmental epithelial proliferation, as indicated by Ki67 expression, is apparent in the luminal epithelial cells, suggesting a capacity for division during pregnancy and lactation. CD3-positive T lymphocytes are present both intraepithelially, suggesting exocytosis, and in foci subjacent to the ductular epithelium. We consider that sinus-like dilatations of the mammary duct may have the potential to give rise to a subset of the mammary gland neoplasms classified as ductal in origin. Milk accumulation in these sinus-like dilatations is likely to provide a niche for bacterial replication in cases of mastitis in rabbits. These structures are an important component of the innate immune system of the mammary gland, both as a physical barrier and as an interface between the milk and mammary immune cells.

Keywords: lactation; mammary gland; mastitis; milk duct; pregnancy; rabbit; sinus; tumour.

Figure 1

Sinus‐like dilatations of the mammary ducts of the rabbit mammary gland. (A) Transverse section approximately 2 mm from the distal aspect of the teat of a lactating rabbit. Six major ducts are apparent (arrowheads). (B) Parasagittal section through the teat canal‐mammary duct junction of a pregnant wild rabbit, estimated 22 days gestation. Arrows indicate transition from a stratified squamous to a bilaminar ductular epithelium. (C,D,E) Parasagittal sections through the teat and mammary tissue of a pregnant wild rabbit, estimated 27 days gestation (C) and a lactating rabbit (D,E). Multiple ducts (diamonds) exhibit sinus‐like dilatations (s), which are supported by thin layers of fibrous tissue (arrows). (E) Parasagittal section of a sinus‐like dilatation demonstrating the double layered epithelial lining (arrowheads). The lumen is to the right of the photomicrograph and the sinus‐like dilation is supported by fibrous tissue (arrow). Scale bars: 500 μm (A), 50 μm (B), 1.5 mm (C), 1 mm (D) and 40 μm (E). H&E stain (A,B,D,E); Massons trichrome stain (C). Images are representative of 10 (A) to 12 (B–E) biological repeats.

Figure 2

The rabbit mammary gland during pregnancy and lactation. (A,B) Immunohistochemical staining for E‐cadherin demonstrates expression at the basal and lateral aspects of luminal mammary epithelial cells forming mammary alveoli and lining the sinus‐like dilatations of the mammary duct. Mammary gland from two different lactating rabbits. Inset shows an apparently binucleated mammary alveolar luminal epithelial cell. Arrow indicates an apparently binucleated epithelial cell lining a sinus‐like dilatation of the mammary duct. (C) Immunohistochemical staining for cytokeratin 14 demonstrates intense cytoplasmic expression in the basal epithelial layer of the sinus‐like dilatations, and rare cytokeratin 14‐positive luminal cells (arrows). Pregnant rabbit, estimated 17–18 days gestation. (D,E) Immunohistochemical staining for Ki67 demonstrates multi‐segmental expression in the luminal epithelial cells lining the sinus‐like dilatations of the duct and in mammary alveolar luminal epithelial cells. Lactating rabbit. (F) Immunohistochemical staining for CD3 demonstrates CD3‐positive T lymphocytes both in clusters between adjacent mammary alveoli, subjacent to the sinus‐like structures, and in intraepithelial foci (arrows). Lactating rabbit. Scale bars: 30 μm (A,B), 50 μm (C,F) and 100 μm (D,E). Haematoxylin counterstain; Lu: lumen of sinus‐like structure. Images are representative of three biological repeats.