Table 4 Histological features of the rat reproductive tract during different phases of oestrous cycle (Westwood, 2008)

Proestrus

Presence of mitotic figures. Polymorphs are occasionally present. There is little or no degeneration or desquamation. Formation of stratum granulosum (indicates onset), superficial mucoid layer and corneum with time. At the end of this phase, cornified cells are prominent with superficial mucoid layer and some desquamation mucoid cells.

Epithelium is cuboidal to columnar. There is presence of mitoses in epithelial cells with little or no degeneration and little infiltration of inflammatory cell. Dilatation is seen towards the end of proestrus

Corpus luteum degenerates. There is the presence of cytoplasmic vacuoles and fibrous tissue proliferation in the central cavity

Estrus

There is gradual shedding of superficial mucoid and cornified layers with reduction in height of epithelium. There is appearance of cell debris with loss of mitotic figures and gradual leukocyte infiltration.

The onset is characterized by appearance of notable degeneration/necrosis of epithelial cells. There is loss of mitotic activity and leukocyte infiltration. Dilatation may persist to late estrus.

Degenerated corpus luteum is usually present. There are small corpus luteum with basophilic cell cytoplasm, central fluid-filled cavity and no fibrous tissue.

Metestrus

At the onset, there is complete detachment of cornified layer. Leukocyte infiltration persists with continued desquamation and loss of stratum granulosum and upper germinativum.

There is continued degeneration of endometrial epithelial cells. Mitotic activity is seen; both mitotic activity and degeneration are seen together.

Corpus luteum may still contain fluid cavity. There is minimal basophilic cells. No fibrous tissue.

Diestrus

The onset is characterized by epithelium with variable leukocyte infiltration, and subsequent epithelial proliferation and thickening. There is no leukocyte infiltration at the later part of this phase.

There is a small, avascular, slit-like lumen. There is low columnar epithelium with few mitoses which increases as the cycle progresses. Stromal oedema is seen at the end of this phase.

Large corpus luteum is seen. There is fibrous tissue formation in the central cavity.