Relationship between uterine morphology and peripheral concentrations of sex steroid hormone in wild Japanese black bears (Ursus thibetanus japonicus)

Developing a better understanding of the reproductive physiology and breeding condition peculiar to wild Japanese black bears (Ursus thibetanus japonicus) is crucial for estimation of their habitat distribution. The aim of this study was to clarify the changes in morphology of the genital organs, cellular proliferation in the endometrium and sex steroid hormone concentrations along with the reproductive cycle in Japanese black bears.Samples were collected from a total of 24 female Japanese black bears (1–15 presumptive years old) that were caught in the wild in Iwate prefecture during the period between August 1999 and September 2005. Twenty-two out of the 24 animals were hunted from May to October. The ovaries from the 24 animals and the uteri from 23 animals were observed macroscopically and histologically to examine the relationship between morphology of the genital organs and the month of the year the animal was caught. The staining pattern of proliferating cell nuclear antigen (PCNA) in the endometrium was characterised. Peripheral concentrations of oestradiol-17β and progesterone were determined by radioimmunoassay.All the animals that had a corpus luteum (n = 12) were captured from August to October. The thickness of the endometrium in the animals captured from August to October (n = 16) was significantly greater than those in animals captured from May to July (n = 5) (P < 0.05). From August to October, the thickness of the endometrium and the ratio of the area of the uterine glands to the area of the endometrium in the animals with a corpus luteum (n = 12) were significantly greater than those without a corpus luteum (n = 4) (P < 0.01). Positive PCNA staining was only observed in the uteri of two animals captured in May. There was a significantly positive correlation between plasma progesterone concentrations and the thickness of the endometrium (ρ = 0.589, P < 0.05). There were also significantly positive correlations between the progesterone to oestradiol-17β ratio (P4/E2 ratio) and the thickness of the endometrium (ρ = 0.710, P < 0.05), and between the P4/E2 ratio and the ratio of the uterine gland area in the endometrium (ρ = 0.626, P < 0.01).These data suggest that the corpus luteum is formed during or just after the breeding season and that the cells in the endometrium and the uterine glands, which proliferate in the early breeding season, grow and develop under the influence of progesterone and oestradiol-17β during the period of delayed implantation in Japanese black bears.