Ultrasound image attributes of the bovine corpus luteum: structural and functional correlates

Nulliparous heifers (n = 58) were studied to determine whether computer-assisted quantitative echotexture analysis of ultrasound images reflects the functional and histomorphological characteristics of the corpus luteum. The ovaries of heifers were examined daily by transrectal ultrasonography from day -2 (day 0 = ovulation) until the day of ovariectomy during metoestrus (day 3; n = 8), early dioestrus (day 6; n = 9), mid-dioestrus (mean, day 10; n = 7), or pro-oestrus (mean, day 18; n = 8; Expt 1). High resolution ultrasound images of corpora lutea were obtained in vitro, and were digitized and analysed using custom-developed computer algorithms optimized for ultrasonography. Cryostat sections of corpora lutea were examined for lipid distribution, and corpora lutea were homogenized to determine the content of progesterone, total protein, cholesterol and triglyceride. In Expt 2, heifers (n = 26) were ovariectomized as in Expt 1, and ovaries were prepared for histomorphometric evaluation. Pixel values (brightness of picture elements) of ultrasound images of corpora lutea were characterized as high during metoestrus, low during early and mid-dioestrus, and increasing again during pro-oestrus (P < 0.05). Changes (P < 0.001) in volume density of luteal cells were characterized as increasing from metoestrus (40.7 +/- 0.4%) to mid-dioestrus (55.8 +/- 2.8%) and decreasing again at pro-oestrus (41.5 +/- 0.9%). The proportion of blood vascular components decreased (P < 0.001) progressively from 31.0 +/- 1.0% in metoestrus to 15.6 +/- 1.1% in pro-oestrus. Pixel values of ultrasound images of corpora lutea were correlated with luteal (r = -0.72, P < 0.05) and plasma (r = -0.71, P < 0.03) progesterone concentration, and to the volume densities of luteal cells (r = -0.75, P < 0.02) and connective tissue (r = 0.69, P < 0.03). Estimates of triglyceride, protein and cholesterol content of corpora lutea were not correlated with pixel values of ultrasound images. Protein and cholesterol content did not change while triglyceride concentration increased during pro-oestrus (P < 0.05). Results support the hypothesis that ultrasound images reflect luteal and plasma progesterone content, and histomorphological characteristics of the corpus luteum.