The characteristics of the changes in the morphology of the corpus luteum of the cow's ovary are that the weight of the corpus luteum increases very quickly at first, and the growth period of the corpus luteum exceeds half of the estrus cycle. From the 3rd to the 12th day of the estrus cycle of the cow, the weight of the corpus luteum and the concentration of progesterone increase very rapidly, and remain relatively stable until the 16th day. Through the observation of cattle B-ultrasound, it can be seen that the diameter of the mature corpus luteum of the cow is larger than the diameter of the mature follicle.

The volume of the corpus luteum of the cow reaches its maximum on the 11th to 15th day of the estrus cycle. The diameter measured on the cattle B-ultrasound is about 20-25cm. The mature corpus luteum is spherical or elliptical, usually slightly protruding from the surface of the ovary, and its size is similar to or slightly larger than the follicle. The newly formed corpus luteum is brown, light yellow on the 7th day of the cycle, golden yellow on the 14th day, and then turns orange-yellow, and finally turns brown-red. The degeneration of the corpus luteum of the cow leaves a dark red remnant on the ovary, which can often last for several months.

The diameter of the follicles of native cattle, low-generation hybrid cattle or young cattle measured by cattle B-ultrasound may be only about 1cm (because the diameter of the ovary is only 1cm). The follicles of estrus cattle fluctuate significantly. As the estrus process progresses, the follicle wall changes from thick to thin, and the feel changes from hard to soft, with a sense of being broken at the touch. After ovulation, there is an ovulation nest at the primordial follicle. A few hours after ovulation, the blood and tissue fluid around the ovulation nest fill and form a blood clot. It feels relatively soft. In estrus, cows generally have no obvious corpus luteum, only a hard degenerate corpus luteum, but some cows may still have a larger corpus luteum, but the corpus luteum at this time has no function. The ovaries on the opposite side of the estrus follicles are mostly static or active.

The corpus luteum is differentiated from the follicles of cows. The follicle endometrial cells and granulosa cells generate small corpus luteum cells and large corpus luteum cells respectively. The weight of the follicle before ovulation is about 200mg. It can also be observed by cattle B-ultrasound that the corpus luteum grows rapidly at this stage. On the 11th day after ovulation, the corpus luteum grows to 4g in the cow. The corpus luteum of dairy cows and fine breed cattle is larger. This explosive growth is like some fast-growing tumors, which comes from the large and small corpus luteum cells that account for 70% of the corpus luteum tissue. Although most of the growth of the corpus luteum tissue is the result of hypertrophy and proliferation of corpus luteum cells, there are also reports that small corpus luteum cells and endothelial cells and fibroblasts also undergo cell division, which has an effect on the proliferation of corpus luteum tissue.

It is worth noting that the weight of the new corpus luteum (3rd day) and the degenerated corpus luteum (19th day) is light. The average value of blood progesterone (P4) concentration is also low and varies greatly. This difference is not due to the number of days in the estrus cycle, but the difference between individuals in the process of corpus luteum formation and degeneration.

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