The morphological changes of the corpus luteum in cow ovaries are characterized by a rapid increase in corpus luteum weight, with the growth period of the corpus luteum exceeding half of the estrus cycle. From the 3rd to the 12th day of the estrus cycle in cows, the increase in corpus luteum weight and progesterone concentration is very rapid, and it remains relatively stable until the 16th day. Through B-ultrasound observation of imported cows, it can be seen that the diameter of mature corpus luteum is larger than that of mature follicles.

On the 11th to 15th day of the estrus cycle in cattle, the volume of the corpus luteum reaches its maximum, with a diameter of about 20-25cm measured by B-ultrasound in imported cattle. 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 in color, light yellow on the 7th day of the cycle, golden yellow on the 14th day, then orange yellow, and finally brownish red. The degeneration of the corpus luteum in cattle leaves a dark red residue on the ovaries, which can often last for several months.

The diameter of follicles in native cattle, low generation hybrid cattle, or young cattle may only be about 1cm (as the ovarian diameter is only 1cm) when measured by imported cattle using ultrasound. The follicles of estrus cows fluctuate significantly. As the estrus process progresses, the follicle walls become thinner and the texture changes from hard to soft, with a feeling of breaking at the touch. After ovulation, there is an ovulation nest at the site of the primordial follicle. A few hours after ovulation, the blood and tissue fluid around the ovulation nest fill up, forming a clot. The texture is relatively soft, and cows in estrus generally have no obvious corpus luteum, only a hard and degenerated corpus luteum. However, some cows may still have a large corpus luteum shape, but at this time the corpus luteum is no longer functional. The ovaries on the opposite side of the estrus follicle are mostly quiescent or active.

The corpus luteum is derived from the differentiation of follicles in cows, and the follicular endometrial cells and granulosa cells respectively generate small corpus luteum cells and large corpus luteum cells. The weight of the follicle before ovulation is about 200mg, and in imported cattle, rapid growth of the corpus luteum can also be observed using B-ultrasound at this stage. On the 11th day after ovulation, the corpus luteum grows to 4g in the cow's body. The corpus luteum of cows and high-quality breeds is larger, and this explosive growth is like some rapidly growing tumors, originating from the large and small corpus luteum cells that account for 70% of the corpus luteum tissue. Although most of the growth of corpus luteum tissue is due to the hypertrophy and proliferation of corpus luteum cells, there are also reports suggesting that small corpus luteum cells, endothelial cells, and fibroblasts also undergo cell division, which plays a role in the proliferation of corpus luteum tissue.

It is worth noting that the weight of the newly formed corpus luteum (day 3) and the degenerated corpus luteum (day 19) is light. The average concentration of progesterone (P4) in the blood is also low and varies greatly, not due to the number of days in the estrus cycle, but rather due to individual differences in the process of corpus luteum formation and degeneration.

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