Bovine corpus luteum and follicles

Keywords: bovine, follicle, ovary, physiology, cycle, ovulation, corpus luteum.

This image provides a partial understanding of anatomy and physiology of ovaries during the estrous cycle. It works together with several other entries in LORI, especially, The estrous cycle, Two and three wave estrous cycles and A follicle and a corpus luteum; ultrasound.

Image size: 1976 x 1513px. Always click on images for larger sizes.

In the upper half of the image, note the size of the ovary compared to the author's fingers. It is fairly large; typical for a cycling cow because much of the volume in an active ovary is due to the presence of luteal tissue; an indication of ovulation and hence, cyclicity. In a non-cycling cow or a prepuberal heifer, the ovary is very small, sometimes barely the length of a finger nail.

Note the tunica albuginea around the entire ovary, thin enough in ruminants (and most other animals) to allow ovulation through the surface of the ovary.

Note:  Mares by contrast, have a thick tunica albuginea that does not allow ovulation through the surface of the ovary. For more on significance of that anatomical quirk, see this entry in LORI

The practical significance of having ovulations occur through the surface of the ovary is that part of the maturing corpus luteum (CL) comes to lie outside the tunica albuginea. The part of the CL that bulges through the tunica albuginea is called its "crown". The presence of a crown on the surface of an ovary makes it possible to know if a cow has ovulated recently. More importantly, it is possible to feel the crown of a CL during transrectal palpation.

Now consider the large follicle growing at the base of the CL in the upper image. In this case, the follicle is close to its periovulatory size; about 18 to 22 mm in diameter.

Note that the mature CL here is co-existing with a large follicle. This is often seen during transrectal ultrasonograhy as well.  Bearing in mind the fact that the CL is producing large amounts of progesterone, ask yourself if progesterone is able to suppress follicle growth. It is well known that progesterone can block the ovulation of a mature follicle, but can it suppress follicle growth? In other words, does progesterone suppress the secretion of follicle stimulating hormone (FSH)? The answer, perhaps obviously, is no; certainly not significantly.  This is why progesterone alone cannot be used to control estrous cycles with precision in any animal.   To control estrous cycles with precision, one must use progesterone (to suppress estrus and prevent ovulation) but other mechanisms must be used to control follicle growth,  To do that, one needs something to suppresses FSH, such as estrogens, androgens and if it were possible, inhibin itself (inhibin is not available for clinical use). Otherwise, follicle populations can be controlled by inducing follicles to ovulate on demand or otherwise, they can to be aspirated or physically destroyed. A new follicle wave will then grow on cue.

The image at the top of this entry could have come from a couple of places in the run of a normal estrous cycle. Using the image below, lets examine that statement. Remember that there are usually two or three waves of follicles during a bovine estrous cycle, sometimes four. This cycle has three waves. That means that two waves of follicle develop and regress during the luteal phase and it is only the follicle/s in the third wave will have the good fortune to ovulate. The other two waves will have been wasted. Why? Let me know if you ever find out.

Again, the image could only have occurred in two places: A or B. At both of those stages of the cycle, there would have been a large follicle in the presence of a mature CL. True, a large follicle could also have been present at point C, in the minutes before ovulation but a mature CL would not have been present. Instead a corpus albicans, a smaller and lighter structure, would have been seen in the image i.e. the product of destruction of a CL (luteolysis) in the absence of pregnancy.

Image size 1016 x 755px. Always click on images for larger sizes.

There is another interesting tidbit to comment on here and that is the presence of that tiny follicle in the first image. In fact, it was probably one of many in the ovary; invisible here. It is emphasized in the oval cutout. Because waves of follicle grow and regress as the estrous cycle progresses, it could belong to almost anywhere in the estrous cycle (see d, e, f &g). Most likely however it was one of those present at e, f or g, when a mature CL was present.

Synchronizing follicle growth in cattle; Ovsynch

Keywords: synchronize, bovine, ovsync, cow, estrous, estrus, cycle, mechanism

Image size: 5363 x 6393px

This diagram, extrapolated from many sources, shows the mechanism of Ovsynch treatment.

The receptive periods during the estrous cycle when GnRH will cause ovulation  are shown as gray vertical bars. If GnRH is given during any of these periods (about 65% of the duration of a three wave estrous cycle) follicles present at that time will ovulate, causing a drop in inhibin, FSH release, and the start of new wave of follicle growth. It is this wave of of follicle growth that is utilized to provide synchronous ovulations about nine days later.

As mentioned, a synchronous response to the first injection of GnRH occurs in about 65% of cows. Nine days after the first injection of GnRH, some of the non-responding cows will either respond to the injection of prostaglandin and show estrus or they may show estrus spontaneously. In this manner, about 65% of cows will respond to the first injection of GnRH and about 35% of the cows (the non-responders) will be in estrus after the injection of prostaglandin and will respond to to the second injection of GnRH, together with the other cows. Collectively this means that about 85% of all the treated cows could respond to Ovsynch treatment.

These figures assume that all the cattle being treated are actually having estrous cycles. In high producing cows, is seldom the case so the response to Ovsynch treatments may often fall short of these calculations.

Sensitive periods for Ovsynch treatment

Keywords: cow, synchronize, bovine, estrous, estrus, cycle, follicle, wave, Ovsynch

A diagram modified from several sources, showing an estrous cycle with three follicle waves. The physiology of two or three follicle waves is important during “Ovsych” treatment (GnRH-Prostaglandin-GnRH) because it is only those follicles that have already been recruited and already growing in a follicle wave that will ovulate after GnRH treatment; an essential step in re-setting the recruitment of follicles so that a mature follicle will be available for ovulation about 8 to 9 days later. This image shows the periods of time (vertical grey bars) during which follicles can be induced to ovulate by GnRH treatment.

Two and three wave estrous cycles

Key words: bovine, estrous, estrus, cycle, follicle, wave, Ovsynch, prostaglandin

A diagram modified from several sources of two and three wave estrous cycles. One cow has two waves of follicle growth with the follicle in the second wave eventually ovulating. The other has three waves, with the last wave providing the follicle that ovulates. The length of the estrous cycle is one or two days longer in cows that have three follicle waves instead of two. A minority of cows have four follicle-wave estrous cycles.

This physiology is important in everyday dairy practice when prostaglandins are used to shorten the inter-estrous period because follicles are more likely to large and ready to ovulate if cows have more follicle waves during the estrous cycle. There is usually only one follicle per wave but once a follicle has grown to its maximum size it begins to regress, it cannot be utilized. Therefore, many follicles and their oocytes are wasted. The physiology of two or three follicle waves is also important during “Ov-sych” treatment to synchronize ovulation in cattle.

A follicle and a luteinized cystic follicle

Keywords: CL,corpus, luteum, cystic, bovine, cycle, infertility, cow, ultrasound

Image size: 640 x 480px

A follicle and a luteinized cystic follicle seen by transrectal ultrasonography in a cow.

Together with its appearance, a history of postpartum anestrous allowed a diagnosis of a luteinized cystic follicle (sometimes referred to a luteal cyst) in this cow.  Cystic follicles and the luteinized structures they develop into, are abnormal structures, associated with an absence of estrous cycles in the early to late postpartum period; almost always in dairy cows. In dairy cows, abnormal estrogen secretion and abnormal LH surges are thought to be important in the syndrome. Cystic follicles are also seen in beef heifers, but usually only after the use of steroid growth promoters.

Clarification: Cystic corpora lutea are normal structures, associated with normal estrous cycles.  Most corpora lutea luteinize completely, obliterating the antrum of the ovulated follicle but many are left with small to large cysts in the center.  This author does not use the term “luteal cyst” (an abnormal structure) because it can easily be confused with the term “cystic corpus luteum” (a normal structure). Instead, the term luteinized cyst (a cystic follicle that has become luteinized) should be used.