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.
