An involuting uterus, late postpartum

Keywords: uterus, vagina, bovine, corpus luteum, postpartum, cervix

The uterus of a cow that had involuted partially after calving. She had also ovulated at least once after calving.

Image size: 1308 x 972px

The presence of a corpus luteum (CL) in the left ovary indicates that she had ovulated recently.  Some may ask: Why is it that a cow would ovulate before her uterus is fully involuted and ready to accept a new pregnancy? The answer is that this cow's calf was removed shortly after birth and was not allowed to suckle i.e. this tract was from a dairy cow, a product of human manipulation. Dairy cows are not subjected to the negative effect of sucking on postpartum anestrus. By contrast, beef cows are suckled and do not ovulate for extended periods after calving (depending on breed and nutrition) usually well after the time that the uterus has taken to involute completely.  In the right ovary of this cow, a small follicles has formed a layer of luteal tissue within the follicle, a sign of low level stimulation with luteinizing hormone (LH). Although it is smaller than most cystic follicles it is essentially just that; a luteinized cystic follicle. It is possible that this cystic follicle could have persisted beyond the lifespan of the CL in the left ovary, causing the cow to have a delayed return to estrus (as is often the case). However it is also possible that it was of a similar age to the CL, failing to mature and ovulate as a twin ovulation.  Finally, note that there is no shortage of small tertiary follicles in these ovaries, waiting to participate in postpartum estrous cycles. This is because the production of follicle stimulating hormone (FSH) recovers far sooner that LH production after a calf is born.

The caruncles in this specimen are small and almost completely involuted yet discoloration in the centers on the caruncles suggests that their involution is not complete. The endometrium between caruncles is sloughed soon after calving but is restored by 10 to 12 days postpartum. The caruncles by contrast, have only sloughed tissue to the level of their capillary beds by that time and it is only by about 35 days  postpartum that caruncles are completely restored with new epithelium. Interestingly, caruncles visible throughout the life of the cow. In fact they are even evident when females calves are developing in utero.

Note the short uterine body i.e. how quickly the horns divide as an object ascends into the tract. This is important for several reasons. First, if one is to preferentially inseminate in one horn or another, the pipette must be directed accordingly, as soon as the internal cervical os has been reached, A similar situation is encountered when one has to collect embryos from one horn or another. Finally, because of the virtual absence of a uterine body, bovine fetuses are highly likely to be in longitudinal presentation at calving. By contrast, mares have long uterine bodies making transverse presentation more likely than in cows (although thankfully, transverse presentations are still unusual in mares). Also, no thought is given to horn selection in mares during routine insemination or embryo flushing.

Note the cervical "rings". Apart from the ring that surrounds the external cervical os, these structures are more akin to crescents than complete rings. One must negotiate these rings with an insemination pipettes during artificial insemination (AI). In mares, these rings do not exist and entry into the uterus is simple. In cattle by contrast, the cervix must be grasped per rectum and the tip of the A.I pipette placed in the fornix of the vagina. Then the external cervical os is then manipulated over the instrument while the instrument itself is eased through the cervical canal. The same situation is encountered when one flushes embryos from the uterus.

Interestingly, the word fornix is derived from the Latin word for archway. In homo sapiens, standing erect, this is perhaps appropriate. In domestic animals however, the arches (around the cervix) have fallen on their sides!

Selected reference:

The mind of an aging theriogenologist.

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.

Postpartum corpus luteum in a cow

Keywords: cow, bovine, postpartum, ovary, corpus luteum, CL

Image size: 1044 x 778px

Corpora lutea (CLs) in the ovary of a cow approximately one week after she had given birth to twins. The cow was euthanized because she had injured herself during calving. This image shows that corpora lutea are still present within the ovary for at least a week after calving; this despite the fact that they are not required for the maintenance of pregnancy for the last four months of gestation. These corpora lutea would have been producing little if any progesterone. In fact, cows can be ovariectomized after the first month of gestation and will maintain pregnancy and can calve normally. Therefore, the role of the ovaries (and corpora lutea in this case) during late gestation cattle is uncertain.

Follicle and a corpus luteum; ultrasound

Key words: wave, bovine, CL, corpus luteum, follicle, cow, ultrasound

Image size: 640 x 480px

A large follicle (the anechoic structure) adjacent to a corpus luteum (CL) in the ovary of a cow; as seen during transrectal ultrasonography. Follicles grow and regress in waves during the bovine estrous cycle. There are commonly two or three waves of follicle growth during the diestrous period. When there are three waves, the estrous cycle is longer (21 to 22 days) than when they are two follicle waves (19 to 20 days). Less commonly,  cows may even have four follicle waves. Therefore the discovery of a large follicle during diestrus (as in this case) does not necessarily mean that a cow is approaching estrus.

Bovine mucometra/hydrometra

Keywords: adhesions, bovine, hydrometra, mucometra

Bilateral peri-uterine adhesions and fibrosis of the perimetrium/myometrium which appear to have caused distortion and mucometra.

Image size: 1023 X 1283 px

Dissection of the tract did not reveal a specific area of occlusion that would have resulted in this condition. It is possible that the inciting cause was fibrosis of the cervix itself. 

This condition should be referred to as mucometra because it is characterized by an accumulation of mucus; not water (hydro). It is not related to a chromosomal abnormality such as that in polled intersex goats i.e. the PIS syndrome. It is a rare condition in cattle, manifested by unilateral or bilateral distension of the uterus with clear fluid. Occasionally the fluid may contain clumps of inspissated tissue the nature of which have not been described.  In studies on slaughter plant specimens, mucometra occurred in about 0.1 to 0.8 percent of cattle.

Mucometra occurs in cattle as a result of adhesions or fibrosis (as seen here). It is also seen in cattle with segmental aplasia or in heifers with persistent hymens. Mucometra has also been linked to chronic cystic ovarian disease (COD) but is by no means, characteristic of that condition.

References:

Fathalla, M. et al 2000 An  abbatoir survey of gross reproductive abnormalities in the bovine genital tract in northern Jordan. Israel. J. Vet. Med. 55. No page numbers. Available on-line.

Roberts, S. J.1986. Veterinary obstetrics and genital diseases (Theriogenology). Published by the author.  ISBN-10: 9997670922. ISBN-13: 978-9997670922

Normal postpartum bovine uterus.

Keywords: postpartum, suckling, beef, dairy, lochia, involution, uterus, estrus, ovulation

Image size: 1751 x 1131 px

A normal completely-involuted bovine uterus. Note to the typical Y-shaped structure with a short body and two long horns. In the left ovary, a follicle approximately 2 cm in size can be seen developing. In the contralateral (right) ovary, there appeared to be no significant structures. This is typical of a postpartum reproductive tract in a suckled beef cow.. Cows (milk but not suckled) may ovulate as soon as 10 or 14 days after calving, long before the uterus has even involuted.

The corpus luteum and corpus albicans.

Keywords: CL, bovine, cow, palpation, progesterone

Image size: 1104 x 728px

The crown of a corpus luteum (CL) in an ovary of a cow.  The CL is a source of progesterone and several other hormones, including relaxin, müllerian inhibiting hormone, inhibin and even GnRH.

A CL is easily palpable per rectum in most cases and is useful for determining if the cow is having estrous cycles and in some cases, assisting one in staging the estrous cycle. The crown of a corpus luteum is seen in all species except horses where the tunica albuginea does not allow the follicle to ovulate through the surface of the ovary.

After ovulation, blood fills the antrum of the follicle, coagulating and forming a corpus hemorrhagicum. As this blood is phagocytosed, the theca granulosa of the ovarian follicle undergoes both hyperplasia and hypertrophy, folding in towards the center of the antrum.

This image shows the size of a mature CL, the author's thumb depressing the cut surface of that structure in a transected ovary.

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In the example shown below, that folding is clearly visible.

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After luteolysis, the corpus luteum undergoes regression and shrinks, forming a corpus albicans. The term "albicans" is derived from the Latin word for white but in domestic animals, this is practically a misnomer. In fact it is only in a few animals (cetaceans for example) where the corpus albicans remains as a large, substantial structure after luteolysis.  In domestic animals it only becomes a small, white body long after luteolysis is complete and in fact, is often invisible to the naked eye by that time.  Indeed, at the end of the luteal phase in domestic animals, when it is commonly referred to as a corpus alicans, the corpus "albicans" (CA) is a dark yellow-to-bright orange structure as shown here.

A CA is seen below, advanced in its luteolysis and degeneration, evidence by rays of fibrous tissue in the structure. Note that this luteal structure has a cavity in the center; a common finding in luteal tissue in cattle. It is not abnormal and is merely a function of luteinization from the theca granulosa growing but never reaching the center of what was previously a follicle. Instead of being referred to as a cystic CL/CA it is often labeled a "luteal cyst".

Image size: 704 x 486px

Cystic follicle in a cow

Keywords:  anestrus, COD, cyst, cystic, follicle, bovine, management, cow, estrous, cycle

Reproductive tract from a cow with cystic ovarian disease (COD); a syndrome associated with high milk production and negative energy balance in high producing dairy cows. COD is usually associated with persistence of large, un-ovulated follicles in the absence of standing estrus, probably because of luteinization (with progesterone production) of the cystic structures due to low pulsatile releases of LH. Nevertheless, this illustration shows that ovulation occasionally occurs in the presence of a cystic follicle. Note the corpus luteum in the left ovary, in the same ovary as the cyst.

In these cases, the cyst may then regress and may even be replaced by other cysts until ovulation becomes predictable and cyclicty returns to normal.

This illustration also suggests that luteinization of cystic follicles may be so limited that they not produce enough progesterone to suppress LH and block ovulation.

The terminology of COD can be confusing, with cystic follicle occasionally being referred to as luteal cysts. In the author's opinion this term should not be used because it can be confused with a structure called a cystic corpus luteum; a non-pathological entity. Instead, when a cyst becomes luteinized it should be called a luteinized cyst rather than a luteal cyst.