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!
The mind of an aging theriogenologist.
