The uterus is comprised of three main layers – the perimetrium, myometrium and endometrium. The perimetrium is a thin membrane which secretes lubricating fluids. The myometrium consists of smooth muscle which facilitates contractions, which are necessary to aid the passage of sperm and also for parturition (giving birth). When oestrogen is high and progesterone is low, the myometrium is firm and in animals which can be palpated (mainly the cow and mare rectally), this can be felt and is one indicator of oestrus. Under the influence of progesterone, the myometrium in most animals (mares being one notable exception) has a much lower degree of tone, allowing the embryo to attach.
The endometrium is the inner lining which thickens during the luteal phase of the animal’s cycle to also facilitate attachment of the foetus and subsequent placentation. The placenta is the organ which connects the foetus with the uterine wall, allowing the exchange of nutrients and waste products between mother and foetus. While developed placentas are a feature of mammals and rodents, very rudimentary placentas are also found in many egg-laying and marsupial animals.
There are various degrees of placentation and they are classified by the distribution of the chorionic villi. This may be diffuse (as in pigs), zonary (found in dogs and cats), discoid (rodents and primates) or cotyledonary (ruminants). These classifications are not absolute – a mare, for example, has a diffuse placenta but with microcotyledons.
Above: Photograph showing placentomes in the cow, which consist of the circular maternal caruncles, which attach to the foetal cotyledons. Below: how placentomes show on an ultrasound scan in a ewe.
In egg-laying animals such as birds and reptiles, a uterus is not necessary as the egg matures outside of the female’s body. The majority of these animals will have only a single oviduct, instead of the scenario discussed previously, where the egg can originate from the left or right ovary and travel down either fallopian tube (Neilsen, No Date).
Watch our video, below, to see how caruncles and cotyledons connect together in the cow. Please note that this was donated by an abattoir. We did not harm a cow for the purpose of making this video.
The mother of this little bull calf was sent to the abattoir – the farmer would sadly not have realised that she was pregnant. She may still have been exhibiting signs of being in oestrus (bulling), because a dominant follicle was found developing on her ovary alongside the corpus luteum. This is very unusual, and without a proper ultrasound examination, there is no way that the farmer could know this. This resulted in the loss of a fertile cow and a perfectly healthy developing calf.
An ultrasound scan would have shown a number of important markers, which would have alerted the farmer to this pregnancy.
All of the above images are taken from the Vet Image Solutions DEFRA-approved bovine training course.
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