Bovine biology series
Part - 44 Life cycle (5/8)
This is our fifth lesson of the life cycle. This month we examine calving.
If ever we wonder about life and what meaning we may bring to its definition, then the birth of a new generation, out of the old, commands our greatest attention and ultimate respect. For I cannot think of any other aspect of true awe than when a female gives birth. And then, indeed, can nurture and provide nourishment for its newborn.
I don’t remember my first experience at calving. I am sure, however, that I was moved by it, in fact watching and participating in the birthing process, as well as most of you do know, is one of life’s moving moments. For some of us, our love of biology and science is born in a calving pen, although our feeble attempts in explaining what is going on is humbling and truly spiritual. Nevertheless, we do know a little.....
We do know that it is the calf that initiates calving by secreting cortisol from its adrenal cortex. The fetal cortisol, in turn, stimulates the shared placenta in secreting prostaglandin. The prostaglandin is the driving hormone that shifts the cow’s progesterone concentration towards increasing the cow’s bloodstream estrogen level. As calving nears, placental prostaglandin levels increase dramatically. Prostaglandin does many things, including: preparing the muscular uterine walls to accept calcium ions that are critical for contraction during calving, regressing the corpus luteum, that most important tissue responsible for progesterone secretion maintaining the pregnancy, and also prostaglandin promotes relaxation of the cervix so it may dilate.
Relaxin is another hormone that helps relax the pelvis of a cow so that the calf has more room for delivery.
So calving is largely controlled by a dramatic shift of hormones, from maintenance of the fetus to more active ones that let muscles begin contracting.
At the time of calving, then, dilation of the reproductive tract is underway. The cow is building a nest, so to speak, finding a comfortable place to stretch her body in an almost paralyzed manner. This stretching out and elongating of the frame helps the cow breathe easier, as well as straightening out the reproductive tract.
Labor is largely controlled by the onset of very high levels of oxytocin, secreted by the cow herself. Oxytocin is thought to be released first as the fetus bumps up against the pelvic canal, a signal that expulsion is eminent. Known as Furgeson’s reflex, the fetus initiates nerve impulses that travel through the cow’s spinal cord to the hypothalamus (brain). The hypothalamus, then, directs the posterior lobe of the pituitary gland in releasing oxytocin so as to initiate muscular contraction, and expulsion is ongoing as the very fast acting oxytocin drives muscles to move the fetus through he cervix and into the vagina. Oxytocin and prostaglandin are responsible for the rhythmic contractions of the uterine muscles.
The placenta is still very much a part of the caretaker role for the fetus. That continues as long as the fetus is still away from airspace first made possible after the delivery is complete.
Probably the most significant and dramatic change from fetus to calf is the entry of air into lungs instead of oxygen supplied by the cow’s bloodstream through placental support. At birth, of course, the placental circulation of blood is cut off. The calf becomes asphyxic from hypoxia, or lack of oxygen. The term asphyxic refers to the stopping of the pulse that results from the placental circulation cut off.
Indeed, the fetus must be delivered outside the vagina as soon as possible. Once outside, the gasping and opening of the lung tissue expands in rather dramatic manner the cavity of space in the new calf for holding air in the lungs. Those first few gasps expand this area.
At or very near the same time, the umbilical cord is severed, no longer is it needed, for the fetus is now a newborn individual, on its own. Its bloodstream is now receptive to the oxygen rich air breathed in through its mouth and nostrils, filling the lung tissue. Also, with respiration comes the exchange of carbon dioxide and water vapor out of the lung tissue.
What about the placenta? Even though its primary job is done, it still has one role to play that of orderly removing itself from the lining of the uterus, the endometrium. As anyone who knows what a retained placenta is all about (and the multitude of problems that go with one of these), this orderly step is extremely important if the uterus is to once again serve as a viable organ for another pregnancy.
Ruminants have what we call cotlyedonary placentation, which means that the placental attachment to the endometrium is accomplished by groups of finger-like projections grouped into cotyledons. These cotyledons attach themselves to specific sites on the endometrium called caruncles. Thus, the fetal cotyledons and the maternal caruncles comprise the attachment tissue called the placentome.
This relationship of shared tissues is ended at calving, in the chemical sense of sharing nutrients and waste products, and the physical form by separating from one another soon after the calf is born. This is the final step of calving, the expulsion of the placental membrane.
The separation of the placenta from the endometrium is a complex interaction of physical relaxation of the protruding villa that clings to the endometrium. If this separation is not complete, the placenta is retained. There are many causes, including vitamin E and selenium deficiency, overt body condition, hypocalcemia, dystocia or multiple births. In some cases, bacterial invasion at the time of calving occurs, and thus metritis is diagnosed.
But if everything proceeds as directed by a wonderful interplay of chemical and physical actions, and some more that we do not even know how to describe or characterize, the cow expels the placenta, the calf begins to breath, and in an hour or less the calf, wobbly at first, then with stable and sure foot, nurses one of four teats for its first meal.
And in truly amazing manner, within 50 days or so the reproductive tract in this cow will once again be ready to start this process all over again.....that is if everything went reasonably well this time.
Next month: all about lactation.
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