Bovine biology series
Part - 22 Reproductive Hormones (2/2)
Reproduction: Hormones
I do not remember the first time I became aware of estrus in a cow. I do, though, remember the importance of it very early on, when we were taught that when one cow mounted another, the numbers of both cows were written down. A few days later, when I happened to observe one of these cows laying down in the free stall, she would expunge a red discharge. I wondered, as a youngster, what might this be all about?
Later as a student in biology class, our High School teacher rather embarrassingly tried to answer my question in front of the class. He stumbled a bit, and then blurted out something about a change in hormones would result in reproductive changes, including the discharge of blood from the reproductive tract.
Of course I became more interested in such things, and later in a graduate level physiology class, I would spend a whole semester studying reproductive physiology. But I never forgot the lessons I learned on the farm.....that cows act differently when hormones change, cows come into heat, and without these working in concert, a pregnancy does not happen.
Could we say, then, that there is a reason why cows come into heat? As directed by the cascading of hormonal events beginning with the hypothalamus and the pituitary glands of the brain, and ending with the ovary and uterus, the period of estrus is defined by the Greek word "oestrus" which means "frenzy and passion". Wow! These two words adequately describe the period of cows in heat, searching out the bull that may meet her desirous passions for sex. Thus we know that in cows, the expressions of mounting and increased activity are overt signals to cows and hopefully a bull that ovulation is near, it is time to get on with the business of fertilization so pregnancy can occur.
What a remarkable system, is this hormonal one. Without it, what might signal the bull to mount the cow? Without it, would bovines survive very long? Not likely. Thus the beauty of biology is found when we consider the adaptations and mechanisms that are in place so that a specie like the bovine can survive. It is, of course, necessary for every specie on this planet.....reproduction. And we know that life itself is one of reproduction. The great majority of animals and plants on this planet live long enough to reproduce, then the mission is complete, and they die away, no longer needed for if they remain too long, valuable resources are consumed and soon the entire specie is in peril.
While this is a fascinating topic about reproduction in general, we must get back to the cow. And the hormones of reproduction.
Let us begin in the head, the site of the brain, and the location of two endocrine glands responsible for much of reproduction.
Very near the base of the brain where the spinal column is attached is the hypothalamus. This endocrine gland is highly sensitive towards the environment. Such as temperature, light and stress. Thusly it is, by and large, an organ of sensory perception, and that means the environment of the cow influences the amount and kind of hormonal product.
The hypothalamus secretes a hormone called GnRH, or gonadotropic releasing hormone. It is here, in this spot, that reproduction originates. The stimuli of the environment and the genetic switches that certainly are the result of generations and generations of development, erupt and signal the hypothalamus to awaken. The protein hormones, such as GnRH are made and stored. At the right time they are released in just the right amounts, making their way to the anterior pituitary gland. This little bit of tissue, also situated at the base of the brain, receives these GnRH molecules.
What message do they carry? As if by the lighting of a lamp, the anterior pituitary is awakened as well. Specifically, cells and groups of cells called gonadotrophs begin making two hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The message, then, is to grow a follicle and prepare the reproductive tract for fertilization and pregnancy.
The target organ for LH and FSH is the ovary, of which there are two in the normal cow. Both are protein hormones, and both are synergistic as they work together in developing the ovary for the release of an egg.
LH and FSH are secreted during the follicular phase of the reproductive cycle. As the cycle enters the 12th or 13th day, a rise in estrogen stimulates the production of LH and FSH. The estrogen is derived from the ovary, and thus the feedback to the brain is to begin the follicular portion of the cycle once more if there is no pregnancy. The FSH stimulates the growth of follicles on the ovaries.
The yellow mass of tissue on the ovary is the site of a growing follicle. The release of prostaglandin by the uterus, a signal that this organ is ready for fertilization, causes it to rupture. The follicle ruptures, the ovum is released, and the animal is ovulated. In a cow this occurs about 8-12 hours after the end of estrus. If the ovum is fertilized, the spot on the ovary becomes a mature yellow body, or corpus luteum. The CL secretes progesterone, a steroid hormone that is required to support the pregnancy. If the ovum is not fertilized the CL shrinks and estrogen once again becomes the steroid hormone preparing the reproductive organs for another estrus cycle.
This is, of course, why cows are bred about 12 hours after the end of estrus. It is at this time in the estrus cycle that ovulation has just occurred. The ovum will live in the fallopian tubes for 24 hours or so, and so the sperm have about one day in which to find this cell and fertilize it.
The role of prostaglandin, then, is to cause ovulation, but only if a follicle is mature enough. That is, if the part of a cycle is too soon after the previous ovulation, there will not be a mature-enough follicle present. The prostaglandin will do no good, as it does not have a receptive target. However, if the cycle period is 14 or more days since the previous ovulation and there is no pregnancy, then a mature follicle should be present on one of the ovaries. The prostaglandin will rupture the follicle and the ovum is released, beginning the fertile period of the cycle.
So two conditions exist as the cow experiences her reproductive cycle. If the ovum is fertilized, then the CL is maintained, and progesterone is the predominant hormone in support of the pregnancy. If the ovum is not fertilized, the CL regresses and shrinks, the output of progesterone is diminished and the ovary, then, secretes estrogen. The estrogen makes its way to the hypothalamus, and the cycle begins again with the onset of GnRH production.
The steroid hormones, estrogen and progesterone, serve different roles and are produced by different tissues and glands. We know that maintenance of the corpus luteum (CL) is absolutely required in order for a normal pregnancy to develop. The CL, sitting atop the ovary, holds in check the cascading of events that begins another reproductive cycle. As if held in a state of rest, the hypothalamus and anterior pituitary gland relax, for their job is done. A pregnancy has happened. The ovaries CL will support it, along with prostaglandin secreted by the placenta.
If, however, the ovum is not fertilized, then the cycle repeats. That is, the ovary secretes estrogen, waking up the endocrine glands of the brain, and the estrus cycle begins again.
We know that fertilization takes place in the fallopian tubes. The ovum, once fertilized, stays here for a few days, until the uterus can be prepared for implantation. The CL, in secreting progesterone, helps in this preparation. Progesterone slows uterine muscle activity and changes the uterine lining. These changes promote the formation of the placenta, the bridge between developing embryo and the cow.
The maternal recognition of implantation and pregnancy is important in order the pregnancy be maintained. Prostaglandin, either secreted by the uterine exogenously or given artificially endogenously, will abruptly end a pregnancy. So the cow must recognize an embryo is in her uterus and thus the secretion of prostaglandin is stopped. It is in the wondrous relationship that is formed as a bond between embryo and cow in the lining of the uterus, endometrium and placental cells that a signal is sent to the brain that a pregnancy exists.
It is this part of reproduction that scientists cannot completely explain. Or said another way, the very special bond between the glob of embryonic cells that is required for the specie to survive, and the body in which it lives, the warm bathing fluids of nutrients and nurturing chemical properties, is a part of life we lack definition for. All that we know is that it works when everything else is OK. For if reproduction is a luxury, and I believe it is, then an embryo and a mother, and the bond of these two generations is quite unique indeed.
But how so very important it is to have.
We switch gears a bit and move away from reproduction into the area of a class of hormones called the catecholamines. These are protein hormones secreted by the adrenal medulla, a different part of the adrenal glands as compared to the adrenal cortex where steroid hormones originate.
Three common catecholamine hormones are dopamine, norepinephrine (noradrenaline) and epinephrine (adrenaline). The latter two are ones more commonly known. All three are derivatives of the amino acid tyrosine.
Epinephrine is secreted by the medulla cortex in response to stress. This hormone causes quick and major changes in the body. One is the mobilization of glucose from muscle cells, so that when a cow is frightened, she can immediately take off running very quickly. Also, fatty acids are released from fat cells and these are used for fuel - energy to move muscles. Epinephrine and norepinephrine can increase the rate and stroke volume (the amount of blood that is pushed into the arterial system with one stroke of the heart). They also can alter the smooth muscle control of some blood vessels, shunting blood away from the skin, gut and kidneys and moving more blood volume to the brain, heart and skeletal muscles. You may recognize this series of descriptions as ones describing the flight or fight syndrome. So we have, then, a mechanism of rapidly preparing the body to run very fast away from a threat or stay put and fight. Both require a change in normal body parameters. The body is not in cruise control; it is ready to do something very quickly. The fuel is the readily available glycogen in the muscles. It is in short supply, though, compared with the reserves of energy found in fat cells. But in times of stress involving rapid movement, that (glycogen) is what it is there for.
The adrenal medulla is connected with the sympathetic part of the nervous system. These nerve sites are throughout the body, many of which are at the surface of the skin. These nerve endings are in close proximity to chromaffin cells, which main role is to secrete acetylcholine, the precursor to the catecholamines. One additional role of norepinephrine specifically is the maintenance of blood pressure.
This class of hormones play an important role if an animal should go into shock. This condition can be expressed in many forms, including hypovolemic shock (insufficient blood volume for the organs and tissues), cardiac shock (insufficient blood volume from each heart stroke), septic shock (overwhelming infection due to toxins or substances that impede the flow of blood from a portion of the body) and anaphylactic shock (immediate reaction to a toxin resulting in respiratory failure and vascular failure (examples are bee stings, snake bites and certain antibiotics in certain hypersensitive individuals)).
The catecholamine hormones wait in residence, but in times of stress, immediately respond in an effort to keep the animal alive. It is one more of the many mechanisms that promote Homeostasis, that inborn need for self-survival.
Next month, we examine another part of reproduction, the gestation period. From fertilization to parturition. The period known as gestation.
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