Bovine biology series
Part - 21 Reproductive Hormones (1/2)
Reproduction: Hormones
We begin a new subseries of lessons this month. For the next few months we will examine male and female reproduction.
This month we begin with some general comments about hormones, and I will be referring to them frequently throughout this subseries on reproduction.
I came to some knowledge of reproduction very early on a farm. There is nothing quite as grand or miraculous as watching a litter of piglets expelled from the cavity of a white sow, or kittens born beneath a pile of hay. I recall many moments during my wonderful childhood in which calves were born; covered with the wet and slippery mucous, then they rather abruptly rose, wobbled a bit, and found mom. Somehow the calf seemed driven to find nourishment, and somehow this newborn creature just knew that it would be found through the opening of a teat engorged with white or pink colostrum.
I also remember one experience very early in my life that I have never forgotten. A cow bred about six months came into the milking parlor, and through the reproductive cavity there was a six month fetus in advanced stage of expulsion: an abortion about to happen.
A few hours later I stood next to this sick cow with the Veterinarian as he identified the probable cause: the Hardjo strain of Leptospirosis. Our vaccination program, while usually protective, had not done so in this case. Disease had won out, and at least for this cow, there must be reason.
I have thought about this abortion many times.
In the context of reproduction, I write of these two experiences for a good reason. Reproduction is required for all species to survive. It is not limited to animals, for plants and bacteria and even solar systems in our cosmos must be reproduced or they die away, the genetic substance of them, be it the speck of viral stuff, the whiff of a mustard seed, the complexity of a Guernsey cow, or the entire hydrogen and ice mass of a solar system should be lost.
To that end, reproduction begets birth. It is a luxury, I am sure. That is, where upon the days of my childhood I witnessed the blessing of birth, such an end could be expressed if reproduction worked, and worked well. If such a stress as a Leptospira organism, or a physical ailment, or a nutritional deficiency was to be expressed, then the wonder of homeostasis, the rule of self-preservation, takes precedence, and the luxury of reproduction is sacrificed.
Consider this: the very act of an abortion or a miscarriage is biology performing self-preservation: Homeostasis. As if the body, the one you and I possess, recognizes the need for survival, and only if everything is working fine, we are fed good enough, we get enough rest, we are relatively free of disease, and our immune, hormonal and blood chemistry parameters are relatively normal, then we can produce the required stuff of reproduction. And in the female, the step of pregnancy, that glorious act fulfilled when birth occurs can happen when the female body itself is healthy.
So I approach reproduction and pregnancy as a luxury in biology. You can be sure that if we truly examine a herd of cows, for instance, if the reproductive parameters fall within normal boundaries, then the herd is relatively well-fed, healthy, free of disease and living in a healthy environment.
Similarly, if cows are not getting bred, if they do not show signs of estrus, if they abort a fetus, or if at calving there are a multitude of metabolic and physiologic problems related to retained placentas or reproductive infection, then something is wrong in such a way that cows are difficult to impregnate or if they are, do not carry to full term, and if they do carry to full term, they may contain such problems that reduce or eliminate the chances of another pregnancy.
We know, for instance, how difficult cows are to conceive after a prolonged retained placenta. We know, too, that nutritional deficiencies can reduce the number of eliminate altogether estrus and therefore cycling of hormones in cows. We know that even if everything goes well, and the animal conceives and she is doing well production-wise, then an invading bacteria or virus that challenges the integrity of the body proper before the immune system can render it or them harmless, will result in the fetal death so that the body, a cow in this case, can survive. Even that is not enough in some cases, for if the reallocation of energy to fight an invading pathogen is inadequate, the cow dies.
Life is not fair. Certainly that is apparent to us as we examine the wonder of reproduction. I ask that you remember throughout this discussion, that reproduction is not a right but a luxury; it is necessary but it is not guaranteed. And it is one of the most challenging facets of biology. Without it we are soon absent, but with it life is a progression of generations. Be it the lowly bacteria living in a soil particle, or the birth of solar system somewhere as we gaze into the cosmos on a night such as this summer one, life is renewed over and over.
Now, the topic of hormones. Finally.
Hormone is derived from the Greek word "hormaein", a word meaning to set in motion, or to spur on. We define hormones as a chemical substance produced in the body by an organ or cells of an organ or by scattered cells. The substance, a hormone, has a specific regulatory effect upon the activity of certain organs or groups of cells or cell types.
Thus, hormones as chemical substances travel via the blood and lymph systems to target locations throughout the body, and these targets are clearly defined, clearly responsive to the hormone, and clearly a change in activity happens.
I have throughout this biology series discussed the exocrine and endocrine systems. By definition, the exocrine system is comprised of organs that secrete substances through or into ducts. Examples are salivary glands, secreting buffers into the mouth cavity as an aid in adjusting rumen acidity. The mammary gland, of course, is an exocrine gland, secreting milk into lumen cavities, which accumulates into ducts leading to the teat sinus.
The endocrine system, however, is inexorably tied to the production of hormones. The glands of the endocrine system are ductless; instead their secretion (hormones) are released directly into the bloodstream or lymph so that metabolism or reproduction can be controlled or altered.
The endocrine glands include the hypothalamus, pituitary, and pineal body in the head, the thyroid, parathyroid and thymus of the throat area, the adrenal glands sitting atop the kidneys, the pancreas near the stomach (the pancreas is an exocrine gland too, secreting bile into the bile duct and protein digesting enzymes into the pancreatic duct for emptying into the stomach), and the gonads, a term that collectively includes the male testes and the female ovaries.
The endocrine glands directly influence the way our bodies perform, exist, sleep, work, digest, reproduce, and eventually die.
For instance, the pancreas accomplishes the production of insulin, and insulin is required for the passage of glucose into cells so that they are fed and therefore live to perform their specific duties.
The pituitary gland, the little bit of tissue situated at the base of the brain near the spinal column, is a tremendously important endocrine organ. The pituitary gland is the one that senses milk letdown and thus secretes oxytocin so that its target organ, the smooth muscles of the exocrine mammary gland, will contract and therefore physically move milk into ducts and into the gland and finally the teat lumen so that the milk volume may be removed.
The pituitary gland's oxytocin is required during the birthing process too, as the smooth muscles lining the birth canal are contracted, thus aiding in the birth of the soon-to-be born calf and the fetal placenta membranes.
The hypothalamus secretes oxytocin and vasopressin, a hormone stimulating the contraction of blood capillaries so that blood pressure is increased. This hormone is extremely important in the case of a general trauma involving blood loss, so that blood pressure is maintained during the course of rapid blood loss. Vasopressin plays a role in adjusting urine output so that blood volume is maintained. In times of heat exposure, for instance, the blood capillaries near the skin open so that fluid in the blood can carry heat away from the body in the form of thermal regulation. This results in the loss of blood volume.
Endurance athletes know this drill too well, for we are told to drink before we begin to sweat in order to compensate for the loss of blood volume. Well, vasopressin as secreted by the hypothalamus plays a role too, increasing the volume of liquid reabsorbed in the kidney so that urine output is reduced.
The problem of lowered blood volume involves several less than desirable consequences, such as reducing the availability of nutrients via the bloodstream that can be delivered to extremely hard working cells and conversely, the removal of waste products, like lactic acid and carbon dioxide gas that are the result of these hard working cells. In essence, regulation of blood pressure and blood volume is critical to Homeostasis.
The thyroid gland, near the throat, secretes two metabolic hormones, thyroxin and triiodothyronine. These two hormones contain iodine for their important role in helping regulate metabolism in the body. The basil metabolic rate, or BMR, is greatly influenced by this endocrine gland. And these two hormones have many targets in the body, most having to do with regulating general cell metabolism as influenced by genetic factors secondarily and environmental factors primarily (such things as lifestyle selection, body fat composition, respiratory rate and general degree of fitness).
Hypothyroidism [too little] is characterized by a reduction in BMR, and clinically is expressed as fatigue and general lethargy, and the body is sensitive to cold temperatures because the core temperature is or can be insufficient to maintain a feeling of warmth that is comfortable.
Hyperthyroidism [too much] is characterized by increased BMR as the result of an over-active thyroid gland and hence over-production of thyro-hormones. Clinically, the body is racing faster than it should, so cells are working overtime. Weight loss, nervousness, sweating (increased core temperature must be dissipated via skin cooling), and muscular weakness are typical signs of hyperthyroidism. An enlarged thyroid gland (Goiter), that swelling in the neck area, is a condition brought about by insufficient intake of iodine in the diet or ration. The thyroid gland increases in size due to hyperplasia, a term used to describe the abnormal proliferation of cells in response to the overcompensation for an organ trying to compensate for an inadequate production of its specific metabolite or hormone....in this case, the iodine hormones.
Goiter is especially prevalent in areas where seafood is scarce. For instance, the mountain range areas of the Andes, Himalayas, the Alps and Pyrenees are inhabited by people lacking sea food given the distance and terrain of securing such food, and also the fact that these higher elevation areas of the planet have not been once part of the sea. The oceans of the world are rich in iodine; hence, the produce of the sea and the land where the sea once flowed contains iodine sufficient for animal intake.
Interestingly enough, large areas of the Pacific Northwest and the regions near the Great Lakes and upper East Coast are areas of endemic goiter deficiencies, driven largely by a low level of iodine in soils.
The adrenal glands, situated near the kidneys, serve dual roles. The first is found in the adrenal cortex. Here, the gland produces and secretes into the bloodstream the sex hormones estrogen and progesterone (as well as host of many other related steroid hormones). Here, too, the gland is influenced by the pituitary gland, whose hormone corticotropin stimulates the formation of steroid hormones.
Corticotrophin is also called ACTH, or adrenocorticotropin hormone. It serves other functions in addition to stimulating adrenal function of steroid hormones. Some call ACTH the stress hormone. It is labeled as such because it causes certain and specific physiological changes in the body during times of stress. For instance, the conversion of glucose from other non-carbohydrate molecules, a process called gluconeogenesis is increased under a stressful situation in which the animal has been depleted of glycogen stores in the liver. Maintenance of sufficient glycogen inventory in the liver is critical, because if blood glucose levels drop, many body functions are slowed or stopped.
On very common one is loss of mental function, and it is this reason our mothers sent us to school with a hearty breakfast. The conversion of that breakfast into glucose and glycogen fueled our brains so that we could learn and create. Glucose is the high octane fuel that is capable of very rapidly being shunted to muscles cells or brain cells via insulin and thus work or thought can be done. There is a very limited supply of glycogen, however, so this precious fuel is used only when necessary. That is why hard working cows or long distance athletes must constantly be fueled, or they run out of glucose, then glycogen, and the result is subclinical ketosis-like symptoms; body turns primarily to fat burning for energy.
We know that with cows under a constant sub-measurable amount of stress, say, a poorly ventilated building or damp free stalls, that ACTH levels are increased so that energy is shunted away from things like milk production and reproduction and more towards keeping muscles fueled with glucose. The affect of such energy fractionation is obvious, in that over time, the overall health and well being of the animal is compromised.
Eventually, the animal succumbs to stress, and clinically the expression of it is found in an impaired immune system that cannot deal appropriately with a pathogen that a healthy, less stressed animal has no problems dealing with.
This is an extremely valuable point. Subclinical stress does take its toll, accumulatively, over time. The body is resilient and can deal with stress to a certain point.
Eventually the body functions are impaired in such a way that the cascading of events, all of which are the direct result of stress and the indirect result of less energy available for routine functions, cause disease and infection, poor performance and unthriftyness, reproductive failure and loss of pregnancy, and eventually the loss of homeostasis. Usually, however, the animal is sold for slaughter long before she dies, for the alert herdsman identifies the symptoms near the front end and before too many cascading events occur; she exits the herd.
The class of steroid hormones that is produced by the adrenal cortex, called glucocorticoids, is also responsible for the control and therefore regulation of carbohydrate metabolism, and to some extent fat and protein metabolism.
Glucocorticoids cause the formation of glycogen and its storage in the liver. This is related to stress as well. Again, in a stressed animal, the body can rapidly deplete glucose and therefore liver glycogen. Replacement therapy of such a condition is accomplished with glucocorticoids, such as hydrocortisone.
This hormone is rapidly converted into cortisol in the body, and its effect upon the liver is to favor the formation of liver glycogen from non-carbohydrate sources. This is, of course, the body’s way of dealing with carbohydrate depletion so that very important body functions like heart muscle contractions; brain functions and breathing muscles are able to function. Again, homeostasis at its best. Hydrocortisone also inhibits the action of insulin; thus less glucose can be used by cellular activity. As such, then, glucose is saved for truly important functions; the peripheral ones like muscle activity in the legs or lactation demands of the mammary gland or fetal demands are reduced or slowed.
Another major function of glucocorticoids is their ability to stabilize or reduce inflammation. When tissue is injured or infected, the typical inflammatory response includes the accumulation of fluids into the tissue spaces, the swelling of tissues due to dilation of blood vessels and the buildup of white blood cells. The administration of hydrocortisone is commonly done to reduce this swelling.
How does this work? Well, hydrocortisone reduces blood vessel diameters, reducing the volume of blood volume in the local area [hyperemia is defined as an excess of blood, a normal part of the inflammatory process]. Hydrocortisone also reduces the exudation [the escape of fluid from blood vessels into interstitial space], as well as decreasing the rate at which white blood cells loose their integrity after performing their critical role of engulfing pathogens and other waste debris that is found in a local trauma area that is inflamed.
Dexamethasone is a particular kind of glucocorticoid that serves a specific role of anti-inflammatory. It is 25 times more powerful than hydrocortisone, and thus serves as an adjunct to antibiotic therapy in dealing with infection and the inflammatory reaction.
A related class of hormones secreted by the adrenal cortex is the mineralcorticoids. The most common one is aldosterone, a steroid hormone responsible for promoting the retention of sodium and bicarbonate and the excretion of potassium and hydrogen ions that are metabolical waste products. Excess aldosterone cause edema and plasma volume expansion.
Aldosterone plays a role in hydration, of course, in that by retaining sodium, this cation has the affinity to hold onto water. So retaining sodium is retaining water.
The target organ is the kidney. Excess potassium is excreted, so in times of excess dietary potassium [an increasingly common metabolic concern in dairy herds fed forages grown on soils having multiple applications of storage manure pond effluent that is high in potassium and is thusly luxuriously incorporated into plant material. Since potassium is largely absorbed in the digestive system, any excess over and above metabolic function obligates the kidney to discharge it into the urine volume], aldosterone plays a role in aiding its exit from the body.
A concern is the balance between discharging excess potassium, thus an obligation of the adrenal cortex to elevate levels of aldosterone, and yet in order to do so more water is consumed (drinking) for this flushing activity, and yet the aldosterone promotes retention of sodium and therefore water. So in the case of hyperkalemia, a condition of excess potassium, aldosterone therapy is called upon, but the body will retain a higher quantity of water.
Mineralcorticoids also enhance the retention of sodium in peripheral tissues: the sweat glands, salivary glands and in the intestinal mucosa, keeping them moist and pliable.
Next month: adrenal cortex steroids and the adrenal medulla hormones, the catecholamines: epinephrine and norepinephrine.
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