Bovine biology series
Part 40 - Life cycle (1/8)
We have concluded a rather complete examination of mammalian biology by reduction of the complex into the simple.....cells and tissues and organs.
Where do we go from here?
Well, for the next seven lessons we will examine the inter-relationships of the cells, tissues, and organs. That is, instead of reducing the mammal into smaller and simpler components (reduction), we will study the complexity of them. In this way, we can then begin understanding the marvelous function of mammals.
In my final lesson, part 48, I will list an index for you and in summary form highlight the totality of the mammal. Thus, we will have completed four years of study, and almost 200 pages of text.
For now, let us move on, building the complex mammal from the beginning.....
What is the beginning? That is "The Question" that I cannot begin to write about here, for such an answer predisposes knowledge that does not exist to us. There is, I believe, always to be an unknowable answer, which part of our history that is in rational thought unexplainable. To know is really to not know. For me, personally, the true pathway of life is more than mere biology, and even though I am a biologist and love the enduring study of cells, tissues, organs, and their function, I know that there is more, an infinite more that I will never know. The pathway, though, of our individual life cycle may be towards the illumination of that infinite more, grasping, searching, contemplating and attempting a feeble understanding of where we came from, where we are going, and what purpose do we have here?
Such questions challenge every thinking individual. They challenge me when I write this series. And they are deeply inset in our individual lives if we will truly undertake such a challenge.
I believe that we can partake of this challenge by studying the life cycle of mammals. We can bring meaning into our own lives by examining the biology of life’s onset, birth, adolescence, puberty, reproduction, maturity, and finally, the return into the organic atomistic mixture.
Before we jump into this rather comprehensive study, I am recalling a fond memory. Standing atop a mountain in Honduras, my colleagues and I gazed into the night sky. I have rarely seen the stars so bright, the air so clean, or the vision of grandeurness so beautifully displayed. I was quite comfortable in knowing that I am, and you are, and everyone is a part of a great existence. While I can never explain, nor can I ever understand much that I see, I continue to try to do so.
Let us begin this challenge together, the illumination of a life cycle, the simple made more complex. Yet, let us be humble too, knowing that there is a part, a great part, which is nevertheless unknowable.
We begin at the organic conception of male and female.....
The testis are the site of spermatogenesis, the formation of sperm cells (spermatozoa) from body or stem cells. During spermatogenesis, the number of chromosomes is halved, the double helix of DNA is unwrapped so that just one of them is found in the newly formed spermatozoa. Within the testis this process takes place in the seminiferous tubule.
Spermatogenesis is begun when the stem cell undergoes mitosis. This first phase of sperm cell production begins in the very lining of the seminiferous tubule, where contained in the basement membrane are the stem cells that divide, divide again, until after four divisions in the bovine bull (only three in man) a primary spermatocyte is formed. Thusly, from one stem cell is formed 16 spermatocytes after the fourth mitotic division.
The second phase of sperm cell formation is meiosis. Chromosome numbers are halved in this phase, as the DNA is unwound and each component is left in a haploid state (meaning halved). The halving of chromosomes is essential during meiosis so that the individual strand of DNA may match up with the similar individual DNA strand contributed by the female.
The haploid spermatozoa have the proper number of chromosomes.....30 in the case of dairy cattle and 23 in humans.
The third, and final step, is the maturation of the spermatozoa. Maturation involves the formation of a tail so that these cells can move about within the female reproductive tract, the formation of mitochondria, so that energy can be supplied to this tail, and the very important acrosome, a specific part of every spermatozoa that functions as a membrane breaker when the spermatozoa reaches the female ooctye (female egg).
Thus, sperm cells, spermatozoa, are made by the millions in the continuing process of spermatogenesis. The resultant spermatozoa are owners of a haploid state of chromosomes, that is, half of normal body or germ cells; they have a tail for moving about and they possess a specific acrosomal head for breaking and entering the membrane enclosing the female oocyte.
The spermatozoa pass from the seminiferous tubules into the epididymides, where they accumulate energy (glucose) into their cellular membranes as they mature.
At the time of ejaculation, the spermatozoa, combined with other seminal fluid, are discharged through the spermatic cord containing excretory ducts through the penis itself. The average ejaculate in a dairy bull is 6 milliliters, but can vary between 2 and 10 mls. The average number of spermatozoa per ejaculate is 8 billion, but this concentration is highly variable depending upon bull health and especially frequency of ejaculation.
Now, the female mammal is quite different in this point: the number of reproductive eggs is set at the time of birth. In quite a remarkable manner by which we do not begin to understand, when the emerging fetus of a mammal is defined as female, written into its genetic code of instructions are the numbers of egg cells, oocytes that will emerge from the ovary. The number is set, as if in recognizing that in the female, the time of sexual reproduction is limited. It is based on a cycle of hormonally controlled interactions, and as such, reproduction can occur in a limited time period every month or cycle....a few hours or even during a specific season.
Whereas, the male can produce millions of spermatozoa every day and thus, be capable of reproducing at a moments notice, the female is only capable of doing so in an extremely limited manner.
We know this to be true, for we can provide thousands of bull units for artificial reproduction over the lifetime of a healthy bull. But the cow can only produce just a handful of calves, unless we alter, in an artificial manner, the ovulating ability of her reproductive cycle. Even then, the female is capable of producing only a set amount of eggs, and we can easily exhaust this supply is carried to the hormonally induced extreme.
We may wonder why the male and female have evolved in this manner. For the primary life function in mammals, as for all of life, is to reproduce, the continuation of DNA into the next generation so that the specie will endure. In the plant kingdom as well as the animal, we know that the basis of the life cycle is reproduction, thus continuation of genetic information into the next generation.
We may wonder, then, why the relationship between the male and female in mammals is so made? Consider this: if in fact the cow ovulated multiple eggs every day, then reproduction would be far simpler in that mating would almost guarantee the fertilization of an egg. What then?
Well, multiple fertilizations obligate the uterus to multiple implantation’s, multiple placenta demands and a huge, huge demand for energy as needed for the growing fetuses. We know that this could not be possible, for twins present a tremendous metabolic and physiologic demand on a cow.
As far as having a cow ovulate ever day without a cycle, this too is unreasonable, for the uterus is not prepared for a fertilized egg every day, and the hormonal relationship of cycling would be lost, thus obligating the cow to almost constant pregnancy.
We know that pregnancy is a both a required part of life, certainly, but it is a luxury at the same time. That is, everything else must be going OK, nutritional demands are met, systemic immune system is healthy, and the reproductive organs are free of infection. Thus, we conclude that there is a very good reason for one or two eggs to be ovulated every 21 days or so, and that for just a few hours in this time period fertilization can occur. We may further conclude that these female eggs are precious for they are just one a few hundred that will be viable if reproduction occurs.
The sites of oocyte....female egg production are the ovaries, of which there are two in mammals. The ovaries develop in the female fetus in such a manner like the mitosis phase in spermatozoa development throughout the life of the male. The entire mitotic period in the female is accomplished during the fetal growth period. At birth, mitosis ends and the number of oocytes are established.
Female puberty is largely controlled by the establishment of cyclic ovarian activity. At puberty, the female ovary is the site of meiosis as the haploid oocytes occurs. The meiosis of haploid cells does not occur in female infancy. This protective measure, in effect, provides for body growth and the establishment of a reproductive organ network so that after fertilization, the resulting fertilized egg can be grown.
The meiosis phase begins in the germ cells (primordial follicle) of the ovary, from which a follicle is developed. Follicles are really a clump of cells, one of which is the haploid oocyte. The others are follicle cells surrounding the oocyte that provide protection and nourishment during the corpus luteal phase of ovulation.
While there can be, and usually are, several follicles growing in various stages of maturation, usually one and occasionally two will be ovulated so that fertilization can occur. Why? Well, one good reason is the matching or coordination with the hormonal system, so that at the time of highest reproductive capability, an ooctye freshly ovulated after the corpus luteum has regressed is ready to go, ready for a spermatozoa and thus, receptive to fertilization. The corpus luteum is a protective organization of cells that encompasses the ooctye, but at the time of highest luteinizing hormone surge, ruptures and releases the oocyte. The oocyte is on its own now, feeding from the ovary, the site of its birth, and the follicle, its protective home until it too must die.
Ovulation is really the rupture of the corpus luteum so that the ooctye is freed. Ovulation generally occurs 24 to 48 hours after the surge of luteinizing hormone produced in the anterior pituitary gland. The female begins that part of her reproductive cycle known as estrus. Ovulation in the cow occurs about 12-18 hours after the onset of estrus. In this time period artificial insemination is most successful: the follicle has just been ruptured and is now free, swimming in a solution particularly agreeable to spermatozoa.
At the time of copulation, semen is deposited in the vagina, and the millions of sperm cells must get through the cervix into the uterus and into the oviducts. This can happen within minutes after ejaculation of semen deposition has occurred.
Spermatozoa are changed here in preparation for potential fertilization. A process called capacitation requires several hours. Capacitation prepares the acrosome for its primary objective.....penetrating the female oocyte. The acrosome is capable of providing enzymes at the time of contact so that the oocyte membrane can be breached or broken into. Spermatozoa must be capacitized if they are to fertilize. These capacitized spermatozoa live 24 to 48 hours.
The oocyte, recently freed from it corpus luteal home, will live 12 to 18 hours after rupture. So there is in both haploid cells, the male spermatozoa and the female oocyte, a window of time for fertilization to occur.
The process of fertilization can be defined as the moment these two meet. The first stage involves the passage of the capacitated spermatozoa through the outermost membrane of the oocyte, the zona pellucida, facilitated by the spermatozoa enzymes.
When this initial penetration occurs, immediately thereafter a zona reaction takes place, closing the successful spermatozoa within this zone so that another spermatozoa cannot enter. Once the spermatozoa is successfully encased within this zone, its head enlarges, as does the nuclei of the oocyte until they come together in the cytoplasm of the oocyte. The membranes eventually fuse, incorporating sperm and egg nuclei into a single cell.
This amazing process is known as syngamy.....the fertilization is complete and now the new cell is called the zygote.....the organic conception complete!
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