Bovine biology series
Part 41 - Life cycle (2/8)
As stated last month we are building the complex animal after years of studying reduction. In this manner a life cycle is comprised biologically.
Know that we cannot, nor can science ever hope to illuminate the entirety of a single animal. For reduction science may find the smallest of particles and describe its function and molecular weight, but it cannot find its purpose.
I am rather of the opinion we leave that to the poets....
In last month’s first complex building lesson, we discovered that the role of parental contribution of haploid numbers of chromosomes is the basis of life’s renewal. That they are joined in forming a zygote is remarkable in itself, for in comprising a life of biological material, the genome found in chromosomal molecules carries the specie forward.....forward into future life once more yearning for existence.
For I am certain that life must yearn for existence. That said, perhaps it is the poet in me that believes a zygote of any specie must yearn and long to live, reach childhood, develop into adulthood, contribute its genes into the pool so that as death overcomes this complexity of life, it does not really die, but is transformed into something else....the next generation.
But for this lesson we may leave the discourse of bringing meaning and justification for life’s existence, for another time. I will, however, write of this someday.
The process is called syngamy, and the result is a diploid zygote after two cells containing haploid (or half) the normal number of chromosomes are joined together. A more common term is ovum fertilization by a sperm cell that results in conception.
In mammals this takes place in the oviduct. At the time of fertilization, a membrane very quickly surrounds the ovum so that other sperm cells cannot enter the fertilized egg.
What happens next is unexplainable, for the first cell division occurs as the beginning of the fetal growth. We can describe some of the chemistry that happens, but what drives this cellular division is not known. For our sake, it is found in the genes that carry the instructions for life’s march either forward or after maturity, backwards, as cells no longer divide and replicate themselves.
The two cells that form within the confines of this membrane are called blastomeres, a term used to name each cell that results in these first few cell cleavages. The word blastomere is derived from the Greek word "blastos", meaning germ, and mere is from the Greek word "meros", meaning part.
Cleavage continues, with each blastomere dividing once more. At day three, eight to 16 cells are made. By the 32 cell stage the cells form a circular structure called the morula (interestingly enough, the word morula comes from the Latin word "morus", which means mulberry). The morula begins its move to the uterus by day three to five.
When the morula is six to eight days old, a cavity or indentation is formed within the structure; at this point of cavitation the morula is now known as a blastocyst (cyst is from the Greek word "kystis", meaning sac). Another term commonly used is embryo, defined as that period of cavitation until there is a differentiation of the organs and the placenta, then we use the term fetus.
One question we might ask is why does fertilization occur in the oviduct? Conditions in this part of the reproductive tract are conducive for egg and sperm health, whereas the uterus is more conducive for sperm cell mobility. The fertilization and subsequent period of zygote and morula growth in the oviduct allows, at the same time, conditions in the uterus to change its conditions by cleaning up sperm cells that remain and changing tissue, hormonal (namely progesterone from the developing corpus luteum) and pH conditions for blastocyst growth.
Progesterone decreases the muscular activity of the uterus and stimulates the secretion of uterine milk from tissue cells (epithelium) lining the uterus.
At 14 days the cow embryo is 70 mm in length (2.8 inches). The nutritive requirements are met by the uterine milk. The embryo freely moves about in this milk until the third or fourth week in the cow (in the human, it is closer to a week).
Of course, the mother, a cow or woman, must recognize that an embryo does exist and that further estrus activity is not needed. This may seem to be rather elementary, but in fact a cascading of events is required for this to occur. Briefly, the corpus luteum on the ovary itself must be maintained so that progesterone levels are sufficient to inhibit PGF-alpha2, prostaglandin. This hormone is made in the uterus itself, so as long as an embryo is present and actually interacts with the uterine wall tissue, prostaglandin secretion is inhibited.
Since implantation on the uterine wall does not occur until the corpus luteum may be regressed, the embryo itself must communicate with the maternal uterine (specifically, the endometrium, the cells lining the inside portion of the uterus) to stay alive. This ensures maintenance of the corpus luteum, production of progesterone, and the inhibition of another estrus cycle.
The embryo continues to grow by rapid cell division. They are nourished by uterine milk. But soon this milk will not sustain the food requirement of the embryo, so it must implant itself on the uterine wall. The embryo is fixed, physically and functionally to a spot on the wall.
Implantation is not a sudden event but rather a slow and carefully orchestrated one. For instance, cells called trophoblasts form on the outside layer of the embryo (tropho is derived from the Greek word "trophe", which means nutrition. These trophoblasts specifically attach themselves to the uterine cell wall. The uterine cell wall (endometrium) has done its part too by forming the right kind of pre-placental tissue.
The trophoblasts, that outermost layer of the embryo, form tissues called chorions (derived from the Greek word "koreon", meaning membrane). The chorionic tissue possess highly vascularized tissue called villa, those finger-like projections that reach out to and touch the uterine wall. Thus intimate contact is made with the uterine wall (endometrium). The villa is grouped as cotyledons. After calving we can see them as round, deeply red structures placed at intervals throughout the light pink membrane.
There are several types of placental membranes; in cows they are called cotlyedonary, in humans they are called discoidal, and in the mare and sow they are called diffuse.
Thus, on the embryo side of pregnancy we have chorionic tissue giving rise to cotyledons as part of the attachment tissue found in the placenta.
On the maternal side, found on the endometrium, are caruncles. These rather frequently found tissues help support and stabilize the placental membrane.
The embryonic membrane develops into a more vascularized membrane so that nourishment can reach the embryo. This membrane is called the allantios. Nutrients diffuse through the maternal blood supply into the allantoic blood vessels. The watery like fluid within the allantios is called the amnioc fluid.
During this period of placentation the embryo is now called the fetus. Concurrently the fetus is now differentiating into specific types of structures; limbs, organs, and the skeleton can be recognized.
The umbilicus is found on the fetal side of the placenta. The umbilical cord is the entire emanation from the navel of the fetus. Several large arteries and one large vein are contained in this cord. These arteries and vein bifurcate and spread out over the fetal portion of the membrane and reach into the maternal side.
Oxygen and nutrient rich blood is supplied by maternal blood flowing into the maternal placental membrane through arteries. These arteries come in close proximity with fetal capillaries that contain fetal blood. Here, the fetal veins pick up the nourishment (oxygen, glucose) in this blood and transport it through the umbilical cord. The fetal cells are fed with this nourishment so that fetal growth can occur. When the fetal cells have used this nourishment, the waste products (carbon dioxide) are dumped into the fetal arteries and exit through the umbilical cord. The fetal arteries enter the maternal placenta, dumping this nutrient depleted blood into the maternal veins for transport back to the heart and eventually the lungs.
If this pathway reads a bit confusing, think of it in a simpler way. For instance, the fetus must receive nourishment and grow. It is, however, contained in a salt like solution, the amniotic fluid. But it must get oxygen and glucose and other nutrients, so the placenta provides these. The maternal placenta is like a lung, where oxygen can be fed to fetal capillary vessels, and also where the waste product carbon dioxide can be discharged. After the fetus is born, of course, its own lungs will do this.
The maternal placenta also serves as a digestive system, where maternal nutrients can be fed to the growing and duplicating fetal cells. Thus the fetus does not have to actually eat, as it does soon after birth. Again, these nutrients enter the growing fetus through the umbilical vein found in the umbilical cord.
Fetal blood is higher in oxygen content concentration than the maternal blood. This is also the case for glucose. This is interesting, I think, because if we think about oxygen transfer in lung tissue as highly efficient, then oxygen transfer in a liquid medium of blood would certainly be far less efficient. Thus the fetal blood must have a higher concentration of oxygen because it cannot yet breathe through its lungs.
The bovine placenta serves another very important role, that of progesterone secretion. This production occurs later in the pregnancy, for in the first half or so, the corpus luteum provides this hormone. Estrogen is also produced in the placenta in the form of estrone. This hormone is produced in conjunction with the fetus. Estrone serves as a precursor for cell growth, especially muscle and connective tissue in the fetus.
A protein hormone, relaxin, is secreted by the corpus luteum. It serves the role of quieting the uterus during pregnancy. As parturition nears, however, this hormone level increases rapidly and its target is the cervix. The cervix must relax and expand so that the fetus may be born.
Placental lactogen is secreted in the placenta. This protein hormone serves an important role by behaving like growth hormone, thus increasing the rate at which fetal growth can take place. It increases protein synthesis, which helps build the muscular system of the fetus.
The fetus grows slowly during the first three months, for during this time the entire above is taking place. Certainly once the placental membranes are securely in place and the cow is healthy, the fetus can grow faster. In a normal Holstein cow the fetus at the end of the first trimester will weigh 15 pounds. At the end of the second trimester the fetus will weigh in at 50 pounds, and at the time of drying off, seven months after conception, and the fetus will have a mass of 60 to 70 pounds of weight.
Over a third of the fetal weight occurs during the dry period. During this time the demand for nutrients from the cow is the greatest, of course. During this third trimester the fetus is clearly defined. As anyone who has seen an aborted fetus during this period knows, the expelled fetus clearly has its limbs, skeletal structure, head, eyes and ears, and even some color pigmentation of the skin.
The causes of abortion are many; by and large they are two folds.
One, in the early part of a pregnancy, if the embryo is not nourished with uterine milk, if the uterine endometrium is not healthy, or if a residual pathogen is present for any reason, the embryo is absorbed and eventually excreted from the reproductive tract as an overt, obviously unhealthy discharge.
An abortion, however, may occur even if the embryo is implanted on the uterine endometrium, and now as a fetus, it may have a fully developed placental membrane. Any hormonal imbalance, loss of progesterone for instance, or a pathogenic organism such as leptospirosis, brucella, vibriosis, and infectious bovine rhinotracheitis, may cause an abortion.
So can a nutritional problem. Excessive plant estrogen’s, pine needles, or a severe lack of dry matter intake may abort a fetus. Abortions serve an important mechanism for alerting the herdsman that something has gone wrong and the cow will sacrifice a fetus in order to stay alive herself.
Next month, we study parturition and the milk feeding period.
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