Bovine biology series
Part - 23 Gestation
Gestation: pregnancy
The word gestation is derived from the Latin word gestatio, from a related word gestare, which means to bear. We define gestation as the period of development of the young in animals. The gestation period begins at fertilization of the female ovum by the male sperm, and ends upon birth. The word pregnancy is also a Latin word, derived from the word praegnans, which means with child (or in the case of a cow, with calf).
Considering reproduction, the female ovum and male sperm cell are quite unique. They each possess exactly half the number of chromosomes as a normal body (somatic) cell. Incorporated into each of these chromosomes is one very long DNA molecule representing thousands of genes. We are genetically defined by our genes, just as all animals are. Simply stated, when the sperm cell unites with an ovum cell of the female, the genes from these two cells unite, or become fertilized. Thus the genes from each parent combine into a fertilized egg specifically found in the cell nucleus. At this point, the fertilized egg is the time of conceptus; gestation begins, and the female is pregnant.
The process of fertilization is called syngamy; the resultant fertilized cell is called a zygote. The zygote begins replicating, of course, and by 6-8 days becomes a blastocyst, or the more popular term embryo. Once placental membranes form and the embryo is replicating enough so that organs and tissues can be differentiated, the embryo then becomes a fetus, and is thusly called the fetus until birth.
Fertilization occurs in the oviduct. The uterus is not ready for the implantation of the embryo quite yet. The uterus must be cleaned up, so to speak, from the environment that was just a few days prior conducive for sperm cell motility and mobility.
Before passage of the embryo into the uterus, progesterone from the developing corpus luteum has been preparing the uterus. Progesterone decreases the muscular activity and tonicity of the uterus. This steroid hormone also favors the formation of uterine milk, a nutritive substance that feeds the embryo until the placental membranes are in place. This implantation upon the uterine wall occurs 2-5 weeks after conception.
The female must very soon recognize that she is pregnant. In some way, the reproductive system must signal the CL to keep going and secret progesterone. Concurrently, prostaglandin must be inhibited; if it is not, abortion will occur. An absolute requirement of pregnancy is the inhibition of prostaglandin (PGF2 alpha is the prostaglandin that initiates regression of the CL). Another signal to the cow that an embryo is present is the elongation of the embryo. This free-floating cellular mass interacts with the lining of the uterus, namely the endometrium. This interaction favors the retention of the CL.
The embryo becomes fixed to the uterine wall, both physically and functionally via the placental membranes. In order for the embryo to be fed and so that it may in turn grow, progesterone is secreted by the CL and to a lessor extent, the placenta. Estrogen is also produced by the placenta and the growing embryo and fetus.
Once implantation has occurred, the fetus and female are joined together via the placental membranes. The female, of course, nourishes the fetus so that it may grow. In turn, the fetus is able to discharge waste products into the placental membranes so that the female can rid them as she does her own.
For parturition, or birth to occur, the uterus must undergo a transformation from a restful, non-physically contracted one towards one that prepares for the birthing process to begin. The fetus or calf that is about ready to be born secretes hydrocortisone (cortisol) from its own adrenal cortex. Thus it is the fetus that begins the process of birth, altering the dynamics of hormonal control from the progesterone that maintains pregnancy to estrogens that stimulate uterine contractions. This process is not sudden, but occurs over a month's period before calving.
When the time of birth is very near, within a few days, the estrogen levels have increased to the point that they stimulate the production of prostaglandin. The CL begins to regress. Thus 24-36 hours before calving, the reproductive tract cannot sustain the pregnancy any longer. The increasing levels of prostaglandin exert a strong contraction influence on the uterine wall. It is the very strong influence of oxytocin that begins the actual uterine contractions. Labor has begun. Oxytocin is produced in the hypothalamus and stored in the posterior lobe of the pituitary gland. It is called upon to contract the smooth uterine muscles (just as it does to contract the smooth muscles cells of the mammary gland).
So it is the fetus that decides when to be born. By secreting cortisol, a cascading of endogenous (within the body) hormonal changes occurs; with finally the level of prostaglandin’s overriding the influence of maintaining the pregnancy. Cortisol is a stress hormone, and we might consider this when animals - females are under stress. Under such times of stress, females can abort due to higher than normal levels of their own cortisol production, in spite of the level secreted by the fetus. That is why moving dry cows or placing them in a stressful environment should be done as carefully as possible or the stressful environment eliminated.
The approximate period of gestation, 282 days, is one that requires just the right mix of endogenous hormonal balance. For if a pregnancy is to be carried to term, then at birth a complex series of events, many of which are controlled by hormones, result in a healthy calf, the newborn that begins life on its own.
Reproduction is a luxury, but without it, life ceases. Maybe that is why we are so amazed at the wonder of a calving pen that in the space of just nine months a calf is born.
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