| ||Fertilization | Cleavage | Implantation | Gestation
This section deals with the process of fertilization, cleavage, implantation and gestation. The most important factor in the reduction of the net calf crop is the failure of the heifer or cow to become pregnant. Factors affecting net calf crop in a disease-free beef herd that was naturally mated over 14 years were summarized by Bellows et al. (1979). Four factors reducing net calf crop (71.0%) were nonpregnancy of females (1 7.4%), perinatal calf death (6.4%), calf deaths between birth and weaning (2.9%), and fetal deaths during gestation (2.3%). Therefore, 60 percent of the reduction in calf crop could be attributed to: failure to mate, fertilization failure and/or embryonic mortality. This is a very conservative estimate because beef cows could be mated two to three times during the 45 to 60 day exposure to bulls.
The entire process of sexual reproduction is centered around the act of fertilization, which consists essentially of the fusion of two cells, the male sperm and the female egg, to form one single cell, the zygote. Each sperm cell or egg contains only half the normal number of chromosomes or genetic material, so that when fusion occurs the new individual contains the full complement of chromosomes specific for that species. In the cow, fertilization takes place at the ampullaristhmic junction of the oviduct. The female egg, which was released from the ovarian follicle surrounded by cumulus cells at the time of ovulation, arrives at the junction one to two days after standing heat, denuded of the cumulus cells and ready for fusion with the sperm. The fertile life span of the ovum is very short (8 to 12 hours). Sperm are deposited in the vagina during natural service and into the uterus during artificial insemination. Although the total number of sperm deposited into the female tract measures in the thousands of millions, the number travelling as far as the ampulla is probably not much more than 1 000 in any mammal. Some sperm reach the site of fertilization very quickly (15 minutes), but in order to fertilize the egg, sperm must go through changes termed capacitation. In cattle, sperm capacitation takes about four hours. The fertile life span of sperm is in the range of 30 to 48 hours.
The sperm must penetrate two membranes on the egg in order to complete fertilization. The first outer membrane that the sperm must penetrate is called the zona pellucida. Passage of the sperm into the zona pellucida is facilitated by proteolytic enzymes released from the sperm head when its acrosome is lost. The zona pellucida undergoes some sort of change after the passage of a sperm which makes it less easy for subsequent sperm to enter. This is called the zona reaction and is one measure of protection against polyspermy. The last stage of sperm penetration into the egg involves the attachment of the sperm head to the surface of the vitelline membrane. This is a vital period in the fertilization process since it is at this time that activation occurs. Stimulated by the close proximity of the sperm, the egg awakens from its dormancy and development begins. The sperm plasma membrane and vitelline membrane of the egg then rupture and fuse with one another to form a continuous cell membrane over the ovum and outer surface of the sperm. The other defense mechanism against polyspermy is shown by the vitelline membrane and is termed the vitelline block. The fertilizing sperm is actively engulfed by the vitelline; but subsequently, the vitelline surface becomes unresponsive to sperm contact and no further sperm are engulfed. The disadvantage of polyspermy to the organism is that it leads to an abnormal number of chromosomes in the embryo, which is a fatal condition.
Once the male and female membranes fuse, the genetic material from each parent is able to join in a process called syngamy to form one new individual. The process of fertilization is now complete and the fertilized ovum undergoes its first cleavage to produce a two celled embryo. Each daughter cell now contains the normal diploid number of chromosomes, half of which have been derived from the egg and half from the sperm. The duration of fertilization from the time of penetration of sperm to the first cleavage is estimated to be 20 to 24 hours in the cow.
Significant pregnancy losses due to fertilization failure do not appear to occur in mature cows, but may account for up to 50 percent of the conception failures in virgin heifers.
Cleavage of the newly fertilized embryo is simply mitotic divisions of one cell into two cells, two cells into four cells, four cells into eight cells, and so on. In each mitotic division, the genetic information is duplicated such that each daughter cell arising from the original cell contains exactly the same chromosomes. The embryo moves from the oviduct into the uterus at about the 8-cell stage, usually 3 to 3 1/2 days after ovulation. At this point, the embryo is free-floating within the uterus, and depends upon uterine secretions for nourishment.
By the 16- to 32-celled stage, the cells of the embryo are crowded together into a compact group still within the zona pellucida. The embryo is now known as a morulla. Fluid begins to collect between the cells, and an inner cavity or blastocyst appears. Once the cavity begins to expand, the embryo is known as a blastocyst. Some embryonic loss occurs during the time of blastocyst formation and may be due to embryonic chromosome defects. In cattle, the blastocyst stage occurs between day seven to eight after ovulation and is the time when embryos are flushed from a donor cow and implanted into recipient cows during embryo transfer. A single layer of large flattened cells, the trophoblast layer, surrounds a knob of smaller cells which lie to one side of the central cavity. knob, or inner cell mass, will give rise to the adult organism while the cells of the trophoblast form the placenta embryonic membranes. In the cow, the zona pellucid shed at about day eight and blastocyst elongation begins a few days later.
Maternal recognition of pregnancy must take place about day 16 to 17, or the uterus will produce prostaglandin F2a which will regress the corpus luteum. If the co luteum regresses, circulating progesterone levels d and the embryo cannot implant. It is now believed that embryo produces a protein or hormone-like substance which signals the uterus of its presence and thereby either blocks secretion of uterine prostaliandin F2a or rend the corpus luteum insensitive to as luteolytic actio Significant embryo losses can occur around the time maternal recognition of pregnancy due to either failure of the embryo to produce the signal or failure of the mother to recognize the signal from the embryo. lf the embryo died, or if the cow fails to recognize a pregnancy, the corpus luteum will regress normally and the cow will return to estrus at about 21 days after mating. Repeat breeders, or cows which fail to conceive after two or more services, should be suspected of having some genetic predisposition to pregnancy failure. Such cows may produce genetically abnormal eggs or may be unable to provide a suitable hormonal environment for normal embryo development to occur. In any case, repeat breeders are rarely "cured" and should be culled for fertility reasons.
While the embryo is undergoing cleavage and blastocyst formation, the uterus is also undergoing changes which prepare the way for implantation. The embryo is said to be implanted when it becomes fixed in position and physical contact with the mother is established. In cattle, the embryo remains in the uterine cavity and whatever attachment it forms with the wall of the uterus before the formation of the placenta is of an extremely loose nature. Progesterone, secreted by the corpus luteum of the ovary, acts to decrease muscular activity of the uterus. In addition, progesterone increases by supply to the uterus and stimulates proliferation of the uterine epithelium, and an increase in uterine milk secretions. By 33 days postmating, the fetal chorionic membrane has formed a fragile attachment with two to four of the cotyledons surrounding the fetus; within a few days maternal caruncles and fetal cotyledons have become so intimately interdigitated that the embryo is being completely nourished through the cotyledons. Growth of the cotyledons is also stimulated by progesterone. The other significant loss of embryos occurs at the implantation stage. Cows losing embryos at this time will return to estrus approximately 40 to 42 days after mating.
The length of gestation extends from the time of fertilization until birth of the offspring and averages 283 days in the cow (ranges from 279 to 285 days). Fetal death after implantation occurs only rarely (less than 2.5,Y,,), and any aboilions must be examined for disease factors.
Changes occur in the female's reproductive tract as pregnancy progresses. The fetal and maternal hormone systems interact throughout pregnancy such that pregnancy not only is maintained but that continued develop- ment and growth of the fetus is assured. Perhaps the most marked example of the ability of the fetus to regulate the mother's system is its ability to program development of the udder so that milk production is synchronized with parturition
The fetal placenta is the major unit producing hormones during pregnancy, and the fetus plus placenta contribute to hormonal changes in the mother's circulation in late pregnancy. Hormone production by the fetus may also affect its own growth and regulate maternal function to assure adequate environmental conditions (oxygen, nutrients, water and minerals).
Swelling (edema) and an increase in blood supply are the major reactions of the vulva to pregnancy, the edema increasing with the progress of pregnancy. These vulval changes occur around the fifth and seventh months of gestation in heifers and cows, respectively. The lining of the vagina is pale and dry during most of gestation but becomes swollen and pliable toward the end of pregnancy.
During pregnancy, cervical secretions increase to produce a very viscid mucus, serving to seal the cervical canal by the so-called mucous plug of pregnancy. Before parturition, this seal breaks down and is discharged in strings. During pregnancy, the external os of the cervix remains tightly closed. A few days before the onset of labor, relaxin is released by the corpus luteum of pregnancy and, in conjunction with increasing estrogen levels, acts to relax the cervix and pelvis.
As pregnancy progresses, the uterus undergoes gradual enlargement to permit expansion of the fetus, but its muscular walls remain quiet to prevent premature expulsion. Three phases can be identified in the adaptation of the uterus to accommodate pregnancy proliferation, growth and stretching. Uterine proliferation occurs before blastocyst attachment and is promoted by high progesterone levels. Characteristic changes of the inside lining of the uterus initiated by progesterone are increased blood supply, growth and coiling of the uterine glands, and leucocyte infiltration. Uterine growth starts after implantation. Uterine growth includes muscular hypertrophy and an extensive increase in connective tissue. Modification of the connective tissue is important both during uterine adaptation to the growing fetus and during involution after calving. The structural changes which take place in the pregnant uterus are reversible but are restored at different rates after parturition. During the period of uterine stretching (the last trimester), uterine growth diminishes while its contents are growing at an accelerating rate.
When the mother recognizes the presence of a viable fetus in the uterus, the corpus luteum persists as the corpus luteum of pregnancy and subsequent estrous cycles are suspended. During early pregnancy this suspension may not be complete as considerable follicular development occurs in the ovaries and some may even reach preovulatory size; however, these follicles eventually become atretic. The corpus luteum of pregnancy in the cow persists at a maximal size and continues to produce progesterone throughout pregnancy. However, from about day 150 to day 250 of gestation, the fetal-placental unit is usually capable of sustaining progesterone production and pregnancy should the corpus luteum regress.
Usually, animals gain weight during pregnancy due to growth of the conceptus as well as increases in maternal body weight. In heifers, nutrient retention due to growth may mask actual weight increase due to pregnancy. Considerable alternation in the distribution of water occurs in pregnancy. Some of this is mechanical and is related to the increase in venous pressure due to the weight of the enlarging uterus. Edema extending from the udder to the umbilicus is frequently observed during late gestation in cows. Therefore, heifers must receive adequate diets so that both the heifer and the fetus grow during pregnancy.
Adapted from Beef Herd Management Reference Binder and Study Guide 408-1