1. Fertilization, Conception and Pregnancy
Lecture 1: Obstetrics
Dr. KHALID MOHAMMED KARAM
Fertilization, Conception and Pregnancy

2. Introduction
Fertilization
conception
Spermatozoa transport
Pregnancy
Formation of the extra-embryonic membranes
The yolk sac
The amnion
The allantois
Placentation
Fetal development and other changes

3. Introduction
In order to achieve good reproductive efficiency it is necessary to maximize the chances of successful pregnancy at a given service or insemination. Therefore it is important to maximize both the fertilization rate and the conception rate and, furthermore, to be able to detect non-pregnant cows as early as possible so that appropriate action can be taken. The present chapter is concerned with the processes of fertilization and conception and the changes that occur during pregnancy.

4. Fertilization and conception
Much of the research carried out on fertilization has been conducted in species other than the cow; however, much of what follows refers to mammalian species in general.
At ovulation the ovum or egg is collected by the fundibular end of the oviduct or fallopian tube. It is transported down the oviduct towards the uterus possibly by a combination of cilial (hair-like) action and muscular contractions. Transport through the oviduct appears to be under the control of ovarian steroid hormones since oestrogens reduce and progesterone increases the speed of passage of ova through the oviducts. Fertilization normally occurs in the ampulla section of the oviduct close to the junction with the isthmus.

6. In the cow, the ovum enters the uterus 4–5 days after ovulation.
Mammalian spermatozoa acquire motility, and part of their capacity to fertilize the ovum, during their passage through the epididymis. At the same time, they undergo changes in metabolic patterns, enzymatic activities, and the ability to bind to zona pellucida surface, electrophoretic properties, and stabilization of some sperm structures. However, before spermatozoa are able to fertilize the ovum, they have to undergo a further series of maturational changes in the female tract. These processes are known as capacitation and the acrosome reaction and are thought to require about six hours in the cow.
This requirement for maturational changes is the main reason why it is preferable to inseminate cows several hours before ovulation

7. Spermatozoa transport
In the case of natural service, semen is deposited in the anterior vagina whereas with artificial insemination it is usual to place it just inside the uterus or in the anterior cervix. Spermatozoa ascend the female tract by both active and passive processes.
Active transport involves activity of the sperm tail or flagella, but clearly its interaction with epithelial surface secretions and cilia is also important. Propulsion of spermatozoa through the uterus appears to be quite rapid and the isthmus of the oviduct acts as a spermatozoa reservoir in many species.

8. Spermatozoa have been detected in the oviducts as little as two minutes after insemination although, again, others have failed to confirm this.
This rapid transport appears to be passive and due solely to uterine contractions
of the female and has been demonstrated to occur even with dead spermatozoa.
It is unlikely that such rapidly transported sperm would have any fertilizing function; most are found to be immotile.
On reaching the ovum, the sperm penetrates any remaining cumulus oophorus by the action of the enzyme hyaluronidase from the acrosome and comes into contact with the zona pellucida.

9. Mobility of the spermatozoa is also important in the process of sperm penetration. Normally, only one sperm is able to pass through the zona, but when more enter, a process known as polyspermy, the resultant embryo is non-viable.
The fusion of the sperm and ovum cell membranes begins at the middle of the sperm head region. The sperm head becomes engulfed by the ova with the loss of the tall. The sperm’s nuclear membrane disappears and the male chromatin comes into contact with the ova cytoplasm. Penetration by the fertilizing sperm (pronucleus) stimulates the resumption of the second meiotic division of the oocyte and the extrusion of the second polar body. Fertilization is completed with the fusion of the haploid male and female pronuclei, a process known as syngamy

10. Pregnancy Early development of the embryo
Gestation is often divided into three stages: (1) the ovum from 0–13 days, (2) the embryo from 14 days, when germ layers begin to form until 45 days, and (3) the fetus from 46 days until parturition.
The ovum begins to divide mitotically, a process known as cleavage, immediately after fertilization is complete. Division continues so that a solid cluster of cells or blastomeres known as a morula (mulberry shape) is formed by five or six days. From about day 6 after fertilization, the ovum begins to hollow out to become a blastocyst. This consists of a single spherical layer of cells, the trophoblast, with a hollow centre, but also with a group of cells, the inner cell mass at one edge. The inner cell mass is destined to form the embryo, whilst the trophoblast provides it with nutrients.

13. At about day 8 the zona pellucida begins to fragment and the blastocyst ‘hatches’. This is then followed by a period of blastocyst elongation. Development of the so-called germ layers begins from about the fourteenth day and characterizes the beginning of the embryo phase. The three germ layers arise from the inner cell mass and are termed the ectoderm, mesoderm and endoderm.
The ectoderm gives rise to the external structures such as skin, hair, hooves and mammary glands and also the nervous system.
The heart, muscles and bones are eventually formed from the mesoderm whereas the other internal organs are derived from the endoderm layer.
By day 16, the embryo is sufficiently developed to signal its presence to the maternal system and prevent the luteolysis that would have occurred if the cow had not been pregnant.
By day 45, formation of the primitive organs is complete and the fetal phase is considered to have begun.

14. Formation of the extra-embryonic membranes
The embryo is able to exist for a short time by absorbing nutrients from its own tissues and from the uterine fluids, but it ultimately becomes entirely dependent on its mother for sustenance.
Therefore the embryo becomes attached to the endometrium by means of its membranes, through which nutrients and metabolites are transferred from mother to fetus and vice versa. The attachment process is known as implantation and may begin as early as day 20, although definitive placentation does not occur until day 40–45

15. The yolk sac
This structure serves to transfer nutritive material from the uterus to the embryo and is only of transitory importance in mammals. It is formed as an out pouching of the developing gut.
It is separated from the uterine wall only by the outer layer of the blastocyst and its blood vessels readily absorb.

16. The amnion
This membrane is composed of a layer of mesoderm and a layer of ectoderm, which grow up and over the embryo and eventually fuse to enclose it in a complete sac. The amnion is usually complete by day 18 and becomes filled with fluid, providing support and protection for the developing embryo. The amniotic sac is the so-called water bag that is often seen protruding from the vulva during first-stage labor.

17. The allantois
The allantois is formed as an out pouching of the developing hind gut. This grows outward, eventually coming into contact with the external layer of cells, the trophectoderm, formerly the trophoblast, to form the chorion or chorion & allantois.
This membrane is usually well developed by day 23 and eventually surrounds the embryo, amnion and allantoic cavity, becoming densely vascularized, with the vessels branching away from the umbilical cord.

18. Placentation
The placenta is formed by intimate contact between the chorion and the endometrium. In ruminant species, the placenta is described as ‘cotyledonary’ since placental attachment occurs only in the discrete areas of the endometrial caruncles.
Exchange of oxygen, carbon dioxide and nutrients between embryo and mother takes place solely through the cotyledons.
By day 32 of pregnancy the allantois and trophectoderm almost fill the pregnant uterine horn and fragile cotyledonary attachment is also taking place there. A 12-week fetus and its associated membranes. At the end of gestation the amniotic and allantoic cavities contain approximately 25 and 15 litres of fluids respectively.

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20. Fetal development and other changes
Fetal growth is exponential throughout gestation, the rate increasing as pregnancy progresses. The average length of gestation is regarded typically as 280–285 days but is to some extent dependent on breed, particularly of the sire. For example, the effect of sire breed on the gestation period of Friesian cows. This shows that the continental beef bulls tend to produce longer gestation periods than Friesian and Hereford bulls.
Pregnancy appears to occur more commonly in the right uterine horn than in the left at a ratio of 60 : 40, with the corpus luteum typically being on the same side, reflecting the slightly more active right ovary as reported by several authors.

21. Approximately 1. 4% of cattle births are twin births
Approximately 1.4% of cattle births are twin births. Twins tend to be born prematurely and to predispose to calving difficulties and retained placenta. In cattle, as opposed to many other species, the fetal membranes of twin calves tend to fuse with each other, resulting in a direct vascular connection between the two fetuses. This means that if one fetus is lost, it is highly likely that both will be, and also predisposes to freemartinism in which testosterone from a male calf interferes with the reproductive tract
development of its female twin. Identical twins, derived from a single fertilized ova, or twins resulting from two ovulations on the same ovary are likely to begin development in the same uterine horn. The resulting overcrowding can increase the chance of loss, unless one of the calves can migrate to the contralateral horn.