Veterinary obstetric ……………………………………………………………………..dr.Hella J. Al-Fatlawy
Physiology of pregnancy
Fertilization and conception
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 . 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. 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
The process of capacitation is stimulated when sperm enter the female
reproductive tract. The acrosome reaction follows capacitation and involves
the fusion of the sperm cell membrane and the acrosome and the formation
of gaps through which the acrosome contents can diffuse. The acrosome
reaction is necessary to allow penetration of the oocyte by the sperm.
Veterinary obstetric ……………………………………………………………………..dr.Hella J. Al-Fatlawy
Spermatozoa transport
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.
Spermatozoa have been detected in the oviducts as little as two minutes after
insemination . 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.
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. The sperm nucleus possesses a
cytoskeletal coat, the perinuclear theca (PT), which is removed from the
sperm head at fertilization. The PT contains an oocyte-activating factor. This
has not been characterized, but is thought to be responsible for triggering the
signaling cascade of oocyte activation.
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.
Following fusion of sperm and egg, the contents of the cortical granules in
the egg release into the perivitelline space causing the zona pellucid to
become refraability of the zona pellucida to prevent the entry of another
spermatozoon after fertilization persisted through to the blastocyst
stage.ctory to sperm binding and penetration. The fusion of the sperm and
ovum cell membranes begins at the middle of the sperm head region. The
Veterinary obstetric ……………………………………………………………………..dr.Hella J. Al-Fatlawy
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 .
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
(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.
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
Veterinary obstetric ……………………………………………………………………..dr.Hella J. Al-Fatlawy
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. This mechanism is described below under ‘Hormonal
changes during pregnancy. By day 45, formation of the primitive organs is
complete and the fetal phase is considered to have begun.
Early development of the fertilized ovum to the blastocyst stage.