Download Formation of the extra

Lecture 1: Obstetrics
Dr. KHALID MOHAMMED KARAM
Fertilization, Conception and
Pregnancy
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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
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.
• Fertilization and conception
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• 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.
• 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
• Spermatozoa transport
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• 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.
• 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.
• 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.
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• 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
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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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• 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.
• 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.