Lesson-6. Fertilization and foetal development.
Fertilization
Fertilization is the fusion of the male and female gamete resulting into a zygote. At
Ovulation the ovum is collected by the fundibular end of the oviduct. It is transported
down the oviduct possibly by a combination of cilial acition and muscular contractions.
Fertilization normally occurs in the ampulla-isthmus junction.
Before spermatozoa are able to fertilize the ovum, they have to undergo a series of
maturational changes in the female reproductive tract ( Capacitation). These process
require about 5-6 hours in the cow. The process of capacitation is stimulated in some way
by the female reproductive tract. The acrosome reaction involves the fusion of the sperm
cell membrane and the acrosome and the formation of gaps through which the the
acrosome contents such as lytic enzymes ( Hyaluronidase) can diffuse.This enzyme
permit the sperm cell to penetrate the cumulus cells surrounding the the ovum. The sperm
cell losses the acrosomal cap and plasma membrane exposing the perforatorium and inner
acrosomal membranes which are responsible for the penetraton of the zona pellucida and
reach the vetelline membrane. After penetration of the vetelline membrane, the nucleus of
the male gamete fuse with the female gamate resulting into formation of a pronuclei. The
chromosome in each pronuclei unite to form the zygote.
Daigram.
Maternal recognition of Pregnancy
Maternal recognition of pregnancy in cattle involves physiological mechanism that
results in protection of corpus lutea from leutolysis by modification or inhibition of
uterine production of Prostagladin. Oxytocin is secreted by the posterior piyuitary and
CL of cattle in pulsatle manner and interacts with the the endometerial receptors to
stimulate episodic secretion of PGF2 which is essential for leutolysis. Progesterone
from CL and Oestrodiol from follicles appear to be responsible for development of
endometerial receptors for oxytocin.
In cattle the bovine tropoplast protein-1 ( bTP-1) are secreted by the trophecoderm of
conceptus between days 10 and 24 days of pregnancy. This antileutolytic protein is
primarily responsible for inhibiting uterine production of PGF2.
Post.
Pituitary
Ovar
y
P+
Endometrial
Receptor
E+
OXY
PFF2
leutolysis
BTP-1
-VE
Conceptus
Figure. Schematic illustration of the Mechanism of maternal recognition of pregnancy in
cattle
DEVELOPMENT OF CONCEPTUS
Gestation is arbitrarily divided into three stages as follows.
0-13 days
Stage of Ovum,
14-45 days
Stage of Embryo
46 until parturition
Stage of foetus
Stage of Ovum
 Following the fertilization, zygote begins to divide mitotically,a process known as
cleavage and, as a result of peristaltic contractions and ciliary currents in the
oviduct, the zygote is propelled towards the uterus and can be found in the uterine
horns by day 4.

By day five or six, a solid cluster of cells or blastomere known as a Morula is
formed.

After day 6 the Zygote begins to hollow out to become a blastocyst, which
consist of a single layer of cells, the trophoblast with a hollow center and also a
group of cells, the inner cell mass at one end.

The inner cell mass is destined to become the embryo while the trophoplast
provide it with nutrient.
Figure. The process of cleavage of the zygote.
Stage of Embryo


By day 8-9 after fertilization, the zona pellucida begins to fragment and the
blastocyst begins to elongate and extend throughout the uterine horn.
Development of the germ layers begins from about the fourteenth day. The three
germ layers arise from the inner cell mass and are term as Ectoderm, Mesoderm,
and Endoderm. The ectoderm gives rise to the external structures such as skin,
hair, hooves and mammary gland and also the nervous system. The heart, muscles
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and bones are eventually formed from the Mesoderm whereas the other internal
organs are derived from the endoderm.
By day 18 the amnion begins to enclose the embryo
By day 22 the heart begins to beat
By day 23, the neural groove closes to form brain and spinal cord and head
becomes recognizable
At the same time the allantois is also well develop
Liver becomes prominent and the forelimb buds appear at about 25th day.
By 26 day, the embryo becomes curved and tail buds appear.
The hind limb buds appear by the 28th day.
Eyes and nostril become evident by 30-45 days
First Placental plate appear by 30th day and allanto-chorionic sac fill the gravid
horn
By 33 days fragile cotyledonary attachment develops
By day 37,facial features become clear
Period of Foetus
 By day 46-54, the appendages elongate and the eyelid closes by day 60.
 Bones ossify by day 70-180.
 By 90th day, hair follicles appear
 By 110th day, teeth development begins
 Growth of Hair takes place in 150-220 days
Foetal growth is exponential throughout gestation, the rate increasing as pregnancy
progresses.
Stage of pregnancy ( Months)
1
2
3
4
5
6
7
8
9
Foetal-crown-rump length ( Cm)
0.8
6
15
28
40
52
70
80
90
Development 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 has to depend on the mother for sustenance. Therefore the
embryo become attached to the endometrium by means of its membranes and by which
nutrients and metabolites are transferred from mother to fetus and vice versa. This
attachment is known as Implantation and may begin as early as day 20 days and is
completed by day 40-45. The attachment to the endometrium of the mother is achieved
through the development of foetal membranes.
The Yolk sac is formed as an outpouching of the developing gut. It is separated from the
uterine wall only by the outer layer of the blastocyst and it readily absorbes nutrients. The
yolk sac serves to transfer nutrient from the uterus to the embryo and is only transitory.
This role is eventually taken over by the allantoises.
The amnion is composed of a layer of mesoderm and a layer of ectoderm which grow up
and over the embryo eventually fusing to enclose it in a complete sac. It is usually
completed 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.
The allantois is formed as an out pouching of the developing hindgut. This grows
outward eventually coming in contact with the trophectoderm to from the chrion or
chorion- allantois. This membrane is well developed by day 23 and eventually surrounds
the embryo, amnion and allantoic cavity, becoming densely vasculaized with the vessels
branching away from the umbical cord.
The allantochorion contacts the uterine caruncles, finger-like processes or villa,
containing capillary tufts , grow out from the allantochorion into the crypts of the
maternal caruncles, which also are surrounded by capillary plexuses. Thus is formed the
characteristic ruminant cotyledon, or placentome, through which the nutritious and
gaseous exchange takes place between mother and fetus. There are about 120 functioning
cotyledons arranged in four rows along each of the uterine horn. In cattle,the placenta is
of cotyledonary type as the villi are grouped into multiple circumscribed areas.
Figure. Development of foetal membranes
Foetal development.
Hormonal changes during Pregnancy.
Oestrogen
The ovarian follicle development continues during the pregnancy due to low levels of
Gonadotrophin secretion. Both ovarian follicles and embryo-plancental unit produces
oestrogen. Plasma milk Oestrone sulphate concentrations rise gradually during pregnancy
in the cow and their measurement is a means of Pregnancy diagnosis.
Progesterone level in plasma and milk rises during the first few days of pregnancy in an
identical manner to that occuring in early luteal phase of the non-pregannt cow. Instead
of declining from about day 17 , high progesterone levels are maintained throughout
pregnancy reflecting the maintenance of CL. Mean plasma level of LH and FSH remain
low during pregnancy.
Gestation Period of different species of animals.
Species of Animals
Gestation period ( Days)
Cattle
280 +- 5 days
Horse
327-357 days
Pig
112-115 days
Sheep
140-155 days
Goat
142-145 days
Dog
60-63 days
Cat
56-65 days
yak
258 days