Download Implantation of Embr..

IMPLANTATION,
FETUS
ETAL MEMBRANES
AND
Implantation is the embedding of the
developing embryo m the lining of
the uterus. In cow implantation
actually begins from 11th to 40th day
post coitum. After fertilization the
zygote travels slowly down the
oviduct; cell division occurs and this
early cleavages of the fertilized egg
are completed in the oviduct, the
young embryo, consisting of from 8
to 16 cells (the blastocyst stage
arrives in the uterus in search of
permanent attachment by about 4
days.
During the first days after its arrival
in the uterus, the embryo is
completely dependent upon uterine
secretions for its energy. The uterine
glands under the influence of
oestrogen
and
progesterone
hormone secrete 'uterine milk" which
is composed of protein, fat and
traces of glycogen. As the blastocyst
increases in size, it can no longer
absorb enough nutritive material by
diffusion, and thus makes it
imperative that it establishes a more
adequate source of nutrition. About
the 8 day zona pellucida begins to
break up, and the cells push
outward.
Layers of cells form and from these
layers grow membranes that soon
will nourish the developing embryo.
To understand the development of
these membranes, it is first
necessary to have some concept of
the three primary germ layers in the
developing embryo. After numerous
cell divisions, the embryo is a hollow
sphere of cells a few layers thick.
This single tissue layer is known as
the ectoderm, and is the origin of the
skin and other structures.
Somewhat later, one side of this
sphere is pushed in to form a double
walled cup-like structure, leaving
ectoderm on the outer surface; the
inner surface is known as endoderm,
Later part of this pinches off into a
tube to form the digestive tract.
Proliferation of cells between the
ectoderm and endoderm gives rise
to a third germinal layer; the
mesoderm. The mesoderm plays an
important part in the formation of
muscles.
From these three germinal layers
arise not only the various tissues of
the developing embryo itself, but
also those surrounding membranes,
which protect it and enable it to
obtain nourishment.
These membranes are collectively
known as the extra-embryonic
membranes.
The period of the embryo between
13th day to 45th day is characterised
largely by the first formation of most
organs and body parts. During this
period the digestive tract, the lungs,
the liver, and the pancreas all
develop from the primitive gut. The
heart and the circulatory system are
started, and between 21st and 22nd
day the heart begins to beat. The
beginnings of the nervous system,
the muscular and skeletal systems
and the urogenital system are
established.
From the sides of the embryo a fold
grows up and over it, fusing at the
top and ultimately enclosing the
embryo in a double-layered sac that
is known as the amnion. This amnion
or water bag as it is commonly called
becomes filled with a clear watery
fluid in which the embryo is
suspended. Its purpose is to form a
protective cushion against eternal
shocks and pressure of the adjacent
body organs and to prevent
adhesions between the surface of
the embryo and the surrounding
membrane. At parturition, the
amnion acts as a wedge to dilate the
cervix% at which time it usually
ruptures, allowing the "waters" to
escape.
As an outpouching of the hindgut of
the digestive tract the allantois is
formed. It enlarges and fills the
space between the amnion and the
outer membrane of the placenta, the
Choron.
The allantois functions as the urinary
receptacle for the embryo and, also,
collects some solid wastes. The
chorion, .the outer membrane,
completely surrounds the embryo,
arnion and allantoic cavity, is rich in
blood vessels and lies in opposition
to the uterine mucosa, through
which, by diffusion and osmosis, an
exchange of gases and nutrients
occurs between the blood vessels of
the foetal circulation and the blood
stream of the mother. The allantois
which fuses with the chorion
becomes the foetal placenta.
The transfer of O2 CO2 and nutrients
is affected by more or less intimate
union of the chorion with the uterine
mucosa of the dam to form the
placenta. The mucosa, or lining of
the uterus is essentially a spongy
network of blood sinuses in which
the finger-like villi of the chorion bury
themselves. Thus, the placenta is
partly maternal and partly foetal in its
origin. There is no mixing of maternal
and foetal blood, however, for both
nutrients and gases must pass
through the membrane of the
placenta The food materials leave
the maternal placenta and enter the
foetal circulation by diffusion through
the maternal and foetal placentae,
after which they enter the blood
vessels of the allatois witch
terminate in the umbilical cord.
Wastes are carried back to the
maternal circulation by the same
system. There are two general types
of placentae among farm animals.
The sow and the mare have a diffuse
placenta; the entire chorion is beset
with finger-like villi which fit into
corresponding depressions in the
uterine mucosa- The cow and the
ewe have a cotyledonary placenta;
the villi are localised in a hundred or
so rosettes, or cotyledons, over the
surface of the chorion. These are
separated by areas of smooth
chorion. The villi of the cotyledons fit
into pits in the spongy button like
cotyledons of the uterus. At
parturition, the chorionic villi of both
types are merely withdrawn, and
there is no extensive destruction of
the uterine tissue.
The extent to which a foetus grows
in the latter stages of pregnancy is
demonstrated by the actual ·weights
obtained by the late Sir John
Hammond
in
one
of
his
investigations
carried
out
on
Shorthorn heifers:
Table
Month of pregnancy
1
2
3
4
5
6
7
8
91/2
Weight of foetus
Or calf (kg.)-- 0.00009 0. 009 0. 090 0.730 1-816 2.724 10.442 16.80 34.00
Correct feeding is important throughout pregnancy, but it is even more important
during the last stages, when the foetus is making very rapid growth.