The union of sperm with
egg cell takes place in fallopian tube of female reproductive
system. After this union diploid Zygote is formed with two sets of
chromosome one set of chromosomes from sperm and another
set from ova.The two sets of genetic material from two
sources recombine to recreate and a unique genetic combination, which
unfolds gradually during embryonic development forming a baby. The baby
inherits some traits from father and some from mother. The period in
which the fertilized zygote develops in to fully matured baby in side
uterus is called gestation period. The gestation period for human
is 9 months.
The fertilized zygote undergoes rapid cell divisions called Clevage which starts 30 hrs after fertilization. The zygote undergoes first mitotic division and forms two identical cells called Blastomeres. The clevage continues to form mass of 16 cells called Morula.The
size of morula is same size as that of zygote. By 72 hrs Morula reaches
uterus. Morula continues dividing and forms ball of 100 cells called Blastula. It is hollow at centre as is called Blastocoel.
It gets implanted in endometrial tissue. In rare conditions one
out of 300 pregnancies, Blastula some where other than uterus in
fallopian tube or abdominal cavity (Ectopic pregnancy) causes severe
complications.
The next stage in
embryonic development is the formation of the body plan.The cells in the
blastula rearrange themselves spatially to form three layers of cells
in a process known as gastrulation. Each of these layers is
called a germ layer, which differentiate into different organ
systems.The three germs layers are the outer ectoderm, , and middle mesoderm and inner most endoderm
Organogenesis is
the process by which the three germ tissue layers of the embryo, which
are the ectoderm, endoderm, and mesoderm, develop into the internal
organs of the organism by cell differentiation, the process by which a
less-specialized cell becomes a more-specialized cell type. This must
occur many times as a zygote becomes a fully-developed organism. In
vertebrates, one of the primary steps during organogenesis is the
formation of the neural system.
The ectoderm
It forms epithelial cells
and tissues, as well as neuronal tissues.It gives rise to the nervous
system and the epidermis. During the formation of the neural system,
special signaling molecules called growth factors signal some cells at
the edge of the ectoderm to become epidermis cells. The remaining cells
in the center form the neural plate.
The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord .
The endoderm
It consists of flattened
cells, which subsequently become columnar. It forms the epithelial
lining of the whole of the digestive tube. It also forms the lining
cells of all the glands which open into the digestive tube, including
those of the liver and pancreas; the epithelium of the auditory tube and
tympanic cavity; the trachea, bronchi, and air cells of the lungs; the
urinary bladder and part of the urethra; and the follicle lining of the
thyroid gland and thymus. Additionally, the endoderm forms internal
organs including the stomach, the colon, the liver, the pancreas,
the urinary bladder, the epithelial parts of trachea, the lungs, the
pharynx, the thyroid, the parathyroid, and the intestines.
The
mesoderm gives rise to the muscle cells, bones and connective tissue in the
body.
Week 3:
The embryo grows to a length of about 2
millimeters long with rounded head and a
tapering tail. There are four extra-embryonic membranes which protect the developing embryo they are the Yolk sac, the Allantois , the Amnion, and the Chorion.
Amnion surrounds
the body of the embryo like a fluid-filled balloon.It allows fetus to be
suspended in amniotic fluid, protecting from mechanical injuries,
allows normal fetal movement and accommodates growth.The normal amount
of amniotic fluid is typically 500 to 1000 ml.Amniocentesis
is procedure of removing a small amount of amniotic fluid by inserting a
needle through the mother’s abdomen and into the amniotic cavity.Fetal
cells present in the amniotic fluid can be cultured and examined for
various chromosomal and metabolic defects.Traditionally after birth of
baby, amnion and and placental tissues were discarded, but these tissues
were found quite promising for stem cell therapy as they contain
pluripotent stem cells that can form variety of tissues.
Yolk sac usually contains reserve food 'Yolk' for developing embryo. It is
well observed in embryos of fishes and amphibians. In human embryo Yolk
sac is devoid of yolk and is vestigial as there is ample supply of
nutrients from mother's blood.
Allantois is
large sac which is vestigial again in human embryos but in most aves
and reptiles it stores urinary wastes and serves respiration.Allantois
is continuous with forming urinary bladder of embryo.
Blood vessels begun to form and by day 20, the embryo has developed
arteries and veins which circulates embryos blood. Cells aggregate along the
embryo's dorsal surface to form neural tube. Neural tube eventually develop into the brain and spinal cord.
Week 4:
The embryo takes curved C
shape as eye stalks and ear pits appear. Upper and
lower limb buds become noticeable. The umbilical cord and
facial areas develop. It`s size is approx 3-4 mm in this stage. Body
tapers to spiraled tail which is prominent in 4 week embryos.
Lung, liver, pancreatic, and gall bladder
buds emerge in the form of mesodermal somites which are
group of differentiating cells.Fourth week is marked by rapid
neural and cardiac development. On either sides of neural tube feature
spinal cord primitive bones develop. Nerves, muscle, and
connective tissues emerge
around the primitive bone formations.
Week 5:
the embryo is about 7 to 9
millimeters long and has all of its internal organs. The mouth,
stomach, and urinary bladder are present. Nose pits and eye lenses
are
visible. A few days after upper limb bud extension, the lower limb
buds
evolve further.The external ears takes their form, and upper limb
buds extend to form paddle like hands. Extensive brain development
occurs at this time, and the
head grows big, causing it to appear large compared to the
body. The umbilical cord becomes more clearly defined.
Week 6:
The trunk of embryo
straightens and upper limb continues developing. Bone, cartilage, and
muscles become defined around the spinal
cord and in the embryonic chest. Early in this week, tooth buds
appear.
These buds will become the baby teeth that are lost in
childhood. Rib cells line up horizontally along the trunk sides,
and skin.The regions of the brain that
will become the cerebral hemispheres are very prominent at this
time. The
embryo appears more human by this point. It is about 11
to 14 millimeters long, and its rate of heart beat is 140 to
150 beats per minute.
Week 8:
Nerve cells in the
brain form at a rate of about 100,000 a minute. The top of the head
becomes more rounded and erect. Between day 52 and day 56, the fan shaped
toes go from being webbed to separated. The fingers are entirely distinct.
The eyelids close over the eyes and become fused shut until about the
twenty-sixth week. External genital sex organ differences begin to
develop. All appearances of the tail are gone. By day 56, the embryo is
roughly 27 to 31 millimeters long.
The first three months of embryonic development are known as the first
trimester. At the end of
the first trimester, the embryo looks like an adult, with all major organs
having been formed. It is about 7.5 cms long. Additional time
in the mother's womb is needed to permit further development of the
organs.
At
the beginning of the second trimester, the growing organism is no
longer called an embryo, but a fetus. Fetal development continues
through
the second and third trimesters until it is ready for birth at the
end of
the ninth month.When fetus reaches the end of 9th month, uterus
contractions begin due to Oxytosin hormone associated with labor pains.
Child birth takes place by rupture of embryonic membranes by dilation
of cervical muscles and vagina. parturition. The process of
normal childbirth is occurs in three stages the dilation of the
cervix, birth of the baby fallowed by detachment and exit of
placenta through vagina. The umbilical cord is clamped in two
places and is cut in between.Respiration initiates normally by usual
first baby cry. The newly born baby given warm water bath and swallowed
amniotic fluid is removed out using a sterilized tube. The baby is
allowed to feed on mother`s first milk called colostrum which has high
concentrations of nutrients and antibodies. Antibodies in colostrum
protect the mucous membranes in the throat, lungs, and intestines of the
baby.
The cells in the
blastula rearrange themselves spatially to form three layers of cells in
a process known as gastrulation.
During gastrulation, the blastula folds upon itself to form the three
layers of cells. Each of these layers is called a germ layer, which
differentiate into different organ systems
Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
The cells in the
blastula rearrange themselves spatially to form three layers of cells in
a process known as gastrulation.
During gastrulation, the blastula folds upon itself to form the three
layers of cells. Each of these layers is called a germ layer, which
differentiate into different organ systems
Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
The cells in the
blastula rearrange themselves spatially to form three layers of cells in
a process known as gastrulation.
During gastrulation, the blastula folds upon itself to form the three
layers of cells. Each of these layers is called a germ layer, which
differentiate into different organ systems
Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
The cells in the
blastula rearrange themselves spatially to form three layers of cells in
a process known as gastrulation.
During gastrulation, the blastula folds upon itself to form the three
layers of cells. Each of these layers is called a germ layer, which
differentiate into different organ systems
Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
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