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1st & 2nd Weeks of development
Dr. Abdul Waheed Ansari
Chairperson & Prof. Anatomy,
Embryonic changes during first week
• Fertilization takes place in the uterine tube.
• The zygote formation is the result of fertilization.
• The zygote divides continuously to form a ball of cells.
• The zona pellucida ruptures after due to increase in cell
• The cells organize into two layers, inner cell mass and
outer cell mass.
• The inner cell mass are embryonic cells, going to form
• The outer cell mass are trophic cells, going to form the
• 1. polar body
• 2. perivitelline space
• 3. zona pellucida
• 4. cell membrane
• The ball of cells is called as morula, a cavity
appears inside, it is renamed as blastula.
• The morula migrates and moves towards the
uterine cavity and the outer cell mass
penetrates the layers of uterus and buries the
developing embryo into the walls of the
• It is called as implantation.
Normal & abnormal sites of implantation
• Normal implantation
zone of the blastocyst is
in the superior and
posterior wall of the
• The fertilized human
through the tube and
implants itself on the
6th day in the
3. In the fallopian tube (most frequent)
5. In the lower uterus segment (later placenta
7. In the pelvic area
As implantation is underway
• There is much activity inside and around the
• A structure called the yolk sac forms from the
cells of the hypoblast.
• The yolk sac is a structure that provides nutrients
from the mother to the embryo before the
placenta is ready to function.
• A membrane called the amnion forms from the
• The epiblast also gives rise to the entire head and
body of the embryo.
Four celled stage
• 1. perivitelline space
• 2. zona pellucida
• 3. blastomeres
• 1. zona pellucida
• 2. perivitelline space
• 1. cavity within
• 2. zona pellucida
1. zona pellucida
2. embryo escaping from the zona (called
3. blastocyst cavity
4. perivitelline space
Inner cell mass-embryoblasts
1. inner cell mass
2. blastocyst cavity
• The inner cell mass will going to
form the entire embryo.
• The inner cell mass will rearrange
into two layers-the bilaminar germ
disk-tall columnar cells lines the
amniotic sac and cuboidal cells will
form the roof of yolk sac.
• These are the changes occurring
during the second week of
• The tall columnar cells are going to
form the epiblast cells and the low
cuboidal cells will form the
The trophoblasts are the outer cell
• The trophoblast cells initially
single layered. Later they
form double layered
• The inner cells are called as
• The outer cells are called as
• The primary villus will form
together-the precursor of
placenta during the second
week of development.
• The secondary villus is the
result when extraembrionic
mesoderm is invading the
• With the migration of fetal
blood vessels into the
secondary villus, it is
renamed as tertiary villus.
• The syncitiotrophoblast cells
manufacture the hormone,
HCG (Human chorionic
gonadotropin) and released
into the urine of mother.
• Approximately 8 days after fertilization, cells from
the growing embryo begin producing a hormone
called human chorionic gonadotropin or hCG.
• This hormone is present in a pregnant woman’s
blood and urine almost immediately and is the
substance detected by most pregnancy tests.
• HCG is commonly called the pregnancy hormone
and its detection is the most frequent way of
• Just 8 to 9 days after fertilization.
Establishment of utero-placental
circulation (Days 11 to 12)
• The syncytial cells penetrate deeper in the stroma
of the uterine wall, they finally erode the
endothelial lining of the maternal capillaries.
• The syncytial lacunae then become continuous
with the sinusoids and maternal blood enters the
• As the trophoblast continues to erode more and
more sinusoids, maternal blood begins to flow
through the trophoblastic lacunar system, thus
establishing the primitive utero-placental