Transcript embryology - Human Anatomy

Week #1: Fertilization………
MORULA
MORULA
to Implantation
A.
Fertilization (1)
• The development of a human
being begins with the fusion
of an ovulated oocyte and
a sperm
fertilization
– The oocyte and sperm are
called germ cells
– Each is haploid
• contains 23 chromosomes
following meiosis
• this is the amount of DNA
needed by somatic
(body) cells
Mitosis
: chromosomes double 1x,
the nucleus (cell) divides 1x
Meiosis
: chromosomes double 1x,
the nucleus (cell) divides 2x
A. Fertilization (2)
• This produces a zygote
–this is a single cell
–it means “united”
–it is diploid
• it contains 46
chromosomes
• like all somatic cells
zygote
B.
• The embryo now
rapidly undergoes a
series of mitotic
divisions
called
cleavage
• This results in an
–increase the
number of cells
Cleavage
C. Cell Differentiation
• By day 4, the embryo
forms a morula
–a ball containing 16cells
–the cells have now
differentitated into
• an inner cell mass
– gives rise to the
embryo proper
(namely YOU)
•an outer cell mass
– gives rise to the
embryonic contribution
to the placenta
morula
(means “mulberry”)
D. Cavitationand the Blastocyst
• Next a fluid-filled
cavity
forms
between the inner
cell mass and the
outer cell mass
• This is called
“cavitation”
• This gives rise to
the blastocyst
inner cell mass
outer cell mass
cavity
“blastocyst”
“blastocyst”= “germ-bag”
E.
Implantation
• On about day 7,
the blastocyst
implants itself into
the wall of the
mother’s uterus
Week #2: BilaminarDisk Formation
•The inner cell mass
now differentiates into
2 distinct cell layers
epiblast
–the epiblast
–the hypoblast
hypoblast
• This is called the
bilaminar
embryonic disk
(1)
Week #2: BilaminarDisk Formation (2a)
• A cavity
forms next to
the epiblast
–It is called the amniotic
cavity
–it becomes filled with
amniotic fluid
–this will eventually
surround and suspend the
embryo and provide
• a shock-free environment
• a constant temperature
–and allow free movement
amniotic cavity
Week #2: BilaminarDisk Formation (2b)
• The cells lining the
amniotic cavity are shed
on occasion
• These cells, along with
proteins, carbohydrates,
fat, enzymes, hormones
and embryonic excretions
can be extracted and
examined
• This procedure is called an
amniocentesis
amniotic cavity
Week #2: BilaminarDisk Formation (3)
• A cavity
also forms
next to the hypoblast and
is called the yolk sac
–it will contribute to
respiratory and
digestive system
structures
amniotic cavity
yolk sac
A few words about the outer cell
mass (1)
•The outer cell mass
–is the embryo’s
contribution to the
placenta
–becomes intimately
associated with the
inner lining of the
uterus in which the
embryo is embedded
A few words about the outer cell
mass (2)
• Where the 2 surfaces
join
–many blood vessels
from the embryo and
from the mom come
close together
–they exchange
nutrients and wastes
via diffusion
–no actual blood flows
between the embryonic
and maternal circulation
Week #3: Gastrulation
(1)…
• With further cell differentiation, a 3rdcell layer
forms between
the epiblastand hypoblast
• The embryo is now a trilaminardisk
• This process is called gastrulation
Week #3: Gastrulation(2)…
• The epiblastis now
called the ectoderm
• The hypoblast is now
called the endoderm
• The middle layer,
composed of cells that
migrated from the
epiblast, becomes the
mesoderm
• These 3 cell layers give
rise to all the cells of
your body!
Week #3: Gastrulation(3a)…
•The ectoderm
will give rise to
–organs that maintain contact with the outside
world
• epidermis (epithelial lining of the skin)
• nervous system
Week #3: Gastrulation(3b)…
•The endoderm
will give rise to
–the epithelial lining of the digestive and
respiratory tracts
–plus outgrowths of the digestive tract that give
rise to the:
• liver
• pancreas
Week #3: Gastrulation(3c)…
•The mesoderm
will give rise to
–connective tissues
•
•
•
•
connective tissue proper
bone
cartilage
blood
–muscle
That is, basically
everything between
the inner and outer
linings of your body!
Week #3: …and Notochord formation
• The formerly round, 3-layer disk elongates
• A rod-like structure forms in the mesoderm
• It is called the notochord
–it extends the length
of the embryo (from head
to tail)
–it will contribute to the axial skeleton
notochord
cross section
Notochord at 5 weeks
• This is to give you a
better idea of the
notochord extending
from head to tail
• It forms the basis
for the axial skeleton
This is our next lecture!
The Axial Skeleton
gives rise to 3
Components:
• the skull
• the vertebral column
• the thoracic cage:
–the sternum
–the ribs
Week #4: Neurulation
• Ectoderm overlying
the notochord now
–invaginates
and
–differentiates
into
the neural tube
–this becomes the
nervous system
• brain
• spinal cord
….
Neural Tube Formation
• Here is a view of
the neural tube as if
forms superficial to
the notochord
Here are the brain and spinal cord in
you!
Week #4:and Formation of the Fetus (1)
• The shape of the embryo changes drastically
– from an elongated disk it rolls up into a
– “C”-shaped…
• (from head to toe)
tube
• (from side to side)
Week #4:and Formation of the Fetus(2)
• The embryo now has
–an outside
• lined by ectoderm
–an inside
• lined by endoderm
ectoderm
–and everything in between
• from mesoderm
–and is now called a fetus
• all the major organ systems
have now been established
endoderm
Week #4:and Formation of the Fetus(3)
• And the fetus now bulges into the
amniotic cavity
Week #4:and Formation of the Fetus(4)
• It remains connected to the placenta
via
the umbilical cord
• It contains very important blood vessels!
Placental Blood Supply
• Oxygenated blood for the fetus is supplied
by the Mom
(high O2
from mom)
Summary
• Week #1:
– Fertilization →zygote formation
– Cleavage →morulaformation
– Beginning of cell differentiation and cavitation→blastocyst
formation
– Implantation
• Week #2:
– Differentiation continues →bilaminardisk formation
– Formation of amniotic cavity and yolk sac
• Week #3:
– Gastrulation→trilaminardisk
– Notochord formation
• Week #4:
–Neurulation
– Major shape changes in embryo
– Formation of the fetus
Four Weeks
• By the end of
the 4thweek of
gestation the
upper
and
lower limb
buds
have
formed
• (The fetus is at
very high risk of
malformations at
this age)
Figure 29–7a, b
Eight Weeks
• By the end of
the 8thweek
the fingers
and toes have
formed
nowface
begins to
• The
look human-like
• (The fetus
remains at very
high risk of
malformations
through the 8th
week)
Figure 29–7c, d
Twelve Weeks
• This is the
period of
functional
maturation of all
organ systems
• (By this age, the
fetus is at lower
risk of
malformations)
Figure 29–7c, d
Fertilization is defined as the
A.
increase in cell number
B.
joining of the sperm and oocyte
C. formation of an inner and outer cell
mass
D.
development of a cavity
E.
formation of a trilaminardisk
The ectoderm gives rise to
A.
muscle
B.
bones
C.
lining of the digestive system
D.
liver
E.
skin and nervous system
The neural tube gives rise to
A.
muscle
B.
connective tissue
C.
skin
D.
liver
E.
brain and spinal cord
At what developmental stage does the
embryo implant into the uterus?
A.
fertilized oocyte
B.
morula
C.
blastocyst
D.
trilaminardisk
E.
fetus
At what developmental stage does the
trilaminardisk form?
A.
fertilized oocyte
B.
morula
C.
blastocyst
D.
gastrulation
E.
fetus