Transcript Development and Differentiation

Development and
Differentiation
What do
you see
in this
picture?
Let’s take
a closer
look
When does the heart begin to beat?
When can a fetus move its arms and legs?
Fertilization has three functions:
• transmission of genes
• restoration of the diploid number of
chromosomes
• initiation of development
Fertilization
• Millions of sperm enter the vagina and
swim into the oviducts
Why are there so many sperm for the one
egg you may ask? Suggestions?
There are Millions of sperm for a
couple of reasons:
• Assures survival in the acidic female
reproductive tract
• ensures that some will enter the
right oviduct that the ovum was
released from.
Steps in Fertilization:
• Contact between sperm and egg
• Entry of sperm into the egg
• Fusion of egg and sperm nuclei
• Activation of development
• Union of the male and female gametes
gives rise to a zygote.
• the sperm acrosome or “head” penetrates
the egg and releases enzymes that
stimulates the egg to become
impenetrable.
Important steps after fertilization:
1. zygote continues down the oviduct
2. Zygote undergoes cell division: 2, 4, 8
cells. No increase in the overall size of
the zygotes= Cleavage
• It is now called an embryo and will
remain an embryo until bone cells begin
to form weeks later!
Human Development before
Implantation:
Development before Implantation:
3. By the time the embryo reaches the uterus
it has formed the Blastocyst- a hollow ball
of cells formed by mitotic division
. The inner cell mass will develop into the
baby, whereas the other cells called the
trophoblast will become the membranes
that nourish the embryo.
Implantation:
• Occurs at the end of the first week.
• This is when the embryo has finished it’s journey
•
enters the uterus and attaches to the
endometrium (which we remember is the
uterine lining!)
The trophoblast (the outside layer of the
blastocyst) secretes human chorionic
gonadotropin (HGC) which pregnancy kits are
designed to pick up!
Gastrulation:
• Happens during the second week of development
• involves a series of cell migrations to positions where
they will form the three primary cell layers.
Ectoderm forms the outer layer and gives rise to the
skin, brain, and nervous system .
Endoderm forms the inner layer and gives rise to
organs and tissues associated with digestion and
respiration
Mesoderm forms the middle layer and gives rise to
muscles, bones, and structures associated with
reproduction
Primary membranes develop from the germ
layers and support, nourish and protect the
embryo.
1. Chorion- is involved in gas exchange
2. Amnion- Provides a liquid environment for the
developing embryo
3. Allantois- collects waste from the embryo
4. Yolk sac- surrounds the yolk.
What do you think the purpose of the yolk is?
Nourishes the embryo during development
Invagination
• Immediately after gastrulation, the body axis
of the embryo begins to appear
• cells that will form the nervous system fold
into a neural tube (which will eventually form
the spinal cord
• The mesoderm forms the notochord which
will eventually form the vertebrae
Neural Development: ~week 3
Ecto=
nervous
Meso=
notochord
Once the
folds fuse
around the
notochord
the embryo is
now called a
NEURULA
The First Trimester:
• Weeks 1-12
• By week 4 limbs, eyes and spine have
begun to form
• At weeks 8-9 bone cells begin to form,
now the neurula is called a fetus
~ 100mm
long by the
end of the
first
trimester
Remember at 8
weeks of age the
baby has developed
it’s sexual
reproductive
organs, so the
doctor can tell
whether the baby is
a boy or a girl!
The Second Trimester
• Weeks 13-24
• You can hear the fetus’ heartbeat with a
stethoscope
• Skeleton is forming
• Begin to feel movement of the baby
• By the end the baby is 300mm long
• Most organs formed but still developing
The Third
Trimester:
•
•
•
•
•
•
•
Weeks 25-38
Size increases rapidly
Fetus moves around a lot
Immune system develops
By the 8th month, eyes are
open
Maternal nutrition VIP
Normally 525mm long
The Placenta:
• Blood-rich organ present only during
pregnancy
• Forms from the chorion
• Fully formed by week 10
• Produces estrogen and progesterone…any
ideas why?
Prevents new follicles from forming,
maintains the endometrium
So, because levels of progesterone and
estrogen are high, does menstruation
occur during pregnancy?
No, because high levels of estrogen INHIBIT
FSH!
• Embryo gets all its nutrients and oxygen from
the placenta
Umbilical cord- gets rid of wastes and connects the
fetus to the placenta
See figure 15.19 on page 511
SEE THIS: (feeding and the third trimester)
http://www.pbs.org/wgbh/nova/miracle/program.h
tml
Teratogens:
• Any agent that causes structural
•
•
•
•
abnormality to the fetus.
Whatever a mother ingests or inhales
can end up in her blood and therefore
the baby’s blood
Ex: Smoke=constrict blood flow to fetus
Ex: alcohol= FAS
EX: x rays= genetic defects
Birth:
• Prostoglandins= hormones that cause the
release of another VIP hormone called
oxytocin
- Both these hormones cause the
uterus to contract which starts labour
1. Dilation stage- cervix
widens, amnion breaks
and the amniotic fluid is
released (2-20 hrs). The
cervix widens to approx.
10 cm.
2. Expulsion- contractions
push baby into the birth
canal. (0.5-2 hrs)
3. Placental Stage- after
baby is born, placenta
and umbilical cord are
expelled from uterus
(called afterbirth).
• Once baby is delivered and is breathing
normally, the umbilical cord is cut and now the
baby will ingest food for nutrients and rid its
own waste.
Caesarean Section:
• Known as a c-section
• a form of childbirth in which a surgical
incision is made through a mother's
abdomen and uterus to deliver one or
more babies
• Done if baby is ‘rump’ first or if
complications arise
Twins- A Unique Occurrence
1. Fraternal Twins
• Form when more than one egg is released from the ovaries.
• Thus the twins can be different sexes because their genetic make
ups are different
2. Identical Twins
•
•
•
•
Form from the one egg
Egg splits in to during early embryonic development
Thus twins are the same sex, with the same genetic material
Still unknown the entire mechanism which causes this to happen