Transcript Fetal Development

FETAL DEVELOPMENT
Chapter 3
DNA
Programs genetic code into nucleus of cell.
Nucleus controls the
development & function of cell.
Normal human chromosomes begin in pairs.
One supplied by mother and the other by
father.
Each body cell contains 46 chromosomes,
made up of 22 pairs of autosomes (body
chromosomes) and 1 pair of sex chromosomes
that determine sex of the fetus.
Biological development is influenced by the
external environment (teratogens,
undernutrition, smoking)
Early prenatal care is essential for optimum
outcome of pregnancy.
Cell Division and Gametogenesis
Nucleus- beginning of cell division
Two types of cell division:
1. mitosis (continuous process by how body grows & develops
and dead body cells are replaced
Each daughter cell contains same number of chromosomes as
parent cell
46 chromosomes = diploid number
Spermatogenesis and oogenesis
2. meiosis (reproductive cells undergo two sequential
divisions) The number of chromosomes in each cell is
reduced by half to 23 chromosomes per cell each including only
one sex chromosome.
46-23 = 22 + 1 sex chromosome (haploid number)
This process is completed in the sperm before it travels toward the
fallopian tube & the ovum if fertilization takes place.
Gametogenesis
Fertilization
When sperm & ovum unite, the
new cell contains 23 chromosomes
from the sperm and 23 chromosomes
from the ovum, returning the diploid
number to 46.
Traits are inherited from both mother and father.
Normally occurs in the outer third of
the fallopian tube, near the ovary
The sperm reaches fallopian tubes w/in 5 minutes.
A chemical change in the membrane around the
fertilized ovum prevent penetration by another
sperm.
Due to short life span of mature
gametes, the ovum is estimated to
survive up to 24 hours after ovulation.
Sperm remains capable of fertilizing
the ovum for up to 5 days after being
ejaculated in the area of cervix.
Fertilized ovum
Sexual Counseling
The sperm survives up to
5 days in the area of the
cervix and pregnancy
can occur as long as 5
days before ovulation.
Sex Determination
Sex is determined at fertilization.
Ovum always contributes an X
chromosome, whereas the sperm can
carry an X or a Y.
When sperm carrying the X
chromosome fertilizes the X-bearing
ovum, a female child (XX) results.
When a Y-bearing sperm fertilized the
ovum, a male child (XY) is produced.
The male determines the sex of the
child.
However, the pH of female
reproductive tract & estrogen levels of
the woman’s body affect the survival
rate of the X-and Y-bearing sperm as
well as speed of their movement.
Inheritance
Each gene is coded for inheritance.
The coded information is responsible
for individual traits.
Genes carry instructions for dominant
and recessive traits.
Dominant traits usually overpower
recessive traits and are passed on to
the offspring.
If only one parent carries a
dominant trait, an average of
50% of offspring will have
(and thus display) that
dominant trait.
If each parent carries a recessive
trait, there is higher chance that one
of offspring will display that trait.
Tubal Transport of the Zygote
Zygote = union of sperm and
the ovum.
Transported through fallopian
tube into the uterus.
Fertilization normally occurs in
the outer third of fallopian tube.
Zygote undergoes rapid mitotic
division, or cleavage.
Cleavage begins with two cells,
which subdivide into four and
then eight cell to form the
blastomere.
The size of zygote does not
increase, rather, the individual
cells become smaller as they
divide and eventually form a
solid ball called the morula.
Ovulation and Fertilization
Stages of prenatal development
After fertilization, the union of ovum
and sperm is called a
1.zygote – goes thru mitotic
division and forms a
2. morula (solid ball)
enters the uterus on 3rd day
& floats there for another 2
to 4 days. The cells form a
cavity, and two distinct
layers evolve.
3. blastocysts (inner layer
of
solid mass of cells), which
develops into the embryo
and embryonic membranes.
Outer membrane develops
into the chorion.
4. embryo
5. fetus
Ovulation and Fertilization
Occasionally the zygote
does not move through
fallopian tube and
instead becomes
implanted in lining of
tube, resulting in tubal
ectopic pregnancy.
Implantation of Zygote
Usually implants in
upper section of the
posterior uterine wall.
Cells burrow in
endometrium.
Endometrium now
called decidua
Development
After implantation the
cells begin to
differentiate and
develop special
function.
The chorion, amnion,
yolk sac, and primary
germ layers appear.
Chorion and Amnion
Thick membrane w/ fingerlike
projections called villi on outmost
surface.
The villi immediately below embryo
extend into decidua basalis on uterine
wall to form the embyonic or fetal
portion of the placenta.
The amnion is a thin structure that
envelops and protects embryo. It
forms the boundaries of amniotic
cavity, and its outer aspect meets
inner aspect of the chorion.
These membranes together form
an amniotic sac filled w/fluid and
allows the embryo to float feely.
Amniotic Fluid
Characteristics:
Clear
Mild odor
Often contains bits of
vernix or lanugo
Increases up to 1000 ml
at 37 weeks
Fetus swallows fluid and
excretes urine into the
fluid.
Functions:
Maintains even
temperature
Prevents amniotic sac
from adhering to the fetal
skin
Allows symmetrical
growth
Allows buoyancy & fetal
movement
Acts as cushion to
protect the fetus and the
umbilical cord from
injury.
Yolk Sac
Forms by 9th day in blastocyst.
Only functions in embryonic period.
Initiates production of RBC, and
continues for about 6 weeks until
embryonic liver takes over.
The umbilical cord encompasses
the yolk sac, and the yolk sac
degenerates.
Embryonic stage is from the 2nd to
the 8th week of development.
Fetus is from the 9th week until
birth.
Germ Layers
After implantation the
zygote in the blastocyst
stage transforms its
embryonic disc into three
primary germ layers know
as: 1. ectoderm,
2. mesoderm, and
3. endoderm.
Each germ layer develops
into a different part of the
growing embryo.
Prenatal Developmental Milestones
The heart begins to
pump blood by 3rd week
when the mesoderm
and neural tube form.
It is at this time that
women suspect they
are pregnant because
they have “missed" their
menstrual periods.
Embryonic and Fetal Development
Week 4
Heart pumps blood
Esophagus & trachea
separate
Stomach forms
Neural tube closes;
forebrain forms
Embryonic and Fetal Development
Week 6
Skull & jaw ossify
Hand & elbows
differentiate
Auditory canal forms
Eye is obvious
Heart has all four
chambers
Nasal cavity & upper
lip form
Embryonic and Fetal Development
Week 8
Embryo has distinct human
appearance.
Tail has disappeared.
Formation of sex organs
(ovaries/testes)
Embryo enters fetal period.
The fetal period begins at the
9th week.
By 10 weeks, the external
genitalia are visible to ultrasound
examination.
Embryonic and Fetal Development
Week 17
No subcutaneous fat present
Visible blood vessels
Eye movement occur
Genitalia visible on
ultrasound
Movements felt by mother
Sucks & swallows amniotic
fluid
Age of viability – By 20
weeks of gestation the lungs
have matured functionally
enough for fetus to survive
outside the uterus. NICU care
is required.
Embryonic and Fetal Development
Week 25
Wrinkled skin, lean body
Eyes are open.
Viable fetus
Quickening
Fetus sleeps & moves
Vernix caseosa present on skin
Lanugo covers body
Brown fat formed
Lungs begin to secrete surfactant
Beginning of respiratory movements.
38-40 weeks of gestation is considered full term.
Folic Acid
Folic acid supplements
can prevent most
neural tube defects
such as spina bifida
Example: L-methylfolate,
prescribed for women who
are not able to absorb folic
acid.
Folic acid is especially
recommended during the
first 2 months of pregnancy
when body organs start to
develop.
Accessory Structures of Pregnancy
Placenta, umbilical cord,
and fetal circulation
Placenta – temporary
organ for fetal respiration,
nutrition, and excretion
Also functions as
endocrine gland.
Forms when chorionic villi
of embryo extend into
blood filled spaces of
mother’s decidua basalis.
Placental Transfer
A thin membrane separates the
maternal & fetal blood and the two
blood supplies do not normally
mix.
No mixing of fetal or maternal
blood occurs.
Two umbilical arteries carry
deoxygenated blood and waste
products away from fetus.
The fetal blood releases carbon
dioxide and waste products and
the umbilical vein takes in oxygen
and nutrients and carries them to
the fetus.
“AVA” = 2 arteries, 1 vein
Placental Hormones
Four hormones
produced by placenta:
1. progesterone
2. estrogen
3. human
chorionic
gonadotropin
(hCG)
4. human
placental
lactogen (hPL)
Progesterone
Functions of:
Maintains uterine
lining for implantation
of zygote
Reduces uterine
contractions
Prepares breasts for
lactation
Stimulates testes to
produce testosterone to
aid male fetus develop
reproductive tract
Estrogen
 Stimulates uterine growth
  Blood flow to uterine
vessels
 Stimulates development of
breast ducts for lactation
Other functions not directly
related to pregnancy are:
 Skin pigmentation
 Vascular changes in skin &
mucous membranes of nose &
mouth
  salivation
Human Chorionic Gonadotropin (hCG)
Hormone “signal” sent
to corpus luteum that
conception has occurred.
Causes corpus luteum
to continue production of
estrogen & progesterone
to sustain pregnancy.
Detectable in blood as
early as 7 – 9 days after
fertilization has occurred.
Human Placental Lactogen (hPL)
Causes decrease insulin
sensitivity & use of glucose
by mother, making more
glucose available to the
fetus to meet growth needs.
Umbilical Cord
Develops w/placenta and blood
vessels
“lifeline” between mother and fetus.
Two arteries carry blood away from
the fetus, and
One vein returns oxygenated
blood to the fetus
“AVA” = 2 arteries, 1 vein
Wharton’s jelly covers & cushions
cord vessels and keeps the three
vessels separated.
Blood vessels are coiled w/in the cord
to allow for movement & stretching
w/out restricting circulation.
Fetal Circulation
By 4th week, circulation
of blood through
placenta to the fetus is
well established.
Because the fetus
does not breathe &
the liver does not
have to process most
waste products,
several physiological
diversions are needed.
Fetal Circulatory Shunts
Ductus venous: diverts
some blood away from the
liver as it returns from
placenta
Foramen ovale: diverts
most blood from the right
atrium directly to left atrium,
rather than circulating it to
lungs
Ductus arteriosus: diverts
most blood from pulmonary
artery into aorta
Circulation before Birth
Oxygenated blood enters the
fetal body through the umbilical
vein.
About half of blood goes to liver
and remainder enters inferior
vena cava through the ductus
venosus. Blood from inferior
vena cava enters right atrium
where most passes directly into
the left atrium through the
foramen ovale.
Small amt of blood is pumped to
lungs by right ventricle. Rest of
blood from right ventricle joins
that from left ventricle through
ductus arteriosus.
http://www.youtube.com/watch?v=IRkisEtzsk
Circulation after Birth
Once infant breathes, fetal shunts
are no longer needed. Blood is
circulated to lungs and liver.
The foramen ovale closes
because pressure in
right side of heart falls
as lungs become fully
inflated and there is little
resistance to blood flow.
The blood oxygen level rises,
causing ductus arteriosus to
constrict.
The ductus venosus closes when
the flow from umbilical cord stops.
Closure of Fetal Circulatory Shunts
Some conditions may
cause the ductus
arteriosus or foramen
ovale to reopen after
birth.
A condition such as
respiratory distress
syndrome may be
caused by reopening of
the foramen ovale.
Multiple Births
Monozygotic twins = identical twins:
same sex, look alike, develop from a
single fertilized ovum.
The result is two identical
embryos, each with its
own amnion but with a
common chorion and
placenta and some
common placental
vessels.
Physical differences between
monozygotic twins are caused by
prenatal environmental factors
involving variations in the blood
supply from the placenta.
One or other may be affected by
inadequate placental nutrition.
Multiple Births
Dizygotic twins = fraternal
twins: may or may not be
of the same sex and
develop from two separate
ova fertilized by two
amnions, two chorions,
and two placentas.
In many twin or multiple
pregnancies, the placenta
may not be able to supply
sufficient nutrition to both
fetuses, with the result
that one or both twins is
smaller than average.
Questions?