Download Stages of Fetal Development

Fetal Development
Stages of Fetal Development
In 38 weeks a fertilized egg matures from a
single cell carrying all necessary genetic
material to fully developed fetus ready to be
born.
Terms:
•
•
•
•
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Ovum - from ovulation to fertilization
Zygote - from fertilization to implantation
Embryo - from implantation to 5-8 weeks
Fetus - from 5-8 weeks until term
Conceptus - developing embryo or fetus and
placental structures throughout pregnancy
Stages of Fetal Development
• Preembryonic stage: fertilization through 2nd week
– Fertilization; cleavage; morula
– Blastocyst and trophoblast (see Box 10-1)
– Implantation
• Embryonic stage: end of 2nd week through 8th week
– Basic structures of major body organs and main external
features (see Table 10-1)
• Fetal stage: end of the 8th week until birth (Table 10-1)
Stages of Fetal Development
Fertilization: The beginning of pregnancy.
• Fertilization is the union of the ovum and a
spermatozoon.(conception, impregnation or
fecundation).
• Fertilization occurs in outer 3rd of a
fallopian tube, the ampullar portion.
• Fertilization has a span of 72 hours (48
hours before ovulation plus 24 hours
afterwards).
• Ovum and surrounding cells are propelled
into fallopian tube by cilia.
• One ovum reaches maturity each month.
• Ovum is capable of fertilization for only 24
hours (48 at the most).
• Sperm reach the ovum and clusters around
the protective layer of corona cells.
• Hyaluronidase (proteolitic enzyme) is
released by sperm and acts to dissolve the
layer of protective cells.
• One sperm is able to penetrate the cell
membrane of the ovum.
• Once it penetrates the zona pellucide the
membrane becomes impervious to sperm.
(Except in the formation of
hydatidiform mole > abnormal growth).
This is what I was trying to remember the
other day in class!!!!!
• Next the chromosomal material of ovum
and sperm fuse and become a zygote.
• 23 chromosomes each = 46 total
• Fertilization depends on:
– maturation of both sperm and ovum
– ability of sperm to reach the ovum
– ability of sperm to penetrate the zona
pellucida and cell membrane and achieve
fertilization.
Implantation:
• zygote migrates toward body of the uterus.
• takes 3 to 4 days
• during this time mitotic cell division, or
cleavage, begins.
• 1st cleavage occurs at about 24 hours.
• continues at a rate of one every 22 hours.
• the zygote reaches the body of uterus with
about 16 to 50 cells.
• bumpy appearance termed morula.
• Morula continues to multiply as it floats
free in the uterine cavity for 3 to 4 days.
• Large cells collect leaving a fluid space
surrounding an inner cell mass and is
termed blastocyte.
• This attaches to the uterine endometrium.
• Trophoblast - cells in the outer ring.
• Embryoblast - inner cell mass later forms
the embryo.
• Implantation occurs in 8 to 10 days after
fertilization.
• The blastocyte brushes against the rich
endometrium (in secretory phase of
menstrual cycle)
• attaches to the surface of the endometrium adhesion
• proteolytic enzymes dissolve tissues
• settles down (burrows) into soft folds invasion
• usually high posterior side of the uterus
• if low = placenta previa
• 50% of zygotes never achieve this.
• A pregnancy ends in 8 to 10 days after
fertilization, before the woman is aware of a
pregnancy.
• Vaginal spotting may occur with adhesion /
invasion due to rupture of capillaries by the
implanting trophoblast cells.
• Once implanted the zygote is called an
embryo.
Decidua:
• uterine endometrium grows in thickness and
vascularity.
• Will be discarded after the birth of the child.
• 3 areas:
– decidua basalis - directly under the
embryo
– decidua capsularis - stretches or
encapsulates the surface of trophoblastmembranes
– decidua vera - remaining portion of lining
Embryonic and Fetal Structures
• Embryo grows and pushes decidua
capsularis like a blanket, enlargement,
contacts opposite uterine wall, fuses with
endometrium.
• At birth this entire inner surface of the
uterus is stripped away, leaving the organ
highly susceptible to hemorrhage and
infection.
Embryonic and Fetal Structures
Chorionic Villi:
• Once implantation is achieved,
trophoblastic layer of cells of blastocyst
begins to mature rapidly.
• 11th to 12th day miniature villi reach out
from trophoblast into endometrium.
• Central core of connective tissue contain
fetal capillaries.
• Syncytial layer produces hormones, hCG,
HPL, estrogen, progesterone.
Embryonic and Fetal Structures
Placenta: (latin for pancake)
• arises out of trophoblast tissue.
• serves as fetal lungs, kidneys, GI, endocrine
• growth parallels fetal growth: 15 to 20 cm
in diameter and 2 to 3 cm in depth at term.
• covers about 1/2 the surface of internal
uterus
Embryonic and Fetal Structures
Circulation:
• 12 th day of pregnancy maternal blood
begins to collect
• by 3rd week-oxygen, nutrients, fluid diffuse
from mother through chorionic villi to villi
capillaries to the developing embryo.
• no direct exchange of blood between
embryo and mother during pregnancy.
• exchange is by selective osmosis through
the chorionic villi.
• Minute breaks do occur
Embryonic and Fetal Structures
• membrane is affected by maternal B/P, pH
of fetal and maternal plasma.
Cotyledon:
• in a mature placenta there are about 30
separate segments. (networked)
• 100 maternal uterine arteries supply the
mature placenta.
• blood flow through the placenta is about 50
mL/min. at 10 weeks to 500 to 600 mL/min
at term.
Embryonic and Fetal Structures
• To accommodate increased blood flow
arteries increase in size.
• Mothers heart rate, total cardiac output, and
blood volume increase to supply the
placenta.
Uterine perfusion:
• placental circulation is most efficient when
mother lies on her left side.
• This lifts the uterus away from the inferior
vena cava,preventing blood from being
trapped in lower extremities.
Embryonic and Fetal Structures
• If the mother lies on her back the weight of
the uterus on vena cava causes supine
hypotension.
• At term a placenta weighs 400 to 600g 1 lb.
• A small or enlarged placenta suggests
circulation to the placenta is compromised.
• Women with diabetes may develop a larger
than usual placenta from fluid collected
between cells.
Endocrine Function
Human Chorionic Gonadotropin:
• 1 st hormone to be produced.
• found in maternal blood and urine shortly
after implantation or first missed period for
100 days from trophoblast.
• analyzed with urine pregnancy test.
• negative within 1 to 2 weeks post delivery.
• hCG functions to keep corpus luteum
producing progesterone, if this fails or
progesterone falls the endometrium will
slough,
Endocrine Function
until 8th week outer layer of cells of
placenta begins to produce progesterone.
• hCG suppresses maternal immunologic
response to not reject the placenta.
Estrogen:
• primarily estriol is produced as a second
product of syncytial cells of placenta.
• contributes to mammary gland development
in preparation for lactation.
Endocrine Function
• stimulates uterine growth to accommodate
fetus.
• assessing amount of estriol in maternal
serum was used to test fetal well being.
Progesterone:
• maintains endometrial lining of the uterus
during pregnancy.
• present in serum 4th week of pregnancy.
• reduces contractility of uterine muscle
during pregnancy(prevents premature labor)
Endocrine Function
Human Placental Lactogen: (HPL)
• both growth-promoting and lactogenic
properties.
• produced by the placenta by 6th week of
pregnancy and increases to peak at term.
• it promotes mammary gland growth in
preparation for lactation in the mother.
• regulates maternal glucose, protein, and fat
levels so adequate amounts are available to
the fetus.
Umbilical Cord
• Formed from the amnion and chorion and
provides a circulatory pathway connecting
the embryo to the chorionic villi.
• Function is to transport oxygen and
nutrients to the fetus from the placenta and
to return waste products from the fetus to
the placenta.
• 21 inches (53cm) long and 3/4 inch (2cm)
thick.
• One vein - carries blood from placental villi
to the fetus.
Umbilical Cord
• 2 arteries - carrying blood from the fetus
back to the placenta villi.
• Wharton’s jelly - gelatinous
mucopolysaccharide gives cord body and
prevents pressure on the vein and arteries.
• The outer surface is covered with amniotic
membrane.
• Blood can be withdrawn from the umbilical
vein or transfused into the vein during
intrauterine life for fetal assessment or
treatment.
• Rate is rapid 350 mL/min. at term.
Umbilical Cord
• Blood flow (blood velocity) can be
determined by ultrasound.
• The rapid rate of blood flow through the
cord makes it unlikely that it will twist or
knot enough to interfere with O2 supply.
• 20% of births a loose loop of cord is found
around the fetal neck (nuchal cord).
• Smooth muscle is abundant in the arteries of
the cord.
• Constriction of muscles after birth
Umbilical Cord
contributes to hemostasis and helps prevent
hemorrhage of the newborn through the
cord.
• The cord contains no nerve supply, so it can
be cut at birth without discomfort to child or
mother.
Membranes and Amniotic Fluid
• The chorionic villi on medial surface of the
trophoblast gradually thin and leave the
medial surface smooth this becomes
chorionic membrane, the outer most fetal
membrane - next to baby.
• Once it becomes smooth, it offers support to
the sac that contains the amniotic fluid.
• The amniotic membrane (amnion) forms
beneath the chorion and becomes adherent
to the fetal surface of the placenta, and give
that surface a typically shiny appearance.
Membranes and Amniotic Fluid
• no nerve supply: no pain when it ruptures.
• Amniotic membrane acts to support and
produce amniotic fluid.
• It produces a phospholipid that initiates tha
formation of prostaglandins which cause
uterine contractions and maybe the trigger
to initiate labor.
• amniotic fluid is constantly being newly
formed and reabsorbed, so it is never
stagnant within the membranes.
Membranes and Amniotic Fluid
• Fetus continually swallows the fluid, it is
absorbed across the fetal intestine into the
fetal bloodstream.
• Umbilical arteries exchange it across the
placenta. Also by direct contact with fetal
surface of the placenta.
• At term amniotic fluid is 800 to 1,200 mL.
• Excessive amniotic fluid-hydramnios
(>2,000mL) this occurs in women with
diabetes R/T hyperglycemia (fluid shift into
amniotic space).
Membranes and Amniotic Fluid
• Reduction in the amount of amniotic fluid oligohydramnios (<300) a disturbance of
kidney function).
• alkaline pH 7.2
• Protective:
• shields against pressure or blow to mother’s
abdomen.
• protects fetus from changes in temperature.
• aids in muscular development with allowing
movement.
• protects cord from pressure, protecting fetal
oxygenation.
Origin and Development
From the beginning of fetal growth,
development proceeds in a cephalocaudal
(head to toe) direction.
• Head first then middle and then lower body
parts. This continues after birth also.
• Body organ systems develop from specific
tissue layers called germ layers.
Primary Germ Layers:
At the time of implantation, the blastocyte has
separated to two cavities in the inner
structure.
Origin and Development
• Amniotic cavity (large) - lined with a
distinctive layer of cells called - Ectoderm.
• Smaller cavity yoke sac which is lined with
entoderm which supplies nourishment only
until implantation. After that, it provides a
source of red blood cells until the
hematopoietic system is mature.
• Ectoderm - CNS, PNS, skin, hair ,nails,
sebaceous glands, sense organs, mucus
membranes of the mouth, anus, nose,
tooth enamel, mammary glands
Origin and Development
• Mesoderm (middle layer)
– support structures - bone cartilage,
muscle, ligament, tendon. Dentin of teeth,
kidneys, ureters, reproductive system,
heart, circulatory system, blood cells,
lymph cells.
• Entoderm (yolk sac)
– lining of pericardia, pleura peritoneum,
GI tract, respiratory tract, tonsils,
parathyroid, thyroid, thymus, bladder,and
urethra.
Origin and Development
Each germ layer of primary tissue develops
into specific body systems.
One reason rubella infection is so serious in
pregnancy is because the virus is capable of
affecting all the germ layers.
All organ systems are complete at 8 weeks’
gestation (end of the embryonic period).
Organogenesis: (organ formation)
The growing structure is most vulnerable to
invasion by teratogens.
Cardiovascular System
One of the 1st systems to become functional
in intrauterine life.
Simple blood cells joined to the walls of the
yolk sac progress to a network of blood
vessels and to a single heart tube forming as
early as the 16th day of life, beating at 24th
day, the septum divides during the 6th or
7th week, heartbeat may be heard with a
doppler at 10th to 12th week.
After the 28th week the heart rate begins to
show a baseline of 5 beats/min.
Cardiovascular System
Fetal Circulation:
• As early as the 3rd week of intrauterine life,
fetal blood has begun to exchange nutrients
with maternal circ across the chorionic villi.
• Fetus derives O2 and excretes CO2 from the
placenta (not lungs).
• Blood enters the cells of lungs.
• Specialized structures in the fetus shunt
blood flow to brain, liver, heart,and
kidneys.
Cardiovascular System
• Blood from the placenta is highly
oxygenated.
• Blood enters through the umbilical vein
(called vein because the direction of blood
flow is toward the fetal heart).
• Carries blood to inferior vena cava through
accessory structures - ductus venosus.
• It receives O2 blood from the unbilical vein
to supply the fetal liver.
• Then, empties into the inferior vena cava.
Cardiovascular System
• From the inferior vena cava blood is carried
to the right side of the heart.
• As blood enters right atrium the bulk is
shunted into the left atrium through an
opening in the atrial septum the foremen
ovale.
• From the left atrium it follows normal circ.
into the left ventricle and into the aorta.
• Deoxygenated blood from the body is
returned to the heart by the vena cava.
Cardiovascular System
• The blood enters the right atrium and leaves
by the normal circ. route.
• A large portion of this blood is shunted
away from the lungs through an additional
structure - ductus arteriosus which is
directly into the aorta and then the
descending aorta.
• Most of the blood flow from the descending
aorta is transported by the umbilical arteries
(arteries even though they are transporting
deoxygenated blood because they are
carrying blood away from the heart).
Cardiovascular System
back through the umbilical cord to the
placenta villi, where new O2 exchange
takes place.
• O2 saturation of the fetus is about 80% of
the newborn’s saturation level.
• Fetal heart rate - 120 to 160 beats/ min. is
necessary to supply O2 to cells when RBC’s
are never fully saturated.
• CO2 does not accumulate because of rapid
diffusion into maternal blood across a
favorable placental pressure gradient.
Cardiovascular System
Fetal Hemoglobin:
• Fetal hemoglobin has greater O2 affinity
and is more concentrated.
• Hemoglobin at birth - 17.1 g/100ml
• Hematocrit - 53%
• Sickle cell anemia - beta hemoglobin chain,
symptoms do not appear for 6 months
because hemoglobin matures then.
Respiratory System
• At the 3rd week respiratory and digestive
tracts exist as a single tube.
• Initially it is solid then canalizes.
• Week 4 a septum begins to divide the
esophagus from the trachea.
• Week 6 lung buds may extend down into
the abdomen.
• Week 7 diaphragm becomes complete.
• Week 24 and 28 - alveoli and capillaries
begin to form. Both must be developed
Respiratory System
before gas exchange can occur in the fetal
lungs.
• Spontaneous respiratory begin at 3 months.
• Specific lung fluid with low surface tension
and low viscosity forms in alveoli to aid in
expansion of alveoli at birth.
• Surfactant a phospholipid substance is
formed and excreted by aveolar cells about
24th week. This decreases alveolar surface
tension on expiration, preventing alvolar
collapse and improves respirations outside.
Respiratory System
• Surfactant has 2 components:
– lecithin - 35th week increased production
– sphingomyelin - early formation of
surfactant
• Surfactant mixes with amniotic fluid (L/S).
• Lack of surfactant is a factor with RDS.
• Interference of blood supply to the fetus
causes increased production of surfactant.
• Hypertension > increases stress increases
steroid level associated with alveolar
maturation.
Nervous System
• Week 3 and 4 formation begins.
• Neural plate - a thickened portion of the
ectoderm is apparent by 3rd week. Its top
portion differentiates into the neural tube,
which will form the CNS (brain and spinal
cord), the neural crest, which develops into
the peripheral nervous system.
• Week 8 - brain waves can be detected on
EEG.
• All parts of the brain form in utero and
growth continues after birth.
Nervous System
• Eye and inner ear develop as projections of
the original neural tube.
• Week 24 ear is capable of responding to
sound
• Eyes exhibit a pupillary reaction.
• Very vulnerable to anoxia during the early
weeks of the embryonic period, all during
pregnancy and at birth.
Endocrine System
As soon as endocrine organs mature
intrauterine life, function begins:
• Adrenal glands supply a precursor for
estrogen synthesis by the placenta.
• Pancreas produces insulin needed by the
fetus(does not cross the placenta from
mother to fetus).
• Thyroid and parathyroid glands play a vital
role in metabolic function and calcium
balance.
Digestive System
• Week 4 digestive tract is separated from the
respiratory tract.
• Grows rapidly from solid to a tube by
canalization.
• Endothelial cells of the GI tract proliferate,
this occludes the lumens.
• Intestines remain in the base of the cord
until 10th week, then after abdominal cavity
growth the intestine returns to the
abdominal cavity.
• Must rotate 180 degrees.
Digestive System
• 16th week meconium forms in the intestines
• meconium consists of cellular wastes, bile
fats, mucoproteins, mucopolysaccharides,
portions of the vernix casosa, lubricating
substances that form on the fetal skin.
• It’s black or dark green and sticky.
• GI tract is sterile before birth.
• Vitamin K is synthesized by action of
bacteria in the intestines. This causes low
levels of vitamin K in newborns.
Digestive System
• Week 32 or the fetus weighs 1,500 g
sucking and swallowing reflexes.
• Week 36 the ability of the GI tract to secrete
enzymes essential to carbohhydrate and
protein digestion.
• Amylase is not mature until 3 months after
birth.
• Lipase may not have developed yet.
• Liver is active throughout gestation.
– It functions as a filter between
Digestive System
the incoming blood and fetal circulation
– deposit for stores of iron and glycogen,
this is immature at birth.
– This leads to hypoglycemia and
hyperbilrubinemia in the first 24 hours
after birth.
Musculoskeletal System
• Week 11 fetus can be seen to move on
ultrasound (mother does not feel it yet)
• Week 20 quickening - movement of the
fetus.
• Week 12 ossification of bone tissue begins
and continues until adulthood.
Reproductive System
A child’s sex is determined at conception.
• Week 6 gonads form (ovaries or testes).
• If testosterone is present male organs
develop.
• In the absence of testosterone female organs
develop.
Urinary System
Rudimentary kidneys are present by 4th week
but are not essential before birth.
• Week 12 urine is formed and excreted into
the amniotic fluid by week 16.
• At term fetal urine is 500mL/day.
• The Loop of Henle is not fully
differentiated until birth.
Integumentary System
• Skin of a fetus is thin and almost
translucent until subcutaneous fat begins to
be deposited about week 36.
• Skin is covered by soft downy hairs
(Lanugo) and a cream cheese like substance
vernix casosa, which is important for
lubrication and keeping the skin from
macerating.
Immune System
IgG maternal antibodies cross the placenta into the fetus at
3rd trimester, giving temporary passive immunity.
• Poliomyelitis, rubella, rubeola, diptheria, tetanus, mumps,
pertussis. Not herpes.
• Passive immunity peaks at birth and decreases over the
next 8 months. At 2 months already has declined
substantially.
• Diphtheria, tetanus, pertussis, poliomyelitis, H influenzae
are started soon after birth.
• Measles last over 1 year.
IgA and IgM antibodies cannot cross the placenta, if present
the fetus was exposed to the disease.