Download HUMAN REPRODUCTION BIOLOGY 269

In pre-embryonic period (as discussed a couple of weeks ago):
(fertilization - two weeks)
Zygote undergoes cleavage to form
morula, then blastocyst which
implants into the endometrium of
the uterus.
Blastocyst forms trophoblast (will
develop into placenta) and
embryoblast (will develop into the
embryo).
Formation of the amniotic cavity
between trophoblast and
embryoblast forces the latter into a
flat, plate-shaped embryonic disk.
During this period, cells of embryoblast and disk have not yet begun to
differentiate into different types.
Trophoblast continues to grow and invade endometrium of mother’s
uterus.
Inner lining of trophoblast forms another membrane, the chorion, which
surrounds embryo (don’t worry about how this forms).
Later, amnion will enlarge and fuse with chorion, forming the membrane
which surrounds the fetus and contains the amniotic fluid in which the
fetus floats.
The next six weeks (2 wks - 8 wks of development) are the embryonic period.
During this period, cells of the embryonic disk differentiate to create primitive
forms of almost all of the organs.
First: Cells of embryonic disk separate into three germ layers:
Endoderm - nearest the blastocyst cavity
(changes name to yolk sac)
Mesoderm - in middle
Ectoderm - nearest amniotic cavity
View From Top,
Showing Ectoderm
View From Edge (Cross-section)
Showing All Three Layers
As embryo develops:
Endoderm will form linings of digestive
and respiratory systems.
Mesoderm will form skeletal, muscular,
urinary, reproductive,
circulatory systems.
Ectoderm will form skin and
nervous system
The first system to begin developing is the nervous system. This
appears as a groove along the ectoderm, which closes over to form a
neural tube.
The neural tube enlarges at one end to form brain;
remains cylindrical at the other end to form spinal cord
~ Day 22 – 23, sides of flat embryonic disk begin to fold toward yolk sac.
By day 28: Embryo has “normal” body shape, amniotic cavity pulled
around to surround it.
Placenta continues to develop
from trophoblast; many blood
vessels develop within it and
form villi which project into pools
of the mother’s blood in her
endometrium.
Embryo floating in amniotic fluid,
stays attached to placenta by
umbilical cord.
Umbilical vessels carry blood
between placenta and embryo.
~ Day 28: Arm buds and leg buds begin developing.
Bones, muscles, and blood vessels
develop from mesoderm as limbs
get bigger, nerves grow out from
spinal cord.
Vertebrae forming
Other organs continue developing
internally.
Length ~ 4mm
(width of pencil)
Late embryonic period (4 – 8 weeks):
Nervous system continues to develop; brain rapidly enlarges and folds
Electrical activity begins
Eyes and ears begin to develop
Limbs continue developing,
fingers and toes separate
Digestive system forms esophagus,
stomach, intestine, liver, pancreas
Lungs bud off from digestive system and grow
Heart folds, divides into two chambers and then four, begins contractions
Kidneys, bladder, gonads develop
Face develops as two halves on side of head; move to front and fuse
End of embryonic period
(8 weeks):
Beginning of fetal period
Body has human shape
Almost all organs have begun developing
Head growth still most pronounced
Cartilage tissue in bones being
replaced by bone tissue
Limbs complete
Some movement of limbs and head
Length ~ 3 centimeters
Mass ~ 1 – 2 grams
Mass of quarter: 5.7 grams
Placenta well developed:
Fetal heart pumps blood from embryo to
placenta through umbilical arteries:
Low in oxygen
High in carbon dioxide & other wastes
In placenta:
Carbon dioxide & wastes diffuse out
of fetal blood into maternal blood
Oxygen & nutrients diffuse from
maternal blood into fetal blood
Blood flows from placenta back to
embryo through umbilical veins:
High in oxygen & nutrients
Low in carbon dioxide & other wastes
Ninth week:
(Organs and systems continue development)
Fetus = 5 cm long, 4 - 5 grams
Facial features well formed
External genitalia develop, but male and female still very similar:
penis / clitoris scrotum / labia
Tenth week:
(Organs and systems continue development)
Fetus = 7 cm long, 8 - 10 grams
External genitalia easily determined to be male or female, but not yet
large enough to visualize by ultrasound
After tenth week:
Fetus increases rapidly in size
Organs and systems continue to develop,
begin functioning
Week 10-12:
Length ~15cm
All major parts of the brain are developing
Head is approximately half of the fetus' mass
Ears fully formed and can detect sound
Tooth buds appear in upper and lower jaws
Eyelids close (will not open again for 16 weeks
Can curl hand and fingers into fist
May suck thumb (stimulates swallowing)
Swallowing amniotic fluid and urinating
Week 13-17:
Length ~20 cm
Nervous system continues to develop rapidly
Skin formed, but thin. Blood vessels easily seen through it
Hair starting to develop
Including eyebrows and eyelashes.
Fingernails and toenails forming.
Eyelids still fused shut.
Ovaries form primordial follicles.
Moves arms, legs, head
Mother can feel movement = “quickening”
Fetal heartbeat can be detected with stethoscope
Week 17-20:
Rapid growth: length ~28 cm; weight ~500 grams (18 oz)
Nervous system continues to develop rapidly
Reflexes begin to develop
Fat forming under skin
Begins sleep / wake cycles
Alveolar air sacs forming in lungs
< 10% of premature babies survive at this stage
even with full medical intervention
Week 21-26 (six months):
Length ~38 cm; Weight ~ 1200 grams (2 lb 11 oz)
Nervous system continues to develop rapidly
Starting to control body functions
Hair present over entire body. Will become longer on scalp
Eyelids ready to open
Lungs producing some surfactant
Testes begin descent through inguinal canal
Swallowing amniotic fluid
Movement of hands, feet, face
More than half of premature babies survive at this stage
with medical intervention, often with no permanent problems
Week 26-28:
Length ~42 cm; Weight ~1500 grams (3 lbs)
Rhythmic breathing motions
Eyelids open and close
Most bones fully formed, but pliable
and capable of rapid growth
Body fat rapidly increasing
90% of premature babies survive at this stage
with medical intervention, usually with
no permanent problems
Week 29-32:
Length 40-48 cm; Weight 2.5 – 3 kg (5.5 – 6.5 lbs)
Body fat significantly increasing
Longer hair on head
Eyes open, respond to light
Fingernails and toenails have reached
ends of fingers and toes
>90% of premature babies survive at this stage
with medical intervention, usually with
no permanent problems
Week 33-37 (9 months):
Can gain as much as half a pound per week
Lungs continue to develop;
Strong respiratory motions
Skin pink, smooth
Testes have reached scrotum
Fingers can intentionally grasp objects, fine
movement of face and eyes
Immune system now competent – can build
antibodies
Nine months:
Ready for birth
During development, a couple of
openings had to form in the heart
and large blood vessels to allow
blood to bypass being sent to the
lungs.
We will talk about what happens
to those in a couple of weeks.
X
X
Following slides for review
X
X