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Embryology

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					   Biology 201
 Human Anatomy


Human Embryology
One of the best ways to understand the structure of
the adult human body is to understand how it
developed.


Similarly:
Human Development: 3 Periods
 Pre-embryonic:
 Embryonic:
 Fetal:
Fertilization:


Resulting cell =
As




Fluid-filled
Pre-embryonic period now over,
Embryonic period begins.




             Fertilization – 2 wks = Pre-embryo
             2 wks – 8 wks = Embryonic period
             8 wks – birth = Fetal period
First:




    View From Top,     View From Edge (Cross-section)
    Showing Ectoderm   Showing All Three Layers
As embryo develops:
The first system to begin developing
~ Day 22 – 23,

By day 28:
~ Day 28:   begin developing.




                 Length ~ 4mm
                 (width of pencil)
Late embryonic period (4 – 8 weeks):
 Nervous system continues to develop;
   brain rapidly enlarges and folds
 Eyes and ears begin to develop
 Limbs continue developing, fingers and toes separate
 Digestive system forms

 Lungs bud off from digestive system and grow
 Heart folds, divides into chambers, begins contractions

 Kidneys, bladder, gonads develop

 Face develops as two halves on side of head; move to
 front and fuse
End of embryonic period
Beginning of fetal period (8 weeks):

 • Body has human shape
 • Almost all organs have begun developing
 • Head growth still most pronounced
 • Limbs complete
 • Length ~
 • Mass ~


                                       Mass of quarter: 5.7 grams
Placenta well developed:
In placenta:
Ninth week:

 (Organs and systems continue development)
 Fetus =




Tenth week:

 (Organs and systems continue development)
 Fetus =
After tenth week:

 (Organs and systems continue
 to develop)
After 17 weeks (four months)
 Skin formed, but thin. Blood vessels easily
 seen through it
 Hair starting to develop. Fingernails and
 toenails forming.
 Eyelids still fused shut.
 Ovaries form primordial follicles.
 May suck thumb.
 Moves arms, legs, head
 Mother can feel movement.
After 26 weeks (six months):
 Hair present over entire body. Will
 become longer on scalp
 Eyebrows and eyelashes forming
 Eyelids almost open
 Lungs producing surfactant
 Testes begin descent through inguinal
 canal
 Swallowing amniotic fluid
 Movement of hands, feet, face
After 34 weeks (eight months):
 Skin pink, smooth
 Longer hair on head
 Eyes open, respond to light
 Fingernails and toenails have
 reached ends of fingers and toes
 Testes have reached scrotum
 Fingers can grasp objects, fine
 movement of face and eyes
Nine months:
 Ready for birth




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posted:3/7/2012
language:English
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