Transcript Human Development

Biology 212
Anatomy & Physiology
Human Development
One of the best ways to understand the structure of the adult
human body is to understand how it developed.
Similarly: abnormalities during development produce
abnormalities in adult structure.
Assuming:
You paired up with an appropriate partner
You elicited normal sexual responses from each other.
A few milliliters of semen,
containing 200- to 600-million
sperm, were deposited high
in the vagina, just below
the cervix of the uterus
Scenario I:
The woman is not within a few days before or after ovulation
Her cervix will be plugged
with thick mucous, so few if
any sperm will enter the
uterus and subsequently
the Fallopian tubes.
The few sperm which might make it into the Fallopian tubes are
not pushed distally (toward the ampulla)
The sperm do not meet an oocyte and die within 48 hours
No fertilization occurs
Scenario 2:
The woman has just ovulated, or she will ovulate within 1 - 2 days.
The mucous plug in her cervix will be thin and watery; sperm can
easily penetrate it
Most sperm will die in her
vagina, but a few million
(1 - 2%) are pulled
through the cervix, into
the uterus, and into the
Fallopian tubes where
they are propelled toward
the ampulla
(+100,000 make it this far)
Sperm meet the oocyte, and fertilization can occur
At ovulation, oocyte is
surrounded by a thick
membrane called the
, and by
follicular cells called the
surrounding
the opening of the
Fallopian tube sweep
this structure into the
infundibulum
Oocyte is viable for
must occur within this time.
, so fertilization
Fertilization occurs within ampulla of Fallopian tube
If fertilization does not
occur, oocyte will
degenerate and die
A few thousand sperm reach the ampulla of the
Fallopian tube within 30 to 60 minutes after
ejaculation
But
They are not capable of fertilizing oocyte
Must undergo processes called
and
which will take 8 to 10 hours after ejaculation
Prior to ejaculation,
membrane around head of sperm is strong
Semen contains chemicals to prevent weakening
Tail is moderately active
After ejaculation, Chemicals
in semen diluted / washed
away.
Secretions from uterus &
oviducts cause membrane
to weaken.
Movement of tail increases.
Occurs as sperm make contact with
corona radiata and zona pellucida
Requires 15 to 60 minutes
Acrosome swells, becomes
leaky, releases digestive
enzymes
(Acrosome swells, becomes leaky,
releases digestive enzymes)
Enzymes digest path
through corona radiata
and zona pellucida
Hundreds of sperm
usually involved in this
digestion
Head of one sperm makes contact with outside of
oocyte, binds onto it, and is quickly taken into it.
Two systems prevent entry
of more than one sperm:
: Plasma membrane
of oocyte changes its electrical
charge, which prevents other
sperm from binding onto it.
: Zona pellucida
swells, pushing other sperm
away from the oocyte.
Once head of sperm enters oocyte, its nucleus is
released into the cytoplasm.
Nucleus of sperm (23 chromosomes) and nucleus of
oocyte (23 chromosomes) meet and fuse together.
Cell now called
, with 46 chromosomes
Human Development: 3 Periods
Fertilization
2 weeks
2 weeks
8 weeks
8 weeks
38 weeks (birth)
Fertilization: Lateral part of oviduct (Fallopian tube)
One to two days after ovulation
Resulting cell =
Zygote undergoes repeated divisions (mitosis), called
, as it is transported through oviduct toward uterus
Eventually forms solid mass of 20 – 30 cells called
, same size as the zygote. This reaches
uterus 3 to 4 days after fertilization
Morula does not embed in uterus immediately.
Stays loosely attached for 4 to 5 days, nourished by
secretions of endometrium.
Continues dividing into +100
cells, forming hollow sphere
called
Fluid-filled cavity in center =
Blastocyst develops mass of cells at one end, called
which will develop into
the embryo.
Cells surrounding cavity, called
will develop into the placenta.
Cells of trophoblast
secrete digestive
enzymes, forming
finger-like projections
into endometrium of
uterus.
Blastocyst burrows into
endometrium, which will
heal over. This process
is called
As trophoblast is causing implantation
and developing into placenta,
Embryoblast is enlarging and
developing into embryo
Fluid-filled
Develops betwee
Embryoblast =
Now called
(12 – 14 days after fertilization)
Pre-embryonic period now over,
Embryonic period begins.
During next six weeks of
period, cells of
embryoblast will differentiate into beginnings of all organs.
Fertilization – 2 wks = Pre-embryo
2 wks – 8 wks = Embryonic period
8 wks – birth = Fetal period
First: Cells of embryonic disk separate into two, and
then 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 epithelial linings of digestive and
respiratory systems.
Mesoderm will form skeletal, muscular, urinary,
reproductive, circulatory systems,
as well as muscle and connective
tissue layers of digestive and
respiratory systems .
Ectoderm will form skin and nervous system
The first system to begin developing is the
This appears as a groove along the ectoderm, which closes
over to form a
The neural tube enlarges at one end to form
remains cylindrical at the other end
to form
~ 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 to form villi which
Embryo floating in amniotic fluid; stays attached to
placenta by
containing
~ Day 28:
and
begin developing.
Bones, muscles, and
blood vessels develop
from mesoderm as limbs
get bigger, nerves grow
out from spinal cord.
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
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 ~ 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:
In placenta:
Carbon dioxide & wastes
Oxygen & nutrients
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
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
After tenth week:
(Organs and systems continue
to develop)
Fetus increases rapidly in size
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