Do Now
Identify each of the cells in the diagram and indicate the
Human chromosome number of each.
Egg (23)
Sperm
(23)
Zygote (46)
How do we go from a zygote (a single cell) to a
multicellular organism? (hint: think about a
process used to increase cell#)
STAGES OF EMBRYONIC DEVELOPMENT
• Series of steps leading from zygote to organism
1. Cleavage – increase in cell# by rapid mitosis, no cell
growth
2. Differentiation – cells get their jobs by “reading” different
parts of DNA, form tissues and organs
3. Growth
Stage 1
• Egg and sperm are produced by
MEIOSIS (gametogenesis).
• They contain half as much
genetic material as parent cells
(in humans = 23)
• The joining of the two nuclei is
called fertilization
• This can occur internally or
externally
Stage 2
1 The sperm
approaches
the egg
SPERM
2
The sperm’s
enzymes
digest the
egg’s jelly
coat
3 The plasma membranes
of sperm and egg fuse
Sperm
head
4 The sperm
nucleus
enters
the egg
cytoplasm
Nucleus
Egg
nucleus
EGG CELL
5 The nuclei
of sperm
and egg fuse
46
•Normal
fertilization:
http://www.stanford.
edu/group/Urchin/nf
ert.htm
Sperm
nucleus
Jelly
coat
• The fertilized
egg or Zygote
now has how
many
chromosomes?
Zygote
nucleus
2-cell stage
Stage 3
• The fertilized egg divides into
two new cells by MITOSIS to
form identical cells
• This is the beginning of
cleavage: rapid increase in cell
#’s by mitosis (stages 3-4)
• NO CELL GROWTH!
• # of chromosomes in each cell:
46
http://fig.cox.miami.edu/~cmallery/150/devel/blastula.gif
Stage 4
• Four Cell Stage: still increasing
in numbers by mitosis (cleavage)
• Number of chromosomes in
each cell: 46
Stage 5
• Blastula: hollow ball stage
• Number of chromosomes in
each cell: 46
• The layer of cells is generally one
cell thick and the cavity is filled
with fluid
• Cell growth starts
Stage 6
• Gastrulation: inward pushing of
blastula to form two layers
(indentation)
• The cell is beginning to grow
before dividing
Stage 7
• Differentiation:
formation of three layers
which will develop into
organs and organ systems
• Development can take place internally or
externally
Twins
• Fraternal Twins – two
separate eggs
fertilized by two
separate sperm
(dizygotic)
•Identical Twins –
single egg is
fertilized , zygote
splits early
(monzygotic), have
identical DNA
• Indian girl - born
fused at the pelvis to a
"parasitic twin" that
stopped developing in
the womb
• She had absorbed the
organs and body parts
of the other fetus, a
condition that occurs
once in 50,000
conjoined twin births.
Fertilization and Cleavage Video Clip
•
http://trc.ucdavis.edu/biosci10v/bis1
0v/media/ch27/cleavage_implant.ht
ml
•
http://www.unc.edu/~fconlon/Danie
lResearch.htm
•
http://raven.zoology.washington.edu
/celldynamics/events/workshops/arc
hive/2003/cytomod_abstracts/GvD_
VDF/images/GvD-VEF-fig1.jpg
• http://www.howstuffwork
s.com/adam-200048.htm
Stem cells – cells without a “job”, removed from
blastocyst before they differentiate