Download 15.1 Fertilization and Embryo Development

15.1 Fertilization and Embryo Development
Timeline of Early Human Development ***p.510 Figure 15.2
DAY
Fertilization 
Cleavage 
Blastocyst 
Implantation 
Gastrulation begins Gastrulation complete
Neurulation begins  Neurulation complete
Differentiation begins 
_____________________________________________________________________________________________
0
1
4 6
14
21
24
Fertilization (Conception) 12-24 hours after ovulation
- sperm must penetrate through protective layers of egg (zona pellucida and corona
radiata)
- only one sperm can enter egg; upon
entry egg plasma membrane depolarizes
and prevents others from entering
- involves the joining of male and female
gametes (sperm and egg)
egg + sperm  single cell zygote
(23) (23)
(46)
- occurs within the oviducts
Cleavage within 30 hours of fertilization
-
zygote divides by mitosis (produces identical cells)
2 cells

4 cells  8 cells
-
Zygote remains the same size despite divisions = cleavage
Blastocyst 3-5 days after fertilization
-
after the zygote reaches 16 cells it is now called a morula
morula reaches uterus and begins to fill with fluid to form a blastocyst (hollow
structure for more cells to develop)

uterine
wall
morula
-
2 parts to blastocyst:
o trophoblast = outer layer of blastocyst; develops into the chorion (will form
part of the placenta – provide nutrients and removes wastes); secretes
hormones and enzymes
o inner cell mass (embryoblast) = forms the embryo
Implantation complete at 14th day
-
around 5th-7th day blastocyst attaches to endometrium (trophoblast secretes
enzymes to penetrate into lining)
implantation complete at 10th – 14th day = now said to be pregnant
hCG (human chorionic gonadotropin hormone) = secreted by trophoblast to
keep progesterone and estrogen levels high to maintain endometrium
Gastrulation during 2nd week
- amniotic cavity forms = space where baby will develop
o inner cell mass flattens into embryonic disk
-
-
gastrulation = embryonic disk forms primary germ layers (will develop into all the
cells, tissues and organs):
outer = ectoderm
middle = mesoderm
inner = endoderm
embryo now called a gastrula
marks the beginning of morphogenesis = events that form distinct structures of the
developing organism “producer of forms”
Neurulation during 3rd week
- mesoderm thickens to develop the notochord (along the back of the embryonic
disk); will become the framework/backbone for the skeleton
- neurulation = nervous system develops from the ectoderm located above
notochord when layer folds inward on each side of notochord, fuses and make
neural tube (brain and spinal cord)
Differentiation
= process that enables a cell to develop a particular shape and perform specific functions
different from other cells
EMBRYONIC PERIOD OF DEVELOPMENT – up to end of 8th week
Time Period
Place of Occurance
Major Events
12-24 hours
Oviduct
Fertilization
Within 30 hours
Oviduct
Cleavage
th
3-5 day
Uterus
Blastocyst
5-7th day
Uterus
Blastocyst attaches to
lining
10 – 14th day
Endometrium
Implantation
nd
2 week
Endometrium
gastrulation
rd
3 week
Endometrium
neurulation
4th week
Uterus
Rapid growth and
differentiation (blood cells
and vessels forming; buds
for appendages appear;
head visible with hint of
eyes, ears, nose)
5th week
Uterus
Eyes open (no eyelids);
brains cells differentiating
th
6 week
Uterus
More brain development;
limbs lengthen and flex;
gonads begin to produce
hormones to influence
external genitalia
development
th
th
7 – 8 week
Uterus
Organs formed; NS
coordinating body
movements, skeleton
forming cartilage; eyelids
develop; external genitalia
forming
End of 8 weeks
Uterus
Embryo now called a
FETUS; 90% organs
developed; looks
recognizably human (size
of a paperclip)