Bio A/H SOURCE : http://www.biology.iupui.edu/biocourses/N100/2k4ch39repronotes.html
Stages of Development of an Animal Embryo
QUESTION How does a zygote – the single 2N cell that results from fertilization - become a multicellular organism
with tissues, organs, and other specialized cells and structures? [Besides Lots and Lots of mitosis!]?
Most animals, including humans, go through different stages of embryonic development: Successful development at
each stage depends on successful completion of the stage before!
1. Gamete formation (n + n): eggs and sperm are produced in specialized tissues (We already discussed Meiosis!)
2. Fertilization: the union of egg and sperm
-Occurs in Fallopian Tubes (usually)
- Fusion of egg and sperm  a new diploid human
zygote results (2n) - the first cell of the new animal...
The Moment of Fertilization
3. Cleavage: The first days and weeks after conception: mitotic cell divisions begin, converting the zygote to a
multicellular organism
Day 1: first cleavage - 1 cell becomes 2
Day 2: second cleavage - 4-cell stage
Day 3: 6-12 cell stage - can test at this stage for genetic diseases
if done by IVF – totipotent cells
Day 4: 16-32 cell stage - solid ball of cells - morula -
Day 5: Solid morula develops into hollow, fluid-filled blastula
The embryo will develop from the inner cell mass, or embryonic
disc
Day 6 -7: Blastocyst attaches to uterus ( implantation).
-The blastocyst secretes HCG - human chorionic gonatotropin
-Can cause "morning sickness"
-Pregnancy test measures this hormone
Bio A/H SOURCE : http://www.biology.iupui.edu/biocourses/N100/2k4ch39repronotes.html
DIFFERENTIATION IS APPARENT HERE -Pluripotent
Days 7-10: Gastrulation: major cellular reorganization into 2
or 3 tissue (germ) layers:
-Ectoderm: skin, nervous system
-Endoderm: lining of gut and internal organs
-Mesoderm: muscles, bones, heart
Gastrula: Early embryo with 3 tissue layers. All cells
in a gastrula have the same DNA; however, different
cells now begin to "turn on" (or "express") different
genes to become different organs.
Days 10 - 14: Pregnancy becomes established
-Fluid filled amniotic cavity starts to form
-Yolk sac starts to form (will make blood cells, germ cells)
-Embryo starts to form from embryonic disc
- Chorion (placenta) starts to form
At the end of this stage, a woman will have just missed her
period!
Days 21 on...
Week 3 - Week 8 (week 10 if you are counting the time from
fertilization (Embryo) - Development of all organ systems
Day 22: Cardiac cells (early heart) begins to beat
Day 24 embryo shown at right
After week 9: The embryo becomes a fetus
WHAT CAN GO WRONG ?
CHROMOSOMAL ISSUES: Roughly one third to one half of all fertilized zygotes never make it beyond week 8. RESULT:
Miscarriage. Some of the critical errors that can cause a miscarriage are:
* Inheritance of a defective set of chromosomes. Errors in meiosis (called nondisjunctions) can produce an egg or
sperm that has an abnormal number of chromosomes or broken chromosomes. This can be lethal (although there are
some diseases that occur with either 45 or 37 chromosomes. About half of the early miscarriages have this type of
“random defect”
* Errors in mitosis after fertilization. A nondisjunction in a dividing blastula may produce one abnormal cell -- but since
the blastula has so few cells, that means a significant fraction of the embryo is defective, preventing further
development (mosaicism can also occur here).
LOCATION ISSUES * Implantation errors. Human embryos have to nestle down in a good home, in the uterus. If the
mother's hormones are not just right, that can prevent implantation, and the otherwise healthy zygote may be sloughed
away. In addition, 0.5 - 1% of all pregnancies are ectopic: the zygote tries to implant in the wrong place, most often in
the fallopian tubes. This is always fatal for the embryo, and has the potential to be fatal for the mother."