Development of Animals
Aka…Embryology
Female Reproductive System
Embryology - study of development of the
embryo
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5 major stages..
1. Gametogenesis - gamete production
2. Fertilization - gamete --> zygote
3. Cleavage - Zygote --> Blastula
4. Gastrulation - Blastula --> Gastrula
5. Organogenesis - Organ Formation
• -i.e. Neurulation- Gastrula --> Neurula
A. Fertilization:
male and female gametes fuse to
form a zygote
1. Funnel-shaped end of a
fallopian tube is the usual
site of fertilization
2. If a sperm penetrates the
egg, fertilization results.
Tiny hair-like cilia lining
fallopian tube propel
fertilized egg, zygote,
through the tube toward
the uterus.
Fertilization
Cell Division
1…2 buckle my shoe
1. Cleavage: process
by which a zygote
divides by mitosis to
form two new cells
a. Two cells will divide to
form 4 cells, and so
on
b. Continue dividing until
a blastula (a.k.a.
blastocyst) is formed
4 cell stage
• 2 cells divide by
mitosis and
become 4
• 2nd mitotic division
• Still the same size
as the previous
cleavage (2 cell)
8 cell
• 1st time it’s obvious that the cell is either a
human or another organism like a worm!
• You can tell it is a deuterostome or a
protostome….. What’s that??
Deuterostomes vs. Protostomes
SAY WHAT???
• 8-CELL STAGE IS KEY DIFFERENCE!!!Deuterostomes- (starfish & vertebrates)
cleavage results in 8 cells sitting directly on
top of each other. This is called radial
cleavage
Protostomes
• Protostomes (clams, worms, & insects)
undergo spiral cleavage- cells divide and they
do not sit on top of each other; they appear
to spiral
2. MORULA – 16 cell
2. Morula: a zygote
consisting of about 12-32
cells in a solid ball that
reaches uterus in about
3-4 days after fertilization
3. Blastula (blastocyst)
formation
3. Blastula = a hollow ball of cells that
forms between 5-8 days after
fertilization
a.
b.
This will attach to the lining of the uterus
cell division continues
Cells start to compact and move to the
edge of the cell, leaving a fluid-filled
space in the center- which is called
the blastocoel
4. Gastrula
• Now gastrulation occurs. The hollow ball of cells
known as the blastula begins to fold inward on
one side
• (imagine that you had a deflated basketball and
pushed it in on one side).
• When the folding occurs, it begins to create a
horse shoe shaped structure that is 2 cell
layers thick..
Gastrulation
Blastula (hollow ball of cells) transformed into the Gastrula (three-layered stage)
• Gastrula: structure made up of 2 layers of cells
with an opening at one end
• The point where the horse shoe almost touches
is called the blastopore (small hole).
• In protostomes this will eventually form the
mouth.
• In deuterostomes this will form the anus.
– Cells that are folding inward form a cavity lined with a
2nd layer of cells
– Gastrulation - sorts all the cells into distinct cell
layers (ectoderm, endoderm, and mesoderm)
Gastrulation forms Three distinct
tissue layers
a. Layer on the outer surface of the
gastrula is called the ECTODERM
– 1. ECTODERM Continues to divide and
eventually forms the SKIN and NERVOUS
tissue
b. ENDODERM
• B. Layer on the INNER
SURFACE of the gastrula
is called the ENDODERM
– 1. ENDODERM Continues
to divide and eventually
develops into the lining of
the digestive tract and
organs associated with
digestion
c. MESODERM
1.Mesoderm is the 3rd cell
layer found in the
developing embryo between
the ECTO and ENDODERM
(“meso” means middle)
2. MESODERM Continues to
grow and divide and
eventually develops into
muscle cells, circulatory,
and excretory cells, bone
cells and respiratory system