Transcript Early Development in Animals

Early Development
in Animals
Chicken or Human??
The Process of Early Development
•
•
•
•
•
•
1.
2.
3.
4.
5.
6.
Fertilization
Cleavage
Morula
Blastula
Gastrulation
Neurulation
• “First call mom before going nuts”
1. Fertilization
• Sperm & egg: each have haploid
genomes
• Fertilization restores diploid state to make
zygote
• But that is not the whole story…
Fertilization
• The cytoplasm and mitochondria are contributed
only by the egg.
– Mitochondrial DNA testing follows the maternal
pedigree
• Some mammalian genes are active in
development only if they come from the egg;
others are only active if they come from the
sperm
– Concept of “genomic imprinting”
– Example of human genomic imprinting
Deletion of Part of Chromosome 15
Angelman Syndrome
When deletion is on maternal
chromosome 15
Prader Willi Syndrome
When deletion is on paternal
chromosome 15
A Look at Fertilization
The 1st sperm to fuse with the egg membrane wins, and there is an
immediate change in the egg membrane to make it impermeable to any
additional sperm.
When that “shutdown” doesn’t work
• Molar pregnancy
2. Cleavage
• Taking the cytoplasm of the zygote and
repackaging it into smaller cells; it’s all
about the yolk.
These are my
Eggs!
Cleavage in Frog Eggs
• Is uneven because the yolk is
concentrated on 1 side of the egg
• All that yolk makes it harder for cleavage
furrow to form
• Cells in vegetal pole are larger than those
in animal pole
The Chicken and The Egg
The Chicken and The Egg
Drosophila (Fruit Flies)
• Mitosis without cytokinesis
• Nuclei migrate to periphery
Mammals have Unique Cleavage
• Rotational cleavage
3. Morula
• Means “mulberry”
4. Blastula (Non-mammalian)
Characterized by central fluid-filled cavity
Blastocyst (Mammalian)
Human Embryo Implantation
5. Gastrulation
• Cells in blastula undergo major
movements and cell divisions
• Begin the formation of 3 distinct embryonic
layers
Ectoderm
Mesoderm
Endoderm
The 3 Embryonic Layers
• Ectoderm
– Skin
– Nervous system
• Mesoderm
– Muscle, bone, kidneys, blood, etc.
• Endoderm
– Liver, lungs, lining of the gut
6. Neurulation
• Forming an internal neural tube from an
external sheet of cells
• Neural plate
neural groove
neural tube
Neural tube becomes spinal cord; Notochord becomes vertebrae
4 Extraembryonic Membranes
(Birds,Reptiles)
• 1. Yolk sac:
surrounds yolk
(provides nutrients)
• 2. Amnion: secretes
the fluid the embryo
is surrounded by
• 3. Chorion:
Outmost of the
protective
membranes
• 4. Allantois: A sac
that holds
nitrogenous wastes
made by the embryo
Extraembryonic Membranes in
Mammals
• 1. Amnion: Secretes amniotic fluid; what
is tested in amniocentesis; what “breaks”
in labor
• 2. Chorion: The chorion, together with
part of the uterine wall, make the placenta.
• 3. Yolk sac: Mammalian don’t have a
yolk.
• 4. Allantois: Not needed as much;
nitrogenous wastes go across the placenta
and are excreted by mom’s kidneys.