Early Development in Animals

Chicken or Human??

The Process of Early Development

• 1. Fertilization• 2. Cleavage• 3. Morula• 4. Blastula • 5. Gastrulation• 6. 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

Endoderm

Ectoderm

Mesoderm

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.