Animal DevelopmentChapters 21 & 47

“Homunculus”

Acrosomal & Cortical Reactions

Cortical Reaction: Wave of Ca+

(So only one sperm gets in)

Preventing Cross-Species Fertilization

• Fertilin protein in sperm fits like a key into a recognition protein on egg

CleavageNote: Size is not increasing

Early Development Stages

• Single cell• Cleavage

– Produces smaller cells– Total size does not increase

• Morula – 16 to 64 cells• Blastula – at least 128 cells• Gastrula – formation of blastopore

– Formation of germ cell layers• Ectoderm, endoderm, mesoderm

– Primitive gut

Sea Urchin Development

Cleavage Stages: rapid mitosis without growth

• Reduces cell size.• All cells receive identical copy of DNA.• Cytoplasm they inherit is not identical,

may contain special substances (determinants: beginning of cell differences).

• Forms mass of cells (morula), then hollow ball called (blastula)

Cleavage in Vertebrate Embryo

Blastula

Gastrulation

Some key stages of development in animals and plants

Review: What are the stages?

Organogenesis• Formation of organs

• 3 kinds of morphogenic changes– Folds– Splits– Dense clustering of cells

• First to develop– Neural tube– Notochord– Skeletal rod

Neurulation

Organogenesis of Chick EmbryoRudimentary organs have formed (56 hours)

Eye

Forebrain

Heart

Somites

Neural Tube

Amniotic Egg & Development ofExtraembryonic Membranes

Human Embryo & Extraembryonic Membranes

• Blastocyst – hollow ball of cells• Trophoblast

– epithelium of blastocyst– Enzymes - implantation– Forms placenta

• Chorion – develops from trophoblast– Outer membrane

• Amnion – develops from epiblast– Fluid-filled cavity

• Yolk sac – blood formation• Allantois – umbillical cord

Extraembryonic Membranes• Amnion• Yolk sac• Chorion• Allantois

Functions of the Placenta

• Fetal blood vessels penetrate pools of maternal blood in endometrium

• Blood does NOT mix, but O2 and CO2 glucose/vitamins diffuse across

• Prevents passage of most large proteins & cells.

Morphogenesis & Changes in Cell Shape

Morphogenesis &Convergent Extention of Cell Sheets

• Cells rearrange themselves

• Narrower & longer

Major Stages of Embryogenesis

Genetic Basis of Development

How do you get from one cell to a multi-cellular animal?

Stem Cells: Undifferentiated Cells

When a cell divides, the new cells don’t get the same stuff

& cells can affect their neighbors

Determination & Differentiation

Segmentation genes are turned on in certain cells

Hox Genes: head to tail organization in animals

Switches

Apoptosis: programmed cell death

Let’s discuss the role of these during development:

• Hox genes• Organizer• Noggin• Sonic hedgehog• ZPA• Switches

Uh oh… How does this happen?

How does this happen?

How does this happen?