1. Animal Development Introduction to animal development
Development requires both cell growth and cell differentiation (different cells express different genes)
How are new organisms produced?

2. Animal Development Stages of development Fertilization
Fertilization = union of gametes (sperm & egg)
3 steps involved in fertilization
Penetration: head of sperm (acrosome) release enzymes to digest glycoprotein layer (zona pellucida) surrounding egg
How are new organisms produced?

3. Animal Development Stages of development Fertilization
3 steps involved in fertilization
Activation: sperm physically contacts egg plasma membrane
final meiotic division produces two egg nuclei, one retained as haploid egg nucleus within egg (mammals)
Cytoplasm rotates toward site of sperm entry; creates gray crescent on opposite side, establishing dorsal-ventral orientation
Produces increase in protein synthesis to prepare for cell divisions
How are new organisms produced?

4. Animal Development Stages of development Fertilization
3 steps involved in fertilization
Nuclei fusion: fusion of sperm nucleus with egg nucleus to form diploid zygote
Two hemispheres of zygote
Animal pole: pole where cells divide faster and are smaller; develops into ectoderm
Vegetal pole: pole where cells contain yolk, divide slower and are larger; develops into endoderm
How are new organisms produced?

5. Animal Development Stages of development Cleavage
Cleavage begins within an hour of fertilization; series of mitotic divisions
Zygote divides into 2, 4, 8, … smaller and smaller blastomeres without increasing size of embryo
Cleavage involves about 12 divisions resulting in solid ball of blastomeres
How are new organisms produced?

6. Animal Development Stages of development Cleavage
Pattern of cleavage divisions depends on amount of yolk in zygote
If little yolk (lancelets), holoblastic cleavage produces similar-sized blastomeres
If lots of yolk in vegetal pole (frogs), holoblastic cleavage produces different-sized blastomeres
How are new organisms produced?

7. Animal Development Stages of development Cleavage
Pattern of cleavage divisions
In birds and reptiles...
Egg all yolk except for small area at one pole, cleavage only occurs in this area (called bastodisc)
Cleavage pattern called meroblastic; embryo forms like a cap on yolk
How are new organisms produced?

8. Animal Development Stages of development Cleavage
Pattern of cleavage divisions
In mammals...
Holoblastic cleavage except concentrated at one pole (inner cell mass analogous to blastodisc)
Cells surrounding inner cell mass (trophoblast) become part of the placenta
How are new organisms produced?

9. Animal Development Stages of development Blastula
Stage characterized by hollow ball of cells
At about 16-cell stage of cleavage, interior cells create osmotic gradient causing water to fill extracellular spaces
fluid-filled part of blastula is blastocoel
For short period of time, cells of mammalian blastula can develop into most of the cells types in the body; these are embryonic stem cells
How are new organisms produced?

10. Animal Development Stages of development Gastrulation
Gastrulation = initial movement of cells in embryo
Invagination: dent inward
Involution: roll inward
Creates anterior-posterior orientation of embryo (bilateral symmetry) and archenteron (gut of embryo)
Cell movement during gastrulation creates three embryonic tissue layers:
Endoderm: digestive, respiratory, and most other organs
Ectoderm: skin, nervous system
Mesoderm: notochord, bones, blood vessels, muscles
How are new organisms produced?

11. Animal Development Stages of development
How are new organisms produced?

12. Animal Development Stages of development Gastrulation
Cell movement during gastrulation:
Migrating cells move by changing shape
migrating cells have surface molecules that adhere to adjacent cells; cells move as a unit
How are new organisms produced?

13. Animal Development Stages of development
Gastrulation in primitive chordates (e.g. lancelets)
Surface of blastula invaginates into blastocoel
2-layered (endoderm, ectoderm), cup-shaped embryo is gastrula
Opening created by cell migration is archenteron
Opening of archenteron is blastopore
How are new organisms produced?

14. Animal Development Stages of development
Gastrulation in most aquatic vertebrates
Gastrulation more complex because of size differences between animal and vegetal cells
Major steps:
Layer of surface cells invaginates at blastopore (dorsal lip of blastopore)
Cells from animal pole involute over dorsal lip, causing inner cells to fill blastocoel and create archenteron
Inner cells involuted over dorsal lip move between layers to form mesoderm
How are new organisms produced?

15. Animal Development Stages of development
Gastrulation in most aquatic vertebrates
Major steps:
Opening of blastopore becomes filled with inner yolk-filled cells creating yolk plug
3 tissue layers established; prepares embryo for tissue differentiation and development of organs
How are new organisms produced?

16. Animal Development Stages of development
Gastrulation in terrestrial vertebrates
Embryo develops in blastodisc or inner cell mass
Lower and upper cells layers differentiate into endoderm and ectoderm without cell movement
Cells of ectoderm invaginate and involute at primitive streak to produce mesoderm
How are new organisms produced?

17. Animal Development Stages of development Neurulation
Ectoderm cells elongate into neural plate as notochord develops from mesoderm below
Induction = one embryonic region influences development of adjacent region
Cells in neural plate changes shape, eventually rolling into neural tube which then develops into brain and spinal cord
How are new organisms produced?

18. Animal Development Stages of development Neurulation
During formation of neural tube, mesoderm creates coelom and some organs
In vertebrates, neural crest forms like ‘roof’ of neural tube
How are new organisms produced?

19. Animal Development Stages of development
Cell migration, organogenesis and growth
Neural crest cells migrate to different areas of embryo
Anterior: produce forebrain, sense organs
Posterior: produce gill arches, muscle somites
Ventral: produce Schwann cells, adrenal medulla
How are new organisms produced?

20. Animal Development Stages of development (review)
How are new organisms produced?

21. Animal Development Stages of development (review)
How are new organisms produced?

22. Animal Development Stages of development (review)
How are new organisms produced?

23. Animal Development Cell communication during development Induction
Certain act as organizers; induce changes in adjacent cells
Organizers produce protein that binds to target cells; cause changes in gene expression (based on concentration)
How are new organisms produced?

24. Animal Development Cell communication during development Induction
Primary induction involves embryonic tissue types: endoderm, mesoderm and ectoderm (e.g. mesoderm of notochord inducing ectoderm to produce neural tube)
Secondary induction involves already differentiated tissue
What would happen if cells of optic stalk moved elsewhere?
How are new organisms produced?

25. Animal Development Cell communication during development
All cells of embryo genetically identical
How does embryo develop specialized cells?
Different genes expressed in different cells
As cells specialize they lose ability to express more and more genes (development involves progressive restriction of gene expression)
Early blastomere are totipotent (capable of expressing all genes)
Some cells (e.g. egg cells) become determined early in development because of their location in embryo
Other cells become committed later in development
How are new organisms produced?