Lecture 8 Flashcards
Schizocoely
Band of mesoderm forms around the gut before a coelom forms
Coelom is formed by mesodermal cells dividing
can form all three body plans
Enterocoely
Coelom Formation,
Mesoderm and coelom form at the same time
Gastrulation begins with one side of the blastula bending inward forming the archenteron (endoderm)
As archenteron elongates, the sides push outward and expand into a pouch- like coelomic compartment
Pouch-like compartment pinches off
What are the three body plans
Acoelomate, pseudocoelomate, and eucoelomate
Cytoplasmic Specification
The cytoplasmic determinants are not evenly distributed…
They partition unequally among blastomeres during cleavage
which Leads to cell differentiation
lesds to mosaic development
Mosaic Development
Cytoplasmic Specification
in the early tunicate embryo. When the four blastomere pairs of the 8-cell embryo are dissociated, each forms structures that it would have formed if it had remained in the embryo.
Regulative Development
Conditional specification (regulative development).
A) What a cell becomes depends upon its position in the embryo. Its fate is determined by interactions with neighboring cells.
B) If cells are removed from the embryo, the remaining cells can regulate and compensate for the missing part.
Induction
capacity of some cells to evoke a developmental response from other cells
What can the inducer and responder influence
each other
• i.e. the position of the cell in the embryo and signals from its neighbours determines its fate.Mechanisms of Development
Mechanisms of Development
Cytoplasmic specification
Conditional specification
What does Cytoplasmic specification lead to
mosaic development
• cytoplasmic determinants distribute unequally in blastomeres
• Individual blastomeres cannot produce whole embryo
What does conditional specification lead to
regulative development
• Neighbouring cells induce cell fate
• Individual blastomeres can produce whole embryo
Metazoa major clades
- Deuterostomia (Deuterostomes)
* Protostomia (Protostomes)
How many metazoan phyla are there
34
similar developmental patterns in metazoa
- Presence of blastula and gastrula stages
* Formation of germ layers
Metazoa
kingdom of all unicellular animals
PROTOSTOMIA
• Members generally possess trochophore larvae or a lophophore
DEUTEROSTOMIA
E.g. Chordata, Echinodermata, Hemichordata
four developmental characters in metazoans
- Fate of blastopore
- Cleavage patterns
- Fate of cells
- Mesoderm formation
Protostome
- “First mouth”. i.e. the first embryonic opening
* Blastopore becomes the mouth
Deuterostome
• “Second mouth” i.e. the second embryonic
opening (no name) becomes the mouth
• Blastopore becomes the anus
Cleavage Patterns in Protostome
Spiral cleavage occurs in most lophotrochozoan protostomes
Blastomeres cleave at a45 degree angle to the animal-vegetal axis
Produces quartets of cells that lie in the furrows between cells of underlying layer
Upper layer of cells appear to be offset from the lower and shift in a spiral fashion
blastomeres pack tightly together compared to radial cleavage
Cleavage patterns in Deuterostome
Radial cleavage occurs in most deuterostomes
Embryonic cells are arranged in radial symmetry around the animal-vegetal axis
First cleavage plane passes through the animal-vegetal axis giving rise to two identical blastomeres
Second cleavage occurs simultaneously in both blastomeres and oriented parallel to animal-vegetal axis but perpendicular to first cleavage
After third cleavage,upper tier of cells sits directly ontop of the tier of cells below
Fate of cells in Protostome
• Mosaic development characterizes most lophotrochozoan protostomes
• Cytoplasmic determinants are partitioned unequally
• If separated, individual blastomeres still form structures that were dictated by the
cytoplasmic determinants
• Needs all the blastomeres to develop properly
Fate of cells in deutersome
- Regulative Development characterizes most deuterostomes
- Fate of a cell depends on interactions with neighbouring cells
- Early blastomeres are capable of producing an entire embryo if separated from other cells
- Remaining blastomeres can compensate for the missing cell
- E.g.human identical twins.
Mesoderm Formation in Protostome
- Coelom is formed by schizocoely
* Protostomes can be coelomate, acoelomate, or pseudocoelomate
Mesoderm formation in deuterosome
- Coelom is formed by enterocoely
* All deuterostomes are coelomate
