Developmental Patterns of Metazoans Flashcards
What are the coelom formation?
Schizocoely (coelom forms by splitting)
Enterocoely (coelom forms by out pocketing)
What is schizocoely?
- band of mesoderm forms around the gut before a coelom forms
- coelom is formed by mesodermal cells diving
- can form all three body plans (coelomate, pseudocoelomate, acoelomate)
What is enterocoely?
- 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
- only forms coelomate body plan
What is cytoplasmic specification?
- eggs cytoplasm is not homogeneous
- contains; organelles, mRNAs, enzymes, transcription factors
- these cytoplasmic determinants are not evenly distributed
- they partition unequally among blastomeres during cleavage
- leads to cell differentiation
How does cytoplasmic specification lead to mosaic development?
Cytoplasmic specification in the early tunicate embryo. When the 4 blastomere pairs of the 8-cell embryo are dissociated, each forms structures that it would have formed if it had remained in the embryo
What is conditional specification?
- cell fate is not fixed until it receives positional information from neighbouring cells
- induction (capacity of some cells to evoke a developmental response from other cells)
- the inducer and responder cells can influence each other
i.e. the position of the cell in the embryo and signals from its
neighbouring determines its fate
How does conditional specification lead to regulative development?
Conditional specification.
A) What a cell becomes depends upon its position in the embryo. Its fate is determined by interactions with neighbouring cells
B) If cells are removed from the embryo, the remaining cells can regulate and compensate for the missing part
What are the Metazoa?
- Kingdom of multi cellular animals
- Currently 34 metazoan Phyla
- Two major clades (deuterostomia and Protostomia)
- Share similar developmental patterns
Presence of blastula and gastrula stages
Formation of germ layers - Other aspects of early development have been used to construct metazoan phylogeny
What are the Protostomia?
Lophotrochozoan Protostome: - members generally possess trochophore larvae or a lophophore - Platyhelminthes - Mollusca - Annelida Ecdysozoan Protostome: - members shed cuticle as they grow - Nematoda - Arthropoda
What are Deuterostomia?
- Chordata
- Echinodermata
- Hemichordata
What are the developmental patterns in Metazoans?
- Protostomes and Deuterostomes are identified by a suite of four developmental characters
Fate of blastopore
Cleavage patterns
Fate of cells
Mesoderm formation - Protostomes are further divided into two major clades (Lophotrochozoa and Ecdysozoa)
What is fate of blastopore?
Protostome:
- first mouth (the first embryonic opening becomes the mouth)
- blastopore becomes the mouth
Deuterostome:
- second mouth (the second embryonic opening becomes the mouth)
- blastopore becomes the anus
What is cleavage patterns?
Protostome:
- spiral cleave occurs in most lophotrochozoan protostomes
- blastomeres cleave at a 45 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
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 on top of the tier of cells below
What is fate of cells?
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
Deuterostome:
- regulative development characterizes most deuterostomes
- fate of 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
What is mesoderm formation?
Protostome:
- coelom is formed by schizocoely
- protostomes can be coelomate, acoelomate or pseudocoelomate
Deuterostome:
- coelom is formed by enterocoely
- all deuterostomes are coelomate
