Lecture 7 Flashcards
What is a gamete
mature haploid male or female germ cell (i.e. egg and sperm)
What is an egg?
• An organic vessel where an embryo develops
(colloquial terminology)
• An ovum (scientific terminology)
Ovum
the female reproductive or germ cell
Egg characteristics
Eggs has components of a normal somatic cell (e.g. cytoplasm, nucleus, organelles)
• Eggs also have yolk (provides energy during development)
• Eggs have a polarity;
• animal pole has most of the cytoplasm (and
the nucleus)
• vegetal pole has most of the yolk,
placement of yolk in egg
Isolecithal
Mesolecithal
Telolecithal
Centrolecithal
Isolecithal:
very little yolk, evenly distributed through egg
Typical of placental mammals
*Also echinoderms, tunicates, cephalochordates, molluscs
Mesolecithal:
moderate amount of yolk concentrated at vegetal pole
Typical of amphibians
Telolecithal:
abundance of yolk densely concentrated at vegetal pole
Typical of birds, reptiles, fish, monotremes, some amphibians
Centrolecithal:
large centrally located mass of yolk
Typical of arthropods (e.g. insects)
General rule of amount of yolk
Lots of yolk (e.g. telolecithal eggs), young exhibit direct development
• Goes straight from embryo to miniature adult
Little yolk (e.g. isolecithal, mesolecithal eggs) young exhibit indirect development
• Passes through larval stage capable of feeding itself • Undergoes metamorphosis to reach adult stage
• e.g. amphibians, echinoderms, tunicates, cephalochordates, molluscs
Why are mammals an exception to amount of yolk rule
because they don’t need a larval stage due to mother giving so much energy , ie direct development
Cleavage
- Embryo divides repeatedly without growth (essentially skips G-phase of mitosis)
- Single large egg cell becomes many smaller cells called blastomeres
- By the end of cleavage, the zygote is called a blastula
Cell division occurs more easily in _____ than yolk
cytoplasm
Two types of cleavage:
- Holoblastic: complete and approximately equal divisions of cells
- Meroblastic: restricted to a small area of egg
Direction of cleavage
- Spiral cleavage
* Radial cleavage
Where are cells formed in most animals
around a fluid-filled cavity called a blastocoel
True or False: most animals go through blastulation
FALSE! ALL ANIMALS go through blastulation, most go through gastrulation
Gastrulation
- Converts the spherical blastula into a two- or three- layered embryo
- The layers are called germ layers
How many germ layers does the blastula and gastrula have?
the blastula has 1 germ layer
the gastrula has 2 or 3
Gastrulation process
• One side of the blastula bends inward in a process called invagination
• The internal pouch that is formed is the gut cavity (also called
archenteron or gastrocoel)
• The opening to the gut cavity is called the blastopore
• Outer layer of cells (lining blastocoel) = ectoderm
• Inner layer of cells (lining gut) = endoderm
• When the gut opens only at the blastopore it is called a blind gut
• Most animals have a complete gut with a second opening, the anus
• The blastopore becomes the mouth in some organisms and the anus
in others
• This difference differentiates Protostomes and Deuterostomes
7.1 Key Events in Animal Development
• In many animals, a third layer forms = mesoderm
• The mesoderm is formed from the endoderm
• Coelom = cavity completely surrounded by mesoderm
one germ layer
Some animals stop at the blastula stage (e.g. some sponges)
Two germ layers
Some animals develop two germ layers (e.g. some sponges, sea anemones and comb jellies)
• diploblastic
Three germ layers
Most animals develop three germ layers (e.g. everyone else!)
• triploblastic
Organogenesis
- Organs develop from specific germ layers
- First event in organogenesis is formation of the nervous system
- Formed from the ectoderm
- Heart is the first functional organ
Growth
• Longest phase in animal development
ex
• The human fetus grows from the size of a single poppy seed (4 weeks) to an 8lb jackfruit (40 weeks)
• Organogenesis takes place in the first 8 weeks
Categorizing Metazoans
- Number of germ layers (diploblastic, triploblastic)
- Presence and formation of coelom (acoelomate, pseudocoelomate, coelomate) and (next: schizocoely, enterocoely)
- Cleavage pattern (radial, spiral)
- Body symmetry (e.g. spherical, radial,
bilateral) - Mechanism of development (mosaic, regulative)
