(1) The Making of a Body and a Field: Part 1 Flashcards
zygote
single fertilized egg
blastocyst
a ball of identical cells (same shape as enbryo bc cells aren’t growing, just dividing)
what is developmental biology
the gradual process of change within cells and tissues of the body; it is directed by intrinsic and extrinsic factors; a process taking us from genotype to phenotype
what are six stages of development?
- fertilization
- cleavage
- gastrulation
- organogenesis
- metamorphosis
- gametogenesis
fertilization
fusion of haploid gametes (sperm and egg); results in a diploid zygote with unique genome
cleavage
period of rapid mitotic cell division to produce multicellular blastula; cells of blastula are blastomeres
gastrulation
period of cellular rearrangement to produce specialized tissues and organs
metamorphosis
major alteration of body plan in a new hatchling or newborn that transforms a sexually immature larval form to a sexually mature adult (ex: tadpoles to frogs)
gametogenesis
formation of haploid gametes by meiosis
what is polarization when it comes to eggs
one side is different from the other
animal pole
animal cap (brown; the white part is the nucleus); the majority of the frog
vegetal pole
provides nutrients
fertilization in Xenopus laevis (African clawed frog)
fertilization is external; mating is amplexus
what does fertilization accomplish (3 things)
- joins haploid sperm and egg
- activates biochemical events in fertilized egg (ACTIVATION); allows the development of the embryo
- cause rearrangements of cytoplasmic structures and contents in fertilized egg (set up axes of future organism)
3 body axes of bilaterians
- ant/post
- dors/vent
- left/right
cleavage in Xenopus laevis (frog)
- mitosis is very rapid so there is no growth between divisions; the embryo becomes multicellular but doesn’t grow bigger2
- biphasic (M & S); no G1 or G2
Gastrulation in Xenopus Laevis
- cells on the surface involute (tuck into center)
- create a dimple, called a blastopore
- blastopore is initiated at a stop that denotes future dorsal side, so upper ridge of tissue is called dorsal blastopore lip
- blastopore becomes deeper, but also extends laterally and ventrally
Organogenesis in Xenopus Laevis: Neurulation
- Neurulation is the formation of nervous system
- forms on outside from ectoderm along dorsal midline
- deepends to form a neural groove
- circularizes to form a neural tube and is covered over by surface ectoderm
3 primary germ layers
- ectoderm
- mesoderm
- endoderm
present in embryos of all triploblastic animals; everything but sponges and comb jellies
ectoderm
outermost layer; skin/nervous system
mesoderm
middle layer; notochord, somites (vertebrae, muscle, dermis of skin), many internal organs
endoderm
innermost layer; lining of digestive tract
archenteron
primitive gut
pre-hatching in Xenopus Laevis
embryo is curled and still enclosed in a transparent membrane called vitelline layer
post-hatching oin Xenopus laevis
embryo breaks free of the vitelline layer and is a free-swimming tailbud-stage larva, which must eat
Metamorphosis in Rana Pipiens
fully aquatic to terrestrial (added forelimbs, hindlimbs, smaller tail, lungs)
Gametogenesis
- is completed in adults
- required for continuation of species
- diploid precurose cells undergo meuosis to produce haploid sperm in males and haploid egg in females
- completed in time for next mating season (spring)
generalities about cleavage
- rapid mitosis cell division without growth in between
- cell cycle is biphasic (only M & S)
- embryo grows no larger, just becomes multicellular with progressively smaller cells
mid-blastula transition (still in blastula phase)
- rate of mitotic cleavage slows
- cell cycle lengthens to include G1 and G2 (cell now actually grows)
- progression through diff. phases reguated by diff. conbos of cdk’s and their associated cyclins
- cdk’s and cyclins that function after mid-blastula transition are synthesized from genome (not from stores mRNAs or original egg cytoplasm)
cleavage patterns
dependent upon the amount and distribution of yolk
holoblastic (isolecithal, mesolecithal); meroblastic (teloecthial, centrolecithal)
dependent upon angle of mitotic spindle and timing of its formation (x,y,z axis)
holoblastic
the entire yolk is cleaved
meroblastic
the yolk isn’t entirely cleaved bc of DENSE yolk
isolecithal
sparse yolk, evenly distributed
humans
mesolecithal
moderate yolk, deposited at vegetal pole
telocithal
dense yolk throughout most of egg
chickens
centrolecithal
dense yolk, centrally located
most insects
influence of yolk
- dense yolk impedes cleavage
- cleavage of entire egg proceeds unimpeded in isolecithal egg
- cleavage of the vegetal pole in mesolecithal eggs (amphibians) occurs at a slower rate
- cleavage of the vegetal pole does not occur at all in telolecithal eggs (chick)
- superficial cleavage occurs in centrolecithal eggs (insects)
cleavage pattern of sea urchin and starfish (echinoderms)
radial but isolecithal
cleavage pattern of mammals
rotational but isolecithal
cleavage pattern of amphibians
displaced radial cleavage but mesolecithal
cleavage pattern of birds/chick
discoidal cleavage but telolecithal
cleavage pattern of insects/drosophila
superficial cleavage but centrolecithal
2 major cell types in early embryos
epithelial and mesenchymal
epithelial cells in early embryo
- tightly packed into sheets
- cells are connected to each other via tight junctions
- serve as a barrier function
- ex: skin
mesenchymal cells in early embryo
- cells are not connected to each other; allows them to move
- not tightly packed
- migratory
- ex: microglia, macrophages
surprising early discoveries from anatomical studies
- cells move around in embryo- sometimes great distances
- cells grow
- cells change shape
- cell divisions differ in number and in plane of orientation
- certain large subsets of cels selectively die (apoptosis)
- cells differ in membrane components and in secreted products
types of cell movement during gastrulation
- invagination
- involution
- ingression
- delamination
- epiboly
