Early Development Flashcards
Stage after gastrulation
Pharyngeal stage
characteristics
Embryo has
* Pharynx
* Central neural tube
* Notochord
* Somites
* Head region
Fertilization
fusion of mature gametes
Cleavage
Series of rapid cell divisions during which the cytoplasm is split between smaller daughter cells (blastomeres)
Results in the blastula
Gastrulation
Slower cell division and dramatic cellular rearrangements
At the end of gastrulation, the gastrula has all 3 germ layers
Ectoderm, mesoderm, endoderm
Ectoderm
outer layer = skin, brain, neural crest
Mesoderm
middle layer = blood, heart, kidney, gonads, bones, muscles, and connective tissue
Endoderm
inner layer = digestive tube and its associated organs including the lungs
Germ layers
Three distinct regions of the embryo that give rise to differentiated cell types and specific organ systems
Ectoderm, mesoderm, endoderm
Organogenesis
Formation of tissues and organs
Many organs actually contain cells from multiple germ layers
Cells in the outer layer of skin (epidermis) are ectodermal while inner layers are mesodermal
Notochord
Rod of mesodermal cells
Signals overlying ectoderm to become the nervous system
Begins developing at 17 days and gone by 7-10 weeks
Example of an embryonic tissue that undergoes programmed cell death
Metamorphosis
Process of changing from immature to sexually mature organism
Gametogenesis
Germ cells are gamete precursors
Process of producing gametes for reproduction
Set aside normally during very early development
Different than somatic cells which are all other cells of the body
Gametogenesis requires meiosis
Meiosis
Chromosomes replicate prior to cell division so each gene is represented 4 times
Replicated chromosomes (called chromatids) are held together by the kinetochore and all 4 chromosomes pair together
* Recombination occurs
chromatid becomes a single chromosome
Result = 4 germ cells with a haploid nucleus
1st division separates chromatid pairs
2nd division splits the kinetochore so each chromatid
becomes a single chromosome
von Baer’s laws
4 generalizations of vertebrate development
von Baer’s first law
The general features of a large group of animals
appears earlier in development than do the specialized features of a smaller group
Developing vertebrates look similar after gastrulation & diversity later
All vertebrate embryos have gill arches, a notochord, a spinal cord, and primitive kidneys
Less general characters develop from the more
general, until finally the most specialized appear
Early on, all vertebrates have a similar skin. Specializations such as scales, feathers, hair etc develop later
von Baer’s third law
The embryo of a given species, instead of passing
through the adult stages of lower (simpler
anatomically) animals, departs more and more
from them
For example, all embryonic vertebrates have gill arches. These are not the same as adult fish gills. Rather, fish elaborate and develop these structures into gills while in mammals these structures develop into the eustachian tubes (ear-mouth connection).
von Baer’s fourth law
Therefore, the early embryo of a higher animal is
never like a lower animal but only like its early
embryo.
Human embryos never pass through a stage where they look like an adult fish or bird. Rather, human embryos, fish embryos, and bird embryos initially share common characteristics and look similar
How does the blastula become a
gastrula which becomes an adult?
A complicated series of cell movements and shape
changes allow the formation of the axis
Fate map
Cells that will give rise to certain tissues have their fate determined during gastrulation
Identification of groups of cells in the gastrula that will become a particular tissue in the adult
Fate mapping method to label individual cells and track them over time
Fluorescent dye labeling
Step 1: Inject cells with fluorescent tracking dye (green)
Step 2: See where they go
Fate mapping method to label individual cells and track them over time
Chimeric organisms
i.e. quail chick chimeras
Chimeric organisms revealed the diversity of tissues derived from
neural crest
Step 1: Transplant cells from quail embryo into chick
Step 2: See where they go!
- How do we identify quail cells in a chick?
1. Differences in nuclear DNA condensation
2. Quail-specific antibodies
- Immunohistochemistry
Neural crest arises from the ectoderm
Adjacent to developing neural tube
Delaminates and migrates away
What do the neural crest become?
Defects in neural crest migration cause cleft palates
The enteric nervous system and peripheral nervous system come from the neural crest.
Cell transplantation showed us a lot
Transplanting the neural crest from a pigmented chicken embryo into an albino results in black feathers
Induction
The ability of one group of cells to change the behavior of another
For the nervous system, it is the ability of existing embryonic tissues to reprogram surrounding pluripotent cells to become the nervous system
Induction gives us the neural plate
What induces the nervous system?
Hensen’s node and notochord
Essential for first steps of nervous system formation
In mammals it forms from a primitive structure called Koller’s sickle at the anterior edge of the primitive streak. It can induce and pattern an embryonic axis (Spemann-Mangold organizer in amphibians)
How do we know that Hensen’s
node is an organizing center?
Hensen’s node is at anterior edge of primitive streak.
Cells migrate into primitive streak during gastrulation
to form the germ layers.
Transplant node to a new animal
Induces second neural tube – primitive nervous system
All vertebrates have an organizer of some sort
- Zebrafish = Shield
- Frog = Spemann Mangold organizer
- Chicken and Human = Hensen’s node
Discovery of the Spemann-Mangold
organizer’s function
Transplanting cells from the blastopore lip (Hensen’s node in mammals) to another embryo resulted in the
formation of a second axis/nervous system in newts.
How does Hensen’s node/SMO induce the nervous system?
Node
Induces notochord and anterior nervous system
Secretes Chordin and Dickkopf
How does Hensen’s node/SMO induce the nervous system?
Notochord
Once formed, participates in process
Secretes Noggin
Notochord essential to pattern posterior spinal cord
How does Hensen’s node/SMO induce the nervous system?
Chordin and Noggin
Paired action of Chordin and Noggin suppress BMP signaling to promote anterior neural fate
BMP = bone morphogenetic protein
Dickkopf suppresses Wnt signaling
Knockout of either Chordin OR Noggin has no effect
Knockout both – you lose facial structures and forebrain
Morphogens and patterning the anterior-posterior nervous system
Anterior structures require what?
Low BMP
Low Wnt
Node antagonizes BMPs and Wnt to promote anterior neural fate
Morphogens and patterning the anterior-posterior nervous system
Posterior structures require what?
High Wnt
High BMP
High FGF
Node antagonizes BMPs and Wnt to promote anterior neural fate
Neurulation
The process by which the neural plate becomes the neural tube
