Development Flashcards
Maternal Inheritance
Nearly all the cytoplasm and mitochondria of the zygote is from the egg
Establishment of Axes in Frogs
- Polarity of egg determines the anterior-posterior axis before fertilization
- At fertilization, pigmented cortex slides over underlying cytoplasm toward point of sperm entry (animal hemisphere). This rotation exposes a region of lighter-colored cytoplasm, aka the gray crescent, which is a marker of the future dorsal side
- The first cleavage division bisects the gray crescent. Once the anterior-posterior and dorsal-ventral axes are defined, so is the left-right axis.
Cytoplasmic determinants and establishment of axes/gradients
Beta-catenin (TF) and GSK-3 (protein kinase) are found throughout cytoplasm but GSK-3 inhibitor is localized at vegetal pole in cortex
What activates GSK-3 in frog eggs?
fertilization
What happens after GSK-3 is activated in frog eggs?
Beta-catenin is phosphorylated by GSK-3 and targeted for degradation
What stops beta-catenin from being degraded?
GSK-3 inhibitor moves to the gray crescent and prevents beta-catenin degradation > higher conc. of beta-catenin on dorsal side than ventral side
What initiates gastrulation?
Beta-catenin turns on genes in the dorsal side of the embryo
Experimental manipulations of beta-catenin
-Depleted: NO gastrulation
-Overexpressed: a second axis of embryo formation induced
*Beta-catenin acts as the primary embryonic organizer (sets up all three axes)
What largely influences the patterns of cleavage?
the amount and distribution of yolk in the embryo
Cleavage in human cells
- 2 cell stage w/ polar body and zona pellucida encircling cells
- 4-cell stage w/ zona pellucida (no polar body)
- 8-cell stage
- Compaction
- Morula + zona pellucida begins to degenerate
- early blastocyst and cells move to one side
- late blastocyst (trophoblast) w/ inner cell mass
What happens if blastomeres are separated in an early stage?
2 embryos result = identical twins (genetically identical/monozygotic twins)
What happens if the blastomeres are not separated but the inner cell mass separates in the later trophoblast?
Conjoined twins
What are fraternal twins?
The result of two separate oocytes fertilized by two separate sperm and are not genetically identical
What does cleavage result in?
Repackaging of egg cytoplasm into cells of blastula; cells get diff amts of nutrients and cytoplasmic determinants
What is gastrulation?
The process by which a blastula is transformed into an embryo w/ three tissue layers: ectoderm, mesoderm, and endoderm
What does the ectoderm become?
brain and nervous system, epidermis of skin
What does the mesoderm become?
skeletal system, gonads, muscle, circulatory system, kidneys
What does the endoderm become?
inner linings of gut, respiratory tract, liver, pancreas, thyroid, urinary bladder, germ cells of sperm and eggs
When does amphibian gastrulation begin?
when cells in gray crescent change shape and bulge inward > dorsal lip of blastopore forms > successive sheets of cells move over dorsal lip into blastocoel > blastopore becomes anus
What do the first endodermal cells do?
Form the lining of the archenteron (primitive gut)
What did Hans Spemann’s experiment reveal?
that cytoplasmic determinants in the gray crescent are necessary for gastrulation
Bird and reptile embryos have modified gastrulation
-to adapt to huge yolk sizes
-cleavage forms a blastodisc on top of yolk
When does gastrulation begin for bird and reptile embryos?
when cells move toward the midline of the blastodisc, forming a ridge called the primitive streak
What is a bird/reptilian blastodisc composed of?
an epiblast (forms embryo) and a hypoblast (forms extra-embryonic membranes)
In birds and reptiles, what forms the endoderm and mesoderm?
Cells migrating through Hensen’s node (forward region of streak) and the rest of the primitive streak
Neuralation process
- Ectoderm thickens and rises above notochord to form neural plate
- Microtubules help elongate the cells of the neural plate
- Microfilaments at dorsal end of cells may then contract, deforming the cells into wedge shapes
- Ridges and a groove forms (cell wedging in the opposite direction causes ectoderm to form a “hinge”)
5.
Neuralation process
- Ectoderm thickens and rises above notochord to form neural plate
- Microtubules help elongate the cells of the neural plate
- Microfilaments at dorsal end of cells may then contract, deforming the cells into wedge shapes
- Ridges and a groove forms (cell wedging in the opposite direction causes ectoderm to form a “hinge”)
- Ridges fuse and neural plate is pinched off, forming a neural tube
In frogs, what does the anterior end of the neural tube become?
the brain
In frogs, what does the remainder of the neural tube become?
spinal cord
What develops during neuralation?
Body segmentation
What forms on both sides of the neural tube?
Somites: blocks of mesoderm
What do somites on the neural tube do?
produce cells that form the vertebrate, ribs, and muscles of the trunk and limbs
What is a key difference of chicken and mammal embryo development?
Bird embryo grows on top of huge yolk mass; mammal embryo occurs on the floor of the “cavern” within inner cell mass of trophoblast
What is an example of induction?
-Development of vertebrate eyes: forebrain forms bulges at both sides
-Signals from optic vesicles induce surface tissue to form the lens placodes which then become lenses
-Developing lens influences the size of the optic cup and induces the surface tissue to develop into cornea and optic cup to form a retina
What is an example of induction and developmental fate mapping?
Nematode worm C. elegans
-epidermal cells have 2 fates: epidermis or vulva
-an “anchor cell” secretes LIN-3 that induces epidermal cells to differentiate into vulva cells, which sets up a developmental gradient
-If anchor cell is removed, no vulva is developed and eggs hatch within the mother and they devour her
What is developmental induction controlled by?
Molecular switches that allow a cell to proceed down 1/2+ alternative developmental channels
More chicken development post-gastrulation and neurulation
- Yolk sac forms (extension of endodermal tissue of hypoblast)
- Yolk is digested by endodermal cells of sac
- Nutrients are transported through blood vessels lining outer surface of yolk sac
- Amnion is formed (ectoderm and mesoderm combine and extend beyond embryo)
- Amnion secretes fluid into amniotic cavity = protection for embryo
More human development
- In preparation for implantation, mammalian blastocyst hatches out of zona
- blastocyst approaches endometrium and begins process of implantation by digesting its way into endometrium and uterine wall
- trophoblast of blastocyst forms villi that interlock w uterine wall (promotes gas and nutrient exchange)
- Inner cell mass separates to form amniotic cavity
Where do gastrulation and neurulation take place?
In the cells beneath the amniotic cavity
Extraembryonic membranes (humans)
- trophoblast cells of blastocyst attach to uterine wall
- trophoblast cells send out villi to increase surface area and contact w/ maternal blood (to get nutrients)
- hypoblast (cells at bottom of embryo) extends to form chorion
- Chorion and other tissues produce placenta
What is the placenta responsible for?
-gas and nutrient exchange of mammalian embryo
-also takes over for corpus luteum and produces estrogen and progesterone
Amniocentesis
a test used to detect defects by sampling cells from the embryo that are in the amniotic fluid
Defects include: Down syndrome, cystic fibrosis, Tay Sachs
