15 - Zygote to Embryo 3 Flashcards
Collectively, the embryonic discs and chorionic sacs are referred to as _____?
The conceptus, which is the membranes plus the embryo.
How are the cranial-caudal and dorsal-ventral axes of symmetry established in an embryo? What about bilateral symmetry?
Through expression of genes involving the primitive node.
L and R sidedness is established by presence of midline structures such as the notochord.
What does induction require?
An inducing tissue, a source of inducing signals, transfer of signal (usually via diffusion) and a competent response.
What is organogenesis? What is morphogenesis?
Organogenesis: formation of organs and organ systems
Morphogenesis: development of form
Describe differential growth that occurs during embryonic development?
Some regions or areas grow fast while others may not grow at all or grow slower by comparison.
How does the neuroectoderm/neural plate form? What is this the precursor of?
Signaling molecules from the notochord and prechordal mesoderm cause changes in overlying ectoderm.
Ectoderm of sides of midline thicken and the cells get taller.
Precursor of CNS.
How does the neural plate change during embryonic development? What is this due to?
It elongates, expands cranially, and narrows caudally.
Due to changes in cell shape, cell number, and cell cycle length.
What is primary neurulation? When does it occur in development?
Process of shaping the neural plate and transforming it into a neural tube.
Between day 18 and 28
What is the bending/folding of the neural plate a result of? What occurs and what forms?
Cell shape change and cell cycle length.
Basal midline or floor plate acts as a median hinge point.
Mesoderm proliferation helps raise folds.
Forms neural tube.
When does the neural tube close during development?
Day 22.
What does the cranial part of the neural tube eventually become? How does this occur?
The brain!
Ventral flexure appears and 3 regions are defined by it:
- prosencephalon (in front of flexion)
- mesencephalon (middle near flexion)
- rhombencephalon (behind flexion)
What are the first appearances of the ears and eyes in an embryo?
Otic dic: ectoderm thickening adjacent to rhombencephalon; promordium of the inner ear.
Optic sulcus: groove associated with the prosencephalon; primordium of the eyeball.
What is the neural crest and how does it change during development? What does it become?
Derived from neural tube, undergoes an epithelial to mesenchyme transformation.
Serves as a precursor for a variety of neural and non-neural tissues such as: -primary sensory neurons
- PNS sheathing cells
- Neurons of autonomic ganglia
- pigment cells.
The intraembryonic mesoderm is organized into what three regions? What is the notochord considered?
Paraxial mesoderm
Intermediate mesoderm
Lateral Mesoderm
Notochord is considered the axial mesoderm.
What happens to the paraxial mesoderm in the head and trunk?
Head: remains unsegmented and is called the head mesoderm
Trunk: becomes segmented and its subdivisions are called somites.
-somites are transient, and formation and reorganization proceeds from cranial to caudal levels.
What are the steps in somite formation from the paraxial mesoderm?
- Fissure formation: mesenchyme aggregate and separate into somite pairs.
- Periodicity: mediated by segmentation clock and wave model
- Epithelialization
- Specification: regional patterning along cranial-caudal axis
- Differentiation: somite cells into precursor compartments to form tissues.
How does the intermediate mesoderm develop (day 19-20)? What does it become?
It forms a solid cord of mesenchyme cells along the back wall in the trunk region called the nephrogenic ridge.
Eventually the ridge tissue begins to differentiate into gonads and kidneys, then it is called urogenital ridge.
How does the lateral mesoderm develop? What does it become (day 20)?
Split into 2 layers by formation of small spaces.
The spaces later coalesce into the intraembronic coelem (body cavity).
Once the coelem is formed through the splitting of the lateral mesoderm, what does it look like?
It looks like an inverted U and the caudal ends of each leg maintain connection with extraembryonic coelem (chorionic cavity).
What happens to the dorsal and ventral layers of the split lateral mesoderm? What are the new names given to each?
Dorsal layer: associates with the overlying surface ectoderm to become the somatic mesoderm (together called somatopleura)
Ventral layer: associates with underlying endoderm to become the splanchnic mesoderm (together called splanchnopleura)
What are precursors to the cardiovascular system? Where does early formation of the CV system begin?
Hemeangioblasts.
Begins in the wall of the umbilical vesicle (yolk sac).
What layer/tissue eventually forms the heart? Where in the embryo does this occur?
The splanchnic mesoderm, in the anteriolateral area of the embryo.
What five things occurred during week 3 in order to establish the primary germ layers.
- Formation of primitive streak
- Epithelial to mesenchyme and cell migration
- Endoderm replaces hypoblast
- Mesoderm inserted between endoderm and ectoderm
- Ectoderm cells remain in the original epiblast.
What five things occurred during week 3 to begin organogenesis?
- Induction of neural plate and form neural tube
- Mesoderm derived mesenchyme organizes into three areas
- Paraxial mesoderm divided into somites
- CV precursors become located in the splanchnic mesoderm
- Embryo begins to fold
During gastrulation, ______ and _____ precursors are removed from the epiblast.
Mesoderm and endoderm
During gastrulation, three primary germ layers are form. What are these layers?
Endoderm: will be inside
Mesoderm
Ectoderm: will be outside
After gastrulation, the embryo is ______.
Trilaminar.
In primary neurulation, the induced ______ causes the ______ to raise, and eventually will close to become the ______?
Neural plate
Neural folds
Neural tube
What is the difference between vasculogenesis and angiogenesis?
Vasculogenesis: vessel formation directly from angioblasts
Angiogenesis: vessel formation by sprouting frol existing vessels
How does the embryo fold in the sagittal plane?
Neural tube elongation is the major growth force that drives folding.
Flat embryo becomes C-shaped and has head fold and tail fold.
How does the embryo fold in the transverse plane? What is formed?
Lateral folds bend downward.
Formation of an outer tube of surface ectoderm and inner tube of endoderm.
Inner tube becomes primitive gut tube that has three components: foregut, midgut, hindgut.
During folding in the transverse plan, what does the midgut retain communication with? What is the gut tube suspended from the body wall by?
The umbilical vesicle via the omphaloenteric (vitelline) duct.
Gut tube suspended by dorsal mesentery.
Describe the four body folds at the end of week three? How does the ventral body wall close?
The head, tail, and two lateral folds are continuous domains of a common circle that converge at the connecting stack forming an umbilical ring.
With embryo growth, the folds are drawn together around the ring (as one would close a sac with a drawstring).
What does folding of the embryonic disc result in? What important structure is formed and what is it made of?
A tubular embryo flexed in the sagittal plate.
Embryo is elevated into the amnionic cavity.
Ventrally the amnion and surface ectoderm converge at the umbilical ring and the stalk becomes the umbilical cord containing vessels and the allantois.
What part of the gut communicates with the yolk sac via the omphaloenteric duct?
The midgut
What four things result from embryo folding?
- Establishment of body form
- Structures relocate from dorsal wall to ventral wall
- Closure of ventral body wall
- Embryo elevated into amnionic cavity
What are characteristics of the embryo at the beginning of the fourth week?
Beating heart; Embryonic circulation established and connected to yolk sac and placenta
Neural tube mostly closed but open at each end
Folding began
Somite pairs continue to form
~2 mm in length
