15 - Zygote to Embryo 3

Question 1

Collectively, the embryonic discs and chorionic sacs are referred to as _____?

Answer 1

The conceptus, which is the membranes plus the embryo.

Question 2

How are the cranial-caudal and dorsal-ventral axes of symmetry established in an embryo? What about bilateral symmetry?

Answer 2

Through expression of genes involving the primitive node.

L and R sidedness is established by presence of midline structures such as the notochord.

Question 3

What does induction require?

Answer 3

An inducing tissue, a source of inducing signals, transfer of signal (usually via diffusion) and a competent response.

Question 4

What is organogenesis? What is morphogenesis?

Answer 4

Organogenesis: formation of organs and organ systems

Morphogenesis: development of form

Question 5

Describe differential growth that occurs during embryonic development?

Answer 5

Some regions or areas grow fast while others may not grow at all or grow slower by comparison.

Question 6

How does the neuroectoderm/neural plate form? What is this the precursor of?

Answer 6

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.

Question 7

How does the neural plate change during embryonic development? What is this due to?

Answer 7

It elongates, expands cranially, and narrows caudally.

Due to changes in cell shape, cell number, and cell cycle length.

Question 8

What is primary neurulation? When does it occur in development?

Answer 8

Process of shaping the neural plate and transforming it into a neural tube.

Between day 18 and 28

Question 9

What is the bending/folding of the neural plate a result of? What occurs and what forms?

Answer 9

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.

Question 10

When does the neural tube close during development?

Answer 10

Day 22.

Question 11

What does the cranial part of the neural tube eventually become? How does this occur?

Answer 11

The brain!

Ventral flexure appears and 3 regions are defined by it:

• prosencephalon (in front of flexion)
• mesencephalon (middle near flexion)
• rhombencephalon (behind flexion)

Question 12

What are the first appearances of the ears and eyes in an embryo?

Answer 12

Otic dic: ectoderm thickening adjacent to rhombencephalon; promordium of the inner ear.

Optic sulcus: groove associated with the prosencephalon; primordium of the eyeball.

Question 13

What is the neural crest and how does it change during development? What does it become?

Answer 13

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.

Question 14

The intraembryonic mesoderm is organized into what three regions? What is the notochord considered?

Answer 14

Paraxial mesoderm

Intermediate mesoderm

Lateral Mesoderm

Notochord is considered the axial mesoderm.

Question 15

What happens to the paraxial mesoderm in the head and trunk?

Answer 15

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.

Question 16

What are the steps in somite formation from the paraxial mesoderm?

Answer 16

1. Fissure formation: mesenchyme aggregate and separate into somite pairs.
2. Periodicity: mediated by segmentation clock and wave model
3. Epithelialization
4. Specification: regional patterning along cranial-caudal axis
5. Differentiation: somite cells into precursor compartments to form tissues.

Question 17

How does the intermediate mesoderm develop (day 19-20)? What does it become?

Answer 17

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.

Question 18

How does the lateral mesoderm develop? What does it become (day 20)?

Answer 18

Split into 2 layers by formation of small spaces.

The spaces later coalesce into the intraembronic coelem (body cavity).

Question 19

Once the coelem is formed through the splitting of the lateral mesoderm, what does it look like?

Answer 19

It looks like an inverted U and the caudal ends of each leg maintain connection with extraembryonic coelem (chorionic cavity).

Question 20

What happens to the dorsal and ventral layers of the split lateral mesoderm? What are the new names given to each?

Answer 20

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)

Question 21

What are precursors to the cardiovascular system? Where does early formation of the CV system begin?

Answer 21

Hemeangioblasts.

Begins in the wall of the umbilical vesicle (yolk sac).

Question 22

What layer/tissue eventually forms the heart? Where in the embryo does this occur?

Answer 22

The splanchnic mesoderm, in the anteriolateral area of the embryo.

Question 23

What five things occurred during week 3 in order to establish the primary germ layers.

Answer 23

1. Formation of primitive streak
2. Epithelial to mesenchyme and cell migration
3. Endoderm replaces hypoblast
4. Mesoderm inserted between endoderm and ectoderm
5. Ectoderm cells remain in the original epiblast.

Question 24

What five things occurred during week 3 to begin organogenesis?

Answer 24

1. Induction of neural plate and form neural tube
2. Mesoderm derived mesenchyme organizes into three areas
3. Paraxial mesoderm divided into somites
4. CV precursors become located in the splanchnic mesoderm
5. Embryo begins to fold

Question 25

During gastrulation, ______ and _____ precursors are removed from the epiblast.

Answer 25

Mesoderm and endoderm

Question 26

During gastrulation, three primary germ layers are form. What are these layers?

Answer 26

Endoderm: will be inside

Mesoderm

Ectoderm: will be outside

Question 27

After gastrulation, the embryo is ______.

Answer 27

Trilaminar.

Question 28

In primary neurulation, the induced ______ causes the ______ to raise, and eventually will close to become the ______?

Answer 28

Neural plate

Neural folds

Neural tube

Question 29

What is the difference between vasculogenesis and angiogenesis?

Answer 29

Vasculogenesis: vessel formation directly from angioblasts

Angiogenesis: vessel formation by sprouting frol existing vessels

Question 30

How does the embryo fold in the sagittal plane?

Answer 30

Neural tube elongation is the major growth force that drives folding.

Flat embryo becomes C-shaped and has head fold and tail fold.

Question 31

How does the embryo fold in the transverse plane? What is formed?

Answer 31

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.

Question 32

During folding in the transverse plan, what does the midgut retain communication with? What is the gut tube suspended from the body wall by?

Answer 32

The umbilical vesicle via the omphaloenteric (vitelline) duct.

Gut tube suspended by dorsal mesentery.

Question 33

Describe the four body folds at the end of week three? How does the ventral body wall close?

Answer 33

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).

Question 34

What does folding of the embryonic disc result in? What important structure is formed and what is it made of?

Answer 34

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.

Question 35

What part of the gut communicates with the yolk sac via the omphaloenteric duct?

Answer 35

The midgut

Question 36

What four things result from embryo folding?

Answer 36

1. Establishment of body form
2. Structures relocate from dorsal wall to ventral wall
3. Closure of ventral body wall
4. Embryo elevated into amnionic cavity

Question 37

What are characteristics of the embryo at the beginning of the fourth week?

Answer 37

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