Embryology Lecture 2 - Early Embryonic Development 2 Flashcards Preview

ESA1 Callum's cards > Embryology Lecture 2 - Early Embryonic Development 2 > Flashcards

Flashcards in Embryology Lecture 2 - Early Embryonic Development 2 Deck (33):

What constituates the Bilaminar disk?

What is it derived from?

What will it later become?

The Epiblast and Hypoblast.

Derived from the embryoblast.

This will later become the trilaminar disk.


What is a germ layer?

A layer from which other developed tissues will arise


What is the ectoderm?

Where is it located?

The outermost layer of cells on the dorsal face of the trilaminar disk


What tissues are derived from the Ectoderm?


Skin derivatives (hair, nails, lens and cornea, mouth and anal lining)

Brain and spinal cord

Peripheral nerves

Retina and Iris


What is the Mesoderm?

Where is it located?

The middle layer of cells in the trilaminar disk


What tissues are derived from the Mesoderm?

Smooth, skeletal and cardiac muscle

Connective tissue



Vasculature and Heart


Urinary system


What is the Endoderm?

Where is it located?

Innermost layer of cell on the ventral face of the trilaminar disk


What tissues are derived from the Endoderm?

Lining of the GI tract

Glands associated with the GI tract (E.g. Liver, pancreas)

Other gut derivatives (E.g. Lungs)


What is the primitive streak?

Why is it important?

A narrow groove that develops on the dorsal surface of the epiblast.

Has an important role in the orientation of the embryo (determining the front and back)



What is the trilaminar disk?

The three germ layers, ectoderm, mesoderm and endoderm


Give a very brief description of the embryonic period of development (3-8 weeks)

Period of greatest change

All major structures and systems formed

Most perilous for the developing child


What 3 cavities are established around the embryo at the start of week 3 of development?

Amniotic cavity immediately dorsal

Yolk sac immediately ventral

Chorionic cavity surrounding


What is gastrulation?

What does it achieve?

The process establishing the three germ layers

Trilaminar disk is formed and axes observed in an adult are set


What three axes are set by the process of gastrulation?





How does gastrulation start?

With the appearance of the primitive streak, node and pit



After the appearance of the primitive streak, what are the two processes that constitute gastrulation?




When and where does the primitive streak form?

Describe the appearance of the primitive streak and associated structures

In the third week of development the primitive streak appears on the dorsal surface of the epiblast

The streak is a narrow groove with bulging edges

Primitive node located at the cranial end of the streak

Primitive pit located at the centre of the node


What happens to the primitive streak after it has formed?

It regresses (disappears)


In which direction does development of the trilaminar disk occur?

Cranial/rostral to caudal


Describe the migration and invagination processes

Cells on the epiblast migrate towards the primitive streak

They then invaginate into the epiblast and displace the hypoblast, this creates the 3 layers of the trilaminar disk

As more cell migrate through the streak they spreak laterally and cephalad (cranially/rostrally)

Mesoderm spreads out between the ectoderm and the endoderm like a sandwich filling leaving two gaps/holes

Cephalad gap corresponds to the mouth

Caudal gap corresponds to the anus


What is the significance of the site of any particular epiblast cell's invagination?

Fate of the invaginating epiblast depends on location of invagination


What is the significance of the notochord?

Notochord defines the phylum 'Chordata'

It's the basis for the axial skeleton/axial skeleton forms around it

It drives the formation of the nervous system (neurulation)

Has an important signalling role



Describe the formation and location of the notochord

Prenotochordal cells of the epiblast migrate through the cranial part of the primitive pit

Forms a solid rod of cells running in the midline of the mesoderm along the same axis as the primitive streak


After the notochord has performed its function, what happens to it?

It regresses (disappears)

Vestigial remnants in an adult is the nucleus pulposus if the intervertebral disks


What axis is defined by the primitive streak?

Front and back i.e. Rostral/caudal


How is correct left/right distinction in later development ensured?

The primitive node contains ciliated cells that only beat to the left

This results in the leftward flow of signalling molecules

This results in side specific signalling cascades that ensure correct left/right development


What is the result of immotile cilia on cells of the primite node?

Can result in situs inversus

Complete mirror image viscera

Can be complete mirror image or partial

No morbidity associated with complete mirror

Problems arise with only partial mirror



Why is gastrulation not complete in some parts of the embryonic disk at the end of week 3 of development?

Because development proceeds cephalocaudally, gastrulation isn't complete in cuadal region until week 4


What is the distinction between monozygotic and dizygotic twins?

Monozygotic = Single fertilised oocyte giving rise to two identical infants

Dizygotic = Two fertilised oocytes


How does monozygotic twinning occur?


Embryo splits after first cleavage

Two embryos, each with own placenta formed


Inner cell mass of the conceptus is duplicated 

Results in two embryos sharing a placenta


How do conjoined twins arise?

Splitting can occur very late

E.g. with the duplication of the primitive streak

Sometimes seperation of the embryos is not complete


Describe teratogensis

The process through which normal embryonic development is disrupted

From weeks 3 to 8 is the most sensitive period to teratogenic effect

Each organ system has a sensitive window depending on when most of its development occurs


Give some examples of teratogens




Certain therapeutic drugs