Ground School

Question 1

Somites form from?

What do somites differentiate to form?

Answer 1

Paraxial mesoderm

Sclerotome, Myotome, Dermatome

Question 2

What is the fate of the following developmental tissue types?

1. Sclerotome
2. Myotome
3. Dermatome

Answer 2

1. Sclerotome undergoes EMT and then becomes the vertebrae and ribs
2. Myotome becomes all skeletal muscles
3. Dermatome undergoes EMT and then becomes the CT and the dermis of the back

Question 3

Myotomes split into what two groups? What are the fates of these subgroups?

Answer 3

1. Epaxial myotome–located dorsally, form the intrinsic muscles of the back and are innervated by dorsal rami
2. Hypaxial myotome= located ventrally, form the anterior wall muscles and limb muscles and are innervated by ventral rami

Question 4

Describe the process of re-segmentation of the somites to form vertebra.

Answer 4

Somites (ball of epithelial cells–>
Differentiate to form sclerotome (plus myotome and Dermatome)–>
Sclerotome cells undergo EMT–>
Transverse fissure divides sclerotome into R/C–>
Sclerotomes migrate to notochord–>
C of one sclerotome associates with R of the sclerotome below–>
Vertebra is made up of 4 segments of different sclerotomes (C/R of left and right) and the caudal part of sclerotomes becomes rostral part of vertebra (and vice versa)

Question 5

What is the fate of the transverse fissure formed during re-segmentation?

Answer 5

It becomes the intervertebral joint space and the mesenchymal cells form part of the intervertebral discs

Question 6

Describe the process of spinal nerve development

Answer 6

Neural crest cells migrate to rostral sclerotome mass–> there they form the dorsal root ganglion and send out processes to the periphery.
Later, motor axons emerge from floor plate of neural tube and they follow the pathways set out by sensory neurons.
The nerves end up in the transverse tissue of the sclerotome and thus eventually in the intervertebral joint space.

Question 7

What precursor forms the muscles of the limb and muscles of the anterior body wall?

Answer 7

Paraxial mesoderm–> somites–>Hypaxial myotome

Question 8

What precursors form the intrinsic muscles of the back?

Answer 8

Paraxial mesoderm–> somites–> epaxial myotome

Question 9

What precursors form the sensory neurons?

Answer 9

Neural crest cells

Question 10

What precursor forms the motor neurons?

Answer 10

Neural tube

Question 11

Describe the fate map of the mesoderm.

Answer 11

Notochord= first mesodermal cells, organizes the embryo axis
Paraxial mesoderm= innermost, forms the somites which go on to form the vertebrae, ribs, skeletal muscles, and back dermis
Intermediate mesoderm= middle part, goes on to form parts of the urinary and reproductive systems
Lateral Plate Mesoderm= outermost, splits into 2:
1. Somatic mesoderm= forms body wall and limbs; forms bones, ligaments, connective tissue, blood vessels
2. Splanchnic mesoderm= forms muscles of the gut, heart, CT of the lung and digestive system

Question 12

Describe the fate map of the ectoderm.

Answer 12

It forms the neural tube, precursor of CNS. Also forms neural crest cells which migrate to mesoderm and undergo EMT (sometimes thought of as 4th germ layer)–> form the PNS, as well as bones and CT of the head
Lateral ectoderm forms epidermis of the skin.

Question 13

What are the two places on a developing embryo where the ectoderm meets the endoderm with no mesoderm in between?

Answer 13

Oropharyngeal membrane (primitive mouth) and Cloacal membrane (primitive anus)

Question 14

What are primordial germ cells?

Answer 14

The are cells within the embryo that go on to form oocytes or sperm; they retain their totipotency

Question 15

Give an overview of the process of gastrulation.

Answer 15

This is the process by which epiblast cells differentiate into the three primary germ layers.
The primitive node and primitive streak form of dorsal side of epiblast epithelium–> epiblast cells move through the node/streak and are displaced into one of the three germ layers (some cells must undergo EMT or EMT/MET)–> destination of where the cells end up determines their fate. Gastrulation occurs cranially to caudally.

Question 16

Describe cranial-caudal folding.

Answer 16

Cranial neural tube grows rapidly–>this forces the cranial part of the embryo ventrally, forming head fold.
The heart, pericardial cavity and septum transversum are displaced ventrally.
As the neural tube grows causally, the tail-fold is formed and body stalk is swept ventrally (it will end up at the umbilicus)

Question 17

What is the septum transversum?

Answer 17

It is a condensation of mesenchyme that will form the diaphragm and induce liver formation

Question 18

What is the allantois?

Answer 18

It is a projection from the hind gut that extends into the body stalk; it eventually becomes the bladder

Question 19

Describe body cavity formation.

Answer 19

Initially, pericardial cavity forms rostral to Oropharyngeal membrane–>the intraembryonic coelom becomes the peritoneal cavity; at first 2 cavities form on either side of dorsal midline and then these 2 cavities merge with the pericardial cavity to form one large cavity (eventually the pericardial cavity and peritoneal cavity split again)

Question 20

What is the AER and what is it’s role?

Answer 20

Apical endodermal ridge= specialized ectodermal epithelial structure

Question 21

What time during pregnancy would a drug have the most detrimental effects?

Answer 21

Embryonic period of development, weeks 3 through 8, because this is the time during which organ systems are developing.

Question 22

If a drug is taken during pregnancy and shown to result in shorter limb length, how is this drug affecting embryonic development?

Answer 22

Limb length is established by proximal-distal growth of the limb. Proximal-distal growth is under the control of the apical ectodermal ridge (AER), an epithelial structure that is located at the distal edge of the limb. The AER sends signals to the underlying mesenchyme, causing it to grow out away from the body. This mesenchyme is derived from the lateral plate mesoderm. The proliferating mesenchyme near the AER is called the progress zone. As the tip of the limb grows out, more proximal cells leave the progress zone and differentiate into the bone and connective tissue of the limb. Decreased proliferation in the progress zone, possibly due to faulty signaling of the AER or a faulty response of the mesenchyme could result in a shorter limb.