Developmental biology 2 (Prof. Dale)

Question 1

How is the yolk distributed in sea urchin eggs ?

Answer 1

Evenly.

Question 2

What are micromeres in the sea urchin egg ?

Answer 2

Micromeres = small blastomeres produced at the vegetal pole.

Question 3

What is the structure of the blastula generated by the sea urchin egg ?

Answer 3

At the 62-cell stage, the blastula consists of a simple epithelium surrounding a fluid filled blastocoel.

Question 4

How does gastrulation take place in the sea urchin embryo ?

Answer 4

• vegetal pole of the sea urchin blastula thickens to form a the vegetal plate –> releases primary mesenchyme cells into the blastocoel
• vegetal plate buckles inwards to form the archenteron + a group of secondary mesenchyme cells form at the tip –> these cells produce pseudopodia that attach to the animal pole + pull the archenteron towards it
• archenteron fuses with the animal pole to form a continuous tube (the gut) with the blastopore forming the anus

Question 5

What causes the buckling of the vegetal plate in the sea urchin embryo ?

Answer 5

It is caused by localised contraction of actin-myosin microfilaments near the apical surface of the blastula epithelium –> apical contraction narrows the apical surface relative to the basal surface, bending the epithelium inwards

Question 6

What is invagination in the see urchin embryo ?

Answer 6

Invagination = inward movement of cells at the vegetal pole following apical contraction

Question 7

How is elongation of the archentron mediated in the sea urchin embryo ?
What is the combined effect on the archentron in this process ?

Answer 7

By convergent-extension, in which cells acquire lamellipodia (made of β-actin) perpendicular to the invaginating vegetal plate and intercalate with each other.
This narrows the archenteron (convergence) + at the same time elongates it (extension).

Question 8

What are the different steps of cleavage divisions in mammalian embryos ?

Answer 8

• mitosis = initiated ~ 24 hours after fertilisation + occurs every 12-24 hours
• at the morula stage (~16 cells) they cells undergo compaction (maximize their contacts)
• cells at the centre = inner cell mass (ICM)
• peripheral cells = the trophoblast
• blastocoel cavity that forms in the centre of the embryo (now called the blastocyst)

Question 9

What are the different fates in the mammalian blastocyst ?

Does this vary between animal kingdoms (phyla) ?

Answer 9

• trophoblast –> forms the chorion (placenta)
• the ICM –> divides into epiblast + hypoblast layers
• hypoblast –> differentiates as extraembryonic endoderm
• epiblast –> forms the amniotic membrane, extraembryonic mesoderm + all tissues (ectoderm, mesoderm and endoderm) of the fetus
  Each germ layer forms similar tissues in all animal phyla.

Question 10

To which tissue types does the ectoderm give rise to ?

Answer 10

Epidermis + nervous system (cartilage/bone in the head)

Question 11

To which tissue types does the mesoderm give rise to ?

Answer 11

Cartilage/bone, connective tissue muscle, kidney, blood, vasculature, heart + gonads

Question 12

To which tissue types does the endoderm give rise to ?

Answer 12

Digestive tract (epithelium), lungs, liver, gall bladder + pancreas

Question 13

How does implantation take place ?

What happens to the ICM ?

Answer 13

• mammalian blastocyst attaches to uterine wall + trophoblast proliferates to form syncytiotrophoblast = highly invasive tissue that penetrates the uterine wall
• formation of lacunae that will fill with maternal blood from uterine capillaries
• ICM divides into epiblast + hypoblast layers
• epiblast –> forms the amniotic membrane
• hypoblast –> forms Heuser’s membrane

Question 14

What is ingression and when does it happen ?

Answer 14

Ingression = process where epiblast cells migrate through the primitive streak and node as individual cells, happens during gastrulation

Question 15

What are the different stages of gastrulation ?

Answer 15

• ingression
• the 1st cells to ingress invade + displace the hypoblast –> forming embryonic endoderm
• subsequent cells migrate into the space between the epiblast / endoderm –> forming the mesoderm
• cells remaining in the epiblast layer form ectoderm

Question 16

What are the 5 types of mesoderm ?

How are these and the notochord formed ?

Answer 16

• cells migrating through the node = axial mesoderm + prechordal mesoderm + notochord
• mesoderm from primitive streak condenses either side of the notochord = paraxial mesoderm, intermediate mesoderm + lateral plate mesoderm

Question 17

How are somites formed and along which axis ?
What are they made of ?
How many are formed ?

Answer 17

• paraxial mesoderm –> forms pairs of somites, sequentially from rostral to caudal
• the number formed varies between species
• somites = formed of a simple epithelium surrounding a central cavity

Question 18

What do somites subsequently divide into ?

Answer 18

In response to signals from surrounding tissues, they subsequently divide into sclerotome, myotome and dermatome layers.

Question 19

How is the lateral-plate mesoderm further subdivided ?

Answer 19

• Sometopleuric (most lateral)

- Splanchnopleuric (more medial)

Question 20

To what organs/tissues do the mesodermal tissues give rise to ?

Answer 20

• Dermatome: dermal (deep) layer of skin
• Myotome: skeletal (striated) muscle
• Sclerotome: vertebrae, ribs, sacrum, coccyx
• Intermediate: kidneys, gonads, reproductive tracts
• Somatopleuric: limb bone, connective tissues, vasculature, blood
• Splanchnopleuric: cardiac mesoderm, smooth muscle,
  connective tissues, vasculature, blood

Question 21

What is the neural plate ?

From what does it form ?

Answer 21

• the neural plate = precursor of the NS

- the ectoderm anterior to node forms the neural plate

Question 22

What is the fate of neural plate cells compared to epidermal cells ?

Answer 22

• cells of the neural plate –> elongate to form a
  pseudostratified columnar epithelium
• cells of the epidermis –> remain cuboidal

Question 23

What is regression ?

Answer 23

Regression = a phenomenon where, as the embryo
expands through cell proliferation, the primitive streak
becomes restricted to the posterior margin of the epiblast

Question 24

When does the neural tube form ?

Answer 24

4th week of human development.

Question 25

How does the neural tube close ?

Which parts of the tube close last ?

Answer 25

• the edges initially meet at only a single site –> at the boundary of the hindbrain and spinal cord (but in reality there may also be a sites in the midbrain and at the anterior boundary)
• the rest of the tube closes like a “zipper” from the initial sites, the caudal neuropore closing last

Question 26

What drives neural tube closure ?

Answer 26

• cell shape changes drive neural tube closure
• prior to neurulation, neuroepithelial cells = cuboidal
• during neurulation –> cells elongate along apical-basal axis to become columnar

Question 27

What forces neuroectodermal cells to adopt a wedge shape ?

Answer 27

Apical constriction –> occurs when the apical actin-myosin cable contracts, forcing cells to adopt wedge shape. This generates the physical force that causes the neural plate to bend.

Question 28

What are hinge points ?

Answer 28

Hinge point = apical contraction sites causing neural plate to bend

Question 29

What are the most common neural tube defects (NTDs) ?
How frequent are they ?
How lethal are they ?

Answer 29

• Anencephaly = rostral neuropore fails to close
• Spina bifida when caudal neuropore fails to close.
• Anencephaly + spina bifida account for up to 95% of all NTD, with equal prevalence
• affects ~1400 UK/year, 90% are terminated

Question 30

Where are neural crest cells formed ?

Answer 30

• neural crest cells –> formed at the lateral edges of the neural plate –> become the roof of the neural tube
• they delaminate from the neural tube to become a mesenchymal population that migrates away form the neural tube to form many different cell types

Question 31

Which cells will mesodermal neural crest cells give rise to ?

Answer 31

• smooth muscle
• osteoblasts
• osteoclasts
• adipocytes
• chondrocytes

Question 32

Which cells will ectodermal neural crest cells give rise to ?

Answer 32

• melanocytes
• Schwann cells
• PNS neurons

Question 33

In which species is the neural crest found ?

Why is this important ?

Answer 33

In vertebreate embryos.

Considered crucial for vertebrate evolution.

Question 34

At what point do mammalian embryos display the basic vertebrate body plan ?
Which organs can we thus see ?

Answer 34

• at the end of neurulation
• we can make out the forebrain (fb), midbrain (mb), hindbrain (hb), trunk (t), a large heart (h), aorta, and umbilical cord

Question 35

What is the phylotypic stage ?

What happens after this stage ?

Answer 35

• Phylotypic stage = the point where vertebrate embryos are most similar to each other at the end of neurulation and share a common body plan
• After –> morphology increasingly diverges

Question 36

What evidence supports that, at the phylotypic stage, vertebrate embryos share the same “molecular anatomy”

Answer 36

Using in situ hybridization, we can see that the mRNA of a gene called krox-20 is localised to two stripes in the hindbrain of mouse, chick, frog, and fish embryos.

Question 37

By which point are most of the organs formed in human embryos ?
When do all these organs become functional ?

Answer 37

• most of the organs are formed during a 4 week period after the formation of the body plan = organogenesis
• most of the organs and tissues –> become fully functional during the fetal period (~30 weeks)
• even then only a very immature infant is born