Developmental biology

Question 1

Why do you use animal models?

Answer 1

To study:
How cells grow (proliferate) and differentiate
How cells create whole bodies (morphogensis)

Question 2

What invertebrate models are there?

Answer 2

Drosophila
Benefits - have short life span, easy to handle and to mutate. Genome is already sequenced

Question 3

What vertebrate models are there?

Answer 3

Anamniotes - e.g. frogs (xenopus laevia) and fish (zebra fish)
- young developed externally from mother, with large, transparent eggs that are easily mutated. Genome is sequenced.

Amniotes e.g. mice and chickens
-close to humans, but internally developed so difficult to be observed. Easily manipulated and can create “knock outs” to see what genes do. Often sequenced.

Question 4

What happens in the first events of life?

Answer 4

1. Diploid primordial germ cells from the ovary/testes undergo meiosis to form haploid germ cells (1n). These fuse to form diploid zygote.
2. Mitotic cell division starts and cells are now called blastomeres. These sit in the zona pellucida. At 16cells they become a morula.
3. The morula rearranges to inner core and a superficial layer.
4. Superficial layer pumps fluid in forming blastocyst cavity, turning embryo into blastula. Innercore becomes mass of embryonic strem cells and outer is trophoblasts.
5. The blastocyst grows and hatches out of zona pellucida. Implants into uterine wall by the trophoblasts invading the endometrium using syncytiotrophoblasts.
6. Now implanted the blastocysts re-organises
   -Epiblast - columnar cells adjacent to amniotic cavity
   - Hypoblast - cuboidal cells facing the yolk sac/blastocyst cavity.

Question 5

What is the naming cycle of the cells?

Answer 5

Primordial germ cells
Zygote
Morula
Blastocyst (in zona pellucida)
Blastocyst (out of zona pellucida in wall)

Question 6

What is the layers of the implanted

Answer 6

Yolk sac > hypoblast > epiblast > amniotic cavity > extraembryonic mesoderm > cytotrophoblast > syncytiotrophoblast > endometrium

Question 7

What is gastrulation?

Answer 7

Where a single layer (epiblast) in the blastocyst forms 3 distinct layers:

-Ectoderm - outer layer. Forms all neural tissue, epidermis and neural crest
-Mesoderm - middle layer. Forms muscle, bones and organs.
-Endoderm - inner layer. Lining of the gut and derivatives.

Question 8

What is the mechanism of primary gastrulation?

Answer 8

Cells in the middle of the epiblast converge in the middle between the epiblast and the hypoblast and form the primitive streak. Cells ingress from the streak and move along.

The first cells to ingress (posterior) contact with hypoblast and become the endoderm (single layer). These undergo mesenchymal to epithelium.

Next cells (anterior) sit on top of endoderm and become mesoderm. These remain mesenchymal.

The streak moves long the embyo and reaches 75% up. It forms the a knot of dividing stem cells (Hensen node in chickens).

The node regresses back along the streak leaving a trail of stem cells. These stem cells form the notochord, which eventually become somites.

Question 9

What is secondary gastrulation?

Answer 9

Primary is head/body and then secondary forms the hindlimbs. The tail bud is the second knot of stem cells and forms only the mesoderm, never the endoderm.

Question 10

What are the layers after secondary gastrulation?

Answer 10

Ectoderm

Lateral plate mesoderm - notochord - LPM

Endoderm

Question 11

What are the steps of primary neurulation?

Answer 11

1. Ectoderm receives signal (BMP4) to develop dermis. Neural plate thickens at the edges by columnarisation (Fgf)
2. Cells in the midline form median hinge point by attaching to notocord forming the neural groove
3. The epidermis either side pushed the neural groove and elevates
4. A neural groove bend is formed at the top as the plate bends due to the dorsolateral hinge point
5. The dorsolateral hinge point fuses together and the ectoderm grows over the top of it.

Question 12

What happens if neurulation fails?

Answer 12

Closure is initiated in 5 places. Failure to close leads to spina bifida and if its the cranial neural tube is encephaly.

Question 13

What are the steps of secondary neurulation?

Answer 13

Again occurs from sacral/caudal region. This time involves mesoderm not ectoderm.

1. Mesenchymal cells form medullar cord
2. Numerous cavities develop forming one lumen/cavitation
3. Primary and secondary tubes fuse

Question 14

What are neural crest cells?

Answer 14

Where the neural plate meets the ectoderm as it grows over, it forms the neurectoderm enducing an epithelial to mesenchymal transition through Fox D3 and Slug signalling.

These mesenchymal cells breakaway and migrate.

Question 15

What is somatogenesis?

Answer 15

The formation of axial musculoskeletal system (vertebrae, ribs, muscles and dermis)

Somites are formed either side of the neural tube as the streak regresses and tube joins. 1 somite = 1 vertebrae.

Paraxial mesoderm form either side of the neural tube, creating mesenchymal tissues (pre-somatic mesoderm) from anterior to posterior end

The pre-somatic mesoderm becomes somites via mesenchymal to epithelial transformation anteriorly.

This can be timed, for example in chick is every 90 mins and controlled by Hairy gene.

Question 16

How do the somites develop?

Answer 16

The split further into sclerotome and dermomyotome.

1. Poterior pre-somatic mesoderm turns into somite epithelial ball
2. Next anterior, the ventromedial portion of the somite turns epithelial back to mesenchymal forming sclerotome. The dorsal portion remains epithelial forming the dermomyotome like a sheet. Shh control.
3. More anterior - dermomyotome forms myotome on top. Remaining cells form dermotome.
4. Most anterior - a fissure develops between middle of sclerotome. Poterior half fuses with anterior half of adjacent sclerotome. Myotome does not resegment.

Question 17

How to limbs/fins develop?

Answer 17

The lateral plate mesoderm sits lateral to the somites.

Forelimbs - at cervical-thoracic boundary
Hindlimbs - at lumbar-sacral boundary

Question 18

What is the mechanism of limbs developing?

Answer 18

1. LPM grows outwards (FGF 10) forming tissue bud of ectoderm and mesenchyme
2. At apex of bud there is an apical ectodermal ridge. This controls 3 things;
   a.) the rate of cell proliferation in underlying mesencyme (proximodistal growth). Distal area signals to adjacent cells to tell them to be humerus, radius/ulna or digits. Controlled by Hox genes (9 for humerus and 9-13 for digits)

b.) Maintains genes controlling patterning of anteroir posterior axis (thumb to little finger). The zone of polarising activity expresses Shh, diffuses through the bud. Highest conc at posterior end (little finger) and leads anterior (thumb)

c.) Maintains genes controlling dorsal-ventral axis (back of hand to palm). Wnt7a expressed on dorsal side, induced Lmx1 expression on mesenchyme. Also controlled by Fgfs in apical ectodermal ridge.

It is then sculpted by controlled apoptosis between digits via BMPs.

Question 19

Fish evolutionary differences?

Answer 19

Fins are in different positions and are unpaired. Consists of structures like radius and ulna but no digits. Have same proximal Hox gene but not distal