Developmental Biology

Question 1

Describe the animal models used in developmental biology and their advantages and disadvantages.

Answer 1

Models – some human similarities and easy to breed & maintain in lab, combo of model species, understanding evolutionary changes

Drosophila – genome sequenced, easy gene mutation, repro fast, short lifespan, large quantities

Anamniotes – no amnion (lower verts), develop outside mum, easy mutation, sequenced genomes, e.g. zebrafish

Amniotes – have amnion, some develop externally to mum (reptiles/birds), closer to humans, mammalian transgenics (knockout) e.g. in mice

Question 2

Describe how cell division and cleavage occurs in the fertilised egg during the formation of the blastocyst.

Answer 2

• 2 haploid cells fuse to make diploid zygote
• Mitotic cell division occurs & fit inside zona pellucida
• Each cell touches zone pellucida
• Blastomeres = primitive undifferentiated dividing cells in cleavage stage
• Morula = 16 cell stage of blastomeres inside the zona pellucida
• Morula turns into a blastocyst

Question 3

Describe changes in the organisation of the blastula following implantation

Answer 3

Blastocyst cavity = Outside of the cell pumps sodium-rich fluid into embryo (from the uterine environment)

Inner cells of blastocyst cavity = Inner cell mass (embryonic stem cells).

Superficial layer cells = trophoblasts (form extra embryonic membrane components e.g. placenta)

Blastocyst hatching = break out of zona pellucida

Question 4

Define gastrulation as the mechanism that generates 3 definitive germ layers

Answer 4

A single layer gives rise to 3 germ layers
Ectoderm - outer
Mesoderm - middle
Endoderm – inner

Blastocyst embryo
Epiblast = 3 germ layers
Hypoblast = extra embryonic membran

Question 5

Describe the elongation of the primitive streak and formation of the node

Answer 5

Elongation of primitive streak
- Starts tail end of embryo
- Cells in the middle of the epiblast converge to the middle of the disk at the tail
end of the embryo
- Cells drop through primitive streak structure into blastocoele gap
- Primitive streak elongates from tail up to clavicle region

Formation of Hensen’s node

- Primitive streak stops elongating at clavicle (75% axis length from tail)
- Cells condense at anterior end (top) of streak to form the Node

Question 6

Explain how cells ingress along the primitive streak and the node and the tissues that these gastrulating cells form.

Answer 6

Cells from epiblast ingress individually through epiblast layer and primitive streak dip

Endoderm = first cells to ingress (posterior cells) drop down on top of hypoblast layer & form a single layer of cells (become epithelial again)

Mesoderm = rest of the cells that ingress afterward, sit between the epiblast and endoderm layers, forms a mass of cells (no layering)

Ectoderm = cells that stay in epiblast layer

Node = Epiblast cells that ingress through the Node (under epiblast layer) migrate anteriorly to form parts of the head

Regressing Node = only mesodermal cells, somites and notochord

Question 7

Describe the contribution of the tailbud to gastrulation.

Answer 7

Secondary Gastrulation
Tailbud = knot of stem cells in posterior of streak
Activation = when regressing node reaches tailbud at hindlimb level
Moves posteriorly = only forming mesoderm notochords and somites

Question 8

Describe how the neural plate forms a tube.

Answer 8

1. Shaping & folding
   
   • Cells in ectoderm change shape & thicken
   • Cells along midline form a hinge (MHP) by attaching to notochord
   • Creates neural groove (dip)
2. Elevation
   
   • Epidermis/ectodermal tissue push on MHP & deepen groove
3. Converge
   
   • Neural tube tissue formed via DLHP (dorsolateral) & ectoderm fold over top
     of groove
   • DLHP – ectoderm & neural tube stuck together
4. Closure
   
   • Tube edges intercalate & fused/pulled tightly together
   • Ectoderm over top for protection

Question 9

Describe how neural crest is born from the neural tube

Answer 9

• NC originate from crest neurectoderm (dorsal edge of neural tube)
  
  • Epidermis + crest neuroectoderm (touching via closure hinging) = Epithelial to
    Mesenchymal transition to Crest neuroectoderm

E –> M transition
- Signalling molecules (BMP4 & Wnt6) release from ectoderm & picked up by
receptors in neuroectoderm (to stop being epithelial)
- New genes expressed (transcription factors FoxD3 & Slug) which change the
cell adhesion properties (cells change from epi to meso)

Question 10

Describe the contribution of neural tube and neural crest to the adult CNS & PNS.

Answer 10

Neural crest derivatives
- PNS, pigment cells, facial bone/ cartilage/ connective tissue

Sympathetic/parasympathetic
- flow of info from CNS to PNS

PNS control
- gut motility, blood vessel diameter (blood pressure), bladder control, energy
release/storage (glucose) in liver

Question 11

Describe the formation of a somite.

Answer 11

Paraxial mesoderm
- blocks of mesodermal tissue both sides of neural tube during regressing node
of primary gastrulation

Pre-somatic mesoderm
- region of undifferentiated somites at posterior end, PSM form mesenchymal
tissue (somites) first anteriorly (oldest)

Segmentation of PSM
- molecular clock, hairy gene cyclically changes pattern expression in PSM =
anterior ends to bud off at correct time

Question 12

Discuss how the somite divides into dermomyotome and sclerotome.

Answer 12

Immature somite
- PSM M–>E transition, somite = epithelial ball w/ tiny meso centre

Somite maturation – first splitting

Sclerotome
- mesenchymal cells (undergone E–>M transition), ball of cells sits in ventral-
medial portion of somite

Dermomyotome
- remains epithelial cells, sits dorsal in somite on top of sclerotome

Question 13

Discuss the contribution of the somite to the adult body plan

Answer 13

1. Somite splits into Sclerotome & Dermomyotome
   
   • Sclerotome = axial skeleton, vertebrae & ribs
2. Dermomyotome spits into myotome & dermatome
   
   • Myotome = body & limb muscles
   • Dermotome = dermis

Question 14

Describe how the proximo-distal outgrowth of the limb is controlled by the AER.

Answer 14

LPM = lateral plate mesoderm
Limb bud – Fibroblast GF10 induces bud to grow outward from a LPM region

AER = apical ectodermal ridge
- controls 3 axis (dorsal ventral, antero-posterior & proximal-distal)
- FGF8 protein in ridge diffuses from ectoderm to other cells in mesenchymal
to grow outward

Proximo-distal outgrowth
- Amount of time mesoderm cells spend in progress zone (area perceiving
AER signals)
- 1 bone in proximal region and multiple bones in distal regions due to distal
spending longer under AER influence
- Hox genes – 1 switched on in Humerus, 3 in radius/ulna, 5 in wrist/hand

Question 15

Describe how the anterior-posterior pattern is controlled by the ZPA

Answer 15

Fgf (in AER progress zone) = ensures shh expression in ZPA

ZPA = zone of polarising activity

Expresses signalling protein sonic hedgehog (shh)

High conc. shh posterior end (centre), lower conc. shh anterior (further away)

Diffusion gradient = different digit types (little finger high conc.) (thumb low conc.)

Question 16

Understand the role of cell death in sculpting the limb.

Answer 16

Apoptosis of mesenchymal between:
- digits, radius/ulna & limb ends (skeletal pattern)

Bone Morphogenetic Proteins (BMPs)
- molecules responsible for apoptosis

Question 17

Describe how changes in gene expression can change the morphology of the limb during evolution.

Answer 17

Fish fins - paired pectoral & pelvic limbs

            - similar hox genes for humerus & radius/ulna
            - evolution produced new distal hox genes = tetrapods having digits

Duck foot – evolved BMPs not to apoptosis mesenchymal cells between digits

Snake limbs – evolution lost forelimbs before hindlimbs

                   - forelimb loss = change in hox gene expression along LMP axis
                   - hindlimb loss = limb bud does not form AER properly

Question 18

Describe the phases of the cell cycle.

Answer 18

Interphase = G1, G0, S & G2

G1 (gap phase) – time between mitosis & chromosome duplication, normal cell activity (cyclin D checkpoint)

G0 – leaves G1 & is transient or permanent

S stage – chromosomes replicate cell contents & form double stranded sister chromatids (cyclin A checkpoint)

G2 (gap phase) – organelles needed for cell division are synthesised (2 of everything) (cyclin A checkpoint)

M phase – mitosis = cells actively dividing in 6 stage process (cyclin B checkpoint)

C phase – cytokinesis = new cells sperate

Question 19

Describe the phases of mitosis

Answer 19

Prophase – chromosomes condense, mitotic spindles begin assembling outside nucleus

Prometaphase – nuclear membrane break down & 2 daughter chromatids joined by centromere (kinetochore)

Metaphase – spindle form via centrioles moving to opposite poles with microtubules & chromatids line up on metaphase plate

Anaphase – centromere (kinetochore) split, move to opposite poles via spindle microtubule contractions

Telophase – nuclear membrane forms around 2 sets of chromosomes, chromosomes unravel, spindle breaks down

Cytokinesis – division of cytoplasm via actin-myosin contractile ring = 2 independent genetically identical cells

Question 20

Understand that cell division must slow or stop to allow differentiation.

Answer 20

Terminal differentiation – division to stop for differentiation to occur (skeletal & nerve)

Slow division to differentiate – only form parent type cells

Specification – capable of autonomous differentiation in isolation but can be reversed. Not “fate committed”. (intermediate stage)

Determination – when the cell differentiates into a specific cell type, even when it is placed amongst cells of a different type. This is irreversible and “fate committed” (past the specification/ intermediate stage)