Developmental biology

Developmental biology + Alimentary 1.5 Development of the GIT

What makes an organisms a good developmental biology model?

• Reproduces quickly and frequently

• Easy to maintain in a lab

• Have some similarity to humans (in order to get funding)

What makes fruit flies good model invertebrates?

• Genome is sequenced

• Short lifespan - develop quickly

• Easy to handle and maintain

• Easily mutated

What makes fruit flies bad scientific models?

They are invertebrates so certain tissues cannot be studied.

What is an anamniote?

Fish and amphibians. They lay eggs in aquatic environments and their foetuses do not have an amnion.

What makes anamniotes good scientific models?

• Genome is sequenced

• They develop externally to the mother

• Large, transparent eggs - easy to study

• Easily mutated

What is a zygote?

A cell formed from the fusion of the male pronucleus and female pronucleus.

What is cleavage stage?

The zygote undergoes 4 rounds of mitotic division to form smaller cells called blastomeres which each touch the zona pellucida.

What is a morula?

An embryo consisting of 16 identical cells

What does the morula reorganise into?

An inner mass of cells and a superficial layer of trophoblast cells

What does the inner cell mass of the morula become?

The embryo

What do the trophoblast cells become?

The extra embryonic membranes

How does a blastocyst form?

Fluid from the external environment is pumped into the embryo to form the blastocyst cavity, within which the inner cell mass floats.

What is blastocyst hatching?

A process of elongation of the blastocyst due to uptake of fluid, which results in thinning of the zona pellucida. This occurs in ruminants.

What is blastocyst hatching?

The process by which the blastocyst breaks out of the zona pellucida.

What 2 layers does the blastocyst reorganise into?

Epiblast (closest to the endometrium)

Hypoblast (closest to the blastocyst cavity)

What is gastrulation?

The process by which the epiblast rearranges into 3 germ layers

What is primary gastrulation?

Formation of the tissues of the head and body to hindlimb level.

What is secondary gastrulation?

Formation of the tail tissues.

What are the 3 germ layers that the epiblast gives rise to?

Ectoderm - outer layer (skin)

Mesoderm - middle layer (muscle, bones, organs)

Endoderm - inner layer (gut)

How does the primitive streak form?

Cells converge at the tail end of the embryo and form an indentation which migrates up the embryo towards the head end

How does the endoderm form?

Cells move from the edge of the embryo towards the midline, where they ingress through the primate streak and form a layer of endoderm along the hypoblast.

How does the mesoderm form?

A second lot of cells move from the edge of the embryo towards the midline, where they ingress though the primitive streak and form a layer of mesoderm on top of the endoderm.

What is the purpose of the node at the end of the primitive streak?

To prevent the streak from progressing any further anteriorly towards the head.

How does the head endoderm form?

Cells ingress through the node and migrate anteriorly to form the head endoderm.

How does the head mesoderm form?

The second lot of cells that ingress through the node and migrate anteriorly form the head mesoderm.

How do the notochord and somites form?

As the primitive streak regresses towards the tail end, cells continue to ingress through the primitive streak, into the mesoderm - where they will form notochord and somites.

What is the tailbud?

A knot of stem cells posterior to the streak.

How does the tailbud form the posterior notochord and somites?

As it moves posteriorly, it leaves behind mesoderm cells which will become notochord and somites.

What is primary neuralation?

The process by which the head and most of the spinal cord form.

What is secondary neuralation?

The process by which the tail portion of the spinal cord is formed.

What is the neural plate?

The section of ectoderm directly above the notochord.

How does primary neuralation occur?

1) Cells at the edge of the neural plate thicken

2) Neural plate cells stick to the notochord forming the median hinge point (MHP)

3) Neural plate ectoderm forms a dip called the neural groove, with the MHP at the bottom

4) The top edges of the neural groove meet to form a tube - the neural tube.

What are the dorsolateral hinge points (DLHP)?

Joints at the either corner of the top of the neural tube, where neural plate sticks together.

How does secondary neuralation occur?

The mesenchymal cells that gastrulated through the tailbud condense to form a solid rod called the medullary cord, which later develops cavities which fuse into one large lumen - the neural tube.

How does neural crest form?

Ectodermal release BMP4 and Wnt6 which turn on Fox D3 and Slug genes in the neural tube causing epithelial to mesenchymal transition. These mesenchymal cells break away and travel along predestined pathways in the mesoderm to form the PNS.

What are somites?

Paired tissue bundles found either side of the neural tube.

What does each pair of somites form?

A vertebrae

In what direction are somites added?

From head end to tail end

What cell type are somites?

Epithelial (they undergo mesenchymal to epithelial transition)

What are the 2 sections of the somite?

The dermomyotome on the top and the sclerotome on the bottom

What causes the scleretome to undergo epithelial to mesenchymal transition?

Sonic Hedgehog

What muscles do the dermomyotome cells form?

The epaxial (back) muscles and hypaxial (stomach) muscles.

What does the scleretome from?

The vertebrae.

Why does the body produce one extra somite?

As the posterior section of one somite fuses with the anterior section of the adjacent somite, meaning half a somite is lost at the head end and half at the tail end.

What germ layer do the limbs originate from?

Lateral plate mesoderm (LPM)

What gene determines the position of the limb bud?

HOX genes

Where are HOX genes expressed?

In the somites

What signalling molecules causes the limb bud to grow?

fgf10 (fibroblast growth factor 10)

What is the Apical Ectodermal Ridge (AER)?

The ectodermal tissue at the end of the developing limb bud

What signalling molecule is secreted by the apical ectodermal ridge?

fgf8

What determines the type of bone formed by mesenchyme in the limb bud?

How long the mesenchyme spends in the progress zone, exposed to fgf8. Shorter = less complex (humerus, femur) Longer = more complex (carpals. tarsals, phalanges)

What gene is expressed by the Zone of Polarising Activity (ZPA)?

Sonic Hedgehog

Which finger forms where there is high concentration of Sonic Hedgehog?

Little finger

Which finger forms where there is low concentration of Sonic Hedgehog?

Thumb

What signalling molecules are responsible for causing apoptosis between the bones of the limb?

BMPs (Bone Morphogenetic Proteins)

What are the Home Office Regulations regarding embryo use in research?

Embryos can be used for 2/3 of their gestation period.

What are the 3Rs of human animal research?

• Replacement - use something other than animals where possible

• Reduction - use fewer animals

• Refinement - use more ethical techniques

What is hyperplasia?

An increase in cell number due to cell division?

What is hypertrophy?

An increase in the size of individual cells.

What are the 4 main phases of the cell cycle?

• G1 phase (normal cell activity)

• S phase (DNA replication occurs)

• G2 phase (cell grows)

• M phase (mitosis)

What is G0 phase?

The exit point of the cell cycle.

What is the centromere?

A constricted region on the chromosome where sister chromatids join together.

What are kinetochore fibres?

Protein complexes assembled on the centromere which act as the attachment point for microtubules during mitosis.

What is the centrosome?

An organelle found in the normal cell’s cytoplasm which microtubules grow out from.

What are the 2 different cytoskeletal machines involved in mitosis?

The mitotic spindle - microtubules

The contractile ring - actin and myosin

What are the 6 stages of M phase (mitosis)?

1) Phrophase

2) Prometaphase

3) Metaphase

4) Anaphase

5) Telophase

6) Cytokinesis

What happens during prophase?

• Chromosomes begin to condense

• Nuclear membrane breaks down

• Mitotic spindle assembles

What happens during metaphase?

• Chromosomes and mitotic spindle attach to each other by the kinetochore

• Single chromatids align on the cell equator/metaphase plate

What happens during anaphase?

• Sister chromatids are separated by the separase enzyme

• Microtubules shorten, pulling the chromosomes to opposite poles

What happens during telophase?

Chromosomes arrive at the poles and decondense.

What happens during cytokinesis?

The contractile ring pinches the membrane inwards at the equator, dividing it into 2 identical daughter cells. Cytokineses starts in anaphase and ends at the end of telophase.

What are the 2 steps that make up differentiation?

Specification and Determination

What is specification in differentiation?

Stem cells are determined to differentiate into a certain cell type, which will not change if the cell is exposed to different environmental factors.

What is determination in differentiation?

Stem cells are determined to differentiate into a certain cell type, which will not change if the cell is exposed to different environmental factors.

What germ layer does the GIT originate from?

Endoderm

What germ layer do the muscle and tissue elements of the viscera originate from?

Mesoderm

What germ layer does the epithelial lining of the viscera originate from?

Endoderm

What are the 3 main regions that the mesoderm is divided into?

• Paraxial mesoderm (closest to somites)

• Intermediate mesoderm (middle)

• Lateral plate mesoderm (outer layer)

How does the gastrointestinal tube form in the early embryo?

1) Endoderm folds and elevates at its edges to form a groove

2) Endoderm edges fold around to create 2 loops which join to make one big tube

3) The tube fuses in the midgut and hindgut region

4) Buccopharyngeal and cloacal membranes seal the ends of the tube

What happens if the buccopharyngeal membrane is still present at birth?

It must be perforated.

What happens if the cloacal membrane is still present at birth?

Surgery or euthanasia

How does the GIT tube develop into the foregut?

1) Tube rotates 90C on its axis

2) Tube bulges out to the left and buds form

3) Bulge tilts downwards

How do the liver, pancreas and gallbladder form?

1) Buds on the developing stomach form these structures

2) The ventral bud becomes the ventral pancreatic bud and duct, gallbladder and liver buds

3) The dorsal bud becomes the dorsal pancreatic bud and duct

4) The ventral and dorsal pancreatic buds fuse to form the pancreas

How does the spleen form in the developing embryo?

From a small bundle of mesodermal cells called the spleen priomordium.

How does the midgut form?

1) Physiological herniation occurs - the midgut loop grows out of the embryo

2) The midgut loop rotates

3) The bottom half of the loop forms the jujunoileal loops

4) The midgut retracts and rotates into the position it will have in the adult abdomen

How does the hindgut form?

1) The urogenital tract and hindgut initially end in a common region - the cloaca.

2) The mesoderm pushes downwards, separating the urogenital canal and anorectal canal