Developmental biology 1 (Prof. Dale)

Question 1

What are some of the oldest known embryos ?

Answer 1

Fossilized embryos from the Ediacaran Doushantuo Formation (551-635 million years old) of South China.

Question 2

What did Samuel Taylor Coleridge say about development in 1803 ?

Answer 2

“The history of man for the nine months preceding his birth would be far more interesting and contain events of greater moment than all threescore and ten years that follow it.”

Question 3

What are the 8 main phases of the human life cycle ?

When do these phases occur (on average) ?

Answer 3

Fertilization (“day zero”) –> cleavage (1-5 days) –> implantation (6-9 days) –> gastrulation (3-4 weeks) –> body plan (4 weeks) –> organogenesis (4-8 weeks) –> fetal development (8-40 weeks) –> birth (9 months)

Question 4

What are model organisms to study embryology ?

Answer 4

The Mus Musculus mouse, the Gallus Gallus chicken, the Xenopus Laevis frog, the Danio Rerio zebra-fish, the Caenorhabditis Elegans worm and the Drosophila Melanogaster fly.

Question 5

What did Barbara McClintock say say at her Nobel Prize for Physiology or Medecine in 1983 about the importance of genes in development ?

Answer 5

“With the tools and the knowledge, I could turn a developing snail’s egg into an elephant. It is not so much a matter of chemicals because snails and elephants do not differ that much; it is a matter of timing the action of genes.”

Question 6

How big is the egg for C. Elegans ?
The mouse ?
Drosophila ?
The zebra-fish ?
Xenopus ?
The chick ?

Answer 6

C. Elegans = 50μm thick
Mouse = 100μm thick
Drosophila = 180μm x 510μm 
Zebra-fish = 700μm
Xenopus = 1.3mm
Chick = 5cm

Question 7

What does the size of the egg depend on ?

Why are mammals an exception to the general rule ?

Answer 7

The size depends on the amount of nutrition the mother provides before the egg is laid and the size of the larval organism it must produce –> small larva require smaller eggs than large larvae
Mammals are an exception to this rule as the embryo continues to receive nutrition from its mother during fetal development.

Question 8

What are the different molecules stockpiled in eggs ?

In what proportions are these present ?

Answer 8

• Yolk Protein (Vitellogenin, ~90% of protein content) = nutrition the the embryo will need to form a larva that is capable of feeding itself
• Proteins required for ‘household’ functions (metabolism, cell division, DNA replication, transcription, protein synthesis etc.
• RNA (messenger, ribosomal, transfer etc.
• Lipids, glycogen etc.

Question 9

From what point onward can the embryo increase in size ?

Answer 9

The embryo can increase in size when it starts feeding itself.

Question 10

How does oogenesis happen in mammals ?

Answer 10

• begins in the fetal ovary but meiosis is not completed until fertilisation
• oocytes are held in prophase I until a few are activated each menstrual cycle, but are then held at metaphase II until fertilized by sperm
• the first polar body contains the nuclear material of the first meiotic division, while the second polar body contains the nuclear material of the second meiotic division

Question 11

When does spermatogenesis happen in mammals ?

Answer 11

• begins at puberty

- production of huge numbers of spermatozoa throughout adult life (>1000 per second)

Question 12

What are the characteristics of mammalian eggs ?

Answer 12

• only ~100 μm in diameter, lacking the large amounts of yolk found in the eggs of other groups of animals
• ovulated oocytes/eggs have not completed meiosis but are held in metaphase II
• the first polar body contains the chromosomes discarded by the first meiotic division
• cortical granules are found just beneath the plasma membrane
• the oocyte/egg is surrounded by a layer of somatic follicle cells derived from the ovary and a membrane called the zona pellucida

Question 13

What are the characteristics of spermatozoa ?

Answer 13

• highly specialised cells formed by meiosis in adult males
• lost most of their cytoplasm and the head contains a highly condensed haploid nucleus, along with a centriole and the acrosome
• the midpiece contains mitochondria and the base of the flagellum, which extends throughout the tail
• males ejaculate millions of sperm (~40M in humans in developed countries VS ~100M in developing countries) yet only one is required to fertilize the egg

Question 14

How does fertilization occur ?

How does the egg prevent polyspermy ?

Answer 14

• contact between sperm and zona pellucida causes the acrosome to burst, releasing enzymes that digest a hole in the zona pellucida
• sperm pass through and fuse with the egg plasma membrane, causing a localized release of calcium that soon spreads throughout the egg ==> Meiosis now reactivated in the egg + cortical granules fuse with the plasma membrane, releasing enzymes that modify the zona pellucida such that new sperm can no longer bind

Question 15

How does fertilization happen in the mouse ?

Answer 15

(A) In the mouse egg chromosomes (fc) are held on the metaphase plate of meiosis II.
(B) Fertilisation introduces the male pronucleus (mp) and its associated centriole (c), stimulating the production of the female pronucleus (fp) and the 2nd polar body (pb).
(C) The two pronuclei fuse to produce the dipoid nucleus with zygotic chromosomes (zc) flanked by maternal and paternal centrioles.
(D) Mitosis begins and chromosomes line up on the metaphase plate.

Question 16

What is the role of cytokinesis ?

How does it work ?

Answer 16

• Cytokinesis = completes mitosis + divides the cytoplasm between both daughter cells
• A contractile ring forms beneath the plasma membrane,
  containing a band of actin and myosin filaments –> always forms in the same plane that was previously occupied by the metaphase plate
• As the actin + myosin filaments slide by one another, the ring contracts and pinches the 2 cells apart

Question 17

Why were Sir Tim Hunt FRS, Sir Paul Nurese and Leland Hartwell awarded the 2001 Nobel Prize in Physiology or Medicine ?
What key experiment enabled them to obtain this prize ?

Answer 17

“[…] for their discoveries of key regulators of the cell cycle.”
Experiment:
- fertilised eggs of the sea clam Spisula solidissima were labelled with 35S-Methionine, samples taken after 45 minutes + thereafter every 10 minutes, loading proteins on a polyacrylamide gel followed by exposure to autoradiographic film
- 1st cleavage occurred around 55 minutes after fertilisation and second cleavage around 30 minutes later
- intensity of different bands changed at each cell cycle, which led to their designation as Cyclins (subsequent work demonstrated that Cyclins are important regulators of the cell cycle in all eukaryotic cells)

Question 18

What is the role of the fertilized egg ?

Answer 18

• Produce hundreds of trillions of cells through the process of mitosis
• Instruct these cells on what cell-type they must become
  (specification and determination)
• Organize them into tissues and organs (morphogenesis)
• Give cells the characteristics required for their function.
  (differentiation)

Question 19

What are the different types of cleavage divisions ?

Answer 19

Holoblastic cleavage: the entire egg is cleaved during each division. (mouse, Xenopus, C. elegans)
Meroblastic cleavage: only part of the egg is cleaved during each division. Two types:
Discoidal cleavage = restricted to a disc of yolk free cytoplasm at the animal pole. (zebrafish, chick)
Superficial cleavage = restricted to yolk free cytoplasm covering entire surface the egg. (Drosophila)

Question 20

Are the 1st mitotic cleavage divisions accompanied by growth ?

Answer 20

NO ! --> 1st mitotic divisions are not accompanied by growth !
Daughter cells (=blastomeres) = half the volume of parent cell.

Question 21

How does holoblastic cleavage happen in the mouse (and most mammals) ?

Answer 21

• Most mammals have small eggs containing very little yolk that is equally distributed in the egg –> these fertilised eggs almost always use holoblastic cleavage.
• Cleavage divisions = not synchronous + the embryo soon has an odd number of blastomeres
• At the 16-32 cell stage, blastomeres undergo compaction whereby they maximize contacts with each other
• They then form a fluid filled cavity = blastocoel and the embryo = blastocyst.

Question 22

How does holoblastic cleavage occur in Xenopus ?

Answer 22

• Xenopus –> large eggs with large amounts of unevenly distributed yolk, which is more abundant in the lightly coloured vegetal hemisphere
• Despite the large yolk content they usually divide fully.
• 1st two mitotic divisions = along the animal-vegetal axis and at right angles to each other + third division is equatorial (producing 4 animal and 4 vegetal blastomeres)
• 1st 12 divisions = synchronous + produce an embryo = blastula w/ a fluid filled cavity in the animal hemisphere = blastocoel

Question 23

How does meroblastic cleavage occur in the zebrafish ?

Answer 23

• Zebrafish eggs –> relatively large amounts of yolk that fills the cytoplasm except for a small region at the animal pole
• 1st three mitotic divisions = meridional but only cleave the yolk free cytoplasm at the animal pole (discoidal cleavage), while the 4th cleavage = equatorial producing 2 layers, 8 cells each = form a mound sitting atop a single large yolky vegetal cell

Question 24

How does meroblastic cleavege in the chick occur ?

Answer 24

• Chick eggs = very large and very rich in yolk, w/ a relatively small yolk free region at the animal pole
• Cleavage divisions = occur in this yolk free region giving rise to the chick embryo
• the embryo uses the yolk to fuel development

Question 25

How does superficial cleavage occur in drosophila ?

Answer 25

• Drosophila eggs = a lot of yolk concentrated w/in the center with a peripheral yolk free periplasm
• the 1st 8 nuclear divisions occur centrally + are not accompanied by cytokinesis
• most nuclei then migrate to the egg periplasm to form the syncytial blastoderm, where they undergo a further 4 nuclear divisions
• plasma membrane then folds inwards + partitions each nucleus into a single cell, to form the cellular blastoderm

Question 26

How long are different cleavage events in different species ?

Answer 26

Drosophila melanogaster (fruitfly) = 8 minutes
Caenorhabditis elegans (nematode) = 20 minutes
Danio rerio (zebrafish) = 20 minutes
Xenopus laevis (frog) = 25 minutes
Gallus gallus (chick) = 1.5 hours
Mus musculus (mouse) = 20 hours

Question 27

Why is there such a variety in cleavage length between species ?

Answer 27

• cleavage divisions in many eggs = very short –> allows embryo to quickly build a larval form capable of feeding for itself + escaping predators
• eggs of placental mammals develop inside their mothers wombs, who provides them with both nutrition + protection –> cleavage divisions + development in general can therefore progress at a much slower pace

Question 28

How are rapid rates of cell division achieved ?

Answer 28

By modifying the cell cycle:

• removing gap phases (G1 & G2)
• shortening S-phase (DNA replication)
• removing cytokinesis (only nuclei divide)

Question 29

What is the Mid-Blastula Transition (MBT) ?

Answer 29

After 12 cell divisions (in Xenopus) the normal cell cycle is introduced + cell division becomes asynchronous = MBT
(also occurs in drosophila + zebrafish embryos)

Question 30

Which genetic material does the embryo use before the MBT ?

What happens after the MBT ?

Answer 30

• cell cycle = so rapid before MBT –> very little time for transcription ==> the oocyte must stockpile the RNA that it needs during the cleavage divisions –> since only the maternal genome contributes to this stockpile, RNA = maternal RNA
• zygotic RNA synthesis increases significantly at the MBT, while maternal RNA is degraded