Week 18 Flashcards
What is an oocyte in embryogenesis?
Female sex cell that has not finished undergoing meiosis
What is an ovum in embryogenesis?
Mature female sex cell. Completes meiosis (on fertilisation)
What is a zygote in embryogenesis?
Fertilised ovum female and male pronuclei. Two sets of info in nucleus
What is pre-organogenesis/Conceptus stage of development?
Cell division. 12-24 hours between cleavages
Polarisation of the structure: eg embryonic pole
Formation of 2 layers (bilaminar germ disc)
What is embryo stage of development?
Gastrulation: formation of three layers (trimlaminar germ disc)
Laying down of basic body plan
Organogenesis
What is fetus stage of development?
Growth
Refinement
Function: cell function
What is Morula in embryogenesis?
Ball of cells (8+). No cell growth. Hyperplasia only.
What is compaction in embryogenesis?
Process of creating a tight ball of cells
What is Blastomere in embryogenesis?
Individual cell that makes up the morula
What is blastocyst in embryogenesis?
Hollow ball of cells with a thickening at one end when viewed as a cross-section
What is a Blastocoel in embryogenesis?
The fluid filled hollow within the blastocyst
What is Gastrulation in embryogenesis?
Cell movements which produce gut and three primary germ layers
How would a twin occur due to morula splitting?
Totipotent morula cell is lost from the morula before compaction occurs. Leading to development of a second fetus.
First few stages after fertilisation?
- Egg released from the ovary, and fertilisation occurs near the end of the fallopian tube.
- Presence of Zona Pellucida until the uterus is reached
- Cell division increases the number of cells in the ball, not the size of the ball.
- Implantation between the secretory glands
- Implantation is more likely in the upper quadrants
What are early stages of zygote division known as?
Cleavage
In cleavage division, where are 4 stages. What are these?
Somatic cell cycle consists of four phases;
S,M, G1, S and G2.
Embryonic cell cycle alternates between S and M phases
(Embryonic growth is by Hyperplasia, not hypertrophy Ie cells divide, but there is no cellular growth (cells get smaller))
How long is cleavage in human and mouse embryogenesis?
Human: 12-24hrs
Mouse: ~10hrs
Sea urchin cleavage embryogenesis?
- Rapid, complete cleavage with equal division = Holoblastic cleavage
- Blastomeres are all the same size (equal).
- Blastomeres are smaller with each division
- Later cleavages are asymmetric
Amphibian cleavage embryogenesis?
Unequal holoblastic division ie
- Complete cleavage
- Unequal size
- Blastomeres in animal pole are smaller than blastomeres in vegetal pole because of the presence of yolk
What is Amphibian cleavage pattern?
First 2 cleavages are perpendicular (equal holoblastic) but nuclei are displaced ‘animal-ward’
Cleavage 3 is perpendicular/equatorial but unequal holoblastic
Gives rise to a more rapidly dividing animal pole
Avian (chick) cleavage embryogenesis?
Meroblastic cleavage
Cleavage does not bisect the yolk
Also found in: reptiles, birds, some fish, insects
Blastula stage of embryogenesis (Blastocysts)?
- A hollow ball of cells with a thickening at one end
- Cavity = blastocoel
- Inner Cell Mass (ICM): group of cells that will contribute to the embryo
- In humans, the blastocyst begins to implant into the uterus and continues to develop
What is blastula stage identifiable in human embryo?
Identifiable at 4-6 days of human gestation
What is the next stage after blastula stage?
Next stage is formation of two cell layers
What is a bilaminar germ disc composed of?
Epiblast: Outer layer - form embryonic tissue, surrounded by amniotic cavity
Hypoblast: Primitive endoderm (inner) giving rise to extraembryonic tissues. Faces onto the yolk sac
(Temporary layer for the bilaminar germ disc)
What stage does gastrulation of embryogenesis take us into?
The embryonic phase of development and beginnings of organogenesis (Week 3)
Bilaminar embryo becomes what in gastrulation?
Bilaminar embryo becomes a three layer embryo. Allowing for the basics of a body plant. Occurs 12-15 days after fertilisation in humans
(“It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.” - Wolpert)
What are the three body layers created in gastrulation?
Ectoderm - Forming skin and nervous system
Mesoderm - Forming internal organs, muscles, bones, etc
Endoderm - Forming internal linings eg Blood vessels, gut lining, etc.
What occurs in Gastrulation in sea urchin?
- Mixture of cell movement and shape change towards vegetal pole. Cells become columnar
True or false: Axes have been specified during blastula stage and not gastrulation in sea urchins?
True, so Gastrulation is a mixture of cell movement and shape change in sea urchins.
Stage 1 of gastrulation in sea urchin?
- Some cells undergo epithelial-mesenchymal transition and move through vegetal pole forming middle layer
Stage 2 of gastrulation in the sea urchin?
- Cells change shape, invaginate to create a hollow (archenteron)
Stage 3 of gastrulation in the sea urchin?
- Mesenchymal cells form a ring around archenteron
- Hollow extends through larva- division and filopodia
Stage 4 of gastrulation in the sea urchin?
- Joins invaginating stomodeum (mouth and upper throat)
Gastrulation in amphibians stage 1?
Grey crescent cells move inward to create a dorsal blastopore
Migration of cells across the animal pole- will eventually cover the surface
Gastrulation in amphibians stage 2?
Some cells push over and through the dorsal lip of the Blastopore- cells change shape to do so.
Cells also undergo epithelial-mesenchymal transition (EMT) as they do this.
Gastrulation in amphibians stage 3?
Invagination and cell movement to form a hollow and archenteron
Expansion of mesodermal tissue under ectoderm
Archenteron extends into blastocoel
Chick and mammalian gastrulation different to amphibian?
Instead of blastopore = (Henson’s Node)
Also have a Primitive streak.
Chick and mammalian gastrulation stage 1?
- Cells move through the primitive streak to create mesodermal and endodermal layers in place of the hypoblast.
Chick and mammalian gastrulation stage 2?
Epiblast cells (top layer) converge on the primitive streak/groove
Chick and mammalian gastrulation stage 3?
Individual proliferating cells move through Primitive Streak and displace the hypoblast to create the endoderm
Chick and mammalian gastrulation stage 3?
Later cells move through to create the mesoderm
Cells spread out laterally and anteriorly
How is the gut formed in mammals during gastrulation?
- Gut formation is one of the key roles of gastrulation (gut lining is endodermal)
- Endoderm rolls, or folds down and pinches off
Stage 1 of gut formation in mammals?
- Surface of the embryo rolls down as the embryo develops and grows. The mesoderm becomes internalised and endoderm is pinched off
Stage 2 of gut formation in mammals?
Gut ends up as a tube within the embryonic cavity, suspended by a stalk of mesoderm
Organogenesis begins with what process?
Neurolation
Neurulation in all vertebrates occurs in the same basic ways:
- Notochord develops from dorsal mesoderm
- Induction of neural plate from ectoderm layer above the notochord
- A neural groove and neural folds form from the dorsal ectoderm
- The neural folds rise up, meet and fuse to form the neural tube
(The neural tube becomes the central nervous system)
Formation of the neural plate organogenesis: How is the notochord formed?
Primitive steak formation (gastrulation) sets up a new axis. Cells move in from the primitive streak and anteriorly to form the notochord.
Notochord incudes epithelium (ectoderm) to become neural.
Where does primary neurulation occur?
In the anterior part of the neural tube
features of Primary neurulation?
Occurs in the Anterior part of the neural tube (NT)
Only the very end of the tube forms a different way in humans.
Neural crest cells sit at the edge of the neural plate
Creation of hinge points:
- Medial: upward
- Dorsolateral: inward
Closure and delamination
Where does the neural crest cells sit?
Edge of the neural plate
What are neural crest cells?
Specialist migratory populations
Migrate on cranial, dorsolateral and ventral pathways
Highly migratory, invasive and proliferative
Contribute to many key structures
- What do neural crest cells contribute to?
- What are the 4 types, what do they contribute to?
- Craniofacial structures, melanocytes (melanoblasts in embryo) and cells that form adrenal medulla, and other structures:
- Cranial NC = cranial neurons, facial mesenchyme
- Cardiac NC: tissue wall of large arteries
- Trunk NC: dorsal root ganglia- sensory, adrenal medulla, pigment cells
- Vagal NC: parasympathetic ganglia
Neural tube creation is consistent across species. What occurs?
Convergent extension to lengthen the cells; no increase in number occurs.
Formation of hinge points: folding purposes
Development requires signals: induction. What three features of embryogenesis do the signals aid?
Growth
Morphogenesis
Differentiation
How do cells ‘know’ where they are on the body plan?
Positional information may be determined by induction (a cells behaviour influences another):
- Cellular communication, both long and short range, can create complex positional information
- Contact with surrounding cells, with underlying tissues and responding to signals from neighbouring/distant cells can affect how a cell will behave (induction)
- Time will also play a role in cell behaviour and fate.
One cell signalling concept is that for the formation of a blastula, gastrula and neurula, they need to create a body plan and the main axes. Explain how cells do this using blastocyst as an example?
Cells on the outside of a blastocyst have contacts on 2 sides, this “Tells” them they are on the outside.
Cells in the Inner Cell Mass have more contacts, defining them as “inner”.
If you move them at early stage the cells will change identity. If you move them later on, cells will “remember” what they are supposed to be.
One cell signalling concept is that induction can be by short or long - range signals. What are short range signals?
Cells can react to their own signals (autoregulation) or to signals from neighbouring cells.
This can be inhibitory or inducing
One cell signalling concept is that induction can be by short or long - range signals. What are long range signals?
- Directional
- often create gradients- where cells will behave differently depending on where they are on the gradient. This type of signal is a morphogen. The most common one in development is Sonic Hedgehog.
- involved in cellular movement ie axon migration
Induction: What does the Spemann-Mangold Organiser explain?
Transplantation of the dorsal lip of the blastopore (organiser) to the opposite side of a host newt embryo. Equivalent structure in mammals is node.
Results in the generation of a secondary (induced) embryo
Using pigmented and non-pigmented embryos means you can identify origins.
Found Mesoderm influences ectodermal behaviour
Induction of neural plate in amphibians ?
BMP-4 expressed throughout in early stages
BMP antagonists are expressed in the emerging organiser
As inhibition of BMP spreads across the surface, neural plate forms in the Animal pole above the organiser where BMP is inhibited
How does the organiser changes cell fate?
Bone Morphogenic Protein (BMP)-4 is uniformly distributed in late blastula and causes VENTRAL development.
Chordin and Noggin secreted by organiser stop action of BMP-4 and allow dorsal development e.g. muscle and notochord from the dorsal mesoderm
What is the first neural plate development marker?
Sox2
Induction of neural plate 2 (amphibians)?
BMP antagonists induce neural fate. But…
Fibroblast Growth Factors (FGFs) are also needed for neural plate formation.
What occurs in Chicks after Fibroblast growth factors are activated?
FBFs activate the churchill gene which causes the activation of Sox2 (the first neural expression plate development marker)
The hand as a model for axes?
Anterior = thumb
Posterior = little finger
Proximal = wrist
Distal = finger tips
Ventral = palm
Dorsal = back of hand
Limb bud structure requires?
Apical ectodermal ridge for limb outgrowth
What occurs if the Apical ectodermal ridge (AER) in the early limb structure lost?
Loss of the AER leads to progressive loss of distal structures- related to timing of loss. Later loss of AER gives minimal loss.
Key signals: Fibroblast Growth Factors
Important regions of the limb
Apical Ectodermal Ridge (AER)
- Ectoderm.
- Stimulates Outgrowth (FGFs)
Progress Zone (PZ)
- Mesoderm
- Proliferation
Zone of Polarising Activity (ZPA)
- Patterning
- Morphogen signal: SHH
Why is SHH important?
SHH sets up a gradient across the limb bud: High concentration at the posterior, low at the anterior.
This means that at different positions along the gradient, cells will be exposed to (“see”) different amounts of SHH.
The result of this is that cells along the gradient will switch on different genes and start to behave differently or develop different identities.
Is SHH relevant to other systems not just the limb buds?
YES!!
- The limb bud is an important model for what happens elsewhere in the body.
- It has all axes in one
- It has the same, or very similar, signaling patterns
- It is external to the body so can be easily manipulated.
- What we learn from the limb can be applied to other systems or to develop treatments and interventions
What is the importance for other developmental patterning events: Neural tube pattering?
SHH expressed in the floor plate, helps to set up neural patterning
What is the importance for other developmental patterning events: Cyclopia/midline devt?
Inhibit Shh, get midline defects
Holoprosencephaly
Cleft palate
One, central, eye
What is the importance for other developmental patterning events: Thalidomide?
Used as an anti-emetic for pregnant women
Resulted in limb defects, CV and other serious problems
Loss of SHH and FGF
What are the components of a spermatozoon?
Head: Acrosome, nucleus, neck
Middle piece: Mitochondrion (spiral shape) (ATP)
Flagellum, Plasma cell membrane
What does fertilisation stimulate?
Egg activation,
Zygote formation
Sea urchin fertilisation overview?
External fertilisation
Large eggs
Transparent embryos
What are the 5 steps of sea urchin fertilisation?
Contact
Acrosomal reaction
Contact and fusion of sperm/egg membranes
Cortical reaction
Entry of sperm nucleus
Stage 1 of sea urchin fertilisation: contact?
Sperm contacts the egg’s jelly coat, triggering exocytosis from the sperm’s acrosome
Stage 2 of sea urchin fertilisation: Acrosomal reaction?
Hydrolytic enzymes released from the acrosome makes a hole in the jelly coat.
Growing actin filaments form the acrosomal process, which protrudes from the sperm head and penetrates the jelly coat.
Proteins on the surface of the acrosomal process bind to receptors in the egg plasma membrane
How do sea urchins have a fast block to polyspermy during fertilisation?
Membrane potential present across sea urchin egg plasma membrane
Fusion of plasma membranes causes
- Ion channels in plasma membrane open
- Na+ influx from sea water
= Membrane depolarisation
Change in plasma membrane –> no more sperm fusion
Transient effect
Explain conspecific fertilisation?
- egg fertilised by sperm from same species
- Bindin receptor on surface of vitelline membrane : bindin on surface of acrosomal process
- More enzymes break down vitelline layer and plasma membrane allowing fertilisation to occur.
Stage 3 of sea urchin fertilisation: Contact and fusion of sperm and egg membranes?
Fusion triggers depolarisation of the membrane which acts as a fast block to polyspermy
Stage 4 of sea urchin fertilisation: Cortical reaction?
Cortical granules in the egg fuse with the plasma membrane.
Secreted contents clip off sperm-binding receptors and cause the fertilisation envelope to form.
Acts as a slow block to polyspermy
Stage 5 of sea urchin fertilisation?
Entry of sperm nucleus
how do sea urchins slow block polyspermy during fertilisation?
Ca2+ wave increases cytosolic conc of Ca2+ where sperm fuses
Cortical granules fuse with plasma membrane and release enzymes:
- break down adhesion between vitelline layer and membrane
- increase osmotic pressure causing water influx
- snip off sperm receptors
- harden fertilisation envelope
Also stimulates egg activation
What is the calcium wave of cortical reaction during sea urchin fertilisation?
Binding of a sperm to the egg activates a signal transduction pathway triggering release of Ca2+ into the cytosol from the ER
What does egg activation cause in sea urchin fertilisation
increase in protein synthesis and metabolic rate (Parthenogenesis)
What occurs in Pronuclei fusion in sea urchin during fertilisation?
- Migration of female pronucleus guided by microtubules from male pronucleus centrosome
- Directly fuse to form zygote
- DNA synthesis begins
Oocyte at ovulation: human fertilisation?
Ovulation releases a secondary oocyte and the first polar body
Surrounded by the zona pellucida (glycoprotein layer) and corona radiata (multicellular)
Both layers must be penetrated by the sperm
How many sperm must interact with the egg for human fertilisation to occur?
Multiple
Steps of human egg fertilisation?
- Sperm contact corona radiata
- Acrosomal reaction to digest zona pellicuda
- Fusion of membranes and sperm enters
- Block to polyspermy
Mammalian sperm-egg molecular interactions: Zona pellucida glycoprotein ZP3 binds to? triggering?
zona pellucida glycoprotein ZP3 binds to Beta 1-4-galactosyltransferase on sperm
triggers acrosomal contents release
Mammalian sperm-egg molecular interactions: Acrosomal enzymes ____ and _____ digest?
acrosomal enzymes acrosin and -N-acetylglucosaminidase digest zona pellucida glycoproteins allowing access cell membrane
Mammalian sperm-egg molecular interactions: what molecules are essential for membrane fusion?
CD9, JUNO, SPACA6 and Izumo1
Mammalian sperm-egg molecular interactions: Izumo1 protein binds to what?
Juno on secondary oocyte plasma membrane
to block polyspermy: release of intracellular Ca2+ –> exocytosis of oocyte secretory vesicles to harden zona pellucida
How is fertilisation and oocyte activated by sperm entry?
Acrosomal enzymes and strong flagella movements from multiple sperm create gaps in the corona radiata and zona pellucida
Only one sperm cell then makes contact with the oocyte membrane –> membrane fusion occurs
Pronucleus formation in human fertilisation?
The entire sperm enters and is absorbed into the cytoplasm
Meiosis II resumes
- Secondary oocyte splits in two haploid cells of unequal size – the ovum and the second polar body
- The female pronucleus develops
Spindle formation and cleavage preparation in human fertilisation?
The male pronucleus develops
Spindle fibres from centrosome appear in preparation for the first cleavage division
Preparation for first mitotic division in human fertilisation?
Each pronucleus enters mitosis separately (DNA replication)
Nuclear membranes break down and pronuclei fuse = zygote
Chromosomes attach to common spindle
Beginnings of cleavage development during human fertilisation?
Chromosomes line up along metaphase plate and prepare to divide
Step 1 of human fertilisation: Sperm contact corona radiata?
Sperm must travel through a layer of follicle cells before reaching the Zona Pellucida (extracellular matrix of egg).
Binding of a sperm to a sperm-receptor induces acrosomal reaction facilitating sperm entry
What is required for a sperm cell to enter a human egg during fertilisation?
Sperm capacitation
What does sperm capacitation refer to?
the physiological changes spermatozoa must undergo in order to have the ability to penetrate and fertilize an egg
