Concepts and Language of Development Flashcards
(53 cards)
How is polyspermy prevented after the sperm penetrates the zona pellucida?
sperm enters oocyte - biochemical changes are triggered in the zona pellucida’s coat proteins
- alters structure = prevents additional sperm from penetration
what features define the one-cell zygote stage post-fertilisation?
- two unfused pronuclei (one from sperm, one from oocyte)
- surrounded by zona pellucida
- fluid-filled perivitelline space with one/two polar bodies
What are polar bodies and when are they produced?
polar bodies = small, non-functional cells formed during asymmetric divisions in oogenesis
- first polar body forms after meiosis I
- second polar body forms only if fertilisation occurs and meiosis II is completed
Why do polar bodies contain little cytoplasm?
asymmetric meiosis division - most of the cytoplasm is retained in the oocyte to support early embryo development
What happens to the size of cells during early cleavage?
total size of the embryo stays the same due to the rigid zona pellucida, so each mitotic division results in smaller cells
what is compaction? when does it occur?
around 12-16 cell stage (morula)
- outer cells flatten and tightly adhere to form an epithelial barrier, setting up for blastocoel cavity formation
what characterises the blastocyst stage?
(day 4/5) - fluid-filled cavity (blastocoele) forms
- outer epithelial layer of trophoblast cells
- inner cell mass that will form the embryo
What must occur for implantation to take place?
zona pellucida must be enzymatically shed (hatching) - allows the blastocyst to interact with and embed into the uterine wall
What do trophoblast cells contribute to after implantation?
begin differentiating into structures that will form the placenta (extra-embryonic structures)
What is formed from the inner cell mass after implantation?
bilaminar embryonic disc (epiblast & hypoblast)
What is the primitive streak and why is it important?
midline structure that forms from the caudal end, where epiblast cells migrate and differentiate—this initiates gastrulation and germ layer formation
what role does Henson’s node play?
signalling centre - secretes growth factors to r regulate behaviour of migrating cells during gastrulation
How are the three germ layers formed during gastrulation?
gastrulation = bilaminar embryonic disc (epiblast and hypoblast) transforms into a trilaminar disc with three distinct germ layers (ectoderm, mesoderm, and endoderm)
- primitive streak forms from caudal/posterior end
- extends anteriorly along embryo midline (establishes body axes & site for cell migration) - Henson’s node as signalling centre (anterior end of PS)
- secretes growth factors (Wnt, Nodal, BMP antagonists)
- Wnt & Nodal promote primitive streak formation, cell migration, mesoderm specification
- BMP antagonists inhibit BMP = promote neural and mesodermal fates.
- directs differentiation and movement of epiblast cells during gastrulation - epiblast cells move laterally, converge towards PS & undergo EMT - break cell-cell junctions and transforming into mesenchymal, migratory cells
- ingression and formation of the germ layers:
- first wave of epiblast cells moves through the streak, displaces the hypoblast, and forms the definitive endoderm
- second wave migrates between the epiblast and the newly formed endoderm, forming the intraembryonic mesoderm
- remaining epiblast cells that do not ingress become the ectoderm, forming the embryo’s outermost layer
(!!) signals from Henson’s node that influence gastrulation?
Wnt, FGFs & Nodal signalling - promote EMT (loss of E-cadherins/cell-cell junctions) & mesodermal specification
BMP antagonists - inhibit BMP; promote neural and mesoderm specification
what is neurulation? what structure does it form?
process that forms the neural tube from the neural plate - later develops into the brain and spinal cord
How does the neural tube close?
through a zipping mechanism from the middle outward, with cranial and caudal neuropores being the last to close
what condition arises due to failure of the NT to close anteriorly?
anencephaly
what condition arises due to failure of the NT to close posteriorly?
spina bifida
What does the ‘cloth purse’ model describe in embryology?
escribes the folding of the flat embryonic disc into a 3D cylindrical body—like tightening a drawstring purse
forms a closed gut tube, connected to the yolk sac via the vitelline duct and to the developing bladder via the allantois
What is mosaic development? What experiment supports mosaic development?
form of development where a cell’s fate is autonomously determined — encoded within the cell itself; unaffected by its environment
Roux (1888): destroyed one cell of a 2-cell frog embryo → surviving cell only formed half an embryo → suggested pre-programmed fate.
- BUT flawed = the dead cell remained in place, possibly interfering with development
- repeated with separation instead - each frog blastomere developed into a full embryo = regulative development confirmed.
What is regulative development? What experiment supports regulative development?
cell’s fate & development is influenced by its position, environment, and interactions with neighbouring cells — not predetermined
Driesch: separated sea urchin blastomeres with a hair - each developed into a complete embryo, showing plasticity and environmental influence
is development purely mosaic or regulative?
no - most embryos use a combination of both; allows for:
- compensation if something goes wrong
- developmental flexibility
- stronger outcomes
what is a fate map? how is it made?
labelling a region of tissue and grafting it into the same position in a host embryo - tissue follows its normal developmental path & structure it forms
What happens when tissue from later-stage embryos is grafted heterotopically (into a different location)?
doesn’t change fate - tissue fate is determined and has an irreversible commitment to a specific developmental path
