Early Devlopment Flashcards
What is fertilization age (also known as coneptual age)?
- measured from the time of fertilization (assumed to be +1 day from last ovulation)
- difficult to know time of fertilization exactly (unless IVF)
What is gestational age?
- calculated from the time of the beginning of the last menstrual period (LMP)
- Determined by fertilization date (+14 days) if known, or early obstetric ultrasound and comparison to embryo size charts
What is carnegie stage?
- 23 stages of embryo development based on embryo features not time
- Allows comparison of developmental rates between species
- Covers the window of 0-60 days fertilization age in humans
What is the time of embryogenic stage?
14-16 days post-fertilization
What is the time of embryonic stage?
16-~50 days post fertilization
What is the time of fetal stage?
~50 to 270 days post-fertilization or ~8 to ~38 weeks
What is the embryogenic stage?
- establishing the early embryo from the fertilized oocyte
- Determining two populations of cells: pluripotent embryonic cells (contribute to fetus)
- Extraembryonic cells (contribute to the support structures eg placenta)
What is the embryonic stage?
- Establishment of the germ layers and differentiation of tissue types
- Establishment of the body plan
What is the fetal stage?
- Major organ systems now present
- Migration of some organ systems to final location
- Extensive growth and acquisition of fetal viability (survival outside the womb)
- 2nd/3rd trimester
How many cells are in an ovulated oocyte?
1
How many cells are in a zygote?
1
How many cells are in the cleavage stage embryos?
2-8
How many cells are in morula?
16+
How many cells are in blastocyst?
200-300
At what cell stage are the genes of the embryo not transcribed until?
Until 4-8 cell stage
What is the embryo dependent on to get through the first divisions?
dependent on maternal mRNAs and proteins
What happens to these mRNA and proteins during oocyte development (i.e. pre-ovulation)
synthesized and stored
What can impair embryonic development?
failure to synthesise, store or interpret these mRNAs and proteins during oogenesis
What happens during the maternal-to-zygotic transition (4-8 cell stage)?
- Transcription of embryonic genes (zygotic genome activation)
- Increased protein synthesis
- Organelle (mitochondria, Golgi) maturation
What does compaction start?
the formation of the first two cell types
What happens around the 8 cell stage or later?
- Outer cells become pressed against zona
- Change from spherical to wedge-shaped
- Outer cells connect to each other through tight gap junctions and desmosomes
- Forms barrier to diffusion between inner and outer embryo
- Outer cells become polarised
What is the compacted morula?
- 2 distinct cell populations:
1. Inner (pink)
2. Outer (green)
What happens in blastocyst?
Inner (pink) cells and outer (green) reorganize with formation of the blastocoel cavity
What doe blastocyst formation establish?
two cell type
What is the zona pellucida?
- Hard protein shell inhibiting polyspermy
- protects early embryo
What is the inner cell mass?
Pluripotent embryonic cells that will contribute to the final organism
What is the trophoectoderm?
Extra-embryonic cells that contribute to the extraembryonic structures that support development
What is the blastocoel?
Fluid-filled cavity formed
osmotically by
trophoblast pumping
Na+ ions into cavity
What happens day 5-6?
- Hatching
- To implant the blastocyts must escape zona pellucida
1. Enzymatic digestion
2. Cellular contractions
What is the separation of embryonic cell lineages I?
morula to:
- Inner cell mass (embryonic)
- Trophectoderm (extra-embryonic)
What happens day 7-9?
peri-implantation
What does the trophectoderm lineage separate further into?
- Cytotrophoblast
2. Syncitiotrophoblast
What do trophoblast cells fuse to form?
syncitiotrophoblast
What does syncitiotrophoblast invasion destroy?
- local maternal cells in the endometrium
- creates interface between embryo and maternal blood supply
What do the cytotrophoblast cells do?
remain individual to provide source of syncitiotrophoblast cells
What do inner cell mass separate further into?
- epiblast
2. hypoblast
What is epiblast?
from which the fetal tissues will be derived
What is hypoblast?
which will form the yolk sac (extraembryonic structure)
What happens day 12+?
Bi-laminar embryonic disc formation
What does syncitiotrophoblasts secrete?
hCG and detection of beta hCG subunit in blood/urie is basis of pregnancy tests
What is the bilaminar (two. later) embryonic disc the final stage before?
gastrulation (day 14-16)
What happens to some cells separated from the epiblast?
Some cells become separated from the epiblast by the formation of a new cavity – the amniotic cavity
What do these amnion cells contribute to?
extra-embryonic membranes
What is a result of these cavities?
- two-layer disc of epiblast and hypoblast, sandwiched between cavities
- embryo is now ready for gastrulation
What does the epiblast separate into?
- Epiblast
2. Amnion
What does the separation of the last epiblast form?
- Ectoderm
- Mesoderm
- Endoderm
What organs are in the germ layer endoderm?
- GI tract
- Liver
- Pancreas
- Lung
- Thyrod
What organs are in the germ layer ectoderm?
- CNS and neural crest
- Skin epithelia
- Tooth enamel
What organs are in the germ layer mesoderm?
- Blood (endothelial cells, red and white BC)
- Muscle (smooth, skeletal and cardiac)
- Gonads, Kidneys and adrenal cortex
- Bone cartilage
What happens in gastrulation?
- First cells to invaginate through the primitive groove form the definitive endoderm
- Remaining cells of epiblast are called ectoderm
- Cells that remain in the space between the ectoderm and definitive endoderm from layer (mesoderm)
What does the formation of the primitive streak result in?
defines head-tail and left-right axes of embryo
What does Invagination of cells into the primitive streak form?
3 germ layers
- Endoderm cells (first through streak) - replace hypoblast
- Ectoderm (reminds of upper (ventral) surface
- Mesoderm sandwich between ends and ectoderm
