Early Development Flashcards
What are the 3 ways to measure embryo-fetal development?
- Fertilisation age —> from day after ovulation
- Gestational age —> from 1st day of last period
- fertilisation age + 14 - Carnegie stage —> 23 stages marking
developmental milestones
- within fertilisation age 0-60
- comparable between species
What are the 3 stages of embryo-fetal development?
1st trimester:
1. Embryogenic
- start: fertilised oocyte
- end: pluripotent embryonic —> form fetus
extraembryonic cells —> support structures
- fertilisation age 14-16 days
- Embryonic stage - end: 3 germ layers formed
- fertilisation age 16-50 days
2nd and 3rd trimester:
3. Fetal stage - organ systems present
- organs migrate to final location
- fetal growth and viability
- fertilisation age 50-270 days
What are the 5 stages of blastocyst formation?
- Ovulated oocyte —> fertilised in fallopian tube
- Zygote (1 cell)
- Cleavage stages embryos (2 —> 4 —> 8 cells)
- embryo size unchanged - Morula (16 cells)
- Blostocyst (200-300 cells)
What are the 6 stages of early embryo development?
- Maternal-to-zygotic transition (1 to 8 cells)
- Compaction (morula)
- Hatching (day 5-6)
- Pre-implantation (day 7-9)
- Implantation
- Gastrulation
What is the maternal-to-zygotic transition?
Zygotic genome activation:
- 1 to 4 cell stage —> embryo uses maternal mRNAs
and proteins (deposited during
oocyte development)
- allows for first 2 divisions
- 4 to 8 cell stage —> embryonic genes transcribed
- inc protein synthesis
- organelle maturation (mitoc + golgi)
What is compaction?
Formation of blastocyst:
1. 8 to 16 cells division —> morula formed —> outer
cells pressed against zona pellucida —> connect via
tight gap junctions —> outer barrier
- Outer cells and inner cells exposed to different
environments —> outer to extraembryonic cells +
inner to pluripotent embryonic cells —>
trophoectoderm and inner cell mass - Inner pluripotent embryonic cells migrate to top
end —> form blastocoel cavity —> Na+ ions
pumped in —> water follows —> fluid filled cavity
What are the 4 regions of a blastocyst?
- Zona pellucida —> outermost layer
- Trophoectoderm —> extra-embryonic cell layer
- Inner cell mass —> pluripotent embryonic cells
- inner cells at top end - Blastocoel —> fluid-filled cavity
What is hatching?
Blastocyst leaves zona pellucida:
- enzymatic degradation
- cellular contractions
—> blastocyst free for implantation
What are the pre-implantation events?
Further differentiation of cells:
Trophoectoderm
- syncitiotrophoblasts —> implantation
- cytotrophoblasts —> source of cells for
syncitiotrophoblasts
Inner cell mass
- epiblasts —> to fetal tissue
- hypoblasts —> to form yolk sac
What is implantation?
Embryo attaches to maternal endometrium:
1. Synciotiotrophoblasts form projections —> invade
endometrium and act as an anchor ( + interface for
blood supply) —> secrete B-hCG (test)
- Epiblasts form amniotic cavity in middle —> bi-
laminar disc between amniotic cavity and blastocoel
What is gastrulation?
Formation of 3 germ layer:
1. Primitive streak forms in epiblasts of bi-laminar disc
—> splits cranial and caudal end (+ right and left)
2. Cranial end of streak expands —> primitive node
—> primitive pit —> primitive groove
3. Epiblast cells fall in —> endoderm
Next set of endoderm cells fall in —> mesoderm
Remaining epiblast layer —> ectoderm
What are the 3 germ layers of an embryo?
- Endoderm —> GI, liver, pancreas, lungs, thyroid
- Mesoderm —> blood, muscles, gonads, kidneys,
adrenal cortex, bone, cartilage - Ectoderm —> CNS, skin, tooth enamel
What is neurulation? (ectoderm)
Formation of CNS
1. Notochord (rod of cartilage-like cells) forms down
embryo midline of mesoderm and ectoderm
2. Notochord signals —> neural plate of ectoderm
forms neural groove (valley) with neural folds
(mountains) containing neural crest cells
3. Neural folds fuse —> neural groove becomes
neural tube (ectoderm rim) —> neural crest cells
migrate through rest of ectoderm
4. Neural tube gradually closes from head to tail (day
23 to 27)
What is somitogenesis? (mesoderm)
Formation of somites (paired blocks of mesoderm)
1. Mesoderm forms around neural tube and
notochord —> pairs (each side) condense and bud
off —> from head down
2. Somites —> sclerotome (vertebrae and rib cartilage)
—> dermomyotome —> dermatome (to
skin, fat, connective)
—> myotome (muscle)
- 44 pairs
- 1 forms per 90 mins
How does the GI tract form? (endoderm)
- Ventral (head and tail) and lateral (sides) folding —>
pinch off some yolk sac —> primitive gut - Primitive gut spits into
- foregut —> to upper duodenum
- midgut —> to first 2/3 colon
- hindgut —> to upper anal canal
