Embryology Flashcards
define embryology
anatomical description diff stages development
morphogenesis
development internal + external form + structure
define developmental biology
embryology + mechanisms involved in regulation
2 cell types
- somatic - of body, limited life
- germ line - for repro, immortal
3 types mammals
- monotremes - lay eggs
- marsupials - short gestation, immature young
- placentals - more mature when born
marsupial embryos still have a placenta
key stages embryo development
- cleavage
- compaction
- formation trophectoderm + blastocyst
- primitive endoderm
- implantation
- visceral + parietal endoderm
- formation primordial germ cells + migration genital ridges
- gastrulation
cleavage
cell division w/o cell growth from 1 cell embryo to 8 cell embryo
cells called blastomeres
zygote
fertilised oocyte
compaction
blastomeres maximise cell-cell contact to form morula
requires cell adhesion mol e-cadherin (epithelial calcium dependent adhesion) expressed on surface
Oct4
transcr factor required cell pluripotency, preventing premature differentiation ESCs
formation trophectoderm
only cells outer surface have free apical surface = transcription factor YAP enters nucleus = higher expression levels Cdx2, inhibiting Oct4 = differentiate to form TE
before all cells had Cdx2 + Oct4
formation blastocyst
TE pumps Na+ inside morula = water in by osmosis = fluid-filled cavity (blastocoel) forms
transcription factor
prot typically found nucleus responsible for turning genes on/off - which ones to mRNA to prots
functions trophectoderm
- pump Na+ into morula to form blastocyst - need Cdx2 to localise channels or not enough Na+ in
- hatching from zona pellucida
- implantation - helps digest through lining endometrium
- maintenance pregnancy
- formation placenta
cell polarisation meaning
intrinsic cell asymmetry - diff types prots diff parts cell
e.g. TE polarised apical-basal axis w 2 diff cell types slight diff prot expression = Na+ channels only on apical surface
blastocyst escapes ZP
- blastocoel expands until embryo pressing against ZP
- mural TE secretes strypsin - lyses hole in ZP
what happens to mural + polar TE after blastocyst escapes ZP
mural -> giant trophoblastic cells
* terminally diffed, non-invasive
polar -> proliferative cytotrophoblastic cells
* outside wall blastocyst + responsible implantation
formation primitive endoderm (=hypoblast)
fibroblast growth factors (FGF) in external environ bind receptors = activated + cytoplasmic part phosphorylated = signalling cascade = genes expressed for transcr factor Gata6
–> suppresses expression Oct4 = differentiate
–>turns genes on make basement mem
functions hypoblast
- deposit basement mem (for compartmentalisation)
- give rise extra-embryonic tissues to support growth
- secrete signalling mols
what are epiblast cells when out of embryo
embryonic stem cells
implantation
blastocyst-uterus adhesion
- initial low-strength interactions bet TE + endometrial epithelial cells
- then stronger contact made by cadherins on surfaces
- TE starts embedding uterus wall
receptivity uterus endometrium
undergoes cyclical changes + only receptive implantation in small window
* acheived by oestrogen + progesterone - level maintained chorionic gonadotropin (CG)
primitive endoderm -> parietal
- TE secretes parathyroid hormone-related peptide (PTHrP), causing diff
- no close connection neighbours so migrate + form layer underlying TE
- loads RER secrete loads ECM prots form thick basement mem (Reichert’s)
primitive endoderm -> visceral
- epiblast cells secrete bone morphogenic prots (BMPs), causing diff
- tall epithelial w microvilli + close connections to make epithelium
- support growth embryo - provide nutrients b4 placenta formed
role Reichert’s membrane
prevents maternal immune cells entering parietal yolk sac + destroying embryo
only in rodents
formation primordial germ cells
extra-embryonic tissues secrete BMPs that instruct differentiation
high levels needed so only cells v. close extra-embryonic tissues exposed sufficient + diff
genital ridges
tissue that will form gonads (ovaries/testes)
how differentiation PGCs to oocytes or sperm determined
gonadal somatic cells in genital ridges all cont retinoic acid but in male gonad express cytochrome P450 enz that degrades RA
RA required for PGCs become oocytes, therefore P450 inhibits oocyte diff
differentiation determined by environment not sex chromosomes
how does retinoic acid cause PGCs differentiate into oocytes
lipophilic hormone
* binds retinoic acid binding prot in cyt
* binding stims complex enter nucleus
* then RA moves to bind retinoic acid receptor - transcription factor in nuc -> causes genes turn on/off
gastrulation defn
pt in embryogenesis where org’s main body plan developed
== formation 3 embryonic germ layers from which all tissues + organs formed
where does gastrulation occur
starts at primitive streak - marks future posterior end embryo
gastrulation 1: formation primitive ectoderm
basement mem induces polarisation epiblast cells form pseudostratified epithelium = prim ecto
visceral endoderm role in gastrulation
anterior part secretes LIM1 to inhibit gastrulation - only happens posterior end embryo
also secretes factors for the head to develop
gastrulation 2: forming 3 embryonic germ layers
cells from primitive ectoderm loosen contact w each other = epithelial to mesenchymal transition (EMT) + migrate through primitive streak
1. some displace visceral endoderm, forming definitive endoderm
2. some positioned bet ectoderm + endoderm, forming mesoderm
cells migrating require expression transcr factor brachyury (T)
what is primitive streak
groove in midline epiblast
epithelial to mesenchymal transition (EMT)
e-cadherin (holds cells together) levels reduced
cells at prim streak can synth + respond FGFs - involved down-reg e-cad
caudal dysgenesis
abnormality due to toxins resulting in insufficient mesoderm
teratogen
toxin or something affecting embryo development
e.g. high levels RA
organs derived from endoderm
- lungs
- thyroid
- gut + liver
internal germ layer
organs/tissues derived mesoderm
- cardiac muscle
- skeletal muscle
- smooth muscle
- kidney
- gonads
- blood cells
tissues/organs derived ectoderm
- skin
- nervous sys
- pigment cells, esp melanocytes
external germ layer
ethical issues w synthetic embryos
illegal experiment human embryos past day 14 - same rules apply?
