Introduction to Embryology Flashcards
Define embryology
Study of formation and development of embryo and fetus
Define embryo
- Organism in process of development
- From 2nd to 8th week after fertilisation
Define fetus
- Developmental period from week 9 post-conception
Define embryogenesis
Progression from a single cell by establishing organ primordia
Why is embryology important?
- Prenatal diagnosis
- Preventing birth defects
- Fertility assistance
What has human genome sequencing allowed scientists to do while studying embryology?
- Cell lineage tracing
- Roles of individual genes and environmental factors in development
What are the three steps of Week 1?
OVULATION
FERTILISATION
MITOSIS AND IMPLANTATION
Describe ovulation in Week 1. PART 1
- Increased production of LH and FSH from anterior pituitary - follicle maturation
- Graafian folic matures - oocyte completes Meiosis I and initiates Meiosis II
- LH causes contraction. Ovarian wall ruptures - oocyte released.
Describe ovulation in Week 1. PART 2
- Fimbriae pass oocyte into fallopian tube which contracts (along with mucosal cilia) moving oocyte towards the uterus
- At site of rupture, corpus luteum forms - secretes progesterone.
- Uterus prepared for pregnancy
Describe fertilisation in Week 1.
- Occurs in ampulla of fallopian tube
- Penetration of corona radiata and then zone pellucida
- Fusion of sperm and oocyte membranes
- Zygote forms
What occurs during cleavage?
- Rapid cell division following fertilisation
- Day 1: Single cell zygote
- Day 2: Two cell stage
- Days 3-4: Eight cell stage (early morula)
- By Day 4: 16-cell stage (late morula). Zone pellucida disappears
What is each cell formed during cleavage known as?
Blastomere
By day 4, morula cells separate into 2 cell masses. What are these masses called and what is the consequence of their formation?
- EMBRYOBLAST - Inner mass
- TROPHOBLAST - Outer mass
- Blastocele forms and embryo now a blastocyst
What is the embryoblast and what happens to it during development?
- Future embryo
- Different into epiblast, hypoblast and AVE and organise themselves
What is the trophoblast and what happens to it during development?
- Future placenta
- Invaginate into endometrium of uterus
What is Week 2 of development characterised by?
- Bilaminar disc formation
- Cell differentiation
What occurs in the trophoblasts during cell differentiation in Week 2?
- Differentiate into cyto/syncytiotrophoblasts
- Holes (trophoblastic lacunae) form
What occurs in the embryoblasts during Week 2?
- Epiblast cell mass separates to form amniotic cavity
- Hypoblast cells extend to form exocoelomic membrane which surrounds exocoelomic cavity (primitive yolk sac)
What has happened by the end of Week 2 (Day 13)? PART 1
- Presence of extra embryonic cavity separating trophoblasts and embryo
- Primary yolk sac becomes secondary yolk sac
- Bilaminar sac forms
What has happened by the end of Week 2 (Day 13)? PART 2
- Amniotic cavity is present and a primitive placenta exists
What occurs in Week 3 of development?
- Gastrulation
- Epiblasts become ectoderm
- Epiblasts that invaginate through primitive streak form the mesoderm
- Hypoblast cells become endoderm
- Cephalo-caudal and left-right axes established
What occurs during Weeks 4-8 of development?
- Endoderm differentiates to form tubes e.g airways, the gut
- Mesoderm differentiates to form connective tissue, muscle and blood vessels
- Ectoderm forms skin, neural tissue
What occurs from Week 9 to birth (the fetal period)?
- Organ maturation
- Placental development
- Increase in weight (particuarly in last 2 months) and length( particularly 3rd-5th month)
- Slowdown in head growth
- Full term 40 weeks after onset of last menstruation (38 weeks from
conception)
How can genomes influence embryonic development?
- Genes transcribed to form proteins which can regulate gene expression
- Proteins can also acts as signalling molecules
What is induction?
- Group of cells and tissues causes change in activity of another group of cells/tissues
What does induction depend on?
- The competence of cells to respond to induction usually through cell signalling
Describe the Sonic Hedgehog Pathway.
- Responsible for vasculature and neural development
- Left-right axis formation
In detail, what occurs in the Sonic Hedgehog Pathway.
- SHH proteins binds to Patched (Ptc) protein receptor
- Binding stops Ptc inhibited of smoothened proteins
- Once activated, these proteins up regulate the GLI transcription factors which regulate expression of target genes
What is the Notch pathway needed for?
- Apoptosis
- Cell proliferation
- CVS and neural system development
In detail, describe the Notch pathway. PART 1
- Notch transmembrane receptors bind to DSL ligands
- Binding cleaves off the Notch intracellular domain (NICD)
In detail, describe the Notch pathway. PART 1
- NICD enters nucleus and binds to DNA binding protein inhibiting transcription of Notch target genes
- Binding of Notch removes inhibition so target genes can be transcribed.
Describe the the planar cell polarity pathway.
- Controls cell reorganisation within tissues
- Causes convergent extension - tissues become longer and narrower
In detail, outline what happens in the PCP pathway.
- Wnt proteins bind to Frizzles receptor.
- Wnt, along with CELSR and VANGL transmembrane proteins activate DVL protein
- DVL regulates kinase signalling causing cytoskeletal changes.
What are the consequences of abnormal PCP pathways?
- Neural tube defects
- Congenital heart disease
