lecture 14 - embryo development Flashcards
What is preformationism
Sperm or egg contains miniature preformed organism
What is Epigenesis
Embryo develops progressively from undifferentiated egg
What are the stages of embryo development
oocyte/sperm maturation
Ovulation/ejaculation
Fertilisation
Implantation
Embryo development
Foetal development
Parturition (birth)
Human gestation
Describe early embryo development/cleavage
Week 1
- Fertilised oocyte, cleaved embryo, fragmented cell, blastocyst
Describe how humans hatch too
Day 6-7
- Expanded blastocyst (trophectoderm) into zona pellucida
Describe implantation
Week 2
- Blastocyst adhere to uterus wall
- bilaminar germinal disc (hypoblast, epiblast)
Describe granulation
Week 3
- Embryo change from blastula to a gastrula (multiple layers in cells)
- Inner cell mass forms two germ layers - Hypoblast (primitive endoderm) - Epiblast (embryonic cells)
- Epiblast form three germ layers - ectoderm (outer), mesoderm (middle), endoderm (inner)
- Cells proliferate, differentiate and move
Describe the germ layers
Endoderm - internal organs (internal)
Mesoderm - connective tissue (middle)
Ectoderm - skin, nerve cells (external)
Describe gastrulation
Week 3
- Formation of primitive streak/groove in epiblast
- Pass through primitive streak and new layers beneath epiblast
- New cell lineages
What forms in week 3
Neural groove -> neural tube (brain and spiral cord)
Somites (balls of mesoderm) -> connective tissue
Notochord - midline structure (not present in adults) - patterning signals by secreting sonic hedgehog
What happens at week 4
Begin formation of
- Gut tube
- Liver
- Genital ridge
- Neural tube, brain vesicles
- Heart begins to beat
Describe embryonic period
Week 5-8
- Heart descends into thorax
- Lung - buds form, descend into thorax
- Liver enlarges
- Ears - external form (cochlear otic vesicle)
- Eyes - retinal pigment present, eye & eyelids develop
- Limbs - buds form & elongate, hands/feet, fingers & toes lengthen
- Brain - cerebellum begins forming
- Skeleton - 33-34 cartilage vertebrae present, ossification of limb bones begins
Describe foetal period
Weeks 9-38
- Organogenesis largely completed
- Extensive growth
- Ongoing differentiation and development of organ system
What is the percentage of survival of a foetus
After 22 weeks - 15% survival
After 28 weeks - 90% survival - 1/3 significant morbidity
What can go wrong in an oocyte
Cellular and mononuclear mechanisms required for fertilisation and early embryo development inherited to oocyte (metabolites, organelles and mRNA)
>50% of fertilised eggs lost early in development
What happens to an oocyte during fertilisation
-cumulus expansion
- zona pellucida binding
- oolemma fusion
- oocyte activation
- Sperm processing
- Pronuclear formation
What happens to sperm during fertilization
- Motility
- Morphology
- Acrosome
- Concentration
What happens if things go wrong in the cleavage stage
First 2 weeks
- Exposure to teratogens usually cause complete loss of conceptus
- Abnormalities result in fertilisation or implantation failure
- No birth defects as no organs/structures developed yet
- Early embryo may compensate for damage
- Subtle effects on long term health (high BP, insulin resistance/diabetes)
What happens if things go wrong in the embryonic period
-Gastrulation, embryo folding, formation of organ & systems
- Active period of development and differentiation
- Most vulnerable to major birth defects
What happens if things go wrong in the foetal period
- mainly growth of organs & structures already developed
- Birth defects less sensitive
- Small size, mental retardation, defects in eyes, ears, teeth & external genitalia
What are periods of susceptibility
- Different organs susceptible to teratogens at different stages
- Heart forms earliest so sensitive sooner
- Complex organs - brain are susceptible longer
