lecture 14 - embryo development Flashcards
(21 cards)
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
