Embryology - 2 Flashcards
What is embryogenesis?
First eight weeks of embryo development post fertilisation
Describe week 2-3 in embryoblast development
Formation of bilaminar disc and development of amniotic sac, yolk sac, extraembryonic mesoderm and chorionic cavity
Describe week 3-4 in embryoblast development
Differentiation of bilaminar disc into trilaminar structure - 3 germ layers through process of gastrulation and neural tube formation
What happens in week 5-8 of embryoblast development?
Organogenesis
What period begins after week 8 of embryoblast development?
Foetal period
What cells are in the inner cell mast of the blastocyst?
Pluripotent cells
What is the fluid filled cavity of blastocyst?
Blastocoele
What does the inner cell mass differentiate into?
Bilaminar epiblast and hypoblast
2 cavities form - yolk sac on side of hypoblast and amniotic cavity on side of epiblast
What does the epiblast differentiate into?
3 germ layers - ectoderm, mesoderm and endoderm in process of gastrulation - day 13 to 16
What does gastrulation start with?
Groove appearing in the caudal end of the epiblast called the primitive streak
How is the endoderm formed?
Epiblast cells migrate into hypoblast layer displacing them
How is the mesoderm formed?
Epiblast cells migrate through primitive streak between the epiblast and hypoblast
What is the gastrula?
Newly formed trilaminar disc with 3 layers which forms the different organ systems
What does the ectoderm give rise to?
Epidermis, CNS, PNS, hair, nails, neuroendocrine organs and enamel of teeth
What does mesoderm give rise to?
Dermis, musculoskeletal structures, CVS, kidneys, ureters, bladder, gonads, adrenal cortex, visceral and parietal linings
What does endoderm give rise to?
Lining of gastrointestinal tract, parenchyma of liver, pancreas, thyroid, parathyroid, tonsils, thymus, bladder and urethra
What are the major subunits of the mesoderm?
Paraxial mesoderm
Intermediate mesoderm - kidneys, gonads and ureter
Lateral plate mesoderm
What are the 2 reasons for multiple pregnancy to occur?
Dizygotic - two or more eggs fertilised and implanted so non-identical
Monozygotic - one embryo splits into 2 so identical
What does the monozygotic twin types depend on?
The timing of the single embryo dividing
What is the type of twin is it if division day is day 1-4?
Dichorionic diamniotic - 20% incidence
Separate placenta and amniotic
What type of twin is it if division day is day 5-8?
Monochorionic diamniotic - 75%
Share placenta and separate amniotic sac
What type of twin is it if division day is day 7-14?
Monochorionic monoamniotic - 5%
Share placenta and amniotic sac
What type of twin is it if division day is more than 14 days?
Conjoined twins - less than 1%
