Mechanisms of Disease During Embryogenesis
Learning Objectives:
Have an appreciation of the complexity involved in embryonic development
Understand the difference between genetic and environmental causes for congenital
disease
Be able to identify examples of birth defects and relate them to a specific
developmental process
2 main periods of human development
Embryonic period: until end of week 8 where most of organogenesis occurs
Fetal period: remaining time in utero, involves growth and modelling
Mechanisms of development (order)
Egg → fertilisation → cleavage → gastrulation → neurulation and somitogenesis →
organogenesis → adult
Overview of Developmental Stages
Day 1: Fertilised zygote which is surrounded by a thick protective membrane called
the zona pellucida
o 2 polar bodies are also produced due to the unequal cell division after a
secondary oocyte divides. So 1 set of chromosomes are pulled aside and form
the first polar body.
Cleavage stages: Fertilised egg moves along uterine tube and divides as it goes along
(1 cell zygote → 2 cell zygote → 4 cell zygote → 8 cell zygote)
Day 3: 16 cell zygote then forms the Morula. The zona pellucida then starts to
dissolve or is shed off the Morula slowly.
Day 4/5: Morula turns into a Blastocyst (32 cells) - this still has the zona pellucida.
They then implant onto the uterine wall (in order to derive nutrition).
o Deriving nutrition, giving away wastes & exchanging gases can be served
through a barrier = this barrier is an organ called the placenta.
Day 6/7: The blastocyst develops a cystic cavity called a blastocele. This contains a
trophoblast cell layer.
o Trophoblast cell layer are cells that form the outer layer of a blastocyst. They
provide nutrients to the embryo & develop into a large part of the placenta.
Gastrulation
Transformation of bilaminar disc (epiblast and hypoblast) → into trilaminar disc
(ectoderm, mesoderm & endoderm)
Hypoblast layer becomes displaced by involuting cells that later form mesoderm &
endoderm
Ectoderm (outer layer)
Epiblast layer → Ectoderm layer → Skin, Nervous Systems & Pituitary
Mesoderm (middle layer)
In the middle of Epiblast and Hypoblast layer → Mesoderm layer → MSK, CVS, Repro
systems & Connective Tissue
Endoderm (inner layer)
Learning Objectives:
Have an appreciation of the complexity involved in embryonic development
Understand the difference between genetic and environmental causes for congenital
disease
Be able to identify examples of birth defects and relate them to a specific
developmental process
2 main periods of human development
Embryonic period: until end of week 8 where most of organogenesis occurs
Fetal period: remaining time in utero, involves growth and modelling
Mechanisms of development (order)
Egg → fertilisation → cleavage → gastrulation → neurulation and somitogenesis →
organogenesis → adult
Overview of Developmental Stages
Day 1: Fertilised zygote which is surrounded by a thick protective membrane called
the zona pellucida
o 2 polar bodies are also produced due to the unequal cell division after a
secondary oocyte divides. So 1 set of chromosomes are pulled aside and form
the first polar body.
Cleavage stages: Fertilised egg moves along uterine tube and divides as it goes along
(1 cell zygote → 2 cell zygote → 4 cell zygote → 8 cell zygote)
Day 3: 16 cell zygote then forms the Morula. The zona pellucida then starts to
dissolve or is shed off the Morula slowly.
Day 4/5: Morula turns into a Blastocyst (32 cells) - this still has the zona pellucida.
They then implant onto the uterine wall (in order to derive nutrition).
o Deriving nutrition, giving away wastes & exchanging gases can be served
through a barrier = this barrier is an organ called the placenta.
Day 6/7: The blastocyst develops a cystic cavity called a blastocele. This contains a
trophoblast cell layer.
o Trophoblast cell layer are cells that form the outer layer of a blastocyst. They
provide nutrients to the embryo & develop into a large part of the placenta.
Gastrulation
Transformation of bilaminar disc (epiblast and hypoblast) → into trilaminar disc
(ectoderm, mesoderm & endoderm)
Hypoblast layer becomes displaced by involuting cells that later form mesoderm &
endoderm
Ectoderm (outer layer)
Epiblast layer → Ectoderm layer → Skin, Nervous Systems & Pituitary
Mesoderm (middle layer)
In the middle of Epiblast and Hypoblast layer → Mesoderm layer → MSK, CVS, Repro
systems & Connective Tissue
Endoderm (inner layer)
