Embryogenesis

• what happens after the neural tube is formed?

  the ectoderm, endoderm and mesoderm go onto forming their structures...

  
  

  for example, surface ectoderm starts to form the:

  - ears and the eyes

  - the sensory epitheliums like those in the nose mouth

  - the epidermis of fetal skin

  - hair, nails, sweat glands, and mammary glands.
• describe what the developing fetus looks like at this point

  - the neural tube is open on both ends (large opening at top- cranial neural pore, and the small opening at the bottom end- caudal neural pore)

  ----> eventually both pores seal up and the neural tube is fully formed

  
• what do the neural crest cells give rise to in the head?

  - Melanocytes

  - Cranial ganglia

  - Craniofacial skeleton

  - Odontogenic mesenchyme (involved in forming the tooth pulp) = ectomesenchyme
• what do the neural crest cells give rise to in the trunk?

  - Melanocytes

  - Sensory neurons

  - Sympathetic neurons

  - Enteric neurons

  - Glia
• what do neural crest cells do?


  migrate and form a new layer of cells between the ectoderm and neural tube.

  - they migrate out within the developing fetus to form a variety of tissues
• what are the neural crest cells?

  migrating cells from the dorsal neural tube
• what is the epithelial-mesenchymal transition?

  a process by which epithelial cells lose their cell polarity and cell-cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells

  
  

  -> mesenchymal stem cells are multipotent stromal cells that can differentiate into a variety of cell types.

  -> neural crest cells are a source of mesenchymal stem cells

  
• how are the body walls formed?

  Lateral sides of the embryo fold up and fuse towards the midline so what used to be lateral ends up ventral, and what used to be medial ends up dorsal

  
• What is Sonic Hedgehog responsible for?

  it is a signaling molecule expressed in the prechordal plate and neural plate

  - allows patterning

  - essential for eye field separation
• give an example of where it is important to pattern/shape the neural tube?

  for the separation of the eye field (Region within open neural plate so cells within there will later on contribute to the eye)

  
  

  - neural plate and prechordal plate need to be present so two eye fields can be created.

  
  

  -> if not it can lead to malformation of the maxilla and mandible
• What is critically important in patterning/shaping the neural tube?

  Signals from axial mesoderm
• where does the neural tube sit?

  between mesoderm and ectoderm
• how does the neural plate fold into the neural tube?


  - dips down forming a normal groove with edges called neural folds.

  - as the groove continues to grow ventral to the ectoderm layer, the neural folds come thioether and pinch of the surface of the ectoderm layer forming the neural tube
• what does the neural plate fold into?


  neural tube

  
• how does neurulation occur?

  the notochord stimulates the cells in the ectoderm layer to thicken and form the neural plate
• What is the neural plate?


  origin of the central nervous system
• What happens while the mesoderm is laid down?

  neurulation:

  Neural plate forms in ectoderm
• what does the time when cells leave the primitive streak define?

  their position along the head-tail axis

  (head to tail)

  
  

  - cells that leave first are rostral (head)

  - cells that leave later are caudal (tail)
• what are the somites?

  Somites are segmental axial structures of vertebrate embryos that give rise to vertebral column, ribs, skeletal muscles, and subcutaneous tissues

  
• what is the notochord?

  The notochord is an embryonic midline structure

  
• what does the position along the primitive streak define?

  the medial-lateral axis of the mesoderm

  (back to belly)

  
  

  ----> cells from tip of the streak move into the notochord

  = midline = dorsal

  
  

  ----> cells slightly more along the streak move into somites

  = paraxial = more ventral than notochord

  
• what are the body axes?

  - head-tail (rostral-central)

  - back-belly (dorsal-ventral)

  
• what do both the mesoderm and endoderm layers form?

  - lungs

  - pancreas

  - liver

  -thymus
• what does the endoderm layer form?

  - Gastrointestinal tract
• What do the different mesoderms form?

  
• What are the different types of mesoderm you can see through a medial-lateral (dorso-ventral) axis?

  mesoderm cells around the notochord differentiate into the different types of mesoderm

  
• what does the mesoderm layer form?

  - Vertebral column

  - Skeleton

  - Skeletal muscle

  - Circulatory system

  - Blood

  - Kidney
• what does the ectoderm layer form?


  - skin

  - nervous system
• describe how gastrulation occurs


  - primitive streak forms (site of gastrulation)

  - cells leave the epiblast and ingress into the primitive streak

  - ingressing cells form mesoderm and endoderm

  - cells that remain at the surface form ectoderm

  
• what is the inner cell mass of the blastocyst divided into?

  - epiblast (single celled layer sheet) = embryo proper

  - hypoblast = extraembryonic structures

  
• what are the 3 germ layers formed from gastrulation?

  - endoderm

  - mesoderm

  - ectoderm
• what is gastrulation?

  blastocyst goes through the generation of 3 germ layers
• what happens after the blastocyst is fully developed?

  it attached to the uterus lining and it becomes fully embedded into the endometrial tissues (tissue lining the uterus).

  Then gastrulation occurs
• what is the source of pluripotent embryonic stem cells?

  the inner cell mass of the blastocyst
• what does the outer group of cells of the blastocyst become?

  the cells that nourish and protect it.

  - the trophectoderm forms into trophoblast which later on will give rise to much of the placenta
• what does the inner group of embryoblast cells of the blastocyst become?

  the embryo
• What is a blastocyst?

  inner cell mass (embryoblast cells) enveloped by outer layer (trophectoderm).

  - also has a blastocoel (fluid filled cavity).

  
• describe what happens after fertilisation

  - the egg and sperm combine to form the zygote.

  
  

  - the zygote undergoes cleavage (it divides).

  first it divide into two cells called blastomeres.

  then it continues to divide until there are 16 cells.

  (the ball of 16 cells is called the morula)

  
  

  - the morula develops into a blastocyst:

  an outer layer cell mass (trophectoderm) forms

  an inner layer cell mass locates to one side of the blastocyst (embryoblast)

  It forms a blastocoel (fluid filled cavity).
• why do we need to study development?

  Understanding normal development is central to:

  • Understanding the causes & genetics of birth defects

  • Stem cell research

  • Regenerative medicine