Human Development

Ear Development:

• Develop from placodes (ectodermal thicking visible on the surface).
• Important for sensory organ formation. 
• Otic placode visible from week 4, disappears from the surface by week 5. 
• Otic placode forms components of the inner ear. 

Eye Development:

• Develops from an optic placode. 
• Visible from the end of the 4th week. 
• It forms a lens. 

Nervous System:

• Apparent after gastrulation, 19-21 days post fertilisation.
• The neural tissue forms the ectoderm. 
• The neural plate will give rise to the brain (cephalic region)
• The neural tube gives rise to the spinal cord (caudal region).

NEURAL TUBE CLOSURE:

• Neural plate rolls up to form neural tube with a neural crest forming at the boundary with ectoderm.
• 23-26 days post fertilisation- Folding of neural ectoderm to form a  tube.
• Failure to do so results in open neural tube disorders.

NEURAL CREST FORMATION:

• Cells at the tips of neural folds do not participate in neural tube formation.
• They remain between the dorsal surface of neural tube and ectoderm.
• Later they will migrate to other locations e.g. melanocytes of skin, bone, cartilage, connective tissue of face.

SOMITE DEVELOPMENT:

• 44 pairs total, added from cephalic to caudal end.
• Added alongside neural tube in pairs.
• Appear first around 19 days post fertilisation.
• New somite pair every 90 minutes.
• Produce muscle, vertebral and rib bones. 

• 4th week- External structures visible.
• They present as buds that protrude outwards.
• Patterning is important to specify proximal, distal, dorsal, ventral digits.
• Forelimb bud presents first, and the hindlimb bud follows. 
• Outgrowth processes from limb buds make hands and feet distinguishable. 
• Condensations of cartilage show precursors of digits. 
• Week 8- Apoptosis between digits causes separation.
• Bone growth is occurring too. 

• 22-23 days post fertilisation, when neural tube is closing. 
• Visible on the ventral surface (anterior).
• Continues through to week 8.

• Grows via branching morphogenesis.
• Endoderm and mesoderm supply most of the alveoli. 
• Ectoderm provides neural innervation. 
• Mesoderm provides musculoskeletal support.

• Grows via branching morphogenesis, starting from ureter.
• Grows in close proximity to genitals.
• Undergoes 3 stages of development:

1. Pronephros day 18
2. Mesenephros day 24
3. Metanephros day 35

• Week 2-3 -Initially arises from endoderm.
• After week 4, other germ layers contribute. 

1. Mesoderm = mesentary, smooth muscle, blood vessels.
2. Ectoderm = enteric nervous system.

• Regions of the gut are divided into 3 cavities:

1. Foregut = oral cavity, oesophagus, trachea, stomach.
2. Midgut = small intestine and pancreas (herniated during development).
3. Hindgut = colon