HUMAN EMBRYOLOGY

1. FERTILIZATION AND ZYGOTE FORMATION2. CLEAVAGE3. GASTRULATION.4. HISTO- AND ORGANOGENESIS

PERIODS OF EMBRYOGENESIS

CHARACTERISTICS OF GAMETES

OOCYTE SPERMATOZOA

CHARACTERISTICS OF GAMETES

FERTILIZATION

FERTILIZATION

FERTILIZATION

CLEAVAGE

CLEAVAGE

IMPLANTATION

EARLY GASTRULATION

EXTRAEMBRYONIC ORGANS FORMATION

LATE GASTRULATION

DIFFERENTATION OF GERM LAYERS

22-23 days pf Human

23-26 days pf Human

Formation of the Neural Tube

• Secondary Neurulation

1. Occurs beyond the caudal neuropore

2. lumbar and tail region

3. Exclusive mechanism for fish

4. Starts with formation of medullary cord

5. Cavitation of cord to form hollow tube

Differentiation of Neural Tube

• Major morphological changes: differentiation of brain vesicles and spinal cord

• Differentiation of neural tube cells• Development of peripheral nervous system

Neural Crest Cells

• Migration:– Epithelial to mesenchyme transition

– Migrational pathways are established by juxtacrine signals:

• Fibronection, laminin in ECM + integrins

• Ephrin proteins: Restrict movement

• Contact inhibition

• Use of existing structures

– Migration ceases when these signals are reversed

Neural Crest Cells

• Differentiation:– Largely based on location along neural tube and their

migration route:

Neural Crest Cells• Differentiation:

– Migration routes along trunk:– Ventral pathway: cells move through anterior portion of somite

toward ventral side of embryo• Cells become: sensory neurons, sympathetic ganglia,

medulla of adrenal gland– Dorsolateral pathway: cells move between epidermis and somite

• Cells become: melanocytes

• Basic organization of the PNS is established by the migratory pathways of the neural crest cells