Changes

*Embryogenesis is driven by three cellular process:

*Embryogenesis is driven by three cellular process:

#Cell division and growth

#'''Cell division and growth'''

#*Fertilisation produces a unicellular zygote which undergoes a series of mitotic divisions to eventually become a multicellular organism. Mitosis produces (i) growth by hyperplasia (ii) an increased number of cells for greater diversity of function, and thus more complex organism.  

#*Fertilisation produces a unicellular zygote which undergoes a series of mitotic divisions to eventually become a multicellular organism. Mitosis produces (i) growth by hyperplasia (ii) an increased number of cells for greater diversity of function, and thus more complex organism.  

#Cell differentiation

#'''Cell differentiation'''

#*As embryogenesis progresses, cells become specialised in structure and function. Regulation of gene expression allows different proteins to be expressed in some cells, allowing different tissues to exists despite all cells containing the same DNA. At the blastomere stage (16 cells) cells are '''totipotent''', meaning they have the capacity to form every adult cell type. As the embryo progresses and enters gastrulation cells become '''pluripotent''', meaning they can form several but not all cell types. Eventually, most cells terminally differentiate and can no longer form any other cell type.   

#*As embryogenesis progresses, cells become specialised in structure and function. Regulation of gene expression allows different proteins to be expressed in some cells, allowing different tissues to exists despite all cells containing the same DNA. At the blastomere stage (16 cells) cells are '''totipotent''', meaning they have the capacity to form every adult cell type. As the embryo progresses and enters gastrulation cells become '''pluripotent''', meaning they can form several but not all cell types. Eventually, most cells terminally differentiate and can no longer form any other cell type.   

#Morphogenesis

#'''Morphogenesis'''

#*During embryogenesis two cell types exist; mesenchymal and epithelial. Mesenchymal cells are single or loosely linked to other cells and irregularly shaped. Epithelial cells are tightly attached to each other or a membrane and have a regular shape (cuboidal or columnar).

#*During embryogenesis two cell types exist; mesenchymal and epithelial. Mesenchymal cells are single or loosely linked to other cells and irregularly shaped. Epithelial cells are tightly attached to each other or a membrane and have a regular shape (cuboidal or columnar).