CONCEPT 15. Development

• What is embryogenesis?

  The development of a multicellular organism from a zygote
• What is a zygote?

  The first cell of an embryo, product of a fertilisation event
• As the zygote undergoes multiple rounds of cellular division, specific cell types will arise in specific patterns and positions along major spatial axes.

  
• The arrangement of cells is organised and cells are arranged according to a body plan.
• What does cell fate establishment refer to?

  Also known as determination, during the early stages of embryogenesis, cells begin to express certain genes and become increasingly more restricted in their fate.
• Cell potency: the cell's ability to differentiate into other cell types
• If a cell has high potency, it has the ability to differentiate into more cell types
• What does cell fate establishment achieve?

  Cells become committed to a specific path of differentiation
• Since determination occurs in a highly ordered pattern, it is suggested that instructive cues are present to determine the pathway of a cell.
• What forms may instructive cues take?

  Cytoplasmic factors or cell signalling molecules
• Cell fate is associated with changes in the pattern of gene expression, dictated by instructive cues from internal signals or interactions with neighbouring or more distant cells
• All cells contain a complete genome, but it is the expression of different genes that give rise to the different cell types.
• What is morphogenesis?

  The process in which cells and tissues organise and arrange themselves to create the final form of the body
• Morphogenic processes: cell division, changing cell shape through expansion, movement of cells, adhering to one another and apoptosis
• The final body is comprised of many types of specialised cells, which come together into coherent tissues and organs that make up the functioning organism.
• If all cells contain the same genome, how are we able to obtain specialised cells?

  Expression of cell different genes that determine the cell's morphology and function
• What are house-keeping genes?

  Genes expressed by all cells, involved in fundamental biological processes such as cellular respiration
• What occurs in morphogenesis of plants?

  • Precise orientation of cell division
  • Cells expand in size
  • No cell movement in embryogenesis
  • Establishment of body plans
• What occurs in morphogenesis of animals?

  • Random cell division/orientation
  • Cells expand and can change shape
  • Cells move and adhere to others forming tissue
  • Establishment of basic body plans
• What growth pattern do animals have?

  Determinate growth where all organs and tissue types are formed during embryogenesis thus predetermined body form increases in size
• What growth pattern do plants have?

  Indeterminate growth where few organs and tissue types are formed thus new organs and tissues are continually generated through the activity of meristems thus a flexible body plan
• Natural selection states that the most successful organisms are more likely to survive and reproduce and is a means of evolution
• Hopeful monsters describe small changes in DNA that lead to large changes in phenotype through the alternation of master genes which regulate the expression of other genes.
• Key stages of animal development

  • Fertilisation: fusion of gametes to produce a zygote
  • Cleavage: zygote divides to form the blastula
  • Gastrulation: body plan and germ layers are established
  • Organogenesis: formation of tissues and organs
  • Metamorphosis: larvae transforms into adult form
• What occurs in cleavage?

  The zygote rapidly divided and the simple arrangement of undifferentiated cells form the blastula
• What occurs in gastrulation?

  Blastula undergoes dramatic cellular rearrangement, establishing the body plan axes and germ layers
• Ectoderm. outer layer of cells that give rise to the skin and nervous system
• Endoderm. Inner layer of the embryo that gives rise to the lining of organs including the lungs, gut and pancreas
• Mesoderm. Middle layer of the embryo that gives rise to muscles, blood vessels and connective tissue including fat cells and bone
• Triploblastic. animals with three germ layers with bilateral symmetry
• Diploblastic. simple animals with two germ layers, lacking the mesoderm with radial symmetry such as jellyfish and hydra
• The more similar the development of two animals is, the closer they are in the evolutionary tree
• Stem cells are self-renewing undifferentiated cells that enable the replacement of cells in tissues where cell death routinely occurs e.g skin and gut lining
• Stem cell division results in the production of a new stem cell and a daughter cell which can differentiate. These cells have an unlimited capacity to divide and produce more cells.
• Totipotent. cells with the potential to differentiate into all cell types e.g zygote
• Pluripotent. cells with the ability to differentiate into all cell types excluding extra-embryonic tissue such as the placenta e.g embryonic stem cells and induced pluripotent stem cells
• Multipotent. cells that can produce several cell types but are restricted e.g intestinal stem cells
• Unipotent. cell that can only produce one cell type e.g skin cells
• Cancer develops from the irregulation of developmental mechanisms including migration and division
• Meristems are plant tissues containing undifferentiated cells which can perpetually divide, facilitating plant growth