Embryology

Zygote: fertilized egg (1 cell), divides through mitosis

Embryogenesis: stages of development between fertilization and birth

Processes of embryogenesis:

·         Fertilization: fusion of the gametes

·         Cleavage: series of mitotic cell divisions after fertilization. The cytoplasm forms many small cells, called blastomeres, which form a sphere (blastula)

·         Gastrulation: Blastomeres change their relative positions; the three germ layers of the embryo form (endoderm, mesoderm, ectoderm)

·         Organogenesis: Cells interact with one another and rearrange themselves to produce tissues and organs

·         Metamorphosis: in some animals, larva become sexually mature adult organisms

·         Gametogenesis: germ cell differentiation occurs

       Epigenesis: the view that organs of an embryo are formed de novo (from scratch) each generation

        Preformation: the view that organs are already present, in miniature form, within an egg and a sperm.

     The Preformation theory is flawed n that it is unable to account for intergenerational genetic variations

Now, most biologists believe that instructions for the formation of the organism are present in a fertilized egg

    Primary germ layers and early organs
        Pander discovered germ layers: ectoderm, mesoderm and endoderm


      Ectoderm: outer layer of embryo, produces epidermis, brain and nervous system
      Mesoderm: generates blood, connective tissue, heart, kidneys, gonads, bones and muscles
     Endoderm: inner layer of embryo, forms epithelium of digestive tube and associated organs

   The four principles of Karl Ernat Von Baer
        1. The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group

     2. Less general characteristics develop from the more general until finally the most specialized appear

       3. An embryo does not pass through the state/formations of other organisms but instead separates itself from them
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        4. The embryo of a higher form never resembles an adult of another form, but only an embryo of its own form 



     Blastospore: dimple, marks fixture dorsal (top) side of embryo. Formed at the beginning of gastrulation.

    The blastospore expands to form a ring. Cells that migrate through it become mesoderm
   The endoderm cells are the large yolky cells in the vegetal hemisphere.

   Notochord: rod of mesodermal cells in the most dorsal portion of the embryo. Signals ectodermal cells above it to form a tube and become the nervous system instead of forming the epidermis

    Neural precursor cells elongate, stretch and fold into the embryo, forming the neural tube
    Mesodermal cells near the neural tube and notochord are segmented into somites

    Somites: precursors to the back, skeleton and spine 

      Sources: "Developmental Biology" Tenth Edition - Scott F Gilbert