EMBRYO

• Embryology


  Study of embryos
• Embryology


  • Deals with the descriptive study of embryos
  • Traditional: a study of structural changes in embryonic development
  • Contemporary: a study of embryonic developmental processes of integrated complex phenomena
• Embryo


  Early stage when the developing animal does not yet resemble the adult of the species
• Developmental biology


  • An analysis of biological development
  • How a single cell zygote (simple) get transformed into a multicellular embryonic stage (complex)
  • How the individual cells of this multicellular embryonic stage, that individual cells that starts with a simple one get transformed into specialized cells with distinct structures correlated with their functions
• Significance of increasing knowledge in embryology/animal developmental biology

  • To understand normal and abnormal development
  • To understand better the mechanism of such development (normal & abnormal)
  • Understanding normal and abnormal development, and the mechanisms of these developments will give you a springboard
• Foundations for


  • Embryology or developmental biology starts with the onset of fertilization and ends with birth, hatching, and metamorphosis
  • Ending in birth in the case of mammals
  • Ending in hatching in the case of avians, reptilians
  • Ending in metamorphosis in the case of amphibians
• Birth, hatching and metamorphosis


  Convenient landmarks in the continuing process of development
• Types of development

  • Ontogenetic development: refers to the development of immune individual from a fertilized oocyte in the case of sexual reproduction or development of immune individual budding off from afferent organism in the case of asexual reproduction
  • Phylogenetic development: pertains to evolutionary development of a species. This is more on the evolution side. Development from a simple form of life into a diversity of more complex forms of life
• In embryology and development biology, starts with gametogenesis
• Gametogenesis


  Generation of the specialized cells that are involved in fertilization, these are the gametes
• Gametogenesis


  • Generating and maturation of oocytes, oogenesis
  • Generation and maturation of the spermatozoa, spermatogenesis
• Mosaic development


  • Where the fate of a cell depends upon specific cytoplasmic determinants in the zygote
  • Cytoplasmic determinants are unequally asymmetrically distributed into the daughter cells during cleavage division
  • Belongs to snails, gastropods, bivalvia
  • Invertebrates like molluscs make use exclusively of mosaic mode development
  • If part of the embryo is removed, certain cell types would be lacking in later stage development or will not develop into a normal larva
  • If a blastomere is isolated, it cannot develop because the other parts were left from the original side of the embryo
• Regulative development


  • Where the fate of the cell depends upon interactions with neighbor cells, not by what piece of cytoplasm it has acquired during cell division
  • Cell to cell interactions, these involve the signaling factors having an influence on the development of the cell or the cell to follow a certain developmental pathway
  • In other words, regulative development, the fate of the cells is not limited early. Cells can give rise to any cell type of the body. They have the total and unlimited potential.
  • A characteristic feature of vertebrates
  • Vertebrates utilize regulative development though in combination with mosaic development
  • When a blastomere is isolated early in cleavage, it can form a new complete individual
• Key processes of development

  • Cleavage division
  • Pattern formation (body axes formation)
  • Morphogenesis
  • Cell differentiation
  • Growth
• Scope of embryology

  • Gametogenesis
  • Fertilization
  • Cleavage
  • Blastulation
  • Gastrulation
  • Neurulation/organogenesis
• Neurulation


  Main goal is the establishment of the precursor of the central nervous system. This is the officially start of organogenesis
• Organogenesis


  • Establishment of body organs
  • Organs are not yet functional
  • Precursor cells of an organ is established, they are not yet functional, they start out as few
  • From the few cells that are of the same kind, they get to increase the population by mitotic division, hence, there becomes a smooth blending of organogenesis with histogenesis
• Histogenesis


  • Histo means tissues
  • In organogenesis, there is the multiplication increase in the population of the cells for that particular organ and as the cells increase in number they form an aggregate tissue
  • And as tissues mature, they slowly acquire specific functions. This is a function in addition to the basic functions of the cell like energy metabolism, protein synthesis, housekeeping functions.
  • The organs starts to form a specific functions
  • The cells gradually undergo differentiation
• Differentiation


  Specialization in specific structures and functions
• Histogenesis, differentiation, pattern formation and morphogenesis - thin line separating these processes, they are almost taking in simultaneously, overlapping processes
• Resulting in fetal growth
• General problems of developmental biology

  • Problem of differentiation
  • Problem of morphogenesis
  • Problem of growth
  • Problem of reproduction
  • Problem of evolution
  • Problem of environmental integration
• Morphogenesis


  Investigates how this regulation of cell fates contributes to the form and structure of the organism and its component parts
• Important basic concepts in embryology/developmental biology

  • Concept of guidelines
  • Concept of fate, potency, determination
  • Concept of capacity and competence
  • Concept of embryonic induction
  • Concept of regulation
  • Concept of inevitability
  • Concept of differentiation
  • The hox genes
• Concept of guidelines


  • Directive influences on embryonic development
  • Preformed guidelines: Present right at the start of ontogeny, Maternal genes/maternal effect genes, Oocyte cytoarchitecture
  • Progressively-formed guidelines: Appear gradually in every step of ontogeny, Zygotic genes - fusion of the maternal and paternal genes that resulted in the nuclear fusion during the fertilization
• Maternal effect genes/factors in amphibian and fish oocyte

  • Balbiani body (at the vegetal pole) - accumulation of mitochondria and cytoplasmic granules (germ granules) containing silenced mRNAs
  • Maternal mRNAs are organized in cytoplasmic granules together with several regulatory proteins responsible for their post-transcriptional processing and thus translational regulation
• Egg cytoarchitecture


  Affects the cleavage pattern of the oocyte
• Anterior-Posterior axis


  • Coupled to Gastrulation
  • Developmental potential and inducing properties of cells in the dorsal lip of blastopore (DLB) change with time
  • Early cells in the DLB → anterior mesoderm → neural tissue
  • Latter cells of the DLB → posterior → induce posterior neural structures
  • Wnt signal activity: High in posterior, Low in anterior
• Concept of fate


  • What cells would become
  • Dependent on: Cell asymmetries, Unequal cytoplasmic determinants, Inductive information, Morphogens
• Oct4


  Required for the maturation of ICM (inner cell mass)
• Cdx3


  Required for the maturation of TE (trophectoderm)
• Concept of potency


  • Ability of a cell to follow a developmental pathway
  • Embryonic stem cells: Unspecialized, Can undergo unlimited self-renewal
  • Totipotent: Total potential → any cell
  • Pluripotent: Can differentiate → any body tissue (cannot support full development of the entire organism/embryo)
  • Multipotent: Differentiate → different cell types within given lineage
  • Unipotent: Fully specialized, Can generate its own specific type
• Concept of determination


  • Gradual commitment to a certain cell fate
  • Geared to follow a certain development pathway
• • Cell lineage determination during embryogenesis and generation of pluripotent embryonic cells
  • The three primary germ layers form during normal development (path 1)
  • Embryonic stem cells from the inner mass (path 2) or
• Morphogens


  Chemical substances in the developing embryo that guide body formation
• Potency


  Ability of a cell to follow a developmental pathway
• Types of potency

  • Embryonic stem cells (unspecialized, can undergo unlimited self-renewal)
  • Totipotent (total potential, can become any cell)
  • Pluripotent (can differentiate into any body tissue, cannot support full development of the entire organism/embryo)
  • Multipotent (can differentiate into different cell types within a given lineage)
  • Unipotent (fully specialized, can generate its own specific type)
• Determination


  Gradual commitment to a certain cell fate, geared to follow a certain development pathway
• Cell lineage determination during embryogenesis and generation of pluripotent embryonic cells

  1. Path 1: The three primary germ layers form during normal development

  2. Path 2: Embryonic stem cells from the inner mass

  3. Path 3: Embryonic germ cells from the gonadal ridge
• Embryonic induction


  Evocative influence of cells, within a cell or a cell with a neighboring cell or between cells, involves capacity and competence