Introduction

• Field of biology that has evolved from embryology
• Embryology is the study of embryos
• Embryology has evolved into developmental biology, which is now more focused on the analysis of biological development
• Developmental biology focuses on how a single cell from a zygote transforms into a multicellular embryonic stage
• Developmental biology focuses on how individual cells transform 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 the mechanisms of development
  • Foundations for the development of new techniques for prenatal diagnosis and treatments
  • Therapeutic procedures to circumvent problems of infertility
  • Interventions to prevent birth defects and address abnormalities
  • Stem Cell Research to help in the improvement of prenatal development and long-term postnatal effects
• Developmental biology starts with the onset of fertilization and ends with birth, hatching, and metamorphosis
• Development is a series of uninterrupted correlated events
• Ontogenetic development is the development of a new individual via sexual reproduction
• Sexual reproduction connotes the development of a new individual from a fertilized oocyte
• Asexual reproduction is a new individual budding off from a parent organism
• Phylogenetic development is the evolutionary development of a species
• Gametogenesis is the production of gametes that will be fertilized
• Major accomplishment of ontogenetic development:

  • Generation of cell number (Growth)
  • Cellular diversity within generation (Differentiation)
  • Cellular order within generation (Morphogenesis)
• Modes of ontogenetic development:

  • Mosaic Development:
  • Fate of an embryonic cell depends on specific cytoplasmic determinants in the zygote
  • Ex. molluscs use exclusively mosaic mode of development
  • Each cell varies in cytoplasmic determinants gained during cell division, leading to limited developmental potential
  • Regulative Development:
  • Fate of a cell depends on interactions with neighboring cells
  • Relies on cell-cell interactions with unlimited potential
  • Characteristic feature of vertebrates
  • Removal of one cell is compensated by remaining cells
• Key processes of development:

  • Cleavage Division
  • Pattern formation (body axes formation)
  • Morphogenesis
  • Cell Differentiation
  • Growth
• Scope of embryology includes:

  • Gametogenesis
  • Fertilization
  • Cleavage
  • Blastulation
  • Gastrulation
  • Neurulation/Organogenesis
  • Histogenesis and Differentiation
  • Fetal Growth
• Fundamental questions addressed in developmental biology:

  • How does the fertilized egg give rise to an adult?
  • How does the adult provide yet another body?
• Problem of Differentiation:

  • How does the same genetic information result in different cell types?
  • How can the fertilized egg generate different cell types?
  • What drives the differentiation of cell types considering all cells are genetically identical and equal?
• Zygote progresses to blastula and then gastrula
• Blastula stage consists of big (macromere) and small (micromere) blastomeres
• Embryo towards the gastrula stage shows the generation of different types of embryonic cells known as primary germ layers: ectoderm, mesoderm, and endoderm
• Problem of Morphogenesis
• Cells are not randomly distributed in the body but are positioned in a specific manner at the right place and time
• Problem of Growth
• Cell divisions and growth are tightly regulated
• Embryonic cells are dictated to stop dividing
• Problem of Reproduction
• Special reproductive cells are set apart during embryonic development
• Only germ cells pass characteristics onto the offspring
• Problem of Evolution
• Changes in development create new body forms
• Importance of distinguishing between analogous and homologous structures
• Problem of Environmental Integration
• Organism's phenotype is influenced by the environment
• Example: Wing color of butterflies is influenced by exposure during their caterpillar stage
• Important Basic Concepts in Developmental Biology
• Concept of Guidelines: Directive influences on embryonic development
• Preformed Guidelines: Acquired early on, even before the start of ontogeny
• Maternal genes/maternal effect genes are present right at the start of ontogeny