CHAP 1

• Two major objectives of development

  • It generates cellular diversity and order within the individual organism
  • It ensures the continuity of life from one generation to the next
• Fundamental questions in developmental biology

  • How does the fertilized egg give rise to the adult body?
  • How does that adult body produce yet another body?
• Differentiation

  The generation of cellular diversity - how a single fertilized egg cell can generate so many different cell types
• Regeneration

  How some organisms can regenerate parts of their bodies, and how stem cells retain this capacity
• Environmental integration

  How the development of organisms is influenced by environmental cues
• Evolution

  How changes in development create new body forms over generations
• For animals, fungi, and plants, the sole way of getting from egg to adult is by developing an embryo
• The developmental biologist usually finds the transient stages leading up to the adult to be the most interesting
• Stages of development between fertilization and hatching (or birth)

  • Fertilization
  • Cleavage
  • Gastrulation
  • Organogenesis
  • Birth
  • Metamorphosis
  • Gametogenesis
• Pronuclei


  The subsequent fusion of the gamete nuclei, on which only half the normal number of chromosomes characteristic for the species gives the embryo its genome
• Gastrulation


  A series of extensive cell rearrangements that result in the embryo containing three germ layers (endoderm, ectoderm, and mesoderm)
• Metamorphosis


  The dramatic change in form and often habits of an animal during its development after birth or hatching
• Cleavage and Gastrulation


  1. The volume of the egg stays the same, but it is divided into tens of thousands of cells

  2. Gastrulation in amphibians begins with the formation of a dimple called the blastopore, where cells migrate inward to form the mesoderm, endoderm and ectoderm
• Organogenesis


  1. The notochord produces chemical signals that redirect the fate of the ectodermal cells above it to become the nervous system

  2. The mesodermal tissue adjacent to the neural tube and notochord become segmented into somites, the precursors of back muscles, vertebrae and dermis

  3. The embryo develops a mouth and anus, and elongates into the familiar tadpole structure
• Triploblastic

  An animal whose body arises from three primary germ layers - ectoderm, endoderm and mesoderm
• Metamorphosis and Gametogenesis

  1. Almost every organ of the tadpole is subject to modification as it transforms into the adult frog

  2. As metamorphosis ends, the development of the germ cells (sperm and egg) begins, with the germ cells undergoing meiosis to become mature and capable of fertilization
• Holoblastic

  Characterized by complete cleavage that divides the whole egg into distinct and separate blastomeres
• Meroblastic


  Characterized by incomplete cleavage as a result of the presence of an impeding mass of yolk material
• William Harvey conclude that all animals even mammals originate "EX OVO OMNIA" ("All from the egg")

  1651
• Harvey was the first to see the blastoderm of the chick embryo
• Harvey first noticed that "island" of blood tissue forms before the heart does
• Preformationism


  The organs are already present in miniature form within the egg or sperm
• Epigenesis was supported by Aristotle and Harvey while preformationism was supported by Maphigi
• Factors determining pattern of embryonic cleavage

  • Amount and distribution of yolk protein within the cytoplasm
  • Factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation
• Isolecithal


  Eggs with sparse, equally distributed yolk
• Gastrulation


  • Cells of the blastula are given new positions and new neighbors
  • The multilayered body plan of the organism is established
  • Endodermal and mesodermal organs are brought to the inside
  • Cells that will form the skin and nervous system are spread over the outside surface
  • The three germ layers (ectoderm, endoderm, mesoderm) are first produced
• Types of cell movements in gastrulation

  • Invagination
  • Involution
  • Ingression
  • Delamination
  • Epiboly
• Ectoderm


  Generates the outer layer of the embryo, produces the surface layer (epidermis) of the skin and forms the brain and nervous system
• Mesoderm


  Becomes sandwiched between the ectoderm and endoderm, generates the blood, heart, kidney, gonads, bones, muscles, and connective tissues
• Cellular processes in morphogenesis


  • Direction and number of cell divisions
  • Cell shape changes
  • Cell migration
  • Cell growth
  • Cell death (Apoptosis)
  • Changes in the composition of the cell membrane or secreted products
• Homologous structure


  Organs whose underlying similarity arises from their being derived from a common ancestral structure
• Between 2% and 5% of human infants are born with a readily observable anatomical abnormality