Animals

• Zoology


  The study of animals
• Embryology


  The study of the process by which animals and plants grow and develop
• Characteristics of animals


  • Eukaryotes
  • Multicellular
  • Heterotrophs
  • No cell wall
• Caspar friedrich wolff: In the embryo of a chicken there is no small chicken, only a bunch of undifferentiated cells, making layers. The material out of which the embryo is constructed is, in an early stage of development, arranged in the form of leaf-like layers. Coined term "epigenesis" (embryo contains material to transform into animal)
• Layers in model embryo


  • Ectoderm
  • Mesoderm
  • Endoderm
• Ectoderm


  External layer that will generate epidermis and nervous system
• Mesoderm


  Intermediate layer that will generate muscles, reproductive organs, bones and most of the internal organs. It surrounds the peritoneal cavity
• Endoderm


  Internal layer that will generate the digestive tube, respiratory tube and urinary tube
• Stages of embryonic development


  • Fertilization
  • Cleavage
  • Gastrulation
• Fertilization


  Male and female gametes merge in a single cell
• Cleavage


  The large cell, the zygote, is divided without increasing size, blastula is formed
• Gastrulation


  Blastula is further divided now with growth. The three germ layers are formed now
• Change is determined: once it begins there is no turning back
• Purposes of fertilization

  • Get back to the diploid cells by merging two haploid nuclei
  • Initiate cell differentiation
• Specific recognition


  Sperm from a species must go to the egg of the same species, done by chemotaxis, important for animals with external fertilisation
• Avoiding polyspermy


  Only one sperm cell should enter the oocyte, done by modification of electrical potential of the membrane, release of a highly concentrated substance, and inflating to remove all sperm cells except the one already inside
• Cleavage


  The zygote is divided without increasing size, starting from one huge cell, division happens until cells are of a normal size. The zygote becomes first the morula (a compact group of cells), then the blastula (an empty sphere)
• Blastomere


  Each cell of the blastula
• Blastocoel


  The cavity inside the blastula
• Zygote polarity


  Two differentiated areas: animal pole where most dividing cells will be found, and vegetal pole where most of the vitellus will be found
• Differentiation patterns


  • Isolecithal
  • Mesolecithal
  • Telolecithal
  • Centrolecithal
• Holoblastic cleavage


  Eggs with little or intermediate amount of yolk divide completely
• Meroblastic cleavage


  Eggs with a lot of yolk cannot divide completely, as the yolk is an impediment for division
• Animals with isolecithal or mesolecithal eggs


  Tend to develop a larval state, as absence of yolk fosters early release so young animal can look for its own food
• Mammals


  Don't have larval state but develop placenta to feed from mother
• Symmetry of cleavage


  How (in what order and position) cell divisions happen
• Gastrulation


  A crucial stage in the life of all organisms, where an extreme reorganization of blastomeres happens, and the blastula becomes the gastrula with three germ layers: endoderm, mesoderm, ectoderm
• Hoboblastic radial cleavage


  Vegetal pole flattens and invaginates, forming the archenteron which will become the digestive tube, and the blastopore which will become the anus or the mouth
• Meroblastic discoidal cleavage


  The blastoderm is elongated towards the caudal region, a line is created in the middle by small invagination (the primitive streak), and cells migrate along this line to form the ectoderm, endoderm and mesoderm
• Hoboblastic rotational cleavage


  Similar to birds and reptiles, indicating phylogenetic closeness. Mammals took yolk away secondarily, in favor of placenta
• Coelom formation


  An important step in gastrulation, the coelom is the main activity in most animals and will hold most of the organs
• Schizocoely


  Part of the tissue near the blastopore is broken and some cells create a new cavity
• Enterocoely


  Tissue from the archenteron is evaginated to form new cavities
• Protostomes


  Mouth first, the blastopore will become the mouth. Schizocoely, triblastic invertebrates
• Deuterostomes


  Mouth second, blastopore becomes the anus. Enterocoely. Echinoderms and vertebrates
• Ontogeny is a recapitulation of phylogeny – Ernst Haeckel
• Animal organization levels


  • Protoplasmic
  • Cellular
  • Cell tissue
  • Tissue organ
  • Organ system
• Animal symmetry


  • Spherical symmetry
  • Radial symmetry
  • Bilateral symmetry
• Internal cavities


  • Acoelomate
  • Pseudocoelomate
  • Coelomate
• Protozoa


  Unicellular, microscopic organisms with all types of symmetry, no embryonic layers, simple or multiple nucleus, movement with cilia, flagella, pseudopods or pulsatile vacuole, contractile vacuoles, terrestrial or marine, independent or symbiotic, free or sessile, ability to encyst, micronucleus for reproduction and macronucleus for non-reproductive functions