Animal Reproduction

• Types of Reproduction:

  Asexual Reproduction has no gamete involvement

  Sexual Reproduction involves the union of gametes (egg and sperm)
• Binary Fission is a type of asexual reproduction involving the division of the body into two or more parts.
• In Budding, the offspring arises as an outgrowth from the parent and is initially smaller than the parent. Failure of the offspring to separate from the parent leads to colony formation.
• Fragmentation and Regeneration involves the breaking of the body intoseveral pieces, followed by regeneration or regrowth of lost body parts
• Hermaphroditism and parthenogenesis are less common forms ofsexual reproduction.
• Gametogenesis is the production , maturation, and differentiation of gametes (egg & sperm)
• 4 Phases of Gametogenesis:

  1. Primordial germ cells (extraembryonic origin and migration of germ cells)
  2. Proliferation by mitosis (increase in no. of germ cells by mitosis)
  3. Meiosis (Reductional in chromosomal number by meiosis)
  4. Structural and Functional Changes (Final structural & functional maturation of egg and sperm)
• Copulation: vital for the unification of gametes
• Fertilization

  • Location can be external or internal
  • External: releases gametes in environment
  • Internal: injection of gametes to female reproductive system
• Sexual systems consist of two components:

  
  

  (1) primary organs, which are the gonads that produce sperm and eggs and sex hormones; and (2) accessory organs, which assist the gonads in formation and delivery of gametes, and may also serve to support the embryo.
• 1st Major Event in Fertilization: Contact and recognition between sperm & egg

  • Chemoattraction of sperm to molecules secreted by egg
  • Exocytosis of acrosome
  • Penetration of sperm to the egg
• 2nd Major Event in Fertilization: Regulation of sperm entry

  • Prevents polyspermy (ability of other sperm cells to fertilize an egg)
• 3rd Major Event in Fertilization: Fusion of Genetic Material

  • Nuclear envelope breaks down
  • Sperm pronucleus fused with egg pronucleus
  • Oocyte completes its 2nd meiotic division
• 4th Major Event in Fertilization: Activation of egg metabolism

  • Metabolic reactions increasesubstantially to start cleavage
• Cleavage

  • From a single zygote we can observe the presence of numerous blastomeres
  • Before cleavage begins an animal-vegetal axis is visible on the embryo
  • Embryo manifests polarity (by the yolk: mostly proteins; nourishes the embryo + has 2 regions)
  • Vegetal pole: yolk abundant
  • Animal pole: little yolk; mostly cytoplasm
  • Animal vegetal axis provides a reference point on the embryo
  • Cleavage is generally an orderly sequence of cell divisions so that one cell divides to form 2 cells -> 4 -> 8 -> and so on
• Classification of Embryo: Amount of Yolk

  • Alecithal: negligible
  • Mesolecithal: moderate
  • Microlecithal: small
  • Polylecithal: abundant
• Classification of Embryo: Distribution of the yolk

  • Isolecithal: even distribution (in echinoderms; placental + marsupial mammals)
  • Mesolecithal: moderate; vegetal pole (amphibians)
  • Telolecithal: high amount; vegetal pole
  • Centrolecithal: centrally located
• Classification of Embryo : Cleavage pattern

  • Meroblastic: incomplete cleavage (telolecithal + centrolecithal)
  • Holoblastic: complete cleavage (Isolecithal + mesolecithal)
• Classification of Embryo: Development of Embryo

  • Direct: highly nourished by yolk; young -> adult
  • Telolecithal eggs
  • Indirect: little yolk; young -> larval stages
  • Isolecithal + Mesolecithal
  • Mammals use placenta to complement absence of yolk
• Blastulation

  • During cleavage the embryo divides repeatedly converting large cells into many smaller cells (blastomeres)
  • Hollow cavity: blastocoel -> give rise to coelom
  • During this period: no growth, only subdivision of mass until normal somatic cell size is attained
  • Development continues beyond the blastula to form one or two germ layers in gastrula stage
  • Blastula -> gastrula (via gastrulation)
• Gastrulation

  • Gives rise to embryonic blastopore [can give rise to mouth or anus]

  protostomia/protostome 1st forms mouth

  • Lophotrochozoa: platyhelminthes + molluska + annelida
  • Ecdysozoa: Nematoda + arthropoda

  deuterostomia/ deuterostome (1st forms anus): Echinodermata + Chordata
• Gastrulation

  • Gastrocoel/archenterm -> gives rise to GIT
  • Converts blastula into 2 or 3 layered embryo
  • Development of germ layer
  • Layers = germ layer + all subsequent body parts develop from one or more layers
• • To form a 2nd germ layer, one side of the blastula bends inward via invagination
  • Germ layers = precursor to matured organs
  • 2 germ layers: outer and inner layer
  • Exhibits diploblastic architecture:  ectoderm (outer) + endoderm (inner) [Cnidaria + ctenophora]
  • Diploblastic = radiata (shows radial symmetry)
  • Triploblastic: ectoderm (outer) + endoderm (inner) + mesoderm (middle) 
  • Bilateria (shows bilateral symmetry)
• Mesoderm formation

  • Acoelomate: lacks body cavity; fills blastocoel; platyhelminthes
  • Pseudocoelomate: 2 body cavities; not lined by mesoderm; nematoda
  • Coelomate: 2 body cavities; gut is lined by mesoderm
  • Schizocoelous: the rest 
  • Enterocoelous: echinodermata + chordata
  • ectoderm : give rise to skin
  • Mesoderm: urogenital system
  • Endoderm: internal organs like liver pancreas and lining of urinary
• Parts of gastrula:

  • Archenteron/ gastrocoel – internal pouch/ gutcavity

  • Blastopore – opening of the gastrocoel

  
  

  • Germ layers:

  • Ectoderm – outer layer

  • Endoderm – inner layer