SEED DEVELOPMENT AND GERMINATION

1.   After fertilization:
     ·    zygote undergoes mitotic divisions
     ·    to form the embryo.

 

 2.   Endosperm muclei:
     ·    undergo mitotic divisions
     ·    to form endosperm tissue.

3.   Endospermic + embryonic development:

     ·    occur in the ovule.
     ·     integuments surrounding the ovule
     ·    will harden to form seed coat.

 

 

4.   Ovary à will become the fruit.

5.   There are two types of seeds:

     ·    monocotyledon (seeds have one cotyledon)
     ·    dicotyledon (seeds have two cotyledons)

6.   Germination:

     ·    emergence of the radicle
     ·     thru seed coat.

7.   Water:

     ·    required for germination
     ·     enters the seed by imbibition.
     ·    initiate embryo to synthesise gibberelin.

8.      Gibberelin will:

     .    diffuse thru endosperm àinto aleurone layer.
     .    initiate production of hydrolytic enzymes.

2.      Hydrolytic enzymes will:

     .    enter endosperm
     .       catalyse conversion of compound (stored in endosperm).

3.      Products (especially glucose) will be:

     .       transferred to the embryo
     .    used for respiration + germination

HUMAN FETAL DEVELOPMENT

1.   After eight weeks of development:

• all the organs + systems à been laid down.
•  embryo =  foetus.

 

2.   Extraembryonic membranes:

• amnion, chorion + allantois
• formed from embryonic cells
• in early stages of embryonic development.

3.   Amnion:
     Ø  develops from the epiblast.
     Ø  encloses embryo in a fluid-filled amnioc sac,
     Ø  protecting embryo (foetus) from:
          ·    abrupt temperature changes
          ·    mechanical impacts.

4.    Chorion:

      Ø  develops from trophoblast.
      Ø   surrounds the embryo + other extraembryonic membranes.

5.   Aallantois:

     ·    develops from part of embryo’s archenteron.

6.   Placenta:

     ·    consists of endometrial tissue + extraembryonic membranes.
     ·    a specialized organ
     ·    allows exchange of materials
     ·    between maternal +  foetal blood streams.

7.      Foetal part of the placenta

• arises from amniotic + chorionic membranes.
• umbilical blood vessels àformed from allantois.

8.      Materials are exchanged across placenta by:

• diffusion,
• active transport
• selective absorption.

9.      Nutrients, oxygen, water + antibodies:

• pass from the mother’s blood
• into the fetal blood circulatory system.

10.  Umbilical artery:

     ·    carries blood
     ·    from aorta of foetus
     ·    to placenta,

11.  Umbilical vein:

     ·    carries blood
     ·   from placenta
     ·   to posterior vena cava of foetus.

12.  Placenta is therefore:

     ·    site of gaseous exchange,
     ·    source of nutrients
     ·    excretory organ
     ·    source of antibodies.

13.  Although foetal + maternal blood streams:
      ·     flow very close to each other
      ·    but blood do not mix.

 

EMBRYOLOGY

1.      Embryonic development;
      ·         is triggered by fertilisation.
      ·         can be divided into three main stages:
             o   Cleavage
             o   Gastrulation,
             o   Organogenesis
 
2.      Cleavage:
      ·         division of the zygote
      ·         into daughter cells.
      ·         embryo (at the end of this stage) = blastula

 

3.      Gastrulation:
      ·         arrangement of embryonic cells
      ·         into three germ layers (ectoderm, mesoderm +  endoderm).
      ·          embryo (at the end of this stage) = gastrula.

4.      Organogenesis:
      ·         formation of organs + organ systems.
      ·         each germ layer àgives rise to different organ systems.