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MTBIOLOGY TODAY | JULY   ‘15 51

Pollen grain

• Pollen grain (microspore) is the first cell of the male

gametophyte.

• Each pollen grain or microspore is a haploid, oval or rounded

structure.  It   has     two     layers:    exine   and intine. The     exine   

appears first   as  a   thin    layer,  but     later   becomes considerably    

thick    and     can     be  differentiated into     two     layers  –  ektexine

(also    called sexine)  and    endexine    (also    called nexine).    

The  ektexine   is   further    differentiated  into     a   basal  foot

layer   a    middle bacula  and  an  upper  tectum. The     bacula  

which   consists    of  radially    extended    rods,   forms   various 

sculpturing pattern on   the     exine.

• The exine is chiefly composed of sporopollenin.

Sporopollenin   is   absent in   the     pollen grains  of  Zostera.

Sporopollenin   is   considered to   be  the     oxidative  polymer 

of   carotenoids    and/or  carotenoid  esters.     It   is  one     of  the    

most     resistant  organic materials.  It   is  not     degraded   by  

any  enzyme.    It   is  not     affected   by   high    temperature,   strong  

acid     or  strong alkali. Because of   the     sporopollenin, pollen  

grains  are well    preserved   as  fossils.

•    The     thin    areas  of   the     exine  are  called germ pores  or  

germ slits. Inner    layer   called intine  is   thin,       smooth and 

is   composed   of  pectocellulose.

•    The     cytoplasm  of   pollen grain   contains    dictyosomes,

mitochondria    and     endoplasmic reticulum.  Pollen  grains  are 

densely cytoplasmic as  long    as  they    are     in  tetrads.    But     soon    

after   their   release from    the     tetrad,     they    undergo considerable    

enlargement and  as  a  result  their   cytoplasm   becomes highly

vacuolated.

•    Pollen     grains  are     released    from    the     anthers either  at  2-celled    stage   or  at  3-celled    stage.  They    are     released    during  

M-phase of   cell    division   cycle    (mitotic   nuclear division    stage)  after    going  through the  G 1 -phase (preparatory    stage   

for  DNA     synthesis),    S-phase (DNA     synthesis  stage)  and  G 2 -phase (preparatory    stage   for     nuclear division).  The     3-celled    

pollen  grains  are  released   during  late     M-phase,   having  two  sperm  nuclei. In   the     2-celled   pollen, the  generative nucleus

remains undivided   and  contains   double  the  amount of   DNA     of  sperm  nucleus.    Here     the     generative nucleus divides in  

the  pollen tube.