Thursday, 19 November 2020

Structure of Embryosac or Female Gametophyte of Angiosperms

 

The Female gametophyte or embryo sac in angiosperms is eight nucleated and seven celled. It is called Polygonum type, since it was first studied in Polygonum divaricatum by Strasburger.

After the last nuclear division in the female gametophyte, the cleavages are such that all the cells of the embryo sac are formed within the wall of the parent megaspore.

The embryo sac has three main parts:

1.     Egg apparatus             2. Antipodals              3. Central Cell




Egg Apparatus:

Three nuclei of the embryo sac towards micropyle develop into an egg apparatus. As these nuclei are covered by thin walls, they are considered as ‘cells’.

The egg apparatus consist of one egg cell and two synergids. The three cells of the egg apparatus an egg cell and two synergids are arranged in triangular fusion.

EGG CELL:

The middle cell of the egg apparatus is big and acts as an egg cell. The egg shows common wall with the two synergids the central cell.

  The wall is thicker in the micropylar region but becomes thinner towards the chalazalside.It is absent at the chalazal end in cotton maize.  At the micropylar end the lateral walls of the egg cell appear to join the central cell wall.

 The egg cell becomes highly polarised early in its development. The polarity is expressed by the aggregation of cytoplasmic elements at the chalazal end of the cell. The micropylar end of the cell is occupied by a large vacuoles.

SYNGERGIDS:

They are elongated cells present at the micropylar end of the embryosac. When two synergies are present they lie in contact with each other and partly embrace the egg. They are pointed or hooked toward the micropyle..

  The wall around the synergies is incomplete. There is a distinct wall around the micropyle one third of the cell which thins towards the chalazal end and finally disappears. As a result the chalazal one third of the cell lacks a wall.

   A prominent structure called filiform apparatus(FA)is present at the micropylar end of each synergid. Recent electron microscopic works have revealed that the filiform apparatus is a mass of finger-like projections of the cell into the cytoplasm..

  The cytoplasm of the synergid is strongly polarised. They are ephemeral structures. In embryosac with two synergid one degenerates before the entry of the pollen tube into the  embryo sac, whereas the other one often called the persistent synergid degenerates shortly after the embryo sac has received the pollen tube discharge.

FUCTIONS:

Looking at the structure and concentration of cell organelles the synergids appear highly active metabolically three function have been ascribed to synergids..

 1) They play an important role in directing the pollen tube growth by secreting some chemotropically active substances..

2) The degenerating synergids forms the seat for pollen tube discharge in the embryosac..

3) Jenser suggested that the filiform apparatus may be aiding the synergid in the absorption and​transportation of materials into the embryosac form the nucellus.

ANTIPODAL CELLS:

The three nuclei arranged at the posterior side of the embryo sac are called the antipodal cells. The antipodal cells exhibit the greatest variation amongst all the cells of the embryosac. Usually they degenerate before or soon after fertilisation without any appreciable enlargement.

  In many plants the antipodals are persistent and show some structural cytological features suggesting their possible role in the nutrition of the embryosac. In the Caltha palustin they persist upto the octant stage of pro embryo. In grasses they undergo a series of mitosis divisions leading to the formation of a large number of antipodal cells. This highest number of antipodal cells known is 300 recorded in Sasa paniculata.

In Zea mays, during additional divisions in antipodals, the walls of many cells remain incomplete leaving protoplasmic continuities between adjacent cells. This results in the formation of multinucleate protoplasm or syncytium.

Haustorial behaviour of antipodal cells is known in many plants. In Argemone mexicana the antipodal cells are much larger than the either the egg or the synergids. After fertilisation they continue to enlarge and persistupto heart shaped stage of the embryo.

  Three main functions have been attributed to the antipodal cells. Often nutritive role had been proposed for the antipodal cells especially where they are persistent. Formation of wall projections in antipodal cells of Maize, Rice, Poppy gives them the appearance of the so called transfer cells and support the suggestion that these cells any be associated with the nutrition of embryosac​.

  The antipodal cells may also store large quantities of starch, lipids, and proteins which are utilised by the developing endosperm embryo. Another role ascribed to antipodals is to produce and secrete substances that control the growth development of endosperm..

CENTRAL CELL:

It is the largest cell of the embryosac and the mother cell of the endosperm.  The enlargement of the embryosac after the last nuclear division is largely due to the inflation of the large central vacuole of the central cell.

  The nuclei of the central cell called polar nuclei are very large and each possesses a conspicuous nucleolus.They are present either in the centre of the cell suspended by cytoplasmic strands, or in the cytoplasm close to the egg apparatus.

 Unlike the egg cell the cytoplasm if the central cell is rich in all cell organelles appears to be the centre of intense synthetic activity. There are plasmids containing starch and sometimes proteins and phytoferrtin. In Capsella the central cell possess numerous sphaerosomes associated with glyoxysomes that probably convert fat into sugar. The central cells contains sufficient food reserves that are available for use during fertilisation and early stages of endosperm development..

   The presence of cell wall projections in the micropylar or chalazal region shows that central cell draws nutrition from the surrounding nucellus or integuments

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