MEGASPOROGENESIS, DEVELOPMENT OF EMBRYO SAC ,
SUBCELLULAR DETAILS OF CONSTITUENT CELLS AND THEIR FUNCTION,
Department of Botany
M.sc. 2 –SEM (2017-2018)
ROLL NO -08
• Usually a single hypodermal initial known as primary archesporial cell is
differentiated at the apex of the nucellus.
• The pri archesporial cell divides periclinally into outer primary parietal cell
or pri wall cell and inner pri sporogenous cell.
• The pri parietal cell may or may not divide.The pri sporogenous cell directly
behave as megaspore mother cell.
• The MMC undergoes meiotic division to form 4 megaspores.
• The 4 megaspores thus formed and are arranged in an axial row forming a
• Usually only one megaspore of the tetrad is functional and grows at the
expense of the other three, which degenerate. The functional megaspore
enlarges and forms the embryo sac.
• The female gametophyte ,also called embryo
sac ,is mostly a 7-celled structure .
• There is a large central cell with two polar
nuclei which later fuse to form the secondary
nucleus. The micropylar end of the central cell
is occupied by the egg apparatus, comprising
an egg cell and two synergids and at its chalazal
end three antipodal cells are present
• The synergids are elongated cells present at micropylar
end of the embryo sac .
• When two synergids are present they lie in contact with
each other,and partly embrace the egg. They are pointed
or hooked towards the micropyle.
• The wall around the synergids is incomplete. there is a
distinct wall around the micropylar one-third of the cell
which thin towards the chalazal end and finally
• The cytoplasm of the synergids is strongly polarized.the
chalazal region of the cell is occupied by one large or
many small vacuoles.
• Synergids are ephemeral structures.In embryo sacs
with two synergids, one degenerates before the
entry of the poller 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.
• In Cotula australis one of the synergids become
swollen and haustorial in nature.synergids haustoria
also occur in Sedum
• Extensive synergids haustoria also been reported in
Cortaderia selloana a member of the poaceae.
• The haustoria arise as finger-like projections from
the filiform apparatus and penetrate the
FUNCTION OF SYNERGID
• (1) They play an important role in directing the pollen tube growth by secreting
some chemotropically active substance.
• (2) The degenerating synergids forms the seat for pollen tube discharge in the
• (3) Jensen (1965)suggested that the filiform apparatus may be aiding the
synergids in the absoption and transportation of materials into the embryo sac
from the nucellus.
• Although much has been said about the structure and function of synergids ,yet
several qestions remain unanswered,such as the nature of metabolic activity of
synergids ,their differential behaviour,entrance of pollen tube through FA,and
their role in opening of pollen tube
• A prominent structure ,called filiform apparatus is present at
the micropylar end of each synergid.
• Its differentiation from the synergid wall was described as early
as 1906 by Habermann.
• The filiform apparatus is a mass of finger-like projections of
the wall into the cytoplasm .
• Structurally ,each projection of the FA has a core of tightly
packed microfibrils(possibly cellulosic) enclosed by a non-
• They are rich in polysaccharides.
• The form of FA IS variable.it may be spherical as in Torenia
or wedge- shaped between the tips of synergids as in Petunia
and helianthus, in Jasione, the FA extends irregularly along
the entire length of the synergids
• The three cells of the egg apparatus (an egg and two
synergids)are arranged in a triangular fashion .the egg
shows common walls with the two synergids and the
• It has a wall similar in structure and extend to that of
synergids.the wall is thicker in the micropylar region but
become thinner towards the chalazal side.
• It is absent at the chalazal end in cotton ,maize,
• At the micropylar end the latteral walls of the egg cell
appear to join the central cell wall .actually the end wall
of the egg cell is the wall of the central cell.
• The egg cell becomes highly polarized early in its development. The micropylar
end of the cell is occupied by a large vacuole. thus the distribution of the vacuole
and cytoplasm in the egg cell is just opposite of that in the synergids.
• The egg may contain small or large amounts of starch ,that is consumed during
fertilization or early embryogeny.
• The egg cytoplasm is rich in ribosomes. The egg of Nicotiana rustica contains
two strongly RNA- positive bodies that multiply and persist till the 4-celled stage
• Antipodal cells are present at the chalazal end.
• The antipodals exhibit the greatest variation amongst all the cells of
the embryo sac.useally they degenerate before or soon after
fertilization ,without any appreciable enlargement.
• In many plants the antipodals are persistent and show some
structural and cytological features suggesting their possible role in the
nutrition of the embryo sac.
• In Caltha palustris they persist up to the octant stage of the
• In grasses they undergo a series of mitotic divisions leading to the
formation of a large number of antipodal cells.
• The highest number of antipodal cells known is 300 in Sasa
paniculata. Zea mays has about 20 antipodal cells ,each with 1-4
nuclei. Multinucleate condition of antipodals cells also occurs in
• The antipodals are rich in ascorbic acid ,oxidases,sulfhydril compounds.
Starch ,lipids and proteins also occur in aboundance. However they show
very low concentrations of RNA and polysaccharides.
• Haustorial behaviour of antipodal cells is known in many plant
• (Grindelia, Haplopappus, Quinchamalium
• (1) Antipodals are nutritive in function it nourishes the embryo sac.
• (2) The antipodal cells may also store large quantities of starch , lipids, and
proteins which are utilized by the developing endosperm and embryo.
• (3) Antipodal cells is to produce and secerte substances that control the
growth and development of endosperm
• It is the largest cell of the embryo sac, and the mother cell of the
• The enlargement of the embryo sac after the last nuclear division is largely
due to the inflation of the large central vacuole of the central cell.the vacuole
in the central cell is the reservoir of sugars, amino acids,and inorganic salts.
• The nuclei of the central cell, called polar nuclei, are very large,and each
possesses a conspicuous nucleolus.
• The two polar nuclei fuse,before or during double fertilization,to form the
• Unlike the egg cell the cytoplasm of the central cell is rich in all cell
organelles and appears to be the centre of intense synthetic activity. There
are plastid containing starch sometimes proteins and phytoferretin.
• In Capsella the central cell possesses numerous sphaerosomes associated with
glyoxysomes that probably convert fat into sugar.
• There are numerous ribosomes usually present as single ribosomes or small
• The wall of the central cell is highly variable from one part of the embryo
sac to the other. The central cell is connected with egg, synergids, and the
antipodals through plasmodesmatal connections.
• The presence of cell wall projections in the micropylar or chalazal region
shows that central cell draws nutrition from the surrounding nucellus or
• The haustorial extensions with parietal prolongation enter the ovular tissue.
In addition to absorption,these extensions secrete enzymes that help in the
digestion of the nucellar tissue.(rifot,1973).
EMBRYO SAC MAJOR TYPES
Depending on the number of megaspore nuclei taking part in the
development, the female gametophytes of angiosperms may be classified
into three main types
MONOSPORIC EMBRYO SAC
• The chief charecterstic of a monosporic embryo sac is that derived from only one of
the four megaspores, as in polygonum type.
• All the nuclei in such an embryo sac are genetically identical because they are derived
through mitossis of a single nucleus .
• There are two type of monosporic embryo sac
1 .POLYGONUM TYPE
• The embryo sac is formed by the chalazal megaspore of the tetrade and eight
• The mature embryo sac comprises a 3-celled egg appratus, three antipodal
cell, and a binucleate central central cell.
2. OENOTHERA TYPE
• Geerts, in 1906, found that in Oenothera lamarckiana the embryo sac is
usually formed by micropylar megaspore of the tetrad, which undergoes
only two nuclear divisions instead of the usual three occuring in Polygonum
type of embryo sac. In this way 4 nuclei are produced which organize into
the two synergids, the egg and a single polar nucleus
• Since the third division is omitted and all the nuclei are situated in the
micropylar part of the embryo sac, there is neither a lower polar nucleus nor
any antipodal cells.
• Also Found in Epilobium, Circaea and others with few exception
BISPORIC EMBRYO SAC
• The bisporic embryo sac developed from one of the two dyads formed as a
result of first meiotic division or meosis one of megaspore mother cell one
of two dyads degnerate both nuclei of the function of dyad take part
information of embryo sac withaout wall formation each nucleus undergoes
two mitotic division so the mature embryo sac 8 nucleate the 8 nuclei are
arranged in to antipodal egg apparatus and polar nuclei as in polygonal type
of embryo sac bisporic type embryo sac are two type-
2. Endymion type
2. ENDYMION TYPE
• The embryo sac is formed by the micropylor dyad cell.
• Also described in Agraphis patula, and appears to be quite characteristics
of certain families, Podostemonaceae, Butomaceae( Butomus). Alimaceae and
the tribe Viscoideae of the Loranthaceae. And also found in several
members of Lileaceae, Balanophoraceae, Amaryllidaceae and Orchidaceae.
TETRASPORIC EMBRYO SAC
• The meotic divison of the megaspore mother cell is not a complete by wall
formation (cytokinesis) hence all the 4 nuclei like in a single cell called
coenomegaspore all the 4 nuclei of coenomegaspore particepate in the
formation of embryo sac is genetically more hetrogenious then the
tetrasporic embryo sac are of many type .
• Under two categories-
Where no nucleare fusion occure
Where nuclear fusion occure
WHERE NO NUCLEAR FUSION OCCURE
1. Adoxa type - The embryo sac is eight nucleate ,formed after a single post
–meiotic meosis ,and its organisation is similor to the Polygonum type.
2. Plumbago type- In this type only one post –meiotic mitosis occure , but
the organization of the mature embryo sac is very defferent from that in
Adoxa type.The mature embryo sac comprises an egg cell and a four
nucleate central cell .the other three nuclei are cut-off as peripharal cell.
3. Penaea type – As a result of two post meiotic mitosis in coenomegaspore
16 nuclei are formed . The mature embryo sac comprises 4 group of 3 cell
each, one group is at the micropylor end , one at the chalazal end and two
arreanged laterally . The remaining four nuclei behave as polar . The
micropylor trid usually function as the egg apparatus.
4. Drusa type- This type of embryo sac is also 16 nucleate . The mature
embryo sac comprises a normal egg appratus ,two polar nuclei and 11
WHERE NUCLEAR FUSION OCCURE
After the second meotic divison 3 megaspore nuclei are of fast fuse to
nucleus of the chalazal and 4 nucleus and micropylor end remains haploid
1.Fritilaria type- After the nuclear fusion both the triploid and haploid nuclei
divide twice forming 4 nuclei each pole .the mature embryo sac consist of an
egg appratus and a central cell one is haploid and other is triploid.
2. Plumbagella type- After nuclear fusion the triploid nucleus at the chalazal
end and haploid nucleus of micropylor end undergoes a single meiotic
division to form two group and and 2 nuclei. 2 nuclei each 2 diploid and
2 haploid nuclei the embryo sac conssist of a haploid egg. A triploid antipodal
and one haploid and one triploid .rarely plant species may show more than
one kind of embryo sac in Delosperma coopri the 3 kinds mono, bi, and
tetrasporic type have been regarded the tempereture is also known to
influenc the embryo sac development most angiosperm as 8 or 7 celled
embryo sac synergid are not present in Plumbago and Plumbagella type no
antipodal are present.
• THE EMBRYOLOGY OF ANGIOSPERM – S S BHOJWANI. SS
BHATNAGAR. P K DANTU
• AN INTRODUCTION TO THE EMBRYOLOGY OF
ANGIOSPERM BY- P. MAHESHWARI
• IMAGES FROM GOOGLE SCHOLAR