Cleavage and its patterns

PRESENTED BY-
SAUMYA SHARMA
M.Sc Life science
Sem -I

Cleavage
 With in few hours of fertilization, the fertilised egg
undergoes a series of repeated mitotic cell divisions,
which occur in rapid succession to produce an increase
in the number of cells without any addition to its
volume.
 This phase of repeated divisions of the egg is known
as- CLEAVAGE OR SEGMENTATION.
 The cleavage was first observed by Swammerdam in
1738 in frog’s egg.
 In 1780, Spallanzani described first two cleavage
planes of toad’s eggs.

Various patterns of Cleavage-
The repeated cleavage furrows produce a number of
blastomeres which exhibit a specific pattern of
arrangement. The following types of cleavage patterns
have been studied in different mammals on various
basis :-
1. On The Basis Of cleavage furrow.
2. On The basis of fate of germ layers
3. On The Basis Of arrangement of the cells

A. On The Basis of Cleavage
furrow-
 Holoblastic equal cleavage-
In this type of cleavage,
the cleavage furrow extends
completely through the entire
length of the egg and bisects
it into equal blastomeres. It
occurs in microlecithal eggs.

 Holoblastic unequal cleavage-
In such type of cleavage the yolk is distributed along
vegetal animal axis. The yolk is sparse in the animal pole
and gradually increases towards the vegetal pole. The
first and second furrow are laid down similar to those as
in holoblastic equal cleavage. Third furrow is not
equatorial but towards the animal pole producing 4
small cells in animal and 4 large cells in Vegetal
hemisphere called macromeres and megameres.
This type of pattern is seen in lower fishes and
amphibians.

 Meroblastic cleavage or Discoidal cleavage-
It occurs in megalecithal or heavily telolecithal eggs,
which have anormous amount of yolk. The active
portion of the egg is confined to a small cytoplasmic
region at the animal pole called the germinal disc or
blastodisc. The cleavage furrows are restricted to the
germinal disc, the yolk remains uncleaved.
The early blastomeres are incomplete and are
continuous with the underlying yolk.
Examples- abairds, reptiles, bony fishes and
monotreme eggs, elasmobranchs.

 Superficial cleavage-
This cleavage occurs in centrolecithal eggs. Here, the
early divisions occur in the surface layer of the egg
and cleavage furrows do not extend into the central
yolk. In centrolecithal eggs , the zygote nucleus lies
in the centre of the egg. It divides repeatdly without
the div of the egg cytoplasm. As as result, a large
number of nuclei are formed . These remain
embedded in the undivided superficial layer of
cytoplasm. The cytoplasm divides by furrow laid
down from the surface towards inner side and
separates into a large number of cells arranges
around central yolk.

1. Indeterminate cleavage –
In indeterminate cleavage, the fate of blastomeres is not
so rigidly sealed, but the blastomeres exhibit plasticity.
For examples, each of the 2 blastomeres of a zygote, if
separated after the first cleavage, can produce one
complete embryo.
This type of cleavage ,in which future of blastomeres is
not predetermined , is knows as Indeterminate cleavage.
Examples – vertebrates.
B. On The Basis Of Fate Of
Germ Layers-

2. Determinate Cleavage-
In some animals (Ascaris), the blastomeres have
a predetermined future, i.e definite blastomeres
are determined to give rise to specific parts of the
embryo. This type of cleavage is known as
Determinate cleavage.
As a result of determinate cleavage, a mosaic type
of blastula is formed. This type of development is
called mosaic development.
Example- Ascaris egg.

C. On The basis Of arrangement of
cells-
1. Radial cleavage-
In radial cleavage, the successive cleavage furrow
cut straight through the egg , at right angles to
one another so that the resultant blastomeres
appear to be arranged radially.
All those eggs which have holoblastic cleavage,
exhibit radial cleavage.
Examples- Synepta eggs.

2. Spiral cleavage-
It is a modification of the radial cleavage . In this,
the mitotic spindles of the third clv in the four
blastomeres are laid down obliquely and are
arranged in a sort of spiral so that the 4
blastomeres of upper tier do not lie over the
corresponding blastomeres of lower tier but
between them. This type of cleavage is known as
spiral cleavage. The turn of spiral may be ina
clockwise or counter clockwise direction.
Examples- Annelids, Molluscs.

Cleavage and its patterns

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