This document discusses cleavage, the process by which a fertilized egg undergoes cell division to form a multicellular embryo. It describes key terms related to cleavage such as furrow, morula, blastula, blastocoel, blastocyst, blastomere, and gastrula. It explains different types of cleavage including holoblastic, where the cleavage furrow extends through the entire egg, and meroblastic, where cleavage is incomplete. Within these categories are subtypes like radial, spiral, bilateral, discoidal, and superficial cleavage that describe the orientation and extent of the cell divisions. Factors like the amount and distribution of yolk in the egg influence the cleavage pattern.
4. Cleavage
After fertilization, the development of a multicellular organism proceeds by a
process called cleavage, a series of mitotic divisions whereby the enormous
volume of egg cytoplasm is divided into numerous smaller, nucleated cells.
Shortly after fertilization, the zygote undergoes cleavage, a series of rapid mitotic
divisions with no period of growth during each cell cycle.
OR
5. FURROW
FURROW:
the cleavage furrow is the indentation of the cell's surface that begins
the progression of cleavage, by which animal and some algal cells
undergo cytokinesis, the final splitting of the membrane, in the process
of cell division.
7. MORULA
The morula stage is the final stage prior to formation of a fluid filled cavity
called the blastocoel cavity.
8. BLASTULA BLASTOCOEL
Cleavage results in the formation of a hollow ball of cells
called a blastula.
After the cleavage has produced over 100 cells, the embryo
is called a blastula. The blastula is usually a spherical layer of
cells (the blastoderm) surrounding a fluid-filled or yolk-filled
cavity (the blastocoel). Mammals at this stage form a
structure called the blastocyst.
OR
9. BLASTODISC
In fish eggs, cleavage occurs only in the blastodisc, a thin region of yolk-free
cytoplasm at the animal cap of the egg. Most of the egg cell is full of yolk. The
cell divisions do not completely divide the egg, so this type of cleavage is
called -----------------
11. Yolk distribution in three kinds of
eggs
In a sea urchin egg, the cytoplasm contains a small amount of evenly
distributed yolk and a centrally located nucleus.
In a frog egg, there is much more yolk, and the nucleus is displaced toward one pole.
Bird eggs are complex, with the nucleus contained in a small disc of cytoplasm that
sits on top of a large, central yolk mass.
14. Meroblastic cleavage. Only a portion of the
egg actively divides to form a mass of cells in this
type of cleavage, which occurs in eggs with
relatively large amounts of yolk.
17. Germ-layer derivation of the major tissue types in animals. The three germ layers that
form during gastrulation give rise to all the organs and tissues in the body, but the
neural crest cells that form from ectodermal tissue give rise to structures that are
prevalent in vertebrates, such as gill arches and bones of the face and skull.
18. Cleavage in a frog embryo. The cleavage planes in the first and
second divisions extend from the animal pole to the vegetal pole,
but the third cleavage is perpendicular to the polar axis. In some
species, the first division bisects the gray crescent, a lighter-colored
region that appears opposite the site of sperm entry.
21. PLANES OF CLEAVAGE
An egg can be divided from different planes during cleavage. Depending on the
position of the cleavage furrow the planes of cleavage are named.
1.Meridional plane: The plane of cleavage lies on the animal vegetal axis. It bisects
both the poles of the egg. Thus the egg is divided into two equal halves.
Or
If the cleavage furrow bisects both the poles of the egg passing through the polar
axis the cleavage plane is said to be meridional.
22. PLANES OF CLEAVAGE
4. Latitudinal plane: It is similar to the equatorial plane, but it lies on either side of
the equator. It is also called as transverse or horizontal cleavage
3. Vertical plane: The cleavage furrows may lie on either side of the meridional
plane. The furrows pass from animal to vegetal pole. The cleaved cells may be
unequal in size.
2. Equatorial plane: This cleavage plane bisects the egg at right angles to the main
axis. It lies on the equatorial plane. It divides the egg into two halves
23. According to the concentration of yolk in an eggs can be divided into 4 types.
1. Alecithal: In the eggs yolk is absent. Eg. True mammals
2. Microlecithal: Eggs have little amount of yolk. Eg . Sea urachin
3. Msolecithal: Moderate amount of yolk is present in the eggs. Eg . Amphibians
4. Poly/Macrolecithal: Large amount yolk is present. Eg. Birds and Reptiles
24. On the Basis of the distribution of yolk
a. Isolecithal or Homolecithal Egg: In isolecithal eggs, the very little amount of yolk
present is uniformly distributed throughout the ooplasm (eg. echinoderms,
Amphioxus, mammals). This condition is usually observed in eggs with very little
amount of yolk.
b. Telolecithal Egg: In eggs containing moderate or large quantity of yolk, the
distribution of yolk is not uniform. lt is concentrated more towards the vegetal pole.
Such a type of egg, in which the yolk is concentrated towards one pole, is called
telolecithal egg.
(eg. reptilian and avian eggs).
C.Centrolecithal: The eggs of insects have their yolk in the center and the divisions
of the cytoplasm occur only in the rim of cytoplasm around the periphery of the cell
(i.e., superficial cleavage).
25. TYPES OF CLEAVAGE
Type of division
There are 2 types of cleavage i.e. Holoblastic and Meroblastic cleavage.
I. Holoblastic cleavage (Greek holos, "complete"): The cleavage furrow
extends through the entire egg.
II. Meroblastic cleavage :
The eggs undergoing partial cleavage, resulting in unequal blastomeres
26. I. Holoblastic cleavage (Greek holos, "complete"): The cleavage furrow extends through the
entire egg.
A. Isolecithal (Greek, "equal yolk"): The eggs have sparse, equally spaced yolk.
Eg. Eggs of sea urchins, mammals, and snails.
In isolecithal egg following cleavage types are occur.
1. Radial cleavage : In this cleavage pattern, division take place in such a manner that all
the blastomeres are placed in a radially symmetrical fashion around the polar axis.
When such an egg is viewed from the poles, the blastomeres seem to be arranged in a
radially symmetric form.
Example: Sponges, coelenterates, sea urchin, sea cucumber, amphioxus
27. Spiral cleavage : The spiral cleavage is diagonal to the polar axis. In this type, the
spindles for the third cleavage, instead of being erect, are oriented diagonally so that
the resulting upper tier of cells is sidewise.
The upper 4 cells are placed over the junction between the four lower cells. The
upper smaller cells are called micro and lower larger cells are known as
macromeres. The spiral cleavage results due to oblique positions of the mitotic
spindles.
This type of cleavage is called the spiral type because the four spindle during the
third cleavage are arranged in a sort of spiral.
Examples: Eggs of annelids, molluscs, nemerteans and some of the planarians.
28. (a–f) Top views of the animal pole. The successive cleavage divisions occur in a
spiral pattern as illustrated.
(g) A typical trochophore larva.
29. 3. Bilateral cleavage: In this pattern of cleavage, the blastomeres are so arranged that the
right and left sides becomes distinct. In this case, two of the first four blastomeres may
be larger than the other two, thus establishing a plane of bilateral symmetry in the
developing embryo.
Examples: Tunicates
4. Rotational Cleavage: It is characterized by a slow rate of division, a unique
cleavage orientation, lack of divisional synchrony, and the formation of a blastocyst.
The blastocyst forms after the blastomeres undergo compaction.
Examples: Mammals, Nematodes.
31. 2. Meroblastic (Incomplete Cleavage):
A. Telolecithal (Dense yolk throughout most of cells)
1. Bilateral cleavage: The following cleavage planes are centered on this axis
and result in the two halves being mirror images of one another.
Eg. Cephalopoda molluscans.
2. Discoidal cleavage: Cleavage are restricted only to the small cytoplasmic
cap at the animal pole resulting in a rounded embryonic or germinal disc is
termed discoidal cleavage.
The eggs of birds and fishes have only one small area of the egg that is free of
yolk (telolecithal eggs), and therefore, the cell divisions occur only in this
small disc of
cytoplasm, giving rise to the discoidal pattern of cleavage
Example: Eggs of elasmobranchs, bony fishes, birds, reptiles and egg laying
mammals.
32.
33. B. Centrolecithal (Yolk in center of egg)
1. Superficial cleavage : Cleavage is restricted to a supeficial peripheral layer of
cytoplasm around yolk, hence the term superficial cleavage.
Or
The eggs of insects have their yolk in the center (i.e., they are centrolecithal), and the
divisions of the cytoplasm occur only in the rim of cytoplasm around the periphery of
the cell (i.e., superficial cleavage).
Example: Centrolecithal eggs of arthropods
