Embryology

1. Cleavage and Fate
Maps
Embryology
Bio 415 LECTURE
4

2. Cleavage stage and and Fate Maps
Cleavage is the first phase of embryonic development by mitotic cell
division.
Introduction
Levels of Cleavage types:
 Full splinting or cleavage.
 Partial splinting or cleavage.
 Surperfacial cleavage or cleavage.
Cleavage stages:
2 cell stage  4 cell stage  8 cell stage 16 cell stage 32 cell
stage Morula  Blastula.
Fate maps:
 In amphibians
 In birds
 In Mammals

3. Diagram explaining the development of embryos within the
Fallopian Tubes (cleavage stages)

4. Cleavage stage

5. • After fertilization and formation of the zygote(the fertilized ova) begins the first
stages of early embryonic formation, which consists of:
A series of indirect divisions (mitotic) known as the phase of cleavage or
(segmentation) where the fertilized egg is divided into several successive divisions
produced Blastomeres.
• At the begging of cleavage the blasts are initially large and then become
smaller with the progress in the process of splinting or cleavage to form blastula
at the end of the cleavage phases.
Cleavage Introduction

6. before the fertilized egg begins the process of splinting, it prepares internally by
increasing the following:
1. Increasing the composition of protein.
2. Multiplies Genetic material or DNA chromosomes.
3. Formation of the fertilization membrane.
4. Formation of the hyaline layer (protection or preservation of the blastomeres
together).
Cleavage Introduction

7. What is cleavage?
Cleavage is a rapid series of mitotic divisions that occur just after fertilization.
There are two critical reasons why cleavage is so important:
1. Generation of a large number of cells:
That can undergo differentiation and gastrulation to form organs.
2. Increase in the nucleus / cytoplasmic ratio:
Eggs need a lot of cytoplasm to support embryogenesis. It is difficult or
impossible for one nucleus to support a huge cytoplasm, and oocytes are one
of the largest cells that exist. One small nucleus just cannot transcribe
enough RNA to meet the needs of the huge cytoplasm.
 A larger nucleus to cytoplasmic ratio is optimal for cell function. Cell
division occurs rapidly after fertilization to correct this problem.

8. 1. During the process of cleavage ,the cell cycle consists of only 2 phases:
 The (S- phase DNA Synthesis) the manufacture of genetic material.
 The cellular division phase (M-phase) only.
2. The cell in cleavage does not go through a first or second interval (G1,G2 phases)
as in the normal cell cycle.
3. The cell division is regular at begging and then followed by multiple divisions that
cannot be pursued.
4. The cleavage leads to a multicellular early embryo
Cleavage character
Fig. Comparing between cell cycle in
early cleavage and somatic cell.

9. 5. Cleavage does not include an increase in the size of the fetus (about the same size
as the ovum ).
6. The embryonic form remains as it is the general form is the same (except the
appearance of the blastula cavity) At the end of the cleavage.
7. The time of the cleavage does not depend on the amount of the yolk, it varies by
the difference of living organisms:
• In the mammals' ova the first cleavage stage takes 24 hours which is the lowest
eggs in the yolk.
• In the amphibians 1st cleavage done within 3 hours.
• In the embryo of birds completes the stages of cleavage before the female lays the
egg.
Cleavage character

10. 1. The first level of splinting passes through the main axis of the fertilized ova starting
from the animal pole (the location of the nucleus to the vegetative pole (produces two
cells or two blastomers).
2. The second level of cleavage: Radial cleavage may be perpendicular to the first
division such as sea urchin and amphibians in both blastomers, and may be rotating
(Rotational cleavage) divided on the axis length and the other on the width of the axis
as in mammals.
3. The third level of splinting: horizontal on the first and second splits and produces
eight cell or blastomers upper group of the animal half pole of the embryo (in which
the nucleus was located) a lower group of the half of the vegetative pole (as in
amphibians)
4. The fourth cleavage: compose from 4 divisions in the upper cell group and another in
the lower cell group produces 16 cells.
5. The fifth cleavage is a set of 16 cell divisions in each blastomear to produce 32 cells
6. The sixth cleavage: a set of 32 cell division to give the stage of Morula.
7. The seventh cleavage is a set of divisions resulting in the blastula stage. the end of
the cleavage stage
Cleavage Levels

11. Radial cleavage Rotational cleavage
Type of cleavage, Radial cleavage in sea urchin embryo And Rotational cleavage as in mammals.

12. Cleavage differs from normal mitoses in 2 respects
1. Blastomeres do not grow in size between successive cell divisions as they do
in most cells. This leads to a rapid increase in the nucleus / cytoplasmic
ratio.
2. Cells undergoing cleavage have mainly S and M phases of the cell cycle
(little or no G1 or G2).
3. Cleavage occurs very rapidly, and mitosis and cytokinesis in each round of
cell division are complete within an hour. Typical somatic cells divide much
more slowly (several hours to days) and even the fastest cancer cells divide
much slower than occurs in a zygote during cleavage.
4. Cleavage differs in different types of eggs. The presence of large amounts of
yolk alters the cleavage pattern, leading to incomplete cleavage that
characterizes birds and reptiles.

13.  Eggs are classified by how much yolk is present
1. Isolecithal eggs (iso = equal) have a small amount of yolk that is equally
distributed in the cytoplasm (most mammals have isolecithal eggs).
2. Mesolecithal eggs (meso = middle) have a moderate amount of yolk, and the
yolk is present mainly in the vegetal hemisphere (amphibians have
mesolecithal eggs).
3. Telolecithal eggs (telo = end) have a large amount of yolk that fills the
cytoplasm, except for a small area near the animal pole (fish, reptiles, and
birds).
4. Centrolecithal eggs have a lot of yolk that is concentrated within the center of
the cell (insects and arthropods).
Two areas of interest:
How does the process of cleavage differ in different organisms?
What mechanisms regulate cleavage?

14.  The pattern of cleavage of the zygote depends upon the pattern of
yolk distribution.
1. Holoblastic cleavage occurs in isolecithal eggs (mammals, sea urchins). The
entire egg is cleaved during each division.
2. Meroblastic cleavage occurs when eggs have a lot of yolk. The egg does not
divide completely at each division. Two types:
a. Discoidal cleavage is limited to a small disc of cytoplasm at the animal
pole. All of the yolk filled cytoplasm fails to cleave (characteristic of
telolecithal eggs such as birds).
b. Superficial cleavage is limited to a thin surface area of cytoplasm that
covers the entire egg. The inside of the egg that is filled with yolk fails to
cleave (centrolecithal eggs such as insects).

15. Typical cleavage patterns of isolecithal, mesolecithal,
telolecithal and centrolecithal eggs

16. • In eggs where the yolk is distributed regularly and equally,
such as most mammals, sea urchins.
• The amount of the yolk is small
• The cells is totally divided
• The whole cells are equal in size (cells in the animal pole are
smaller).
sea urchins. cleavage
1- Holoblastic equal cleavage

17. Amphibians have mesolecithal or telolecithal eggs and
undergo holoblastic cleavage with unequal blastomeres
 Amphibian eggs have a lot of yolk, however, they are still able to undergo holoblastic
cleavage, but produce unequal blastomeres
 The 1st cleavage is meridional, as is the 2nd.
 The 3rd cleavage is equatorial. The cleavage is displaced toward the animal pole due to
the yolk. This results in 4 small animal blastomeres and 4 large vegetal blastomeres.
 Morula (morum = mulberry) at the 16 to 32 cell stage the embryo is called a morula
because it looks like a mulberry. Witch will transfer to blastula stage.

18. The blastula formation = the last stage of cleavage
 Blastula = will form at the end of cleavage stage, it from the 128 cell stage
onward the amphibian embryo is a blastula.
 Its has a cavity called the blastocoel between the animal and vegetal pole.
 The outer surface of the amphibian blastula has cells connected by specialized
cell junctions.
1. Tight junctions create a seal that isolates the outside of an embryo from the
inner layer. Tight junctions polarize the apical and basal surfaces. The basal
portions of cells start secreting into the blastocoel.
2. Desmosomes attach the blastomeres together on the outside.
3. Gap junctions connect all surface blastomeres.

19. Fig: The cleavage stages in amphibian embryo

20.  In eggs with a large amount of yolk, the levels of cell division or cleavage
do not pass through cells and there are two types:
a- Discoidal
Is incomplete as in reptiles and birds where the cell division is confined to
the highest area of the egg. Where the fertilization occurs in the active
cytoplasm (germinal disk). (or blastoderm -cytoplasm free of the yolk and
floats on it - in the form of a small disk)
b - Partial cleavage ( or Meroblastic)
It is incomplete cell division or cell spliting as in fish where the cleavage
occurs at the top of the egg or some of the surface dividing cell are
complete, while the underneath dividing cells are incomplete, and
connected with the yolk.
2- Discoidal or Meroblastic Cleavage

21. Discoidal cleavage in birds

22. Meroblastic cleavage in fish

23. 3-Superficial Cleavage
 Superficial Cleavage occur in
insect
 The Yolk in the central of the
ova
 The several cleavage hapend
in the nuclouse And the
neclous moved to the
cytoplasm area.
 In the surface of the ovum
and each neoulous suronded
by cytoplasm.
 The cells form in the surface
of the egg.

24. • The researchers were able to draw a virtual map on the phase of the blastula
stage.
• The blastula are distributed into 3 areas (animal pole, vegetal pole and the
middle of the blastula), they follow the fate of each particular group of areas
of the blastula when they grow up later.
• The researcher (Vogt) were able to drown the fate map by special vital die
to stain the 3 area of blastula stage (its not toxic to the embryo cell), (the
vital die Neutral red, Nile blue sulphate, and Janus green),then they follow
the fate of each stained group of cell.
1. The animal pole will develop to the ectoderm witch will give the neural
tube and the skin (epidermis).
2. The vegetal pole (the lower part of the blastula) will form the endoderm
witch will develop and give the elementary canal,lungs.
3. The middle of the blastula will give the mesoderm that will develop and
give the dermis, heart, muscles, bones, kidney. See fig.
Fate Maps

25. Fig: Frog Embryo Fate Maps at the blastula stage

26. Fig: Different Embryos Fate Maps at the blastula stage

27. Fig: Embryos Fate Maps at the blastula stage