Cleavage Amphioxux to Mammals
Transcript of Cleavage Amphioxux to Mammals
DR IRAM IQBAL
Cleavage is the series of mitotic cell divisions of the zygote that result in the formation of early embryonic cells blastomeres.
The size of the cleaving zygote remains unchanged because at each succeeding cleavage division the blastomeres become smaller.
Untill the 8-cell stage, they form a loosely arranged clump.
Division of zygote into blastomeres begins approximately 30 hours after fertilization.
Cleavage normally occurs as the zygote passes along the uterine tube toward the uterus.
During cleavage zygote is normally within the rather thick zona pellucida.
After the third cleavage, blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions.
This process, compaction, segregates inner cells, which communicate extensively by gap junctions, from outer cells.
Compaction is mediated by cell surface
adhesion glycoproteins.
Compaction permits greater cell-to-cell
interaction and is a prerequisite for
segregation of the internal cells that
form the inner cell mass or embryoblast
of the blastocyst.
L., morus, mulberry
Approximately, 3 days after fertilization, cells of the compacted embryo divide again to form a 16-cell morula (mulberry).
Inner cells of the morula constitute the inner cell mass, & surrounding cells compose the outer cell mass.
About the time the morula enters the
uterine cavity, fluids begin to penetrate
through the zona pellucida into the
intercellular spaces of the inner cell
mass.
Gradually, the intercellular spaces
become confluent, and finally, a single
cavity, the blastocoel forms.
At this time, the embryo is a blastocyst.
Cells of the inner cell mass , now called the embryoblast, are at one pole, & those of the outer cell mass, or trophoblast, flatten & form the epithelial wall of the blastocyst.
The zona pellucida has disappeared, allowing implantation to begin.
2 4 6 8 10 12 14 16 18 20 22 24 26 28
2 4 6 8 10 12 14 16 18 20 22 24 26 28
Follicular Phase Luteal Phase
ProgesteroneEstrogen
FSH
LH
19th Nov 2009 19
19th Nov 2009 20
SPIRALSTRAIGHT
19th Nov 2009 21
VERTEBRATES NONVERTEBRATES
ANAMNIOTA
AMNIOTA
Un born young ones are nourished in the uterus by means of PLACENTA
Consist of fatty &aluminous substance
which has been transported to the
cytoplasm and stored as rounded
granules. It provide nutrition to the
developing embryo.
No ovum is totally devoid of yolk.
Yolk is usefull in classifying eggs.
Based on relative abundance of yolk
Small
Medium
Large
Mechanism of development is related to
distribution of yolk within the cell
1 .Isolecithal,(equal yolk)
Those ova that contain little yolk tend to have it
dispersed rather uniformly.
Invertebrates & a ll but lowest mammals
2.Telolecithal (yolk at end)
As yolk become more abudent it tend to
concentrate in one hemisphere .
Various invertebrates
All vertebrates
Marsupial mammals
If yolk is moderate in amount as in
amphibians the egg is termed Mediallecithal.
The large yolk rich egg of bony fish, reptiles
birds are termed Megalecithal
3, Centrolecithal ,
Yolk is massed centrally and is
surrounded by a peripheral shell of clear
cytoplasm.eg in arthropods
This is the series of mitotic cell divisions of the
zygote that results in the formation of early
embryonic cells blastomeres. The size of the
cleaving zygote remains unchanged because at
each succeeding cleavage division the blastomeres
become smaller.
Cleavage is a fractioning process, which provides
building units of convenient size, rather than a
process of truly constructive development.
On the basis of the abundance and distribution of yolk, cleavage is classified as follows:
Entire ovum divides; holoblastic ova.
1. EQUAL: In isolecithal ova;
blastomeres are of
approximately equal size; e.g.,
amphioxus,
marsupials
placental mammals.
UNEQUAL: In moderately telolecithal ova;
yolk accumulated at the vegetal pole retards mitosis, and fewer but larger blastomeres form there;
lower fishes amphibians.
Protoplasmic region alone cleaves; meroblastic ova.
1. In highly telolecithal ova; mitosis is restricted to the animal pole;
higher fishes reptiles, birds monotreme
mammals.
1. In Centrolecithal
ova; mitosis is
restricted to the
peripheral
Cytomlasmic
investment; limited
to the arthropods.
Classification according to division plane is as follow:
Classification according to axis of a spindle is as follow:
Fish shaped animal Representative of low chordates Egg:
0.1 mmContains small amount of yolkIsolecithal ova
Cleavage: Total and nearly equalBegins 1 hour after fertilization
Fig. 50-4 (a-d), p. 1085
Polar body
cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, cell stages
Cleavage in Amphioxus
This group represnts animals whose eggs have a fair amount of yolk.
Egg:1-10mm in diaYolk concentrated toward the
vegetal poleModerately telolecithal
Cleavage:Total Unequal
Animal pole
Vegetal pole
Blastocoel Wall Cavity
Egg:LargeGreat amount of yolkHighly Telolecithal type of egg
Cleavage:PartialDiscoidal
Cleavage produces a modified
blastula (named a discoblastula) in
which the cellular cap is termed the
blastoderm.
The space between the blastomeres
and yolk mass is often called a
blastocoele.
Fig. 50-7b, p. 1086
Epiblast
Hypoblast
Blastocoel
Yolk
Eggs: Microscopic in size Isolecithal
Cleavage: Within the zona pellucida Total Nearly equal Begins in uterine tubes Completed in the uterus
This picture is of the unfertilized egg. It can be differentiated from the zygote by the presence of a
large, conspicuous nucleus (large arrow) with obvious nucleolus (smaller arrow) and by the lack of
a fertilization membrane.
This shows the zygote (fertilized cell). It is recognized by the presence of the fertilization membrane surrounding it
and the peripheral, fluid-filled perivitelline space
This is the two cell stage. By this stage, the zygote has completed its first cleavage, which is both equal and
holoblastic (i.e. the entire ovum is divided into cells). The division (cleavage) has passed through the animal-vegetal
axis, producing two similar blastomeres.
This is the four cell stage. This second cleavage also passes through the animal-vegetal axis, but perpendicular
to the first cleavage . Four equal-sized blastomeres are the end result.
The eight cell stage. Here, the third cleavage has occurred in the equatorial plane . upper four
blastomeres (the animal pole) are slightly smaller than the lower four blastomeres (the vegetal pole).
The thirty-two cell stage is seen here. After the 16 cell stage, the cleavages become more difficult to follow, due to the increasing
number of cells and to the division of blastomeres becoming asynchronous. Cleavage continues, forming a mass of cells which organizes itself into the blastula. The lighter area in the centre of
the embryo is the beginning of the blastocoel.
This shows the early blastula. With continuing cleavage, the cells in centre begin to lose contact with one another, and a
central fluid-filled cavity (the blastocoel) forms. This blastocoel is surrounded by a single layer of cells, forming the
hollow sphere know as the blastula
This shows the late blastula. Like the early blastula, it is characterized by a single layer of cells surrounding the
central hollow area - the blastocoel (B). The blastomeres are seen to be smaller and are individually not as
obvious. The blastomeres at the vegetal pole (VP) are taller than those at the animal pole (AP), making the
vegetal pole appear slightly thicker.
Developmental Anatomy, A Textbook and Laboratory Manual of Embryology By LESLIE BRAINERD AREY, Revised 7th Edition The Developing Human, Clinically Oriented Embryology by KEITH L. MOORE 8th EditionLANGMAN’S Embryology 10th Edition, By T.W.SADLER Google Search for Images