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.

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Follicular Phase Luteal Phase

ProgesteroneEstrogen

FSH

LH

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SPIRALSTRAIGHT

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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