Cleavage in human embryo:
After the fertilization process is done the resulting zygote undergoes rapid and repeated mitotic division which result in the formation of bunch of small cells . This process in known as cleavage.
Cleavage is different from normal cell division in which the cells become smaller and smaller but the total volume of the embryo remains constant.
Cleavage of the zygote occurs in the fallopian tube of the uterus.
It takes 30 hours to complete the first cleavage which divides the single celled zygote longitudinally in to two blastomeres of un equal size and then followed by another division after 10 hour from the 1st cleavage in right angle to the plane of 1st cleavage resulting in to four blastomeres. During early cleavage the embryo slowly moves towards uterus.
Morula stage-
In fourth day the embryo reaches the uterus during this the embryo reches 16- 32 called stage called morula ,
This is the solid mass of cell ,which still surrounded by zonapellucida that never allows its attachment to the inner wall of the uterus.
Blastocyst stage-
Further cleavage result in to blastula or blastocyst . The outer layer of morula absorbs fluid which stored in a cavity within the cell mass is called blastocoel or blastocyst cavity ,and each individual cell in the mass of cells is referred to as blastomeres.
The blastocyst contains two types of cells- an outer epithelial layer known as trophoblast which surroundes the inner cell mass.
The end of blastocyst which contains inner cell mass is known as embryonic pole and the opposite is called abembryonic pole . Cells of inner cell mass gives rise to embryonic proper and extra embryonic membranes the cells of trophoblast gives chorion and placenta.
The extra embryonic membranes comprises of - amnion,yolk sac, and allantois.
1. Amnion:
It is a ectodermal derivative , which forms proactive fluid filled capsule around embryo known as amniotic fluid.
2. Yolk sac:
This is a endodermal derivative, which have no function in mammalian embryo but it performs nutritive function in others.
3. Allantois:
It is derived from endoderm . It performs waste removal action in mammals.
Patterns of cleavage
The pattern of cleavage is different in different animal groups. This is determined by the amount and distribution of yolk in cytoplasm, which determines where cleavage can occur and the relative size of blastomeres. Basically cleavage is inhibited by yolk .
The yolk is present in the vegetal pole.
There are only four types of eggs are present according to the amount of yolk present in a egg,they are
1. Megalecithal egg in which more amount of yolk is present. e.g birds and reptiles.
It further divided in to telolecithal,where the egg is un evenly distributed all through the cytoplasm, concentrated towards vegetal pole and
centrolecithal egg ,yolk concentrated at the center of the shell.
2. Mesolecithal egg contains moderate amounts of yolk e.g. amphibians. And
3. Microlecithal egg ,in which little amounts of yolk is present. e.g.echinoderms,annelids and tunicates.
4. Alecithal egg contains no yolk e.g. mammals.
Blastomeres that are formed in animal pole are smaller, known as micromeres and blastomeres in vegetal pole, which are large in size compared to animal pole is known as macromeres.
In mammals, which have no yolk, tow equal blastomeres are formed during cleavage.
Cleavage in microlecithal, mesolecithal and alecithal eggs
The cleavage which occurs in eggs with moderate or little yolk is known as holoblastic cleavage.
In this type the cleavage occurs throughout the whole egg.
Holoblastic rotational cleavage is seen in mammals.
The pattern of holoblastic cleavage is of four types- radial,spiral,bilateral and rotational.
1. Radial cleavage- Occurs in deuterostomes like echinoderms, in which the division planes are situated at right angle to each other and it places the two daughter cells over the top of other.
2. Spiral cleavage- In this cleavage the blastomeres are organized in a spiral way around the embryos pole to pole axis. This may be equal or unequal .
Spiral cleavage occurs in annelids and moluscas.
3. Bilateral cleavage- In this cleavage the bisection of the zygote creates two equal halves, left and right halves. The two halves are mirror image of one another.
In bilateral holoblastic cleavage the blastomeres are completely separated in compare to bilateral meroblastic cleavage in which blastomeres are not separated.
This is seen in tunicates.
4. Rotational cleavage- This cleavage divides the zygote in to two daughter cells during first division along the meridonial axis. From these two daughter cells one divides meridonially and other divides equatorially in further division.
Mammals displays rotational holoblastic cleavage and due to the absence of yolk the embryo is immediately implanted in uterine wall for nutrition.
Cleavage in macro or megalecithal egg
Megalecithal eggs are found in fishes,reptiles and birds with small amounts of cytoplasm in animal hemisphere.
In megalecithal egg , cleavage occurs in animal hemisphere and a blastodisc is formed , in further division the blastodisc becomes blastoderm. This type of cleavage is called meroblastic cleavage where the division is incomplete.
Meroblastic cleavage may be discoidal cleavage ( in telolecithal egg) or superficial cleavage ( in centrolecithal egg).
In superficial cleavage, occurs only in the rim of cytoplasm around the periphery of the cell. The cytoplasm surrounding the yolk shows division in to cells but the central yolk remains unaffected. This is seen in drosophilla.
Cleavage may be divided in to determinate and indeterminate according to potentialities of early blastomeres.
In determinate cleavage, early blastomere is destined to become a particular portion of embryo. In indeterminate cleavage each cell have equal ability to make normal embryo, if one cell is removed the remaining will develop into a complete individual if implanted into uterus.
Determinate cleavage is seen in ascaris where as indeterminate cleavage found in human.
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