POST FERTILIZATION

EVENTSAnushi JainRoll No. : 08

Paper IVMSc Part II

INTRODUCTION» Embryogenesis is the process by

which the embryo forms and develops.

» During the course of development, cells divide, migrate, and specialize.

» Embryogenesis starts with the fertilization of the egg cell by a sperm cell, (spermatozoon). Once fertilized, the ovum is referred to as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo.

POST FERTILIZATION EVENTS

» Post-fertilization events refers to all the events of sexual reproduction taking place after fertilization, that is, after the formation of zygote.

» The major event taking place in this stage is embryo-genesis, i.e., development of the embryo.

» Post fertilization basically involves :i. Early embryonic development,ii. Establishing multi-cellularity,iii. Formation of blastula,iv. Formation of embryonic germ layer.

After fertilization, cleavage occurs. Cleavage is a series of mitotic division which results in formation of smaller cells,

called blastomeres.

The rate of mitotic division slows down, the blastomeres then undergoes movements wherein they change their position relative

to one another. This is known as gastrulation which involves formation of the three germ layers, i.e., The ectoderm, the

endoderm and the mesoderm.

Once the three germ layers are established, the cells interact to with one another and rearrange themselves to form tissues and

organs. This is known as organogenesis.

EMBRYONIC GERM LAYERS» Gastrulation is the process of highly

coordinated cell and tissue movements whereby the cells of the blastula are dramatically rearranged.

» During gastrulation, these cells are given new positions and the multilayered body plan of the organism is established.

» The cells that will form the endodermal and mesodermal organs are brought inside the embryo, while the cells that will form skin and nervous system are spread over its outside surface.

» Thus, three germ layers, i.e.,

ectoderm, endoderm and mesoderm are formed during the gastrulation.

» Gastrulation involves some combination of different type of movements :

FORMATION OF GERM LAYER

» At a very early stage in development of embryo, the embryo acquires a three layered disc form called “The embryonic disc”.

» The three layers that constitute this embryonic disc are : ectoderm, mesoderm, and endoderm.

» The blastocyst is a spherical cyst that is lined by flattened trophoblast cells, and inside it there is a mass of cells, the inner cell mass.

Some of the inner cell mass differentiate into flattened cells, that come to line its free surface.

This constitute the Endoderm.

The remaining cells of the inner cell mass become columnar and form the second germ layer, the

Ectoderm.

A space appears between the ectoderm and trophoblast, called

amniotic cavity, filled with amniotic fluid.

The flattened cells that arises from the endoderm spreads and line the

inside of the blastocystic cavity. This lining is called Heuser’s membrane. In this a way cavity is formed called

the primary yolk sac.

The cells of the trophoblast give rise to a mass of cells called extra

embryonic mesoderm.

Small cavities appear in the extra embryonic mesoderm. Gradually, these

cavities join together to form larger cavity, i.e., chorionic cavity (extra embryonic

coelom). This caused the extra embryonic mesoderm to split into two layers, i.e.,

splanchopleuric and somatopleuric extra embryonic mesoderm.

The developing embryo is now suspended into the extra embryonic coelom and it is attached to the blastocyst by connecting

stalk.

At this stage, two important membranes are formed, i.e., chorion

and amnion.

With the appearance of extra embryonic mesoderm and later of the extra embryonic

coelom, the yolk sac becomes smaller in size and is called secondary yolk sac. Also

there is a change in the nature of lining cells, they are no longer flattened but become

cubical.

The cells of the endoderm then form the prochordal plate which determines the

central axis of the embryo and enables to distinguish between head end and tail end.

After the formation of prochordal plate, the ectodermal cells begin to proliferate, and

form primitive streak. The cells that proliferate in this region pass sideways and

form intra embryonic mesoderm or secondary mesoderm.

TRACKING OF MIGRATING CELLS

» Cell migration is a central process in the development and maintenance of multicellular organisms.

» The blastula possesses polarity and bilateral symmetry. It contains cell areas, which becomes germ layer and give rise to different parts of the embryo.

» Tracking of these cells in blastula is used to denote/study which specific region will become which part of the embryo.

» Tracking the migrating cells helps to map the location of ectoderm, mesoderm and endoderm.

» It helps to understand the patterning of the early embryo as this plays a key role in ensuring the correct development of the embryo.

» This process showing the fate of each part of blastula is called as a “FATE MAP”.

Fate map can be constructed by following methods :A. By using vital stainsB. By using carbon particles

A. BY VITAL STAINS :» This method was devised by Walter

Vogt in 1929. » It consists of soaking a piece of agar

in a vital dye (Nile blue sulphate, neural red, bismark brown), and then applying the piece of agar to the surface of the embryo in necessary position,

» Dye diffuses from agar and the cells of the embryo to which the agar was applied takes up the sufficient dye to produce a stain on the surface of the embryo.

» Once the cells were effectively labeled, the agar chip could be

removed and the embryo was allowed to develop normally.

» With this method, it was possible to discern movements of particular cell populations and the ultimate organ or tissue into which they integrated.

» Recently, cellophane has been used as stain carrier instead of agar.

B. BY CARBON PARTICLES :» The cells of the developing

embryo can be marked by another method independent of vital stains, i.e., by carbon particles.

» This method was developed by N. Spratt in 1946.

» In this method, tiny particles of carbon are applied to the surface of the embryo.

» These particles stick to the surface of the embryo and hence can be used as markers enabling to track the movement of the cells and

draw the fate maps.» Radioactive labelling of the

blastomeres is the most technique used.

» Such techniques helps in clear understanding of gastrulation and representation of the destiny of the particular area of the blastula.

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