Genetics and epigenetics of ADHD and comorbid conditions
Zygote 2
1.
2. Zygote from Greek word zygotos “joined” or “yoked”, from
Zygoun “to join” or “to yoke”
It is a eukaryotic cell formed by fertilization between two
gametes.
The genetic material of sperm and egg combine to form a
single cell called zygote.
The zygote’s genome is a combination of the gamete, and
contains all of the genetic information necessary to form a
new individual.
4. •The progressive changes which are undergone
before an organism acquires its adult like form
constitute the Embryonic development.
•The study of development of an animal from fertilized
egg to birth or hatching is called embryology.
•The process of embryonic development comprises following stages:-
Fertilization •Cleavage
•Blastulation •Gastrulation
•Neurulation •Organogenesis
•Growth •Differentiation
5. •Fertilization takes place when the spermatozoon has successfully
entered the ovum and the two sets of genetic material carried out
by gametes fuse together, resulting in a zygote. This process of
fusing of gametes is known as fertilization.
Fig.1.Sperms entering the ovum
6. •Following fertilization, the zygote undergoes a mitotic division
Called cleavage.
•The beginning of the cleavage process is marked when the zygote
divides through mitosis into two cells. This mitosis continues and the
first two cells divide into four cells, then into eight cells and so on..
•The first division results in formation of two identical
cells called blastomere. Mitotic division occur repeatedly.
the cells get smaller and smaller with each division.
•When the cells number around sixteen the solid sphere of cells
it is referred as a morula.
7. •The process of forming blastocoels and blastulas is termed
as blastulation.
•The hollow–sphere embryo which develops at the end
of cleavage is called blastula. Surrounded by single
layer of cells.
•The cells keep pulling away from central area, forming a
Fluid cavity knows as blastocoel.
This embryonic stage in mammals is called blastocyst.
•The blastocyst is a fluid-filled hollow sphere composed of a single
Layer of large, flattened cells trophoblast cells & a small cluster of
20 to 30 rounded cells, called the inner cell mass, located inside.
Fig. cleavage in human zygote
9. •Gastrulation is other major phase of embryonic development
which is characterized by differentiation of embryonic layers.
•In this process, the embryo is transformed from a hollow ball of
cells, the blastula into three layered stage called the Gastrula.
•The three layers are :-
i. Ectoderm[it forms the nervous system & the skin epidermis]
ii. Endoderm[it forms the epithelial lining of digestive, respiratory &
urinogenital system & associated glands]
iii. Mesoderm[it forms the virtually everything else i.e. bones, muscles,
kidneys etc]
A blastopore develops where
the cells have ingressed, in
one side of the embryo.
10. Fig.1. showing the cleavage process from
single celled zygote to gastrula
Fig.2. showing gastrula of mouse
11. •The ectoderm gives rise to epithelial and neural
tissue, and the gastrula is now referred to as the neurula.
•It is the process of folding of neural plates to neural folds
and then it transformed into neural tube & this takes place
during the fourth week.
12. •The process of formation of organs is called
organogenesis.
•The masses of cells of the three germinal layers split up
into smaller groups of cells, called primary organ
rudiments that further divided into secondary organ
rudiments. The embryo begins to show somewhat
similarity to the adult animal or to larva if development
include larval stage.
13. • The increase in mass of embryo is achieved by
synthesis of new nuclear material & cytoplasm and
by cell multiplication and this process is known as
growth.
•During this period the organ rudiments of the
embryo begin to grow and greatly increase in their
volume. In this way the embryo gradually achieves
the size.
14. •The differentiation refers to the events by
which parts become different from one
another and also different from what they
were originally.
