Human embryocleavage

موضوع: Human embryocleavage الأحد أغسطس 28, 2011 2:00 pm

Human
embryocleavage

Cleavage
of human zygote begins 30 hours after fertilization in uterine tube (tubal
period) and lasts during next 6 days (till 7^th). Since 5-6 day
embryo enters the uterine space (uterine period). As a result multicellular
organism is developing. His name is “blastula” which consists of blastomers.
During all this period of time blastula has a constant size and is surrounded
by tunica of fertilization.

Cleavage process directly depends on the type of oocyte, his yolk inclusions
volume and disposition). Human oocyte is olygolecital II isolecital that is why
human zygote has full (complete) subequal
asynchronic cleavage. It means all the zygote is dividing into
blastomers (fully) and all the blastomers, which appear during the cleavage,
are almost equal in size. The differences between blastomers may be seen just
after the first division: one of the blastomers is a little bit smaller and
lighter than the other one. The blastomeres are oval and lie parallel to each
other. The nucleus of each cell becomes invisible about 1^st hour
before the next division.

The division of two-cell stage is dichotomus, but the blastomeres do not divide
synchronously. At this stage, as at subsequent stages, the larger light cell
divides first. The larger cell divides 40 h after fertilization and three-cell
stage is formed. So, different blastomers are dividing at different time
(asynchronically), their amount changes in such way: 2, 3, 5 …

After the nine-cell stage the blastomeres change their shape and tightly align
themselves against each other to form a compact ball of cells. This phenomenon
is called compaction. It is mediated by cell surface adhesion glycoproteins
(ovomorulin). Compaction permits greater cell-to-cell interaction. When there are 12 to 32 blastomeres, the developing
conceptus is called morula. The
spherical morula exists about 3 days after fertilization been rolling in the uterine tube.

About 72 hours after fertilization human morula contains 12 blastomeres. The
12-cell stage consists of: 11 small light peripheral cells, which surround a
larger (dark) centrally
placed one. Smaller cells are more numerous and divide more rapidly. They form
outer cell mass. Larger cells divide slower and they few in number. Internal
cells of the morula are called inner cell mass.

The morula is surrounded by tunica of
fertilization, which is of great importance in keeping the blastomeres in a
restrained and compacted cluster. Morula remains enclosed by
abovementioned tunica through which it is nourished by diffusion of oxygen and
low molecular metabolites from uterine tube secretions (histiotrophic
nutrition).

During
2-3-4^th days blastula consists
of blastomers (till 32), which are tightly connected one to each other and it
looks like a raspberry. It is morula. It
rolls along the uterine tube to uterine cavity and reach it at the 5^th
day. At that time blastula consists of larger dark blastomers (embryoblast), which are surrounded with
smaller slightly flattened and light ones – trophoblast
and contains the space inside (blastocel).

The blastocyst cavity (blastocel) appears between the trophoblast and the inner
cell mass and separates them except of one side where they remain in contact.
Such type of blastula is called “blastocyst”, it appears as a result of full subequal asynchronic cleavage. The wall of blastocyst (trophoblast)
is known as blastoderm. It has abembryonic pole (over embryoblast) and opposite vegetative pole.

The next 2-3 days is the stage of free
blastocyst because it has no tight contact with maternal organism.
During this time it moves over the surface of endometrium untill abembryonic
pole will attach to to mucosa between ducts (necks) of uterine glands, which
are actively producing mucous.

Embryoblast

Tunica of fertilization

Blastocel

Trophoblast

Early blastocyst

Late blastocyst

موضوع: رد: Human embryocleavage الأحد أغسطس 28, 2011 2:01 pm

Human
cleavage normally occurs as the zygote passes along the uterine tube toward the
uterus. Blastomeres become smaller with each cleavage division until they rich
nuclear-cytoplasmic ratio typical for somatic cells. In the blastocyst stage
zona pellucida becomes thinner and disappears. The trophoblastic cells, which
have the capacity to invide the mucosa, to come into direct contact with the
endometrium.

» الكونغ فو History Channel Human Weapon Kung Fu.720P