Blastulation
(Formation of Blastula):
The process of rearrangement of
blastomeres and formation of central fluid filled cavity in the morula is
called blastulation.
Site: It
occurs in upper part of uterus.
Duration:
It is completed after 5 to 6 days of
fertilization.
Morula continuously absorbs the uterine
fluid.
The morula increases in size from 0.15
mm to 0.3 mm in diameter.
The outer layer of cells becomes flat
called trophoblast (Gr. Trophos = to feed) or trophoectoderm.
It absorbs the uterine fluid and
collects between inner mass cells, and trophoblast.
The space between inner mass cells and
trophoblast is called central cavity of blastocoel or blastodermic vesicle.
The inner mass of megameres shifts to
dorsal surface and attaches itself to trophoblast and now looks like a knob at
one pole.
The knob gives rise to the embryo and is
called embryonal knob.
The side of blastocyst to which
embryonal knob is attached is known as the embryonic or animal pole, and
opposite side as the abembryonic pole/ Vegetal pole.
The trophoblast does not take part in
the formation of the embryo proper.
It remains external to the embryo and
gives rise to the extraembryonic membranes, namely, chorion and amnion.
The trophoblast cells in contact with
the embryonal knob are known as cells of Rauber.
These cells form only protective and
nutritive extra-embryonic membranes.
As a blastocyst is formed, zona
pellucida becomes thinner and finally disappears.
Implantation
(Nidation):
The attachment of blastocyst to the
uterine wall is called implantation.
Site: It
occurs on the endometrium of the uterus (near the fund us).
Period: It
starts about 6-7 days after fertilization. It is continues for 3-4 days and is
completed by the 9th or 10th after fertilization. It is under control of
hormones.
During
implantation:
i. Zona pellucida dissolves
ii. The exposed trophoblast or
tropho-ectoderm comes in contact with endometrial lining in the region of
embryonic pole,
iii. The trophoblast develops two layers
in the region of contact between the blastocysts and endometrium. The outer
layer is called syncytiotrophoblast and inner cytotrophoblast.
iv. Trophoblast cells secrete lytic
enzymes, cause breakdown of some endometrial cells, absorb nourishment and
divide themselves to form villi
v. Villi penetrate endometrium for
fixation and absorption of nourishment.
vi. Now the embryo is completely hurried
in the endometrium of uterus.
Once this process of implantation has
occurred, the conception is complete and the pregnancy starts. After
implantation the syncytiotrophoblast cells secrete human chorionic gonadotropin
(HCG) which stimulates corpus luteum to secrete estrogen and progesterone.
Progesterone suppresses next
menstruation.
By second month, the trophoblast
develops into a membrane called chorion which takes over the function of corpus
luteum and continues to secrete estrogen and progesterone.
Progesterone also causes increased
secretion of mucus in the cervix of the uterus that forms a protective plug
during pregnancy.
(REFER DIAGRAMS FROM TEXTBOOK)
VERY SHORT
ANSWER QUESTIONS
1.
What is trophoblast or trophoectoderm?
2.
What are the different terms of
blastula cavity?
3.
What are cells of Rauber?
4.
What is blastocyst?
5.
Explain the term embryonic pole?
6.
What is abembryonic pole?
7.
What is the contribution of trophoblast
in embryo development?
8.
When does zona pellucida disappear?
9.
What is the rice of blastula?
10.
When blastocyst formation is completed?
11.
What is the source of nourishment for
blastula?
12.
What is implantation?
13.
When implantation take place?
14.
Name the layers developed by
trophoblast?
15.
How syncytiotrophoblast is produced?
16.
Which type of enzymes is released by
syncytiotrophoblast?
No comments:
Post a Comment