2. Formation of Germ Layers

6. Highlights
• Fertilization .
• Sex determination.
• Cleavage .
• Formation of germ layers.

7. Highlights
• Fertilization .
• Morula = 16 cells ovum
• Trophoblast =
outer cell layer that covers the inner cell mass in
the Morula.
• Blastocyst =
morula with the inner cell mass is separated from
trophoblast by fluid.

8. Highlights (continue)
Embryonic disc =
• the cells of the inner cell mass multiply, and
arranged to form an embryonic disc having two
germ layers.
Amniotic cavity =
• a cavity appears on the ectodermal side of the
disc.
Yolk sac =
• a cavity appears on the endodermal side .

9. Highlights (continue)
• Extra-embryonic mesoderm =
this separate trophoblast from the amniotic
cavity and the yolk sac.
• Extra-embryonic coelom =
a cavity appears and splits the Extra-amniotic
mesoderm into Somatopleuric layer and
Splanchnopleuric layer.

10. Blastocyst with an inner cell mass and trophoblast.

12. Highlights (continue)
Somatopleuric (parietal) layer in contact with
• trophoblast.
• Splanchnopleuric (visceral) layer in contact
• with yolk sac.
• Chorion =
a membrane consists of trophoblast and
underlying Somatopleuric mesoderm.

13. Highlights (continue)
• The cells forming the wall of the amniotic cavity
form the Amnion.
• The connecting stalk =
A mesoderm attaches the amniotic cavity to
trophoblast, into which the extra-embryonic
coelom does not extended

14. Highlights (continue)
• Prochordal plate =
a rounded area near one edge of ectoderm in the
embryonic disc where ectoderm and endoderm are
not separated by mesoderm.
• Primitive streak = an elevation is seen on the embryonic disc
• If a line is drawn between the primitive
streak and the prochordal plate the
embryonic disc is divided into right and
left halves.

15. Highlights (continue)
Mesoderm =
• 3rd germ layer from cell of the primitive
streak (move between ectoderm and endoderm).
Cloacal membrane =
• a round area caudal to the primitive disc. It
is made up only of ectoderm and endoderm.

16. Details

17. Fertilization
• Fertilization of the ovum occur in the ampulla
of the uterine tube.
• As soon as the spermatozoon enters the ovum,
2nd meiotic division is completed.
• Ovum becomes the FEMALE PRONUCLEUS.
• The head of spermatozoon becomes the MALE
PRONUCLEUS. (head separates from the middle piece and tail).

18. Fertilization (continue)
• Pronuclei lose their nuclear membrane.
• These 46 chromosomes undergo changes like those
in a typical mitotic division having two cells.
• There is NO one-cell stage of the
embryo

19. Sex Determination
 The sex of a child is determined at the time of fertilization.
All ova contain 22 - X chromosome.
Spermatozoa contain either 22-X or 22-Y.
If the sperm is X-bearing,
• The zygote has 44-XX chromosomes. The offspring is a girl.
If the sperm is Y bearing,
• The zygote has 44-XY chromosomes. The offspring is a boy.
It is clear that one chromosome of each of the 23
pairs is derived from the mother and other from the
father.

20. Test Tube Babies
In vitro fertilization =
• outside the body.
In vivo =
• within the body

21. Cleavage

23. Age (in days) Developmental events
2 Embryo is two-cell stage
3 Morula is formed
4 Blastocyst is formed
8 Bilaminar disc is formed
14 Prochordal plate and primitive streak is seen
16 Intra-embryonic mesoderm/ 3 layered disc

24. Cleavage
• The process of subdivision of ovum into smaller cells.
• As cleavage proceeds the ovum comes to have 16 cells = morula.
• Morula ; looks like mulberry.
still surrounded by zona pellucida.
consist of inner mass + trophoplast.(outer layer).
The inner cell mass gives rise to the embryo proper,
and is therefore, called the Embryoblast.
The cells of the trophoblast help to provide nutrition
to the embryo.

25. Cleavage (continue)
• Some fluid passes into the morula separates the
inner cell mass from the trophoblast.
• As the quantity of fluid increases, the
morula acquires the shape of cyst.
• The cells of the trophoblast become flattened,
and the inner cell mass comes to be attached to
the inner side of the trophoblast only,
The morula has become a (blastocyst).

26. Cleavage (continue)
• Morula has now become blastocyst.
• The cavity of blastocyst is called blastocoele.
• The side of the blastocyst to which the inner cell mass
is attached is called the Embryonic pole (or animal ).
• While the opposite side is the abembryonic pole.
• Zona pellucida prevent implantation of the blastocyst
at an abnormal site.

27. Function of Zona Pellucida
Trophoblast :
able to stick to uterine (or other) epithelium.
able to eat-up other cells
can invade and burrow into tissues.
 Zona Pellucida prevent it from sticking to the epithelium
during the travel of embryo down the uterine tube.

28. Function of Zona Pellucida (continue)
The embryo receives nutrition partly from the
substances stored in the ovum, and partly by
diffusion from uterine secretion.
Additional source of nutrition is achieved
when the blastocyst sticks to the uterine
endometrium, and gets implanted to it.
Zona pellucida disappears soon after the
morula reaches the uterine lumen.

29. Function of Zona Pellucida (continue)!!!!!!!!!!!!!!!!
Zona pellucida lacks histocompatibility
antigens and acts as a barrier that separate
maternal tissues from the embryo,
After disappearance of Zona pellucida
::::::::::.??????!!!!!!!!!

30. Function of Zona Pellucida (continue)
• (various immunosuppressive cytokinase and
proteins are produced by the implanting
embryo.
• This block the recognition of embryo as
a tissue foreign to the mother).

31. Formation of Germ Layers.
Blastocyst gives rise to the tissue and organs of the
embryo, also gives rise to a number of structures
that support the embryo and help it to acquire
nutrition.
At a very early stage the embryo proper acquires
the form of three-layered disc, this is called
EMBRYONIC DISC.
 The embryonic disc (embryonic area, embryonic
shield, or germ disc) is consist of three layers:
1. Endoderm.
2. Ectoderm.
3. Mesoderm .

32. Formation of Germ Layers.(continue)
 Blastocyst further changes:
1.Endoderm =
1st germ layer. Formed from flattened cells of the inner mass.
2.Ectoderm =
2nd germ layer. Formed from columnar cells of the inner mass.
3.Amniotic cavity = (filled by amniotic fluid or liquor amnii)
a space between ectoderm (below) and the trophoblast (above).
Amniogenic cells form the roof, while the floor is formed by ectoderm.
4.Primary yolk sac =
The blastocystic cavity is lined on all sides by cells of endodermal origin.
This lining membrane is called (Heuser’s membrane).
This cavity is called the primary yolk sac.
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33. Formation of Germ Layers. / Blastocyst further changes (continue)
5.The cells of the Trophoblast give origin to a mass of
cells called the EXTRA-EMBRYONIC MESODERM (or
primary mesoderm).
EXTRA-EMBRYONIC MESODERM:
does not give rise to any tissues of the embryo itself.
lies outside the embryonic disc.
lies between the trophoblast and the lining of yolk sac.
separates the wall of the amniotic cavity from the trophoblast.
6. Small cavities appear in the EXTRA-EMBRYONIC MESODERM,
these join together to form EXTRA-EMBRYONIC Coelom
(chorionic cavity).

34. Formation of Germ Layers. / Blastocyst further changes (continue)
The EXTRA-EMBRYONIC MESODERM is split into two layers.
a) Somatopleuric
(parietal) extra-embryonic mesoderm = lining inside of the
trophoblast, and outside of the amniotic cavity.= chorionic plate.
b) Splanchnopleuric
(visceral) extra embryonic mesoderm = lining the outside of
the yolk sac.
 Connecting stalk =
part of EXTRA-EMBRYONIC MESODERM attaching the
embryo, amniotic cavity and the yolk sac to the wall
of the blastocyst.

35. Formation of Germ Layers. / Blastocyst further changes (continue)
7.Formation of chorion and amnion:
Chorion =
formed by parietal extra-embryonic mesoderm.
Amnion =
derived from trophoblast.
chorion and amnion play an important role in
child birth (parturition).
8.Secondary yolk sac:
Primary yolk sac become much smaller and its lining
becomes cuboidal and called secondary yolk sac.

36. Formation of Germ Layers. / Blastocyst further changes (continue)
9. At this stage,
the embryo proper is a circular disc composed of
two layers of cells (ectoderm and endoderm).
10. Prochordal plate:
cubical cells of the endoderm become
columnar at one circular area. (enable us to
distinguish its future head and tail ends).

37. Formation of Germ Layers. / Blastocyst further changes (continue)
11. Primitive streak:
some ectodermal cells lying along the central axis, begin to
proliferate, and form a rounded elevation that bulges into
the amniotic cavity, at first it is rounded then becomes a
linear structure in the central axis of the disc.
12. Intra-embryonic mesoderm:
Primitive streak cells coming between ectoderm and
endoderm. These cells form the intra-embryonic mesoderm
(secondary mesoderm), which is the third germ layer.
Gastrulation:
the process of formation of primitive streak, and of intra-
embryonic mesoderm by the streak.

38. Formation of Germ Layers. / Blastocyst further changes (continue)
13.Bucco-pharyngeal membrane:
it is the end result of prochordal plate.
14.Cloacal membrane:
an area caudal to the primitive streak, where
ectoderm and endoderm remain in contact
(like prochordal plate, no mesoderm separate them).

39. Alternative View of Formation of Germ Layers
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
• Epiblast = ectoderm.
• Hypoblast = endoderm.
• All the three germ layers are derived from
the epiblast ????????!!!!!!!!!!!!!!!!!!!!!!!!!
• Other workers consider that the prochordal
plate and the neural crest are from the intra-
embryonic mesoderm.

40. Use of Stem Cells in the Treatment of Disease!!!!!!!!!
• Embryonic stem cells =inner cell mass.
• If exposed to certain factors, the stem cells can form
various types of adult cells.
• Parkinson’s disease/ Alzheimer disease/ Diabetes/ myocardial
infarction/ Blood diseases/ Severe burns/ Osteoporosis/
Spinal cord injury.--------!!!!?????? IMMUNE REACTION.
• The nucleus of patient cell is introduce into the
embryonic stem cell. ???? ETHICAL OBJECTION.
• Adult stem cells are difficult to culture in laboratories.

41. Time Table
Pre-organogenesis period = 14 days
 Development of the embryo from fertilization to the
formation of the bilaminar disc, where no organs are yet
recognizable.
 Anomalies during this period result in death.
Gastrulation : (3rd to 8th week)
 Establishment of the primitive streak and formation of the
intra-embryonic mesoderm.
 Most congenital anomalies are produced by teratogens
acting during this period.

42. Age (in days) Developmental events
2 Embryo is two-cell stage
3 Morula is formed
4 Blastocyst is formed
8 Bilaminar disc is formed
14 Prochordal plate and primitive streak is seen
16 Intra-embryonic mesoderm/ 3 layered disc

43. Thank You
• Next lecture = we will continue from the slide
number 32, and continue further
development of embryonic disc.
• Placenta = assignment