- Mammalian eggs are among the smallest in the animal kingdom. After sperm entry, the male and female pronuclei fuse without forming a zygote nucleus. Cleavages are slow, about 12-24 hours apart. - Following the third cleavage, the blastomeres compact into a ball and further divide. The morula forms, with an inner cell mass and outer trophoblast cells. Cavitation occurs as trophoblast cells secrete fluid. - The blastocyst implants in the uterus. The inner cell mass forms the embryo and associated structures, while trophoblast cells form the chorion and fetal part of the placenta. Gastrulation begins through a primitive streak, forming

1. GAMETE FUSION AND EARLY
DEVELOPMENT IN MAMMALS
Dr.J.JOONU
Asst.Prof,
Department Of Zoology,
Bishop Heber College,
Trichy-17.

2. MAMMALIAN EGG
• Mammalian eggs are
among the smallest in the
animal kingdom, making
them hard to manipulate
experimentally.
• The human zygote, for
instance, is only 100 μm
in diameter—barely
visible to the eye and less
than one-thousandth the
volume of a Xenopus
egg.

3. Gamete fusion
• In mammals the entry of sperm is tangential to the
eggs surface. Once inside the egg, the sperm
nucleus becomes male pronucleus.
• The sperm centrosome produces asters and
contacts the female pronucleus. Male & female
pronuclei migrate towards each other.
• They remain adjacent to each other;
• their nuclear envelopes break down but instead of
forming a zygote nucleus the chromatin
condenses into chromosomes orienting them on a
common mitotic spindle.

5. The unique nature of mammalian
cleavage
• The mammalian oocyte is released from the ovary
and swept by the fimbriae into the oviduct .
• Fertilization occurs in the ampulla of the oviduct,
a region close to the ovary.
• Meiosis is completed at this time, and first
cleavage begins about a day later .
• Cleavages in mammalian eggs are among the
slowest in the animal kingdom—about 12–24
hours apart.

7. • The first cleavage is a normal meridional
division; however, in the second cleavage, one
of the two blastomeres divides meridionally
and the other divides equatorially .
• This type of cleavage is called rotational
cleavage (Gulyas 1975).
• The third major difference is the marked
asynchrony of early cell division.

8. Compaction
• Following the third cleavage, however, the
blastomeres undergo a spectacular change in
their behavior.
• They suddenly huddle together, maximizing
their contact with one another and forming a
compact ball of cells

9. Compaction process

10. • The cells of the compacted 8-cell embryo divide to
produce a 16-cell morula (Figure 11.22E).
• The morula - small group of internal cells surrounded
by a larger group of external cells (Barlow et al. 1972).
• The descendants of the external cells become the
trophoblast (trophectoderm) cells -it forms the tissue of
the chorion, the embryonic portion of the placenta.
The chorion –
• fetus to get oxygen and nourishment from the mother.
• It also secretes hormones that cause the mother's uterus
to retain the fetus,
• It produces regulators of the immune response so that
the mother will not reject the embryo.

11. • These cells generate the inner cell mass (ICM),
gives rise to the embryo and its associated yolk
sac, allantois, and amnion.
• By the 64-cell stage, the inner cell mass
(approximately 13 cells) and the trophoblast cells
-separate cell layers.
• Thus, the distinction between trophoblast and
inner cell mass blastomeres - the first
differentiation event in mammalian development.
• This differentiation is required for the early
mammalian embryo to adhere to the uterus

12. MAMMALIAN BLATOCYST
Implantation of the mammalian blastocyst into the
uterus. (A) Mouse blastocysts entering the uterus.
(B) Initial implantation of the blastocyst in a rhesus
monkey.

13. CAVITATION PROCESS
• a process called cavitation, the
trophoblast cells secrete fluid
into the morula to create a
blastocoel.
• The inner cell mass is positioned
on one side of the ring of
trophoblast cells.
• the blastocyst, is another
hallmark of mammalian
cleavage.

15. • This fetal organ—the chorion—is derived
primarily from embryonic trophoblast
cells, supplemented with mesodermal
cells derived from the inner cell mass.
• The chorion forms the fetal portion of the
placenta. It will induce the uterine cells to
form the maternal portion of the placenta,
the decidua. The decidua becomes rich in
the blood vessels that will provide
oxygen and nutrients to the embryo.

16. The origins of early mammalian tissues are summarized in .
• The first segregation of cells within the inner cell mass results
in the formation of the hypoblast (sometimes called the
primitive endoderm) layer .
• The hypoblast cells delaminate from the inner cell mass to line
the blastocoel cavity, where they give rise to the
extraembryonic endoderm, which forms the yolk sac.
• The remaining inner cell mass tissue above the hypoblast is
now referred to as the epiblast. separate the embryonic epiblast
from the other epiblast cells, which form the amnionic cavity .
• Once the lining of the amnion is completed, it fills with a
secretion called amnionic (amniotic) fluid, which serves as a
shock absorber for the developing embryo while preventing its
desiccation.
• The embryonic epiblast is believed to contain all the cells that
will generate the actual embryo, and it is similar in many ways
to the avian epiblast.

18. • Gastrulation begins at the posterior end of the embryo,
and this is where the node forms
• the mammalian mesoderm and endoderm migrate
through a primitive streak, the migrating cells of the
mammalian epiblast lose E-cadherin, detach from their
neighbors, -migrate through the streak as individual
cells (Burdsall et al. 1993).
• Those cells migrating through the node give rise to the
notochord - integrated into the endoderm of the
primitive gut (Jurand 1974; Sulik et al. 1994).
• These cells can be seen as a band of small, ciliated cells
extending rostrally from the node .
• They form the notochord by converging medially and
folding off in a dorsal direction from the roof of the gut.

19. • The original type of trophoblast cells constitute
a layer called the cytotrophoblast, whereas the
multinucleated type of cell forms the
syncytiotrophoblast.
• The cytotrophoblast initially adheres to the
endometrium -adhesion molecules.
• These cells also contain proteolytic enzymes
that enable them to enter the uterine wall and
remodel the uterine blood vessels so that the
maternal blood bathes fetal blood vessels

21. The ventral surface a the 7.5-day
mouse embryo, seen by scanning
electron microscopy.
The presumptive notochord cells are
the small, ciliated cells in the
midline that are flanked by the larger
endodermal cells of the primitive
gut.
The node (with its ciliated cells) is
seen at the bottom. (B) The
formation of the notochord by the
dorsal infolding of the small, ciliated
cells

22. Human embryo after 50 days of
gestation

23. Chorionic villi to maternal blood
supply

24. Formation of Twins
Fraternal twins – result of two
separate fertilization events.
Identical twins – formed from
a single embryo
Identical twins – produced by
separation of early
blastomeres or by the
separation of inner cell mass
into two regions within the
same blastocyst
Identical twins – 0.25%
33%- Dichorionic before the
formation of trophoblast at
day 5.

25. TWINS