1. Embryonic development involves several key phases including fertilization, cleavage, blastulation, gastrulation, and organogenesis which establish the basic body plan and three germ layers.
2. During blastulation, the blastocyst forms as cells differentiate and fluid accumulates, leading to hatching from the zona pellucida.
3. Gastrulation further rearranges cells into three germ layers- endoderm, mesoderm, and ectoderm- from which all major organs and tissues will develop.
3. Phases of Embryonic Development
Steps
1. Fertilization
2. Cleavage
3. Blastulation
4. Gastrulation
5. Organogenesis
4. Process whereby two sex cells (gametes) fuse together to
create new individual with genetic potentials derived from both
parents.
Have two separate activity:
1. Combining of genes derived from the two parents.
2. Creation of new organisms.
Thus
The first function is: Transmit genes from parents to
offspring.
The second is : initiate reactions in the egg cytoplasm that
proceed development.
Also, Restoration of the diploid number of chromosomes
reduced during meiosis.
Fertilization
5. It requires 3 critical events
1. Sperm migration between cumulus cells
2. Sperm attachment and migration
through the zona pellucida
3. Fusion of sperm and ovum plasma
membranes
(Hafez et al., 2006)
6. • Mitotic cell division of zygote without an increase in cell mass.
• Initial nutrients come from the cytoplasm with continued development- by
oviductal and uterine secretions known as .
• Mitotic divisions transform a one-celled embryo(zygote) into a multicellular
embryo.
• After the first cleavage, the cells are referred to as
• The blastomeres of the early developing zygote are
• Once the zygotes reach the 8-16 cell stage, they are called
7. During late morula stage, blastomere cells begin to differentiation into two
distinct populations: the inner and outer cells.
Inner cells develop gap junctions
- Important for intercellular communication
Outer cells develop tight junctions
- Alter permeability of the outer cells allowing for fluid accumulation
inside of the embryo
Inner cell mass gives rise to the embryo proper.
Trophectoderm or Trophoblast forms the chorion .
• Blastocyst undergoes cellular hyperplasia and fluid accumulation.
• Results in the formation of the blastocoele (fluid-filled cavity).
• - release of blastocyst from the zona pellucida. Day 4 to 8
days post ovulation.
10. Blastocyst Elongation
• Rapid growth of the conceptus occurs during the second week of gestation.
• Elongation is logarithmic and filamentous.
, the blastocyst has extended into the contralateral
uterine horn.
- elongation is slow and takes days to finish.
- elongation is very and occurs within a couple of hours.
Pig embryos can become as long a meter in length.
- does not elongate but increases in diameter 2 to 3 mm per day.
Embryo become large and spherical like a baseball
12. Sp. 2-cell 4-cell 8-cell Morula Blastocyst Hatching
Cow 24 h 1.5 d 3 d 4-7 d 7-8 d 9-11 d
Ewe 24 h 1.3 d 1.5 d 3-4 d 6-7 d 7-8 d
Mare 24 h 1.5 d 3 d 4-5 d 6d 8 d
Sow 14-16 h 1.0 d 2.5 d 3.5 d 5-6 d 6 d
(Hafez et al., 2006)
15. • Rearranges the cells of blastula into a three-layered
(triploblastic) embryo, called a gastrula, that has a primitive
gut.
• Characterized by cell movement.
• loss of zona ICM Embryonic disc
16. • The formation of three primary embryonic germ layers
– Endoderm (inner)
– Mesoderm (middle)
– Ectoderm (outer)
1. Ectoderm- forms exterior tissues including:
Nervous system and mammary glands
2. Mesoderm - forms structural tissue including:
Muscle, Circulatory system, and reproductive
system
3. Endoderm - forms internal organs including:
Digestive system, liver, and endocrine glands
18. FORMATION OF THE PLACENTA
Occurs after embryo hatching and involves massive growth of the conceptus
A. Development of extra embryonic membranes
- Filamentous/thread like structures in the pig. Sheep and cow, but spherical in the horse
B. Used by fetus to attach to the uterus
C. Discarded after birth
D. Consists of 4 membranes
1.
- Develops from trophoblast and endoderm
- Filled with fluid and serves to protect the embryo from mechanical perturbations
2.
- Like amnion, develops from trophoblast and endoderm
- Gives rise to fetus portion of the Placenta and becomes outermost layer of the
placenta
- Does not contain blood vessels
19. 3. Allantois
- Originates from splanchnic mesoderm and rises
forms from the hindgut
- If fuses with the chorion forms the allanto-chorion
and becomes fetus portion of the placenta
- Contains blood vessels
- From splanchnic mesoderm & rises from midgut of
embryo
- Site of primordial germ cells
- It regresses as the allantois develops
- Contains blood vessels
20. Placental Attachment
• Attachment or fusion of the placenta to the
endometrium of the uterus.
Cow 22
Ewe 15
Mare 37
Sow 13
(Hafez et al., 2006)
22. Types of Placental Attachment (Based on chorionic villous
pattern & maternal-fetal barrier)
26. Cotyledonary placenta, characterized by the large number
of discrete button-like structures called cotyledons
In ruminants the
connection is characterized
by the presence of
placentomes consisting of:
Cotyledon – fetal
attachment coming from
the fetus.
Caruncle –maternal
attachment coming from
the uterine mucosa.
Male and female pronuclei, along with the first and second polar bodies are present in ootid. Fusion of male and female pronuclei into a single diploid nucleus constitutes syngamy. The single-celled embryo (zygote) undergoes cleavage (mitotic division) to give rise to two daughter cells called blastomeres. Mitotic divisions continue until a morula is formed. The morula develops into a blastocyst consisting of an inner cell mass (ICM), a blastocoele cavity and a trophoblast. Finally, the rapidly growing blastocyst “hatches” from the zona.
The primitive endoderm forms beneath the inner cell mass and begins to grow downward (arrows). B) As the primitive endoderm grows, an evagination in the ventral inner cell mass forms the yolk sac. C) The newly formed primitive endoderm fuses with the trophoblast to form a double membrane called the chorion. The chorion pushes upward and begins to surround the embryo. At the same time a new sac, called the allantois (A), begins to form the primitive gut. D) The Yolk Sac (YS) regresses and the allantois expands. The chorion nearly surrounds the embryo. E) When the leading edges of the chorion fuse, a complete sac, called the amnion, surrounds the embryo and forms the amnionic cavity. The yolk sac continues to regress while the allantois expands, making contact with the chorion. The allantois and chorion eventually fuse, forming the chorioallantoic membrane