Mammalian development begins with fertilization inside the female's body. The zygote then undergoes cleavage divisions that are slower than in other animals. Around the 8-cell stage, the cells compact together and later form two distinct cell types - the inner cell mass and trophoblast cells. The trophoblast cells go on to form the blastocyst, with an inner cell mass surrounded by trophoblast cells. The blastocyst hatches from the zona pellucida and implants in the uterus, where the trophoblast cells invade the uterine tissue and the inner cell mass forms the embryo.

1. EARLY
MAMMALIAN DEVELOPMENT

2. Name Arsalan aslam 4602
Sahar ali 4606
S.Ashal Fatima 4623
Topic of presentation Early mammalian
development
Presented to Ahmad Raza Azam

3. • Contents
• Early mammalian development
• Unique nature of mammalian cleavage
• Fertilization
• Type of cleavage
• Early genome activation
• Stages of development
• Compaction
• Morula stage
• Cavitation and blastocyst formation
• Escape from zonapellucida
• Implantation

4. Early mammalian development
1.Small in size
• Mammalian eggs are among the smallest in the
animal kingdom.
• The human zygote is only 100µm in diameter and
hardly visible to the eye.
2.Less in number
• Mammalian zygotes are produced less in number.
• Fewer than ten eggs are ovulated by a female at a
given time.
3.Internal fertilization
• The development of mammalian embryos is
accomplished internally rather than in the external
environment.

5. The unique nature of mammalian cleavage
• Mammalian cleavage is different from most other
patterns of embryonic cell division.
• Before fertilization the mammalian oocyte is released
from the ovary and swept by the fimbriae into the
oviduct.

6. a) Fertilization
• Fertilization occurs in the ampulla of the
oviduct, a region close to the ovary.
• Meiosis is completed after sperm entry
• First cleavage begins about a day later.

8. b) Slowest cleavages
• Cleavage in mammalian eggs are among the slowest in
the animal kingdom (12-24 hours).
• The cilia in the oviduct push the embryo toward the
uterus the first cleavages occur along this journey.
c) Rotational cleavage
• First cleavage is meridional division.
• In Second cleavage one of the two blastomeres divides
meridionallt and other divides equatorially called
rotational cleavage.

11. d) Asynchoronous cell division
• Mammalian blastomeres do not all divide at the same time.
• Mammalian embryos do not increase exponentially from 2- to
4- to 8-cell stages.
e) Early activation of the genome
• The mammalian genome is activated during early cleavage, and
produces the proteins necessary for cleavage to occur.
• Example: In the mouse and goat, the switch from maternal to
zygotic control occurs at the 2-cell stage.

12. Stages of development of mammalian embryo
• Most research on mammalian development has focused
on the mouse embryo.
• Mice are relatively easy to bread
• Can be housed easily in laboratories

14. Compaction
• Mouse blastomere through 8 cell stage form a loose
arrangement with plenty of space between them.
• After the third cleavage, cell adhesion proteins like
E-cadherin become expressed and blastomeres
suddenly clump together and form a compact ball of
cells this phenomenon is called compaction.

16. • This tightly packed arrangement is stabilized by tight
junctions that form between the outside cells of the
ball, sealing off the inside of the sphere.
• The cells within the sphere form gap junctions,
thereby enabling small molecules and ions to pass
between them.

18. Morula
• The cells of the compacted 8-cell embryo divide to
produce a 16-cell morula (consist of small group of
internal cells).
a) Inner cell mass (ICM)
• The internal cells are called inner cell mass.
• They give rise to the embryo.
b) Trophoblast
• Most of the descendants of the external cells become
the trophoblast (trophectoderm) cells. They produced
no embryo. Form the tissue of chorion.
• The formation of the trophoectoderm is the first
differentiation event in mammalian development.

20. c) Totipotent
The earliest blastomere can form both trophoblast and the
embryo precursors cell these very early cells called
totipotent.
d) Pluripotent
The inner cell mass is said to be pluripotent .
Each cell of ICM can generate any cell type in the body.

21. e) Cavitation and blastocyst formation
• Initially, the morula does not have an internal cavity. However,
during a process called cavitation.
• The trophoblast cells secrete fluid into the morula to create a
blastocoel.
• The plasma membranes of the trophoblast cells contain a sodium
pump (a Na+/K+-ATPase) facing the blastocoel, and these proteins
pump sodium ions into the central cavity. This accumulation of
sodium ions draws in water osmotically, thus enlarging the
blastocoels.
• The inner cell mass is positioned on one side of the ring of
trophoblast cells
• The resulting structure, called the blastocyst, is another hallmark of
mammalian cleavage.

24. Escape from the zonapellucida
• While the embryo is moving through the oviduct en route to
the uterus, the blastocyst expands within the zona pellucid.
• The zona pellucida prevents the blastocyst from adhering to
the oviduct walls. When such adherence does take place in
humans, it is called an ectopic or tubal pregnancy.
• A trypsin like protease secreted by the trophoblast seems
responsible for hatching the blastocyst from the zona.

26. Implantation
• Once out, the blastocyst can make direct contact
with the uterus. The uterine epithelium
(endometrium) “catches” the blastocyst on an
extracellular matrix containing collagen, laminin,
fibronectin, hyaluronic acid, and heparan sulfate
receptors.
1. Loose attachment
L_selectin on the trophoblast cell adhering to
sulfated polysaccharides on the utrine cells.

27. 2. Stable attachement
• The trophoblast cells synthesize integrinns on the
trophoblast and uterine collagen, fibronectin and
laminin and they synthesize heparin sulfate
proteoglycan precisely prior to implantation.
• p-cadherins on the trophoblast and utrine
endometrium also help to attach the embryo to the
uterus.

28. 3. Digestion of uterine tissue
• Once in contact with the endometrium, the
trophoblast secretes another set of proteases,
including collagenase, stromelysin, and
plasminogen activator. These protein-digesting
enzymes digest the extracellular matrix of the
uterine tissue, enabling the blastocyst to bury itself
within the uterine wall

30. Formation of Extraembryonic Membranes
1. cytotrolphoblast
2. synctiotrophoblast
3. blood vessels and umbilical cord
4. chorion and decidusa-placenta formation

31. CYTOTROPHOBLAST
• A layer of trophoblast cells
• Adhers to endometrium (initially)
• ContainProteolytic Enzymes
 remodeling of utrine Blood vessels
• Screate Paracrine factors
 attract maternal blood vessels
 gradually displace vascular tissues (become lined
with trophoblast cells

33. Synctiotrophoblast
• Trophoblast cells undergo nuclear division without
cytokinesis
• Multinucleated cells form synctiotrophoblast
• help in progression of embryo into utrine wall
(by digesting utrine tissue)

34. BLOOD VESSELS & UMBILICAL CORD
• Mesodermal tissues extend outward from embryo
• Yolk Sac + Primitive Streak derived cells form
Extraembryonic mesoderm
• Extraembryonic mesoderm joins the trophoblastic
extensions give rise to blood vessels
 carry nutrients from mother to embryo
• Narrow connecting stalk forms the vessels of umbilical
cord

36. CHORION & DECIDUA (placenta formation)
 Fully developed extraembryonic organ
 trophoblast tissue + blood vessels containing
mesoderm
 Fuses with decidua to creat placenta
 placenta
 Maternal portion (decidua)
 Fetal portion (chorion)

38.  Embryo is encased in amnion & Chorion
 blood vessels to and from chorion are observable
• blood vessels increased large area exposed to mother
blood
 villi are projected on outer surface of chorion
• diffusion of soluble substances occur through villi
 mother→nutrients & oxygen
 fetus→ wastes (CO2 & urea)

41. THANK YOU