This document discusses the cellular basis of morphogenesis, the biological process responsible for the development of an organism's shape through controlled cell distribution during embryonic development. Key processes involved in morphogenesis include cell sorting, epithelial-mesenchymal transition, cell-cell communication, the role of cell adhesion molecules, the extracellular matrix, and cell contractility, all of which affect how tissues and organs form. Understanding these mechanisms is crucial for advancements in developmental biology, medicine, and regenerative therapies.

1. Department of zoology
Analysis of development
assignment
Submitted to
Prof. Muhammad Tariq
by
Muhammad Mohsin
Roll # 851
Topic:
Cellular basis of
morphogenesis
Contents.
 Introduction
 Cell sorting

2.  Differential Adhesion hypothesis
 Epithelial mesenchymal transition
 Cell-cell communication
 Cell adhesion molecules (CAMs)
 Extracellular matrix
 Cell contractility
Morphogenesis:
Morphogenesis is a biological process that
causes a tissue or organ to develop its shape by
controlling the spatial distribution of cells
during embryonic development. The term
"morphogenesis" is derived from two Greek
words: "morphē," meaning form or shape, and
"genesis," meaning origin or creation. It
encompasses the intricate series of events and
cellular processes that lead to the establishment
of an organism's final, three-dimensional
structure. It may be referred as the precise and
ordered arrangement of cells.
As the cells in our body are organized in a
specific manner and the formation of functional
structures is not because of the random
distribution rather they are organized into
organs and tissues through specific and
sequenced steps. Cells divide, migrate,
communicate and die. The formation of fingers
is always at the top of our hands never in the
middle, our eyes always in our head. This

3. creation of ordered form is called
morphogenesis. It is the process by which
organism develops its shape and form. The fixed
number of divisions of a cell, the presence and
transmission of genetic information from parent
to offspring and many of the stages in the
development of an organism are sequenced. This
sequenced cascade of events is called as
morphogenesis. Morphogenesis is a complex and
highly regulated process that varies across
species and its critical for the formation of
functional and integrated biological structures.
Understanding the molecular and cellular
mechanism underlying morphogenesis is a
fundamental goal in developmental biology.
Here are the key steps of morphogenesis:
 Embryonic development: morphogenesis
primarily occurs during embryonic
development, although it can also play role
in tissue repair and regeneration in adults.
 Cellular processes: morphogenesis
involves various cellular processes such as
cell division, cell differentiation, cell
migration and cell death. These processes
contribute to the formation of tissues and
organs.
 Signaling pathways: cell signaling
pathways like those involving proteins and
genes, play role in morphogenesis. They

4. guide cells to specific locations and
determine their fate.
 Spatial organization: morphogenesis
involves the spatial organization of cells
and tissues, ensuring they are correctly
positioned within the developing organism.
This involves the processes of axis
formation and patterning.
 Genetic control: genetic information
encoded in an organism’s DNA is
responsible for various events of
morphogenesis. Mutations or disruptions in
these genes can lead to developmental
abnormalities.
 External influences: environmental factors
such as nutrition, temperature and
chemicals can influence morphogenesis.
Teratogens are the substances that can
disrupt the normal development.
 Evolutionary significance: morphogenesis
is critical for the evolution of species.
Changes in the genes and processes
controlling morphogenesis can lead to the
development of new traits and adaptations.
 Regeneration: in some organisms like
certain amphibians and starfish,
morphogenesis also plays a role in
regeneration. These animals can regenerate
the lost body parts by the reorganization of
the cells

5.  Medical implications: understanding the
morphogenesis is essential in field like
developmental biology and medicine. It
provides insights to birth defects, tissue
engineering and regenerative medicine.
This process occurs during embryonic
development and continues throughout
organism life.
Cellular basis of morphogenesis.
Cell sorting:
Cell sorting is a critical process in embryonic
development. It refers to the process by which
cells rearrange often driven by differential
adhesion between cells to establish different
tissue layers, structures or organ primordia. The
process of cell sorting plays a pivotal role in
shaping and organizing the tissues and organs
during embryogenesis. Segregation of the germ
layers (ectoderm, mesoderm and endoderm) is
the key process in the embryonic development
that leads to the formation of tissues and
organs. Various factors are involved in the cell
sorting that are discussed below:
1. Differential adhesion hypothesis. We
know that all cells have different proteins
on its surface that leads to the formation
of different structures of tissues and
organs during development. This concept

6. was first introduced by the embryologist
Malcolm Steinberg in the mid-20th
century. It is a key factor in processes
such as tissue segregation, cell sorting,
and tissue patterning during
embryogenesis and organogenesis.
Townes and Holtfreter in 1965 performed
an experiment to understand the
differential cell affinity of various cells in
embryonic development. They took
Presumptive epidermal cell and neural plate cells.
They want to know weather if these cells are
mixed randomly either they will arrange in precise
manner or not. They performed a series of
experiments and got the same thing for ectoderm,
endoderm and mesoderm. So they gave the
concept of differential cell affinity that
“differential affinity reffers to the varying
affunity of cell-cell or cell-extracellular matrix
interaction that play a crucial role in shaping the
developing embryo and determining tissue

7. organization”
Inner ectoderm has positive affinity for mesoderm
and negative affinity for endoderm. Outer
mesoderm has positive affinity for ectoderm while
inner mesoderm has positive affinity for the
endoderm. So, arrangement of germ lines is due to
the selective affinity of cells. Such selective
affinities were also observed by Buocaut in 1974
as he injected the cells from different germ layers
into the body cavity of the amphibian gastrulate.
He founded that these cells migrate to their
appropriate germ layers. Endodermal cells again
set into the host endoderm and the ectoderm cells
were only found in the host ectoderm. thus,
selective affinity is important for cells in
determining the position of various cells.
2.Epithelial mesenchymal transition.
EMT is a biological process that occurs in both
normal and pathological conditions. It involves
the transfer of epithelial cells into mesenchymal
cells, which have different properties and
functions. In EMT, epithelial cells lose the cell-
to-cell adhesion and polarity, gaining migratory
and invasive characteristics. This process is
important during the embryonic development.
This is an important process as it helps in the
following events:

8.  Gastrulation: this process enables the
single layered blastula to into the three
germ layers: ectoderm, endoderm and
mesoderm. Cells in the epiblast undergo
EMT to migrate inward and make three
layers.
 Neural crest formation: EMT is also
important in the formation of neural crest
and we know that neural crest is very
important structure that give rise to
various cells such as neurons, glial cells
and pigment cells. Neural crest cells
undergo this process to form neural tube
and migrate to different locations in the
embryo.
 Organogenesis: this process is involved in
the formation of various organs e.g., in the
formation of heart, EMT play an important
role in the transformation of the
endocardial cells into mesenchymal cells
that contribute the formation of heart
valves.
It is a highly regulated process that helps in
shaping the body and form various tissues and
organs.
3. Cell-to-cell communication
As the embryo passes the eight-cell stage, the
cells become differentiated from one another
due to cell-to-cell communication. This

9. communication regulates the developmental
process by various types of signaling chemicals
that travel to the target cells to bring a specific
response. The most important signaling
mechanisms in the multicellular organisms are
paracrine, endocrine, autocrine and direct
signaling.
From the earliest development the various
developmental processes such as cell adhesion,
migration, differentiation and division are
regulated by the signals from one cell that is
detected by the other cell. These cellular
communications are responsible for the
organogenesis. The development of vertebrate
eye is a good example of cell-to-cell
communication. Light entering through the
transparent cornea focused by the lens on the
retinal cell to bring the response and all this
coordination shows that eye can’t work without
impairing this function of different tissues.
Coordination in the adjacent cells sometime
cause to change their shape, mitotic divisions or
cell fate. This type of interactions between cells
of close range and of different histories and
properties is called as induction. There are at
least two components to every interaction. The
tissue that produces the signal that alters the
function or behavior of another cell is called as
inducer. Often the signal is a secreted protein
called as paracrine factor. These are composed

10. of proteins produced by cell or cluster of cells
and have the ability to alter the behavior and
differentiation of cells. Paracrine factors are
secreted into the extracellular space and they
affect their neighbor cells while the hormones
affect the specific tissues at some distant part of
the body. Responders are the cells that are
affected by the paracrine factors and are
altered. Cells of responding tissue must have
receptor protein for the inducing factor and the
ability to bring the response. The ability to
respond to specific signal is called as
competence. Reciprocal induction, also known
as tissue interaction or tissue induction, is a
fundamental concept in developmental biology.
It refers to the process by which two or more
tissues or cell types communicate with each
other and influence each other's development
and differentiation. Reciprocal induction plays a
crucial role in the formation and patterning of
various organs and structures during embryonic
development. The formation of the vertebrate
eye involves reciprocal interactions between the
optic vesicle and the overlying surface
ectoderm. Signals from the optic vesicle induce
the formation of the lens placode in the surface
ectoderm. In response, the lens placode releases
signals that guide the development of the optic
vesicle into the retina. In tooth development,
reciprocal interactions between the oral

11. epithelium and the underlying neural crest-
derived mesenchyme led to the formation of
specific tooth structures, such as enamel,
dentin, and pulp.
Following are the methods of chemical signaling
in the cell
 Chemical signaling
 Juxtracrine signaling
 Paracrine signaling
 Endocrine signaling
 Autocrine signaling
 Gap junctions
 Extracellular matrix signaling
 Morphogens
 Cell-cell adhesion
 Signaling transduction pathway
4. Cell adhesion molecules (CAMs)
Cell adhesion molecules are diverse group if cell
surface proteins that mediate the physical
interactions between adjacent cells or between the
cells and the extracellular matrix. CAMs are
essential for various developmental processes
including gastrulation, tissue morphogenesis,
organ formation and neural development. They
can be classified into various categories including
cadherins, integrins, selectins and
immunoglobulin superfamily CAMs.

12. o Cadherins are calcium dependent CAMs that
mediate homophilic interactions between
cells. They are especially important in tissue
morphogenesis and maintaining tissue
integrity. Cadherins are critical for processes
like gastrulation, neurulation, and tissue
compartmentalization. N-Cadherin is a
specific type of cadherin that plays a
significant role in cell adhesion in the nervous
system. It is essential for neural cell
migration, axon guidance, and the formation
of neuronal connections. E-Cadherin is a
prominent cadherin involved in cell adhesion
and tissue organization during embryonic
development. It plays a crucial role in the
formation of adherents junctions, which help
hold epithelial cells together and are vital for
tissue integrity.
o Integrins are heterodimeric receptors that
connect cells to the extracellular matrix.
Integrins are a family of cell surface receptors
that mediate cell adhesion to the extracellular
matrix (ECM) and are involved in cell
signaling. They play a role in processes like
cell migration, tissue remodeling, and organ
development. Integrins are crucial for
processes such as embryonic development,
tissue repair, and immune responses.
o Selectins are involved in leukocyte rolling and
adhesion during inflammation. While they are

13. not as directly associated with developmental
processes, they play a role in immune cell
trafficking and can indirectly affect tissue
development during inflammation.
o igSF CAMs play role in diverse process such as
axon guidance and synapse formation.
o NCAMs are a group of cell adhesion molecules
specific to neural tissues. They are involved in
processes related to neural development, such
as axon guidance, synapse formation, and
neural cell migration.
CAMs regulate cell adhesion and migration
ensuring that cells are properly positioned during
development. They also play role in the extensive
movements of cells during the process of
gastrulation to form the three germ layers
ectoderm, mesoderm and endoderm. They also
have role in neural development as N-cadherin is
involved in the axon guidance and synapse
formation. The tissue repair and regeneration
through the life of an organism also require the
CAMs. They facilitate the migration and the
adhesion of the cells at the wound.
Homophilic binding.
Homophilic binding also known as the homotypic
binding is a type of molecular interaction in which
identical molecules or receptors on the surface of
adjacent cells bind to each other. For example,

14. homophilic binding between cadherin molecules
on the surface of neighboring cells.
Heterophilic binding.
Heterophilic binding also known as heterotypic
binding is a type of molecular interaction in which
different molecules or receptors on the surface of
adjacent molecules bind to each other. For
example, during nervous system development
axon guidance molecules on the surface of
growing axons interacts with receptors in the
target cell through the heterophilic binding.
5. Extracellular matrix.
The extracellular matrix is a complex network
of proteins and carbohydrates that surrounds
and support cells within tissues. Like the
genetic information stored on DNA ECM also
play role in the developmental process. It
primarily composed of proteins that provide the
integrity to the cell and tissues. The ECM is a
passive structural scaffold and influences the
cell behavior. The ECM is a complex network of
proteins and carbohydrates that surrounds cells
in tissues and provides mechanical support, as
well as important cues for cell behavior. Cells
interact with extracellular matrix through
surface receptors like integrins which transmit

15. signal that regulate cell adhesion, migration and
differentiation.
During embryogenesis, the ECM guides the cell
movement and tissue morphogenesis. It
provides directional cues to cell to migrate to
their appropriate locations, ensuring the proper
developmental process. Organogenesis also
follows the same process and interact with the
extracellular matrix.
ECM components in stem cell niches regulate
the self-renewal and differentiation of stem
cells. Stem cells interact with the ECM to
maintain their multipotent state and to
differentiate into multiple cell types when
needed. The specific composition and
organization of the ECM vary among different
tissues and developmental stages. Proteins like
collagen, fibronectin, laminin, and
glycosaminoglycans (e.g., hyaluronic acid) are
major components of the ECM. Changes in ECM
composition and structure can have profound
effects on tissue development and can
contribute to developmental disorders and
diseases. Any disturbance in the regulation of
the extracellular matrix communication can lead
to developmental disorders.
6.Cell contractility.
Cell contractility is a fundamental cellular
process that plays a crucial role in

16. morphogenesis which is the process of
shaping and organizing tissues and organs
during embryonic development contractility
refers to a cell s ability to generate
mechanical forces through the contraction of
its cytoskeleton particularly actin filaments
and myosin motor proteins these mechanical
forces are instrumental in driving various
morphogenetic processes here’s how cell
contractility contributes to morphogenesis
cell shape changes one of the most obvious
ways in which cell contractility contributes to
morphogenesis is by altering cell shape actin
and myosin filaments generate contractile
forces allowing cells to change their shape
and size this is particularly important during
processes like gastrulation where cells
undergo dramatic shape changes to form germ
layers and establish body axes tissue folding
and invagination contractile forces within
groups of cells can drive tissue folding and
invagination for example during neural tube
formation in vertebrates cells at the neural
plate border contract their apical surfaces
leading to the invagination of the neural tube
convergent extension convergent extension is
a process in which cells intercalate and
elongate causing tissues to narrow along one
axis and lengthen along another this process
is essential for tissue elongation and axis

17. elongation during development such as in the
development of the vertebrate neural tube and
the elongation of the notochord cell sorting
cell contractility also plays a role in cell
sorting where cells with similar adhesive
properties tend to aggregate together
differential contractility can lead to cell
segregation into distinct tissue layers or
domains within a developing structure lumen
formation in processes like tube formation e g
blood vessel or digestive tract formation
contractility helps create and maintain
lumens the hollow cavities within tubes or
organs contractile forces can squeeze out the
central region of a cell cluster forming a
lumen cell migration contractility is crucial
for cell migration which is essential for tissue
patterning and organogenesis cells generate
traction forces to move within tissues and
contractile forces are involved in cell
detachment pulling and pushing during
migration apical constriction apical
constriction is a process where cells contract
their apical surfaces often driven by
contractile actomyosin networks this is
important in processes like tissue
invagination where specific cells at the
leading edge constrict to create inward
bending tissue tension cell contractility
contributes to tissue tension which helps

18. maintain tissue integrity and organization
balanced contractility within tissues is
essential for maintaining shape and
preventing deformations disruptions in cell
contractility can lead to severe developmental
defects and diseases for example defects in
neural tube closure can result from aberrant
cell contractility leading to neural tube
defects like spina bifida understanding the
molecular mechanisms underlying cell
contractility and its regulation is critical for
comprehending how tissues and organs form
during embryonic development and for
exploring potential therapeutic interventions
for developmental disorders and diseases
References.
[1]: Developmental biology by Scott F. Gilbert
and Michael J.E Barresi
[2]: Bard, JBL (1990) morphogenesis: the
cellular and molecular process of developmental
anatomy Cambridge university press.
[3]: www.slideshare.com
[4]: Thierry J.P. Epithelial-mesenchymal
transitions in tumor progression. Nat. Rev.
cancer.2002;2:442-454

19. [5]: seminars in cell and developmental biology
volume 107, November 2020, pages 147-160