ZOOHCC - 601: Developmental Biology (Theory)
Fertilization (External and Internal): Changes in gametes
External and internal fertilization are two different reproductive
strategies used by organisms to achieve successful fertilization. These
strategies involve specific changes in gametes (sperm and eggs) to
facilitate the fusion of genetic material. Let's explore the changes
that occur in gametes during external and internal fertilization:
External Fertilization
External fertilization occurs when the fusion of gametes takes place
outside the body of the organism. This strategy is commonly observed in
aquatic environments, where organisms release their gametes into the
water. The changes in gametes during external fertilization are as
follows:
Sperm Changes
Sperm Activation: When released into the water, sperm undergoes
activation. This process involves an increase in motility, allowing the
sperm to swim towards the eggs.
Chemotaxis: Sperm may be attracted to chemical signals released by the
eggs. This helps guide the sperm towards the vicinity of the eggs.
External Fertilization Adaptations: Sperm cells may have adaptations
such as streamlined shape, flagella for swimming, and an acrosome
containing enzymes for penetrating the egg's protective layers.
Egg Changes:
Egg Release: The eggs are released into the water, often in large
numbers to increase the chances of successful fertilization.
Protective Layers: Eggs may have protective layers, such as jelly coats
or membranes, to prevent desiccation and provide mechanical protection.
Chemical Signals: Eggs release chemical signals known as attractants to
guide sperm towards them
Internal Fertilization
Internal fertilization occurs when the fusion of gametes takes place
inside the reproductive tract of the organism. This strategy is common
in terrestrial animals and some aquatic organisms. The changes in
gametes during internal fertilization are as follows:
Sperm Changes
Enhanced Motility: Sperm cells undergo modifications to enhance their
motility and increase their chances of reaching the egg. They may
possess flagella or other structures for swimming.
Capacitation: Sperm cells undergo a process called capacitation within
the female reproductive tract. This process involves changes in the
sperm's plasma membrane, enabling it to penetrate the egg.
Acrosome Reaction: During fertilization, the acrosome at the tip of the
sperm releases enzymes that help the sperm penetrate the egg's
protective layers.
Egg Changes:
Zona Reaction: The zona pellucida, a protective layer surrounding the
egg, undergoes changes upon sperm contact. These changes prevent
polyspermy, the fertilization of an egg by multiple sperm.
Cortical Reaction: Upon fertilization, the egg releases cortical
granules, which modify the zona pellucida to further prevent polyspermy.
Egg Activation: Fertilization triggers changes within the egg, such as
activation of metabolic processes and initiation of cell division.
In both external and internal fertilization, the changes in gametes are
aimed at increasing the likelihood of successful fertilization and
subsequent development of the embryo. These changes are crucial for the
fusion of genetic material, the prevention of polyspermy, and the
initiation of embryonic development.
Blocks to polyspermy
Polyspermy refers to the fertilization of an egg by more than one
sperm. In most species, polyspermy is prevented through a series of
mechanisms known as "blocks to polyspermy." These blocks ensure that
only a single sperm fuses with the egg, allowing for successful
fertilization and the development of a viable embryo. Here are the
main blocks to polyspermy:
Fast Block
The fast block is the first line of defense against polyspermy and
acts immediately upon fertilization.
When the sperm fuses with the egg, it triggers a rapid change in the
egg's electrical potential, resulting in a membrane depolarization.
This depolarization creates a temporary electrical barrier that
prevents additional sperm from fusing with the egg.
Slow Block:
slow block
Also known as the cortical reaction, is a more sustained mechanism
that reinforces the prevention of polyspermy.
After the sperm fuses with the egg, it triggers a series of
intracellular changes within the egg.
Calcium ions are released into the egg's cytoplasm, leading to several
reactions:
Planes and patterns of cleavage
Planes and patterns of cleavage refer to the ways in which a
fertilized egg undergoes cell division during early embryonic
development. These divisions help in the growth and differentiation of
cells, leading to the formation of a multicellular organism. The
planes and patterns of cleavage can vary among different organisms.
Here are some common types:
Radial Cleavage: In radial cleavage, the cell divisions occur along
both the vertical and horizontal axes, resulting in daughter cells
that are aligned directly above or below each other. This pattern is
observed in organisms such as echinoderms and some annelids. Radial
cleavage leads to the formation of a layered arrangement of cells.
Spiral Cleavage: In spiral cleavage, the cell divisions occur at an
oblique angle to the vertical axis, resulting in a spiral arrangement
of daughter cells. This pattern is observed in organisms such as
mollusks and some annelids. Spiral cleavage leads to a characteristic
spiral arrangement of cells in subsequent developmental stages.
Bilateral Cleavage: In bilateral cleavage, the cell divisions occur
predominantly along the vertical axis, resulting in daughter cells
that are aligned next to each other. This pattern is observed in
organisms such as arthropods and chordates, including humans.
Bilateral cleavage leads to the formation of a bilateral symmetry in
the later stages of development.
Rotational Cleavage: In rotational cleavage, the cell divisions occur
at an oblique angle to each other, resulting in a rotational
arrangement of daughter cells. This pattern is observed in some
organisms, such as certain annelids. Rotational cleavage leads to the
formation of a twisted arrangement of cells.
Types of Blastula
The blastula is a stage of embryonic development characterized by a
hollow ball of cells called blastomeres. The blastula stage follows
the cleavage of the zygote and precedes the formation of the gastrula.
The types of blastula can vary based on different factors such as the
arrangement of blastomeres and the presence or absence of specific
features. Here are some common types of blastula:
Coeloblastula: In coeloblastula, the blastomeres are arranged as a
single layer of cells surrounding a central fluid-filled cavity called
the blastocoel. This type of blastula is found in organisms like sea
urchins and some amphibians.
Stereoblastula: In stereoblastula, the blastomeres are arranged as
multiple layers of cells, forming a solid mass with no central cavity.
Stereoblastula is observed in some insects, annelids, and certain
bivalve mollusks.
Discoidal Blastula: In discoidal blastula, the blastomeres form a
disc-like structure with a blastocoel confined to a small area at one
pole. This type of blastula is found in birds, reptiles, and monotreme
mammals.
Superficial Blastula: In superficial blastula, the blastomeres divide
without undergoing complete cytokinesis, resulting in a single large
multinucleated cell layer called the syncytium. This type of blastula
is observed in insects such as Drosophila.
Blastocyst: Blastocyst is a specialized type of blastula found in
mammals. It has an outer layer of trophoblast cells that will
contribute to the placenta and an inner cell mass that will give rise
to the embryo. The blastocoel is often present in a small cavity
within the blastocyst.
Fate maps (including Techniques)
Fate mapping is a technique used in embryology to determine the
developmental fate of cells during embryonic development. It involves
labeling or tracing specific cells or groups of cells to track their
lineage and understand their contribution to different tissues and
organs in the developing embryo. Here are some common techniques and
approaches used in fate mapping:
Vital Dyes: This technique involves injecting or applying vital dyes,
such as vital fluorescent dyes or vital dyes that change color upon
chemical reaction, to specific cells or regions of the embryo. As the
embryo develops, the labeled cells or tissues can be visualized under
a microscope to track their migration and differentiation.
Genetic Labeling: Genetic labeling techniques utilize transgenic
animals or genetic manipulations to express fluorescent proteins or
other markers specifically in certain cell populations. By introducing
genetic markers into the embryo, researchers can track the labeled
cells and observe their fate throughout development.
Cell Transplantation: In this technique, cells from one embryo or
region are transplanted into another embryo or specific location
within the same embryo. By labeling the donor cells before
transplantation, their fate can be tracked as they integrate into the
recipient embryo and contribute to different tissues and organs.
Lineage Tracing: Lineage tracing involves permanently marking specific
cells or populations of cells with genetic or molecular markers that
are heritable and can be passed on to daughter cells during cell
division. This allows the tracking of cell lineages and the
determination of the ultimate fate of labeled cells.
Time-lapse Imaging: Time-lapse imaging techniques utilize advanced
imaging technologies, such as confocal microscopy or live-cell
imaging, to capture sequential images of developing embryos at
specific time intervals. This enables researchers to observe cell
movements, divisions, and differentiation in real-time and track the
fate of labeled cells.
Retroviral and Lentiviral Tracing: Retroviral and lentiviral vectors
can be used to deliver genetic markers into specific cells or regions
of the embryo. The viral vectors integrate into the host genome and
pass on the genetic marker to daughter cells during cell division.
This allows for long-term lineage tracing and fate mapping of labeled
cells.
These techniques and approaches provide valuable insights into the
developmental potential and lineage relationships of cells during
embryonic development. They help researchers understand the complex
processes involved in tissue and organ formation, as well as the
mechanisms underlying developmental disorders and diseases.
Early development of frog and chick up to gastrulation
Frog Development:
Fertilization: External fertilization occurs in frogs. The male
releases sperm onto the eggs as the female lays them in water.
Fertilization takes place externally, and the sperm swim to the eggs
to penetrate and fuse with them.
Cleavage: After fertilization, the zygote undergoes rapid cell
divisions called cleavage. The cleavage divisions produce a solid ball
of cells called a morula.
Blastula Formation: Cleavage continues, and the morula transforms into
a hollow ball of cells called a blastula. The blastula consists of an
outer layer of cells called the blastoderm and an inner fluid-filled
cavity called the blastocoel.
Gastrulation: Gastrulation marks the beginning of the formation of the
three germ layers: ectoderm, mesoderm, and endoderm. The process
involves cell movements and rearrangements. The blastula undergoes
invagination, in which cells at one pole of the blastula fold inward,
forming a structure called the blastopore.
Germ Layer Formation: During gastrulation, cells from the blastula's
surface move inward through the blastopore. The cells that enter
through the blastopore form the endoderm, which gives rise to the
digestive tract and associated organs. The remaining cells on the
outside of the embryo form the ectoderm, which will develop into the
skin, nervous system, and other structures. The mesoderm, which
develops between the ectoderm and endoderm, gives rise to muscles,
skeleton, blood, and other organs.
Chick Development:
Fertilization: Internal fertilization occurs in birds. The male
deposits sperm into the female's reproductive tract, and fertilization
takes place inside the female's body.
Cleavage: After fertilization, the zygote undergoes cleavage
divisions, similar to frogs. However, chick cleavage is characterized
by the presence of a large yolk, which is concentrated at one end of
the embryo and provides nutrients for development.
Blastoderm Formation: The cleavage divisions create a disc-shaped
structure called the blastoderm on top of the yolk. The blastoderm
consists of the blastodermic disc, which contains the future embryo,
and the area pellucida, a transparent region in the center of the
disc.
Gastrulation: Gastrulation in chicks involves the formation of three
germ layers, similar to frogs. The cells in the area pellucida begin
to invaginate, forming a groove called the primitive streak. The
primitive streak serves as the equivalent of the blastopore in frogs.
Germ Layer Formation: Cells migrate and move along the primitive
streak. Some cells move inward and give rise to the endoderm, which
forms the digestive tract and associated organs. Other cells remain on
the surface and become the ectoderm, which develops into the skin,
nervous system, and other structures. The mesoderm forms between the
ectoderm and endoderm, giving rise to muscles, skeleton, blood, and
other organs.
After gastrulation, the development of both frogs and chicks continues
with the differentiation and further organization of the germ layers
to form various organs and structures.
Embryonic induction and organizers
Embryonic induction and organizers play crucial roles in the
development of embryos, coordinating cell interactions and guiding the
formation of different tissues and organs. These processes involve the
secretion of signaling molecules and the establishment of specific
regions called organizers that influence nearby cells. Let's explore
these concepts further:
Embryonic Induction:
Embryonic induction refers to the process by which one group of cells
influences the development of neighboring cells. During induction,
certain cells release signaling molecules, known as inducers, which
interact with the target cells and trigger specific cellular
responses. These responses can include changes in gene expression,
cell differentiation, and morphological transformations.
Induction can occur between cells within the same germ layer (e.g.,
mesoderm inducing mesoderm) or between different germ layers (e.g.,
mesoderm inducing ectoderm). It plays a fundamental role in
establishing the body plan and specifying different tissues and organs
during embryogenesis.
Organizers:
Organizers are specialized regions within the developing embryo that
have the ability to induce neighboring cells and direct their
development. These regions produce key signaling molecules, called
morphogens, which have concentration gradients that provide positional
information to surrounding cells.
Two well-known examples of organizers are the Spemann organizer in
amphibian embryos (e.g., frogs) and the Hensen's node in chick and
mammalian embryos. The Spemann organizer, located in the dorsal lip of
the blastopore, releases signaling molecules, including members of the
bone morphogenetic protein (BMP) and Wnt families, which are critical
for dorsal-ventral patterning and the formation of the central nervous
system.
Similarly, Hensen's node, a structure that forms at the anterior end
of the primitive streak in chick embryos, serves as an organizer. It
releases nodal signaling molecules, among others, which influence the
formation of the anterior-posterior axis and induce the formation of
the notochord and other mesodermal structures.
These organizers exert their influence through the secretion of
specific signaling molecules, which diffuse and create concentration
gradients. Cells in different regions of the embryo receive different
concentrations of these molecules, leading to distinct developmental
fates and tissue differentiation.
Overall, embryonic induction and organizers play vital roles in
embryogenesis by orchestrating cell communication and providing
instructive signals for proper tissue and organ formation. They
contribute to the establishment of the body plan and the development
of specialized structures in a coordinated and precise manner.
