ZOOHCC - 601: Developmental Biology (Theory)
Fate of Germ Layers – Fate Map
The fate of germ layers refers to the developmental potential and the
specific tissues and organs that each germ layer gives rise to during
embryogenesis. The three germ layers—ectoderm, mesoderm, and endoderm—form
during gastrulation and contribute to the formation of various tissues and
organs in the developing embryo. A fate map is a graphical representation
that illustrates the regions and structures derived from each germ layer.
Let's explore the fate of germ layers and the corresponding fate map:
Ectoderm:
The ectoderm is the outermost germ layer. It gives rise to several
structures, including:
Epidermis: The outermost layer of the skin.
Nervous System: The brain, spinal cord, and peripheral nerves.
Sensory Organs: The eyes, ears, and olfactory epithelium.
Neural Crest Cells: These cells migrate from the neural tube and
differentiate into various structures such as craniofacial bones, pigment
cells, and peripheral nervous system components.
Epithelial Linings: The epithelial linings of the mouth, nose, anal canal,
and sweat glands.
Mesoderm:
The mesoderm is the middle germ layer. It gives rise to diverse
structures, including:
Skeletal System: Bones, cartilage, and connective tissues.
Muscular System: Skeletal muscles, smooth muscles, and cardiac muscles.
Circulatory System: Blood cells, blood vessels, and the heart.
Reproductive System: Gonads, reproductive ducts, and external genitalia.
Kidneys: Renal tissues and the urinary system.
Dermis: The deeper layer of the skin.
Mesenchyme: Mesenchymal cells that differentiate into various connective
tissues.
Endoderm:
The endoderm is the innermost germ layer. It gives rise to several
structures, including:
Respiratory System: Lungs, trachea, and bronchi.
Digestive System: Epithelial linings of the digestive tract, liver,
pancreas, and gallbladder.
Endocrine System: Thyroid, parathyroid, thymus, and adrenal glands.
Bladder and Urinary Tract: Epithelial linings of the urinary bladder and
urethra.
Epithelial Linings: The epithelial linings of various internal organs.
Fate Map:
A fate map is a diagram that illustrates the regions derived from each
germ layer in the developing embryo. It provides a spatial representation
of the different tissues and organs that arise from specific regions of
the embryo. Fate maps are typically generated through experimental
techniques such as labeling cells with dyes or tracers and observing their
subsequent development.
Fate maps are useful tools for understanding embryonic development and
studying the relationships between different cell populations and tissues.
They help researchers visualize and track the fate of cells during
development, allowing for the identification of lineage relationships and
the study of cell differentiation and patterning.
It's important to note that fate maps can vary between different species
and developmental stages, reflecting the specific embryonic patterns and
structures unique to each organism.
Extra-embryonic membranes in birds and mammals
Both birds and mammals possess extra-embryonic membranes, which are
specialized structures that develop alongside the embryo during early
development. These membranes serve various functions, including
protection, nutrient exchange, waste removal, and gas exchange. However,
there are some differences in the specific names and characteristics of
these membranes between the two groups.
Birds:
In birds, the extra-embryonic membranes are as follows:
Amnion: The amnion is a fluid-filled sac that surrounds and cushions the
developing embryo. It helps protect the embryo from mechanical shocks
and provides a moist environment for its development.
Chorion: The chorion is an outer membrane that surrounds the entire
embryo and its associated structures. It is involved in gas exchange,
allowing the exchange of oxygen and carbon dioxide between the embryo
and the environment.
Allantois: The allantois is a membrane that grows from the embryo's
hindgut. It primarily functions as a storage site for metabolic waste
products such as uric acid. In birds, it also plays a role in
respiration and acts as a site for nutrient absorption from the
eggshell.
Yolk Sac: The yolk sac is responsible for providing nutrients to the
developing embryo. It is attached to the embryo's ventral side and
contains the yolk, which is a rich source of proteins, lipids, and other
nutrients.
Mammals:
In mammals, the extra-embryonic membranes are slightly different due to
the unique reproductive characteristics of this group:
Amnion: Similar to birds, the amnion forms a fluid-filled sac that
surrounds and protects the developing embryo. It helps prevent
desiccation and provides mechanical cushioning.
Chorion: The chorion in mammals plays a crucial role in establishing a
connection between the developing embryo and the maternal uterus. It
contributes to the formation of the placenta, which allows for the
exchange of nutrients, gases, and waste products between the maternal
and fetal bloodstreams.
Allantois: In mammals, the allantois fuses with the chorion to form the
chorioallantoic membrane. This composite structure becomes intimately
associated with the maternal uterus and participates in nutrient uptake,
waste removal, and gas exchange.
Yolk Sac: In mammals, the yolk sac is relatively small and does not play
a significant role in nutrient provision, as the developing embryo
receives its nutrients directly from the mother through the placenta.
It's important to note that the specific characteristics and functions
of the extra-embryonic membranes can vary between species within birds
and mammals. The structures described above represent a general overview
of the extra-embryonic membranes found in these groups.
Implantation of embryo in humans
In humans, after fertilization occurs in the fallopian tube, the
resulting zygote undergoes several divisions to form a structure
called a blastocyst. The blastocyst consists of an outer layer of
cells called the trophoblast and a cluster of cells called the inner
cell mass.
Implantation is the process by which the blastocyst attaches to and
embeds itself into the lining of the uterus (endometrium). It is a
critical step in human development and marks the beginning of
pregnancy. The implantation process typically occurs around 6-10 days
after fertilization and involves the following steps:
Reaching the Uterus: After fertilization, the developing embryo
travels through the fallopian tube toward the uterus. It undergoes
further divisions and growth during this journey.
Contact with the Uterine Wall: Upon reaching the uterus, the
blastocyst makes contact with the thickened and prepared uterine
lining (endometrium). This contact triggers changes in the trophoblast
cells, preparing them for implantation.
Attachment: The trophoblast cells on one side of the blastocyst begin
to adhere to the receptive endometrial lining. This initial attachment
occurs via interactions between specific molecules on the trophoblast
and the endometrium.
Invasion: The trophoblast cells undergo further changes and invade the
endometrial tissue. They proliferate and penetrate the uterine lining,
creating spaces for the implantation process.
Formation of the Placenta: The invading trophoblast cells
differentiate into two distinct layers: the outer syncytiotrophoblast
and the inner cytotrophoblast. The syncytiotrophoblast produces
enzymes that facilitate further invasion and remodeling of the
endometrium. The developing placenta, which connects the fetus to the
maternal blood supply, forms from the trophoblast cells.
Maternal Recognition of Pregnancy: During implantation, the
trophoblast cells release hormones such as human chorionic
gonadotropin (hCG). This hormone is detected in the mother's blood and
urine and serves as a signal for the body to maintain the endometrial
lining and support the ongoing pregnancy.
Implantation is a complex and precisely regulated process that enables
the embryo to establish a connection with the maternal uterus and
receive the necessary nutrients and support for further development.
Placenta (Structure, types and functions of placenta)
The placenta is a vital organ that develops during pregnancy in
mammals, including humans. It serves as the interface between the
mother and the developing fetus, providing essential functions for the
growth, development, and survival of the unborn baby. The placenta is
responsible for nutrient and gas exchange, hormone production, waste
elimination, and immune protection. Let's explore the structure,
types, and functions of the placenta:
Structure of the Placenta
The placenta is composed of both fetal and maternal tissues and
consists of the following components:
Chorionic Villi: These finger-like projections are derived from the
outermost fetal membrane called the chorion. Chorionic villi contain
fetal blood vessels that exchange nutrients, oxygen, and waste
products with the maternal blood.
Decidua Basalis: This is the part of the uterine lining (endometrium)
that lies beneath the developing embryo. It forms the maternal side of
the placenta and contains maternal blood vessels.
Placental Membrane: The placental membrane separates the fetal and
maternal bloodstreams while allowing for the exchange of substances.
It consists of layers of cells from both the chorionic villi and the
decidua basalis.
Types of Placenta:
Placentas can vary in structure and organization across different
mammalian species. In humans, there are two main types of
placentation:
Discoid Placenta: This is the most common type of placenta in humans.
It is disc-shaped and approximately 15-20 centimeters in diameter. The
chorionic villi develop on one side of the placenta, which is in
direct contact with the decidua basalis.
Hemochorial Placenta: Humans have a hemochorial placenta, which means
that the maternal blood comes into direct contact with the fetal
chorionic villi. This type of placenta allows for efficient nutrient
and gas exchange between the maternal and fetal circulatory systems.
Functions of the Placenta:
The placenta performs various vital functions during pregnancy:
Nutrient and Gas Exchange: The placenta facilitates the transfer of
oxygen and nutrients, such as glucose, amino acids, vitamins, and
minerals, from the mother's bloodstream to the fetus. It also removes
waste products, such as carbon dioxide and urea, from the fetal blood
and transfers them to the maternal circulation for elimination.
Hormone Production: The placenta produces hormones essential for
maintaining pregnancy and supporting fetal development. These hormones
include human chorionic gonadotropin (hCG), progesterone, estrogen,
and placental lactogen. They help regulate maternal metabolism,
suppress the mother's immune response against the fetus, and prepare
the breasts for lactation.
Waste Elimination: The placenta facilitates the removal of waste
products, including carbon dioxide and urea, from the fetal
bloodstream. These waste products diffuse from the fetal blood vessels
in the chorionic villi to the maternal blood in the decidua basalis,
which carries them away for elimination.
Immunological Protection: The placenta acts as a barrier against
harmful substances and pathogens, protecting the fetus from the
mother's immune system. It allows for the transfer of maternal
antibodies to the fetus, providing temporary passive immunity.
Endocrine Function: In addition to hormone production, the placenta
also acts as an endocrine organ by regulating maternal blood supply
and maintaining appropriate blood pressure levels during pregnancy.
The placenta plays a critical role in supporting the developing fetus
throughout pregnancy by providing essential nutrients, oxygen,
hormonal regulation, and immune protection. Its structure and
functions are crucial for the healthy growth and development of the
unborn baby.
In human females, the type of placenta is known as a hemochorial
placenta. The term "hemochorial" refers to the direct contact and
interaction between the maternal blood and the fetal chorionic villi
within the placenta.
In a hemochorial placenta, the fetal chorionic villi contain blood
vessels that are bathed directly by the maternal blood. This intimate
contact allows for efficient exchange of substances between the
maternal and fetal circulatory systems, including oxygen, nutrients,
hormones, and waste products.
The placenta in humans is discoid in shape, approximately 15-20
centimeters in diameter, and usually located on the uterine wall. It
consists of chorionic villi derived from the fetal side and the
decidua basalis, which is part of the maternal uterine lining.
The hemochorial nature of the human placenta allows for effective
transfer of substances between the mother and the developing fetus,
ensuring the necessary supply of nutrients and oxygen for fetal growth
and development. It also facilitates the removal of waste products
from the fetal circulation, which are then eliminated by the mother's
body.
