Fertilization and Mitosis in Embryology

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Questions and Answers

What is the primary characteristic of mosaic development in embryos?

Cells do not become determined until late in development.

All cells are identical and can perform any function.

Removal of any cell does not affect the overall structure.

Each blastomere contributes certain aspects to the adult animal. (correct)

In regulated specification, what primarily determines blastomere fates?

The size of each blastomere

Information from the environment and neighboring cells (correct)

The genetic code of each blastomere

Random mutations during cell division

What happens if blastomeres separate into two groups in species with regulative development?

One group will not develop at all.

Both groups cannot survive.

Each group can become an independent embryo. (correct)

Development is halted until cells recombine.

Which germ layer is responsible for forming structures such as the nervous system and skin?

Ectoderm Signup and view all the answers

What is the function of the archenteron during gastrulation?

To undergo convergent extension to form a gut-like structure. Signup and view all the answers

What layer forms the lining of the digestive and respiratory tracts?

Endoderm Signup and view all the answers

In humans, what is a consequence of being able to remove a blastomere without harming the embryo?

It reflects regulative development abilities. Signup and view all the answers

What structure results from the invagination of the vegetal pole during gastrulation?

The anus Signup and view all the answers

What role does the gray crescent play during development?

It specifies body axes and other events. Signup and view all the answers

What initiates the cytoplasmic reorganization after fertilization?

The sperm centriole Signup and view all the answers

What is the role of bottle cells during amphibian gastrulation?

They bulge inward to form the dorsal lip. Signup and view all the answers

What is the significance of the gray crescent in amphibian development?

It contains necessary cytoplasmic factors for normal development. Signup and view all the answers

During which stage does the embryo become a solid ball of small cells?

Cleavage Signup and view all the answers

What is characteristic of complete cleavage?

Blastomeres are similar in size. Signup and view all the answers

How do the organizer cells influence embryonic development?

By inducing different structures at various stages. Signup and view all the answers

What occurs during discoidal cleavage?

The embryo forms a blastodisc atop the yolk. Signup and view all the answers

Which germ layer is directly responsible for the initiation of the nervous system?

Ectoderm Signup and view all the answers

What process occurs when cells elongate and intercalate during gastrulation?

Convergent extension Signup and view all the answers

What results from the interaction between GSK-3 and β-catenin?

Higher concentration of β-catenin specifies the dorsal-ventral axis. Signup and view all the answers

In superficial cleavage, what happens to the plasma membrane?

It grows inward around the nuclei to form cells. Signup and view all the answers

In relation to anterior–posterior axis development, what role do organizer cells play?

They express growth factor antagonists at the appropriate times. Signup and view all the answers

What is a major outcome of cytoplasmic movement after fertilization?

Changes in the distributions of crucial developmental signals. Signup and view all the answers

What is formed when cells pass over the dorsal lip on the midline during gastrulation?

Chordamesoderm Signup and view all the answers

What is the primary outcome of gastrulation in embryonic development?

The generation of multiple tissue layers. Signup and view all the answers

What is the primary role of extraembryonic membranes in vertebrates?

To facilitate nutrition, gas exchange, and waste removal Signup and view all the answers

Which of the following membranes is responsible for protecting the embryo by forming a fluid-filled cavity?

Amnion Signup and view all the answers

Which extraembryonic membrane is involved in waste storage?

Allantoic membrane Signup and view all the answers

In placental mammals, which structure is primarily responsible for facilitating the contact with maternal blood?

Trophoblast Signup and view all the answers

How are Hox genes arranged in relation to their function in the embryo?

They are expressed in the order of their arrangement on the chromosome Signup and view all the answers

What is formed from the hypoblast cells in early mammalian development?

Chorion Signup and view all the answers

Which of the following is NOT a function of the chorion?

Protects the embryo from temperature changes Signup and view all the answers

What is the purpose of the trophoblast's adhesion molecules?

To attach the trophoblast to the uterine wall Signup and view all the answers

What is the primary function of the placenta during pregnancy?

It exchanges nutrients, gases, and wastes between mother and embryo. Signup and view all the answers

What does the amnion produce and what is its significance during pregnancy?

It surrounds the embryo and is filled with amniotic fluid, crucial for cushioning. Signup and view all the answers

What role do blood vessels in the umbilical cord play during fetal development?

They carry nutrients and oxygen from the mother and remove waste products. Signup and view all the answers

During which trimester of pregnancy does the heart begin to beat?

First trimester. Signup and view all the answers

What major developmental milestones occur during the first trimester?

Heart begins to beat and limbs begin to form. Signup and view all the answers

What is the duration of a typical human gestation period?

266 days. Signup and view all the answers

Which trimester sees significant elongation of limbs and development of the nervous system?

Second trimester. Signup and view all the answers

Why is the first trimester considered the most critical period for fetal development?

It is when the fetus is most susceptible to external harm. Signup and view all the answers

Study Notes

Fertilization Activates Development

The sperm enters the animal hemisphere of the egg.
The cortical (outer) cytoplasm rotates toward the site of sperm entry.
The gray crescent forms opposite the site of sperm entry.
The gray crescent specifies body axes and other developmental events.
The centriole from the sperm initiates cytoplasmic reorganization.
Microtubules in the vegetal hemisphere form a parallel array to guide movement of cytoplasm and organelles.
Cytoplasmic movement changes the distributions of critical developmental signals.
The protein kinase GSK-3, an inhibitor of GSK-3, and the protein β-catenin interact, resulting in a higher concentration of b-catenin on the dorsal side.
The higher concentration of b-catenin on the dorsal side specifies the dorsal–ventral axis of the embryo.

Mitosis Divides Up the Early Embryo

Cleavage: early cell divisions with no cell growth.
The embryo becomes a solid ball of small cells.
Blastocoel: a central fluid-filled cavity that forms in the ball.
The embryo becomes a blastula and its cells are called blastomeres.
The pattern of cleavage varies with species.
Complete cleavage occurs in eggs with little yolk; the blastomeres are similar in size.
In frogs, the vegetal pole has more yolk; division is unequal, and daughter cells in the animal pole are smaller.
Incomplete cleavage occurs in eggs with a lot of yolk when cleavage furrows don’t penetrate the egg completely.
In discoidal cleavage, the embryo forms as a blastodisc that sits on top of the yolk.
Superficial cleavage is a variation of incomplete cleavage in insects. The plasma membrane grows inward around nuclei and forms cells.
Blastomeres become determined - committed to specific fates - at different times in different species.
In mosaic development, each blastomere contributes certain aspects to the adult animal (autonomous specification).
If one blastomere is removed, a portion of the embryo will not form.
Regulated specification: in most vertebrates, blastomere fate is determined by information from the environment and neighboring cells.
Regulative development: other cells compensate for any lost cells.
In humans, one blastomere can be removed for genetic analysis without harming the embryo.
In species with regulative development, if blastomeres separate into two groups, each can become an embryo.
Monozygotic twins come from the same zygote and are identical.
Non-identical twins develop from two eggs fertilized by two sperm.

Gastrulation Generates Multiple Tissue Layers

Gastrulation: massive movements of cells transform the blastula into an embryo with multiple tissue layers and distinct body axes.
In triplobastic animals, three germ layers (tissue layers) form.
Endoderm layer: becomes lining of digestive and respiratory tracts, pancreas, thyroid, and liver.
Ectoderm: outer layer; becomes the nervous system, eyes, ears, and skin.
Mesoderm: middle layer; contributes tissues to many organs, including heart, blood vessels, muscles, and bone.
In sea urchins, gastrulation begins with invagination.
Archenteron cells extend, flatten, interdigitate, and migrate over one another to form a long thin tube (convergent extension).
The mouth forms where the archenteron meets the ectoderm.
The blastopore is the opening formed by invagination of the vegetal pole, and becomes the anus.
Amphibian gastrulation: Bottle cells in the gray crescent bulge inward and form the dorsal lip.
Cells move over the lip and into the blastocoel— involution.
Convergent extension occurs when cells elongate as they move, and intercalate (move in between each other).
Hans Spemann experimented with bisecting fertilized salamander eggs. Results differed depending on how the eggs were bisected.
Spemann found that cytoplasmic factors, such as those in the gray crescent, are necessary for normal development.
The organizer (dorsal lip) induces the beginnings of the body plan.
The organizer changes its activity in order to induce different structures.
The first organizer cells to enter the embryo become head endoderm and head mesoderm.
Later cells induce structures of the trunk; the last cells induce tail structures.
Organizer cells express appropriate growth factor antagonists at the right times to achieve different patterns of differentiation on the anterior–posterior axis.

Organs Develop from the Three Germ Layers

Gastrulation produces an embryo with three germ layers that influence one another through inductive tissue interactions.
Organogenesis: organs and organ systems develop.
Neurulation: initiation of the nervous system— occurs early in organogenesis.
One group of cells that passes over the dorsal lip on the midline becomes chordamesoderm.
The notochord forms from the chordamesoderm.
The notochord induces the overlying ectoderm to form the neural plate.
The neural plate folds, forming the neural tube.
The neural tube will become the brain and spinal cord.
Hox genes are expressed along the anterior–posterior axis of the embryo in the same order as their arrangement on the chromosome.
Different segments of the embryo receive different combinations of Hox gene products, which act as transcription factors.

Extraembryonic Membranes Nurture Avian and Mammalian Embryos

Extraembryonic membranes surround the vertebrate embryo.
Extraembryonic membranes function in nutrition, gas exchange, and waste removal.
In mammals, they interact with the mother’s tissues to form the placenta.
In the chick embryo, four membranes form:
- Yolk sac: encloses yolk within the egg.
- Yolk is digested by cells of the yolk sac and the nutrients are transported to the embryo.
- Allantoic membrane: forms the allantois, a sac for waste storage.
- Amnion: surrounds the embryo, forming the fluid-filled amniotic cavity that protects the embryo.
- Chorion: forms a continuous membrane just under the eggshell; reduces water loss and exchanges gases.
In placental mammals, the trophoblast is the first extraembryonic membrane.
Trophoblast cells interact with the endometrium; adhesion molecules attach them to the uterine wall.
The trophoblast burrows into the uterine wall, and sends out villi to increase contact with maternal blood.
Hypoblast cells form the chorion.
The placenta develops from the chorion and uterine tissues.
The placenta exchanges nutrients, gases, and wastes between mother and embryo.
The epiblast produces the amnion, which surrounds the embryo and is filled with amniotic fluid.
Rupturing of the amnion and chorion and loss of the amniotic fluid (the “water breaks”) herald the onset of labor in humans.
Allantoic tissues contribute to formation of the umbilical cord, by which the embryo is attached to the chorionic placenta.
Blood vessels in the umbilical cord carry nutrients and oxygen from the mother to the developing fetus, and carry away wastes, including CO2 and urea.
Gestation, or pregnancy, in humans is about 266 days (9 months) and is divided into trimesters.
In the first trimester, the embryo becomes a fetus:
- Heart begins to beat by week four.
- Limbs form by week eight.
The first trimester is a time of rapid cell division and tissue differentiation.
This is when the fetus is most susceptible to damage from radiation, drugs, chemicals, and pathogens that cause birth defects.
Embryos can be damaged before the mother even realizes she is pregnant.
Second trimester: limbs elongate; fingers, toes, and facial features form; nervous system develops.
Third trimester: internal organs mature.
Birth occurs when the lungs are mature.
The potential for serious effects from exposure to environmental factors exists throughout pregnancy.

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Description

This quiz covers the essential processes of fertilization and early mitosis in embryology. It outlines the steps from sperm entry to the early cleavage stages of the embryo, highlighting key events such as cytoplasmic movement and the formation of body axes. Test your knowledge on these crucial developmental stages.