BIOL #22: Animal Development

Question 1

Embryonic Development: Overview

Answer 1

Gametogenesis: formation of gametes (egg and sperm) in adult animals.

Fertilization: fusion of egg and sperm, which forms a zygote.

Stages of Embryonic Development:

Cleavage: series of cell divisions producing a multi-celled embryo – the beginning of embryogenesis.

Gastrulation: formation of body tissue layers in the multi-celled embryo.

Organogenesis: local changes in cell shape and large-scale changes in cell location begin the generation of organs.

Question 2

Embryonic Development: Genetics

Answer 2

As an embryo develops, specific patterns of gene expression direct cells to adopt distinct fates (e.g. heart muscle cell vs neuron).

Although animals display widely different body plans, they share many basic mechanisms of development and use a common set of regulatory genes.
- For example, the gene specifying heart location in humans has a nearly identical counterpart with the same function in fruit flies (Drosophila) = Tinman gene

Model organisms have become very useful to researchers studying developmental biology since it is considered unethical to manipulate developing human embryos:

• Frog
• Drosophila
• Chick
• Nematode
• Sea urchin

Question 3

Gametogenesis

Answer 3

Gametogenesis is the formation of gametes in the reproductive organs of adult organisms.

Gametes are haploid reproductive cells.
- In animals, male gametes are called sperm and female gametes are called eggs.

Both sperm and egg contribute chromosomes—usually a haploid genome containing one allele of each gene―resulting in a diploid offspring.

However, egg cells contribute more than just chromosomes and are much larger than sperm cells.

Question 4

Sperm Structure and Function

Answer 4

The sperm cell is composed of four main structures:

The head contains the nucleus and the enzyme-filled acrosome.
- Acrosome enzymes allow the sperm to penetrate the egg’s barriers.

The neck encloses a centriole.
- After fertilization, the centriole will fuse with a second centriole that is contributed by the egg, to form the centrosome.

The midpiece is packed with mitochondria, which produce the ATP (energy) necessary for movement.

The tail has a flagellum that acts as a propeller.

Question 5

Egg Size & Nutrients

Answer 5

Egg cells are relatively large and nonmotile compared to sperm cells.
- Oogenesis results in production of a large egg cell due to unequal partitioning of cytoplasm and organelles during rounds of cytokinesis.

Their large size is due to the nutrient storage that is required for early embryonic development.

The quantity of nutrients varies across species:

• The relatively small mammalian egg only has to supply nutrients for early development, as embryos start to obtain nutrition through the placenta shortly following fertilization.
• Egg-laying species produce much larger eggs; the yolk of the egg is the embryo’s sole source of nutrition prior to hatching.

Question 6

Egg Structure & Function

Answer 6

The eggs of many species contain cytoplasmic determinants (RNA or proteins) that control the early events of development.

Many eggs also contain cortical granules, small enzyme-filled vesicles that are activated during fertilization.

The vitelline envelope, a fibrous, mat-like sheet of glycoproteins, surrounds the egg.
- Mammals have an unusually thick vitelline envelope called the zona pellucida.

Some species have a jelly layer, a thick, gelatinous matrix around the vitelline envelope for further protection.

Question 7

Fertilization

Answer 7

Fertilization occurs when a haploid sperm cell and egg cell fuse, forming a diploid zygote (a fertilized egg).

Many conditions must be met before a zygote can form:

• Gametes must be in the same place at the same time.
• Gametes must recognize and bind to each other.
• Gametes must fuse together.
• Fusion must trigger the onset of development.

Sea urchins are a model system for studying fertilization – they produce large numbers of gametes and undergo external fertilization, allowing fertilization to be easily observed.

Question 8

Fertilization in Sea Urchins

Answer 8

When a sperm head contacts the jelly layer, hydrolytic enzymes from the sperm’s acrosome digest through the egg’s jelly layer and vitelline envelope – this is known as the acrosomal reaction.

The plasma membranes of sperm and egg fuse when the sperm head contacts the surface of the egg cell.
- Protein molecules on the tip of the acrosome bind to specific receptor proteins on the egg plasma membrane – this is an important “lock-and-key” recognition mechanism for species-specific fertilization to occur.

Contact between the tip of the acrosome and the egg membrane receptors leads to fusion of the sperm and egg plasma membranes.

Fertilization is complete when the sperm nucleus and centriole enter the egg, and the sperm and egg nuclei fuse to form the zygote nucleus.

Question 9

Polyspermy

Answer 9

Animals employ different mechanisms to avoid polyspermy, fertilization by more than one sperm.

For some animals, the first line of defense is the change in membrane potential that is stimulated by fertilization.
- After fertilization, ion channels open in the egg’s plasma membrane. Sodium ions diffuse into the egg and cause depolarization, a decrease in membrane potential in which the inside of the egg to become relatively more positively charged than before fertilization.
+ Sperm have positively charged surface proteins and are likely repelled when depolarization occurs, blocking other sperm from entering the egg.
- This depolarization occurs 1-3 s after a sperm binds to an egg, resulting in a fast-block to polyspermy – however, it is a short-lived block.
- This mechanism occurs in sea urchin development but has NOT been found to occur in mammals.
+ A fast-block to polyspermy is advantageous in organisms in which external fertilization occurs and hundreds of sperm may encounter an egg within several seconds.

Question 10

Fertilization Envelope

Answer 10

An important line of defense against polyspermy for many animals, including sea urchins and mammals, is the creation of a physical barrier that is stimulated by fertilization.
- After fertilization, a calcium (Ca2+) -based signal is rapidly induced and propagated throughout the egg, resulting in the formation of a fertilization envelope, which keeps away additional sperm.

Question 11

Cortical Reaction

Answer 11

This slower but longer-lasting block to polyspermy is called the cortical reaction.
- Within seconds of fertilization by a sperm, the increased Ca2+ level causes cortical granules underneath the egg’s plasma membrane to release enzymes that modify egg cell receptors and vitelline layer, preventing binding by additional sperm.

Question 12

Egg Activation

Answer 12

The events of fertilization allow for the combination of two haploid sets of chromosomes but also initiate metabolic reactions that trigger the onset of embryonic development.
- There is a marked increase in the rates of cellular respiration (ATP production) and protein synthesis in the egg following fertilization.

Manipulation studies suggest that the same rise in Ca2+ levels that cause the cortical reaction resulting in a fertilization envelope also cause egg activation.

The stages of fertilization are similar across many species although the timing of events differ, as well as the stage of meiosis the egg has reached by the time it is fertilized.
- e.g. oogenesis is complete before fertilization occurs in the sea urchin, however, mammalian oogenesis completes after fertilization occurs.

Question 13

Cleavage

Answer 13

Cleavage is the set of rapid cell divisions that take place in animal zygotes immediately after fertilization.

Cleavage is the first step in embryogenesis, the process by which a single-celled zygote becomes a multicellular embryo.

Cleavage partitions the egg cytoplasm without any additional growth of the zygote – the cell cycle consists primarily of the S (DNA synthesis) phase and the M (mitotic) phase (G1 and G2 phases are absent).

The cells created by cleavage divisions are called blastomeres.

The first cleavage divisions produce a solid ball of cells called a morula.

Further cleavage divisions then produce a hollow ball of cells, called the blastula, with a fluid-filled cavity called the blastocoel.

The increase in number of cells sets the stage for morphogenesis to occur.
- Note that during cleavage, the ball of cells does not change in overall size.

Question 14

Morphogenesis

Answer 14

After cleavage, the rate of cell division slows down as the normal cell cycle is restored.

Morphogenesis, which consist of the cellular and tissue-based processes by which the animal body takes shape, begins.

Morphogenesis consists of two processes: gastrulation and organogenesis.

Question 15

Gastrulation

Answer 15

Gastrulation includes dramatic reorganization of the hollow blastula into a two- or three-layered embryo called a gastrula.

During gastrulation:

• A set of cells at or near the surface of the blastula move to an interior location.
• Cell layers are established and body axes become apparent (e.g. head vs tail).
• A primitive digestive tube is formed.

The cell layers produced by gastrulation are collectively called the embryonic germ layers.

In the late gastrula:

• The ectoderm forms the outer layer
• The mesoderm forms the middle layer
• The endoderm lines the embryonic digestive compartment or tract

Some animals, such as cnidarians (e.g. hydra), only form an ectoderm and endoderm (diploblasts). Organisms with bilateral symmetry, such as vertebrates, form all three layers (triploblasts).

Question 16

Gastrulation in Frog Embryos

Answer 16

During frog embryo development, gastrulation begins with the formation of an opening called a blastopore in the blastula.

Cells from the periphery move inward through the blastopore, forming a tube-like structure, called the archenteron, that will eventually become the digestive tract.

• This rearrangement of tissues sets up the axes (head and tail) for the organism.
• The frog’s anus will develop from the blastopore and the mouth will eventually develop at the other end of the archenteron.

Question 17

Germ Layers

Answer 17

Each germ layer contributes to a distinct set of structures in adult animals.

Some organs and organ systems may be derived from more than one germ layer.

Question 18

Organogenesis

Answer 18

Organogenesis is the process of tissue and organ formation that begins once gastrulation is complete and the embryonic germ layers are in place.

Organogenesis is somewhat different between vertebrates and invertebrates, however, the underlying mechanisms, involve many of the same cellular activities:

• Cell migration
• Cell signaling between different tissues
• Cell shape changes

During organogenesis, cells proliferate and become differentiated, meaning that they become a specialized cell type.

Determination refers to the process by which a cell or group of cells becomes committed to a particular fate.

Differentiation refers to the resulting specialization in structure and function after determination.

• Differentiated cells have a distinctive structure and function because they express a distinctive suite of genes.
• Differentiation typically involves the production of cell-specific proteins.

Question 19

Neurulation

Answer 19

Neurulation involves the first steps in the formation of the brain and spinal cord in vertebrates.

Early in organogenesis, the rod-like notochord appears in the dorsal mesoderm.
- This structure is unique to the animal group called the chordates, which includes humans and other vertebrates.

The notochord functions as a key organizing element during organogenesis.
- In many chordates, as organogenesis continues, the notochord cells undergo apoptosis and disappears.

Signals from the notochord trigger reorganization of the dorsal ectodermal cells, leading to neural tube formation.

The neural tube is the precursor to the brain and spinal cord.

Once the neural tube forms, neural crest cells form, which subsequently migrate to other parts of the embryo to form a variety of tissues (e.g. peripheral nerves, teeth, skull bones).

Mesodermal cells become organized into blocks of tissues called somites, which form on both sides of the neural tube down the length of the body.

Parts of the somites dissociates and migrate individually to new locations in the body.

Question 20

Somite Maturation and Determination

Answer 20

Somite cells form a variety of structures, but are initially not determined, meaning they can become any of the somite-derived elements of the body.

As the somite matures, somite cells become irreversibly determined, and will eventually differentiate into a specific cell type based on their location within the somite.

During the process of determination, somite cells respond to signals from nearby tissues.

These signals diffuse away from cells in the notochord, the neural tube, and nearby ectoderm and mesoderm to act on specific populations of target cells in the somite, resulting in differentiation of the somite cells.

Question 21

Myoblast

Answer 21

A myoblast is a cell that is determined to become a muscle cell but has not begun producing muscle-specific proteins.

Question 22

MyoD

Answer 22

Researchers have found that MyoD is the protein that causes muscle cell differentiation.

MyoD is a regulatory transcription factor that binds to enhancers upstream of muscle-specific genes during gene transcription.

During organogenesis, the neural tube signals specific somite cells to begin MyoD production such that these muscle cells begin expressing muscle-specific proteins.

Question 23

Cytoplasmic Determinants & Differentiation

Answer 23

Cytoplasmic determinants are proteins and RNA that are found in specific locations within the egg cytoplasm, so they end up in specific populations of blastomeres.

By dividing the egg cytoplasm to precisely distribute cytoplasmic determinants to certain cells, cleavage initiates the step-by-step process that, in combination with signals received from other cells, results in the differentiation of cells later in development.

Question 24

Common Development Processes

Answer 24

How does a single fertilized egg cell develop into an embryo and then into a baby and eventually an adult?

A few fundamental principles are common to all developmental sequences observed in multicellular organisms:

• Cell division (proliferation)
• Programmed cell death (apoptosis)
• Cell movement and differential expansion
• Differential gene expression and cell differentiation
• Cell-cell interactions

Question 25

Cell Proliferation

Answer 25

For an individual to develop from an undifferentiated mass of cells, its cells have to proliferate – divide and make more cells.

The location, timing, and extent of these cell divisions are tightly controlled by interacting layers of regulation.

Most cells stop proliferating at maturity. However, there are some specialized, undifferentiated cells that continue proliferating throughout the organism’s life.

• In plants, these cells are called meristems.
• In animals, these cells are called stem cells.

Question 26

Apoptosis

Answer 26

Apoptosis, programmed cell death, is a carefully regulated aspect of normal development.

Abnormal apoptosis – either too much or too little – can lead to disease or deformation.

Examples:

• Apoptosis of tail cells occurs when a tadpole undergoes metamorphosis to become a frog.
• Apoptosis of the webbing between embryonic digits occurs in many bird and mammal species.

Question 27

Cell Movement and Expansion

Answer 27

In addition to dividing, many animal cells have to move for normal development to occur.

Because of their cell walls, plant cells do not move, but changes in the orientation of cell division controls the direction of subsequent cell proliferation and cell expansion.

During gastrulation in animals, cells in different parts of an early embryo rearrange themselves into three distinctive types of embryonic tissues which later form specific organs.

During organogenesis, the cytoskeleton fibers extend and contract, which allows many cells to “crawl” to specific locations throughout the body.

Question 28

Cell Differentiation

Answer 28

During development, most cells must undergo differentiation in order to become a specialized type of cell.

Differentiation is a progressive, step-by-step process. Cells are initially determined, or committed, to a specific developmental pathway and later become differentiated.
- Involves production of cell-specific proteins

Question 29

Cell-Cell Interactions

Answer 29

During development, the most important cell-cell interactions involve sending and receiving signals.

Cell-cell signals change patterns of gene expression and are essential for changing cell activity during development.

Question 30

Differential gene expression

Answer 30

Differential gene expression, the expression of different genes in different cell types, is key to cell differentiation during development.

Every diploid cell formed during an animal’s development has the same genome – all cells contain the same genes but express different subsets of these genes.

A gene can be regulated at multiple levels:

• Transcription
• RNA processing (e.g. alternative splicing)
• Translation
• Post-translation

Transcription is the fundamental level of control in differential gene expression during development.

In eukaryotes, transcription is controlled primarily by the presence of proteins called regulatory transcription factors.

The fate of a cell depends on timing (the current stage of development of the organism) and its spatial location (where it is in the body of the organism).

Spatial location in early development is determined by three major body axes:

• Anterior-posterior
• Ventral-dorsal
• Left-right

Cell-cell signals tell cells where they are in time and space. This information activates transcription factors that turn specific genes on or off, resulting in differentiation.

Question 31

Master Regulators Set Up the Major Body Axes

Answer 31

Pattern formation is the series of events that determine the spatial organization of an embryo.

Certain early signals act as master regulators, setting up the major body axes of the embryo.
- These master regulators activate a network of genes that send signals with more specific information about the spatial location of cells.
- In Drosophila embryos, the bicoid protein is a regulatory transcription factor that forms a concentration gradient and provides cells with information about their position along the anterior-posterior axis.
+ Anterior cells receive a high concentration of bicoid protein, the posterior cells receive a low concentration.

As development proceeds, a series of signals arrive and activate genes that specify finer and finer control over what a cell becomes.

Question 32

The Overall Function of Regulatory Genes

Answer 32

Regulatory genes act in a sequence, triggering gene cascades that provide progressively detailed information about where cells are located in time and space.

Cells receive unique positional information because the identity and concentration of signals and transcription factors vary along the three major body axes.

Each level in a regulatory cascade provides a more specific level of information about where a cell is.

As regulatory cascades proceed, a cell’s fate becomes more and more finely determined.

Question 33

Common Signaling Pathways

Answer 33

During development, the same regulatory transcription factors and cell-cell signals are used in a variety of contexts.

Multicellular organisms have a tool kit of common signals, signal transduction pathways, and regulatory proteins that are used over and over during development.
- An example of a group of genes in this tool kit is the Hox gene cluster, which includes transcription factors that function in patterning the body axis.

This tool kit can direct the development of dramatically different structures because its tools are deployed at different times and in different locations.
- Cellular responses are context-dependent