BIOL #22: Animal Development Flashcards
Embryonic Development: Overview
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.
Embryonic Development: Genetics
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
Gametogenesis
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.
Sperm Structure and Function
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.
Egg Size & Nutrients
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.
Egg Structure & Function
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.
Fertilization
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.
Fertilization in Sea Urchins
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.
Polyspermy
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.
Fertilization Envelope
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.
Cortical Reaction
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.
Egg Activation
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.
Cleavage
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.
Morphogenesis
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.
Gastrulation
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).
Gastrulation in Frog Embryos
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.
Germ Layers
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.
Organogenesis
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.
Neurulation
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.
Somite Maturation and Determination
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.
Myoblast
A myoblast is a cell that is determined to become a muscle cell but has not begun producing muscle-specific proteins.
MyoD
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.
Cytoplasmic Determinants & Differentiation
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.
Common Development Processes
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
