Between fertilization and birth, the developing organism is known as an embryo. The concept of an embryo is a staggering one. The development of an embryo starts with a single cell. The embryo has to respire before it has lungs, digest before it has a gut, build bones when it was pulpy, and form orderly arrays of neurons. One of the critical differences between a developing organism and a machine is that a machine will start to function after it is built. Every multicellular organism has to function even during its development. Multicellular organisms do not spring forth fully formed. Rather, they arise by a relatively slow process of progressive change that we call development. In nearly all cases, the development of a multicellular organism begins with a single cell—the fertilized egg, or zygote, which divides mitotically to produce all the cells of the body. The study of animal development has traditionally been called embryology, after that phase of an organism that exists between fertilization and birth. But development does not stop at birth, or even at adulthood. Most organisms never stop developing. Each day we replace more than a gram of skin cells (the older cells being sloughed off as we move), and our bone marrow sustains the development of millions of new red blood cells every minute of our lives. Some animals can regenerate severed parts, and many species undergo metamorphosis (such as the transformation of a tadpole into a frog, or a caterpillar into a butterfly).
Development accomplishes two major objectives. First, it generates cellular diversity and order within the individual organism; second, it ensures the continuity of life from one generation to the next. Put another way, there are two fundamental questions in developmental biology. How does the fertilized egg give rise to the adult body? And, how does that adult body produce yet another body? These huge questions can be subdivided into several categories of questions scrutinized by developmental biologists:
The question of differentiation
A single cell, the fertilized egg, gives rise to hundreds of different cell types—muscle cells, epidermal cells, neurons, lens cells, lymphocytes, blood cells, fat cells, and so on. This generation of cellular diversity is called differentiation. Since every cell of the body (with very few exceptions) contains the same set of genes, how can this identical set of genetic instructions produce different types of cells? How can a single fertilized egg cell generate so many different cell types?1
The question of morphogenesis
How can the cells in our body organize into functional structures? Our differentiated cells are not randomly distributed. Rather, they are organized into intricate tissues and organs. During development, cells divide, migrate, and die; tissues fold and separate. Our fingers are always at the tips of our hands, never in the middle; our eyes are always in our heads, not in our toes or gut. This creation of ordered form is called morphogenesis, and it involves coordinating cell growth, cell migration, and cell death.
The question of growth
If each cell in our face were to undergo just one more cell division, we would be considered horribly malformed. If each cell in our arms underwent just one more round of cell division, we could tie our shoelaces without bending over. How do our cells know when to stop dividing? Our arms are generally the same size on both sides of the body. How is cell division so tightly regulated?
The question of reproduction
The sperm and egg are highly specialized cells, and only they can transmit the instructions for making an organism from one generation to the next. How are these germ cells set apart, and what are the instructions in the nucleus and cytoplasm that allow them to form the next generation?
The question of regeneration
Some organisms can regenerate every part of their bodies. Some salamanders regenerate their eyes and their legs, while many reptiles can regenerate their tails. While mammals are generally poor at regeneration, there are some cells in our bodies—stem cells—that are able to form new structures even in adults. How do stem cells retain this capacity, and can we harness it to cure debilitating diseases?
The question of environmental integration
The development of many (perhaps all) organisms is influenced by cues from the environment that surrounds the embryo or larva. The sex of many species of turtles, for instance, depends on the temperature the embryo experiences while in the egg. The formation of the reproductive system in some insects depends on bacteria that are transmitted inside the egg. Moreover, certain chemicals in the environment can disrupt normal development, causing malformations in the adult. How is the development of an organism integrated into the larger context of its habitat?
The question of evolution
Evolution involves inherited changes of development. When we say that today’s one-toed horse had a five-toed ancestor, we are saying that changes in the development of cartilage and muscles occurred over many generations in the embryos of the horse’s ancestors. How do changes in development create new body forms? Which heritable changes are possible, given the constraints imposed by the necessity of the organism to survive as it develops?
The questions asked by developmental biologists have become critical in molecular biology, physiology, cell biology, genetics, anatomy, cancer research, neurobiology, immunology, ecology, and evolutionary biology. The study of development has become essential for understanding all other areas of biology. In turn, the many advances of molecular biology, along with new techniques of cell imaging, have finally made these questions answerable. This is exciting; for, as the Nobel-prize winning developmental biologist Hans Spemann stated in 1927, “We stand in the presence of riddles, but not without the hope of solving them. And riddles with the hope of solution—what more can a scientist desire?” So, we come bearing questions. They are questions bequeathed to us by earlier generations of biologists, philosophers, and parents. They are questions with their own histories, questions discussed on an anatomical level by people such as Aristotle, William Harvey, St. Albertus Magnus, and Charles Darwin. More recently, these questions have been addressed on the cellular and molecular levels by men and women throughout the world, each of whom brings to the laboratory his or her own perspectives and training. For there is no one way to become a developmental biologist, and the field has benefitted by having researchers trained in cell biology, genetics, biochemistry, immunology, and even anthropology, engineering, physics, and art.
The Cycle of Life
For animals, fungi, and plants, the sole way of getting from egg to adult is by developing an embryo. The embryo is where genotype is translated into phenotype, where inherited genes are expressed to form the adult. The developmental biologist usually finds the transient stages leading up to the adult to be the most interesting. Developmental biology studies the building of organisms. It is a science of becoming, a science of process. One of the major triumphs of descriptive embryology was the idea of a generalizable animal life cycle. Modern developmental biology investigates the temporal changes of gene expression and anatomical organization along this life cycle. Each animal, whether earthworm or eagle, termite or beagle, passes through similar stages of development: fertilization, cleavage, gastrulation, organogenesis, birth, metamorphosis, and gametogenesis. The stages of development between fertilization and hatching (or birth) are collectively called embryogenesis.
1. Fertilization involves the fusion of the mature sex cells, the sperm and egg, which are collectively called the gametes. The fusion of the gamete cells stimulates the egg to begin development and initiates a new individual. The subsequent fusion of the gamete nuclei (the male and female pronuclei, each of which has only half the normal number of chromosomes characteristic for the species) gives the embryo its genome, the collection of genes that helps instruct the embryo to develop in a manner very similar to that of its parents.
2. Cleavage is a series of extremely rapid mitotic divisions that immediately follow fertilization. During cleavage, the enormous volume of zygote cytoplasm is divided into numerous smaller cells called blastomeres. By the end of cleavage, the blastomeres have usually formed a sphere, known as a blastula.
3. After the rate of mitotic division slows down, the blastomeres undergo dramatic movements and change their positions relative to one another. This series of extensive cell rearrangements is called gastrulation, and the embryo is said to be in the gastrula stage. As a result of gastrulation, the embryo contains three germ layers (endoderm, ectoderm, and mesoderm) that will interact to generate the organs of the body.
4. Once the germ layers are established, the cells interact with one another and rearrange themselves to produce tissues and organs. This process is called organogenesis. Chemical signals are exchanged between the cells of the germ layers, resulting in the formation of specific organs at specific sites. Certain cells will undergo long migrations from their place of origin to their final location. These migrating cells include the precursors of blood cells, lymph cells, pigment cells, and gametes (eggs and sperm).
5. In many species, the organism that hatches from the egg or is born into the world is not sexually mature. Rather, the organism needs to undergo metamorphosis to become a sexually mature adult. In most animals, the young organism is a called a larva, and it may look significantly different from the adult. In many species, the larval stage is the one that lasts the longest, and is used for feeding or dispersal. In such species, the adult is a brief stage whose sole purpose is to reproduce. In silkworm moths, for instance, the adults do not have mouthparts and cannot feed; the larva must eat enough so that the adult has the stored energy to survive and mate. Indeed, most female moths mate as soon as they eclose from the pupa, and they fly only once—to lay their eggs. Then they die.
6. In many species, a group of cells is set aside to produce the next generation (rather than forming the current embryo). These cells are the precursors of the gametes. The gametes and their precursor cells are collectively called germ cells, and they are set aside for reproductive function. All other cells of the body are called somatic cells. This separation of somatic cells (which give rise to the individual body) and germ cells (which contribute to the formation of a new generation) is often one of the first differentiation to occur during animal development. The germ cells eventually migrate to the gonads, where they differentiate into gametes. The development of gametes, called gametogenesis, is usually not completed until the organism has become physically mature. At maturity, the gametes may be released and participate in fertilization to begin a new embryo. The adult organism eventually undergoes senescence and dies, its nutrients often supporting the early embryogenesis of its offspring and its absence allowing less competition. Thus, the cycle of life is renewed.
An Overview of Early Development
Cells in the cleavage-stage cells are called blastomeres.In most species (mammals being the chief exception), both the initial rate of cell division and the placement of the blastomeres with respect to one another are under the control of proteins and mRNAs stored in the oocyte. Only later do the rates of cell division and the placement of cells come under the control of the newly formed organism’s own genome. During the initial phase of
development, when cleavage rhythms are controlled by maternal factors, cytoplasmic volume does not increase. Rather, the zygote cytoplasm is divided into increasingly smaller cells—first in half, then quarters, then eighths, and so forth. Cleavage occurs very rapidly in most invertebrates, probably as an adaptation to generate a large number of cells quickly and to restore the somatic ratio of nuclear volume to cytoplasmic volume. The embryo often accomplishes this by abolishing the gap periods of the cell cycle (the G1 and G2 phases), when growth can occur. A frog egg, for example, can divide into 37,000 cells in just 43 hours. Mitosis in cleavage-stage Drosophila embryos occurs every 10 minutes for more than 2 hours, forming some 50,000 cells in just 12 hours. The pattern of embryonic cleavage peculiar to a species is determined by two major parameters:
(1) the amount and distribution of yolk protein within the cytoplasm, which determine where cleavage can occur and the relative sizes of the blastomeres; and
(2) factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation.
Cell cleavage is accomplished by a remarkable coordination between the cytoskeleton and the chromosomes. This integration of part and whole is becoming better understood as better imaging technologies become available.
Above provides a classification of cleavage types and shows the influence of yolk on cleavage symmetry and pattern.
Gastrulation: “The most important time in your life”
According to embryologist Lewis Wolpert (1986), “It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.” This is not an overstatement. Gastrulation is what makes animals animals. (Animals gastrulate; plants and fungi do not.) During gastrulation, the cells of the blastula are given new positions and new neighbours and the multilayered body plan of the organism is established. The cells that will form the endodermal and mesodermal organs are brought to the inside of the embryo, while the cells that will form the skin and nervous system are spread over its outside surface. Thus, the three germ layers—outer ectoderm, inner endoderm, and interstitial mesoderm—are first produced during gastrulation. In addition, the stage is set for the interactions of these newly positioned tissues.
Gastrulation usually proceeds by some combination of several types of movements. These movements involve the entire embryo, and cell migrations in one part of the gastrulating embryo must be intimately coordinated with other movements that are taking place simultaneously. Although patterns of gastrulation vary enormously throughout the animal kingdom, all of the patterns are different combinations of the five basic types of cell movements—invagination, involution, ingression, delamination, and epiboly.
In addition to establishing which cells will be in which germ layer, embryos must develop three crucial axes that are the foundation of the body: the anterior-posterior axis, the dorsal-ventral axis, and the right-left axis.
The embryo is organized into germ layers—three distinct regions of the embryo that give rise to the differentiated cells types and specific organ systems.
The dividing cells of the fertilized egg form three distinct embryonic germ layers. Each of the germ layers gives rise to myriad differentiated cell types (only a few representatives are shown here) and distinct organ systems.
The germ cells (precursors of the sperm and egg) are set aside early in development and do not arise from any particular germ layer.
These three layers are found in the embryos of most animal phyla:
The ectoderm generates the outer layer of the embryo. It produces the surface layer (epidermis) of the skin and forms the brain and nervous system.
The endoderm becomes the innermost layer of the embryo and produces the epithelium of the digestive tube and its associated organs (including the lungs).
The mesoderm becomes sandwiched between the ectoderm and endoderm. It generates the blood, heart, kidney, gonads, bones, muscles, and connective tissues.
Each of the three germ layers generally gives rise to the same organs, whether the organism is a fish, a frog, or a chick. The notochord is the rod of mesoderm that separates the embryo into right and left halves and instructs the ectoderm above it to become the nervous system
Two types of microscopy are used to visualize the notochord and its separation of vertebrate embryos (in this case a chick) into right and left halves.
The notochord instructs the ectoderm above it to become the nervous system (the neural tube at this stage of development). To either side of the notochord and the neural tube are the mesodermal masses called somites, which will form vertebrae, ribs, and skeletal muscles. (A) Fluorescence micrograph stained with different dyes to highlight nuclear DNA (blue), cytoskeletal microtubules (red, yellow), and the extracellular matrix (green). (B) Scanning electron micrograph of the same stage, highlighting the three-dimensional relationship of the structures.
Keeping track of moving cells: fate maps and cell lineages
There are two major types of cells in the embryo: epithelial cells, which are tightly connected to one another in sheets or tubes; and mesenchymal cells, which are unconnected or loosely connected to one another and can operate as independent units. Within these two types of arrangements, morphogenesis is brought about through a limited repertoire of variations in cellular processes:
• Direction and number of cell divisions. Think of the faces of two dog breeds— say, a German shepherd and a poodle. The faces are made from the same cell types, but the number and orientation of the cell divisions are different. Think also of the legs of a German shepherd compared with those of a dachshund. The skeleton-forming cells of the dachshund have undergone fewer cell divisions than those of taller dogs.
• Cell shape changes. Cell shape change is a critical feature of development. Changing the shapes of epithelial cells often creates tubes out of sheets (as when the neural tube forms), and a shape change from epithelial to mesenchymal is critical when individual cells migrate away from the epithelial sheet (as when muscle cells are formed).
• Cell migration. Cells have to move in order to get to their appropriate locations. The germ cells have to migrate into the developing gonad, and the primordial heart cells meet in the middle of the vertebrate neck and then migrate to the left part of the chest.
• Cell growth. Cells can change in size. This is most apparent in the germ cells: the sperm eliminates most of its cytoplasm and becomes smaller, whereas the developing egg conserves and adds cytoplasm, becoming comparatively huge. Many cells undergo an “asymmetric” cell division that produces one big cell and one small cell, each of which may have a completely different fate.
• Cell death. Death is a critical part of life. The embryonic cells that constitute the webbing between our toes and fingers die before we are born. So do the cells of our tails. The orifices of our mouth, anus, and reproductive glands all form through apoptosis—the programmed death of certain cells at particular times and places.
• Changes in the composition of the cell membrane or secreted products. Cell membranes and secreted cell products influence the behavior of neighboring cells. For instance, extracellular matrices secreted by one set of cells will allow the migration of their neighboring cells. Extracellular matrices made by other cell types will prohibit the migration of the same set of cells. In this way, “paths and guiderails” are established for migrating cells.
Last edited by Admin on Wed Jan 16, 2019 12:40 pm; edited 18 times in total