Chapter 8 - Summary
Cells in a multicellular creature
Large numbers of cells coordinate together to produce multicellular creatures like ourselves. In a single organism we may find nerve cells, muscle cells, skin cells, brain cells and many others too, each type specialised to perform a different task. Yet multicellular creatures develop from just a single cell. How does each one know whether to become nose or hair, nose cell or a hair cell? Developmental biology now has a number of well-documented cases for you to look at, each one casts light on in this complex and fascinating process.
In many cases we may say that development begins when male and female gamete (sperm and egg) unite. To watch a fertilised animal egg divide repeatedly and develop into the adult organism is a wonderful thing! The zygote (newly fertilised egg) divides in a predictable fashion and, as the size of the cluster of cells increases, cells differentiate into particular types. In time, the cell-cluster will assume the shape and composition of the adult organism.
From a genetic point of view, each gamete carries half of the DNA of the zygote, the DNA that contains plans for ALL the different types of cell to be found in the adult organism. We say that the zygote is totipotent. The big questions for developmental biologists are firstly how each cell knows what kind of cell it needs to become and secondly how the information in its DNA is selectively accessed.
Immediately the fertilised egg starts to divide we can ask: "which of the two cells will become the head and which the tail?" It soon becomes apparent that the egg was not a round blob with no sense of head or tail, of up or down, but in fact that it contains plenty of asymmetrically placed elements that could easily specify an axis. In fact, such an asymmetric distribution of determinants has been found to do just this in the egg cell of the nematode and in the syncytial blastoderm of the fruitfly.
The fate of a cell is not always determined by factors that it has picked up when it was created. Cells also may rely upon interaction with the cells around them: the extraordinary life-cycle of the slime-mould Dictyostelium bears witness to this. Since interactions in a embryo are so difficult to isolate, experimenters distinguish between tissue that knows what to become by itself (autonomously specified) and tissue whose future is determined by the cells that surround it (conditionally specified). Good examples are to be found in the newt embryo and the development of the fine structures of the kidney.
The development of an organism is not entirely controlled from within. Environmental factors do play a part, in some organisms in specific and well-defined ways. As, for example, in the case of butterfly polyphenisms, or neural development/wastage.
In a fully-grown organsim, we find whole communities of cells called organs that have dedicated functions. This is the egg. It is a huge sack of proteins in which the single cell of the embryo lurks. The wrinkled appearance may be an artifact, produced by the heavy dehydration required to prepare a sample for the SEM (the egg was soaked in alchohol).
For the majority of the pictures in this sequence the image is about 0.3mm (300µm) across.
The way that animals grow and their range of functions is far more remarkable than plants. There are four or five distinct stages in animal development, outlined in the diagram below.
After the egg is fertilised almost immediately it undergoes a series of mitotic divisions without changing significantly in volume. By appearance this stage is known as cleavage - the resultant cells called blastomeres and the entire cluster, the blastula. Next the blastomeres undergo a series of dramatic movements, changing their positions relative to each other. This rearrangement is called gastrulation and it results, typically, in three regions of cells callsed germ layers: ectoderm (outside), mesoderm (middle) and endoderm (inside). Each of these areas will give rise to different organs or parts of the body.
After gastrulation, the cells begin to divide and grow significantly in a phase called organogenesis in which the organs of the body are produced. Sometimes the outside of an organ is produced by one germ layer and the inside by another. Some cells are set aside: they are the germ cells that will eventually become the gonads (the ovaries or testes i.e. the gamete producing cells). Gamete production does not generally begin until the animal is physically mature. Non-germ cells are called somatic.
When the creature is fully-developed it either hatches from an egg or is "born". In some creatures (such as frogs) there is a subsequent stage of vigorous change called metamorphosis in which the young creature goes through a further transformation (from tadpole to young frog). Once the animal reaches its mature phase it is ready to begin reproducing - initiating the whole cycle one more time. In a mature animal a number of processes of cell development continue to run: for example, red blood cells are constantly generated in the bone marrow and worn skin is replaced.
Juvenile homology.
If you look at the variety of life forms that live on Earth you can classify them into different groups for the sake of convenience and understanding. You could employ many different criteria in this act of taxonomy (....), including placing those that are genetically most similar together, but by far the most convenient criteria rely upon a casual appraisal of the most apparent visual characteristics. So, for example, we distinguish between plants and animals, between animals that fly and that walk, animals that breathe air and those that let water circulate through their gills.
It turns out that, if you examine the way the creatures in this tree DEVELOP, you will discover that related creatures are indistinguishable in their most juvenile stages, that they only subsequently differentiate themselves.
One of the difficulties of Developmental Biology is that you have to study a system that is constantly changing. The states and behaviours of the cells change on an hourly or a minute to minute basis. If there is one aspect of the system that is regarded as a constant it is the DNA. It is assumed that, since the cells divide by Mitosis at every step of the process, that the DNA contents of every cell is entirely the same. This hypothesis has been tested on a few occasions. The first was when the DNA was extracted from a mature cell of a frog and introduced into a frog egg. After many attempts a new frog grew from that egg, an identical clone of the adult, proving that the DNA in that adult cell was totipotent, that it was capable of producing every aspect of an entirely new frog.
The instructions contained in the DNA are, in effect, the proteins that stretches of DNA encode. Those proteins, in turn, form structural features or, as enzymes, control processes that develop the cell. We call each important stretch of DNA a gene and the steps involved in producing finished proteins from DNA, we call gene expression. Gene expression can be regulated at every stage: from the loosening of the chromatin to expose the DNA to RNA polymerases to the transport of the RNA out of the nucleus; from the degradation of that RNA in the cytosol to the secretion of the protein to its ultimate location.
The egg is NOT just a "blob".
Development begins the moment fertilisation has occured. However it's important to remember that the union of sperm and egg is hardly the coming together of two amorphous spheres. Whilst the sperm has a clear structure of its own the egg too has plenty of inner structure. In fact an egg is a very particular kind of cell in its own right. Large, as cells go, it typically contains proteins (as food for the developing embryo), ribosomes and tRNA (to help protein synthesis get off to a good start), mRNA (to specify some of the first proteins that will be formed) and protective molecules (to protect the dormant cell against damage). More that that it will contain "developmental control factors" localised in different regions that will steer the egg to develop in a particular way and ensure that, after cleavage, one blastomere is by no means identical to another.
A different case of cytoplasmic determination is found in the early stages of development of the fruit fly Drosophila. In Drosophila, numerous duplications of the cell nucleus occur without the formation of cell membranes between them. The result is what is called a syncytial blastoderm - a structure peculiar to the development patterns of certain insects.
When the fertilised egg cleaves the blastomeres are not all necessarily the same: if, for example, the cytoplasm that was divided up between them was not, in fact, uniform. This can lead to the programming of each cell to its own unique fate. The development of the nematode worm is a clear example of cytoplasmic determination. We can follow clearly the ongoing localisation of certain determinants called P-granules that will decide which cells become germ-line cell.
Cells interact with each other.
It is easy to think of cells as self-contained units, but in fact, they often interact and co-operate with each other. The extraordinary case of the slime mould Dictyostelium, makes the point very well. A strange organism, when food is plentiful Dictyostelium exists as hundreds of roving amoebae. When food is scarce, however, those amoebae congregate to form a multicellular slug which is capable of (hopefully) moving away. In turn that slug grows a fruiting bud that scatters spores which will in turn form more amoebae.
If the same experiment is tried later on in the development of the same embryo, the results are exactly the opposite - now the tissue has become autonomously specified. Clearly, cell fate is only conditionally specified for a time before the cell gathers some autonomous momentum of its own.
Experiments in developmental biology are complex.
The more complicated the organism, the more likely the systems of determination are to be complex. In many creatures cell-fate is not pre-programmed but rather depends upon interactions with the cells that the cell finds itself adjacent to. It is easy to invent ways in which this could happen. However, experimentally, it is often hard to discover what is actually going on. So, experimenters classify cells as autonomously specified if, separated from the rest of the embryo, they continue down the path of developement they would normally have followed. Then experimenters call them conditionally specified if, when the cell is transplanted from its original place in the embryo, the course of its development changes too.
Adult organanisms often contain structures that are very precisely put together. Consider, for example, your eye: how light must pass through a lens, be focussed onto a retina assembled with nerves, then the signal carried away. There is no room for error and it would take a hand far more accurate than the most patient watchmaker's to make this. From a developmental perspective, once the embryo has reached a high stage of development, a good way of ensuring that the details of the structure turn out right is to have the cells of the relevant tissues "checking" with each other that they are doing the right thing. A good example of this can be seen in the development of the kidney.
The kidney is an organ that serves as what is known to the engineer as a "counter-current multiplier". The body's waste products that are circulating in the blood stream enter the kidneys and pass through very fine blood vessels that wrap around tiny capsules called Bowman's capsules. The impurities diffuse through the membrane of the capsules to form a fluid we know as our urine.
When you look at the overall structure of a kidney, one of its striking features is the fine tree of nephrons terminating in Bowman's capsules. Surely this is, from a developmental perspective, a nightmare to create? The answer is that this part of the kidney is formed from two different types of tissue, which we, for simplicity, will call the yellow and the red tissues (they will carry the urine and the blood respectively). When the yellow tissue meets the red tissue, it is induced to bud. When the red tissue meets the yellow tissue, it is prompted to start forming Bowman's capsules.
Environmental Regulation
Polyphenism
In certain organisms genetic switches respond to specific environmental cues. There is a species of butterfly that produces eggs twice a year. Those that hatch in spring produce caterpillars that live off oak catkins. The caterpillars have a yellow-brown coluour and a beaded shape that camouflages them perfectly. Those eggs that hatch in the late summer develop into caterpilars that, by contrast, look like oak twigs and feed on the leaves. Why are the caterpillars so completely different? The cue to develop differently is diet - whether they are eating catkins or leaves determines their pattern of development.
We call the ability of a species to have two or more distinct phenotypes polyphenism. Several butterflies have been found that show polyphenism with respect to their markings, often switched by a combination of temperature and hours of daylight - good indicators of season and hence the kind of environment in which they will have to live. Another example of polyphenism you may be half-familiar with is the case of the queen bee. She alone is responsible for laying the eggs that will develop into the next generation of bees. All of the larva develop into worker bees with the exception of one, the one that is fed the highly nutritious "royal jelly". That larva switches developmental pathway, becomes tens of times as big, and becomes the next queen bee.
Why is it that scientists study peculiar creatures such as frogs in such detail: why don't they concentrate on something more interesting like us?
In the case of developmental studies, one of the organisims that has been (and is being) highly scrutinised is the fruitfly, Drosophila Melanogaster. It is much tinier than the average fly about your home, it is the very small fly that occasionally inhabits your garbage bin in the kitchen. In the laboratory, Drosophila breed vigorously on a banana mix foodstuff and Drosophila has been one of the perfect organisms for genetic study for nearly a century now. Why? Short generation time (??? days); polytene chromosomes that enabled the visual localisation of genes before molecular techniques; finally, few people object ethically to the kind of treatment to which flies are subjected - they much more likely would with mammals. For these reasons a great deal is known about the genetics of Drosophila and, if we are to study something as complex as Development, then it is a great advantage to work with an organism whose genetics have already been well figured out.
There follows the hope, that if we can understand the principles of the development of the fly we will be better placed to assemble the story of human development. One might imagine there would be a slender chance that knowledge about the fruitfly could be of much use in getting to know ourselves. It is now beginning to look as though, on the contrary, there is a remarkably degree of homology (sameness) between the genetic methods of the fly and the human.
During the course of its development, the Drosophila egg becomes a larva which goes through four molts, becoming progressively larger with each shedding, before entering a pupal stage. In the pupa the larva metamorphoses (fundamenally changes shape) into the adult fly. You can think of the larval stages as the "fattening up" of the developing fly (the larva feeds voraciously and the molts are necessary for its hard skin to accomodate it); the 5th molt or shedding of the skin of the pupa is also called the imaginal molt (Latin "Imago" means "adult"). An obvious differences between the adult and larva are all the various limbs that have appeared. These develop from flat discs of cells apparent on the larva called imaginal discs. The development of the imaginal discs is stimulated by a growth hormone released within the pupa and this process may also be stimulated in vitro. There are imaginal discs for the legs, antenna, halteres ......?????? In the following sequence we are going to look at how the legs evert from their imaginal discs in the Drosophila's metamorphosic stage.
Subtle changes
The physical structure of a multicellular creature may respond to environmental cues in all sorts of ways. We so take for granted the growth or wastage of muscles according to usage that it is easy to forget that exercise may be changing not only the size and number of muscle cells, but their potential for work as well. For example, regular workouts will actually increase the numbers of mitochondria present in the relevent cells as well.
As we "learn" we do so by the physical modification of our nervous system. A child has far more brain cells than an adult and learning tends to take the form of a pruning of plethora or neural pathways that are already there.
Conclusion
The end product of the developmental process is usually an organism which we can regard as being partitioned into a number of functionally distinct regions or organs. For example in the human being there is the digestive system involving the teeth and mouth for chewing, the throat for carrying food to the stomach, the stomach for digesting then the long alimentary canal where food components are absorbed. Finally the anus where the remains are expelled. Then there is the circulatory system in which the blood is the transport medium for the products like food and oxygen that need to be circulated and where the heart is the pump that drives it.
Then we have the lungs for transferring the gaseous components of metabolism (oxygen and carbon dioxide) to and from the air. The nervous sytem for sensing and responding. The kidneys for filterming impurities. The endocrine system for controlling long term chemical priorities.
Just as cell distribute function into organelles so vast communities of cells divide them between organs. For most purposes a doctor or a surgeon is concerned not with cells themselves but with the wellbeing of individual organs. Today it is possible to transplant organs such as hearts or kidneys from one person to another. Now you can see that the recipient is not receiving "just another heart" but a carefully functioning community of cells that are completely alien to their body, that have an utterly different parentage and genetic background. It is perhaps indicative of the autonomy (independance) with which the organs of the body work that such operations are possible at all.