Introduction:

Field of developmental biology has the history which spans the last 500 years. In the last ten years, our understanding of developmental mechanisms has grown exponentially by using modern methods of genetics and molecular biology, often shared with experimental embryology and the use of molecular markers, rather than only morphology, to recognize critical populations of cells and their state of discrimination. Three major principles have emerged.

First, mechanisms of development are extremely conserved, both among developing rudiments of the variety of organ systems and among varied organisms. This conservation takes place both at the level of tissue and cellular mechanisms, and at molecular level.

Second, development of organ rudiments is affected by surrounding tissues through interactions known as inductive interactions. Such interactions are mediated by extremely conserved growth factors and signaling systems.

Third, development is the life-long procedure and can be reawakened in events like wound healing and regeneration, and in certain diseases. Advances in understanding normal development give hope that diseases in which development runs amuck, like cancer, may soon be preventable and completely treatable.

Cell differentiation:

Differentiation is a development of cell types, from what is initially one cell -zygote or spore. The development of cell types like nerve cells takes place with the number of intermediary, less differentiated cell types. The cell stays a certain cell type by maintaining the particular pattern of gene expression. This depends on regulatory genes, like for transcription factors and signaling proteins. These can be included in self-perpetuating circuits in gene regulatory network, circuits which can involve several cells which communicate with each other. External signals can modify gene expression by activating the receptor that triggers the signaling cascade which affects transcription factors. For instance, the withdrawal of growth factors from myoblasts makes them to cease dividing and instead differentiate in muscle cells.

Growth:

Growth is the improvement of the tissue or organism. Growth continues after embryonal stage, and takes place through cell proliferation, enlargement of cells or accumulation of extracellular material. In plants, growth results in the adult organism which is strikingly different from embryo. Proliferating cells be inclined to be distinct from differentiated cells. In some tissues proliferating cells are limited to specialized areas like the growth plates of bones. But some stem cells migrate to where they are required, like mesenchymal stem cells that can migrate from bone marrow to form like muscle, bone or adipose tissue.

Metamorphosis:

Most animals have the larval stage, with the body plan different from that of adult organism. Larva suddenly develops in the adult in the procedure known as metamorphosis. For instance, caterpillars (butterfly larvae) are specialized for feeding whereas adult butterflies (imagos) are specialized for flight and reproduction. When caterpillar has grown sufficient, it turns in the immobile pupa.

Anatomical Approach to Developmental Biology or Principles of Development:

1. Organisms should function as they form their organs. They have to utilize one set of structures while constructing others.

2. The major question of development is how egg becomes the adult? This question can be broken down in component problems of differentiation (How do cells become different from one another and from their precursors?), morphogenesis (How is ordered form is produced?), growth (How is size regulated?), reproduction (How one generation creates another generation?), and evolution (How does change in developmental procedures create novel anatomical structures?).

3. Epigenesis happens. New organisms are developed de novo each generation from comparatively disordered cytoplasm of egg.

4. Preformation is not in anatomical structures, but in instructions to create them. Inheritance of the fertilized egg comprises genetic potentials of organism.

5. Preformed nuclear instructions comprise the ability to respond to environmental stimuli in specific ways.

6. Ectoderm gives rise to epidermis, nervous system, and pigment cells.

7. Mesoderm produces the gonads, kidneys, heart, bones, and blood cells.

8. Endoderm forms the lining of the digestive tube and respiratory system.

9. Karl von Baer's principles define that general characteristics of large group of animals appear earlier in embryo than do the specialized features of the smaller group. As each embryo of the given species develops, it diverges from adult forms of other species. Early embryo of the higher animal species is not like the adult of a "lower" animal.

10. Labeling cells with dyes illustrates that few cells differentiate where they produce, while others migrate from their original sites and differentiate in the new locations. Migratory cells comprise neural crest cells and precursors of germ cells and blood cells.

11. Community of embryonic structure reveals community of descent.

 Life cycles and the evolution of developmental patterns:

Traditional ways of categorizing catalog animals according to the adult structure. But, as Bonner indicated out, this is an extremely artificial method, as what we consider the individual is generally just a short slice of life cycle. Life cycle has to be adapted to environment that is made up of nonliving objects and other life cycles. Take, for instance, life cycle of Clunio marinus, the small fly which inhabits tidal waters along coast of western Europe. Females of the species live only 23 hours as adults, and they should mate and lay their eggs inside this short time. To make matters even more precarious, egg laying is confined to red algae mats which are exposed only during lowest ebbing of spring tide. Such low tides take place on four successive days shortly after new and full moons (that is, at approx 15-day intervals). Thus, life cycle of these insects should be coordinated with tidal rhythms and daily rhythms such that insects emerge from their pupal cases during few days of the spring tide and at the right hour for its ebb.

One of the main successes of descriptive embryology was the thought of a generalizable life cycle. Each animal, whether earthworm, eagle, or beagle, passes through similar phases of development. Life of the new individual is started by fusion of genetic material from two gametes the sperm and the egg. This fusion, known as fertilization, stimulates egg to begin development. The phases of development between fertilization and hatching are jointly known as embryogenesis.

Throughout animal kingdom, the incredible variety of embryonic types exists, but most patterns of embryogenesis are variations on 5 themes:

1. Immediately following fertilization, cleavage takes place. Cleavage is the series of very rapid mitotic divisions in which huge volume of zygote cytoplasm is divided in several smaller cells.

2. After rate of mitotic division has slowed down, blastomeres experience dramatic movements in which they alter their positions relative to one another. Series of extensive cell rearrangements is known as gastrulation, and embryo is said to be in gastrula stage.

3. Once three germ layers are set up, the cells relate with one another and reorganize themselves to generate tissues and organs. This process is known as organogenesis. Several organs have cells from more than one germ layer, and it is not strange for outside of the organ to be derived from one layer and inside from another. Also during organogenesis, certain cells experience long migrations from their place of origin to final location. Migrating cells comprise precursors of blood cells, pigment cells, lymph cells, and gametes.

4. Several species have specialized portion of egg cytoplasm that gives rise to cells which are precursors of gametes (the sperm and egg). Gametes and their precursor cells are jointly known as germ cells, and they are reserve for reproductive function. All the other cells of body are known as somatic cells. This separation of somatic cells (that give rise to individual body) and germ cells (that contribute to formation of the new generation) is frequently one of the first differentiations to occur during animal development. Germ cells finally migrate to gonads, where they differentiate in gametes. The development of gametes, called gametogenesis, is usually not completed until the organism has become physically mature.

5. In several species, organism which hatches from egg or is born in world is not sexually mature. Certainly, in most animals, the young organism is the larva which may look considerably different from adult. Larvae frequently comprise phase of life which is used for feeding or dispersal. In several species, the larval stage is the one which lasts the longest and adult is the brief stage solely for reproduction. In silkworm moths, for example, the adults don't have mouthparts and can't feed.

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