development

Anthropocene

Anthropocene

No previous geological era or epoch includes humans in its definition, and in the scales of geological time, the appearance of homo sapiens on the global stage is a mere blip. The human self-image that unfolds in the modern period has insisted on a separation between homo sapiens and the world, between nature and culture. The concept of the Anthropocene is a challenge to that peculiar form of narcissism. Human societies and their material artifacts are evaluated just like other events in the history of the Earth. The claims to human exceptionalism are set aside. A single geo-history replaces the two accounts of life on earth: natural history and human history.

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embryo

Development is the process that transforms an egg into a growing embryo and eventually into an adult form.

How is this process to be understood? According to Scott Gilbert, the aesthetic of embryology separates it from other areas of biology. It is an aesthetic informed by the ordered, directional change manifest during the life of individual organisms, as they develop from a single, fertilized egg into complex patterns of diifferent, yet interacting cell, tisssues, and organs.

A few questions have dominated the study of embryology:

First of all, how is the extraordinary process of development regulated? How does a single-celled organism turn into a highly differentiated one with millions or even billions of cells? Do Genes control development? see genotype / phenotype

Is the final form of the organism set from the start? Or are there different paths of development available to the embryo? (For a discussion of preformism and epigenesis, see epigenesis}

What is the relation between the sequence of development and the process of evolution? Why do embryos of different species look so similar, and how do they end up so different?

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induction

How can a cell "know" to respond to the presence or absence of an enzyme? What is it that induces genes to work only when needed?

Using a fine baby's hair, taken from his own daughter, Hans Spemann tied off and separated the two halves of a two-celled newt embryo. The cells on either side of the knot gave rise to normal newt tadpoles. When Spemann divided the egg differently, by tying it perpendicular to the furrow between the two cells of the embryo, he obtained a dramatically different result. Only one side made a normal tadpole, while the other made a disorganized mass of belly tissue. This eventually lead to the recognition that a region of the embryo, called the dorsal lip of the blastopore, was critical for the organization of the embryo. If this region of the embryo was removed, the embryo formed a blob of tissue lacking structures that normally form on the top (dorsal) side of the animal. In 1924, Spemann proved that a graft could induce host tissues adjacent to it to completely change their fate and to form a second embryo in relation to the graft. If the dorsal lip was transplanted to the presumptive belly region of another developing embryo, it organized a second embryonic axis, and two embryos formed that were joined together. Spemann dubbed this region the "organizer" because he deduced that it organized the dorsal parts of the embryo into neural structures and could induce development of another embryonic axis. All organizers share the property of influencing the formation of pattern, or morphogenesis, in tissues or cells. The basic interpretation of their special activity is that the cells of organizers produce substances that can influence the development of other cells. Such substances have been dubbed morphogens. It has long been thought that morphogens produced in one site diffuse outward and form concentration gradients from their source. The idea then is that cells surrounding the source respond to the amount of morphogen they experience. The affected area is also called the zone of polarizing activity (ZPA). Recent advances in embryology have correlated these zones with the expression of specific genes (toolkit genes)

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