<center>https://s-media-cache-ak0.pinimg.com/originals/0b/36/d1/0b36d1710d5f5cf63580f01c053c563a.jpg</center>

<br><br>

# <center>We know that a single cell develops into a fully formed organism, but how? </center>

Today we describe the two basic puzzles of development,and outline the earliest attempts to solve these puzzles. We then provide an overview of the factors that influence development, the process by which development occurs, and the early developmental outcome common to all organisms. 
 
<br><br>

 <center>http://s3.amazonaws.com/engrade-myfiles/4067527801313135/gametezygotefertilization.jpg</center>

## Sexual reproduction involves the production of haploid gametes, which come together to form a diploid zygote—a single cell that will develop into a full organism. 


The question of how a zygote develops into a complete organism has been at the center of biology since before Darwin’s time.   It is not surprising that this question should be so central, given the magnitude of the problem. A vertebrate animal, such as a human, may be composed of up to 10^14—one hundred trillion—cells!  These cells may be specialized into hundreds of distinctly different cell types, each with one or more unique functional roles to play in the organism.  Furthermore, these many kinds of specialized cells must occur in the right proportions and must be found in the right positions in the organism. 

<br><br>

 <center>http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/14707620/U2CP3-4B_RegulProteinsLoop_ksm.jpg</center>

## The process of development raises two broad questions, or puzzles.  



The first question is one of diversity.  All organisms start from a single cell, and by and large, those cells all look pretty much the same. Yet zygotes from different species develop into organisms as diverse as oak trees and earthworms or sea urchins and giraffes.  In addition, each of these organisms has many different kinds of cells, each performing different functions and each occurring in different proportions and positions in the organism.  How do both the diversity we observe between species and the diversity we observe within a single species arise from a single, relatively uniform cell? 

The second question is one of consistency.  In spite of the potentially enormous complexity of a completely developed organism and in spite of the intricate developmental processes involved, things go wrong relatively rarely in development.  For example, every giraffe zygote develops into a slightly different giraffe, but giraffes as a whole are remarkably alike. How is this consistency in development maintained?

<br><br>

 <center>https://c7.staticflickr.com/4/3025/2567878118_797d39d096.jpg</center>

## The answers to both questions must be found in the same set of developmental mechanisms.


The most fundamental question, then, is what dictates the way a single cell turns into many different kinds of cells.  The Greek philosopher Aristotle originally proposed that organisms must develop from a single uniform cell. This view is referred to as epigenesis. The alternative view—also ancient—was that an egg cell included an entire organism in miniature. This is known as preformation. Preformation became the dominant view among scientists by the 18th century, largely because it required no special mechanisms or mysterious “force” to create the parts of an organism.  Preformation had its difficulties, most notably the problem of how an entire miniature organism could fit into something the size of a single cell. This was not originally seen as much of a problem, because modern cell theory did not mature until the second half of the 19th century. 

As microscopes improved in the 18th and 19th centuries, however, biologists, including Caspar Friedrich Wolff, began to describe the development of organisms in detail, and it became apparent that fertilized eggs did not contain entire organisms.  Preformationists could always claim that these kinds of observations were simply not adequate because of the limitations of the microscopes. Nonetheless, the weight of evidence from early embryologists eventually held sway, and by the late 19th century, the idea of epigenesis became universally accepted.  

The acceptance of epigenesis, however, raised the fundamental question of how a single cell is able to give rise to many different kinds of cells.   Early embryologists could only propose that there must be some unknown force of nature (the “essential force”) that would control development.  As biologists in the late 19th and early 20th centuries began to understand how the cell worked, especially the process of mitosis and genetic mechanisms, the idea of an essential force was quickly replaced by more specific theories of developmental mechanisms.  

<br><br>

 <center>https://s-media-cache-ak0.pinimg.com/564x/ea/7b/39/ea7b3926259af2330a7cedd110924cb7.jpg</center>

## Several sets of factors influence development.

The earliest embryologists did not know about genes or genetics, but we know today that the zygotes of different species have different genomes; thus, it seems reasonable that genomic differences must underlie both diversity and consistency in development. The emergence of the structure of an organism clearly depends on genetic information, but genes alone cannot fully account for all aspects of development.  


One obvious reason this is true is that development in multicellular organisms does not simply produce a large group of identical cells.  A zygote has a given set of genes that is accurately copied and transmitted to its daughter cells. If genes were solely responsible for development, every daughter cell would be the same because we expect them to have the same genes.  We know that different cells can express different genes, but what ultimately is responsible for determining which genes are expressed? 


Clearly, something external to the genome must be responsible for differences that arise among cells in the development of a multicellular organism. Factors external to genes that affect gene expression may be collectively referred to as the environment.  Gene expression may be modulated by environmental factors that are completely external to the developing organism, such as temperature and nutrition. These factors can affect development to the extent of producing completely different forms under different conditions, as in the case of the moth Nemoria arizonaria. 

<br><br>

<center>![Imgur](http://i.imgur.com/6aP5Jpw.png)</center>

## Although the external environment clearly influences the development of organisms, two other sets of factors control the orderly progress of development. 

* Though outside the organism’s genes, these environmental factors are internal to the organism.   

* The first crucial environmental factor is the cellular contents of the fertilized egg. These stored factors are called cytoplasmic determinants, or maternal determinants because they necessarily come from the mother.  

* The second crucial environmental factor is interactions among cells. Cell-to-cell interactions become increasingly important as cells become more specialized and assume their proper positions and proportions.  

* The end result is that the development of a complex organism is due not just to the organism’s genetic blueprint, but also to the way the environment—defined on several levels—influences the expression of that blueprint. 

<br><br>

<center>http://bio1151.nicerweb.com/Locked/src/Locked/media/ch12/12_02-CellDivFunctions-L.jpg</center>

## Development depends on three main processes. 

1. The first process is cell division—clearly, for a single cell to produce the many cells that make up an organism, it must divide properly and in the proper amounts. 

2. The second process is differentiation, in which cells become specialized, developing specific structures and functions. Differences in gene expression play an essential role in differentiation. 

3. The third main process of development is called morphogenesis, which literally means “creation of form.” 

<br><br>

<center>http://www.mun.ca/biology/desmid/brian/BIOL3530/DEVO_01/ch01f16.jpg</center>

## Morphogenesis refers to a set of mechanisms that change the shape and organization of an organism’s structure. 


One morphogenetic mechanism involves cells changing their shape. Changes in cell shape cause tissue layers to bend and fold, creating more complex shapes out of simpler ones.  A second morphogenetic mechanism is cell movement. Cells move by reaching out cellular protrusions that then drag the rest of the cell along. Molecules on the surfaces of cells, such as glycoproteins, help cells adhere selectively to other cells during movement.  A third morphogenetic mechanism involves cells that are preprogrammed to die (a process called apoptosis). Programmed cell death is important, for example, in creating the spaces between our fingers and toes. 

<br><br>

<center>http://www.fiddlercrab.info/morphology/anterior-posterior.png</center>

## The specific details of development vary greatly from species to species; however, the earliest events in development always create asymmetries that establish the basic body plan of the organism. In plants, the fundamental asymmetry is along the vertical root-shoot axis; in animals, the most fundamental asymmetry is along the head-to-tail (anterior-posterior) axis. 


<br><br>


------------------------------

## Sources: 
[1](https://en.wikipedia.org/wiki/Developmental_biology)
[2](https://www.tocris.com/pharmacologicalBrowser.php?ItemId=187888)

--------------------------



##  Image Credits:
```
https://s-media-cache-ak0.pinimg.com/originals/0b/36/d1/0b36d1710d5f5cf63580f01c053c563a.jpg
http://s3.amazonaws.com/engrade-myfiles/4067527801313135/gametezygotefertilization.jpg
http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/14707620/U2CP3-4B_RegulProteinsLoop_ksm.jpg
https://c7.staticflickr.com/4/3025/2567878118_797d39d096.jpg
https://s-media-cache-ak0.pinimg.com/564x/ea/7b/39/ea7b3926259af2330a7cedd110924cb7.jpg
https://ka-perseus-images.s3.amazonaws.com/11e04c5dcf1ecd2885cd7718bbf3513780940a60.svg
http://bio1151.nicerweb.com/Locked/src/Locked/media/ch12/12_02-CellDivFunctions-L.jpg
http://www.mun.ca/biology/desmid/brian/BIOL3530/DEVO_01/ch01f16.jpg
http://www.fiddlercrab.info/morphology/anterior-posterior.png
Bird [Giphy](https://giphy.com/gifs/blue-5lDik5jPpRw2Y)
```


-----------------------------

<center> <a href='https://steempay.io/payment?&receiver=pjheinz&amount=0.000&currency=SBD&callback=www.steemit.com/@pjheinz'>
<img src='https://steempay.io/media/badges/donate/badge_steem-donate--light-v1.0.png'>
</a></center> 
# <center>BTC: 1JQWNxziArfaCtxsP3tUQ4VSTBY9jresmt  ![Imgur](http://i.imgur.com/chUOWyh.png) https://media2.giphy.com/media/5lDik5jPpRw2Y/200.gif </center>