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Development on the Cellular Level

•  Adding cells through cellular division
•  Differentiating cells to serve different functions
•  Mechanics of differentation: induction and gene expression
•  Establishing the body axes and patterns
•  Cell migration
•  Genes provide positional information
•  Programmed cell death

More to come
More subject specific images will be added to this resource page.

Adding cells through cellular division
•  Symmetric cell division of stem cells ensures that a constant pool of stem cells is available by giving rise to two identical daughter cells both endowed with stem cell properties.
•  Asymmetric division of stem cells results in the production of only one stem cell and a progenitor cell with limited self-renewal potential.
•  Progenitor cells that are produced via asymmetric cell division will go through additional rounds of cell division until they are terminally differentiated into a mature, specialized cell.
•  Asymmetric division can be controlled by both intrinsic and extrinsic factors.
•  Intrinsic factors involve differing amounts of cell-fate determinants being distributed into each daughter cell, while extrinsic factors involve interactions with neighboring cells and the micro and macro environment of the precursor cell.

Differentiating cells to serve different functions
•  The three major cell types in the mammalian body include germ cells (which develop into gametes), somatic cells (diploid cells that develop into a majority of the human body) and stem cells (cells that can divide indefinitely).
•  In human development, the inner cell mass exhibits the ability to differentiate and form all tissues of the body; however, they cannot form an organism.
•  The various types of stem and progenitor cells included in the body that will differentiate to develop more specialized cells includes: hematopoietic stem cells, mesenchymal stem cells, epithelial stem cells and muscle satellite cells.
•  To develop a multicellular oragnisms, cells must differentiate to specialize for different functions.

Mechanics of differentation: induction and gene expression
•  Different types of stem cells exhibit varying abilities to differentiate into specialized cells (from the most unlimited stem cell to the most restricted): totipotent, pluripotent, multipotent to oligopotent.
•  Totipotent cells have the potential to differentiate into any of the cells needed to enable an organism to grow and develop; pluripotent cells have the potential to differentiate into any type of human tissue but cannot support the full development of an organism.
•  A multipotent stem cell has the potential to differentiate into different types of cells within a given cell lineage or small number of lineages, while an oligopotent stem cell is limited to becoming one of a few different cell types.
•  The process of cellular differentiation is under strict regulation by transcription factors which can either activate or repress expression of genes that will affect the proteome of the cell and thus, provide the necessary components it needs to become a specialized cell.
•  All cells contain the same complement of DNA, or genome, but once differentiation occurs, it is the changes in the proteome that will distinguish one cell type from another.

Establishing the body axes and patterns
•  The three axes of the animal body are established in development via the expression of specific sets of genes that regulate which cells will develop into specific structures.
•  During development, the dorsal cells are genetically programmed to develop into the notochord and define the axis.
•  The neural tube can develop in two ways: primary or secondary neurulation, which are used by organisms in varying degrees to establish the neural tube that will develop into the central nervous system (brain and spinal cord).
•  Specific patterns along the neural tube that are established via secretion and production of specific signaling molecules (such as Wnt, Shh, BMP and retinoic acid) play a key role in patterning the dorsal and ventral axes.

Cell migration
•  The disruption or dysfunction of cell migration processes can lead to formation of various diseases such as metastasis, tumor formation and vascular disease.
•  In prokaryotic organisms, and some eukaryotic cells such as sperm cells, cell migration occurs via the use of a cilia or flagella to propel forward.
•  In eukaryotic organisms, cell migration is a much more complex process and can include, but is not excluded to, changes in the cytoskeleton, motor proteins, blebbing, and cytoplasmic displacement; it involves both external and internal signals that mediate these processes.

Genes provide positional information
•  Organogenesis results in the formation of the various organs in the body; however it will only occur if specific sets of genes are expressed to determine ultimate cell type.
•  The ability of specific cells to migrate to the the edge of the ectoderm is highly regulated by specific gene expression and allows for differentiation into epidermal cells; in contrast, the cells which remain in the center will develop into the neural plate.
•  The expression of specific sets of genes will also regulate the reorganization of the mesoderm into distinct groups of cells, called somites, which develop into the ribs, lungs, spine muscle and notochord.

Programmed cell death
•  Programmed cell death can provide an advantage to an organism during development, for instance by maintaining homeostasis and protection against potentially disruptive issues which may arise during the life of a cell.
•  Apoptosis is a process of programmed cell death that is regulated by numerous biochemical events and appears to be genetically mediated.
•  Autophagy is a process of programmed cell death that is characterized as a catabolic process via formation of an autophagolysosome which degrades damaged cellular contents.
•  Necrosis occurs when cellular death is caused by external factors and is characterized as an alternate form of programmed cell death, called necroptosis.


Key terms
•  anencephaly a lethal birth defect in which most of the brain and parts of the skull are missing; absence of the encephalon
•  apoptosis a process of programmed cell death 
•  autologous derived from part of the same individual (i.e. from the recipient rather than the donor)
•  autophagy a type of programmed cell death accomplished through self-digestion
•  blastocyst the mammalian blastula formed during development where the inner cell mass can be found which forms the embryo
•  bleb an irregular bulge in the plasma membrane of a cell
•  chemotaxis the movement of a cell or an organism in response to a chemical stimulant
•  differentiate to produce distinct cells, organs or to achieve specific functions by a process of development
•  extracellular matrix All the connective tissues and fibres that are not part of a cell, but rather provide support.
•  gastrulation the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells
•  inner cell mass a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals
•  laminar of fluid motion, smooth and regular, flowing as though in different layers
•  metastasis the transference of a bodily function or disease to another part of the body; specifically the development of a secondary area of disease remote from the original site, as with some cancers
•  morula a spherical mass of blastomeres that forms following the splitting of a zygote; it becomes the blastula
•  neural tube hollow longitudinal dorsal tube formed in the folding and subsequent fusion of the opposite ectodermal folds in the embryo that gives rise to the brain and spinal cord
•  neurulation the process by which the beginnings of the vertebrate nervous system is formed in embryos
•  notochord a flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine
•  organogenesis the formation and development of the organs of an organism from embryonic cells
•  pluripotent able to develop into more than one mature cell or tissue type, but not all
•  pluripotent able to develop into more than one mature cell or tissue type, but not all
•  progenitor cell a biological cell that, like a stem cell, has a tendency to differentiate into a specific type of cell, but is already more specific than a stem cell and is pushed to differentiate into its “target” cell.
•  proteome the complete set of proteins encoded by a particular genome
•  proteome the complete set of proteins encoded by a particular genome
•  somite one of the paired masses of mesoderm distributed along the sides of the neural tube that will eventually become dermis, skeletal muscle, or vertebrae
•  totipotency the ability of a cell to produce differentiated cells upon division
•  transcription the synthesis of RNA under the direction of DNA

Neural Tube Formation
The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.

In this five-week old human embryo, somites are segments along the length of the body.

Cell Migration
Phase images of BSC 1 cells migrating in a scratch assay in the absence of serum over a period of 15 hours.

Cell Differentiation
Mechanics of cellular differentiation can be controlled by growth factors which can induce cell division.In asymetric cell division the cell will be induced to differentiate into a specialized cell and the growth factors will work in tandem.

Programmed Cell Death
This histological section of a foot of a 15-day-old mouse embryo, visualized using light microscopy, reveals areas of tissue between the toes, which apoptosis will eliminate before the mouse reaches its full gestational age at 27 days.

This video describes the process of apoptosis, or programmed cell death.

Vertebrate Axis Formation
Animal bodies have three axes for symmetry:lateral-medial (left-right), dorsal-ventral (back-belly), and anterior-posterior (head-feet).

Symmetric and Asymmetric Division
This diagram illustrates stem cell division and differentiation, through the processes of (1) symmetric stem cell division, (2) asymmetric stem cell division, (3) progenitor division, and (4) terminal differentiation.Stem cells are indicated by (A), progenitor cells by (B), and differentiated cells by (C).

Hematopoiesis: the differentiation of multipotent cells
The process of hematopoiesis involves the differentiation of multipotent cells into blood and immune cells.The multipotent hematopoietic stem cells give rise to many different cell types, including the cells of the immune system and red blood cells.

Transcription Factors Regulate Gene Expression
While each body cell contains the organism’s entire genome, different cells regulate gene expression with the use of various transcription factors.Transcription factors are proteins that affect the binding of RNA polymerase to a particular gene on the DNA molecule.

Stem Cells
Pluripotent, embryonic stem cells originate as inner cell mass (ICM) cells within a blastocyst.These stem cells can become any tissue in the body, excluding a placenta.Only cells from an earlier stage of the embryo, known as the morula, are totipotent, able to become all tissues in the body and the extraembryonic placenta.

Neural Tube
Transverse section of half of a chick embryo of forty-five hours’ incubation.The dorsal (back) surface of the embryo is toward the top of this page, while the ventral (front) surface is toward the bottom.(Neural tube is in green.)

Neural Tube Formation
The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.

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•  Wikipedia. “Programmed cell death.” CC BY-SA 3.0 http://en.wikipedia.org/wiki/Programmed_cell_death
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•  Wiktionary. “apoptosis.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/apoptosis
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•  Wikipedia. “Cell migration.” CC BY-SA 3.0 http://en.wikipedia.org/wiki/Cell_migration
•  Wiktionary. “metastasis.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/metastasis
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•  Wiktionary. “chemotaxis.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/chemotaxis
•  Wiktionary. “bleb.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/bleb

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