Cellular Development and Differentiation

First stage

Biology
Lec 7
28/12/2015

د. بثينة

Cellular Development & Differentiation
Early development characterized by the rapid proliferation of embryonic cells, which then differentiate to produce many specialized types of cells that make up the tissues and organs of multicellular animals. As cells differentiate, their rate of proliferation usually decreases, and most cells in adult animals are arrested in the G0 stage.
A few types of differentiated cells never divided again, but most cells are able to resume proliferation as required to replace cells that have been lost as a result of injury or cell death. In addition, some cells divide continuously throughout life to replace cells that have a high rate of turnover in adult animals.
A special characteristic of cell growth and cell division, is cell differentiation, which means changes in physical and functional properties of cells as they proliferate in the embryo to form the different bodily structures. The earliest and simplest theory for explaining differentiation was that the genetic composition of the nucleus undergoes changes during successive generations of cells in such a way that one daughter cell inherits a different set of genes from that of the other daughter cell.
Therefore, the present idea is that instead of loss of genes during the process of differentiation, there occurs selective repression of different genetic operons. This presumably results from the buildup of different repressor substances in the cytoplasm, the repressor substances in another cell acting on a different group of genetic characteristics.
The main types of molecular processes that control cellular differentiation involve cell signaling. Many of the signal molecules that convey information from one cell to another during the control of cell differentiation are known as growth factors.
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Cell Proliferation in Adults:

The cells of adult animals can be grouped into three general categories with respect to cell proliferation:
(1)- A few types of differentiated cells, including lens cells, nerve cells and cardiac muscle cells in humans, if they are lost they can never be replaced.
(2) - In contrast, most cells in adult animals enter G0 stage of the cell cycle but resume proliferation as needed to replace cells that have been injured or have died. Cells of this type include skin fibroblasts, smooth muscle cells, the endothelial cells that line blood vessels, and the epithelial cells of most internal organs, such as liver, pancreas, kidney, lung, prostate and breast.
(3)- Other types of differentiated cells, including blood cells, epithelial cells of the skin, and the epithelial cells lining the digestive tract, that have short spans and must be replaced by continual cell proliferation in adult animal cells. In these cases, the fully differentiated cells do not themselves proliferate. Instead, they are replaced via the proliferation of cells that are less differentiated, called stem cells.
Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to, signals. These changes are largely due to highly-controlled modifications in gene expression. A cell that is able to differentiate into many cell types is known as pluripotent. Such cells are called stem cells in animals. A cell that is able to differentiate into all cell types is known as totipotent.
Pluripotent stem cells undergo further specialization into multipotent progenitor cells that then give rise to functional cells. Examples of stem and progenitor cells include:
Hematopoietic stem cells :from the bone marrow that give rise to red blood cells, white blood cells, and platelets
Mesenchymal stem cells from the bone marrow that give rise to stromal cells, fat cells, and types of bone cells.
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Epithelial stem cells: that give rise to the various types of skin cells
Muscle satellite cells: that contribute to differentiated muscle tissue.









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The central problem is the study of development is the question of how a single cell, the fertilized egg give rise to the many cell types of
the mature organism.
Fertilized egg 2-cell stage
4-cell stage (blastomere) 8-cell stage
four are pigmented smaller, located at the animal pole ( micromeres).
four are unpigmented larger, located at the vegetal pole ( macromeres).
8-cell stage morula stage
blastula stage gastrula stage




Development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells
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called the inner cell mass. The cells of the inner cell mass go on to form virtually all of the tissues of the human body.

Blastula Stage

The cells then rearranged to form three germ layers, which give rise to different tissues.
The outer germ layer (ectoderm) from which arise the nervous system and epidermal layer of skin.
The inner germ layer (endoderm) form the epithelial lining of the digestive tract and respiratory passage and contributes the essential
tissue of associated organs., such as liver and pancreas.
3- The middle germ layer (mesoderm) gives rise to the most of the cells of organism, such as those found in the muscles, skeleton, connective tissues, blood, kidneys, gonads, and certain other organs.
The process of functional and structural specialization of these cells is
called differentiation. For example muscle cell precursors elongate
into spindle- shaped cells that synthesize and accumulate myofibrillar proteins. The resulting cell efficiently converts chemical energy into contractile force.
Morphological modification during differentiation are accompanied by chemical changes. In the example given, formation of the muscle cell results from the synthesis of several specific proteins such as actin & myosin.

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Cells are not always restricted to a single activity and frequently perform two or more specialized function, for example. Intestinal epithelial cells both absorb nutrient & synthesize digestive enzymes such as disaccharidases and peptidases.
Cellular Specialization:
To consider how differentiation might be accomplished by an individual cell in order to differentiate during embryonic development, cells must make a series of small shifts in their potential, as for example, when a cell of (blastocytes) becomes a cell belonging to the endoderm, these then proliferate to make more cells of their own kind of another shift is made, and member of this group may become either part of the gut wall or the lung. If the former occurs then a third shift results, and the cell becomes either absorptive or secretory.
It is also generally agreed that the basic codes in the genetic material
do not change with development, but that different regions of the
genome are turned on and other turned off as cells develop. The genes
are said to be differentially expressed, as the cells are progressively determined.
It is important, however, to recognize that a cell may have several functions and be a member of more than one cell group
for example:
There are four groups of contractile cell:
(1)- Muscle cells, muscle cells are the main type and comprise:
striated (voluntary) muscle, cardiac & smooth (involuntary) muscle.
(2)-Myoepithelial cells, cells are an important component of certain
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secretory glands.
(3)- Myofibroblasts, have a contractile role in addition to being able to secrete collagen.
(4)- Pericytes are smooth, muscle-like cells that surrounded blood vessels.






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