Lec.9 Lec.9 Dr: Buthaina Al-SabawiDr: Buthaina Al-Sabawi

Cellular Development & Cellular Development & DifferentiationDifferentiation

Early development characterized by the rapid Early development characterized by the rapid proliferation of embryonic cells, which then proliferation of embryonic cells, which then differentiate to produce many specialized differentiate to produce many specialized types of cells that make up the tissues and types of cells that make up the tissues and organs of multicellular animals. As cells organs of multicellular animals. As cells differentiate, their rate of proliferation usually differentiate, their rate of proliferation usually decreases, and most cells in adult animals decreases, and most cells in adult animals are arrested in the are arrested in the G0G0 stage. stage.

A few types of differentiated cells never A few types of differentiated cells never divided again, but most cells are able to divided again, but most cells are able to resume proliferation as required to replace resume proliferation as required to replace cells that have been lost as a result of cells that have been lost as a result of injury or cell death. In addition, some cells injury or cell death. In addition, some cells divide continuously throughout life to divide continuously throughout life to replace cells that have a high rate of replace cells that have a high rate of turnover in adult animals.turnover in adult animals.

A special characteristic of cell growth and A special characteristic of cell growth and cell division, is cell cell division, is cell differentiationdifferentiation, which , which means means changeschanges in in physicalphysical and and functional functional propertiesproperties of cells as they proliferate in the of cells as they proliferate in the embryo to form the different bodily embryo to form the different bodily structures. The earliest and simplest structures. The earliest and simplest theory for explaining differentiation was that theory for explaining differentiation was that the genetic composition of the nucleus the genetic composition of the nucleus undergoes changes during successive undergoes changes during successive generations of cells in such a way that one generations of cells in such a way that one daughter cell inherits a different set of daughter cell inherits a different set of genes from that of the other daughter cell.genes from that of the other daughter cell.

Therefore, the present idea is that instead Therefore, the present idea is that instead of loss of genes during the process of of loss of genes during the process of differentiation, there occurs differentiation, there occurs selectiveselective repressionrepression of different of different genetic operons.genetic operons. This presumably results from the buildup This presumably results from the buildup of different repressor substances in the of different repressor substances in the cytoplasm, the repressor substances in cytoplasm, the repressor substances in another cell acting on a different group of another cell acting on a different group of genetic characteristics.genetic characteristics.

Cell Proliferation in AdultsCell Proliferation in AdultsThe cells of adult animals can be grouped The cells of adult animals can be grouped into three general categories with respect into three general categories with respect to cell proliferationto cell proliferation::

((11- )- ) A few types of differentiated cells, A few types of differentiated cells, including lens cells, nerve cells and including lens cells, nerve cells and cardiac muscle cells in humans, if they are cardiac muscle cells in humans, if they are

lost they can never be replacedlost they can never be replaced . .

((22- )- ) In contrast, most cells in adult In contrast, most cells in adult animals enter the G0 stage of the cell animals enter the G0 stage of the cell cycle but resume proliferation as needed cycle but resume proliferation as needed to replace cells that have been injured or to replace cells that have been injured or have died. Cells of this type include skin have died. Cells of this type include skin fibroblasts, smooth muscle cells, the fibroblasts, smooth muscle cells, the endothelial cells that line blood vessels, endothelial cells that line blood vessels, and the epithelial cells of most internal and the epithelial cells of most internal organs such as the liver, pancreas, kidney, organs such as the liver, pancreas, kidney,

lunglung..

((33- )- ) Other types of differentiated cells, Other types of differentiated cells, including blood cells, epithelial cells of the including blood cells, epithelial cells of the skin, and the epithelial cells lining the skin, and the epithelial cells lining the digestive tract, that have short spans and digestive tract, that have short spans and must be replaced by continual cell must be replaced by continual cell proliferation in adult animals. In these proliferation in adult animals. In these cases, the fully differentiated cells do not cases, the fully differentiated cells do not themselves proliferate. Instead, they are themselves proliferate. Instead, they are replaced via the proliferation of cells that replaced via the proliferation of cells that

are less differentiated, called are less differentiated, called stem cellsstem cells..

Differentiation dramatically changes a Differentiation dramatically changes a cell's size, shapecell's size, shape, , membrane potentialmembrane potential, , metabolic activitymetabolic activity, , and responsiveness to, and responsiveness to, signalssignals. . These changes are largely due to These changes are largely due to highlyhighly--controlled modifications incontrolled modifications in gene gene expressionexpression. . A cell that is able to A cell that is able to differentiate into many cell types is known differentiate into many cell types is known asas pluripotentpluripotent. . Such cells are calledSuch cells are called stem stem cellscells in animalsin animals. . A cell that is able to A cell that is able to differentiate into alldifferentiate into all cell typescell types is known asis known as totipotenttotipotent. .

The central problem is the study of development is the The central problem is the study of development is the question of how a single cell, the question of how a single cell, the fertilized eggfertilized egg give give rise to the many cell types of the mature organismrise to the many cell types of the mature organism..

Fertilized egg 2-cell stageFertilized egg 2-cell stage

44--cell stage )blastomere( 8-cell stagecell stage )blastomere( 8-cell stagefour are pigmented smaller, located at the animal four are pigmented smaller, located at the animal pole ) pole ) micromeres(micromeres(..

four are unpigmented larger, located at the vegetal four are unpigmented larger, located at the vegetal pole )pole ) macromeres macromeres(( . .

88--cell stage morula stagecell stage morula stage

blastula stage gastrula stageblastula stage gastrula stage The cells then rearranged to form three germ The cells then rearranged to form three germ layers, which give rise to different tissueslayers, which give rise to different tissues..

11-- The outer germ layer The outer germ layer ))ectodermectoderm(( give rise to give rise to skin and the nervous systemskin and the nervous system..

22-- The inner germ layer The inner germ layer ))endodermendoderm(( give rise give rise to the cells lining the digestive tract and to to the cells lining the digestive tract and to internal organs such as liver and pancreasinternal organs such as liver and pancreas..

33 - -The middle germ layer The middle germ layer ))mesodermmesoderm(( give give rise to connective tissues muscle and rise to connective tissues muscle and blood cellsblood cells..

The process of functional and structural The process of functional and structural specialization of these cells is called specialization of these cells is called differentiationdifferentiation. For example muscle cell . For example muscle cell precursors elongate into spindle- shaped precursors elongate into spindle- shaped cells that synthesize and accumulate cells that synthesize and accumulate myofibrillar proteins. The resulting cell myofibrillar proteins. The resulting cell efficiently converts chemical energy into efficiently converts chemical energy into contractile forcecontractile force..

http://en.wikipedia.org/wiki/Image:Cell_differentiation.gif

Morphological modification during Morphological modification during differentiation are accompanied by chemical differentiation are accompanied by chemical changes. In the example given, formation of changes. In the example given, formation of the muscle cell results from the synthesis of the muscle cell results from the synthesis of several specific proteins such as several specific proteins such as actinactin & & myosinmyosin..

Cells are not always restricted to a single Cells are not always restricted to a single activity and frequently perform two or more activity and frequently perform two or more specialized function, for example. Intestinal specialized function, for example. Intestinal epithelial cells both absorb nutrient & epithelial cells both absorb nutrient & synthesize digestive enzymes such as synthesize digestive enzymes such as disaccharides and peptidasesdisaccharides 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.

Specialized cell FunctionMuscle cellsMovementNerve cellsConductivityPancreatic acinar cellsSynthesis and secretion of enzymesMucous gland cellsSynthesis and secretion of mucous

substancesSome adrenal g land,testes & ovarycells

Synthesis and secretion of steroids

Cells of the kidney and salivarygland ducts

Ion transport

Macrophages and some white b loodcells

Intercellu lar digestion

Sensory cellsTransformat ion of physical andchemical stimuli into nervousimpulses

Cellular functions in some specialized cells

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.

The outer epithelia and, as well as seen, the central nervous system develop from ectoderm.

The epithelial linings of respiratory passages and gut and the glandular cells of appended organs such as liver & pancreas are of endodermal origin .

The smooth muscular coats, connective tissues and vessels supplying these organs,skeleton, skeletal muscle,and organs of excretion and reproduction are of mesodermal origin.

An important exception is in the case of the gonads) testis &ovary( in which the precursor cells of sperm and ova, the so-called primordial germ cells, are now believed to be of yolk sac and possibly endodemal derivation.

It is important, however, to recognize that a cell may have several functions and be a member of more than one cell group fore 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 secretory glands.

(3 -)Myofibroblasts, have a contractile role in addition to being able to secrete collagen.

(4 -)Pericystes are smooth, muscle-like cells that surrounded blood vessels.