Cytology and Cell Biology

Introduction:-
Histology:- is the study of the tissues of the body and how these tissues are arranged to
constitute organs, by the use of microscopes s(light and electron) and stained sections of
tissues.
The main objective in studying histology is to identify the specific features of tissues
and function of organs.
The human body is composed of systems, which are made of organs. Organs are made of
tissues which are composed of cells.
The cell size is measured by special units and examined by microscopes.
Methods of study in histology:-

Methods of study in
histology
The human or animal tissue can be studied as: smears, spreads, sections or teased
material. Several methods are applied for this purpose, such as:
1. Microtechniques: using fixed, stained material, examined by light
microscope.
2. Electron microscopy: a fixed material is studied and high magnification is obtained.
3. Autoradiography: A method used to study the tissue synthetic activities by the use of
radioactive isotopes by the use of light or electron microscopy.
4. Cell and tissue culture: to study the living cells and tissues in vitro (outside the
body) and the effect of single molecule or substance on one type of them.
5. Cell fractionation:
By this method, the cell components and organelles are separated according to their
sedimentation coefficient. This method is used to study the chemical composition and
function of the separated components in vitro.
6. Histochemical and cytochemical techniques:
These methods are used to localize substances in tissue sections based on the specific
chemical reactions between macromolecules and production of insoluble colored substances
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 that could be seen by the microscope (LM or EM). Examples of the substances that can be
studied, ions, nucleic acids, and proteins.

The Cell:-

THE CELL
The cell is the smallest structural and functional unit of all living organisms.
Cells serving the same functions are grouped together and united by intercellular
substance to form tissue.
There are two types of cells: prokaryotes and eukaryotes.
The predominantly single-celled organisms of the domains bacteria and Archaea are
classified as prokaryotes (pro- = before; -karyon- = nucleus). Animal cells, plant cells, fungi,
and protists are eukaryotes (eu- = true).
Compared to prokaryotic cells, eukaryotic cells are larger and their nuclei are distinct,
surrounded by nuclear envelope to protect the genetic material, chromatin.

All cells share four common components:

1. A plasma membrane, an outer covering that separates the cell’s interior from its surrounding
environment.
2. Cytoplasm, consisting of a jelly-like region within the cell in which other cellular
components are found.
3. DNA, the genetic material of the cell.
4. Ribosomes, particles that synthesize proteins.
However, prokaryotes differ from eukaryotic cells in several ways.

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This figure shows the generalized structure of a prokaryotic cell

This figure shows a typical animal cell

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.This figure shows a typical plant cell

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 Cell Size:-

At 0.1–5.0 µm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells,
which have diameters ranging from 10–100 µm.

This figure shows the relative sizes of different kinds of cells and cellular components. An
adult human is shown for comparison

 Shapes of cell:- Cells can also differ in shape

1. Squamous: flat, thin, scale-like cells.

2. Spheroid:round to oval cells.

3. Polygonal:irregular angular shapes, with more than 4 sides.

4. Discoid:disc shaped.

5. Cuboidal:squarish.

6. Fusiform (spindle shaped): thick middle with tapered ends.

7. Columnar: taller than wide.

8. Fibrous:long, slender.

9. Stellate: star shape.

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 Colors of cell:- Cells can also differ in color:

o Most cells are colorless

o Some cells are pigmented:

• Red blood cells, muscle fibers

• Melanocytes, retinal pigment, epithelium cells.

 Components of cell:- Organelles are tiny structures within cells that perform specialized
tasks and are often surrounded by their own membrane. These organelles perform the
functions necessary to keep the cell alive.

1. Cytoplasm:

▪ Viscous fluid containing organelles

▪ Components of cytoplasm

– Interconnected filaments & fibers

– Fluid = cytosol

– Organelles (not nucleus)

– Storage substances.
2. Cell membrane:

▪ Outer membrane of cell that controls movement in and out of the cell.

▪ Double layer of phospholipids with embedded proteins.

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3. Cell wall:

▪ Most commonly found in plant cells & bacteria

▪ Supports & protects cells

▪ Plants – mostly cellulose

▪ Fungi – contain chitin.

4. Nucleus:

▪ Directs cell activities

▪ Separated from cytoplasm by nuclear membrane

▪ Contains genetic material – DNA.

5. Nuclear membrane:

▪ Surrounds nucleus

▪ Made of two layers

▪ Openings allow material to enter and leave nucleus.

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6. Chromosomes:

▪ In nucleus

▪ Made of DNA

▪ Contain instructions for traits & characteristics.

7. Nucleolus:

▪ Inside nucleus

▪ Most cells have 2 or more

▪ Directs synthesis of RNA

▪ Forms ribosomes.

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8. Ribosomes:

▪ Each cell contains thousands

▪ Small dark granules of protein and RNA free in cytosol or on surface of rough ER

▪ Interpret the genetic code and synthesize polypeptides (Make proteins).

9. Endoplasmic reticulum:

▪ Helps move substances within cells

▪ Network of interconnected membranes

▪ Two types:

– Rough endoplasmic reticulum

– Smooth endoplasmic reticulum.

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10.Golgi bodies:

▪ Synthesizes CHO’s, processes proteins from RER and packages them into golgi vesicles

▪ Golgi vesicles

– irregular sacs near golgi complex that bud off cisternae

– some become lysosomes, some fuse with plasma membrane and some become secretory
vesicles.

▪ Secretory vesicles → Store a cell product for later release.

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11.Vacuoles:

▪ Membrane bound storage sacs

▪ More common in plants than animals

▪ Contents:

– Water

– Food

– Wastes

12.Vesicles:

▪ Storage organs found in animal cells.

▪ These tend to be much smaller then plant vacuoles.

13.Mitochondria: (singular = mitochondrion) are often called the “powerhouses” or

“energy factories” of a cell because they are responsible for making adenosine triphosphate
(ATP).

▪ Produces energy through chemical reactions – breaking down fats & carbohydrates

▪ Controls level of water and other materials in cell

▪ Recycles and decomposes proteins, fats, and carbohydrates.

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14.Lysosomes:

▪ Package of enzymes in a single unit membrane, variable in shape

▪ Functions:

– intracellular digestion - hydrolyze proteins, nucleic acids, complex

carbohydrates, phospholipids and other substrates

– autophagy - the digestion of worn out organelles and mitochondrion

– autolysis - programmed cell death

– glucose mobilization - lysosomes in liver cells break down glycogen.

15.Chloroplasts:

▪ Usually found in plant cells.

▪ Contains green chlorophyll.

▪ Where photosynthesis takes place.

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 Extracellular Structures:-
 Extracellular matrix:

Human cells, like other animal cells, do not have a rigid cell wall. Human cells do have an
important and variable structure outside of their cell membrane called the extracellular
matrix. Sometimes this matrix can be extensive and solid (examples = calcified bone
matrix, cartilage matrix), while other times it consists of a layer of extracellular proteins and
carbohydrates. This matrix is responsible for cells binding to each other and is incredibly
important in how cells physically and physiologically interact with each other.

 Flagella:

Many prokaryotes have flagella, allowing, for example, an E. coli bacteria to propel its way
up the urethra to cause a UTI (Urinary Tract Infection). Human cells, however (and in fact
most eukaryotic cells) lack flagella. This makes sense since humans are multicellular, and
individual cells do not need to swim around. The obvious exception to this is with sperm,
and indeed each sperm is propelled by a single flagellum. The flagellum of sperm is
composed of microtubules.

 Cilia:

Cilia are especially notable on the single-celled protozoans, where they beat in synchrony to
move the cells nimbly through the water. They are composed of extensions of the cell
membrane that contain microtubules. When present in humans they are typically found in
large numbers on a single surface of the cells, where rather than moving cells, they move
materials.

 Intercellular Junctions:
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 Cell junction is the connection between the neighboring cells or the contact between the cell
and extracellular matrix. It is also called membrane junction.

Types of intercellular junctions:-

1. Tight junction: This is also called (Occluding junctions),these

junctions prevent the movement of ions and molecules from one cell to another cell.

It is also called (zonula occludens) . In this type of intercellular junction, the outer

layer of the cell membrane of the neigh bouring cells fuse with each other. This type

of junction is present in the apical margins of epithelial and endothelial cells in

intestinal mucosa, wall of renal tubule, capillary wall and choroid plexus.

2. Anchoring junctions:

Anchoring junctions are the junctions, which provide strength to the cells by acting

like mechanical attachments, i.e. these junctions provide firm structural attachment

between two cells or between a cell and the extracellular matrix.

Anchoring junctions are responsible for the structural integrity of the tissues, and

are present in the intercalated disks between the branches of cardiac muscles and

epidermis of skin.

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The firm attachment between two cells or between a cell and the extracellular matrix

is provided by either actin filaments or the intermediate filaments. Depending upon

this, anchoring junctions are classified into four types:

1. Actin filament attachment

i. Adherens junction (cell to cell).

ii. Focal adhesion (cell to matrix).

2. Intermediate filament attachment

i. Desmosome (cell to cell).

ii. Hemidesmosome (cell to matrix).

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3.Communicating Junctions:

Cell junctions which permit the movement of ions and molecules from one cell to

another cell. Gap junction and chemical synapse are the communicating junctions.

 Gap Junction:

Gap junction is the intercellular junction that allows passage of ions and smaller

molecules between the cells without movement into interstitial fluid. It is also

called nexus. It is present in heart, basal part of epithelial cells of intestinal mucosa,

etc.

 Chemical Synapse:

Chemical synapse is the junction between a nerve fiber and a muscle fiber or between

two nerve fibers, through which the signals are transmitted by the release of chemical

transmitter

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 Diffusion, Facilitated Diffusion, Osmosis, and Active Transport

Cell membrane shows selective permeability and forms a barrier between internal and
external cell environments.

➔ we need carriers to enter the cell

• Permeable to oxygen, carbon dioxide, water, steroids, and lipid-soluble chemicals

• Larger molecules enter cell by specialized transport mechanisms.

How molecules move through the membrane?

Diffusion is the movement of molecules from an area where the molecule is in

high concentration to an area where the molecule is in lower concentration.

Facilitated diffusion is the movement of a molecule from an area of high concentration to

an area of lower concentration with the help of a protein channel or carrier. In the facilitated
diffusion both amino acids and glucose are shown entering the cell facilitated by a protein
carrier. In a cell membrane there would be proteins specific to each molecule and the carriers
would not be shared in this way.

Osmosis is the diffusion of water through a semi-permeable membrane. Water

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moves from an area of high water molecule concentration (and lower solute concentration)
to an area of lower water molecule concentration (and higher solute concentration). The
osmosis shows water moving into a cell through a channel. Water molecules can be
transported in this way, but can also diffuse directly through the membrane lipid bilayer.

Active transport is the movement of molecules from areas of low concentration to

areas where the molecule is found in higher concentration. This movement is not
spontaneous and requires ATP energy and a protein carrier.

The ATP is used to drive conformational changes in the protein to pump molecules
against their concentration gradient. This process occurs continuously in nerve cell
membranes with sodium-potassium pumps.

Comparison of the ways molecules move into and out of cells.

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 Cell physiology: Endocytosis & Exocytosis

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Endocytosis: The proccess whereby a cell ingests macromolecules, particulate matter, and
other substances from the extracellular space is referred to as endocytosis.

Is a type of active transport that moves particles, such as large molecules, parts of cells, and
even whole cells, into a cell.
Types of endocytosis:
Phagocytosis ("cellular eating") Ingestion of large particles, such as bacteria
Pinocytosis ("cellular drinking") ingestion of extracellular fluid into the cell

Exocytosis: is release of material from the cell.

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