Cell structure and functions

CELL STRUCTURE AND FUNCTIONS.
DR. ABHIMANYU CHATURVEDI
M.D.S
1ST YEAR
I.T.S CENTRE FOR DENTAL
STUDIES AND RESEARCH
DEPARTMENT OF CONSERVATIVE
DENTISTRY
&
ENDODONTICS
SEMINAR NUMBER 1

Contents:
Introduction
History
Types of cells
Anatomy of cells:- Prokaryotic
Eukaryotic
Difference between cells of Plants and Animals.
Components of a cell (Structure & Function) :-
Plasma Membrane
Cytosol
Cell organelles
Cell inclusions
Cell Division:- Mitosis
(Somatic cell division)
Reproductive Cell Division
Fertilization
Meiosis
Transcription
Translation
Specialized Cells of Human Body
Cells and Ageing
Cells of dental interest
Cell Death
Conclusion
References

CELL STRUCTURE & FUNCTION
I. CELL : THE BASIC UNIT OF LIFE:-
• Cell is the basic unit of life, & the structural & functional unit of an organism.
• It is the smallest unit capable of independent existence & performing the essential functions of life.
• All life begins as a single cell. Many organisms are made of a single cell e.g. Amoeba, Chlamydomonas,
Acetabularia, Bacteria, yeast, etc.
• A multi-cellular organism is made of many cells. Here, cells are the building blocks of the body or basic units
of body structure.

Robert Hooke -An English Mathematician & Physicist.
Robert Hooke, in 1665 took a piece of cork of Spanish Oak and prepared thin slices
which was then observed under a microscope.
Honey – Comb structure was observed with a number of box-live compartments,
each having a pore & separated from others by diaphragms.
He called these compartments Cellulae (singular- cellula), now known as Cells
(Latin – cella, meaning hollow space or compartments).
This is mentioned in his book Micrographia, Chapter: ‘Observe XVII
HISTORY : DISCOVERY OF CELL:-

Antonie Philips van Leeuwenhoek:-
 He was a Dutch tradesman & scientist.
 Father of Microbiology.
 He was the first so observe & describe & sketch single celled organism which he
called animalcules, which are now referred to as ‘micro organisms’.
 He observed bacteria (1976), protozoa, spermatozoa (1676), Red blood cells,
banded pattern of muscle fibres (1682), the infusoria (protists) (1674).

Robert Brown (1831):-
 A Scottish Botanist & Paleobotonist.
Discovered preserve of nucleus in cells of orchid root
Felix Dujardin (1835):-
 A French Biologist.
 Discovered living semi-fluid substance of cells & called it
Sarcode
.
Matthias Jakob Schleiden (1838):-
 A German botanist.
 Found all plant cells to have a similar structure – Cell wall, a
clear jelly-like substance & a nucleus.

Theodor Schwann (1838):-
 A German physiologist.
 Discovered that animal cells lacked cell-wall.
 He contributed to the cell theory:-
 All living things or organisms are made of cells & their
products.
 Defined cell as membrane enlocked, nucleus containing
structure.
Purkinje (1839) & Von Mohl (1838, 1846):-
 Renamed Sarcode, or jelly-like substance of cells as
PROTOPLASM (Greek:- protos – first, plasma - form)

• Cell membrane was discovered by
SCHWANN but was provided with a
name by NAGELI & CRAMER (1855).
• Rudolph Virchow (1855) , a
German doctor & biologist was
the first so accept the work of
Robert Remak, who showed the
origins of cells was the division
of pre-existing cells.

• On the basis of organisation of DNA, cells are of 2
types :-
• Prokaryotic
• Eukaryotic
Prokaryotic cells
Characteristics:-
• Nuclear material:- DNA is naked & lies variously
coiled in the cytoplasm, a.k.a GENOPHORE,
NUCLEAR BODY OR NUCLEOID, or prochromosome.
• Additional small circular DNA entities called
plasmids maybe present.
• Nuclear components absent.
• Cell wall presents in bacteria & cyanobacteria but
absent in mycoplasma or PPLOs (pleuro-
pneumonia like organisms).

1) Single stranded flagella/fimbriae may be present.
2) The photosynthetic thylakoids are not organised into chloroplasts & lie freely in cytoplasm.
3) Membrane lined cell organelles like mitochondria, E.R, Golgi apparatus, lysosomes, microtubules,
microfilaments and centrioles are absent.
4) Complex gas vacuoles present.
5) Ribosomes are 70s.
6) One-Envelope system
7) Cyclosis absent
8) Mitotic spindles not formed during cell division.
9) Sexual reproduction absent.
10) Low DNA content
11) Transcription and Translation occur in cytoplasm.
12) Respiratory enzymes usually lie in close contact with cell membrane.
13) Endocytosis & Exocytosis absent.
14) Nitrogen fixation occurs only in some prokaryotes.

Eukaryotic Cells:-
1) Has organised nucleus.
2) Membrane covered cell organelles present.
3) Cell wall present in case of plants, fungi & protists (without
muramic acid.)
4) Flagella if present are 11 stranded with differentiation of
axoneme & sheath.
5) The nucleus is differentiated into nuclear envelope,
chromatin, one or more nucleoli & nucleoplasm .
6) Nuclear DNA is linear, associated with histones, extra
nuclear DNA is commonly circular.
7) Most of the cell DNA lies within the nucleus, a small
quantity is found in the plastids & mitochondria.
8) Transcription occurs in nucleas whereas translation occurs
in cytoplasm.
9) Respiratory enzymes present in cytoplasm & mitochondria.

10.Endocytosis & Exocytosis quite
common.
11.Cyclosis present.
12.Ribosomes are of 80s, 70s ribosomes
occur in plastids and mitochondria.
13.True or Sap vacuoles found.
14.Thylakoids, if present, grouped inside
chloroplasts.
15.Sexual reproduction present.
16.Spindle apparatus produced during
nuclear division.
17.Nucleus contains more than one
chromosome.

Right: Colorized micrograph of a prokaryotic cell of the
bacterium.
Left: Colorized micrograph of a eukaryotic cell of the
green algae

PLANT CELL ANIMAL CELL
1. Cell wall present.
2. Has a definite form.
3. Larger in size.
4. Cannot change shape.
5. Cannot change position.
6. Plastids present.
7. Chloroplasts(chlorophyll containing
plastids) present.
8. A large center vacuole present
9. Nucleus lies on one side in
peripheral cytoplasm.
10.Nucleus is elliptical.
11.Fewer mitochondria.
12.Plants do not bust if placed in
Hypotonic solution due to cell wall.
1. Cell wall absent
2. Definite form less common
3. Smaller in size
4. Can often change shape.
5. Many cells can change position
6. Plastids absent.
7. Absent.
8. Many small vacuoles present.
9. Nucleus usually in centre.
10.Nucleus rounded.
11.Numerous mitochondria.
12.Unless contractile vacuole present,
animal cells will burst.

13. Centrioles absent.
14. Spindle apparatus nuclear division is anastral.
15. Golgi apparatus consists of a number of distinct or
unconnected units called DICTYOSOMES.
16. Cell cannot do phagocytosis.
17. Lysosomes are rare.
18. Glyoxysomes maybe present
19. A plant cell produces all the materials needed by it.
20. Crystals of inorganic substances occur inside cells.
21. Reserve food is generally starch & fat.
22. A tissue fluid does not bathe cells.
23. Adjacent cells may be connected through
plasmodesmata.
24. Cytokinesis occurs by cell plate.
13. Centrioles present.
14. Spindle is amphiastral.
15. Golgi apparatus is either localised or consists of a well
connected single complex.
16. It can ingest materials through phagocytosis.
17. Lysosomes present.
18. Absent.
19. An animal cell cannot synthesize certain amino acids,
fatty acids, vitamins & coenzymes.
20. Usually do not occur.
21. Usually glycogen & fat.
22. Tissue fluid having Nacl bathes cells.
23. Cells connected through a number of cell junctions.
24. Cytokinesis takes place by cleavage.

COMPONENTS OF A CELL:-
1.Plasma Membrane
2.Cytosol
3.Organelles
4.Nucleus
5.Cell Infusions

1)Plasma Membrane:-
i. Also called cell membrane / plasma lemma / bio membrane.
ii. Quasifluid (acts partially like fluid and partially like solid),
elastic, pliable & film-like thin partitions over and inside
cytoplasm.
iii. Average is 75 A (50-100A).
iv. Selectively permeable for solutes but semi permeable for water.
v. Dynamic in nature. Any injured part of Membrane is repaired
within no time.
vi. Appear trilaminar or tripartite under electron microscope.
Composition:-
a) Lipids (20-79%)
b) Proteins (20-70%)
c) Carbohydrates (1-5%)
d) Water (20%)

a. Lipids:-
• Phosphoglycerides or phospholipids.
• Lipid molecules are amphiatic or
amphipathic i.e, they posses both polar
hydrophilic & non polar hydrophobic
ends.
• Hydrophilic region is the head &
hydrophobic part contains 2 tails of fatty
acids.
• Hydrophobic tail usually occur towards the
centre of members resulting in formation
of a lipid bilayer .

b.Proteins :-
• They can be fibrous or globular, structural,
carrier, receptor or enzymatic.
• These also posses both polar & non polar side
chains.
• Polar hydrophilic linkages are towards outer
side.
• Non polar hydrophobic linkages are either
kept folded inside or used to established
connections with hydrophobic part of lipids.

c) Carbohydrates:-
• These are branched or unbranched
oligosaccharides
e.g.- hexose, fucose,
hexoamine, sialic acid etc.

FLUID – MOSAIC MODEL:-
1. The plasma membrane has 2 layers (a bilayer) of phospholipids (fats with phosphorus
attached) which at body temperature is like vegetable oil.
2. Membrane is not solid but Quasifluid.
3. Protein molecules occur at places both inside ( intrinsic, integral proteins) and on outer
side of lipid bilayer (extrinsic, peripheral protein )
4. Protein icebergs in a sea of lipids.
5. Integral proteins pass into lipid bilayer & establish hydrophobic bonds with lipid
molecules. Some run through out the lipid bilayer called tunnel protein or
transmembrane proteins.
6. These tunnels form channels for passage of water & water soluble substances, which
posses selective properties for passage of different ions .
7. Proteins are held by both polar & non-polar side chains.

8. The extrinsic proteins are located more so on cytosolic face than
on external face ( e.g. spectrin)
9. Extrinsic proteins are attached covalently to phospholipid head
or non-covalently to transmembrane protein.
10.Proteins provide structural & functional specificity to
membranes
11.Proteins may shift laterally hence providing flexibility &
dynamism to membrane.
12.Proteins may function as enzymes, permeases, carriers.
13.Some lipids & extrinsic proteins present on outer side possess
small carbohydrate molecule to form glycolipids &
glycoproteins. They constitute glycocalyx or cell coat.
14.Conjugated oligosaccharides function as :-
a) Recognition centres.
b) Sites of attachments.
c) Antigens.
d) Provide negative charge to outer surface.

FUNCTIONS OF CELL MEMBRANE:-
1. Compartmentalisation.
2. Protection from injury
3. Providing organic connections between adjacent cells
(plasmodesmata & gap junctions)
4. Providing selective permeability barrier.
5. Transporting solutes.
6. Responding to external stimuli.
7. Energy transduction.
8. Secretory, excretory & waste products thrown out by
exocytosis.

Membrane transport:-
Occurs by:-
a. Passive transport
b. active transport
c. bulk transport
Passive transport :-
• No energy needed to move particles.
• Transport according to concentration gradient.

1. PASSIVE DIFFUSION OR TRANSPORT ACROSS CELL MEMBRANE:-
• Cell membrane plays a passive role.
a) NEUTRAL SOLUTES AND LIPID SOLUBLE SUBSTANCES:-
• Movement through simple diffusion along
concentration gradient, from higher concentration to
lower concentration.

b) Open channel transport :-
 Channels in form of tunnel proteins.
 Water channel or aquaporins allow water & water
soluble gases (CO2 & O2 ) to pass e.g.:- osmosis.
 Filtration is diffusion under pressure through a
membrane.

c. Facilitated diffusion:-
• Occurs through agency of gated ion channels &
permeases.
• Energy not required.
• Transport along concentration gradient.

i) Ion channels :-
 Highly specific.
 A specific channel for each ion.
 Do not move in dissolved state.
 Most ion channels are gated.
Depending upon stimulus required for opening the ion
channels gated, they are of 3 types:-
 Voltage gated.
 Mechanical gated.
 Ligand gated.
Movement is according to concentration gradient.
Rate of passage is high.

ii. PERMEASES:-
 Function as facilitated pathway for movement of
substances.
 Rate of transport in stereospecific.
 Saturation effect recorded.
iii. CONTRASPORT:-
 Often occurs against concentration gradient.
 It’s a membrane transport that accompanies active
transport of some substances. Eg:- glucose (with Na+)

2.ACTIVE TRANSPORT:-
 Uphill movement of materials across membrane
where solute particles more against chemical
concentration electro-chemical gradient.
 Energy in form of ATP required.
 Sodium, potassium, hydrogen, calcium, iodine,
chlorine etc. are some of ions transported.
 Carrier proteins-embedded proteins change shape to
open & close passage across membrane.

3) BULK TRANSPORT:-
a) ENDOCYTOSIS:-
i. Pinocytosis.
ii. Phagocytosis.
b) EXOCYTTOSIS:
PINOCYTOSIS or POTOCYTOSIS:
• Minute detachable vesicle formed of fluid matter and substances
dissolves ion it of diameter 100-200 nm.
• Also called cell drinking.
• As soon as solute or ligand particle form complexes with receptor
sites, plasma membrane invaginates.
• Invagination deepens and gets pinched off as a vesicle called
pinosome.
• Pinosome migrates inwards and if digestion is involved then
lysosomes are required.

PHAGOCYTOSIS:-
 Also called cell eating.
 Detachable vesicles formed
around solid matter, called
phagosomes, by
invagination of plasma
membrane.
 Phagosome diameter is 1-2 um.
 Fusion with lysosomes to
produce digestive vacuole.
 Digested food diffuses in
cytoplasm.

B. EXOCYTOSIS:-
 Exportation of substances from
cell via plasma membrane to
extra cellular fluids.
 Important mainly in 2 cells:-
(a)Nerve cells for neurotransmitter
release.
(b)Secretory cells- e.g. – insulin

3) CYTOPLASM:-
(A) Cytoplasmic matrix or Cytosol (Hyaloplasm).
• Clear fluid part of cytoplasm which can exist in either
sol and/or gel. (plasmasol, Plasmagel).
Functions:-
1. Providing raw materials to cell organelles for
functioning.
2. Exchange of materials between cell organelles.
3. Biosynthesis of fats, nucleotides, some carbohydrates,
proteins, coenzymes etc.
4. Catabolic activities:- glycolysis, anaerobic respiration,
& pentose pathway type of respiration occurs in
cytosol.
5. Distribution of various materials inside cell via
cytoplasmic streaming.

4) Cell organelles:-
Endomembrane System:-
 Endoplasmic reticulum.
 Golgi complex
 Lysosomes
 Vacuoles.

ENDOPLASMIC RETICULUM:-
 Discovered by Porter & Thompson (1945).
 It’s a 3-D, complicated & inter connected
system of membrane–lined channels running
through cytoplasm, called Cisterns.(Flat
interconnected sac-like).
Types:-
 Smooth E.R.:- Ribosomes absent.
A.K.A. Agranular E.R.
 Rough E.R. :- Ribosomes present.
A.K.A. granular E.R.

Structure:-
1. Cisternae :- Flat interconnected sac-
like.
2. Vesicles :- oval, rounded sacs,
A.K.A. MICROSOMES
3.Tubules:- Tube-like extension
connected to either cisternae or vesicles.

Functions:-
 Provides large surface area for various physiological activities.
 Provides mechanical support to colloidal cytoplasmic matrix.
 Provides quick intracellular transport.
 Provides precursors of different secretory substances to Golgi apparatus.
 Gives rise to vacuoles.
 Rough E.R stores newly formed / synthesized molecules & forms glycoprotein when sugar group is added.
 Smooth E.R is site of synthesis of fatty acids, phospholipids and steroids.
 Smooth E.R as SARCOPLASMIC RETICULUM stores Ca++ for release during muscle contraction.
 Detoxification of various chemicals.

GOLGI COMPLEX (APPARATUS):-
 Made up of smooth membrane saccules or cisternae, a
network of tubules with vesicles and vacuoles.
 First seen by George in 1867 but named after Camillo
Golgi who in 1898 recognized its reticular structure.
Functions:-
Secretion - All glandular cells depend upon Golgi
complex for concentrating & packaging their products
inside a soluble protein coat.
 Formation of glycoproteins & glycolipids.
 Formation of sialic acid and galactose.
 Fat transport.
 Mediation of hormones produced by endocrine glands.
 Formations of lysosomes & secretory vesicles.

LYSOSOMES:-
 Small vesicles bounded by a single
membrane containing hydrolytic enzymes
in form of minute crystalline or semi
crystalline granules of 5-8 nm.
 Are called suicide bags.
 Work in acidic pH (pH=5)
 Formed by Golgi complex.

Functions :-
 Intracellular Digestion :- food obtained via
phagocytosis is digested by Lysosomes.
 Extracellular digestion by release of enzymes via
exocytosis.
 Defence- lysosomes of leucocytes devour foreign
proteins, bacteria & toxic substances.
Autophagy:- In metamorphosis of many animals, certain
embryonic parts are digested for growth of other parts.
Autolysis:-
 Formation of Thyroxine- active hormone thyroxin in
thyroid is formed through hydrolysis of thyroglobulin
by lysosomes.

VACUOLES:-
 Are non-cytoplasmic areas present inside
cytoplasm separated by specific membrane.
 Sap vacuoles– Fluid filled vesicles separated
by tonoplasts.
 Contractive vacuoles– occur in algal cells &
protistans.
 Food vacuoles – Formed by fusion of
phagosomes & a lysosome.
 Air vacuoles–
. Pseudo vacuole.
. Gas vacuoles.

MITOCHONDRIA:-
• Power house of cell.
• Contains 2 membranes and 2 chambers, outer &
inner.
• Contains enzymes that catalyse chemical
reactions of cells that generate ATP.
Function:-
• Are miniature biochemical factories where
respiratory substrate and food are completely
oxidized to CO2 & water. Energy liberated is initially
stored in form of reduced coenzymes & prosthetic
groups.
• Inner chamber has enzymes for synthesise of
fatty aids.
• Synthesise of many amino acids.
• May store & release calcium when required.

RIBOSOMES:-
(Palade Particle)
• Are naked ribonucleo proteins protoplasmic
particles (RNP) to length of 200-340 A & diameter of
170-240 A.
• Function As sites of protein or polypeptide
synthesis.
• Are protein factories.
• Sub spherical in outline.
• Membrane absent.
• Found freely in cytosol and attached to E.R via
SRP protein.
Function:-
1. Protein synthesis.
2. Free Ribosome synthesizes protein for use inside
cell.
3. Attached Ribosome synthesize protein for
transport.

NUCLEUS:-
 A specialized double membrane bound
protoplasmic body containing all the genetic
information for controlling cellular
metabolism & transmission to the posterity.
 Its the largest cell organelle.
 Found in the region of maximum metabolic
activity. Commonly situated in the geometric
centre of cell .
 Spherical or oval in shape.

a. Nuclear Envelope:-
• separates nucleus from cytoplasm.
• Made up of lipoprotein & trilaminar
membrane.
• Has pores or perforations which control
passage of substances to inside or outside of
nucleus.
b.Nucleoplasm :-
• its transparent semifluid & colloidal
substances filling the nucleus.
• Contains nucleosides & enzymes.
c. Nuclear matrix:-
• A network of fine fibrils of acid proteins
functioning as scaffold for chromatin.

d. Chromatin :-
• Hereditary DNA - protein fibrillar
complex.
e. Nucleolus :-
• Naked, round or slightly irregular
structure attached to chromatin at the
NOR (nuclear organiser region).
• Has 4 components :-
I. Amorphous matrix.
II. Granular portion (proteins & RNA,2:1)
III. Fibrillar portion (nucleonema)
IV. Chromatin portion.
• Principal site for development of
ribosomal RNA.
• Stores nucleoproteins.

I. Chromatin:-
• Hereditary material.
• Bear genes.
• Contains genetic information required for
growth & development.
II. Nucleus controls cell metabolism.
III. Formation of ribosomes in nucleolus.
IV.Directs cell differentiation.
V. Replication of nucleus essential for cell
replication.
FUNCTIONS:-

CHROMOSOMES:
• Rod shaped or thread-like condensed chromatin
fibres which are hereditary vehicles as they store &
transmit coded hereditary information.
• There are 2 chromosome halves or chromatids
attached to each other by CENTROMERE.
Chromosomes are of 4 types (based on
position of centromere) :-
(a) Telocentric – (centromere terminal at area of
telomere)
(b) Acrocentric – Centromere inner to telomere.
(c) Submetacentric – Centromere submedian
(d) Metacentric – Centromere median.
• Chromosomes contain a coiled filament called
Chromonema.

Functions:-
 Contains genes. All hereditary information
located in genes.
 Control synthesis of structural proteins thus
helping in cell division & cell growth.
 Control cellular differentiation.
 Can replicate themselves or produce their carbon
copies for passage to daughter cells & next
generation.
 Produce nucleoli for synthesis of ribosomes.

 Form a link between offspring &
parents.
 Determine the sex of individual.
 By process of crossing over, they
introduce variations.
 Mutations are produced due to change
in gene Chemistry.

CYTOSKELETON:-
• Extremely minute, fibrous & tubular
structures which form the structural
frame-work inside cell.
• 3 main types of protein filaments:-
i. Microfilament.
ii. Intermediate filaments.
iii. Microtubules.
Function:-
a. Maintain the integrity of cells.
b. Cause Cyclosis.
c. Responsible for change in plasma
membrane during endocytosis &
exocytosis.

FLAGELLA & CILIA:-
 Fine hair like movable protoplasmic processes of cells
capable of producing a current in fluid medium for
locomotion & passage of substances.
 Flagella are longer (100-200 um) but fever.
 Cilia are smaller (5-20 um ) but numerous .
Function:-
1) Help in locomotion in flagellate & ciliated organisms.
2) Create current for obtaining food from aquatic
medium.
3) In land animals, cilia of respiratory tract help in
eliminating dust particles in incoming air .
4) Can function as sensory organs .
5) Internal transport - passage of eggs in oviduct, passage
of excretory substances in kidneys.

CENTROSOMES & CENTRIOLES:-
CENTROSOMES:
 Dense area of cytoplasm with radiating microtubules.
 Contribute in building mitotic spindles.
CENTRIOLES:
 Minute submicroscopic microtubular subcylinders with a configuration of 9 triplet fibrils and ability to form
their own duplicates without having DNA and a membranous covering.

FUNCTIONS:
 Help in cell division by forming microtubules-
organising centres (MTOCs).
 Centrioles can be transformed into basal bodies which
give rise to cilia and flagella.

CELL INCLUSIONS:-
 They are non-living substances present in cells
 Also called Ergastic Bodies.
 Maybe present in soluble or insoluble state, can be organic or
inorganic in nature.
 3 categories:
a) Reserve food.
b)Excretory or Secretory products.
c) Mineral matter.

 Melanin: present in skin, hair, eyes and meninges for
protection against UV rays.
 Glycogen: present in liver, skeletal muscles for quick
energy, break down occurs in liver into glucose.
 Triglycerides: present in fat cells, broken down to utilize
ATP.

CELL DIVISION:-
 It’s a mean of cell multiplication or formation of new cells from pre-existing cells.
 It occurs in 3 ways:-
a) Amitosis
b) Mitosis
c) Meiosis.
 In prokaryotes, the cell division is known as Binary Fission.
 The cell cycle is divided into 2 major phases:-
a) Interphase
b) Mitotic phase.

INTERPHASE:
 Latin – inter – between, Greek – phasis – aspect.
 It’s a series of changes that takes place in a newly
formed cell and its nucleus before it becomes
capable of division again, hence it’s called
intermitosis.
 It has G1, S, and G2 phases.

G1 phase:
 It’s the longest phase (post mitotic gap
phase).
 Cell is metabolically active and grows
continuously.
 RNA and protein are synthesized.
 Nucleotides, amino acids for histone
synthesis and energy rich compound
are formed.
 Cell organelles increase in number.

S-PHASE:-
 Synthetic phase.
 Chromosomes replicate
 For this their DNA molecules function as templates
and form carbon copies.
 DNA content doubles and duplicate sets of genes
are formed.
 New chromatic fibres are formed.
 Also known as INVISIBLE PHASE of M-STAGE
since chromosomes prepare for equitable
distribution later on.
 Subunits of kinetochores are synthesized.
 Centrosomes begin to divide.

G2 PHASE:-
 Pre-mitotic phase.
 Synthesis of DNA stops.
 Formation of RNA and protein
continues
 Preparation of cell to undergo cell
division.

MITOSIS:-
 Greek – mitos – thread or fibril.
 It’s that type of division in which chromosomes replicate
and become equally distributed both quantitively and
qualitively into 2 daughter nuclei so that the daughter
cells come to have the same number and type of
chromosomes as are present in parent cell.
 First observed by Strasburger in plant cells (1870).
 Boveri and Fleming (1879) in animal cells.
 Mitosis occurs in formation of somatic body cells and
hence often called somatic cell division.

 Consists of 2 steps:-
a) Karyokinesis
b) Cytokinesis.
KARYOKINESIS:
 Indirect nuclear division.
a) PROPHASE: (Greek – pro – first, phasis - stage)
- Early prophase: nucleus becomes spheroidal.
- Viscosity of cytoplasm increases.
- DNA molecules condense to form elongated chromosomes.
Middle Prophase:-
- Chromosomes shift towards periphery, hence a clear
central area is seen.
- Chromosomes shorten and thicken.
- Chromatid threads seen.
- Nucleoli found to be attached to one or more
chromosome.

Late Prophase:-
- Fine fibres start appearing around
nucleus.
- Nucleoli degenerates.
- 2 asters (centriole pairs and astral
rays) come to lie in area of future
spindle poles.
- Spindle poles are organised without
asters in plant cells.
- 2 spindle poles begin to get
connected by fine fibres.
Prometaphase:-
- Nuclear Envelope degenerates.
- Differentiation between cytoplasm
and nucleoplasm disappears.
- E.R and Golgi complex disorganize.
- Spindle apparatus fully organized.

b) METAPHASE:-
• Greek: meta- after or second, phases – stage
• Discontinuous fibres coming from 2 spindle poles get connected
to 2 centromere surfaces or kinetochores of each chromosomes by
means of corona having molecular motors.
• Chromosome fibres tighten, which brings chromosomes to
equator of spindle known as Congression.

c) ANAPHASE:-
• Greek: Ana – up, phasis – stage.
• 2 sub stages, A and B.
ANAPHASE A:-
Centromere of each chromosome divides into 2 so that each
chromatid comes to have its own centromere.
2 chromatids repel and separate to become daughter
chromosomes.
Daughter chromosomes move towards poles of spindle along
their tractile fibrils’ path.
ANAPHASE B:-
At end of anaphase, 2 groups of chromosomes are formed, one
at each pole of spindle.
Spindles elongate.
Spindle fibres disappear from near the poles but remain intact
near the middle.

TELOPHASE:-
Greek: telos – end, phasis – stage.
Cytoplasmic viscosity decreases.
Nuclear envelope reappears and
encloses chromosomes.
Chromosomes resume chromatic
fibres.
Nucleoli reappears.
Mitotic spindle breaks up.

CYTOKINESIS (D-PHASE):
Greek: kytos – hollow or cell, kinesis –
movement.
Division of protoplast of a cell into 2
daughter cells after nucleus division or
karyokinesis, so that each daughter cell
comes to have its own nucleus.
Mitochondria and plastids undergo
division by cleavage or fission.
Cleavage furrow forms around the
centre of a cell, progresses inwards and
separates cytoplasm into 2 and usually
equal portions.

REPRODUCTIVE CELL DIVISION:
• In sexual reproduction, each new organism is produced by union and fusion of 2 different
germ cells, one produced by each parent.
• Gametes (germ cells) are the secondary oocytes produced in the female gonads
(ovaries).
• Sperm, produced in the male gonads (testes).
FERTILIZATION:
• The union and fusion of gametes is called Fertilization, and thus the cells are called
ZYGOTE.
• Somatic cells contain 23 pairs of chromosomes that is 46 chromosomes.
• A gamete has 23 chromosomes.
• 2 chromosomes that makes up a pair are homologous chromosomes or Homologs.
• These homologs appear similar except the sex chromosome that is the X & Y
chromosome.
• In females, the homologous pair of chromosomes consists of 2X chromosomes.
• In males, the pair consists of X & Y chromosomes.
• The other 22 pairs of chromosomes are called Autosomes.

MEIOSIS
• Greek: meioum or meio – to lessen.
• It’s a double division which occurs in a diploid
cell (or nucleus) & gives rise to 4 haploid cells (or
nuclei) each having half the number of
chromosomes as compared to parent cells.
KARYOKINESIS:
MEIOSIS 1:-
Prophase 1:-
a) Lepotene: Greek: leptos – slender, tainia – band,
Nema – thread.
Nucleus enlarges.
Chromatin fibres become visible.
b) Zygotene: Greek: zygon – yoke or tied, tainia –
band.
Synaptonemal Complex: it’s a ladder like
structure with transverse protein filaments connecting
the 2 lateral elements.
The complex formed by a pair of synapsed
homologous chromosomes are called Bivalent or
Tetrad.

Pachytene: Greek: pachys – thick, tainia –
band.
Crossing over occurs. It’s a process of
exchange of genetic material or chromatid
segments between 2 homologous
chromosomes.
d) Diplotene: Greek: diplos, tainia – band.
Nucleoprotein fusion complex of the
synapsed chromosomes dissolve partially, the
remaining points where chromosomes are
attached is called Chiasmata.

Diakinesis: Greek: dia –
through, kinesis – movement.
Chiasmata shifts towards
end of chromosomes.
This phenomenon is called
Terminalisation.
Nuclear envelope
disintegrates.

METAPHASE 1:-
• A colourless bipolar spindle apparatus appears in
region of degenerated nucleus. It consists of fine fibres.
• The fibres converge towards 2 ends called Poles.
• Each chromosome gets detached to spindle pole by
means of tractile fibrils.
ANAPHASE 1:-
• The homologs break their connection and separate
out. This process is called Disjunction, and separated
chromosomes are called Dyads.
• Chromosomes move towards spindle poles along path
of their tractile fibrils.

TELOPHASE 1; CYTOKINESIS; MEIOSIS
2:-
 These are similar to those that occur in
mitosis.
 The net effect of reduction division is that
each resulting daughter cell contain the
haploid number of chromosomes.

 Transcription is the process by which the genetic information encoded
in DNA is copied onto a strand of RNA.
 Three forms of RNA are made from the DNA template:
• Messenger RNA (mRNA), which directs synthesis of a polypeptide chain
• Ribosomal RNA (rRNA), which comes together with ribosomal proteins
to make up ribosomes
• Transfer RNA (tRNA), which binds to amino acids during translation.
Each tRNA can bind specifically to many of the amino acids.
TRANSCRIPTION:

 RNA synthesis involves separation of the DNA
strands and synthesis of an RNA molecule in the
5' to 3' direction by RNA polymerase, using one of
the DNA strands as a template.
 In complementary base pairing, A, T, G, and C on the
template DNA strand specify U, A, C, and G,
respectively, on the RNA strand being synthesized.

 When RNA synthesis is complete, mRNA leaves the nucleus
and enters the cytoplasm, where translation occurs.
TRANSLATION
 Just as a DNA molecule provides the template for making an
mRNA, so mRNA provides a template for synthesizing a
protein.
 Translation is the process whereby the nucleotide sequence in
a molecule of mRNA specifies the amino acid sequence for a
protein molecule.
 In the mRNA molecule, each set of three consecutive
nucleotide bases is called a codon and specifies one amino
acid.

SPECIALIZED CELLS OF HUMAN BODY

SPECIALIZED CELLS OF HUMAN
BODY
 NERVE -Brain, Spinal Cord, Peripheral Nerves
 EPITHELIAL - Skin, lumen Of Bodies
 EXOCRINE - Secrete Products Through Ducts
 ENDOCRINE - Secrete Directly Into Blood Stream
 BLOOD - RBC,WBC.
106

CELLS AND AGING:-
 Cellular aging is the result of a progressive decline in cellular function
and viability caused by genetic abnormalities and the accumulation of
cellular and molecular damage due to effects of exposure to exogenous
influences.
 Aging is a regulated process influenced by a limited number of genes.
1. DECREASED CELLULAR REPLICATION:-
 Normal human fibroblasts when placed in tissue culture have limited
division potential.
 After a fixed number of divisions, all somatic cells become arrested in
a terminally non-dividing state known as Senescence.
 One probable mechanism in human cells is that with each cell
division, there is incomplete replication of chromosome ends
(telomere shortening) which ultimately results in cell cycle arrest.

2. ACCUMULATION OF METABOLIC AND GENETIC DAMAGE:-
 Cellular life span is determined by a balance between damage resulting from
metabolic events occurring within cell and counteracting molecular responses
that can repair the damage.
 One group of potentially toxic products of normal metabolism are Reactive
Oxygen Species.
 They cause covalent irredification of proteins, lipids and nucleic acids.
 This maybe an important cause of Senescence.
 Free radicals may have deleterious effects on DNA leading to breaks and
genome instability thus affecting all cellular functions.

CELL DEATH:-
1. NECROSIS.
2. APOPTOSIS.
1. NECROSIS:
 Focal death along with degradation of tissue by hydrolytic enzymes liberated by cell.
 Accompanied by inflammatory reaction.
 Osmosis occurs resulting in swelling of cell.
 The cell then proceeds to blebbing and this is followed by Pyknosis in which in which
nuclear shrinkage transpires.
 The Karyorrhexis occurs in which the nucleus is dissolved into the cytoplasm.
 The second path of necrosis is shown to occur after apoptosis and budding where nuclear
breaks into fragments known as Karyolysis.

CAUSES:-
 Hypoxia.
 Chemical and physical agents.
 Microbial agents.
 Immunological injuries.
MORPHOLIGICAL TYPES OF NECROSIS:-
• Coagulative necrosis.
• Liquefactive necrosis.
• Caseous necrosis.
• Fat necrosis.
• Fibrinoid necrosis.

APOPTOSIS:-
 It’s a pathway of cell death that is induced by a tightly regulated suicide program in which the
cells activate enzymes that degrade the cell’s own nuclear DNA and cytoplasmic proteins.
 Initiation of Apoptosis occurs principally signals from 2 distinct pathway.
a) Intrinsic or mitochondrial pathway.
b) Extrinsic or Death receptor initiated pathway.

INTRINSIC PATHWAY:-
• The mitochondria releases cytochrome C into the cytosol which binds to the
protein called apoptosis activating factor – 1 thus creating wheel – like
hexamer called APOPTOSOME.
• Apoptosome is able to bind caspase – 9 which in turn amplifies the death
signal.
EXTRINSIC PATHWAY:-
• Death receptors are members on TNF receptor family which can deliver
the apoptic signals.
• The best known death receptors are TYPE 1 TNF RECEPTOR.
• When death receptor ligand binds to the receptor site, active caspase – 8
enzyme is triggered which mediates the execution phase of apoptosis.

THE EXECUTION PHASE:
• Caspase once activated, cleave an inhibitor of cytoplasmic
DNAse and makes it active.
• This enzyme introduces the characteristic cleavage of DNA
into nucleosome sized particles/pieces.
• Structural components of nuclear matrix is degraded and
fragmentation of nuclei promoted.
• Cell shrinkage occurs.
• Pyknosis occurs.
• Cell membrane shows buds.
• Cell breaks into several vesicles called APOPTIC BODIES.

STEM CELLS
• A cell that has the ability to
continuously divide and produce
progeny cells that differentiate into
various other types of cells or tissues.
• These have the potential to treat
many diseases, including Parkinson's,
Alzheimer's, diabetes and cancer.

Classification of stem cells
 On the basis of origin
- Embryonic stem cells
- Somatic/ Adult/ Post-natal/ Mesenchymal stem cells
 On the basis of source
- Autologous ( Obtained from the same individual)
- Allogenic
- Xenogenic
- Syngenic/ Isogenic (Having the same or closely similar
genotypes.)
 On the basis of potency
- Totipotent
- Pluripotent
- Multipotent
- Unipotent
117

Sources of stem cells in human dental pulp
• Dental pulp stem
cells (DPSC)

•Stem cells from Human Exfoliated deciduous teeth
(SHED)

•Stem cells from apical papillae (SCAP)

•Periodontal ligament stem cells (PDLSC)

•Dental follicle progenitor cells

Niche – The specialized microenvironment
housing adult stem cells and transient
amplifying cells.
Two types:-
The cervical loop of rodent incisor for epithelial stem cells
(EpSC).
A perivascular niche in adult dental pulp for Mesenchymal
stem cells (MSC)

Use of stem cells for tooth formation in vitro
and ex vivo. A tooth germ can be created in vitro
after co-culture of isolated epithelial and
Mesenchymal stem cells. This germ could be
implanted into the alveolar bone and finally
develop into a fully functional tooth.
Use of stem cells for tooth formation

Construction of a bioengineered
tooth. The
association of tooth-derived stem
cells with defined
scaffolds in the presence of growth
factors allows the
creation of tooth specific constructs
such as crown and
root of missing parts of an injured
tooth. These
biological constructs could be used
in dental clinics as
substitutes for metal implants,
crowns and restorative
dental materials.
Construction of a bioengineered tooth

DRAWBACKS:
• The use of animal cells for human diseases is
restricted by immune rejection risks.
• A reliable source of Epithelial Stem cells is to be
determined.
• Isolating autologous stem cells requires a source of
easily accessible cells without need for a surgery.
• Procedure is costly, time consuming and
incompatible with the treatment of extensive tooth
loss.

CELLS OF DENTAL INTEREST AND SIGNIFICANCE:
AMELOBLASTS:
• These are the cells that deposit enamel.
• They are 4-5 micrometer in diameter and about 40 micrometer in
length.
• They secret the matrix protein and are responsible for creating and
maintaining an extracellular environment favorable to mineral
deposition.
• Enamel formation begins at early crown stage of tooth development
and involves the differentiation on inner enamel epithelium first at tips
of cusp outlines formed in the epithelium.
• The secretory end of ameloblasts ends in a 6 sided pyramid-like
projections known as TOMES’ PROCESS.

FUNCTION:
Amelogenesis- formation of enamel

Functional stages in the life cycle of
Ameloblasts
PRESECRETORY STAGE:
1. Morphogenic stage
2. Differentiation stage.
SECRETORY STAGE
MATURATION STAGE:
1. Transitional phase
2. Maturation proper.

CLINICAL CONSIDERATIONS
STAGE OF AMELOBLAST
 Formative stage
 Maturative stage
 Protective stage
 Desmolytic stage
DEFECT
 Matrix formation is affected, enamel
hypoplasia.
 Hypo calcification of enamel.
 Enamel resorbed or covered by a
layer of cementum.
 Failure of tooth eruption.

ODONTOBLASTS:
• Most distinctive cells of dental pulp.
• Are arranged in a PALISADE PATTERN in crown of mature tooth.
• Forms a layer lining the periphery of the pulp and have a process
extending into the dentin.
• Are columnar in shape and measure approx. 50 nm in height (in
crown of fully developed tooth.)
• In mid portion of pulp, they are more cuboid, and in the apical part,
more flattened.
• FUNCTION:
• Dentinogenesis: formation of dentin
• They also secret tertiary dentin when irritated.

FIBROBLASTS:
• These cells are greatest in number in the pulp.
• Stellate in shape.
• With age, the fibroblasts become flattened, spindle
shaped with dense nuclei.
• FUNCTION:
• They make the structural fibers and the ground
substances.
• They are responsible for collagen fiber formation.

UNDIFFERENTIATED ECTOMESENCHYMAL CELLS:
• These represent the pool from which connective tissue cells of
the pulp are derived.
• Depending upon the stimulus, these cells may give rise to
odontoblast & fibroblasts.
• Found throughout the cell rich area & pulp core & often are
related to blood vessels.
• Appear as large polyhedral cells possessing a large, lightly
stained, centrally placed nucleus.

CEMENTOBLASTS:
• Soon after HERTWIG’S EPITHELIAL ROOT SHEATH becomes interrupted
and ectomesenchymal cells from the inner portion of dental follicle
then can come into contact of predentin; infiltrating dental follicle
cells receive a reciprocal inductive signal from the dentin and/or the
surrounding HERs cells transform into ameloblasts.
• They form follicle cells around root.
• FUNCTION:
• CEMENTOGENESIS: formation of cementum.
• They synthesize collagen and protein polysaccharides which make
up the organic matrix of cementum.

OSTEOBLASTS:
• Are mononucleated cells.
• Arise from pluripotent stem cells.
• Are plump, cuboidal cells (when very active) or slightly
flattened cells.
• Are responsible of production of organic matrix of bone.
• They cover periodontal surface of alveolar bone.

CEMENTOCLASTS:
• Resemble osteoclasts.
• Occasionally found in normal functioning periodontal ligaments.
• Are mononucleated cells and cause resorption of cementum.

OSTEOCLASTS:
• Are large and multinucleated cells formed by
fusion of precursor cells similar to circulating
monocytes.
• These are cells that resorb bone.
• Occupy the bays in bone (HOWSHIP’S LACUNAE)
or surround the ends of a bone spicule.
• Resorption of bone occurs in 2 stages:
First the mineral is removed from the narrow zone
of/at the bone margin, then the recognizable
exposed organic matrix is disintegrated.

CONCLUSION
 All living things are made of cells.
 Cell is the structural and functional unit of all living things.
 Cell biology research has reached great heights, whereby it has
become possible to maintain, grow and manipulate cells outside
the living organism by genetic engineering techniques which have
been applied in numerous fields, e.g.: Research, Biotechnology and
Medicine.

REFERENCES
• Trueman’s Elementary Biology volume 1.
• Ten Cate’s – Oral Histology, 6th edition.
• Orban’s – “Oral histology and embryology”,11th edition.
• Robbins and Cotran’s Pathologic Basis of Disease, 8th edition.
• http://www.nature.com/cdd/journal/v16/n1/pdf/cdd2008150a.pdf
• Santos A. Susin; Daugas, E; Ravagnan, L; Samejima, K; Zamzami, N;
Loeffler, M; Costantini, P; Ferri, KF et al. (2000). "Two Distinct Pathways
Leading to Nuclear Apoptosis". Journal of Experimental Medicine 192 (4):
571–80. doi:10.1084/jem.192.4.571. PMC 2193229. PMID 10952727

Cell structure and functions

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