The basic functional unit of life is the cell. Robert Hooke first observed cells in 1665 when examining a piece of cork under an early microscope. He saw many tiny compartments that reminded him of monk's cells, so he called them cells. Later scientists like Antonie van Leeuwenhoek observed bacteria and other microorganisms using improved microscopes. Over centuries, scientists discovered that cells are the fundamental unit of structure and function for both plants and animals, eventually formulating the cell theory. Today, cells remain the focus of extensive research that has led to advances in fields like stem cell therapy and personalized medicine.

1. CELL
The basic functional unit of life

2. The discovery of the cell would not have been possible if not for
advancements to the microscope. (1595 – Zacharias Janssen
credited with 1st compound microscope) Interested in learning
more about the microscopic world, scientist Robert Hooke
improved the design of the existing compound microscope in
1665. His microscope used three lenses and a stage light, which
illuminated and enlarged the specimens. These advancements
allowed Hooke to see something wondrous when he placed a
piece of cork under the microscope. Hooke detailed his
observations of this tiny and previously unseen world in his
book, Micrographia. To him, the cork looked as if it was made of
tiny pores, which he came to call “cells” because they reminded
him of the cells in a monastery.

3. The microscopes of his day were not very
strong, but Hooke was still able to make an
important discovery. When he looked at a thin
slice of cork under his microscope, he was
surprised to see what looked like a honeycomb.
Hooke made the drawing in the figure below to
show what he saw. As you can see, the cork was
made up of many tiny units, which Hooke called
cells.

4. Robert Hooke sketched these cork cells
as they appeared under a simple light
microscope.

5. In observing the cork’s cells, Hooke noted
in Micrographia that, “I could exceedingly plainly
perceive it to be all perforated and porous,
much like a Honey-comb, but that the pores of
it were not regular… these pores, or cells,…were
indeed the first microscopical pores I ever saw,
and perhaps, that were ever seen, for I had not
met with any Writer or Person, that had made
any mention of them before this…”

6. Not long after Hooke’s discovery, Dutch scientist
Antonie van Leeuwenhoek detected other hidden,
minuscule organisms—bacteria and protozoa. It
was unsurprising that van Leeuwenhoek would
make such a discovery. He was a
master microscope maker and perfected the design
of the simple microscope (which only had a single
lens), enabling it to magnify an object by around
two hundred to three hundred times its original
size. What van Leeuwenhoek saw with
these microscopes was bacteria and protozoa, but
he called these tiny creatures “animalcules.”

7. Van Leeuwenhoek became fascinated. He went
on to be the first to observe and describe
spermatozoa in 1677. He even took a look at the
plaque between his teeth under the
microscope. In a letter to the Royal Society, he
wrote, "I then most always saw, with great
wonder, that in the said matter there were
many very little living animalcules, very prettily
a-moving.”

8. In the nineteenth century, biologists began
taking a closer look at both animal and plant
tissues, perfecting cell theory. Scientists could
readily tell that plants were completely made
up of cells due to their cell wall. However, this
was not so obvious for animal cells, which lack a
cell wall. Many scientists believed that animals
were made of “globules.”

9. German scientists Theodore Schwann and
Mattias Schleiden studied cells of animals and
plants respectively. These scientists identified
key differences between the two cell types and
put forth the idea that cells were the
fundamental units of both plants and animals.

10. However, Schwann and Schleiden
misunderstood how cells grow. Schleiden
believed that cells were “seeded” by the nucleus
and grew from there. Similarly, Schwann
claimed that animal cells “crystalized” from the
material between other cells. Eventually, other
scientists began to uncover the truth. Another
piece of the cell theory puzzle was identified by
Rudolf Virchow in 1855, who stated that all cells
are generated by existing cells.

11. At the turn of the century, attention began to
shift toward cytogenetics, which aimed to link
the study of cells to the study of genetics. In the
1880s, Walter Sutton and Theodor Boveri were
responsible for identifying the chromosome as
the hub for heredity—forever linking genetics
and cytology. Later discoveries further
confirmed and solidified the role of
the cell in heredity, such as James Watson and
Francis Crick’s studies on the structure of DNA.

12. The discovery of the cell continued to impact
science one hundred years later, with the
discovery of stem cells, the
undifferentiated cells that have yet to develop
into more specialized cells. Scientists began
deriving embryonic stem cells from mice in the
1980s, and in 1998, James Thomson isolated
human embryonic stem cells and
developed cell lines. His work was then published
in an article in the journal Science.

13. It was later discovered that adult tissues, usually
skin, could be reprogrammed
into stem cells and then form other cell types.
These cells are known as induced pluripotent
stem cells. Stem cells are now used to treat
many conditions such as Alzheimer’s and heart
disease.

14. The discovery of the cell has had a far greater impact on
science than Hooke could have ever dreamed in 1665. In
addition to giving us a fundamental understanding of the
building blocks of all living organisms, the discovery of the
cell has led to advances in medical technology and
treatment. Today, scientists are working on personalized
medicine, which would allow us to grow stem cells from
our very own cells and then use them to understand
disease processes. All of this and more grew from a single
observation of the cell in a cork.

15. FORMULATION OFTHE
CELLTHEORY

16. In 1838, Theodor Schwann and Matthias Schleiden were
enjoying after-dinner coffee and talking about their studies
on cells. It has been suggested that when Schwann heard
Schleiden describe plant cells with nuclei, he was struck by
the similarity of these plant cells to cells he had observed in
animal tissues. The two scientists went immediately to
Schwann’s lab to look at his slides. Schwann published his
book on animal and plant cells (Schwann 1839) the next
year, a treatise devoid of acknowledgments of anyone
else’s contribution, including that of Schleiden (1838). He
summarized his observations into three conclusions about
cells:

17. 1.The cell is the unit of structure, physiology,
and organization in living things.
2.The cell retains a dual existence as a distinct
entity and a building block in the construction
of organisms.3 true
3. Cells form by free-cell formation, similar to
the formation of crystals (spontaneous
generation).4. false

18. We know today that the first two tenets are
correct, but the third is clearly wrong. The
correct interpretation of cell formation by
division was finally promoted by others and
formally enunciated in Rudolph Virchow’s
powerful dictum, Omnis cellula e cellula,: “All
cells only arise from pre-existing cells”.

19. MODERN CELLTHEORY

20. 1. All known living things are made up of cells.
2.The cell is structural & functional unit of all living
things.
3. All cells come from pre-existing cells by division.

21. 4. Cells contains hereditary information which is
passed from cell to cell during cell division.
5. All cells are basically the same in chemical
composition.
6. All energy flow (metabolism & biochemistry) of
life occurs within cells.

22. The study of the structure and function of
cells continues today, in a branch of biology
known as cytology. Advances in equipment,
including cytology microscopes and reagents,
have allowed this field to progress, particularly
in the clinical setting.

23. ATimeline
1595 –6 Jansen credited with 1st compound microscope
1655 – Hooke described ‘cells’ in cork.
1674 – 7 Leeuwenhoek discovered protozoa. He saw
bacteria some 9 years later.
1833 – Brown descibed the cell nucleus in cells of the
orchid.
1838 – Schleiden and Schwann proposed cell theory.
1840 – Albrecht von Roelliker realized that sperm cells
and egg cells are also cells.
1856 – N. Pringsheim observed how a sperm cell
penetrated an egg cell.

24. 1858 – RudolfVirchow (physician, pathologist and
anthropologist) expounds his famous conclusion: omnis
cellula e cellula, that is cells develop only from existing cells
[cells come from preexisting cells]
1857 – Kolliker described mitochondria.
1879 – Flemming described chromosome behavior during
mitosis.
1883 – Germ cells are haploid, chromosome theory of
heredity.
1898 – Golgi described the golgi apparatus.
1938 – Behrens used differential centrifugation to separate
nuclei from cytoplasm.
1939 – Siemens produced the first commercial transmission
electron microscope.

25. 1952 – Gey and coworkers established a continuous human cell
line.
1955 – Eagle systematically defined the nutritional needs of
animal cells in culture.
1957 – Meselson, Stahl andVinograd developed density gradient
centrifugation in cesium chloride solutions for separating nucleic
acids.
1965 – Ham introduced a defined serum-free medium. Cambridge
Instruments produced the first commercial scanning electron
microscope.
1976 – Sato and colleagues publish papers showing that different
cell lines require different mixtures of hormones and growth
factors in serum-free media.

26. 1981 –Transgenic mice and fruit flies are
produced. Mouse embryonic stem cell line
established.
1995 –Tsien identifies mutant of GFP with
enhanced spectral properties
1998 – Mice are cloned from somatic cells.
1999 – Hamilton and Baulcombe discover siRNA
as part of post-transcriptional gene silencing
(PTGS) in plants

27. What Are Animal Cells?
•All living beings are made up of one or more cells,
which are the structural and functional units of
life. The activities of an organism are determined
by the activities of the cells that make up its
structure. A cell is the basic unit of life, and all of
life's activities are present in every cell of an
organism's body.What does a cell look like?

28. •Animal cells are generally smaller than plant
cells. Another defining characteristic is its irregular
shape.This is due to the absence of a cell wall. But
animal cells share other cellular organelles with plant
cells as both have evolved from eukaryotic cells.

29. A typical animal cell
comprises the following cell
organelles:

31. Cell Membrane
•A thin semipermeable membrane layer of lipids and
proteins surrounding the cell. Its primary role is to
protect the cell from its surrounding. Also, it controls the
entry and exit of nutrients and other microscopic entities
into the cell. For this reason, cell membranes are known
as semi-permeable or selectively permeable
membranes.

32. 7.Nucleus
•It is an organelle that contains several other sub-
organelles such as nucleolus, nucleosomes and
chromatins. It also contains DNA and other
genetic materials.

33. 8.Nuclear Membrane
•It is a double-membrane structure that
surrounds the nucleus. It is also referred to as
the nuclear envelope.

34. Centrosome
•It is a small organelle found near the
nucleus, which has a thick centre with
radiating tubules. The centrosomes are
where microtubules are produced.

35. 4.Lysosome
•They are round organelles surrounded by
a membrane and comprising digestive
enzymes which help in digestion,
excretion and in the cell renewal process.

36. 2. Cytoplasm
•A jelly-like material which contains all
the cell organelles, enclosed within the
cell membrane. The substance found
within the cell nucleus, contained by the
nuclear membrane is called the
nucleoplasm.

37. 9. Golgi Apparatus
•A flat, smooth layered, sac-like organelle
which is located near the nucleus and
involved in manufacturing, storing, packing
and transporting the particles throughout
the cell.

38. 10. Mitochondrion
•They are spherical or rod-shaped
organelles with a double membrane.
They are the powerhouse of a cell as
they play an important role in
releasing energy.

39. 11. Ribosome
•They are small organelles made up of
RNA-rich cytoplasmic granules, and
they are the sites of protein
synthesis.

40. 9.Endoplasmic Reticulum (ER)
•This cellular organelle is composed of
a thin, winding network of
membranous sacs originating from
the nucleus.

41. 12.Vacuole
•A membrane-bound organelle present
inside a cell involved in maintaining
shape and storing water, food, wastes,
etc.

42. ANIMAL CELL
TYPES

43. ANIMAL CELLTYPES
• Skin Cells
• Melanocytes, keratinocytes, Merkel cells and Langerhans cells
• Muscle Cells
• Myocyte, Myosatellite cells,Tendon cells, Cardiac muscle cells
• Blood Cells
• Leukocytes, erythrocytes, platelet
• Nerve Cells
• Schwann cell, glial cells
• Fat Cells
• Adipocytes

44. Skin Cells
•KERATINOCYTES
The keratinocytes become more mature
or differentiated and accumulate keratin as they move
outwards. They eventually fall or rub off. They form four
distinct layers, described in the table below from the
most superficial to the deepest.
As the most dominant cell type constituting the
epidermis, keratinocytes play multiple roles essential
for skin repair.

45. Layer Cell type
Stratum corneum (horny layer)
•Called corneocytes or squames.
•Dead, dried-out hard cells without nuclei.
Stratum granulosum (granular layer)
•Cells contain basophilic granules.
•Waxy material is secreted into
the intercellular spaces.
Stratum spinulosum (spinous, spiny or prickle cell
layer)
•Intercellular bridges called desmosomes link the
cells together.
•The cells become increasingly flattened as they
move upward.
Stratum basale (basal layer)
•Columnar (tall) regenerative cells.
•As the basal cell divides, a daughter cell migrates
upwards to replenish the layer above.

46. Melanocytes
•Melanocytes are found in the basal layer of
the epidermis. These cells produce a pigment
called melanin, which is responsible for
different skin colour. Melanin is packaged
into small parcels (or melanosomes), which
are then transferred to keratinocytes.

47. Langerhans cells
•Langerhans cells are immune cells found in
the epidermis and are responsible for helping
the body learn and later recognise new
‘allergens’ (material foreign to the body).

48. MERKEL CELLS
Merkel cells are cells found in the basal layer of
the epidermis. Their exact role and function
are not well understood. Special
immunohistochemical stains are needed to
visualise Merkel cells.

50. Myocytes
- sometimes called muscle fibers, form the bulk of muscle tissue.
They are bound together by perimysium, a sheath of connective
tissue, into bundles called fascicles, which are in turn bundled
together to form muscle tissue. Muscles are composed of long
bundles of Myocytes (Muscle fibers). Myocytes contain thousands
of myofibrils. Each Myofibril is composed of
numerous sarcomeres, the functional contracile region of a
striated muscle. Sarcomeres, in turn, are composed of
myofilaments of myosin and actin, which interact using the
sliding filament model (powered by molecular motors) and cross-
bridge cycle to contract.

51. Myosatellite cells
•Myosatellite cells, also known as satellite cells, muscle
stem cells or MuSCs, are small multipotent cells with
very little cytoplasm found in mature muscle.[1] Satellite
cells are precursors to skeletal muscle cells, able to give
rise to satellite cells or differentiated skeletal muscle
cells.

53. Tendon cells
•Tendon cells are known as tenocytes and
these reside between the parallel aligned collagen
fibers. Tenocytes are unlikely to be a uniform
population of cells because they differ
considerably in nuclear morphology on light
microscopy and when grown in culture

54. Cardiac muscle cells (cardiomyocytes)
- Cardiac muscle cells (cardiomyocytes) are striated,
branched, contain many mitochondria, and are under
involuntary control. Each myocyte contains a single,
centrally located nucleus and is surrounded by a cell
membrane known as the sarcolemma.

56. BLOOD CELLS

57. Blood Consists of:
•Red blood cells(erythrocytes): responsible
for carrying oxygen from the lungs throughout the circulatory
sysem
• Oxygen carried by hemoglobin: a protein part of the blood
• Do not repair themselves
• New cells are made in bone marrow
• Dead cells are removed by the spleen and liver
• One animal may have a trillion blood cells!

58. Blood Consists of:
•White blood cells(leukocytes): responsible
for fighting disease and removing harmful substances from the
body
• Four different kinds found in blood
• Some cells surround and digest infectious bacteria
• Produce antibodies: a kind of protein that destroys bacteria, viruses,
and other invasive substances
• WBC counts go up if there is an infection

59. •Platelets(thrombocytes): the structures in blood
that are responsible for clotting
• Disk-like shape
• Creates scabs
• Without them, an animal might bleed to death from a wound

61. NERVE CELL

62. The Schwann cell
- plays a vital role in maintaining the
peripheral nervous system (PNS). Schwann cells
are derived from neural crest cells, and come in
two types either myelinating or non-myelinating
Schwann cells. Both play a pivotal role in the
maintenance and regeneration of axons of the
neurons in the PNS.

63. Glial cells
- exist in the both central nervous system
(CNS) and the peripheral nervous system (PNS).
- The most notable glial cells include
oligodendrocytes, astrocytes, microglia, and
ependymal cells in the CNS and schwann cells,
satellite cells, and enteric glial cells in the PNS

64. Astrocytes
Astrocytes are the most abundant cells in the brain. They
are star-shaped cells that provide physical and nutritional support
for neurons. Functions include: clean up brain "debris"; transport
nutrients to neurons; hold neurons in place; digest parts of dead
neurons; regulate content of extracellular space; promote
synaptic connections; clear excess neurotransmitters; ensure the
continued function of neurons

65. •Astrocytes also
•Help to maintain the permeability of the blood-
brain barrier where they sense glucose and ion
levels inside the brain and regulate their flow into
or out of it[5].
•Are involved in gliosis in response to injury.[4]
•Regulate extracellular fluid transport known as
the glymphatic pathway (functionally represents
the brain’s lymphatic system).[7]

66. Oligodendrocytes
- are the myelin-forming cells of the CNS. . Myelin is
composed of layered phospholipid membranes and
serves to support and insulate axons, allowing for faster
impulse transduction. Saltatory conduction occurs as the
impulses jump across sodium ion-rich nodes of Ranvier.
One oligodendrocyte myelinates multiple axons.
Oligodendrocytes are incapable of replication upon injury

67. Astrocytes AND Oligodendrocytes

68. Ependymal Cells
Ependymal cells, which create cerebral spinal fluid
(CSF), line the ventricles of the brain and central
canal of the spinal cord. These cells are cuboidal to
columnar and have cilia and microvilli on their
surfaces to circulate and absorb CSF.

70. Radial Glia
Radial glial cells are at crucial in brain development, being
the progenitors for neurons and macroglia (oligodendrocytes and
astrocytes) and providing pathways for migration of neurons from
the ventricular surface to their final positions in the brain.
Radial glia also increased synapse stability, improve brain
plasticity and play a role in neuroprotection.

71. Interneuron(green)-radial(red)glial interactions, developing cerebral cortex

72. Microglia
Microglia phagocytose and remove foreign or damaged
material, cells, or organisms.[8] They are small, relatively sparse
cells. They act as the brain's resident cleanup squad by
phagocytosing apoptotic cells, plaques, and pathogens. In the
resting healthy brain, microglia are highly dynamic, moving
constantly to actively survey the brain parenchyma. [11] Microglia
seem to be particularly involved in monitoring the integrity of
synaptic function, optimizing different brain circuits to enable
cognitive development (microglia cells eliminate previously-
formed synapses that are no longer useful).

73. Schwann Cells
Schwann Cells (glial cell of the PNS)
surrounds neurons, keeping them alive and covering
them with a myelin sheath. They play essential roles in
the development, maintenance, function, and
regeneration of peripheral nerves.

74. Satellite Glial Cells
Satellite Glial Cells ensheath the somata of neuron bodies in
sensory, sympathetic and parasympathetic ganglia. They are
thought to have a similar role to astrocytes in the central nervous
system (CNS). They supply nutrients to the surrounding neurons
and also have some structural function. Satellite glial cells bundle
the axons close together by surrounding them, keeping them
from touching each other by squeezing its cytoplasm between
the axons.

75. Satellite and Schwann cells, PNS

76. Adipocytes or Fat Cells
Adipocytes, also known as lipocytes and fat cells, are
the cells that primarily compose adipose tissue, specialized in
storing energy as fat.[1] .
There are two types of adipose tissue, white adipose tissue (WAT)
and brown adipose tissue (BAT), which are also known as white
and brown fat, respectively, and comprise two types of fat cells.

77. Fat Cells

78. A hen’s egg can be seen easily. Is it a cell or a
group of cells?

80. The egg of a hen represents a single
cell and is big enough to be seen by the
unaided eye.

81. BRICKS WALL ONION PEEL/SKIN

82. ORGANISMS SHOWVARIETY IN CELL
NUMBER, SHAPE AND SIZE

83. A hen’s egg can be seen easily. Is it a cell or a
group of cells?

84. A hen’s egg can be seen easily. Is it a cell or a
group of cells?

86. THE CELL
• Both, bricks in a building and cells in
• the living organisms, are basic
• structural units [Fig. 8.2(a), (b)].The
• buildings, though built of similar bricks,
• have different designs, shapes and sizes.
• Similarly, in the living world, organisms
• differ from one another but all are made
• up of cells. Cells in the living organisms
• are complex living structures unlike
• non-living bricks.

87. References
• https://education.nationalgeographic.org/resource/history-cell-discovering-cell/
• https://bitesizebio.com/166/history-of-cell-biology/