2. Cells and Tissues
Essential reading for those who have not done ANHB 1101 and / or ANHB 1102.
Strongly recommended for all others!
In this chapter :
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Structure and function of a mammalian cell.
Cell division.
Tissues – cell groups with specific functions.
What is histology?
The link between histology, gross anatomy and embryology.
In the previous chapter we have noticed some functions of the organ systems of the body. Even in one
system, different organs have different functions; even though they conform to the larger functional
theme of the system. Once again we take the example of the digestive system. The stomach begins the
digestion of proteins. It requires a very acidic medium to be able to this; therefore it also produces
hydrochloric acid. The small intestine completes the process of digestion and it absorbs most of the
digested food to pass on to the liver and then to circulatory system. These two simple examples are
enough to make us wonder, what is it that makes all this possible?
The fundamental answer to this question is – “cells’. Cells are the structural and functional units of the
body. It is amazing that all cells are ‘built’ to a common plan; and yet, differences amongst them enable
them to perform highly specialised, diverse functions.
The enormous variety of functions that cells perform can be summarised into just four fundamental ones.
A group of cells with a common functional theme is called a tissue; so there are four primary types of
tissues. An organ like the stomach shows all four tissue types organised in a specific manner – it is the
organisation that makes the stomach what it is.
This chapter aims at explaining how this happens. And there is more, because not all cells in the body live
throughout our lifespan. Cells can die a natural death or may be victims of disease processes. In some
cases cells are in fact ‘programmed to die’. Whatever the cause, dead cells need to be replaced. This is
achieved by cell division. We therefore also take a look at this process.
The section on cell division includes concepts which are relevant to areas outside histology as
well, and is placed at the end of this chapter.
Finally, we explain what this branch of anatomical science really means to us, in the context of this
integrated unit.
Methods of study and special tools of study.
This section gives the bare essentials required for understanding the theoretical / descriptive aspects of
the topic. You will find more information in a special chapter on the subject of methods of preparation of
material for microscopic study.
The scale. The study of cells and tissues is on the different scale altogether. When we speak of sizes of
cells, the day-to-day units of measurement (millimetres, centimetres etc) are too cumbersome. The unit of
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length we use is one-millionth of a metre (10 m), or one-thousandth of a millimetre (10 mm). This unit
is called ‘micrometre’ or just ‘micron’, and it is abbreviated as µm (or just µ).
Imagine… a 5 cent coin has a thickness of just over 1 mm. If you could cut 1000 equal circular
slices out of it, the thickness of each slice would be a little over 1 µm.
Diameters of human cells can range from less than 5 µm to over 100 µm, though a large majority would
fall in a smaller range.
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The microscope. Any microscope is essentially a magnifying instrument. In this unit we confine
ourselves to a ‘compound’ microscope which uses ordinary light. It has a source of light, a lens to direct a
beam of light to the material under study and two lens systems to magnify the object. The beam of light
must pass through the object before it can be magnified.
The material we study. The object of our study must be very thin for light to pass through it. The
commonest way of achieving it is to cute very thin slices and fix them to glass plates. The thin slices are
called ‘sections’. Most of the sections are between 5 µm and 8 µm in thickness. At this thickness,
different components of cells have very little contrast among them to allow us to see them clearly. We
therefore colour them (‘stain’ them). There are many ways of staining these sections, the commonest
method uses two stains, haematoxylin and eosin. Haematoxylin makes some parts of the sections appear
blue, while others appear pink because they are coloured by eosin. Sections stained by this method are
called H & E sections. We often use the term ‘routine sections’ for such material, since this is a common
method of staining.
At this stage, keep these few points in mind! These are explained in a little more detail with
illustrations on the unit website.
Structure of a generalised cell.
In this section we look at the basic plan of cell structure, without speaking of special features of different
cells. We assume that such a generalised cell is more-or-less spherical and therefore appears rounded in a
two-dimensional picture.
Every cell is bounded by a very thin layer, the cell membrane. Contained within is a fluid called
cytoplasm (cyto = cell). Within the cytoplasm is the controlling centre of the cell, the nucleus. The word
‘nucleus’ means a nut. Think of a stone fruit!
A cell pretty much does all the basic things a living organism does – eats food, uses energy produced by
oxidation, reproduces. Growth, reproduction and repair involve production of molecules of life – proteins,
carbohydrates and fats being the principal ones. Besides, many cells produce such molecules for delivery
outside the cell. Let us understand the structural components of a cell in this context.
Fig. 1 Diagram of a generalised cell.
N : nucleus, n : nucleolus, C : clumps of
chromatin (This nucleus is euchromatic).
M : mitochondria. rER : rough endoplasmic
reticulum. The dots on the walls of the sacs
are ribosomes. The green circles emerging
from the rER are the products of rER, not yet
ready to be delivered.
G : Golgi complex.
Ex : exocytosis. En : endocytosis.
The colours in this illustration are
representative of any staining method!
not
Use this diagram in conjunction with the
description that follows.
The cell membrane.
The cell membrane is made largely of proteins and fats. It has a highly complex molecular structure. The
important concept for us is that the cell membrane is not just a physical boundary. Many substances can
pass across the membrane, in both directions. Passages of substances across this membrane are controlled
by complex biochemical phenomena. Many of these processes are highly energy-intensive.
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The Nucleus.
In routine sections (H & E) the nucleus appears blue. It may appear like a condensed dot or large, pale
with clumps of blue. The blue material is called chromatin (‘coloured’). Chromatin is a mixture of DNA
(deoxyribonucleic acid), some proteins and RNA (ribonucleic acid).
The DNA content of the nucleus is in the form of ‘supercoiled’ chains condensed to form chromosomes
(= ‘coloured bodies’). Chromosomes are present in pairs, one member of each pair from the father, and
one member from the mother of the individual. All nucleated human cells, except germ cells or gametes,
have 23 pairs. Only one of the 23 pairs has dissimilar looking chromosomes – these are the sex
chromosomes, labelled X and Y. Males have one X and one Y chromosome, females have two X
chromosomes in their cells.
DNA contains all the information required for the functioning of the cell, and indeed for the entire
organism. A highly active cell is usually involved in a lot of protein synthesis, which requires uncoiling
of DNA. The nucleus of an active cell is therefore large and pale, with clumps of uncoiled chromatin.
Such a nucleus is called euchromatic (= having ‘good’ chromatin) nucleus.
The nucleus of an inactive cells has almost all chromatin ‘packed up’, and appears as a compact dot. Such
a nucleus is called heterochromatic (= having ‘other’ chromatin).
In a euchromatic nucleus, there is usually a blue dot (or two) close to the centre. This is called the
nucleolus (= ‘little nucleus’). The nucleolus is the site of RNA synthesis, which is an essential step in
protein synthesis. In this unit we are mainly concerned with appearances and their interpretation, not
so much with the biochemical aspects of protein synthesis.
The Cytoplasm.
The cytoplasm contains a number of organelles (= ‘little organs’) with specific functions. Here we
consider three of these, which are of great functional importance. The organelles described here are all
bounded by membranes similar to the cell membrane.
Endoplasmic Reticulum. We take the processes of protein synthesis, described above, to the cytoplasm.
Protein synthesis takes place in membrane bound stacks of sacs. The cavities of these sacs are continuous
with each other. They also communicate with pores in the nuclear envelope. Because these sacs form a
network within the cytoplasm, they are said to form the endoplasmic reticulum (ER). They are studded
with tiny particles called ribosomes containing RNA. This gives the reticulum a granular or rough
appearance in EM pictures. It is therefore called rough or granular ER. Under the light microscope with
routine stains, the granules are invisible – rER is simply seen as a blue area in the cytoplasm.
Proteins which are synthesised are within the cavity of rER. When they are released, a portion of the
membrane forming the ER detaches itself. Such a detached packet is called a vesicle.
It is worth noting that proteins which are used by the cell for its own purpose are made by ribosomes
scattered in the cytoplasm (free ribosomes). Proteins which are to be delivered outside the cell are made
in rER. ER without ribosomes is called smooth ER (sER), where non-protein molecules are synthesised.
Golgi Complex or Body. Proteins which are delivered outside the cell need further processing,
conveniently compared with packaging. This is done in another membrane-bound organelle, the Golgi
body. This organelle usually does not stain well. In cells in which it is prominent, it is seen as a somewhat
diffuse, pale area. Vesicles delivered by rER simply attach to the Golgi complex and the membranes
become continuous. At the other end of the Golgi complex the membrane again forms vesicles, which
attach to the cell membrane and the release the protein outside the cell. Expulsion of such contents is
called exocytosis (= “out of the cell’). Follow the sequence from rER to the exterior in Fig. 1.
Mitochondria. Mitochondria are tiny, rod-like bodies, not easily visible with the light microscope and
routine stains. They are the site of biochemical processes that yield energy. With the EM, they show a
wall of a double membrane, the inner one being folded into a number of crests (cristae). Appearances
suggestive of large numbers of mitochondria arranged in a parallel manner are seen in some cells.
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Besides these, most cells have some degree of mechanical strength given by protein filaments forming a
cytoskeleton, fine tubules (microtubules), membrane bound droplets (vesicles) containing enzymes,
ingested material or material to be thrown out. One of the processes by which a cell ingests (takes in)
large molecules is illustrated in fig. 1. Note how the cell membrane forms an envelope around the particle
to be ingested. The pouch so formed then detaches from the cell membrane, forming a vesicle inside the
cell. This process is called endocytosis, illustrated at ‘En” in fig. 1.
Other organelles, specific to certain cell types will be mentioned when relevant.
Form and function – diversity among cells.
The shape and size of a cell reflects a number of features. Among these are – the amount of cytoplasm,
organelles and other components present within the cytoplasm and the size of nucleus. The shape and size
often give some indication of the function/s of a cell. Cells can be flat and thin, spherical or oval, like
blocks or like columns and even resemble long threads. Cells of similar type and function can also be
arranged in different ways. Yet, cells can be considered as forming masses of four different types, each
adapted to a set of functions. Finer distinctions among each type exist.
The ‘primary’ tissues of the body.
We now take a comprehensive look at organisation of cells with similar functions. A group of cells
directed at a common function is called a tissue. This does not mean that in a given tissue all cells are
identical. They do share some features. The following explanations will make this clearer.
1. Epithelium or epithelial tissue.
An Epithelium is a sheet of cells arranged in a compact manner. The cells of an epithelium are usually in
contact with each other. Indeed, their membranes are joined together by what are known as cell junctions.
Some types of cell junctions do not allow anything to pass between the cells. Anything that passes
through the cells is controlled by the cells.
This structural feature is responsible for the most fundamental functional property of this
tissue – it forms a selective barrier.
To understand this function, we take some illustrative examples.
 Consider the cavity of the stomach or the intestine. Only digested food or other essentials can be
allowed travel from the cavity across the epithelium. The cells of the epithelium have mechanisms to
ensure this. If there were gaps in the epithelium, unwanted substances could pass through them. The
junctions between the cells ensure that this does not happen.
 The lungs have millions of microscopic cavities. These are lined by an epithelium. Only gases
(oxygen / carbon dioxide) can pass easily across the cells.
 A part of the skin is an epithelium which ensures that dust, micro-organisms and other harmful
substances do not enter the body. For that matter, the epithelium of the skin is also partly
‘waterproof’ – prevents water from entering the body when we shower!
These are some examples, in somewhat oversimplified language, but they illustrate the principle.
To summarise and to generalise, we may say that epithelia cover the body surface or line the cavities of
the body. The compact arrangement of cells ensures the integrity of the tissue as a sheet.
Besides this primary function, subtypes of epithelial tissue can perform a number of other functions.
These functions are also reflected in their structure. Let us expand on the same three examples.
 Epithelium of the stomach or the intestine can digest food by producing enzymes and also absorb
digested food. Enzymes are proteins. They must be synthesized from raw materials (mainly amino
acids) within the cells. From our understanding of cell structure, this requires significant amount of
rER, mitochondria and Golgi complexes. The must have a considerable amount of cytoplasm, with
the organelles arranged such that the enzyme molecules are released into the cavity of the organ.
Such cells have shapes like pillars, and indeed are called columnar cells. The epithelium formed by
such cells has a single layer of such cells.
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 Epithelium lining microscopic cavities in the lungs allow gas exchange between air and blood.
Consider that this exchange must be completed in the short time of breath cycle! The epithelium
here is necessarily made of flat, thin cells arranged as a single layer.
 The skin is subject to considerable friction with objects around us. Friction may knock some of the
cells off the surface of the skin. To prevent exposure of deeper structures, the epithelium must have
many layers of cells, and a plan to replace lost cells. The epithelium of the skin also forms a
waterproof covering. This is made possible by the fact that on the surface there are layers of dead
cells containing a special kind of protein and some fats.
An epithelium with a single layer of cells is known as a simple epithelium, one with many
layers is a stratified epithelium. (Stratum = layer).
It is relevant to point out at this stage that an epithelium is like a cloth. It can be ‘tailored’ to the shape
and size of the cavity that it lines, just as a straight sheet of cloth can be tailored into a simple tank-top or
a highly complex wedding dress!
Fig. 2. In a epithelium, cells are compactly arranged to form sheets (A and B). Sheets
can be shaped to fit surfaces they cover or cavities they line (C).
The variety of form and function in epithelial tissue is amazing. We learn more about it in a later chapter.
2. Muscle tissue.
Muscle tissue is for movement. The characteristic of this tissue is elongated cells with special protein
threads (filaments) in the cytoplasm. The two main protein filament types have the property of sliding
against each other. When the filaments slide, the cell shortens.
These cells are arranged in a parallel manner, forming bundles. When many cells in a bundle ‘contract’ in
this manner, the entire mass undergoes shortening.
The arrangement of the filaments within the cells confer additional properties to muscle. Based on these
properties we can consider three subtypes of this tissue.
 Muscle tissue attached to bones brings about movements at joints (with some exceptions), and is
called skeletal muscle. Most skeletal muscle is under voluntary control. Cells of skeletal muscle can
be very long (some of them many centimetres in length), though their diameters are still in the
‘micrometre’ range.
 Muscle tissue in internal organs like the intestines is involuntary. The cells of such muscle are
elongated, but much shorter than those of skeletal muscle. Besides being involuntary, this type of
muscle has other structural and functional features as well.
 Muscle cells in the heart have special features of their own.
Fig. 3. Muscle cells can be quite long. Left : Diagram to show cylindrical skeletal
muscle cells. Right : a single heart muscle cell (highly magnified).
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3. Nervous tissue.
Nervous tissue is meant for carrying ‘information’ from one part of the body to another. this is done by
means of electrical activity in the membranes of the cells. The cells (‘neurons’) of this tissue are
remarkable. They maintain different electrical energy levels (potentials) on the inner and outer sides of
their membranes. Changes in these potentials can make the cell send a wave of an electrical event along
the length of the cell. Such electrical waves are used as signals, and such signals play a great role in
controlling a number of body processes. These cells are called neurons or nerve cells. Nervous tissue also
has other essential cells which have a supporting role.
Nervous tissue forms a major control system of the body, the nervous system.
Fig. 4. A single, representative
neuron. Neurons can vary
greatly in size and shape.
Note : Electrical activity is also seen in other types of cells, notably muscle cells. What makes
neurons special is that they can use electrical activity to transmit information over long distances
in the body and control the behaviour of other parts of the body.
With this basic understanding, let us return to the example of the intestine.
 The intestine is a hollow tube. It has a lining of epithelium, which separates the contents from
the wall of the intestine, produces enzymes for digestion and also absorbs digested food.
 For moving the contents of the intestine, there is a layer of muscle outside the epithelium.
In keeping with the tubular form of the intestine, both epithelium and muscular layers are in fact
like two tubes; the epithelial tubes fits inside the muscular tube.
 There are nerve cells which control these activities. The nerve cells are scattered in the wall.
(In many parts of the digestive system, there are other control mechanisms as well, but we
shall focus on the nerve cells at this stage).
 However, if the two main layers of the wall (epithelium and muscle) are left to themselves,
they might slide and separate from each other.
The two tubes must be connected to each other. The job of connecting these layers is done by the
fourth type of primary tissues.
4. Connective tissue.
The fourth type of tissue is aptly named ‘connective tissue’. One can imagine that joining together implies
some mechanical strength. If the two tubes tend to slide, the joining tissue must resist such movement. In
other parts of the body, such tissues are under even greater stresses.
Most cells are too delicate to resist forces that act on parts of the body – imagine lifting a 20 or 30 kg
weight with your hand!
In connective tissue, cells produce strong materials which fill the spaces between cells. The basis of
these strength-providing materials is fibres (threads) of proteins. The arrangement of these fibres varies
greatly in different situations. They may form delicate networks like loosely woven cloth, strong
networks like tough linen, they may form ropes and ties and even strong structural supports. The strongest
connective tissue is bone, and it has calcium compounds arranged along fibres to make it strong.
Fibres are not the only kind of intercellular material. In all connective tissues, there is a jelly-like material
filling up any remaining space. This ‘ground substance’ has many functions. Among others, they can also
play a role in mechanical strength of a connective tissue.
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Connective tissue is often the defining factor in the shape of an organ – indeed, the shape and form of the
entire body is determined by the bony skeleton.
With the exception of the brain and spinal cord, connective tissue is found everywhere in the body either
as large masses seen in gross anatomical specimens or as microscopic layers.
The facts that connective tissue is ubiquitous, that it has such varied functions (we have mentioned just
one aspect above), that it is the defining factor in the organisation of the body… these are the reasons that
we are going to study connective tissue before all the others in this unit.
It is important to realise that the descriptions given above are meant as an introduction to those
who may be unfamiliar with these concepts. Bear in mind that we are going to use appropriate
biological language and discuss these tissues on more firm ground in this unit.
Cell specialisation / differentiation.
From this discussion it should be obvious that cells with different functions differ from each other in
more ways than just shapes and sizes. The proportion of different organelles and even their molecular
biology must differ. Since an organism begins its journey as a single cell, there must be considerable
specialisation among cells as they grow in numbers. Every cell has a full stock of genetic information that
the organism requires. Yet, in a specialised cell, some functions are suppressed and some functions are
enhanced. This is achieved by the process of cell differentiation. The first cell that the organism begins as,
and a few generations of its descendants can form virtually any cell in the body. Such cells are described
as ‘pluripotent cells’. With further structural changes in the embryo, their capacity for wider
differentiation is reduced. This concept will be better understood when we study the tissues in greater
depth. At this stage we can still consider one example. Connective tissues exhibit a great variety of
function and form – fibrous masses, firm gristle (cartilage), bone and even soft tissues like fat. The
principal cells of all these are specialised to produce intercellular material specific to each. In other
words, they are differentiated. Yet, during embryonic life they arise from a single cell type. We shall
more such examples as we go along.
Histology : what is it about…
The term histology comes from the Greek word histos meaning web or tissue. Histology is thus the study
of tissues. In common usage, the study of the tissues outlined above is called basic histology. The
organisation of these tissues into organs and systems is called systemic histology.
The focus of this unit is on basic histology. Systemic histology is studied in ANHB 2214 (“Organs and
Systems”). Often we do need to consider the general plan of some organs to understand the functioning of
the primary tissues. However this is done in this unit specifically to understand some concepts and terms.
The place of basic histology in this unit.
Histology is the link between structure and function. It explains how structural components of the body
like muscles, bones, cartilage and tendons behave under normal circumstances, and in injury or disease.
In this unit, histology is not studied in isolation. On innumerable occasions you will notice that gross
anatomy is seen and understood through the medium of microscopic structure and function.
Histology depends heavily on the special methods of preparation of the materials, we do need to know the
basic facts about the preparation and interpretation of the material. Such technical aspects are examinable
only in principle, not in detail.
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(Cell division on the next page…)
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Cell division.
An increase in the number of cells is an essential part of growth and repair. This accomplished by cell
division. Cell division of a different kind is the key to the formation of gametes (reproductive cells). Cell
division is a complex process; and apart from the mechanism, introduces a large terminology.
Here we outline the principal steps in cell division, highlighting those concepts which are of
relevance to this unit. The interested student is advised to refer to appropriate resources for
further details. If you have studied cell biology earlier, you may find the description here fairly
elementary. It may still be worthwhile to note the relevant core concepts. A few terms must be
introduced to facilitate this discussion, but the core points are summarised at every stage.
Cell division involves distribution of genetic material between the two new cells (often called ‘daughter
cells’) and a division of the cytoplasm. The key events relate to the nucleus.
We have mentioned earlier that the major component of chromatin in a nucleus is DNA, arranged in a
‘supercoiled’ manner to form chromosomes. The steps involved in ‘supercoiling’ are beyond the scope of
this unit. Suffice it to say that each chromosome stores specific amount of genetic information. In the
non-dividing cell, chromosomes cannot be seen individually. Chromosomes, when in their coiled or
compact form, have two ‘arms’ joined by a knot-like portion called centromere.
As mentioned earlier, all nucleated human cells have 23 pairs of chromosomes. To keep illustrations
simple, only one pair of chromosomes will be shown in the diagrams in this section.
Fig. 5 shows a single pair of compact chromosomes. In most chromosomes, the centromere separates two
arms of unequal lengths. The short arm is conventionally labelled ‘p’, the long arm as ‘q’. The two
members of the pair are called homologous chromosomes. We inherit one member of the pair from the
father (paternal chromosome), the other member from the mother (maternal chromosome).
p
p
Fig. 5. A pair of homologous chromosomes (diagrammatic).
The members are shown in different colours to indicate the fact that one of
them is the paternal chromosome, the other is maternal. Note that their overall
length is identical, so is the position of the centromere. They both have genetic
material for the same characteristics, though the paternal and maternal genes
may express variants of the same characteristics (they may be different “alleles”).
q
q
Histologically we rarely see chromosomes in this compact form. In a nondividing cell, chromosomes are uncoiled to variable degrees and may be
‘tangled’.
Before a cell divides, all DNA in its nucleus must be copied (“replicated”), so
that each daughter cell receives one full set. This is illustrated in the next figure.
Fig. 6. A pair of replicated homologous chromosomes.
Note that the two copies of each chromosome are joined at the
centromere. Each copy is now called a chromatid. The two
chromatids of each chromosome are called ‘sister chromatids’.
p
p
p
p
q
q
q
q
Thus, the paternal chromosome has two identical sister
chromatids and so does the maternal chromosome.
Chromosomes in this form are visible only during cell division. A
chromatid in this stage is the chromosome of a non-dividing cell.
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All body cells, with one exception, divide by a process called mitosis. The exception is a tiny group of
cells on their path to become gametes (reproductive cells, sperms and oöcytes, described in the next
chapter). These cells divide by a process called meiosis.
During mitosis, each daughter cell receives the same amount of genetic material (number of
chromosomes) as the non-dividing parent cell.
During meiosis, the number of chromosomes is halved. Each daughter cell receives 23 single
chromosomes instead of 23 pairs.
To indicate the full complement (paired chromosomes) we use the term ‘diploid’. The diploid number
for human cells is 46 (23 pairs).
To indicate 23 single chromosomes we use the term ‘haploid’.
Both the processes have a number of steps (phases). A detailed description of these steps is beyond the
scope of this unit. However, some key points are :
 Before cell division, replication of chromosomes takes place.
 During the early phases of cell division, the replicated chromosomes undergo coiling and
gradually become visible. The envelope of the nucleus disappears.
 Two small organelles appear at two opposite ends (‘poles’). These are called centrosomes, and
they are connected to the centromeres of the chromosomes by tubules of proteins. Shortening
of the tubules pull the dividing chromosomes to the two poles of the cell.
Here we are mainly interested in the manner in which the chromosomes split.
We consider division by mitosis (mitotic division) first. It is worth repeating that a single pair of
chromosomes is shown in these diagrams. The process is the same for all 23 pairs. In appearance,
chromosomes differ in sizes and proportions of the arms. In the case of sex chromosomes, the Y
chromosome appears shorter than the X chromosome – they are not identical.
Mitosis. (Fig. 7)
During mitosis, the replicated chromosomes, each with two chromatids, are arranged in one plane. (This
phase is called metaphase). Each replicated chromosome splits at the centromere, and the sister
chromatids are pulled apart to the opposite poles of the cell. At the poles the nuclear envelopes form again
and we see a large cell with two nuclei. Soon the cytoplasm divides too, and we have two daughter cells,
each with a diploid number of chromosomes.
Remember : the chromatids as seen during division are the chromosomes of the daughter cells!
A
B
C
Fig. 7. Mitosis. In A, note the replicated chromosomes lined up.
The chromatids separate and move towards the ‘poles’ (B, C).
Note identical pairs in D. E : diploid daughter cells. D is purely
diagrammatic : at this stage the chromosomes condense into
nuclei and are not visible as shown here.
D
E
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Meiosis. (Figs. 8 and 9)
Meiosis is a process involving two divisions. The first division (Meiosis I) has two features of
importance.
1. Replicated homologous chromosomes (maternal and paternal chromosomes of the same pair) are
arranged next to each other as if in an embrace. Their corresponding arms cross at some points. At the
points of crossing, they exchange equivalent parts of their lengths – that is, they exchange genetic
material. This process is called crossing over. When they separate, each of the homologous chromosomes
has a mixture of paternal and maternal genetic material. The exact lengths and therefore the proportion of
material exchanged differs from pair to pair, and even for the same pair in different divisions. This means
that each chromosome, after crossing over, is a unique mix of maternal and paternal genes.
2. The chromatids do not separate during meiosis I. Instead, replicated chromosomes move to opposite
ends of the cell, one member of each pair moving to one end, the other member to the other end.
Thus, at the end of meiosis I, there are two daughter cells, each with a haploid number of replicated
chromosomes. Because of crossing over, we do not call these chromosomes as maternal or paternal!
A
B
C
Fig. 8. Meiosis I. A : Homologous chromosomes in contact (‘synapsis’). Note points of
crossing. B : each chromatid of each homologous chromosome has a different mix of
maternal and paternal portions. At the end of this division (C), each daughter cell has a single,
replicated chromosome. (Chromosomes are shown larger in this figure for clarity!)
In meiosis II, the two cells divide. The chromatids of the single replicated chromosomes separate. This
may sound like mitosis, but remember that it is a single replicated chromosome that splits, and that the
single chromosome has a mixture of genetic material from the two homologous chromosomes. The
colours in fig. Will make it clear that the four resulting chromosomes are similar, yet unique.
Fig. 9. Meiosis II.
Note the chromosomes of
the four haploid cells.
Compare these with the
end products of mitosis.
In both stages of meiosis
chromosomes do line up in
one plane. This is not
shown in these figures in
order to focus on the
important
process
of
crossing over.
To summarise :
 Meiosis comprises two divisions.
 The end products are four haploid cells.
 The unpaired chromosomes differ from each other in the mixture of parental genetic material.
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Meiotic cell division has important implications which cross the boundaries amongst the fields of
histology, embryology and genetics and even have evolutionary consequences.
Histologically, this process is seen during gametogenesis. In routine histological material we have no way
of observing meiosis, but the fact that the four cells so formed are ‘different’ from other body cells has
certain consequences which we study towards the end of the unit.
Embryologically, the union of two haploid gametes leads to the formation of a diploid cell which marks
the beginning of life of the offspring.
Genetically, the process of crossing over and the consequent uniqueness of gametes creates room for an
immense variation among children of the same parents. Consider that even parents have similar mixtures
of genes from their parents, and that this chain of variations can go back a long way into the past.
There are certain genes that do keep together despite such variations, but this leads into a specialised area of genetics
which is beyond the scope of this unit.
The great spectrum of variations thus created, and the fact that small molecular changes (mutations) can
occur during the processes of replication and crossing over are reflected in the concepts of gene pools in
population genetics. These are some of the pillars on which the edifice of evolution stands.
For this unit, we restrict ourselves to the histological and embryological aspects.
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