The Cell

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The Cell

• A cell may be conveniently describe as a mass of
  protoplasm enclosed within a membrane (or plasma
  membrane) containing a subdivision, the nucleus,
  which is bounded by the nuclear membrane. The
  portion of lying between plasma and nuclear
  membrane is known as the cytoplasm. Within the
  cytoplasm a variety of fine structures called
  organelles may be identified. These are
  specialized structure with individual functions
  and consist of the living materials of the cells.

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• Cell
• Cytoplasm Nucleus
• Organelles Organelles
• Cell membrane Nuclear membrane
• Mitochondria Nucleolus
• Golgi apparatus Chromatin granules
• Centrioles
• microtubules (cytoskeleton)
• cytosols
• Inclusions
• Glycogen
• Secretion granules
• Fat

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Cell membrane

• This is a semi-permeable membrane which
  permits the selective passage of substances to
  and from the cell. The exchange of materials
  through the membrane is due to osmotic pressure
  exerted by the intercellular fluid and
  cytoplasmic ground substance or, by an active
  mechanism. Electron microscopy studies shown that
  the membrane contains three layers which are
  thought to be composed of protein and lipid
  molecules.

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Cytoplasmic Organelles

• Endoplasmic reticulum
• Cytoplasm is organized into a network of fine
  branching tubules known as the endoplasmic
  reticulum (ER). These tubules are lined by a
  membrane which in places is coated granules of
  ribo-nucleoprotein (ribosomes), and is known as
  rough or granular ER. It is associated with
  protein synthesis. Parts of the membrane which
  are not coated with ribosomes are called smooth
  or agranular ER and are thought to associated in
  some cells with fats and similar substances. This
  ER can not be resolved by light microscopy.

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Golgi apparatus

• This is specialized area of smooth
  endoplasmic reticulum comprising membranous canal
  and vacuoles. It may by distinguished by its
  selective reaction with silver salts and osmium
  tetroxide. Secretory products are concentrated
  around this area where they may be possibly
  combine with synthesized carbohydrate components.

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Mitochondria

• These small filamentous or granular bodies may
  be distributed evenly through out the cytoplasm
  or accumulated in selected sites according to
  cell type. The number of mitochondria may be vary
  large as much as 2500 have been found liver
  cells. Mitochondria vary in length up to 7 µm and
  are between 0.5-1.0 µm in diameter. Mitochondria
  have been described as a power house of the cell
  and appear to be concerned with cell respiration
  and enzymatic activity. They are rapidly affected
  by autolysis being some of the first structure to
  disappear after death the cell. Acetic acid
  causes destruction and distortion of mitochondria
  and should be avoided in fixing solution. EM
  shows these organelles bound double membranes.
  The innermost is reflected to run across the
  inside of mitochondria at several point to form a
  shelf-like cristae, and these shape varies in
  cells of different function. This can also be
  demonstrated in fixed and unfixed tissue using
  special technique.

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Lysosome

• A minute spherical organelle with a diameter of
  about 0.25 µm. Lysosomes are bounded by a single
  membrane and contain hydrolytic enzymes, i.e.
  enzyme which break large complex molecules into
  smaller molecules. Rupture of the lysosome
  membrane release the enzyme and causes eventual
  destruction of the cells. This process is known
  as autolysis. Lysozymes are abundant in
  leucocytes and macrophages in which they are
  brought to play an important part in phagocytosis
  (intracellular destruction) of bacteria and
  nutrient particles.

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Centrosome

• The cell centre or centrosome is present in all
  cells although not readily visible except during
  the cell division. It is seen in sections as
  clear area of cytoplasm less than 1.0 µm
  diameter, often lying in a concavity of the
  nucleus, and containing two dark dots, the
  centrioles. E. microscopy reveals the centrioles
  to be short cylindrical bodies whose walls are
  composed fine fibres arranged longitudinally like
  bundle of twigs. Centrioles are thought to be
  associated with the formation of fibrillary
  materials e.g. cilia, the hair like process which
  extend from the parted centrioles upon which
  chromosomes arrange themselves during cell
  division.

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Cytoplasmic inclusions

• Non living substances that may be seen in the
  cytoplasm of cells are referred to as inclusions.
  They usually consist of stored nutrients,
  materials produced by the cell, or ingested
  particles. Common seen inclusions are
• 1.Glycogen
• Accumulation of glycogen are stored in
  the cytoplasm of liver cells and skeleton muscle.
  In stained section it is seen either as fine
  granules or as larger amorphous masses.
• 2.Fat
• Fat is generally stored in fat cells but
  it may also occur normally or pathologically in
  other cells. It accumulates in the form of
  minutes globules which tend to fuse together to
  form larger globules, often distending the
  cytoplasm and displacing the nucleus to the
  periphery
• of the cells.

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• 3. Secretion granules
• These are product of cell synthesis and are
  found in the cytoplasm of specialized cells which
  have a secretory function. They are dispersed
  through out cytoplasm as small as globules which
  on fixation usually become coagulated to form
  granules.
• 4. Pigments
• They are frequently seen in cytoplasm of
  cells and may be either endogenous or exogenous
  in nature. Endogenous pigments such as melanin
  and haemosiderin are produced in the body.
  Exogenous pigments are particles of foreign
  matter such as coal which are ingested by
  phagocytosis and absorbed.
• Artificial pigments produced as a result of
  fixation or precipitation of the staining
  solutions may be present. Such pigments can be
  easily identified and removed.

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Nucleus

• Nuclear membrane
• The nucleus contains most of the genetic
  material of the cell. It is bounded by two
  membrane each rather similar to the cytoplasmic
  membrane.
• Chromatin
• Aggregation of material with an affinity for
  basic dyes are scattered through out the nucleus
  these are known as chromatin granules. The
  intense staining reaction of these granules and
  of the chromosome which appear during cell
  division is due to nucleoprotein content.
  Nucleoprotein is composed of basic proteins and
  nucleic acid and the chief nucleic acid present
  in chromatin is deoxyribonucleic acid (DNA).

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• Nucleolus
• This is a small spheroid body present within the
  nucleus of most cells. It contains high
  proportion of ribonucleic acid (RNA) and it is
  thought be concerned with the synthesis of
  proteins.
• Chromosomes
• These are small thread-like bodies which are seen
  within the nuclei during cell division. Each
  chromosome has a bifid structure formed by two
  chromatids lying side by side and linked at one
  point, the centromere.
• Normal somatic (body) cells in man contain 46
  chromosomes arranged in pairs, one of each pair
  derived from father and other from the mother.
  Because of this pairing they are known as the
  diploid set and consist of one pair of sex
  chromosomes and 22 pair of somatic chromosomes
  (autosomes). The sex chromosomes in the female
  are similar to each other and are designated by
  the symbol XX whilst in the male they
  dissimilar and are designated XY

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• The mature female and male germ cells, namely the
  ovum spermatozoon, contain only one set up of
  chromosomes, i.e. 23 and these are known as
  haploid set.
• The chromosomes of the ovum consist of 23
  autosomes and 1 X chromosomes. The chromosomes
  of spermatozoon consist of 22 autosomes and 1 sex
  chromosome which may X or Y, as half of the
  spermatozoon contain an X chromosome and the
  other half Y chromosome.
• On fusion of the ovum and spermatozoon, the
  diploid set of chromosomes is formed and the sex
  of the resulting embryo is determined according
  to the sex chromosomes carried by the
  spermatozoon.
• Cytoskeleton (cell framework)
• The internal network of protein filaments and
  microtubules in an animal or plant cell that
  controls the cells shape and movement. These
  microtubules are 180 Å in thickness and play in
  cell division (mitotic spindles)
• Cytosols (fluid part of cytoplasm)
• The fluid component of a cells cytoplasm
  excluding organelles and other structures.

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