Cell organelles, histology Cytology, cell junctions

1. M.Sc Hamid AL-Tameemi
Lecture 2

2. Cell Organelles
Cytoplasm of a typical cell contains various structures
with specific functions that are referred to as organelles.

3. The Nucleus
The nucleus constitutes the central, more dense, part of
the cell. It is usually rounded or ellipsoid. Occasionally it
may be elongated, indented or lobed. It is usually 4-10
μm in diameter.
The nucleus contains inherited information that is
necessary for directing the activities of the cell. With the
EM the nucleus is seen to be surrounded by a double
layered nuclear membrane or nuclear envelope.

5. Endoplasmic Reticulum
The cytoplasm of most cells contains a system of
membranes that constitute the endoplasmic reticulum
(ER). The membranes form the boundaries of channels
that may be arranged in the form of flattened sacs (or
cisternae) or of tubules.

6. ER functions
1- Rough ER represents the site at which proteins are
synthesized.
2- Smooth ER is responsible for further processing of
proteins synthesized in rough ER.
3- It is also responsible for synthesis of lipids, specially
that of membrane phospholipids (necessary for
membrane formation).
4- Products synthesized by the ER are stored in the
channels within the reticulum.

7. Ribosomes
Ribosomes are present in relation to rough ER. They may
also lie free in the cytoplasm. They may be present singly
in which case they are called monosomes; or in groups
which are referred to as polyribosomes (or
polysomes).
Each ribosome consists of proteins and RNA
(ribonucleic acid) and is about 15 nm in diameter. The
ribosome is made up of two subunits one of which is
larger than the other.
- Ribosomes play an essential role in protein synthesis.

9. Mitochondria
They are so called because they appear either as granules
or as rods (mitos = granule; chondrium rod). The
number of mitochondria varies from cell to cell being
greatest in cells with high metabolic activity (e.g., in
secretory cells).
The mitochondrion is bounded by a smooth outer
membrane within which there is an inner membrane,
the two being separated by an intermembranous
space. The inner membrane is highly folded on itself
forming incomplete partitions called cristae.

11. It also contains some RNA and DNA: these are believed
to carry information that enables mitochondria to
duplicate themselves during cell division. An interesting
fact, discovered recently, is that all mitochondria are
derived from those in the fertilized ovum, and are
entirely of maternal origin.

12. Golgi Complex
When examined with the EM the complex is seen to be
made up of membranes similar to those of smooth ER.
The membranes form the walls of a number of flattened
sacs that are stacked over one another. Towards their
margins the sacs are continuous with small rounded
vesicles .
Material from ER reaches the Golgi complex through
vesicles.

14. Material synthesized in rough ER travels through
the ER lumen into smooth ER.
1- Some proteins are phosphorylated here.
2- Here sugar residues are added to proteins to form
protein-carbohydrate complexes.
3- Proteolysis of some proteins converts them from
inactive to active forms.
4- various substances are sorted out and packed in
appropriate vesicles.

15. Phagosomes
Solid ‘foreign’ materials, including bacteria, may be
engulfed by a cell by the process of phagocytosis. In this
process the material is surrounded by a part of the cell
membrane. This part of the cell membrane then
separates from the rest of the plasma membrane and
forms a free floating vesicle within the cytoplasm. Such
membrane bound vesicles, containing solid ingested
material are called phagosomes

17. Pinocytotic vesicles
Some fluid may also be taken into the cytoplasm by a
process similar to phagocytosis. In the case of fluids the
process is called pinocytosis and the vesicles formed are
called pinocytotic vesicles.
Exocytic vesicles
exocytic vesicles, and the process of discharge of cell
products in this way is referred to as exocytosis (or
reverse pinocytosis).

19. Secretory granules
The cytoplasm of secretory cells frequently contains
what are called secretory granules. The appearance, size
and staining reactions of these secretory granules differ
depending on the type of secretion. These vesicles are
derived from the Golgi complex.

20. Lysosomes
These vesicles contain enzymes that can destroy
unwanted material present within a cell. Such material
may have been taken into the cell from outside (e.g.,
bacteria); or may represent organelles that are no longer
of use to the cell. Lysosomes belong to what has been
described as the acid vesicle system.

21. Peroxisomes
These are similar to lysosomes in that they are
membrane bound vesicles containing enzymes. The
enzymes in most of them react with other substances to
form hydrogen peroxide which is used to detoxify
various substances by oxidising them.
Defects in enzymes of peroxisomes can result in
metabolic disorders associated with storage of abnormal
lipids in some cells (brain, adrenal).

22. Contacts between adjoining cells
In tissues in which cells are closely packed the cell
membranes of adjoining cells are separated, over most of
their extent by a narrow space (about 20 nm). This
contact is sufficient to bind cells loosely together, and
also allows some degree of movement of individual cells.