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HISTOLOGY AND HEMATOLOGY
CYTOLOGY
Learning objectives
i.
At the head of this section you should;
ii.
Know the fine structures of the cell.
iii.
Know the valius organell and their functions.
The cell is the smallest unit of living matter of a multi-cellular living organism. Shape,
structure and size of cells vary with the specific functions of the cells. There are spherical,
oval, columnar, cuboidal and squamous cells. Some cells are spindle shaped; others are star
shaped (stellate) with numerous cytoplasmic processes. Changes in activity are associated
frequently with changes in shape (e.g., from squamous to cuboidal or cuboidal to
columnar). Pronounced differences in cell size exist, varying from cells observable only at
the highest level of magnification of the light microscope (LA1) (e.g., microglial cells) to
those visible with the naked eye, such as the mature oocytes.
STRUCTURAL ORGANIZATION OF THE CELL
The various functions of the organism are reflected in the structural organization of the
cells that perform these functions. Thus, cells whose primary function is protein synthesis
possess
a
well-developed
rough-surfaced
endoplasmic
reticulum
(ER)
and-or
polyribosomes and those with high secretory activity characteristically have a large Golgi
apparatus. However, in spite of these differences, all cells have the same general structural
organization, consisting of a nucleus and surrounding cytoplasm in which are found the
various organelles (Fig. 1-1). The cell is the smallest unit of living matter of a multi-cellular
living organism. Shape, structure and size of cells vary with the specific functions of the
cells. There are spherical, oval, columnar, cuboidal and squamous cells. Some cells are
spindle shaped; others are star shaped (stellate) with numerous cytoplasmic processes. Changes
in activity are associated frequently with changes in shape (e.g., from squamous to cuboidal
or cuboidal to columnar). Pronounced differences in cell size exist, varying from cells
observable only at the highest level of magnification of the light microscope (LA1) (e.g.,
microglial cells) to those visible with the naked eye, such as the mature oocyte
Nucleus
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Because the structural organization of the nucleus varies during the different phases of cell
life, the following description applies only to the nucleus as it appears between two cell
divisions, i.e., the interphase nucleus during the duplication phase.
Nuclear configuration and size vary considerably in different cell types. The nucleus is
usually spherical in cuboidal cells, whereas an elongated ovoid nucleus is common to
columnar and spindle-shaped cells. Other cells have bean- or kidney-shaped nuclei,
common to monocytes, or multi lobulated and horseshoe-shaped nuclei, characteristic of
neutrophilic (polvmorphonuclear) leukocytes.
Although most cells of the mammalian organism have only one nucleus, it is not unusual
to find bi- or even multinucleated cells such as those in liver (hepatocytes) and bone
(osteoclasts). The nucleus consists of chromatin embedded in the nucleoplasm and is
bounded by the nuclear envelope. One or more nucleoli may be present, depending on the
degree of differentiation.
NUCLEAR ENVELOPE
The nuclear envelope cannot always be clearly distinguished with the light microscope. The
nuclear envelope is pierced by numerous circular "pores" around which are fused the two
nuclear membranes. The pores are closed by-a thin septum. It is believed that the pores play
a role in the exchange of material between the nucleus and the cytoplasm.
CHROMATIN
Chromatin is dispersed within the nucleo-plasm. It is deoxyribonucleic acid (DNA) combined
with histones and other structural proteins. Chromatin forms highly coiled, long strands, too
fine to be detected with the light microscope. However, it is designated heterochromatin
when portions of the chromatin strands condense or clump in a given area, and is readily
detected with the light microscope as dark clumps or particles located on the inside of the
nuclear envelope or in the vicinity of the nucleolus. The remaining empty-appearing spaces
are filled with diffuse chromatin or euchromatin .There are two sex chromosomes designated
X chromosome and Y chromosome. Cells from male animals have one X and one Y
chromosome, whereas cells from females have a pair of X chromosomes.
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Granular
vesicle
Golgi
(secretory
apparatu
granule)
s Nuclear pore
Nuclear
Centriol
e envelope
•
Mitochondri
on
Basement membrane
Granular
endoplasmic
reticulum
FIG. 1-1.
Schematic drawing of the general organization of a cell.
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NUCLEOLUS
The nucleolus is a conspicuous, spherical, basophilic body that functions in the synthesis and
assembly of RNA It consists of anastomosing strands of compact filaments associated with
RNA granules the size of ribosomes in a meshwork of filaments.
Cytoplasm
Cell organelles and cell inclusions are embedded in the cytoplasmic matrix or hyaloplasm. In
spite of its structure less appearance, the cytoplasmic matrix is an important part of the cell
because it consists of polypeptides, proteins, enzymes, ions and water.
ORGANELLES
The organelles are variously organized structures that function in cell metabolism. They are;(1)
The cell membrane, (2) Granular endoplasmic reticulum, (3) a granular endoplasmic reticulum,
(4) Golgi apparatus.
(5) Mitochondria. (6) Lysosomes (7) Centrioles.
The cell inclusions are:(1) Microfilaments, (2} microtubules, (3) Glycogen, (4) Lipid, (5) Yolk.
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