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Histology

Cells

basic structural and functional units of life.
All living organisms are cellular in nature,
e.g.: amoebas with only one cell
or human, animals and big plants
with many cells (multicellular).*There are 50
to 100 trillion cells in the human body.

Plasma Membrane

outer thin and flexible membrane of the cell which separates
the intracellular from extracellular compartment (fluid). Heads/hydrophilic
Tails/Hydrophobic

Integral Proteins

Are the most abundant proteins in
the membrane, most extend entirely
through the membrane (transmembrane)
but some protrude from one side of the
membrane. Could act as receptors.

Peripheral Proteins

are mainly on the cytoplasmic side.
They support the cytoplasmic side of
the membrane by a network of filaments.

Glycocalyx

is a short chain of carbohydrate (sugars)
projected out from the external surface
of glycoproteins or glycolipids.
This functions in cell-to-cell binding and
recognition.
Found on the inner surface of small intestine

Functions of the Plasma Membrane

1- Serves as an external cell barrier against substances
and forces outside the cell.
2- Externally facing proteins act as receptors (for hormones,
neurotransmitters etc.) and in cell to cell recognition.
3- Acts in transport of substances into or out of the cell.

Passive Process

substances can pass freely through the lipid bilayer
down their concentration gradient i.e.: from more concentrated region
to the less concentrated region. No energy (ATP) is needed.

Diffusion

movement of small, uncharged molecules like oxygen, Co2
and fat soluble molecules across the membrane

Active Process

substances move against a concentration gradient from
a lower to a higher concentration, ATP is needed.

Active Transport

most larger water-soluble or charged
molecules, such as glucose, amino acids and ions are transported
by a pump or carrier and involve the integral proteins.

Bulk Transport

Large particles and macromolecules pass
through the membrane by this mechanism. There are generally two types: exocytosis and endocytosis.

Exocytosis

Membrane-lined cytoplasmic vesicles fuse with the plasma membrane and release their contents to the outside
of the cell. e.g.: mucus and protein secretions from the glands in the body.
Proteins extending from the vesicle membrane vSNAREs, bind with plasma membrane proteins, the tSNAREs (t for target), this causes the lipid layers of the vesicle and cell membrane to join together.

Endocytosis

Brings large molecules into the cell, through an initial infolding part of the plasma membrane that encloses them to form cytoplasmic vesicles. Clathrin protein, found on the cytoplasmic side of the infolding is responsible for deforming the membrane.
There are 3 types:
phagocytosis, pinocytosis and receptor-mediated endocytosis

Phagocytosis (cell eating)

Here, parts of plasma membrane form pseudopodes and flow around large molecules such as bacteria or cellular debris and engulf it. By this way, a membranous vesicle, called a phagosome is formed. Phagosomes mostly fuse to the lysosomes for enzymatic break down of phagosomal contents. White blood cells have such phagocytic activity.

Pinocytosis (cell drinking)

Is fluid phase endocytosis.
In this phase, a small infolding of the plasma membrane surrounds a small quantity of extracellular fluid containing dissolved molecules. This is the main function of cells lining the small intestine, absorption of the nutrients.

Receptor mediated endocytosis:

Is a selective mechanism. Specific molecules such as insulin and other hormones, enzymes and low density lipoproteins (LDL, molecules that carry cholesterol in the blood to the body's cells) are brought into the cells by first attaching to a receptor on the membrane before being taken into the cells in a protein coated vesicle. Contents of the vesicles are released by binding to lysosomes and the receptors are recycled back to plasma membrane. Viruses and some toxins use the same mechanism to enter the cells.

Familial hypercholesterolemia

Is an inherited disease in which the cells lack the receptors that bind to cholesterol binding LDLs. As a result, cholesterol cannot enter the cells and builds up in the blood, causing hypercholesterolemia and atherosclerosis which lead to stroke or myocardial infarction.

Cytoplasm

cellular region between the nucleus and plasma membrane.
It consists of cytosol, or cytoplasmic matrix which is a viscous fluid containing water, ions and enzymes, inclusions containing stored nutrients and pigments and organelles.

Ribosomes

Are dark staining granules with no membrane. Are site of protein production. They consist of two subunits: protein and ribosomal RNA (rRNA, ribonucleic acid). Free ribosomes make the protein used in the cytosol. They attached on the surface of rER make the proteins used for cell membrane or exported out of the cell. Amino acids on them are linked together to form protein. This process is called Translation and is dictated by DNA of the nucleus. Such instructions are carried to them by messengers called messenger RNA (mRNA).

Rough Endoplasmic Reticulum (rER):

Is a ribosome-studded system of membrane-walled envelopes in cytosol, called cisternae.Ribosomes on this make proteins which enter the cisternae and are secreted
by the cell in vesicles. Ribosomes also make the proteins of the cell membrane.

Smooth Endoplasmic Reticulum (sER):

Is a network of membranous system of sacs and tubules in the cytosol. It has no ribosomes and is involved in the synthesis of lipids and steroids, lipid metabolism and drug detoxification.

Golgi Apparatus

Is a stack of 3-10 disc-shaped envelopes or cisternae which are bound by membrane.
Cisternae have a cis (convex) and a trans (concave) face.
It sorts the products of rER and packs them in membrane bound vesicles and sends
them to their proper destination.
Secretory granules and lysosomes also arise from this.

Mitochondria

Are rod like organelles covered by two membranes in the cytoplasm.The inner membrane is folded into projections called cristae. Mitochondria are the main energy generator of the cell and are the main site of ATP synthesis.

Lysosomes

Are spherical, membrane-walled sacs containing digestive enzymes called Acid hydrolases.They are site of intracellular digestion and they destroy (digest) deteriorated organelles and substances brought into the cells by vesicles.They fuse with phagosomes and empty their enzymes into phagosomes, breaking down their contents. Phagocytic cells have a lot of these.

Tay sachs

is an inherited disease,
infants lack specific enzymes in the
lysosomes responsible for break down of
certain glycolipids. As a result, glycolipids
accumulate in the cell membrane specially
on neurons, resulting in mental retardation,
blindness, spastic movements and death
within 1.5 years from birth.

Peroxisomes

Are membrane-walled, enzyme-containing sacs.
They contain oxidase and catalase enzymes. Oxidases use oxygen to neutralize aggressively reactive substances called free radicals, by converting them to hydrogen peroxide. Hydrogen peroxide, although reactive and dangerous, it is converted to oxygen and water by catalases which break down poisons like alcohol, phenol and formaldehydes that have entered the body. Liver and kidney have many.

Cytoskeleton

Is a network of rods running throughout the cytosol to support the cellular structure
and generates movements of the cell. There are 3 types:
Microtubules, microfilaments and intermediate filaments

Microtubules

Are cylindrical structures made of tubulin proteins. They radiate out from the centriosome region close to the nucleus. They give the cell its shape and they organize the distribution and transport of various organelles within the cytoplasm. Secretory granules, lysosomes etc., attach to microtubulres and pulled by motor proteins called: kinesins and dyneins. They also form the centrioles. They are labile.

Microfilaments

Are fine filaments of contractile protein called actin. They are labile. Actin interacts with another protein called myosin, and generates contractile forces within the cell. It is involved in muscle contraction, and other types of cellular movements such as amoeboid movements and extension of pseudopods.

Intermediate Filaments

are tough insoluble and stable protein fibers
which act to resist tension placed on the cell.

Centrosome

is a spherical structure in the
cytoplasm near the nucleus. It consists of
an outer cloud of protein called: matrix
and an inner pair of centrioles.
Matrix protein is involved in the elongation
of microtubules and mitotic spindle of
microtubules radiates from it in dividing
cells.

Cytoplasmic Inclusions

Impermanent structures in the cytoplasm
such as lipid droplets and glycogen
containing glycosomes.

Centrioles

are in the core of centriosome.
These are paired cylindrical bodies
perpendicular to one another and each
composed of nine triplets of microtubules.
They organize a microtubule network
during mitosis to form the spindle and asters. They also form the bases of cilia and flagella

Tissues

are collection of structurally similar cells with related function.

There are four basic tissues in the body:

1- Epithelial tissue: for covering
2- connective tissue: for support
3- muscle tissue: for movement
4- nerve tissue: for control

Epithelial tissue and glands

sheets of cells that cover body surfaces and cavities.
Their function is to protect the body (such as the skin) or sensory reception such as the olfactory epithelial cells, absorption, such as the internal covering cells of
the intestine, ion transport and filtration, like the cells covering various tubules in the kidney. Glands are also covered internally by epithelial cells which function in secretion of their products.

Special Characteristics of Epithelial Cells

High cellularity
Little extracellualr material,
They have special cell junction ,
They are avascular but are innervated
They have the ability to regenerate

Classifications of Epithelial Cells

1- Squamous (flat)
2- Cuboidal (cube like)
3- Columnar (tall and rod-like)
&
1- Simple (only one layer), 2- Stratified (multiple layers)

Simple Squamous Epithelial Cells

Location:
Serous membranes: (Peritoneum, pericardium and pleura),
Kidney corpuscles, air sacs of lungs, lining of heart

Simple Cuboidal Epithelium

Simple Columner Epithelium

Pseudostratified columnar epithelium

Nonkeratinized squamous epithelium

Stratified keratinizing epithelium (skin)

Stratified Columner

Stratified Cuboidal

Transitional Epithelium

Glands

are one or more cells specialized to secret a
product (mostly proteins) by exocytosis.

Classifications:
exocrine (external secretion)
or endocrine (internal secretion), depending on where
they release their products and as unicellular or
multicellular on the basis of their cell number.

Endocrine or ductless glands

secrete hormones, which enter the circulation and reaches the target tissue to have their effects,an example is the pancreas which has an endocrine as well as exocrine parts. The endocrine part produces insulin and glucagon and other hormones. The exocrine part secretes enzymes responsible for food breakdown and digestion.

Exocrine glands secrete

their products onto body surfaces or body cavities. Goblet cells are examples of mucus secreting unicellular exocrine glands. Multicellular exocrine glands are classified by the structure of their ducts as simple, or compound and by the structure of their secretory units as tubular, alveolar (acinar), or tubuloalveolar.

Epithelial surface features

Some apical epithelial cells might have microvilli.
Microvilli in intestinal mucosa increase the absorption area.
Apical epithelial cells might have cilia. Cilia contains a core of nine pairs of microtubules encircling one middle pair Cilia beat to move fluid such as mucus in the respiratory air ways. Microtubules in cilia generate the ciliary movements.

Features of the lateral cell surfaces

1- Adhesion proteins link plasma membranes of adjacent cells
2- Contours of adjacent cell membranes
3- Special cell junctions:
tight junctions, adhering junctions and desmosomes

Tight junctions (zona occludens):

1- Found close to the apical region of the cells
2- Close off the extracellular spaces
3- Prevent molecules from passing between cells of epithelial tissue
4- Some proteins in plasma membrane of adjacent cells are even fuse together due to this junction.

Desmosomes :

1- two disc-like plaques connected across intercellular space
2- Plaques of adjoining cells are joined by proteins called cadherins
3- Proteins interdigitate into extracellular space
4- Intermediate filaments insert into plaques from cytoplasmic side

Gap junctions:

1- passageway between two adjacent cells
2- they let small molecules move directly between neighboring cells
3- cells are connected by hollow cylinders of protein

Features of the basal epithelial surface:

1- Epithelial cells lie on a protein sheet called basal lamina.
2- It acts as a filter and base on which regenerating epithelial cells can grow.
3- The basal lamina and some underlying reticular fibers , form the ticker basement membrane.

Connective tissue

*Is the most diverse and abundant tissue in the body.
*Cells in this are separated by large amount of extracellular matrix.

Matrix (except blood)

consists of fibers, ground substance and tissue fluid.

Interstitial fluid (tissue fluid)

is watery, occupying extracellular matrix
and derives from the blood.

Ground substance

is viscous and consists of sugar and protein molecules, these are made and secreted by fibroblasts.

fibers in the matrix of connective tissue are produced by

fibroblasts

4 basic subtypes of connective tissues

1- Connective tissue proper
2- Cartilage
3- Bone tissue
4- Blood

the mesenchyme

Connective tissues have a common embryonic origin

Loose areolar connective tissue

Most widespread connective tissue,
surrounds capillaries and underlies
most epithelia.

Loose Areolar Connective tissure Functions:

1- supports and binds other tissues with its
fibers (collagen, elastic and reticular);
2- holds tissue fluid in its jelly-like matrix;
3- fights infections with its blood-derived
defense cells (macrophages, neutrophils,
plasma cells); Mast cells (in inflammation)
4- stores nutrients and fat in fat cells;

Fibroblasts

spindle or star shape cells, produce the fibers of the connective tissues

Collagen Fibers

the strongest and most abundant type of fiber and allow connective tissue to withstand tension. They are made up of thinner fibers called: unit fibrils which are composed of still thinner strands that are strongly cross-linked together.

Scurvy/vitamin C Deficiency

Vitamin C, in citrus fruits is necessary for proper cross-
linking of the collagen fiber molecules. Its deficiency leads to weakening of collagen and connective tissues leading to blood vessel rupture (bleeding from the gums), poor healing etc

Elastic Fibers

thin and long fibers, have elastin which allows them to function like rubber bands.

Reticular Fibers

a special type of collagen unit fibrils. Are short and cluster into networks

Macrophages

big nonspecific phagocytic cells with pseudopods in our body. They eat bacteria and foreign bodies and cell debris.

Plasma cells

antibody producing cells. Abs bind to foreign molecules which marks them for destruction by macrophages.

Neutophils, lymphocytes and eosinophils

these are white blood cells that migrate from blood into tissues. They have various functions, e.g.: neutrophil helps in phagocytosis.

Mast Cells

oval cells near blood vessels. They contain secretory granules containing mediators for inflammation such as histamine (vasodilator) or heparin etc.

Fat Cells

they have a giant lipid droplet in their cytoplasm, they store energy

Adipose Connective Tissue

Reticular Connective Tissue

Dense connective tissue:

Contains large amount of collagen fibers
which resists tremendous pulling forces.

Dense irregular connective tissue

Collagen fibers run in various directions.
Are found in the dermis and organ capsules.

Dense regular connective tissue:

Collagen fibers run in parallel directions
and are separated by rows of fibroblasts.
This tissue is subject to higher tension and
is found in tendons, ligaments and fascias.

Cartilage

Firm, flexible tissue
Contains no blood vessels or nerves
Matrix contains up to 80% water
Cell type - chondrocyte

Types of Cartilage

1- Hyaline cartilage
2- Elastic cartilage
3- Fibrocartilage

Hyaline cartilage:

Elastic Cartilage

Fibro Cartilage

...

Membranes

consist of combine epithelial tissues plus underlying connective tissues. They cover broad areas within body.

Cutaneous membrane (skin):

is dry and covers the body surface

Mucous membranes:

are moist and line the hollow internal organs

Serous membranes:

are slippery and line the body cavities such as: pleura, peritoneum and pericardium.

Skeletal Muscle tissue

Cardiac Muscle tissue

Smooth Muscle tissue

Neurons:

are excitable cells and transmit electrical signals.

Supporting cells:

(neuroglial cells) don't conduct impulses.

Inflammatory response:

non-specific, local response, limits damage to the injury site

Immune response:

takes longer to develop and very specific, destroys particular
microorganisms at site of infection

Acute inflammation

heat, redness/swelling, and pain.
Chemicals signal nearby blood vessels to dilate.
Histamine increases permeability of capillaries.

Edema:

accumulation of fluid
Helps dilute toxins secreted by bacteria
Brings oxygen and nutrients from blood
Brings antibodies from blood to fight infection

Regeneration

: replacement of destroyed tissue with same type of tissue
(epithelia and blood tissue are only tissues capable of doing this.)

Fibrosis

proliferation of fibers: fibrous or scar tissue
*Tissue repair in a skin wound involves both types.

At the end of second month of development

Primary tissue types have appeared
Major organs are in place

In adulthood:

Only a few tissues regenerate
Many tissues still retain populations of stem cells

With increasing age:

Epithelia thin
Collagen decreases
Bones, muscles, and nervous tissue begin to atrophy
Poor nutrition and poor circulation - poor health of tissues

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