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DEVELOPMENT
OF SALIVARY GLANDS
OF SALIVARY GLANDS
· The ontogenetic development of glandular tissue –
Epithelial-Mesenchymal interactions.
Epithelial-Mesenchymal interactions.
· The mesenchyme – role in both the initiation
and growth of the glandular bud & also the cytodifferentiation of
glandular cells.
and growth of the glandular bud & also the cytodifferentiation of
glandular cells.
·
The functional glandular tissue develops as
an epithelial outgrowth of the buccal epithelium, which invades the underlying
mesenchyme.
The functional glandular tissue develops as
an epithelial outgrowth of the buccal epithelium, which invades the underlying
mesenchyme.
·
Individual salivary glands – proliferation of oral
epithelial cells – form a focal thickening that grows into the
underlying ectomesenchyme.
Individual salivary glands – proliferation of oral
epithelial cells – form a focal thickening that grows into the
underlying ectomesenchyme.
·
Continued growth formation of a small bud connected
to the surface by a trailing cord of epithelial cells.
Continued growth formation of a small bud connected
to the surface by a trailing cord of epithelial cells.
·
The mesenchymal cells condense around the bud.
The mesenchymal cells condense around the bud.
·
Clefts develop in the bud, forming two or more new
buds.
Clefts develop in the bud, forming two or more new
buds.
·
A continuation of this process, called branching
morphogenesis successive generations of the buds and a hierarchic ramification
of the gland.
A continuation of this process, called branching
morphogenesis successive generations of the buds and a hierarchic ramification
of the gland.
·
The connective tissue stroma (capsule & septa)
& blood vessels from the mesenchyme.
The connective tissue stroma (capsule & septa)
& blood vessels from the mesenchyme.
·
Signaling molecules – members of the FGF protein family;
Shh; TGF-β; & their receptors – major role in the development of branches.
Signaling molecules – members of the FGF protein family;
Shh; TGF-β; & their receptors – major role in the development of branches.
·
As the epithelial bud forms, those portions of the
bud closest to the stomodeum differentiate into the main excretory duct
of the gland – the most distal portions of the bud form the secretory
end pieces or acini.
As the epithelial bud forms, those portions of the
bud closest to the stomodeum differentiate into the main excretory duct
of the gland – the most distal portions of the bud form the secretory
end pieces or acini.
·
The origin of the epithelial buds – ectodermal –
parotid & minor salivary glands; and endodermal – submandibular and
sublingual glands.
The origin of the epithelial buds – ectodermal –
parotid & minor salivary glands; and endodermal – submandibular and
sublingual glands.
·
The differential contraction of actin filaments at
the basal & apical ends of the epithelial cells – provides the physical
mechanism, underlying the formation of the cleft.
The differential contraction of actin filaments at
the basal & apical ends of the epithelial cells – provides the physical
mechanism, underlying the formation of the cleft.
·
The deposition of extracellular matrix components
within the clefts, serves to stabilize them.
The deposition of extracellular matrix components
within the clefts, serves to stabilize them.
·
The development of a lumen within the branched
epithelium – occurs 1st in distal end of the main cord & in branch cords.
The development of a lumen within the branched
epithelium – occurs 1st in distal end of the main cord & in branch cords.
·
Then in the proximal end of the main cord, &
finally in the central portion of the main cord.
Then in the proximal end of the main cord, &
finally in the central portion of the main cord.
·
The lumina form within the ducts before they develop
within the terminal buds.
The lumina form within the ducts before they develop
within the terminal buds.
·
Studies – the lumen formation – apoptosis of
centrally located cells in the cell cords.
Studies – the lumen formation – apoptosis of
centrally located cells in the cell cords.
·
After the lumen is developed in the terminal buds,
the epithelium – 2 layers of cells.
After the lumen is developed in the terminal buds,
the epithelium – 2 layers of cells.
·
The cells of the inner layer – finally
differentiate into the secretory cells of the mature gland (either mucous or
serous).
The cells of the inner layer – finally
differentiate into the secretory cells of the mature gland (either mucous or
serous).
·
Cells of the outer layer –
contractile myoepithelial cells around the secretory end pieces and
intercalated ducts.
Cells of the outer layer –
contractile myoepithelial cells around the secretory end pieces and
intercalated ducts.
·
As the epithelial parenchymal components increase in
size and number – associated mesenchyme (connective tissue) is diminished.
As the epithelial parenchymal components increase in
size and number – associated mesenchyme (connective tissue) is diminished.
·
A thin layer of connective tissue remains,
surrounding each secretory end piece & duct of the adult gland.
A thin layer of connective tissue remains,
surrounding each secretory end piece & duct of the adult gland.
·
Thicker partitions of connective tissue (septa) –
continuous with the capsule, invest the excretory ducts & divide the gland
into lobes and lobules.
Thicker partitions of connective tissue (septa) –
continuous with the capsule, invest the excretory ducts & divide the gland
into lobes and lobules.
·
The parotid glands – corners of the stomodeum – 4th
to 6th week of embryonic life.
The parotid glands – corners of the stomodeum – 4th
to 6th week of embryonic life.
·
The submandibular glands – floor of the mouth at the
end of the 6th week or the beginning of the 7th week in utero.
The submandibular glands – floor of the mouth at the
end of the 6th week or the beginning of the 7th week in utero.
·
The sublingual gland – lateral to the submandibular
primordium at 8th week, IUL
The sublingual gland – lateral to the submandibular
primordium at 8th week, IUL
·
The minor salivary glands bud from the buccal
epithelium – 8 to 12 weeks of the embryonic life.
The minor salivary glands bud from the buccal
epithelium – 8 to 12 weeks of the embryonic life.
·
The cells of the secretory end pieces & ducts
attain maturity – last 2 months of gestation.
The cells of the secretory end pieces & ducts
attain maturity – last 2 months of gestation.
·
The glands continue to grow post-natally, upto 2
years of age.
The glands continue to grow post-natally, upto 2
years of age.
STAGES OF
DEVELOPMENT: 6 stages
DEVELOPMENT: 6 stages
STAGE I: BUD
FORMATION:
FORMATION:
·
Induction of Oral Epithelium by Underlying
Mesenchyme – results in tissue thickening and bud formation.
Induction of Oral Epithelium by Underlying
Mesenchyme – results in tissue thickening and bud formation.
·
The growing bud is separated from the condensation
of mesenchyme by a basal lamina.
The growing bud is separated from the condensation
of mesenchyme by a basal lamina.
STAGE II: FORMATION
AND GROWTH OF EPITHELIAL CORD:
AND GROWTH OF EPITHELIAL CORD:
·
A solid cord of cells – from epithelial bud by cell
proliferation.
A solid cord of cells – from epithelial bud by cell
proliferation.
·
Condensation and proliferation – in the surrounding
mesenchyme, closely associated with the epithelial cord.
Condensation and proliferation – in the surrounding
mesenchyme, closely associated with the epithelial cord.
·
The basal lamina is found between the cord
and the mesenchyme.
The basal lamina is found between the cord
and the mesenchyme.
·
It is composed of glycosaminoglycans, collagen, and
glycoprotein.
It is composed of glycosaminoglycans, collagen, and
glycoprotein.
·
It plays a role in influencing morphogenesis and
differentiation of the salivary glands throughout their development.
It plays a role in influencing morphogenesis and
differentiation of the salivary glands throughout their development.
STAGE III: INITIATION
OF BRANCHING IN TERMINAL PARTS OF EPITHELIAL CORD AND CONTINUATION OF GLANDULAR
DIFFERENTIATION:
OF BRANCHING IN TERMINAL PARTS OF EPITHELIAL CORD AND CONTINUATION OF GLANDULAR
DIFFERENTIATION:
·
The epithelial cord proliferates rapidly &
branches into terminal bulbs (presumptive acini).
The epithelial cord proliferates rapidly &
branches into terminal bulbs (presumptive acini).
·
The growth in length of the solid epithelial cords
& the differentiation of the berry-like, terminal bulbs occurs in this
stage.
The growth in length of the solid epithelial cords
& the differentiation of the berry-like, terminal bulbs occurs in this
stage.
STAGE IV: DICHOTOMOUS
BRANCHING OF EPITHELIAL CORD AND LOBULE FORMATION:
BRANCHING OF EPITHELIAL CORD AND LOBULE FORMATION:
·
The branching continues at the terminal portions of
the cord – extensive tree-like system of bulbs.
The branching continues at the terminal portions of
the cord – extensive tree-like system of bulbs.
·
Connective tissue differentiates around the branches
– extensive lobulation.
Connective tissue differentiates around the branches
– extensive lobulation.
·
The glandular capsule – from mesenchyme and
surrounds the entire glandular parenchyma.
The glandular capsule – from mesenchyme and
surrounds the entire glandular parenchyma.
STAGE V:
CANALIZATION OF PRESUMPTIVE
DUCTS:
CANALIZATION OF PRESUMPTIVE
DUCTS:
·
Canalization of the epithelial cord – a hollow tube
or duct, by the 6th month in all three major salivary glands.
Canalization of the epithelial cord – a hollow tube
or duct, by the 6th month in all three major salivary glands.
·
Two main theories to explain the mechanism of
canalization:
Two main theories to explain the mechanism of
canalization:
o Different
rates of cell proliferation between the outer and inner layers of the epithelial
cord.
rates of cell proliferation between the outer and inner layers of the epithelial
cord.
o Fluid
secretion by the duct cells increases the hydrostatic pressure and produces a
lumen within the cord.
secretion by the duct cells increases the hydrostatic pressure and produces a
lumen within the cord.
STAGE VI: CYTODIFFERENTIATION:
·
Final stage of salivary gland development –
histodifferentiation of the functional acini and intercalated ducts.
Final stage of salivary gland development –
histodifferentiation of the functional acini and intercalated ducts.
·
Mitotic activity shifts from the epithelial cord to
the terminal bulb portion.
Mitotic activity shifts from the epithelial cord to
the terminal bulb portion.
·
The initiation of cytodifferentiation of salivary
gland acinar cells is dependent on pre-programmed development occurring in
early stages of morphogenesis.
The initiation of cytodifferentiation of salivary
gland acinar cells is dependent on pre-programmed development occurring in
early stages of morphogenesis.
·
Epithelial-mesenchymal contact – for
cytodifferentiation.
Epithelial-mesenchymal contact – for
cytodifferentiation.
·
Once this contact takes place, exocrine cell
differentiation occurs in the absence of mesenchyme.
Once this contact takes place, exocrine cell
differentiation occurs in the absence of mesenchyme.
·
There is a “partial coupling” of the
independently controlled processes of gland morphogenesis and
cytodifferentiation.
There is a “partial coupling” of the
independently controlled processes of gland morphogenesis and
cytodifferentiation.
·
Full differentiation of functional secretory
components – apparent at birth – not complete until the onset of a solid diet
& the presence of masticatory stimuli.
Full differentiation of functional secretory
components – apparent at birth – not complete until the onset of a solid diet
& the presence of masticatory stimuli.