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Hair
follicle dermal papilla
The dermal papilla arises from the cells of the dermis and remains
continuous with the connective tissue sheath of the follicle.
In the larger hair follicles, as in man and in the guinea pig,
the connective tissue sheath is relatively prominent, but mouse
hairs, which are relatively fine, these sheaths are very thin.
The sheath consists structurally of an outer layer of longitudinal
fibers and an inner layer of circular fibers.
The term ‘papilla’ is not consistently used in scientific
documentation and has led to some confusion. Some authors suggest
that the dermal papilla is the bulb or its basal layer; others
interpret
it as the combined basal epithelial layer of the bulb and dermal
papilla; and to others still, the papilla denotes only the mesodermal
pear shape, which is enclosed by the bulb. However, the term dermal
papilla should be used to designate only the connective tissue
element which is enclosed by the bulb of the follicle during anagen,
and which forms a compact ball of dermal cells underneath the "hair
germ" during telogen (the period of quiescence in the hair
follicle cycling process). The dermal papilla is attached to the
connective tissue sheath by a basal stalk.
The establishment of a dermal papilla during early stages of
embryo formation (embryogenesis) is vital to the subsequent development
of all hair follicles and associated modified structures. The
dermal papilla is a group of specialized dermal fibroblast cells,
which collect in the dermis just below the epidermis. In humans,
this initial aggregation begins when the embryo is approximately
3 months old. The papilla cells at this stage are only loosely
collected and appear like long, spindle shaped cluster. The development
of a dermal papilla in the skin marks the site (the dimple) for
future development of a hair follicle. It is the dermal papilla,
which directs and dictates the embryonic generation of a hair
follicle and it also retains this instructive ability throughout
the life of the hair follicle.
Hair follicles are not vascularized in the early stages of their
development. When the follicles enlarge and contain hair, capillary
networks develop nearby, and capillary loops are formed in the
dermal papillae. Therefore large hair follicles have a rich network
of blood vessels (vascularization), the vascularization is less
in smaller ones and non-existent in the follicles of lanugo (downy
hair on fetuses and new born babies).
The papilla of a normal hair follicle is pointed at its summit.
Pili multigemini is a phenomenon where, from one follicle, two
or more follicles develop. Follicles in such a condition show
papilla that is split into two or more parts, ranging from one
papilla with two apices to completely separated papillae.
Hair dermal papilla cells are specialized mesenchymal (stem)
cells that exist in the dermal papilla, and play pivotal roles
in hair formation, growth, and cycling. In a growing follicle,
the papilla is large and the cells are far apart. The nuclei are
big and ovoid and stain a pale color. The cytoplasm of the cells
has fluid – filled bubbles called vacuoles, evident when
stained with basic dyes. Between the cells are the hyaline ground
substance and a loose argyrophilic frame-work.
In the resting follicle, the dermal papilla is a flattened, compact
ball of cells with dense, round nuclei and barely visible cytoplasm.
Some pigment granules may be scattered between the cells. It is
generally believed that during catagen there is a reduction in
the number of papilla cells, and that in early anagen the cells
divide and increase in number. In full anagen though the dermal
papilla cells do not proliferate. Given that hair follicles can
remain in anagen for up to 6 years, that means dermal papilla
cells probably survive for a long time.
There is no ‘loss of the papilla’ during the resting
phase of the follicle, and during telogen, the dermal papilla
becomes isolated in the dermal area. Indeed the bulb as such is
not present in this phase but the mesodermal component remains
as a smaller rounded ball of dermal cells. The occasional mitotic
figures found in the papillae of human hair follicles usually
prove to be in the nuclei of endothelial cells. The number of
papilla cells remains constant, and what changes do occur are
mostly due to fluctuations in the number of endothelial cells,
to the changes in the size of its cells, and to the changes in
the intercellular substances. Degeneration of papilla cells is
rare, and cells remain intact even after X-irradiation.
Cytochemical studies carried out to evaluate the chemical composition
and activity of the cells during the different stages of the hair
follicle cycle have been successful in clearing some grey areas
on the differentiation and growth dynamics of hair follicle structures.
The enzyme alkaline phosphatase, which has been studied extensively
in the rat and the mouse, is abundant in the dermal papilla of
active hair follicles in all species studied, including humans,
and is expressed throughout the entire hair growth cycle. Because
of this continuous expression of alkaline phosphatase in dermal
papilla cells, many research studies involving hair follicle anatomy
involve tested alkaline phosphatase expression in the skin to
determine the number and distribution of dermal papilla cells.
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| Hair follicle
dermal papilla |
Hair
follicle dermal papilla references
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