Hair
follicle embryogenesis overview
The establishment of a dermal papilla is vital to
the development of all hair follicles and associated modified
structures like
sebaceous
glands. The dermal papilla is a group of specialized dermal fibroblast
cells, derived from the embryonic mesoderm. These dermal papilla
cells begin to
aggregate
in the dermis just below the epidermis. For humans, this initial
aggregation begins when the embryo is approximately 60 days old.
The dermal papilla cells at this stage are only loosely collected
and are long, spindle shaped cells.
The development
of a dermal papilla marks the site for future development of a
hair follicle.
In humans, development of hair follicles first
begins on the head, particularly on the eyebrows along with
the lower and upper lip of an
embryo.
From these first few hair follicles on the face, the development
of the hair follicles gradually expands symmetrically
to
cover the entire embryo save for the palms of the hands and soles
of the feet. Skin areas further away from the head, especially
those
on the limb extremities, take longer to develop embryonic hair
follicles. It can take up to 110 days before hair follicles can
be seen to
start developing on the arms and legs.
Above the dermal papilla an epidermal plug, or peg, of cells
develops and proliferates growing into the dermis towards the
dermal papilla. The epidermal
plug of cells
does not grow straight down into the dermis but at a slight angle.
The mesoderm derived dermal papilla and the ectoderm derived
epidermal plug "communicate" with the result of further
proliferation of epidermal matrix cells and eventual differentiation
into the various
sheath
and hair fiber structures.
This gradual differentiation of the hair plug first begins with
the development of three distinct buds of cells on the same side
of the down growth one above the other. The cell bud closest to
the epidermis may develop into an apocrine (sweat type) gland. However,
this only happens in a minority of hair follicles, primarily those
on the scalp, genitals and anus. Isolated apocrine glands may develop
in connection to hair follicles elsewhere but they are usually few
and far between. Most of these upper cell buds regress as the hair
follicle matures.
The cell bud in the middle will gradually develop into the oil
producing sebaceous gland while the cell bud below it forms what
is called the hair follicle "bulge". The bulge is the area of
the hair follicle to which a small muscle will attach. The "arrector
pili" muscle
develops separately from the hair follicle within the mesoderm/dermis.
It grows up towards the epidermis and down towards the hair follicle
bulge region. Eventually it will attach to both the epidermis
at
one end and the hair follicle at the other. The bulge is also
important as this is the location where the stem cells reside in
adult hair follicles.
As the epidermal plug of cells penetrates down into the dermis,
mesodermal cells congregate around it and develop into a fibrous
follicular sheath and/or collagen capsule to encase the epidermal
cells. As the epidermal plug comes into contact with the dermal papilla the
growth is still downward. The epidermal plug seems to push the dermal papilla
down into the dermis as it grows to its full size. It may penetrate
up to 5mm below the skin surface (more in adults).
The dermal papilla develops into a more identifiable structure
of rounded cells containing organelles vital for product synthesis,
although
the cells themselves at this later stage of development are non-proliferative.
The dermal papilla cells communicate with the plug cells and
these epidermal cells begin to arrange themselves into concentric
layers above the dermal papilla. The layers eventually differentiate
into the
hair fiber and the inner and outer root sheaths encasing the fiber.
These layers begin to keratinize higher up the hair follicle while
the cells close to the dermal papilla remain undifferentiated
and continue to
multiply. The multiplying cells can't push out into the dermis
because of the surrounding fibrous capsule. The only way out is
up. The
cells push up and away from the dermal papilla pushing other cells
in front of them. As the cells mature they are incorporated into
the layers
of the hair follicle, become keratinized, die and eventually are
extruded from the surface of the skin.
Hence, development of a hair follicle requires a continuum through
induction, initiation, elongation and differentiation. The
basic hair follicle structures are complete throughout
the skin of an embryo by 160 days.
 |
|
This
diagram shows the eight stages of hair follicle development
as defined by embryologists. The diagram shows the accumulation
of dermal papilla cells below undifferentiated epidermis
which is then stimulated to grow down into the dermis as
a hair "peg". Interaction between the hair peg and the dermal
papilla cells promotes differentiation into a mature hair
follicle. Click the picture for a full size diagram (very
large and 113kb in size).
|
Hair
follicle embryogenesis overview references
- Paus R,
Muller-Rover S, Van Der Veen C, Maurer M, Eichmuller S, Ling G,
Hofmann U, Foitzik K, Mecklenburg L, Handjiski B. A comprehensive
guide for the recognition and classification of distinct stages
of hair follicle morphogenesis. J Invest Dermatol. 1999 Oct;113(4):523-32.
- McElwee KJ, Hoffmann
R. Growth factors in early hair follicle morphogenesis. Eur J
Dermatol. 2000 Jul-Aug;10(5):341-50.
- Holbrook KA, Minami SI. Hair follicle embryogenesis
in the human. Characterization of events in vivo and in vitro.
Ann N Y Acad Sci. 1991 Dec 26;642:167-96.
- Pinkus H. Embryology of hair. In: The biology
of hair growth. Montagna W, Ellis RA (eds). Academic Press Inc,
New York, 1958, pp 1-32.
- Chase HB. Growth of the hair. Physiol Revs.
1954;34:113-126.
- Fraser DA. The development of the skin
of the back of the albino rat until the eruption of the first
hairs. Anat Rec. 1928;38:203-223.
- Koelliker A. Zur Entwicklungsgeschichte
der äusseren haut. Z wiss Zool. 1850;2:67-92.
|
