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Types
of hair produced in embryogenesis
In humans, follicle formation begins on the head in the fourth
fetal month of gestation, and then moves downward to the remainder
of the body. Follicles first produce soft, short and fine hairs,
which are called “lanugo” hairs. Lanugo hair is typically
shed while the baby is still in the womb between the 32nd and
36th week of gestation, but in one third of babies, lanugo hair
can be retained until after birth. The newborns can have a hairy
covering all over of non-pigmented lanugo hair. Though this can
sometimes be a shock to the parents, it is nothing to worry about
and does not indicate any problem. If lanugo hair is still present
at birth, the lanugo hair will be shed within the first month
or so after birth.
Lanugo hair is replaced by small and almost imperceptible hairs
called vellus hairs over most of the body. Vellus hairs increase
tactile perception of the skin surface by acting as sensitive
pressure detectors. Larger follicles on the scalp generate into
terminal hairs, although up to 10 percent of scalp follicles can
remain in a vellus state. The growth of sclap hair in newborns
can be quite variable. While some are born with a full head of
pigmented terminal hair, others have relatively little hair. For
these babies, the hair follicles are growing terminal hair, but
it has not yet become visible above the skin. Within a few months
from birth, the scalp and body hair follicles should have sorted
themselves out and will be producing vellus or terminal hair as
appropriate. No hair follicles form naturally after birth in humans,
though other animals can produce new hair follicles. For example,
red deer grow new hair follicles each year as part of the velvet
covering their antlers.
Inductive
events in the formation of the hair follicle
In the
very early development of the human embryo, there is no hint of
anything resembling a hair follicle. The only cells in
the early human embryo are precursor cells – stem cells
- that are nudged into becoming specialized bone, nerve, internal
organ or skin cells by the action of genes and the growth-and-development
molecules produced by genes. The path to hair follicle development
begins in a group of stem cells that are directed to become specialized
cells of the skin, including hair follicles. The exact nature
of these molecular signals is the topic of research all over the
world. Experiments in mice have revealed much about the signals
and genes that play roles in the formation of hair follicles in
the embryo.
Early research studies demonstrate that embryonic hair follicles
are formed by a series of interactions between the epithelium
(the outer, protective, nonvascular layer of the skin) and the
underlying mesenchymal tissue. Mesenchyme is the part of the embryonic
mesoderm, consisting of loosely packed, unspecialized cells set
in a gelatinous ground substance, from which connective tissue,
bone, cartilage, and the circulatory and lymphatic systems develop
as well as the skin. In mammals the epithelium develops into the
epidermis and part of the mesenchyme becomes the dermis and subcutaneous
fat layer of the skin.
- The first signal is probably from the dermis, which instructs
the overlying epithelium to form an appendage. Regions of epidermal
cells then proliferate and form local thickenings (placodes)
of the epidermis. The very first signal involved in hair follicle
development is not known.
- The epidermal placodes then respond
by sending a message into
the mesenchyme, causing an aggregation of cells in the underlying
mesenchyme. The structure thus formed is called the dermal condensate
and later develops into the hair follicle dermal papilla. The
establishment of a dermal condensate during early stages of
embryo formation is vital to the subsequent development of all hair follicles
and associated structures.
- Finally, a signal from this dermal
condensate initiates proliferation and differentiation of
epithelial placode cells, ultimately
leading to formation of a mature 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.
Formation of the epithelial placode and primary hair germ
In the human fetus, the first morphological indication in the
genesis of embryonic hair follicles is the formation of placode
and hair germ in the epidermal compartment and the tissue condensation
in the mesenchymal compartment underneath. The embryonic hair
follicle develops from a small collection of cells, called the
epithelial placode, which first appears within an otherwise homogeneous
epithelium from around 9 weeks gestation onwards.
Cells within the epithelial placode proliferate to form the "primary
hair germ" (also known as primitive hair germ or follicle
germ) whose progeny eventually generate the entire epithelial
portion of the hair follicle. The periderm is a superficial layer
of cells, which develops from the primordial single-cell-layered
epidermis and forms a transient covering for the epidermis during
much of the remainder of its development. In the early stages
of gestation, the epidermis often consists of only two layers,
i.e. the germinal cells and periderm, but it may show development
of a third (intermediate) layer. In the case of secondary follicles
the epidermis is usually multilayered.
The
follicle peg
From its point of origin, the hair germ grows
obliquely downward and forward into the mesenchyme in the shape
of a solid column
of epithelial cells, and is called the hair peg. The outer cells
of the hair peg are columnar and arranged radial to the long axis.
Cells in the center at first have no definite polarity, but soon
become arranged longitudinally. The advancing end is the broadest
part of the peg and forms either a straight, transverse plate
or is slightly concave because of pressure against the compact
ball of mesodermal cells, the future dermal papilla. The entire
column is enveloped in a sheath of mesodermal cells contiguous
with those of the papilla.
Melanin pigment from the neural crest melanoblast cells migrate
to a region above the somite. In the developing vertebrate embryo,
somites are masses of mesoderm distributed along the two sides
of the neural tube and that will eventually become dermis, skeletal
muscle and vertebrae. The melanocytes then move through the dermis
into the epidermis, mingling with the keratogenic hair matrix
cells. In mice, they populate the hair germ, and move throughout
the follicle peg as the hair germ elongates. The same or similar
is also probably true for humans.
The
bulbous peg
As the follicle grows longer, differentiation into the different
components of the adult hair follicle begins. The bulbous peg
is formed by elongation of the follicle peg. The deepest portion
of the follicle peg surrounds the dermal condensate and forms
a bulbous structure. A solid column of epithelial cells, with
radially arranged cells at its base, forms the matrix of the follicle.
These cells are progenitor cells, which will give rise to all
the layers of the inner root sheath and the hair shaft. In addition
to this, functionally active melanocytes migrate to the innermost
layers of the matrix.
Two solid epithelial swellings begin to appear on the posterior
side of the follicle. The uppermost bulge differentiates into
the sebaceous gland. The sebaceous gland at first is a solid hemispherical
knob, practically devoid of glycogen. As the sebaceous gland gets
larger, its basal cells begin to contain some glycogen. At first,
the sebaceous gland has no duct. Only at a later stage when the
gland begins to form lobes, a septum divides the follicle into
the tube for the hair and into a short keratinizing tube leading
to the sebaceous gland. The central cells of this gland accumulate
lipid and produce sebum, which contributes to the “vernix
caseosa”, secreted by the fetus's sebaceous glands in utero,
coating the fetus and providing antibacterial properties.
The lower bulge of cells in the developing bulbous peg always
remains solid and its cells grow rich in glycogen together with
the rest of the follicle. This bulge becomes the attachment site
for the arrector pili muscle. It is hemispherical in shape, or
it may have a slanting lower end and a shelf-like upper contour.
Epithelial stem cells that can generate the lower follicle, epidermis
and sebaceous gland in the adult localize to this bulge. In the
armpit and perianal regions, a third bulge will form, and this
will become the apocrine sweat gland.
The arrector pili muscle develops through the alignment of mesenchymal
cells at some distance from the follicular wall, and become secondarily
connected with the early colleagenous fibers of the mesodermal
root sheath in the region of the bulb. Since the arrector pili
muscle attaches to the posterior side of the hair follicle, contraction
of the muscle shifts the hair shaft into a more vertical orientation,
making the hair “stand on end”, as it were.
The innermost regions of the hair peg become the cortex and cuticle
of the hair fiber, while the outer layers become parts of the
inner root sheath that covers the hair shaft. As a result of continued
proliferation by the stem cells directly over the dermal papilla,
the hair fiber is pushed upwards within the follicle and it starts
to produce hair keratins. The hair shaft then extends through
the hair canal, an opening that is thought to be formed by the
apoptosis of the cells at the center of the hair peg where the
hair peg meets the skin. The outer root sheath surrounds the hair
follicle (much like a sleeve) and it is continuous with the epidermis.
Differentiation of various parts
With all the essential components of the pilosebaceous follicle
now apparent, growth and differentiation continues and altogether
7 segments of hair follicle can be recognized: The first segment
is the bulb, comprising the matrix of the hair and its sheaths
and the mesoderm derived papilla. The lower follicle is the segment
from the upper end of the bulb to the area of the bulge. Between
the bulge and the sebaceous gland, there is a shorter stretch
called the isthmus. This is followed by the area of the sebaceous
gland. From the point of its opening into the follicular lumen
to the base of the epidermis is the infundibulum, which continues
within the epidermis as the hair canal. All these segments eventually
are traversed by the hair and inner root sheath. In addition,
the arrector pili muscle and apocrine glands are also present.
Formation of the mature hair follicle
As soon as an embryonic follicle attains its definitive length,
mitotic activity in the cone of cells in the upper part of the
bulb increases, and the differentiation of these cells produces
hair. A second concentric cone of cells surrounds the first and
becomes the future internal root sheath. The inner cone produces
the cortex and hair cuticle, but no medulla exists in fetal hair.
The cone of the internal root sheath extends upward and protects
the tip of the hair as it grows into the hair canal. In the upper
part of this canal, the internal root sheath breaks; and later
the hair emerges on the surface of the skin. When fully developed,
the hair shaft is a solid cylindrical structure containing varying
amounts of pigment. Its diameter remains relatively small and
often is not larger than the thickness of the root sheath wall.
Pigment is present in the hair as small brown granules.
Hair
follicle embryogenesis references
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