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Observation of a System of Linear Loops Formed by Re-Growing Hairs on Rat Skin.

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THE ANATOMICAL RECORD 291:858–868 (2008)
Observation of a System of Linear
Loops Formed by Re-Growing Hairs
on Rat Skin
Beijing Normal University, College of Life Science, Beijing,
People’s Republic of China
This paper details linear hair re-growth patterns observed in rats.
Adult rats were shaved and observed. The first wave of hair re-growth
did not distribute everywhere, but along specific craniocaudally-oriented
lines. The hair-lines were 2–15 mm wide and ran from the head, through
the torso to the limbs, and were symmetrical along the left and right
sides of the body. The symmetric hair-lines from both sides of the body
converged around the mouth, nose, and at the pubic region or ventral
midline to form a system of hair-loop-lines (HLLs). The loops can be differentiated into four main patterns. The Dorsal Loop and the Lateral
Dorsal Loop run along the dorsum and hindlimb. The Ventral Loop and
Lateral Ventral Loop travel along the thorax, abdomen, and forelimb.
These hair-lines coincide with our previously observed sympathetic-substance lines (SSLs) in the rat’s skin. Histological observation indicates
that rat hair follicles along the hair-lines were at anagen phase. The catecholamine histofluorescent check showed abundant sympathetic nerve
fibers beneath the hair-lines. After the rats’ hairs were dyed, and selected
portions shaved, re-growth was only observed on the shaved portions,
indicating that the linear hair growth closely correlated with the shaving.
Lastly we examine the cause of the preferential re-growth and briefly discuss the purpose and physiological role of the HLL. Anat Rec, 291:858–
868, 2008. Ó 2008 Wiley-Liss, Inc.
Key words: hair-loop-line; sympathetic-substance line; environmental stimulation
The presence of hair is a common characteristic of all
mammals. Primary data on hair growth come from the
fundamental studies by Dry (1926) on hairy mice. Since
then, hair growth in skin regions has been characterized
by growth phases succeeded by a resting phase. Hair
change occurs in three to four growth cycles (G1–G4) up
to the 126th day of the life in a normal hairy mouse. A
normal G1 cyclic is completed between the 8th and 10th
day in hairy mice (Dry, 1926; Andreasen, 1953; Borum,
1954). The G2 cycle ends on approximately the 45th day
of a normal hairy mouse (Andreasen, 1953; Borum,
1954), in contrast to the rat, whose G2 cycle ends
between the 44th and 50th day of its life (Butcher,
1934). The G3 cycle ends on approximately the 100th
day. No other distinct regular hair pattern can be noted
in normal hairy-pigmented mice after the G3 cycle (Dry,
1926). Hair cycles in rodents may be different, dependÓ 2008 WILEY-LISS, INC.
ing on the species. It is especially true in mice (Johnson,
1958; Eaton, 1976; Militzer, 2001).
In one of our previous studies, using whole body
macro-autoradiography, with an iodine 125-labeled tyrosine, we localized the distribution of iodine-125-catecholamine in the rat skin. The images on the film showed
Grant sponsor: NSFC.
*Correspondence to: Li-yuan Liu, College of Life Science, Beijing Normal University, Beijing, 100875, China. Fax: +86-1013693121193. E-mail:
Received 16 August 2007; Accepted 27 February 2008
DOI 10.1002/ar.20702
Published online in Wiley InterScience (www.interscience.wiley.
various pairs of symmetrical lines running from the
head, through the back of the animal, and to the hind
limbs, that we termed sympathetic substance-lines
(SSLs). Furthermore, we proved that the substance-lines
came from dense nets of noradrenergic nerve fibers
innervating the arrector pili muscles beneath the line
(Liu et al., 2005). In an accidental case involving observations of skin hyperaemia to NA released from the
SSL, an adult rat’s hairs were shaved in a large region
(this was researched and reported in a separate study).
After 2 days, we noticed that the first wave of hair regrowth was located along a line from the head through
the back to the hind limbs. Thus, we researched such
hair-lines in detail, and the cause of growth.
Wistar white rats, 100–120 days old, weighing 200–
280g were selected. There were 20 males and 10
females. The animal experiments described in this study
were performed in accordance with the ‘‘Principles of
Laboratory Animal Care’’ (NIH publication No. 86–23,
revised 1985).
Hair-Line Observation
The rats were anesthetized with pentobarbital sodium
(40 mg/kg, intraperitoneal injection). All of the rats’
hairs were carefully shaven by razors. The rats were fed
in an air conditioned room (approximately 22–248C) in
summer. Hair growth rates were observed daily. When
adequate hair-lines appeared, the rats were anesthetized
again and photographed or scanned by a scanner. Some
rats were recorded every 3–5 days to observe the various
phases of hair re-growth. After all shaved hairs grew to
normal length (approximately 30 days), the rats were
killed with an overdose of pentobarbital sodium, and the
skins were made into specimens.
Histological Check
In the four rats anesthetized with pentobarbital sodium, skin tissue samples were cut from the hair-lines
and shaved bare areas of the rat’s back 3–5 days after
the hairs were shaved. Frozen sections, 15 mm thick,
were sliced vertically along the direction of the hair
root. The sections were stained by the general hematoxylin–eosin method (H&E). The sampled rats were killed
with an overdose of pentobarbital sodium.
Catecholamine Histofluorescent Check
When the rats’ re-growth hair-lines appeared significantly after shaving, skin tissue samples were obtained
from more than 10 rats anesthetized with sodium pentobarbital. Samples were cut from the hair-lines and the
shaved bare region of the rat’s back. Frozen sections,
25 mm thick, were sliced obliquely, in which the section
was parallel with the transverse plane and along the
direction of the hair shafts, at approximately 45 degrees
to the skin surface, so as to obtain the entire section of
the hair follicle’s AP muscle group. Sections were then
processed with our modified sucrose-phosphate-glyoxilic
acid (SPG) method (De La Torre, 1980; Liu et al., 2005).
After covering the specimens with paraffin oil, catecholamine histofluorescence was observed and photographed
with an Opton fluorescence microscope (excitation range:
355–425, emission suppression filter: 460 nm, 50 W mercury vapor lamp).
Shave and Growth
To study the cause of growth, several methods have
been performed.
Dyeing hairs. The hairs of the five rats were dyed
with 1% methyl violet liquid. Methyl violet is a nontoxic
cudbear, or dye. After 10 days’ observation, the dyed hairs
were shaved in alternating rows of skin to form both
shaved and unshaved semicircuitous zones. The shaved
rats were photographed. After 10 days’ additional growth,
photographs were taken again.
Chemical sympathectomy.
Six shaved rats
(three males and three females) were be intracutaneously injected immediately with the neurotoxin 6-hydroxydopamine (6-OHDA, 1 mg in 0.2 ml) along the dorsal-lateral line (a hair-line always passed) at the upper
right back when the hairs were shaved. Control injection
is equal volume saline at the left upper back. The regrowing hairs were observed as above.
Ten rats (five males and five
females) were shaved and observed as above, but the
room temperature was keep above 328C in summer. Otherwise, six rats (three males and three females) were
shaved and observed as above, but the thorax were
wrapped with a circle of thick cotton to preserve heat.
Hair-Line Observations
One to 2 days after the hairs were shaved, new hairs
erupted in 25 of 30 rats. In some rats, hairs began to
erupt 3–6 days after shaving. The first wave of hair regrowth did not distribute evenly, but grew along craniocaudally oriented lines or zones, which we have termed
hair-lines (Fig. 1, 2). The hair-lines were approximately
2- to 15-mm wide and symmetrical along both left and
right sides of the body. The hair-lines usually began to
appear at the lateral back, extending toward the head
and limbs after 3–10 days. During this time, no hair regrew on other areas of the skin. The boundary between
the hair-line and shaved areas were clear and welldelineated. Hairs began to grow in those shaved regions
after 10–20 days. After all hairs arrived at normal
length, the boundaries disappeared.
The wider hair-lines were mainly distributed along
the lateral dorsal skin, decreasing in size along the
limbs and head. The hair-lines always formed several
close loops, which we have termed hair-loop-lines
(HLLs). HLL are distinguished into four patterns,
according to courses and numbers of the loop. Finally,
all patterns converge with each other around the mouth
and nose. Due to the presence of these loops, we have to
suggest a starting point for sake of description.
Dorsal lateral loop.
The first pattern of the
HLL starts along the posterior axillary line of the rat
(Fig. 2 2). In the cranial direction, the hair-line runs up
Fig. 1. Hair-loop-lines appeared 4 days (left, right) and 9 days (middle) after shaving a rat’s hair (No.
20). A small circle appeared on the top of the head. (1) dorsal loop; (3) ventral lateral loop. Scale bar 5
20 mm.
to the face, terminating at the upper lip just above the
mouth, and joining the hairs around the nose where the
hair-lines from both side connect together. In the caudal
direction, the hair-line travels along the anterior lateral
portion of the hindlimb. In the middle of the crus, the
hair-line coils around the front to the medial portion,
then courses upward along the posterior medial portion
of the hindlimb toward the groin. Thus a U-shape hairline forms along the front surface of the crus. Finally,
the hair-line transverses across the pubic region and
merges with the opposing symmetric hair-line. Thus, the
hair-line consists of a loop around the lateral surface of
the back, hindlimb, and pubic region. We refer to this
loop as the dorsal-lateral loop (DLL), because it is more
proximal to the back (Fig. 3 DLL). This was the largest
loop, observed among almost all rats.
Dorsal loop. The second pattern of the HLL starts
along the scapular median line on the rat’s back (Fig. 1,
2 1). Near the cranial direction, the hair-line runs
upward toward the underside of the ear, coiling toward
the cheek, just below the eye. The hair-line then terminates above upper lip, where the hair-lines from both
sides connect together. In the caudal direction, the hair-
line runs downward around the root of the tail and
anus, merging along both sides at the posterior spermary (male) or perineum (female). This hair-line also
gives a fine branch at the hip. This branch runs along
the lateral posterior aspect of the hindlimb toward the
calcaneal tendon, where the hair-line surrounds the
ankle, forming a small loop. Thus one large loop and
two small loops are formed around the head, back, tail,
and hindlimbs. We refer to these loops as the dorsal loop
(DL; Fig. 3 DL). The small loop around the ankle we
termed the ankle loop. Sometimes, the two hair-lines
along both sides of the back converge in the mid-dorsal
line over the anterior dorsal region forming an H- or Xshaped pattern in the latter growth (not first appeared).
In the case of the DL, a small circular hair-line always
exists at the top of the head between the ears, which we
have termed the top loop (TL; Fig. 1). Another hair loop
pattern, a wide hair-band distributed along the dorsal
midline of the rat’s back, can be regarded as a variety of
the DL. At the neck, the hair-band divides into left and
right branches. The two branches coil around the root of
the ears and unite to form one hair-line between the
eyes. A local loop is then formed around the head and
neck. The united hair-line runs along the midline of the
nose, terminating above the nose. At the sacral region,
Fig. 2. New growth hairs distributed along hair-lines and hair-looplines after shaving rat’s hairs. c: No. 24 rat after shaved 6 days showing three hair-lines in the lateral view. d: No. 16 rat 11 days after shaving showing the part of the ventral loop in thorax. a: No. 20 rat show-
ing the wrist loop. b: No. 14 rats 9 days after shaving showing the
part of the dorsal lateral loop in the pubic region. (1) Dorsal loop; (2)
dorsal-lateral loop; (3) ventral-lateral loop; (4) ventral loop. Scale bar 5
20 mm.
Fig. 3. The schematic presentation of hair-loop-lines along a rat’s skin. DLL, dorsal-lateral loop; DL,
dorsal loop; DL1, DL2, the two varieties of DL; VLL, ventral-lateral loop; VL, ventral loop; VL1, VL2, the
varieties of VL.
the hair-band branches into two ramifications, which
coil around both sides of the tail and anus, and merge to
form a small narrow loop. Similar to the previous DL,
two hair-lines branch from the hip and run toward the
ankle, forming the Ankle Loop. These may be regarded
as the two separate hair-lines along both sides of the
back in the above DL combine together. Therefore, the
hair-line along the dorsal midline is always wider
(approximately 20 mm). The H- and X-shaped hair-line
in the above DL may be regarded as a transitional stage
from the two lines to the single band line (Fig. 3 DL1,
DL2). In a specific case, only one hair circle was
observed on the top of the head with a short hair-line on
the sacral region.
Ventral lateral loop.
The third HLL pattern
starts from the rat’s underlip below the mouth and runs
down through the lateral neck to the shoulder (Fig. 1, 23).
The hair-line coils around the posterior shoulder and travels along the anterior axillary line to the lateral groin. The
hair-line then obliquely crossed the hypogastric region to
the front of the urethra, where the hair-lines from both
sides merge with each other. A hair-line or hair-band
always branches from the shoulder and runs obliquely
from the anterior lateral portion of the forelimb to the dor-
sal lateral wrist, where the hair-line circles the wrist,
forming a small loop. Under the lip, the hair-lines of both
sides connect to each other, thus forming loops around the
thorax, abdomen, and forelimb. We refer to these as the
ventral-lateral loop (VLL; Fig. 3 VLL). The small loop
around the wrist we call the Wrist Loop, a phenomenon
observed in the every rat. In many cases, the inner boundary of the VLL at the abdomen was unclear, because
shorter hairs always grew at the abdomen. Otherwise, the
various parts of this loop usually united with part of the
previous DLL. The united hair-lines, therefore, become
wider. In a few cases, the hair-line at the hypogastric
region protrudes downward toward the front thigh, a Ushape-like protrusion of hairs which appears to be formed
by the merging of the DLL with the VLL (Fig 3).
Ventral loop.
The fourth HLL pattern remains
unclear in many rats, usually growing later than others.
This hair-line begins at the small protrusion below
the chin and runs obliquely through the upper thorax to
the anterior medial portion of the forelimb (Fig. 2 4). At
the distal forelimb near the wrist, the hair-line turns toward the posterior medial portion of the forelimb and
back toward the lateral axilla, forming a U-shape. The
hair-line then coils upward slightly, before curving
TABLE 1. Appearance rate of the type of the hair-lines in all 30 rats
Fig. 4.
A long dorsal zone 1 A pair of wide lateral lines
A short dorsal zone 1 A pair of wide lateral lines
A short dorsal zone 1 Two pairs of lateral lines
Two pairs of parallel lines on the back
Most of the back is bare
The histological photo of hair follicles along the hair-line (a) and in the bare area (b). Scale bar 5 40 mm.
downward toward the thorax, descending along the clavicular midline toward the umbilical region of the abdomen, where the hair-lines from both sides merge with
each other. Below the chin, the hair-lines of both sides
begin at the same point. Thus forming a loop around the
thorax, the medial portion of the forelimb, and the upper
abdomen (Fig. 3 VL). We refer to it as the ventral loop
(VL). This loop is described representationally, because
the outer hair-line boundaries more or less follow the
VL routes, while the inner boundaries remain unclear
due to the short hairs spread the abdomen and thorax.
In some rats, the hair-line is distributed as a rhombic or
cruciate shape on the thorax (Fig. 3 VL1, VL2). The
hair-lines on both sides are closed and combine together
at the midline of the abdomen and thorax, similar to the
hair-lines at the midline of the rat’s back.
We drew the schematic figures of all HLL patterns in
Figure 3. All four HLL patterns (DL, DLL, VLL, and
VL) typically appear in all rats. However, in many rats,
some parts of the DLL and VLL unite together. Both
DL1 and DL2 patterns are found in many rats in place
of DL patterns, and can be thought of as variants of DL.
The same is true for the VL1 and VL2 varieties of VL.
The DL and VLL are analogous, because the DL runs toward the hindlimb, forming a circle around the ankle,
while the VLL runs toward the forelimb, forming a circle
around the wrist. Also the DLL and VL are similar, as
the DLL runs toward the hindlimb, turning back toward
the pubic region forming a U-shape, while the VL runs
toward the forelimb, turning back to the thorax, forming
another U-shape. The DL and DLL are mainly distributed around the dorsal skin and hindlimb. The VLL and
VL are mainly distributed around the ventral skin and
forelimb. The dorsal hair-lines are always clearly
delineated, whereas the ventral hair-line are often hard
to recognize.
The emergence of hair-line types of all 30 rats are
listed in Table 1. The use of parentheses (DLL1VLL)
indicates that parts of the DLL and VLL united together
in some rats. The VL1 and VL2 are not listed as the
hair-line types are difficult to judge.
Regarding the difference between the sexes, only the
hair re-growth in female rats somewhat were latterly
than the males. No different pattern of HLL has been
Common Histological Check
When new hair-lines appear in shaved rats, the hair
follicles in the new hair-lines are always in anagen
phase. The bulb of the hair follicles is extremely
enlarged and deeply inserted into the hypodermis (Fig.
4a), but the hair follicles in the shaved bare areas are
short, small, and in the phases of telogen, proanagen, or
catagen (Fig. 4b).
Catecholamine Histofluorescent Check
In the sections from the re-growth hair-lines, numerous radiant clusters or parallel fascicles of catechol-
Fig. 5. Photomicrographs of catecholamine-containing fluorescent sympathetic fibers beneath the regrowing hair-line of the rat skin. The fibers form clusters and fascicles in the arrector pili muscles. Scale
bar 5 40 mm.
amine-positive nerve fibers could always be seen (Fig.
5). But in the sections from the shaved bare region, the
catecholamine positive nerve fibers are seldom and usually appear in simple small bundles.
Shave and Growth
Dyeing hairs. When rats were shaved in semicircuitous patterns, the purple hairs are easily photographed and distinguished. Among the alternating
hair and skin patterns, no white hairs could be found
anywhere in any length (Fig. 6a). At 3 to 5 days after shaving, longitudinal hair growth was observed
along the hairline patterns as described above, but
only in the shaved regions. Ten days after shaving,
the new hair growth nearly reached the same length
as the purple, unshaved hairs (Fig. 6b), yet the purple, unshaved hairs along this same hair-line did not
grow at all, nor did the shaved areas outside of the
Chemical sympathectomy. In five of six rats, the
hair-line appeared through the injected point. The new
hairs grew around the injected point. There did not
show any visible differentiation between the druginjected point and the saline-injected point, and between
the injected area and noninjected area.
Temperature. The appearance of hair-line of rats
fed in the 328C room were approximately 5 days (5.1 6
2.5). When compared with previous rats fed in lower
temperature (2.5 6 1.9) by t-test, the results is significant (P < 0.05). The results from the cotton-wrapped
rats were difficult to judge, because it is quite difficulty
in keeping the cotton attached steadily to the bodies of
active rats.
We have observed a series of new hair re-growth phenomena in rats, with postshaving hair re-growth rates
differing significantly from one body region to another.
The first obvious feature of the hair-line is that it passes
throughout the entire surface of the body. The second
significant feature is that these patterns occur in loops.
We may conclude that a system of linear loops occurs
inherently in the rat’s skin. The HLLs encompass all
body surfaces and occur in regular and geometrical
forms. Given these results, the question remains: Why
does hair re-growth emerge in lines, and why do these
lines form loops?
Although line-like hair growth has been illustrated by
Dry in hairy mice and by Militzer’s photos in nude mice,
the linear hairs are short, local, irregular, and in unstimulated states (Dry, 1926; Militzer, 2001). In Johnson’s
observation, the hair growth of the waves in albino rats
first erupted at the ventral (whereas in this study, the
hair-lines usually began to appear at the lateral back),
and spread laterally to the neck, mid-side and flank
regions during its first and second week. The growth
waves from each side converge in the mid-dorsal line
over the anterior dorsal region 3–4 weeks after the first
growth wave (Johnson, 1958). From the dorsal view, it
seemed like a pair of hair zones running along the dorsal-lateral line of the rat and a H-shape formed (similar
to the dorsal DLL in this study). Because the DLL
appeared in all rats, Johnson’s dorsal inner border
should be the homology with the DLL. But the H-shape
only appeared in some rats in this study. Also in Johnson’s observation, the top of the head was finally covered
by the hair growth of the waves (the uncovered region is
similar to the inner area of the Top Loop in this study).
The hair-lines described in this study occur in regular
loops and throughout the entire body surface. Growth
Fig. 6. The influence of shaving on hair growth in a rat. The head, leg, and the left body were not
dyed. a: At 10 days after dyeing, the hairs were semi-circuitously shaved. No new hairs grew. b: At 10
days after shaving, white hairs grew longitudinally along only the hair-line, and only in the shaved regions.
Scale bar 5 20 mm.
from several loops and bare areas (as opposed to waves
from ventral to whole body) is a new phenomenon. They
do not follow any known nervous, vascular, or lymphatic
structure in the skin. Although the procedure of shaving
have accomplished carefully, partially skin injury also
occurred as visible scratches in rats where partial regrowth had occurred. Thus those scattered small injury
did not shown any influence on the integral hair re-growth,
especially the linear pattern of hair re-growth. According to
our preliminary experiments, postshaving linear hair
growth could be observed in mice and rabbits.
The linear courses of hair-lines coincide with the
arrays of previously mentioned SSLs (Fig. 7; Liu et al.,
2005). In Figure 7, A1 and A2 are a pair of labeled linear arrays that run from the head, through the posterior
axillary lines down to the medial portion of the feet,
that correspond with the DLL. B1 and B2 are another
pair of labeled linear arrays that run from the head
through the scapular median line to the lateral portion
of the hind limbs, that correspond with the DL. C1 and
C2 are a third pair of labeled arrays that begin at the
head, pass around the shoulder, and then run along the
anterior axillary lines, corresponding with the VLL. The
C1 and C2 expanse at the shoulders occur as wide hairlines in all rats. The expanse at the lateral groin coincides with the protrusion of the VLL hair-line in only a
Fig. 7. Autoradiograms of the whole rat skin show labeled arrays
of sympathetic substance-lines (SSL) after intraperitoneal injection of
I-tyrosine. The dashed line is the dorsal midline. a: A male rat with
two pairs of SSL (A1-A2 and B1-B2). b: A female rat with three pairs
of SSL (B1-B2, C1-C2, and D1-D2). Note the hindlimb portion of C1C2 is not continuous to the line in the torso with a small break. c: The
schematic representation of all SSL in the rat skin.
few rats (Fig. 1). The short line at the hindlimb labeled
C1 and C2 had a small break with a line from the back
(Fig. 7b), which we erroneously regarded as part of C1
and C2. Now we know that C1 and C2 are really the
VLL and do not belong to the hindlimb. Therefore, the
label of C1 and C2 at the hindlimb in Figure 7 is mistaken. The D1 and D2 are labeled linear arrays in the
forelimbs, which correspond with VLL in the forelimb.
M1 corresponds with DL2, in which the two hair-lines of
both sides combine together along the dorsal midline.
M2 corresponds with part of the VL along the ventral
The SSL samples come from the incised skin decorticated from the rat, and are spread out into a plane.
Some lines appear at the edge of the incision. Some lines
might be disturbed. The skin in the distal part of the
limb and the spermary are always imperfect. Here, the
hair-lines displayed many details in the live rats, which
were unseen in the autoradiography films. For example,
the U-shape on the hind limb is difficult to confirm in
the film. The lines in the pubic area are always damaged
in the decorticated skin samples. The ankle loop and the
wrist loop are almost unrecognizable, due to dense,
irregular images at the distal limb in the film. We can
confirm that the dense areas at the distal limbs are the
wrist and ankle loops; and that the expanded region at
the perineum is where both sides of the hair-line meet
(Fig. 7a,b). We did not try to correct the details of the
SSL according to the HLL, because the SSL lines represent our previous judgment, in which we have done our
best. Thus the HLL form is very clear and more intact
than the SSL. In comparison with complex, expensive,
and dangerous autoradiographic treatment, hair-line
observations are quite easy, allowing us to gather much
data from many rats. The HLL helps us understand the
SSL perfectly. But regretfully, the hair-line is obviously
wider than the SSL. The catecholamine histofluorescent
check showed abundant sympathetic nerve fibers
beneath the re-growth hair-lines, identical with our previous observation in the SSL, indicating that the hairline grows along the SSL.
Because the SSL arrays coincide with hair-lines, the
hair follicles (or the arrector pili muscles along the hairlines) are innervated by a great number of sympathetic
endings. We are observing the phenomenon that when
the sympathetic innervating a local hairs along the hairline is stimulated and the local hairs stand up, these
pilomotor will spread along the hair-line and finally
form a pilomotor-line, which include some unknown
physiological function (for example, pain). These will be
reported latter in details. Here, we just want to say that
the HLL not only a superficial phenomenon of hair regrowth, but also involve the internal function of the
Cause of Linear Hair Growth
Age and sex may be an important variable affecting
hair growth. It was well established by Dry in 1926 that
the synchronous hair growth patterns in rodents change
dramatically from hair morphogenesis. In this extensive
study, hair pattern development in the first three generations was provided in great detail for the mice. Because
the HLL patterns were already very complex, all rats
used in our experiment had already past their first 100
days of hair growth. We do not account for differences
involving age in this study. However, primary data suggest the numbers of HLL patterns in younger rats were
less than the older rats. In the old rats (>6 month), the
re-grown HLLs were delayed and discontinuous.
One may ask why we never noticed that the hairs
along the hair-line were longer than those along the
side. The reason for this is because we believed that cutting, shaving, or plucking hairs would not alter growth
rate or hair character. Roersma and Veldhuis (2001)
have observed that plucking of an anagen hair is not
limited to reset of the follicle to telogen or continued
hair growth, but that a temporary reduction of the mitotic activity is very likely to occur. Here, when the rats’
hairs were shaved, linear hair patterns noticeably
appeared. Furthermore, the histological observation
proved that the hair follicles along the hair-lines
remained in active anagen growth states. Thus, the hair
grew more quickly after being shaved than we expected.
Why then, did hair growth along the line appear after
being shaved? We have explored these questions in
many ways. Hair growth and density are influenced by
many factors, but most importantly by nutrition and
hormones. But in our unpublished experiments, the
injection of testosterone to the shaved female rats
seemed to have no influence.
The clinical and experimental observations have long
suggested that the skin sympathetic nerves have
‘‘trophic’’ functions in the hair follicle development,
growth, and/or cycling (Paus et al., 1997). Schotzinger
and Landis (1990) demonstrated that the thoracic hairy
skin in developing and adult rats receives abundant catecholaminergic sympathetic innervation. Botchkarev
et al. (1999) have reported that the number of adrenergic nerve fibers and b2-adrenoreceptor expression of the
hair follicle epithelium in murine skin increases during
the early growth phase of the hair cycle (anagen). Subcutaneous injections of the noradrenaline-depleting
agent guanethidine, or of the neurotoxin 6-hydroxydopamine (6-OHDA), induce a premature onset of anagen in
the lower back skin of mice (Peters et al., 1999). Newborn mice were subcutaneously injected once with 6OHDA, and distinct areas of hairless and thinning of
skin were observed surrounding the sites treated with 6OHDA (Asada-Kubota, 1995). Combining our SSL observations, we deduced that the sympathetic innervation
might be the dominant factor to linear hair re-growth.
Thus, the influence of the chemical sympathectomy on
HLL re-growth was observed in our experiment. But no
positive results could be observed.
The sympathetic in the SSL must be active, and the
arrector pili muscles along the SSL also must be active.
Thus, the hair follicles along the SLL (or the HLL) must
be stimulated continually. This mechanical stimulation
and/or trophic functions are important factors to promote the hair growth and keep the hair follicles in an
anagen phase. Thus, the tentative chemical sympathectomy cannot change the existent active growth state of
the hair follicle nor disturb new hair-growth after
shaving along the HLL. As reported, the rat hair
growth was promoted by irradiation using Super Lizer
(infrared ray). The mRNA expression levels of hepatocyte growth factor (HGF) and HGF activator measured
by reverse transcriptase-polymerase chain reaction
were high in the skin extracted from the irradiated
rats (Miura et al., 1999). HGF as a mitogenic factor
for dermal fibroblasts may be the factor in different
hair lines (or zones) of the HLL and non-HLL. These
may relate to the neurotransmitter with HGF in the
hair growth.
When we considered whether hairs along the SSL or
HLL grew actively during normal states, the dyeing
method was selected. Our results indicate that the hairs
along the hair-line do not grow in normal states, but
grow significantly after being shaved. The unshaved
hairs, however, did not grow at all. Of the shaved hairs,
only those along the hair-line grew quickly. This finding
indicates that the environmental influence on the
shaved skin stimulated hair re-growth, or that the SSL
or HLL is sensitive to a change of environment.
We deduced, therefore, that the primary environmental factor to the shaved skin should be temperature. As
a fact, the hair re-growth along the hair-line was significantly delayed when the room temperature was approximately 328C. We attempted to wrap the shaved area
with thick cotton to preserve heat. But the results were
difficult to judge.
As a sum, the morphological differences of distribution
between the HLL and non-HLL are genetically fundamental, and the abundant sympathetic innervations
along the HLL are stimulators for the active growing
phase of hair follicles. Temperature triggers fast regrowth after shaving. We may deduce that the HLL
function to help the body adapt to environmental
change. But how does the environment and/or temperature stimulate hair growth? How does HLL work in a
normal body? And how do the HLLs develop in embryos?
To make it clear, there is a long way to go. After all,
here, indicated again, the skin is not an even sheet
enclosing the body, but it includes a complex linear system that is divided into different parts/zones. We are
sure that the hair-line is not just a superficial phenomenon of hair re-growth, but should involve some important physiological function of the body.
Combining the HLL and the SSL, the experimental
study on hair and skin sympathetic in rats should consider the difference between the HLL and the non-HLL.
For example, when the test and control areas are
arranged in the HLL and the non-HLL, respectively or
alternatively, the results will be quite chaotic. Indeed,
this is not a problem; just shaving an area of hairs and
observing 1–3 days.
Thanks to Mr. Peter Walters for his English corrections.
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observations, loops, skin, rat, growing, system, former, hair, linear
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