[Reprint from The St. Louis Medical and Surgical Journal, Sept., IStoo.] 
The Human Epitrichium-Its Physiological Functions and 
its Role in the Causation of Skin Disease. By A. H. 
' Ohmann-Dumesnil, A.M., 31. D., Professor of Dermatology 
and Syphilology in the Marion-Sims College of Medicine of 
St. Louis. 
There is no doubt whatever that the subject of histology is far 
from being exhausted, and more especially in connection with 
development. Pathological investigations, more especially in 
connection with bacteriology, have, in later years, occupied so 
much of the attention of investigators, that they seem to have 
made the latter lose sight of the fact that we are not as yet 
thoroughly conversant with the morphology and development of 
normal structures, and, as a natural result, histology has not 
advanced with the same rapid strides as other branches of inves- 
tigation connected with the art of medicine. And yet it is diffi- 
cult to imagine a more fascinating study, especially when it is 
taken up in connection with teratology and pathology, for in it 
lies that of which we are always in search-an explanation for 
the peculiar conditions or abnormal processes observed. 2 
It may not be inappropriate to introduce the subject proper by 
a short description of the epitrichium. This layer of the skin 
was discovered and first described by Weicker.* In his exami- 
nations of the embryos of the Bradypus or sloth he found it to 
consist of a continuous membrane which overlies the hairs, hence 
the name epitrichium from epi and tricliion. This led to further 
investigation on his part, and he found it to exist in various 
mammals and in man. The presence of this curious membrane 
in reptiles was clearly shown by Kerbert,f whilst Jeffries* and 
Gardiner showed its presence in birds. These authors are not 
the only ones who have written on the subject, but it seems to 
have awakened but little interest among dermatologists if we are 
to judge from their works. So far as 1 know there is no syste- 
matic treatise on diseases of the skin which devotes any serious 
attention to this particular subject, and yet it is one of more 
than ordinary importance in the consideration not only of the 
anatomy of the skin, but in the pathogenesis, pathology and 
treatment, as well as prognosis, of some affections of the integu- 
ment. Nearly all the work in connection with the epitrichium 
has been done by pathologists and embryologists. It is for this 
reason that I propose to devote some little time to the consider- 
ation of the anatomy of the epitrichium, such as I have found it 
in my investigations. I also desire to say a few words in regard 
to the physiological functions of the epitrichium, as this is also 
a subject not touched upon in the works devoted to physiology. 
The epitrichium may be described, in general terms, to be a 
temporary epithelial layer of the skin which disappears at birth, 
the time when its functional necessity has disappeared. It over- 
lies the stratum corneum of the epidermis, and is anatomically, 
as well as physiologically, distinct. From the investigations 
which have been made by competent observers, and to whom 
allusion has been made above, it is a structure which is not lim- 
ited to mammals, but occurs in birds and reptiles as well, and 
doubtless it also occurs in amphibians and fishes. This con- 
stancy of occurrence is a circumstance which should certainly 
entitle this little-known structure to more consideration, not only 
at the hands of anatomists and embryologists, but especially of 
♦Haut von Bradypus, Halle, 1864. 
tKerbert (C.), Skin A. f. in A., xiii. 205-262, 1877. 
(J. A.), Proc. Boston Soc. Nat. Hist., xxil. 203-241, 1883. 3 
dermatologists. In this paper I propose to describe this struc- 
ture in the human being only, as an attempt to do more would 
require too much space at this time, and is certainly deserving 
of separate consideration in a paper devoted to the comparative 
anatomy of the structure. 
I do not intend to trace the development of the epidermal sys- 
tem in the human being, as it is much the same in all amniota. 
Those who are desirous of doing so can consult Bowen,* Minot,f 
Lander,! Curtis,$ Unna, || and others. In the human embryo 
the epitrichium may already be distinguished at the age of two 
and one-half months, and from this time on it is not difficult to 
demonstrate its existence. It will be found to consist of one 
layer of cells with well-marked nuclei overlying another layer 
of cells which are flatter and provided with smaller nuclei, the 
whole resting on Bowen's basal membrane. As the embryo 
grows older the cells of the epitrichium grow flatter and larger, 
and their contained protoplasm shrinks towards the centres of 
the cells. 
I have had occasion to make a number of preparations of the 
skin of embryos of various ages, and taken from portions of the 
integument removed from the scalp, behind the ear, and from 
the groin. I have observed that at all ages, from two and one- 
half months up to eight months the epitrichium could be made 
out quite distinctly. The cells in the younger specimens are 
larger and fuller, whereas in the older subject they flatten, be- 
coming fusiform in cross-section, but showing distinct nuclei 
which are highly refracting, and suggesting the presence of 
eleidin. In cross-section the stratum corneum can be made out 
as well as the stratum mucosum. The primitive arrangement of 
the cells and their grouping for the formation of the hair-buds, 
as well as the budding of the sebaceous glands, is very plain. 
In the specimens from the older embryos the formed hair and 
the epitrichial cells, which have been pushed aside by the growth 
of the former, are plainly discernible. A horizontal layer of the 
*Bowen (J. T.), Epitrichium A. A., xxii. 685-882. 
tMinot (Charles Sedgwick), Skin Am. Nat.. 575-578; Human Embryology, 548-554, 
1894. 
JLander (R.), " Nails," His. Arch., 103-143, 1884; "Nails," His. Arch., 273-306, 1886: 
"Cornification," His. Arch., 51-96,1888. 
gCurtis (F.), "Nails," Jour, de 1'Anat. et Physiol., xxv. 125-186. 
|Unna (P. G.), "Anatomy and Development of the Skin," Ziemssen's Cyclopedia 
1885. 4 
integument, separated by maceration, shows very plainly the 
presence of the epitrichial layer overlying that of the stratum 
corneum. At first the cells of the former are distinguished from 
those of the latter with difficulty, but, as development progresses, 
the epitrichial cells assume a more distinctly polygonal shape, 
the nucleus is more indistinct, and the protoplasm concentrates, 
as it were, at its centre. A certain amount of shrinking also 
takes place, so that the cell is actually flattened despite the fact 
that the protoplasm has all gathered about one point. It is by 
no means a difficult matter to demonstrate the difference between 
these cells in integument derived from a fetus from six to eight 
and one-half months old. One prominent feature is that the 
cells of the epitrichial layer are much larger than those of the 
underlying structure, and this disparity increases with the 
progress of development. 
An interesting question arises as to whether the epitrichium 
finally merges into the horny layer or not. This idea has been ad- 
vanced by some very competent authorities, but it is open to serious 
objections. If it becomes part of the horny layer, it should cer- 
tainly be so already at the time the hair emerges from the skin, 
and yet it is not. It is raised up by the growing hair. Further- 
more, the cells of the epitrichium never become perfectly flat- 
tened like those of the horny layer, and are essentially of a 
different nature, both morphologically, histologically and chemi- 
cally. Moreover, it can be separated from the horny layer at 
any period, and this separation occurs at or about the time of 
birth, so that a consolidation of the two seems hardly probable. 
To attempt an explanation of the physiological functions of 
the epitrichium is by no means an easy task, and that it possesses 
such there can be no question or doubt. It may be argued that 
it is simply a persistence of type, but this is simply an explana- 
tion which is no explanation, and which does not explain, for 
even if it be such in man it must have some function in animals. 
In the Bradypus it is much more developed, and is separated 
from the embryo, thus constituting another amnion it is claimed. 
But, it may be asked, of what utility would such another amnion 
be? From an examination of this peculiar membrane it would 
seem that it is impermeable to water-at least it does not take 
up aqueous stains very readily, and from this the conclusion 
might be drawn that its function is to protect the skin from 5 
maceration through the action of the liquor amnii. And if 
death of the embryo supervene, maceration of the embryo will 
take place, although more slowly than when denuded skin is 
placed in water. Of course, this prevention of maceration is 
aided by another condition, viz.: the retention of sebum upon 
the surface of the skin, and this is effected by the limiting action 
of the epitrichium which overlies the sebum or vernix caseosa 
which has formed. If this fat be carefully removed from the 
skin of a new-born infant, and its covering membrane be care- 
fully mounted, it will be found to consist of a single layer of 
cells having all the characteristics of the epitrichium, which 
naturally leads to the conclusion that a complementary function 
is to retain the sebum, and thus act further in preventing 
maceration. 
A question which naturally suggests itself in connection with 
the epitrichium is in regard to the length of time it persists, and 
as to whether it is cast off, and if so, at what period of fetal life 
does this occur. As the embryo increases in age the cells of the 
epitrichium become flattened, and in this process the outlines be- 
come polygonal and the edges closely adherent to one another. 
This change takes place at those parts and at the time that the 
hairs and sebaceous glands assume form and the cells composing 
their first buds group themselves in a definite manner. As is 
well known, the development of the hairs and fat-glands is 
rapid, so that by the time the shafts of the former emerge and 
the secretion of the latter finds its way to the surface, the epi- 
trichium has become a single continuous, definite membrane, 
composed of one layer of flat epithelial pavement cells, rather 
large, but quite resistent. In the locality where the hair devel- 
opment is strongest, such as the scalp, the epitrichium is first 
raised, and in those places where the secretion of fat is most 
active it is raised to the greatest degree. Where neither hairs 
grow nor sebum is secreted, there is apt to be no separation at 
all, and this is best exemplified in the case of the nails. The 
eponychium, from epi and onyx, as Unna has denominated it, is 
analogous to the epitrichium, and may be found in earlier fetal 
life. Later on it disappears, having, in all probability, been 
cast off in the course of the growth of the nails. The fact that 
the epitrichium is found over the entire integument in earlier 
fetal life, and that it becomes gradually separated in the later 6 
life of the embryo, is evidence of the fact that it is finally cast 
off at that period when it is no longer physiologically necessary. 
Furthermore, the fact that even at birth it may be found over- 
lying the accumulated secretion of sebum is further evidence 
that the final casting off is a comparatively late process. 
From the fact that, in early fetal life the epitrichium can be 
easily demonstrated in the embryo, and furthermore that it is a 
distinct structure composed of a different sort of cells from those 
which form the outermost layer of the stratum corneum, we 
must conclude that it has functions of its own. In early devel- 
opmental life it is closely adherent to the underlying layer of 
cells, but later on it separates in the manner which has just been 
given. But, should there be an arrest of development of the 
hairs and sebaceous glands, the raising of the epitrichium does 
not occur, and this membrane adheres to that overlying the 
stratum corneum, and it does not become separated from it for 
a period of time, almost, if not, coincident with birth. In the 
meantime the developmental impulse which should have ex- 
pended itself upon the hairs and sebaceous glands is diverted to the 
horny layer, and it undergoes a rapid and marked multiplication of 
cells, making it thicker and assuming a horny appearance. That 
the epitrichium does not become a component part of the stratum 
corneum may be easily demonstrated by means of selective stains 
which will decolorize in the cells of the epitrichium and will not 
in those of the stratum corneum. A question which suggests 
itself in this connection is as to whether the thickening of the 
horny layer is due to a persistent adherence of the overlying 
membrane of which we are speaking. It is evidently not, but 
depends rather upon the arrest in development of the hair and 
sebaceous glands, and this arrest is possibly dependent upon 
pressure exercised from without, which condition would also act 
as an important factor in producing an adhesion between the 
epitrichium and the stratum corneum. 
When the process is one which is generalized the thickening of 
the horny layer is so also, and in consequence we have the differ- 
ent varieties of ichthyosis known as xerosis, ichthyosis simplex 
and ichthyosis congenita, or so-called harlequin fetus. These 
are simply different varieties of the same process, whereas ich- 
thyosis hystrix and ichthyosis sebacea are somewhat analogous, 
but have superadded an exaggerated secretion of sebum. But 7 
it must certainly be true that the ichthyotic process owes its 
origin to the same cause that the other forms do, and there is 
added thereto a certain development of the sebaceous glands, 
whose functions not only continue, but are exaggerated by the 
stimulation exerted by the pressure due to the thickening of the 
stratum corneum. In all of the former ichthyotic conditions 
there is evident lack of development of the sebaceous glands, 
and in all forms of ichthyosis there is a deficiency in the number 
as well as size of all the lanugo hairs. The skin is harsh and 
dry, or rugous and spinous, and it persists in this condition de- 
spite the best directed treatment, which can only mitigate it at 
best. 
Should the epitrichium be adherent in localized areas only we 
have as a result a localized and limited ichthyosis which does not 
extend beyond its primary limits. A peculiarity observed in 
connection with localized ichthyosis is the fact that it partakes 
of the nature of the portion of integument which is affected. If 
it be in the palms and soles the thickening is much greater than 
if it occur upon a flexor surface, and upon this latter it is never 
as well marked as upon the extensor portions of the integument, 
as is daily shown by observation. A peculiarity which has been 
noted in connection with both localized and generalized ichthyosis 
is the fact that it rarely if ever affects the face and axilla?. 
This is explainable by the further fact that in these localities the 
sebaceous glands develop early and rapidly and attain a compar- 
atively large size, as they also do in connection with the hair of 
the scalp, this latter being also singularly free from the ichthyotic 
process. 
From an anatomical examination of the epitrichium in its dif- 
ferent stages, and from a critical examination of the various 
points involved in a consideration of the question, it seems to 
me that there can be no doubt whatever that the epitrichium 
plays an important role in connection with the development of 
the human integument. It not only does this, but it also plays 
a by no means insignificant part in the production of some kera- 
togenous skin diseases, and it is far from improbable that lichen 
pilaris is in some way indebted to the same structure for its 
origin. There are other processes of a similar pathological 
nature which certainly should be investigated more closely. 
Like ichthyosis, they are of frequent occurrence and much more 8 
common than is usually supposed, so that the investigations of a 
thorough and convincing nature could be easily made. No at- 
tempt to include any considerations of this kind would be in or- 
der at this time in this paper, as it is the intention of the writer 
to deal with general considerations only, special ones requiring 
more space than could be given at this time. 
An explanation for the existence of the diseases of the skin 
which have been alluded to has been attempted by various au- 
thors, but it is one in which no satisfaction can be found. It is 
that ichthyosis is hereditary, and this assumption is based upon 
observations of one ichthyotic parent having one or more chil- 
dren similarly affected; or, the condition being seen through 
several successive generations in members of the same family. 
It may be possible that an anatomical peculiarity is transmitted 
from parents to children, but that a pathological process may be 
is a proposition repugnant to reason. If we assume that an ana- 
tomical peculiarity is transmissible from parent to child we can 
readily understand that an arrested or incomplete development 
or an exaggerated development may occur, and, in connection 
with the subject under consideration, there could occur a trans- 
mission of the developmental peculiarity in the sebaceous glands 
whereby they would not attain their proper form, size and secre- 
tory functions, thereby permitting the epitrichium to adhere to 
the stratum corneum, and this, in turn, act as the active cause 
of ichthyosis and allied conditions. This might be taken as a 
partial explanation, but the subject is so extensive that each 
portion of it, from its very nature, should be taken up separately 
and thoroughly discussed.