THE EPIDERMIS AND ITS DERIVATIVES.
In many of the Coelenterata the outermost layer of the blastoderm is
converted as a whole into the skin or ectoderm. The cells composing it
become no doubt in part differentiated into muscular elements and in
part into nervous elements, &c.; but still it may remain through life
as a simple external membrane. This membrane contains in itself
indefinite potentialities for developing into various organs, and in
all the true Triploblastica these potentialities are more or less
realized. The embryonic epiblast ceases in fact, in the higher forms,
to become converted as a whole into the epidermis, but first gives
rise to parts of the nervous system, organs of special sense, and
other parts.
After the formation of these parts the remnant of the epiblast gives
rise to the epidermis, and often unites more or less intimately with a
subjacent layer of mesoblast, known as the dermis, to form with it the
skin.
Various differentiations may arise in the epidermis forming protective
or skeletal structures, terminal sense organs, or glands. The
structure of the epidermis itself varies greatly, and for Vertebrates
its general modifications have been already sufficiently dealt with in
chapter XII. Of its special differentiations those of a protective or
skeletal nature and those of a glandular nature may be considered in
this place.
Protective epidermal structures. These structures constitute a general
cuticle or an exoskeleton of scales, hairs, feathers, nails, hoofs,
&c. They may be entirely formed from the epidermis either as (1) a
cuticular deposit, or as (2) a chitinization, a cornification, or
calcification of its constituent cells. These two processes run into
each other, and are in many cases not easily distinguished. The
protective structures of the epidermis may be divided into two groups
according as they are formed on the _outer_ or the _inner_ side of the
epidermis. Dermal skeletal structures are in many cases added to them.
Amongst the Invertebrata the most widely distributed type of
exoskeleton is a cuticle formed on the outer surface of the epidermis,
which reaches its highest development in the Arthropoda. In the same
class with this cuticle must be placed the molluscan and brachiopod
shells, which are developed as cuticular plates on special regions of
the epidermis. They differ, however, from the more usual form of
cuticle in their slighter adhesion to the subjacent epidermis, and in
their more complicated structure. The test of Ascidians is an abnormal
form of exoskeleton belonging to this type. It is originally formed
(Hertwig and Semper) as a cuticle on the surface of the epidermis; but
subsequently epidermic cells migrate into it, and it then constitutes
a tissue similar to connective tissue, but differing from ordinary
epidermic cuticles in that the cells which deposit it do so over their
whole surface, instead of one surface, as is usually the case with
epithelial cells.
In the Vertebrata the two types of exoskeleton mentioned above are
both found, but that developed on the inner surface of the epidermis
is always associated with a dermal skeleton, and that on the outer
side frequently so. The type of exoskeleton developed on the _inner
side_ of the general epidermis is confined to the Pisces, where it
appears as the scales; but a primitive form of these structures
persists as the teeth in the Amphibia and Amniota. The type developed
on the _outer_ side of the epidermis is almost entirely[149] confined
to the Amphibia and Amniota, where it appears as scales, feathers,
hairs, claws, nails, &c. For the histological details as to the
formation of these various organs I must refer the reader to treatises
on histology, confining my attention here to the general embryological
processes which take place in their development.
[149] The horny teeth of the Cyclostomata are structures
belonging to this group.
The most primitive form of the first type of dermal structures is that
of the placoid scales of Elasmobranchii[150]. These consist, when
fully formed, of a plate bearing a spinous projection. They are
constituted of an outer enamel layer on the projecting part, developed
as a cuticular deposit of the epidermis (epiblast), and an underlying
basis of dentine (the lower part of which may be osseous) with a
vascular pulp in its axis. The development (fig. 235) is as follows
(Hertwig, No. 306). A papilla of the dermis makes its appearance, the
outer layer of which gradually calcifies to form the dentine and
osseous tissue. This papilla is covered by the columnar mucous layer
of the epidermis (_e_), from which it is separated by a basement
membrane, itself a product of the epidermis. This membrane gradually
thickens and calcifies, and so gives rise to the enamel cap (_o_). The
spinous point gradually forces its way through the epidermis, so as to
project freely at the surface.
[150] For the most important contributions on this subject from
which the facts and views here expressed are largely derived,
_vide_ O. Hertwig, Nos. 306-308.
[FIG. 235. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYONIC SHARK,
TO SHEW A DEVELOPING PLACOID SCALE. (From Gegenbaur; after O.
Hertwig.)
_E._ epidermis; _C._ layers of dermis; _d._ uppermost layer of
dermis; _p._ papilla of dermis; _e._ mucous layer of epidermis; _o._
enamel layer.]
The scales of other forms of fishes are to be derived from those of
Elasmobranchii. The great dermal plates of many fishes have been
formed by the concrescence of groups of such scales. The dentine in
many cases partially or completely atrophies, leaving the major part
of the scale formed of osseous tissue; such plates often become parts
of the internal skeleton.
The teeth, as will be more particularly described in the section on
the alimentary tract, are formed by a modification of the same process
as the placoid scales, in which a ridge of the epithelium grows
inwards to meet a connective tissue papilla, so that the development
of the teeth takes place entirely below the superficial layer of
epidermis.
In most Teleostei the enamel and dentine layers have disappeared, and
the scales are entirely formed of a peculiar calcified tissue
developed in the dermis.
The cuticle covering the scales of Reptiles is the simplest type of
protective structure formed on the outer surface of the epidermis. The
scales consist of papillæ of the dermis and epidermis; and are covered
by a thickened portion of a two-layered cuticle, formed over the whole
surface of the body from a cornification of the superficial part of
the epidermis. Dermal osseous plates may be formed in connection with
these scales, but are never of course united with the superficial
cuticle.
Feathers are probably special modifications of such scales. They arise
from an induration of the epidermis of papillæ containing a vascular
core. The provisional down, usually present at the time of hatching,
is formed by the cornification of longitudinal ridges of the mucous
layer of the epidermis of the papillæ; each cornified ridge giving
rise to a barb of the feather. The horny layer of the epidermis forms
a provisional sheath for the developing feather below. When the barbs
are fully formed this sheath is thrown off, the vascular core dries
up, and the barbs become free except at their base.
Without entering into the somewhat complicated details of the
formation of the permanent feathers, it may be mentioned that the
calamus or quill is formed by a cornification in the form of a tube of
both layers of the epidermis at the base of the papilla. The quill is
open at both ends, and to it is attached the vexillum or plume of the
feather. In a typical feather this is formed at the apex of the
papilla from ridge-like thickenings of the mucous layer of the
epidermis, arranged in the form of a longitudinal axis, continuous
with the cornified mucous layer of the quill, and from lateral ridges.
These subsequently become converted into the axis and barbs of the
plume. The external epidermic layer becomes converted into a
provisional horny sheath for the true feather beneath.
On the completion of the plume of the feather the external sheath is
thrown off, leaving it quite free, and the vascular core belonging to
it shrivels up. The papilla in which the feather is formed becomes at
a very early period secondarily enveloped in a pit or follicle which
gradually deepens as the development of the feather is continued.
Hairs (Kölliker, No. 298) are formed in solid processes of the mucous
layer of the epidermis, which project into the subjacent dermis. The
hair itself arises from a cornification of the cells of the axis of
one of the above processes; and is invested by a sheath similarly
formed from the more superficial epidermic cells. A small papilla of
the dermis grows into the inner end of the epidermic process when the
hair is first formed. The first trace of the hair appears close to
this papilla, but soon increases in length, and when the end of the
hair projects from the surface, the original solid process of the
epidermis becomes converted into an open pit, the lumen of which is
filled by the root of the hair. Hairs differ in their mode of
formation from scales in a manner analogous to that in which the teeth
differ from ordinary placoid scales; _i.e._ they are formed in
inwardly directed projections of the epidermis instead of upon free
papillæ at the surface.
Nails (Kölliker, No. 298) are developed on special regions of the
epidermis, known as the primitive nail beds. They are formed by the
cornification of a layer of cells which makes its appearance between
the horny and mucous layers of the epidermis. The distal border of the
nail soon becomes free, and the further growth is effected by
additions to the under side and attached extremity of the nail.
Although the nail at first arises in the interior of the epidermis,
yet its position on the outer side of the mucous layer clearly
indicates with which group of epidermic structures it should be
classified.
Dermal skeletal structures. We have seen that in the Chordata skeletal
structures, which were primitively formed of both an epidermic and
dermic element, may lose the former element and be entirely developed
in the dermis. Amongst the Invertebrata there are certain dermal
skeletal structures which are evolved wholly independently of the
epidermis. The most important of these structures are the skeletal
plates of the Echinodermata.
Glands. The secretory part of the various glandular structures
belonging to the skin is invariably formed from the epidermis. In
Mammalia it appears that these glands are always formed as solid
ingrowths of the mucous layer (Kölliker, No. 298). The ends of these
ingrowths dilate to form the true glandular part of the organs, while
the stalks connecting the glandular portions with the surface form the
ducts. In the case of the sweat-glands the lumen of the duct becomes
first established. Its formation is inaugurated by the appearance of
the cuticle, and appears first at the inner end of the duct and thence
extends outwards (Ranvier, No. 311). In the sebaceous glands the first
secretion is formed by a fatty modification of the whole of the
central cells of the gland.
The muscular layer of the secreting part of the sweat-glands is
formed, according to Ranvier (No. 311), from a modification of the
deeper layer of the epidermic cells.
The Mammary Glands arise in essentially the same manner as the other
glands of the skin[151]. The glands of each side are formed as a solid
bud of the mucous layer of the epidermis. From this bud processes
sprout out, each of which gives rise to one of the numerous glands of
which the whole organ is formed. Two very distinct types in the
relation of the ducts of the glands to the nipple are found
(Gegenbaur, No. 313).
BIBLIOGRAPHY OF EPIDERMIS.
_General._
(304) T. H. Huxley. "Tegumentary organs." Todd's _Cyclopædia of Anat.
and Physiol._
(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." _Archiv f. mikr.
Anat._ Vol. XV. 1876. _Vide_ also Kölliker (No. 298).
_Scales of the Pisces._
(306) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d.
Zähne d. Selachier." _Jenaische Zeitschrift_, Vol. VIII. 1874.
(307) O. Hertwig. "Ueber d. Hautskelet d. Fische." _Morphol.
Jahrbuch_, Vol. II. 1876. (Siluroiden u. Acipenseridæ.)
(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u.
Polypterus)." _Morph. Jahrbuch_, Vol. V. 1879.
_Feathers._
(309) Th. Studer. _Die Entwick. d. Federn._ Inaug. Diss. Bern, 1873.
(310) Th. Studer. "Beiträge z. Entwick. d. Feder." _Zeit. f. wiss.
Zool._, Vol. XXX. 1878.
_Sweat-glands._
(311) M. S. Ranvier. "Sur la structure des glandes sudoripares."
_Comptes Rendus_, Dec. 29, 1879.
_Mammary glands._
(312) C. Creighton. "On the development of the Mamma and the Mammary
function." _Jour. of Anat. and Phys._, Vol. XI. 1877.
(313) C. Gegenbaur. "Bemerkungen üb. d. Milchdrüsen-Papillen d.
Säugethiere." _Jenaische Zeit._, Vol. VII. 1873.
(314) M. Huss. "Beitr. z. Entwick. d. Milchdrüsen b. Menschen u. b.
Wiederkäuern." _Jenaische Zeit._, Vol. VII. 1873.
(315) C. Langer. "Ueber d. Bau u. d. Entwicklung d. Milchdrüsen."
_Denk. d. k. Akad. Wiss. Wien_, Vol. III. 1851.
[151] For a very different view on this subject _vide_ Creighton
(No. 312).