THREAD-CELLS AND EPlDERMrS O I' MYXINE.
The
THREAD-CELLS and
EPIDERMIS of MYXINE.
BLOMFIELD, B.A.
By
355
J.
E.
(With Plate XXX.)
T H E observations here recorded were begun some years
ago by the advice and with the. supervision of Professor
Lankester in the Botanical Gardens at Oxford, and by his
kindness in supplying me with specimens and the use of the
Zootomical Laboratories of University College, London, I
have been enabled to verify and extend them.
The epidermis of the various Cyclostomes has been the
object of researches by several different observers, but the
most exhaustive account is contained in a paper by M.
Alexandre Fcettinger, which appeared in March, 1876, in the
' Bull, de l'Academie royale de Belgique/ 2me s£rie, lx,
No. S. This thesis embodies the results of former writers,
and gives a very complete account of the epidermis as seen
in Petrornyzon Jluviatilis and P.planeri and the larval condition of Ammocoete. And since this description will serve
us as a standard of comparison for the similar structures in
Myxine, it will be necessary to give a short account of his
researches before entering on the immediate subject of this
paper.
The epidermis of Petromyzon is composed of a series of
cells, of which M. Foettinger recognises four chief varieties:
The ordinary epidermic cells.
The caliciform or goblet cells.
The club cells.
The granular cells.
The ordinary epidermic cells, arranged in series and pressing one against the other, form the major part of the epidermis. They possess a nucleus, and in the lower strata very often
two nuclei. Their shape varies according to the situation in
which they are found. In the deeper layers—that is, those
next the dermis, they are prismatic or stalked, resting on
an expanded base, and in the more superficial region the
external series of cells has its free border thickened and
perforated by fine pore canals.
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J. E. BI.O.MP1ELD.
The caliciform, or goblet cells, are similar to those found
in other parts in other animals, as described by F. E. Schultze,
and do not call for a lengthened description.
The club cells are much larger than the other elements of
the epidermis, and are at once recognised by the yellow
colour they assume after treatment with picrocarmine.
They appear to consist of a membrane enclosing the yellow
contents, which have a lamellated arrangement. At one
pole of the oblong cell two nuclei are always present. As
regards their origin and fate, M. Foettinger observed that
they originated in the deep layer of the epidermis among
the cells resting on the dermis, from which they were soon
capable of being distinguished by their increasing size and
tendency to yellow coloration with picric acid. As maturity
is being reached, both these characters become more marked,
and the cell itself slowly makes for the superficial layers of
the epiderm. When it has reached this point it raises the
superficial cells and is discharged to the exterior, where it
may be seen resting, devoid of nucleus, on the superficial
cells. As it progresses to the surface it leaves a finely
granular mass behind to mark its course.
These bodies were supposed by former writers to be muscular (Max Schultze) or nervous (Kolliker) in nature ; but,
as M. Fcettinger states, there can be little doubt that their
function is glandular.
The granular cells are spherical or ovoid in shape, and
lie embedded among the ordinary epidermic cells. Each
possesses a nucleus surrounded by coarsely granular fluid
material, which forms the cell substance, and the whole is
enclosed by a membrane. From the side of the cell next
the dermis several fine processes are seen, which dip down
among the lower cells, and may reach the epidermis; in
which case they terminate in an expanded base, similar to
the epidermic cells found in this situation. Picrocarmine
is the best reagent to distinguish them from the surrounding elements, as it stains them a bright red. They originate
from ordinary epidermic cells in the deepest layer of the
epidermis, and grow upwards, always remaining in contact
with the dermis by their processes, till they reach the external layers of the epidermis, where they serve some
purpose at which M. Fcettinger will not hazard a guess.
General Epiderm of Myxine.
We will now pass on to the consideration of the epidermis
in Myxine. On examining a section of the body-wall of a
Myxi-ne which has been stained with picrocarmine—for this
THREAD-CEM.S AND KIM HEIIMIS OP MYXINE.
357
is the staining fluid which is most useful from its differential
colouring of the various structures—there is seen (cf. fig. 1)
most externally the epidermis, succeeded by the dermis,
which in its turn rests on a peculiar subcutaneous connective tissue, much resembling the notochordal variety of that
group in its general appearance, and these two latter structures merit a short account.
The subcutaneous tissue is rather remarkable. In a
specimen which has been in spirit below the dermis there is
seen an irregular network composed of spherical or hexagonal meshes, each mesh being composed of fibrous tissue
continuous with and running into the dermis, and holding
somewhere in its lumen a granular nucleus with nucleolus,
which sometimes takes up a central at other times a parietal
position (PI. X X X , fig. 1 c).
If now a specimen be examined which has not been subjected to the action of spirit, but preserved in picric acid,
this network is found to be filled with an oily yellow liquid,
occupying the spaces in which the fibres and nuclei may be
traced as before.
This tissue is to be regarded as adipose tissue, in which
the primitive cells, supported in a framework of fibres from
the dermis, have by a process of cell secretion become
•metamorphosed, leaving only the fibrous stroma and nuclei
floating in the oil to testify to their former condition.
The dermis is a structure composed of layers of fibrous
tissue, which take up the carmine of the picrocarmine and
assume a bright pink colour. Here and there between the
laminae may be seen pigmented cells (fig. 1 b).
The epidermis rests on the dermis (fig. 1 a), and is composed
of several layers of cells, among which a difference may at
once be recognised by the use of a staining fluid, such as borax
carmine, which divides it into two portions, of stained and
unstained matter, much in the same way that the epidermis
of higher vertebrates is distinguished into the stratum mucosum and stratum corneum, the stained portion being composed of living protoplasmic cells and the unstained portion
of formed cells, whose protoplasm has disappeared in the
process of producing the particular body which in the case
we are considering is mucus and in the higher vertebrates
keratin.
That the substance is mucus in the case of Myxine is
probable from the shape of the cells, which conform in
character to the typical mucus or goblet cells found in the
alimentary and respiratory tracts. These goblet cells are
arranged in series two or three deep. They vary slightly
358
J. E. BI.OMFIKLD.
in shape, being sometimes pyriform, at others caliciform,
but in all the shrivelled remains of the nucleus can be
seen at the base. The cells of the most external row
sometimes present a beaded appearance on their free surface,
which reminds one of the external cells in Petromyzon,
which have their side much thickened.
In the deeper parts of the unstained layer young goblet
cells may be seen, which possess a normally-sized nucleus,
and are capable of staining; and between these and the
mature mucus-cell all stages are recognisable—in fact, there
are few tissues in the animal kingdom that afford such good
examples of goblet cells and their formation (fig. 2 a).
The ordinary epidermic cells need not detain us. The
lowest series (fig. 2 b) have an expanded base, and the higher
form transitions to the mucus cells; but embedded among
them are two larger kinds, which merit a longer description.
The first kind are stained yellow by the picric acid (fig. ] d).
They occur at all parts of the epidermis, and may sometimes
be seen protruding between the superficial cells at the surface.
In shape they are irregular, but always oblong, appearing
to have their form determined by the ordinary cells which
surround and firmly embed them, for it is impossible in teased
preparations to obtain them quite isolated. Their appearance is coarsely granular, and no obvious nucleus is present,
but often a small red spot may be detected, which would
appear to be the remains of that structure. From the fact
that they stain yellow with the picric acid of the picrocarmine one is lead to the idea that they correspond to the
" club cells " of Petromyzon described above ; and though
they are not so complicated in their structure, yet their
behaviour with picric acid and similarity of their fate—for,
like those cells, they evidently grow towards the surface and
are discharged—leave little room to doubt that they are
homologous.
The second kind of cell (fig. % e) alluded to above would
seem to correspond to the granular cells of Petromyzon,though
the correspondence is not so well made out. In most of my
sections, which were prepared from specimens hardened in
chromic acid or Muller's fluid, these cells appear as clear
spherical spaces embedded among the ordinary epidermic
cells which are compressed and flattened in their immediate
neighbourhood.
The periphery is bounded by a membrane, and in the
interior is a finely granular mass containing a deep-stained
spot, which appears to be the remains of a nucleus. From
the granular mass processes of similar substance radiate
THHEAD-CELLS AND EPIDERMIS OF MYXINE.
359
towards the periphery, giving the whole the appearance of
a spider.
In some specimens, however, which had been placed in
alcohol without previous treatment with chromic acid, the
appearance presented by these cells was different. For then
they seemed to be filled with a coarsely granular material,
exceedingly greedy of the carmine of the picrocarmine,so that
they quickly became stained of a bright pink hue, in which
it was impossible to make out the presence or absence of a
nucleus (see fig. 8 e). This difference in the cells, according to their previous treatment, seems to show that they
contain a liquid material, which is precipitated by alcohol,
but not by chromic acid or Miiller's fluid. If these cells correspond to the granular cells (cellules granuleuses) of
Petromyzon, one would expect to find processes from them
towards the dermis, but this I have not been able to do. As
regards the function of these cells, I am in no better position than M. Foettinger to offer suggestions; but from the
behaviour of similar cells in the glands, which will be considered in the sequel, it seems probable that they contribute
some part to the mucus, possibly the more liquid portions of
that secretion.
The Lateral Glands of Myxine.
Myxine is characterised by the large amount of mucus
which it is capable of ejecting, so that it has earned the
specific name of " glutiuosa," a term, the appropriateness of
which may be judged by the fact that two individuals thrown
into a bucket of water are capable of gelatinising the whole
with their secretion. This secretion conies from a series of
glands which are placed on each side the body, and open by
a series of pores, easily seen on the exterior. Each gland
lies beneath the dermis, embedded in the subcutaneous
tissue, and having the lateral bands of muscle on its inner
side, which are pushed inwards by the distended gland, and
appear capable of exercising an influence in ejecting the
contents, though to what extent this is actually the case it
is hard to say in the absence of direct experiment.
These glands are to be regarded as solid involutions of
the integument in which the epidermic cells have acquired a
more specialised structure in connection with the production
of mucus. They are lined by a fibrous wall, which, in sections
of a small and young gland, appears to be composed of several
layers, though in the fully-developed organ the layers are
fused into one. Eesting on the wall and filling the gland
are the peculiar yellow cells, the characteristic elements of
360
J. E. BLOMFIELU.
the secretion, supported by a stroma, and it is possible to
trace the continuity of the cells in the gland with those of
the epidermis through the stalk of the invagination, so that
the fibrous wall is obviously continuous with the dermis, and
the cell elements of the gland with those of the epidermis.
If the contents of a gland be squeezed out and received
on a slide {cf, fig. 5) and examined, they are seen to be
composed of a quantity of large oblong yellow cells, which
appear to be unwinding, and giving origin to long coiled
and twisted threads running in various directions across the
field of the microscope. Besides these are others bearing a
considerable resemblance to spiders, which appear to form a
stroma for the support of the other cells. These " spider
cells " are found in specimens preserved in spirit.
Of these two structures, the first, which we will designate
"thread cells" {cf. fig. 6), are of particular interest, from the
peculiarity of their construction. In shape they do not
exhibit much variation. They are always oblong, sometimes larger at one end than the other, but generally more
or less ovoid. They stain very readily with picric acid of a
bright yellow hue, and under the action of carmine exhibit
a nucleus or remains of such at one of the poles (the head).
This, which in the unstained cell seems to be of the nature
of a vacuole, after long treatment with staining fluids, is
seen to be a nucleus, often containing one well marked and
comparatively large nucleolus. On careful focussing when
examining the mature cells it is seen that the yellow material is divided into threads, which are wound round a central
core of granular matter. The exact mode in which the
thread is wound is rather hard to determine, but it seems
that most superficially it is arranged transversely to the long
axis of the cell, while in the next layer it runs parallel with
it (fig. 7). When the cell is mature and while it is in the
gland, or after its discharge, it begins to unwind. This
process begins at the pole opposite the nucleus, and it seems
that the cell is unravelled by a single thread. These
threads are found in the mucus at the surface of the body
of the animal in association with epidermic and goblet cells.
In sections of a young gland {cf. fig. 8) where the elements are not mature, it is possible to trace the origin of
the thread cells from the ordinary epidermic cells. As has
been stated above, the glands are to be regarded as solid
involutions of the epidermis, in which the various epidermic
cells undergo a change in connection with the secretion of
mucus, and these two elements—the " thread cells " and
" spider cells"-—maybe seen to have their origin from ordi-
THREAD-CELLS AND EPIUERMIS OF MYXINE.
361
nary epidermic cells. After staining with picrocarmine, it
is easy to trace the development of the thread cells, because
as the yellow-staining material is deposited in the cell the
red tint gives place to a yellowish red, and then becomes
altogether replaced by yellow, except the nucleus and a
small zone of granular matter around it, which retains its
pink tint. As the deposition of the yellow matter progresses striae make their appearance, which indicate the
commencement of the thread-like arrangement that obtains
in the mature cells (fig. 9).
It seems a matter for surprise that these curious cells
should have hitherto escaped description, but as far as I am
aware there is only one drawing of them, which occurs in
Leydig's ' Manual of Histology' (French translation, p. 225),
in which they are figured with threads appended to their
tails and described as nerve cells, the thread being regarded
as the fine termination of a nerve.
The second kind of cell, the " spider cell," would appear
to represent the granular cells (Foettinger) of the epidermis,
which, when enclosed in the glands, undergo a process
whereby they lose their contents, and nothing is left save
the membrane and nucleus, and a granular material round
the latter. It may be that these cells are concerned in the
nourishment of the thread cells, or, as seems more probable,
they themselves contribute a constituent of the secretion,
probably the more liquid part. When in the gland they form
a sort of weak stroma, holding together the thread cells.