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Feathers, Scales, and Hair ( VERY LONG)



Message text written by INTERNET:Dinogeorge@aol.com

"At this point there is simply no way to tell. One might think, since
lizards
and snakes are covered with scales and birds are covered with feathers,
that
scales are somehow "more primitive" than feathers. But scales and feathers
appeared independently--scales in lepidosaurs, feathers in archosaurs--so
that neither is a derived state of the other. It is entirely possible that
some kind of keratinous "pre-pre-feather" appeared in the most primitive
archosaurs and subsequently became modified into "things that look like
scales but aren't" in pre-avian archosaurs, "things that look like feathers
but aren't" in such animals as _Longisquama_, spikes and plates in
sauropods
and ornithischians, pre-feathers in the earliest avian dinosaurs, and true
feathers in later theropod dinosaurs and birds.

It is also possible that scales and feathers developed independently from
the
same kind of dermal structure in the common diapsid ancestor of both
lizards
and birds. I suppose that tests of the chemical composition of scales, croc
scutes, and feathers in extant reptiles would shed some light on the issue.



        This may be of interest to those on this thread. It's taken from:

Torrey, T.W.  1967.  _Morphogenesis of the Vertebrates_, pp. 137-159.  (I
have no further info on it, although I understand the book's been through
many versions since this one -- I haven't had time to check it out yet!) 
The ASCII graphics are my poor attempts to replicate the ones in the book;
I apologize if they're unclear.  You may need to switch your e-mail program
to a monospaced font (Courier or Monaco) to see them properly.  It may also
be helpful to remember here that:

* epidermis is ectodermal in origin
* dermis is mesodermal (mesenchymal) in origin 

        I post this only to note how different feathers are from hair and
from scales.  I leave it to the reader to assess how they think this
applies to phylogeny.

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EPIDERMAL SCALES:  Epidermal scales are derivatives of the stratum corneum,
the keratinized surface component of the epidermis.  because the stratum
corneum is best expressed in terrestrial vertebrates, epidermal scales are
lacking in scales and are found only rarely in amphibians.  It is possible
that the ancient armored amphibians were equipped with epidermal scales,
but in modern forms they are virtually absent.  The nearest approach to
them are such localized thickenings of the stratum corneum as the "warts"
of toads.

        In contrast to amphibians, epidermal scales are extremely well
developed in reptiles and are found to be of two general types.  One
variety occurs in snakes and lizards, the other in crocodiles and turtles. 
In the former type a scale is initiated embyonically by an elevation of
epidermis into which the subjacent dermis projects [1].  The dermis and
stratum germanitivum subsequently retract, leaving the flat, cornified
scale at the surface [2]

[1]            *       *        
             *.*     *.*          
            *..*    *..*
           *...*   *...*
       ****......***....****
       ......................


[2]        *        *
         **        **
        ***       ***
       ****      ****
      *****     *****
    **....*   **....*
   **......***.......***
  ......................

where * is the combined stratum corenum and germinativum and . is the
dermis

Although the final scale is purely epidermal, the dermis appeares to be
involved in the initiation.  Generally speaking, the scales of snakes and
lizards are arranged in longitudinal rows...  (SNIP)

        The epidermal scales of turtles and crocodiles do not overlap as do
those of lizards and snakes, and each is an exclusive product of the
stratum germanitivum without participation of the subjacent dermis.  In
turtles the epidermal scales arise on top the bony armor beneath, but their
pattern does not correspond to the bony plates.  (Scales also cover the
neck and legs.)  Each scale is produced independently through the activity
of the stratum germinativum.  During the life of the turtle, the
germinativum layer under each scale periodically creates a new scale of a
diameter slightly larger than the preceding one.  The older scale is then
pushed away from the surface.  The older scale may be worn or peeled off,
but commonly the successive generations of scales simply pile up one upon
the other.  Because each new scale is larger than its predecessor, there is
created an impression of "growth rings" indicative of the number of scales
produced.  The entire body of crocodilians is also covered by epidermal
scales which, earing away steadily, are replaced as needed.

        Birds and mammals have a limited heritage of the epidermal scales
possessed by their reptilian ancestors.  Such as they have are of the
overlapping type and are formed in the same manner as those of snakes and
lizards.  In birds they are confined to the legs and feet.  When they do
occur in mammals, it is usually on the tail, as in the beaver, rat, mouse,
and opossum.  The pangolin, or scaly anteater, of Asia and Africa, however,
has a complete body covering of horny scales, and epidermal scales are
superimposed on the dermal bony plates investing the armadillo.

FEATHERS:  (SMALL SNIP)  Feathers are believed to have been derived from
reptilian scales, but the similarities between them are evident only during
early embryogenesis.  (SNIP)  The embryogeny of feathers, especially
contour feathers, involves many details (and some disputations)... (SMALL
SNIP)  The first indication of a feather, like the scale of a snake or
lizard, is found in an aggregation of dermal cells immediately beneath the
epidermis.  Continued proliferation of the dermal aggregation results in a
conical elevation, the dermal papilla, ensheathed by epidermis [1].  Thus
formed, the feather germ grows rapidly outward as a tapering epidermal
cylinder filled with a vascular mesodermal core [2].  Meanwhile, the
epidermis surrounding the base of the papilla sinks inward so that the
feather germ comes to project from a pit...termed a follicle.  From this
point onward, down and countour feathers show developmental differences.  

[1]     ***    ***    ***
        ...*  *...*  *...*
        ....* *....* *....*
        .....*......*......*

[2]              *
                *.*
                *..*
               *....*
               *....*
               *....*
               *....*
              *......*
   *****     *........*    *****
   ....*    *..........*   *....
   .....*   *..........*   *....
   .....*    *........*   *.....
   ......*    *......*   *......
   .......*   *......*  *.......
   ........***........**........
   .............................
 

symbols as above.

(SNIP)

        Although feathers are the products of the epidermis, the first
indication of a prospective feather is the appearance of a mesodermal
papilla.  One might be prone to dismiss the dermis with the generalization
that it serves a temporary purpose in bringing a necessary blood supply to
the developing feather and is thereafter eliminated.  But a whole series of
exquisite experimental analyses have shown that the original dermal papilla
plays a vital role in initiating the morphogenetic activities of the
ectodermal epidermis.  It has been demonstrated, for instance, that
embryonic skin ectoderm which normally is not concerned with feather
formation will produce feather germs when prospective dermal papillae are
grafted beneath it.  Or if a feather is plucked from the adult bird and the
dermal papilla left in the floor of the follicle is the excised, a new
feather will not form from the epidermal cells of the follicle.  From these
and related experiments, it seems clear that the dermal papilla provides a
kind of morphogenetic stimulus that is required if epidermis is to form a
feather [induction].

        But the epidermis is more than a passive partner.  If a mesodermal
papilla is removed from a follicle on the rump of an adult hen and
implanted in place of a papilla on the wing, it becomes covered by wing
epidermis and induces tis epidermis to form a feather.  The feather,
however, is of the _wing_ variety.  Thus, although the mesodermal papilla
provides the necessary inductive stimulus, the epidermis not only forms the
feather out of its own substance but furnishes the feather's distinctive
character.  Interestingly enough, the local competence of the epidermis
appears to be acquired early in embryonic life, as shown by the following
experiment.  if mesoderm in the shoulder of an early chick embryo is
replaced by mesoderm from the thigh, the shoulder ectoderm tkes on the
specificity of thigh ectoderm, for the hatched chick possess a patch of
thigh feathers on its shoulder.  The early lability of epidermis is also
shown by an experiment in which it is brought under the influence of
scale-inducing dermis.  in this instance scales rather than feathers are
produced, pointing out not only to the adaptibility of epidermis but to the
likely close evolutionary relationship between epidermal scales and
feathers.

HAIR:  (SMALL SNIP)  Hair, like feathers, are also entierly epidermal in
origin and composition.  But unlike feathers whose source in reptilian
scales seems well established, the phylogenetic origin of this distinctive
mammalian integumentary structure remains uncertain.  Some investigatores
feel that hairs, too, came from reptilian scales, but because of the
relatively inconspicuous role played by the dermis in the embyrogeny of a
hair, most workers doubt the validity of the scale-hair homology and look
to other possible derivations.  (SNIP)

        Unlike feathers, whose development is initiated by a preliminary
aggregation of dermal cells, the embryogeny of a hair begins with the
epidermis alone.  At the site of a future hair, an epidermal nodule is
formed by a local proliferation of the stratum germinativum [1].  The
nodule then extends downward into the dermis as a tongue of tissue whose
deeper end becomes enlarged to form a bulb [2].  This bulb is soon molded
into an inverted cip into which the vascular dermis pushes and aggregates
to form a dermal papilla.  Meanwhile, the deep-lying portion of the
epidermal tongue splits so as to separate a central strand from a
surrounding epithelium.  The central stand becomes the root and shaft of
the hair; the surrounding tissue provides the wall of the follicle.  (SMALL
SNIP)

[1]       --*-*--*----*
          ***-**--****-
          ****-**-*****
          ...*****.....
          ...*****.....
          ....***......
          .....**......



[2]      --*-*--*----*
         ****-**--****-
         *****-**-*****
         ....*****.....
         ...*****......
         ...** **......
         ..*   *.......
         .* * *........
         * ** *........
        * ** *........
        * ** *........
        * ** *........
       * ** *........
       * ** *........
      * ** *........
      * *@* *.......
      **@@* *.......
      *@@@@**........


where * is stratum germinativum, - is epidermis, and @ is dermal papilla


        Although developing hairs have not been subjected to experimental
analysis comparable to that in feathers, there are some hints that
inductive mechanisms may again be operative.  One piece of evidence derives
from the fact that in the development of sensory vibrissae [whiskers], in
contrast to general body hairs, the epidermal nodule is preceded by a
subdermal condensation of dermis, just as a feather primordium is preceded
by a dermal papilla.  Moreover, it has been shown that embyonic epidermis
of the mouse, which normally would produce hair, when cultivated in the
absence of mesoderm, fails to form hair primordia.  It is possible,
therefore, that hairds, like feathers, require an inductive stimulus
furnished by the subjacent mesoderm.

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                _,_
           ____/_\,)                    ..  _   
--____-===(  _\/                         \\/ \-----_---__
           /\  '                        ^__/>/\____\--------
__________/__\_ ____________________________.//__.//_________

Jerry D. Harris                         (505) 841-2809
Fossil Preparation Lab                
New Mexico Museum of Natural History        
1801 Mountain Rd NW                           
Albuquerque  NM  87104-1375             102354.2222@compuserve.com