Phylogenesis of skin on chordates and the disorders

Table of contents
Abstract
Introduction
Phylogenesis of skin on Urochordates
Phylogenesis of skin on Cephalochordates
Phylogenesis of skin on Vertebrates
Disorders of the human skin
Bibliography

ABSTRACT
The skin of vertebrates has numerous and diverse
derivatives, either located within the epithelial sheet
itself (glands) or extending above its surface (teeth, scales,
feathers, hairs, etc.). Many of them have a modular
structure and constitute structural–functional units.
Ontogenetically, all skin derivatives are of ectomesodermal
origin, and their morphogenesis is subject to metabolic
control, heterochronies (divergence in the timing of
origination and development), and regulation by means of
tissue interactions and molecular signaling via similar
pathways.

The diversification (origination of morphological diversity) of skin
derivatives within the same morphological type is explained by the
development of new generations of ectomesodermal structures
separated by heterochronies and regulated by changes in the
gradients of molecular signaling pathways under the influence of
environmental factors.
Evolutionary relationships between the majority of skin derivatives
are obscure, except for teeth and glands associated with sensory
organs that have evolved together with these organs. Apparently,
many chordate skin derivatives (scales, feathers, hairs, and
glands) originated as novelties at nodal stages of phylogeny and
subsequently evolved convergently or in parallel.

INTRODUCTION
The problem of origination and diversification of skin derivatives in
vertebrates is still a challenge to zoologists. The main difficulty in
solving it is that, in contrast to the situation with other organ
systems (musculoskeletal, nervous, excretory, etc.), it is hardly
possible to trace evolutionary relationships between the derivatives
characteristic of representatives of different classes.
The discussion of the problem is accompanied by the advancement
of numerous hypotheses and conjectures that cannot yet clarify the
evolutionary scenario concerning the origination, morphogenesis,
diversification, and homology or historical divergence of these
structures.

Skin derivatives are numerous and diverse. They can be
divided into appendages, or structures extending above the
skin surface, and glands, which lie within the skin.
However, this division is obviously tentative. For example,
hair and feather follicles lie in the dermal or even
hypodermal layer, with only the keratinous shaft of these
structures protruding outward. Glands lie within the skin
but often have accompanying skin or keratinous
protrusions. The same applies to keratinization. Scales,
feathers, and hairs are keratinous structures, whereas
glands are not keratinized.

PHYLOGENESIS OF SKIN ON UROCHORDATES
The Urochordata , also known
as Tunicates and Ascidians ,
are more commonly known as
"sea squirts."
They belong to the same
phylum as the vertebrates
although adults do not have a
backbone.
The name tunicates comes
from the tunic (skin derivative)
surrounding their body.

The tunic is a thick, usually
opaque, covering protecting
their barrel-shaped bodies from
predators. It is made from a
material very similar to cellulose.
On the inner surface of the tunic
is a thin epidermis, it is this which
secretes the tunic. On the inside
of the epidermis is a thicker
dermis (body wall) and bands of
muscles.

The tunic is composed of
proteins and complex
carbohydrates, and includes
tunicin, a variety of
cellulose. The tunic is unique
among invertebrate
exoskeletons in that it can
grow as the animal enlarges
and does not need to be
periodically shed.

PHYLOGENESIS OF SKIN ON CEPHALOOCHORDATES
Cephalochordate, also called
Acrania, any of more than two
dozen species belonging to the
subphylum Cephalochordata
of the phylum Chordata.
Cephalochordates and
vertebrates have a hollow,
dorsal nerve cord, pharyngeal
gill slits, and a notochord.

In Subphylum
Cephalochordata the skin of
the lancelet has only two
regions: the outer epidermis
and the inner dermis. The
epidermis is very thin and
formed by single layered
stratified epithelium. The
dermis consists of the
connective tissue, which is not
well developed.

The skin is rich in sensory
nerve endings that probably
help produce a sense of touch
and are important for
burrowing. A number of
cephalochordates have some
photosensors near the front
and back ends of their body, but
in general (unlike vertebrates)
they lack any eyes or organs to
sense gravity.

PHYLOGENESIS OF SKIN ON VERTEBRATES
In birds, the skin is thin and dry,
glands are absent, except an oil
gland on the tail. The stratum
corneum is keratinized.
In mammals the skin is thick,
consists of epidermis, dermis and
subcutaneous fiber, and enriched
with glands. External epidermis is
a thin layer consisting of flat cells.
The corneal layer is composed of
thin scale-like dead cells without
nuclei.

Fish have scales and contain mucous glands, their secretion reduces water friction.
The skin of amphibians has no scales, epidermis is smooth, thin, moist and slimy covered by the film of mucus secreted by large mucous or alveolar glands.
The skin of reptiles is rough and dry without any glands because most of them inhabit hot and dry areas. Epidermis consists of external corneal and inner malpighian layers.

The human skin is the outer
covering of the body and is the
largest organ of the
integumentary system. The
skin has up to seven layers of
ectodermal tissue and guards
The underlying muscles, bones,
ligaments and internal organs.
There are two general types of
skin, hairy and glabrous skin
(hairless).

DISORDERS OF THE HUMAN SKIN
Rash: Nearly any change in the skin’s appearance can be called a rash. Most rashes are from simple skin irritation; others result from medical conditions.
Dermatitis: A general term for inflammation of the skin. Atopic dermatitis (a type of eczema) is the most common form.
Eczema: Skin inflammation (dermatitis) causing an itchy rash. Most often, it’s due to an overactive immune system.
Psoriasis: An autoimmune condition that can cause a variety of skin rashes. Silver, scaly plaques on the skin are the most common form.
Dandruff: A scaly condition of the scalp may be caused by seborrheic dermatitis, psoriasis, or eczema.

Acne: The most common skin condition, acne affects over 85% of people at some time in life.
Cellulitis: Inflammation of the dermis and subcutaneous tissues, usually due to an infection. A red, warm, often painful skin rash generally results.
Skin abscess : A localized skin infection creates a collection of pus under the skin.

Rosacea: A chronic skin condition causing a red rash on the face. Rosacea may look like acne, and is poorly understood.
Warts: A virus infects the skin and causes the skin to grow excessively, creating a wart. Warts may be treated at home with chemicals, duct tape, or freezing, or removed by a physician.
Melanoma: The most dangerous type of skin cancer, melanoma results from sun damage and other causes. A skin biopsy can identify melanoma.
Basal cell carcinoma: The most common type of skin cancer. Basal cell carcinoma is less dangerous than melanoma because it grows and spreads more slowly.
Seborrheic keratosis: A benign, often itchy growth that appears like a “stuck-on” wart. Seborrheic keratoses may be removed by a physician, if bothersome.

BIBLIOGRAPHY
https://www.webmd.com/skin-problems-and-treatments/picture-of-theskin#:~:text=Skin%20has%20three%20layers%3A,of%20fat%20and%20connective%20tissue.
https://www.encyclopedia.com/plants-and-animals/zoology-and-veterinary-medicine/zoology-general/cephalochordata
https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/urochordata
https://www.researchgate.net/publication/226888918_Skin_derivatives_in_vertebrate_ontogeny_and_phylogeny
Andrews' Diseases of the Skin by William D. James; Timothy Berger; Dirk Elston