10. Excretion ( releasing of sweat by the sweat glands)
SKIN IS FUNCTIONALLY A UNIT WITH 3 PARTS:
3. Basement Membrane Complex
1. Outermost layer
2. An Ectodermal derivative
3. Often glandular
The epidermis is avascular (contains no blood vessels ) and is nourished by diffusion from the dermis . The main type or the four principal types of cells which make up the epidermis are keratinocytes , melanocytes , Langerhans cells and Merkels cells . It is a keratinized stratified squamous epithelium.
5 layers of the epidermis
The stratum corneum ("horny layer") is the outermost layer of the epidermis (the outermost layer of the skin ). It is composed mainly of dead cells that lack nuclei . As these dead cells slough off, they are continuously replaced by new cells from the stratum germinativum (basale).
Cells of the stratum corneum contain keratin
The thickness of the stratum corneum varies according to the amount of protection and/or grip required by a region of the body.
The stratum lucidum ( Latin for "clear layer") is a thin, clear layer of dead skin cells in the epidermis, and is named for its translucent appearance under a microscope .
It contains a clear substance called eleidin , which eventually becomes keratin .
This layer is found beneath the stratum corneum of thick skin, and as such is only found on the palms of the hands and the soles of the feet.
The keratinocytes of the stratum lucidum do not feature distinct boundaries and are filled with eleidin, an intermediate form of keratin.
Thin skin, which covers the rest of the body, lacks a definite stratum lucidum and stratum granulosum.
The stratum granulosum layer typically contains 1 to 3 rows of squamous cells with many small basophilic granules in their cytoplasm.
contain also large amounts of filaggrin , a protein thought to serve in bundling keratin.
Membrane-coated lamellar granules (0.1 to 0.3 micrometers) are also present and contain lipids.
The lamellar granules are exocytosed in this layer to generate a waterproof barrier. This waterproofing also prevents life-sustaining nutrient transport, and thus leads to the characteristic cell death of the outer layers of keratinized epithelium.
the stratum spinosum is a multi-layered arrangement of cuboidal cells that sits beneath the stratum granulosum .
Adjacent cells are joined by desmosomes , giving them a spiny appearance when the cells shrink during the staining process while the desmosomes hold firm.
Their nuclei are often darkened (a condition called pyknosis ), which is an early sign of cell death.
Cells of the stratum spinosum actively synthesize intermediate filaments called cytokeratins, which are composed of keratin . These intermediate filaments are anchored to the desmosomes, joining adjacent cells to provide structural support, helping the skin resist abrasion.
( also stratum basale or basal cell layer ) is the layer of keratinocytes that lies at the base of the epidermis immediately above the dermis . It consists of a single layer of tall, simple columnar epithelial cells lying on a basement membrane . These cells undergo rapid cell division, mitosis , to replenish the regular loss of skin by shedding from the surface. About 25% of the cells are melanocytes , which produce melanin , which provides pigmentation for skin and hair.
The epidermis is divided into many layers where cells are formed through mitosis at the innermost layers.
They move up the strata changing shape and composition as they differentiate and become filled with keratin .
They eventually reach the top layer called stratum corneum and become sloughed off, or desquamated. This process is called keratinization and takes place within weeks. The outermost layer of epidermis consists of 25 to 30 layers of dead cells.
The Dermis (=corium)
1. Innermost layer
2. A mesodermal derivative
3. Neural Crest gives rise to dermal
armor chromatophores or if present
4. Contains a neural and vascular supply
5. Provides tensile strength and physiologic support
It contains the hair follicles , sweat glands , sebaceous glands , apocrine glands , and blood vessels . The blood vessels in the dermis provide nourishment and waste removal to its own cells,
In human skin it has two layers :
1. Papillary Layer
2. Reticular layer
The papillary region is composed of loose areolar connective tissue .
It is named for its fingerlike projections called papillae , which extend toward the epidermis.
The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
The reticular region lies deep to the papillary region and is usually much thicker.
It is composed of dense irregular connective tissue,some elastic fibers and adipose tissue may be present and receives its name from the dense concentration of collagenous , elastic , and reticular fibers that weave throughout it.
These protein fibers give the dermis its properties of strength, extensibility, and elasticity.
Located within the reticular region are also the hair roots , sebaceous glands , sweat glands , receptors, nails , and blood vessels .
The Basement Membrane Complex
In between epidermis and dermis
Outer single layer = Basal Lamina
Inner layers = Basal Lamella
Structures of the Dermal Layer
Collagen and Elastin
Form from stratum germinativum
the types of glands would vary on the ff:
a. Types of glands based to composition
b. Types of glands based as to method of secretion
c. Types of glands based as to type of secretion
Types of glands based to composition
= Single specialized and interspread among the epidermal cells
= commonly known as mucous cell
2. Multicellular cells
= form from the ingrowths of the stratum germinativum into the dermis
= nourished by blood vessels
Types of unicellular glands
Club cells = Elongated, binucleated unicellular glands.
= contain chemicals that
excite alarm or fear
2. Granular cells = narrowed base and wide apical end for secretions
= absent in lampreys
4. Sacciform cells = hold a large, membrane- bound toxic secretory products
Types of Multicellular Glands based on shape:
Tubular glands –
a. Simple tubular glands
- short, blind tubes
located in the dermis
example: thumb pads of male frog : mental glands of salamander : ceruminous glands in the ears of mammals
b. simple coiled tubular gland
= long narrow tube, coiled distal end located in the dermis
= openings are
referred to as pores
of the skin
ex: sweat glands
C. Simple branched tubular glands
= divides at its distal ends into two or more branches
= terminals may or may not be coiled
Ex. Large sweat glands in the armpit
D. Compound tubular glands
= consist of a varying number of simple tubular glands.
2. Saccular glands
a. Simple saccular glands
= with only one expanded bulb or acinus at the end of the duct.
ex: mucus gland and poison gland
b. Simple branched saccular gland
= with several acini arranged along a single excretory duct.
ex : sebaceous gland
c. Compound Saccular glands
= consist of several simple saccular glands called lobules
Ex. Mammary glands
Types of glands based as to method of secretion
Cytogenic – whole cells; testes and ovaries
- They perform their exocrine or cytogenic function by producing living cells
2. Holocrine – product is entire cell contents
- The secretory cell is released and as it breaks apart, the contents of the cell become the secretory product.
- involves death of the cell
3. Merocrine – product moves through cell membrane often by exocytosis; salivary, pancreas
- These are moved to the apical surface where the vesicles coalesce with the membrane on the apical surface to release the product. Most glands release their products in way
4. Apocrine – product is cytoplasm at tip of cell;
the apical portions of cells are pinched off and lost during the secretory process.
Types of glands as to type of Secretion
Mucous glands = secretes mucus
Serous glands = sudoriferous glands
= secretes watery subtances
3. Sebaceous Gland = oil glands
= secrete oily substances
ex : uropygial glands in birds
ceruminous glands and Meibomian glands in humans
Three types of chromatophores
1. Melanophores (brown or black pigment)
= contains melanin
2. Lipophores ( xanthophores with yellow pigment and
3. E rythrophores with red pigment)
4. Iridophores or Guanophores (reflective)
Dermis produces a dermal scale in many
a scale ( Greek lepid , Latin squama )
Serve as exoskeleton of vertebrates for protection of the body
Grouped into epidermal and dermal scales based on mode of development
The scales would vary from every class of vertebrate
Types of scales based on origin
- formed from stratum germinativum
- characteristic of terrestrial tetrapods
- usually replaced
- include the scutes of turtle and snakes
2. Dermal scales
- located in the dermis and mesenchymal in origin
- characteristics of fishes
SURVEY OF VERTBRATE SKIN
Epidermis limited to columnar cells and mucous cuticle
Epidermis is more complex with club and granule cells
1. Epidermis is very thin, with 2 cell types….epidermal cells and unicellular glands (mucous)
2. Mucous cuticle on surface
3. Microridges to hold mucous in place
4. Dermis contains chromatophores
Types of scales of fishes
Placoid Scales – consist of basal plate embeded in the dermis with caudally directed spine projecting through the epidermis
- plate and spine are compose of dentin
- each spine is also covered with enamel
2. Cosmoid scales
- True cosmoid scales can only be found on the extinct Crossopterygians .
- The inner layer of the scale is made of lamellar bone.
- On top of this lies a layer of spongy or vascular bone and then a layer of dentinelike material called .
- The upper surface is keratin . The coelacanth has modified cosmoid scales that lack and are thinner than true cosmoid scales
3. Ganoid scales
- Ganoid scales can be found on gars (family Lepisosteidae ) and bichirs and reedfishes (family Polypteridae ).
- Ganoid scales are similar to cosmoid scales, but a layer of lies over the cosmine layer and under the enamel.
- They are diamond-shaped, shiny, and hard.
Ganoid scales of a Florida Gar Lepisosteus platyrhincus Dekay, Scanning electron micrograph of a Florida Gar ganoid scale. Note peg at upper left which articulates with adjacent scal
4. Leptoid scales
Leptoid scales are found on higher order bony fish and come in two forms, ctenoid and cycloid scales.
As they grow, cycloid and ctenoid scales add concentric layers.
The scales of bony fishes are laid so as to overlap in a head-to-tail direction, a little like roof tiles, allowing a smoother flow of water over the body and therefore reducing drag .
Two leptiod scales ctenoid and cycloid scales
6. Cycloid scales
Cycloid scales have a smooth outer edge, and are most common on more primitive fish with soft fin rays, such as salmon and carp .
Cycloid scales of a Red Firefish
Ctenoid scales have a toothed outer edge, and are usually found on more derived fishes with spiny fin rays, such as bass and crappie
Scales entirely of lamellar bone
Annuli and Circuli
Paradise Fish Macropodus opercularis
5. Rhomboid scales
- rhomboid in shape
- bone is hard, shiny, inroganic substance known as ganoin
Placoid scales or dermal denticles
Outer enamel; inner dentin
Epidermis does not cover scales
Outer enamel, intermediate dentin, bony core
Dermal scales of three basic types….
Ganoid (Gars, Bichirs)
Outer enamel (=ganoin), inner bone
Cycloid and Ctenoid (Teleosts that bear scales)
Epidermis with thin stratum corneum and very little keratin; Leydig cells
Dermis with chromatophores, poison glands and mucous glands
Scales are rare
Epidermal scales, with thick outer layer of keratin
Thinner “hinge” region
Inner layer of epidermis regenerative sloughing
Outer scale surface ( Oberhäutchen ) often sculpted …m icroepidermatoglyphics
Dermis with chromatophores in many
Dermis may possess Osteoderms
1. Epidermis thin and bilayered… stratum corneum and
2. Dermis well-vascularized and innervated
3. Very few glands
4. Unique epidermal feathers (of keratin) with basic
Barbs, barbules and hooklets
Basic feather types:
= Flight, Down, Filoplume and Contour
= Feathers probably arose as epidermal scale modifications
Epidermal chromatophores produce pigments which are carried into feather during development, but feather surface provides structural color
Types of Feathers
Contour feathers give the bird its characteristic smooth round shape. They also give the bird its visual coloring and provide a first level of defense against physical objects, sunlight, wind and rain. They are very important.
Down feathers are smaller and lack the barbules and their accompanying hooklets so they are not zipped together and do not look as neat. In fact they are soft and fluffy. They provide most of the insulation and are so good at this that mankind for many years collected down feathers from various birds to put into sleeping bags and jackets to help keep us warm.
Semiplumes are half-way between a contour feather and a down feather. These occur between the contour feathers and help to supply insulation and a certain amount of form as well.
Filoplumes are very small and have only a very few barbs at their tips. They are believed to have a sensory function, helping birds keep their feathers in order.
Bristles have practically no barbs at all and are stiff. They occur around the eyes and mouths of some birds and are protective in function. They are particularly evident in the honey buzzard ( Pernis apivorus ) for instance, which feeds on the nests and young of social bees and wasps and needs protection around its beak from the stings of the adult bees and wasps.
Epidermal feather primordium, dermal papilla, “collar” and eruption
Feathers grow in tracts, and are connected together in the dermis by tiny feather muscles
Epidermis with 5 layers:
Stratum corneum – outer, keratinized
Stratum lucidum – no organelles
Stratum granulosum – keratin development
Stratum spinosum – developing ells
Stratum basale – germination layer
Epidermal glands present in dermis:
Sebaceous (oil) – Holocrine
Sudoriferous (sweat) – Merocrine
Gland types based on fate of product:
Exocrine – ducted; product into ducts
Endocrine – ductless; product into blood
Integumental derivatives result from on of three processes:
I. Functional Epithelial Extinction (FEE) = which leads to “Structured Ectodermal Derivatives”
II. Ectodermal-Mesodermal Interaction (EMI)
= which leads to “Structured Ectodermal- Mesodermal Derivatives”
Dermis well-vascularized and innervated
= Hair produced in epidermis, and unlike scales and feathers is an ingrowth of epidermis into the dermis
Root and Shaft
Cuticle, Cortex and Medulla
= Fur (pelage ) is a thick covering of hair
Guard hairs – longer, coarser
Underfur – shorter, finer
= Hairs moved by arector pili muscles
III. Delamination (DEL)
= which leads to “Structure Mesodermal Derivatives”
I. Structured Ectodermal Derivatives
A. Integumental glands
Mucous – Fishes (unicellular
Poison – Fishes (unicellular/multicellular)
Amphibian, one bird (multicellular)
Venom – Reptiles (modified salivary)
Platypus (modified sweat?)
Salivary – Primarily tetrapods
Musk (scent) – Reptiles, Mammals
Preen (uropygial) – Birds
Sebaceous(oil) - Mammals
Ceruminous (wax) – Mammals, Turkey
Sudorifereous (sweat) – Mammals
Mammary – Mammals (modified sebaceous?)
Photophore Glands – Deep sea fishes
B. Keratinized integument
A manifestation of FEE several processes:
1. Shedding: Continuous loss of small flakes or cell groups. Probably in all vertebrates…even in areas of specialized thickenings (callouses, palmar and plantar surfaces).
2. Sloughing: Periodic loss of large complete sheets of skin.
Many fishes (mucous cuticle)
Most amphibians (with autophagy)
Reptiles (may be accompanied by autophagy)
Some seals, whales, elephants, cervid velvet
3. Molting: Periodic loss of specialized keratinized
Hair…..including baleen, quills
Shell-breaker (=egg caruncle)
epidermal structure of birds, turtles, crocodilians, tuatara (Egg Tooth of lizards and snakes is a true tooth)
4. Retention: Rather permanent specialized
keratinized ectodermal derivatives.
a. Rattlesnake rattle
True horn: bony spike from skull sheathed in keratinized
never branched (except for
d. Claws, Nails, Hoofs
All containing Unguis and Subunguis
e. Digital caps (amphibian “claws”)
f. Local thickenings – Tori, friction ridges
II. Structured Ectodermal-Mesodermal Derivatives
Composite structures derived from an interaction between Ectoderm and Mesoderm, such as
1. Dermal Scales
III. Structured Mesodermal Derivatives
Structures derived primarily from Mesoderm, such as