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BIOLOGY 453 Exam 1: Lecture 3


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Epidermis: function, germ layer, tissue type, major layers?
* Function - barrier to external environment
* Embryonic Germ Layer - somatic ectoderm
* Tissue Type - epithelial - tight junctions between cells; normally non-vascular
* Major Layers
o Stratum germinativum/basalis
+ mitotically active, migrates up; always present
o Stratum corneum
+ ONLY tetrapods; outermost layer of skin, if it is present
+ made of dead cells, hardened with keratin - a scleroprotein
+ functions
# hardening agent protects skin from abrasion
# thick layers reduce water loss; addition of waxes/lipids further waterproof skin
Exocrine glands - define and 3 types?
* Define - gland that have ducts to surface of organ (i.e. skin in this case)
* secretion methods (we'll use these terms primarily with mammals)
o holocrine - whole cell destruction, releases contents & cell membrane - very sticky
o apocrine - top part of cell pinches off with secretion - relatively sticky
o eccrine/merocrine - exocytosis or active transport - watery secretions
Dermis: function, germ layer, tissues, mineralized tissues?
* Function - attach epidermis to underlying muscle, cushioning of deeper organs...
* Embryonic Germ Layer
o mesoderm typically
o epimere or somite - dermatome; hypomere - somatic mesoderm layer
* Non-Mineralized Tissues
o Define - scattered cells in a matrix of fluid, gel that has extracellular proteins; vascularized
o Dense Connective Tissue - lots of extracellular collagen fibers, strong
o Dermal Papillae
+ specialized structure within dermis
+ highly vascularized, loose, fibrous connective tissue
+ functions -
# induces formation of epidermal & dermal scales
# nourishes teeth
# forms hair & feather follicles...
* Mineralized Tissues - connective tissue with mineral deposits - hydroxyapatite (CaCO3 & CaPO4)
o high mineral content increases stiffness & strength, resists deformation
o collagen fibers in matrix reduce cracking of hard material, gives tolerance to torsional effects
o enameloids & dentine layers grow in laminar sheets - rows of cells secrete mineral deposits & move away from hardened tissue layers
Hypodermis (Subcutaneous): function, germ layer, tissue type?
* Embryonic Germ Layer - mesoderm; epimere or somite - dermatome; hypomere or somatic mesoderm
* Tissue type - adipose tissue (special type of connective tissue)
* Functions
o cushion, i.e.prevent mechanical injury to underlying muscle or organs
o energy storage in some taxa
o insulation to retain body heat (in endotherms)
Chromatophores: germ layer, tissue type, functions?
# Embryonic Germ Layer - neural crest = ectomesenchyme
# Tissue type - specialized large epithelial cells with numerous branches or projections
# Location in Skin - upper dermis &/or epidermis
# Functions - protective & warning color, sexual attraction, hardening of other tissues
Pigment Migration - qualitative or physiological change
# neuronal or neuro-hormonal control
# color changes occur by amoeboid movement of entire chromatophore or
# rapid change in dispersion of pigment within chromatophore
# rapidly alterations in skin color controlled by Melanocyte-stimulation hormone or melatonin
# common in fishes, amphibians, reptiles
Pigment Injection - morphological or quantitative change
# hormonal control often regulated by photoperiod in mammals (seasonal color changes in fur)
# pigment vesicles (melanosomes) from chromatophore are injected into other cells (keratinocytes) of epidermis or other epidermally derived features such as feathers/fur
# common in birds & mammals
Frog Skin Color
* typically green colored
* long wavelength (blues) light passes through iridophores & is absorbed by deeper melanophores
* shorter wavelength light (green-yellow) is diffracted & refracted back by iridophores
* only green light escapes absorption/blockage by yellow color screen of lipophores
* other colors (blue, yellow, black..) are possible where chromatophore units are non-overlapping or absent
Human Skin Color: cause for diff in black/white skin?
same number of melanophores per mm3 across all races

* black skin
o higher rate of activity of melanophores
o pigment is well dispersed in epidermal cells
* white skin
o less active melanophores
o pigment concentrated above cell nuclei only
selective pressures responsible for skin color evolution
# Tradeoff between needs for Vitamin D production & maintenance of folic acid (folate) & UV exposure

* folic acid - necessary for normal fetal development (neural tube closure) & sperm production
* Vit. D - necessary for normal bone growth & development

# UVA - penetratse skin deeply into the dermis & damagse folic acid being transported inside blood vessels.
# UVB - penetrates less deeply, into epidermis only, stimulates keratinocytes to convert cholesterol into previtamin D.

* kidneys convert previtamin D into Vit. D.

# UVB - stimulates melanocytes to produce more melanin pigment which is packaged into melanosomes.

* Melanosomes are taken up by keratinocytes to shield keratinocytes from damage to their cellular DNA.

# Lighter skin color selected for in higher latitudes, less UV exposure; darken skin selected for in lower latitudes
Fish Epidermis
# relatively thin, usually made of living cells
# often a cuticle on surface: non-cellular mucopolysaccharide
Fish Mucous glands
* substance made - mucus - glycoprotein, varied forms, gelatinous
* size or type
o unicellular - single celled glands are abundant in skin
+ Taxa - all "fishes"
+ Functions
# decrease turbulence & drag during swimming
# protect body from microbial or parasite invasion
# osmoregulation - reduce water exchange through skin
# aestivation - e.g. lungfish build mucus cocoons when lakes dry out
o multicellular - large, many cells, has visible duct to surface
+ Taxa - hagfish slime gland
+ Function
# produce copious, thick slime
# protects hagfish from predation by other vertebrates
Fish Granular Glands
* substance made - alkaloids (toxins), other chemicals
* Unicellular - single cells glands
o by with injury
o Taxa - lamprey & several Actinopterygii
o Functions - alarm nearby individuals regarding predators, triggered injury of an individual
* Multicellular - uncommon
o Taxa - several Actinopterygii & Sarcopterygii
o Functions
+ poison glands; associated with fin spines
+ alarm substances too in some species
Fish Dermis: Fibrous connective tissue
Fibrous Connective Tissue

* regular collagen layers at 45 deg. angles to body axis
* layered, helical arrangement like plywood increases strength
* resists forces that deform skin, and aids locomotion
Dermal "Bony" Scales/Armor: Evolutionary origins, pressures, trends?
* Evolutionary Origins
o scientists are not certain if first vertebrates had dermal scales or not
o certainly conodonts had bony "teeth" like structures
o but hagfishes may be close to the earliest ancestral "craniate" & they lack scales
* 2 Hypotheses for its Evolution
o early selective pressure?
+ first vertebrates lacked large amounts of bony internal skeleton
+ small scales could be used to store Ca+2 a mineral in short supply in aquatic environments & needed for variety of normal metabolism functions
o later selective pressure?
+ first vertebrates were small filter feeders or scavengers
+ many larger invertebrate predators existed at time
+ quickly may have acquired larger dermal armor for protection
* Trends in Bony Scale Evolution - after dermal armor appeared
o scales quickly became lighter & more flexible as gnathostome fishes evolved
+ fish became more predaceous themselves & benefited from improved speed
o reduce size (width) of scale - esp. in tail to make tail more flexible to improve propulsion
o decreased number of layers (tissues) in scales - e.g. lose spongy bone & lose dentine..
o decreased thickness (depth) of scale - reduce thickness of enamel & lamellar bone layers
Cosmoid or Dermal Armor
Bony Scales * taxa - fossil Cephalaspidomorphi, Placodermi, fossil Sarcopterygii
* layers - enameloid + cosmine (dentine) + spongy bone + lamellar bone
* spongy bone is more open & cellular
o lamellar bone is in dense, flat sheets & may be acellular
* body arrangement - large plates over head, smaller posteriorly
# Placoid (Dermal Denticles)
* taxon - Chondrichthyes
o evolved from cosmoid type; paedomorphosis may have produces the reduction in scale size
* layers - enameloid + thick dentine + pulp cavity (perhaps fibrous bone at base????)
o spiny tip of "triangular" scale pierces epidermis
* arrangement - isolated, over entire body; but nearly absent in some rays
* functions
o lightweight, improve body flexibility
o some designs can minimize turbulence in water
Rhomboid (Ganoid/Cosmoid)
* taxa - fossil & a few living Actinopterygii, fossil Sarcopterygii
* "primitive" layers - text & lab manual disagree with other texts on tissues present & with illustration in handout - ignore this type
* "derived" layers - enameloid or ganoine + a thin lamellar bone
* arrangement - entire body covered with small, equal sized, scales that show little/no overlap
* function - protective but body is still limited in flexibility & "stiff"
Elasmoid (cycloid & ctenoid)
* taxa - modern Actinopterygii (derived from ganoid scale type) & modern Sarcopterygii show independent origins of cycloid scales
* layers - thin acellular, fibrous bone
* arrangement - entire body covered, but absent or nearly absent in some
* functions
o scales overlap to improve body flexibility
o they are very light weight
o still provide some protection

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