Epidermis and derivatives Neural tube brain and spinal cord



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Biol211, W07

Clara Tsui



Embryonic Layer

Fate

Ectoderm

Epidermis and derivatives

Neural tube – brain and spinal cord

Neural crests – teeth, pharyngeal cartilage and bone, sensory nerves


Mesoderm

Notochord

Dermatome – dermis, dermal bone, mesenchyme

Myotome – trunk muscles

Sclerotome – vertebrae

Mesomeres – urogenital system


Endoderm

Gut and associated organs

Extra-embryonic structures

Ectoderm = chorion

Mesoderm = amnion

Endoderm = allantois (evagination of gut)
The Integument – from ectoderm and mesoderm


Melanophores

Chromatophores

Black/brown

In epidermis

Amniotes


Many different colours

In dermis

Non-amniotes

Can change colour quickly


Fish integument – scales are dermal



  • ganoid scales – enamel covered

  • elasmoid scales – proteinaceous, no mineral content (teleosts)

    • used to reconstruct a fish’s life without killing it (salmon)

  • dermal denticles/placoid scales – enamel covered

  • cosmoid scales – enamel covered (lungfishes)

Amphibian



  • mucous/poison/granular glands and chromatophores are found in the dermis

  • no scales

Reptile


  • no mucous glands

  • “scutes” are large epidermal keratin scales overlying dermal plates

  • scent glands release scent

Bird


  • reptile-like keratin scales on their legs

  • feathers are thought to be derived from keratin scales

  • innovations: keratin beak and uropygial gland

  • no scent or mucous glands

Mammal


  • epidermal glands: sebaceous, scent, sweat, mammary

  • hair is not homologous to feathers or scales

  • the only vertebrates that sweat

  • keratin structures: hair, scales, unguis, horns, baleen, quills

  • antlers are bony imposters!


Teeth – evolved from bony armour of Agnathan fishes
Definitions

Homodont

Teeth all the same size and shape within a species

Heterodont

Differentiated teeth (shape and size)

  • canines, incisors, premolars, molars

  • an adaptation to endothermy because endotherms have to eat a lot?

  • eutherian mammals: 3.1.4.3

  • humans: 2.1.2.3

Pleurodont

Teeth anchored to inner side of jaw

Acrodont

Teeth fused to the jaw at their base

Thecodont

Teeth anchored into jaw in sockets

  • humans; most mammals are not like this!

Shark


  • teeth are anchored in integument, shed continuously

  • new teeth are formed in a fold of epidermis called dental lamina

  • enamel comes from above (epidermis) from ameloblasts

  • dentine comes from below (dermis) from odontoblasts

Mammals


  • dental lamina is an embryonic structure

  • a primordium remains in jaw for a replacement after tooth is formed

  • carnivores have fewer and larger teeth, large cheek teeth (premolars + molars)

  • herbivores have fewer teeth, lack canines; cheek teeth are ridged and folded for grinding

  • diastema: open space between teeth; facilitates movement of tongue

  • adaptations: open-rooted teeth (rodents); hypsodont/high-crowned teeth (horses); twelve molars (elephants)

Chondrocranium – in Chondrichthyes and embryos


Visceral skeleton – formed from neural crest

  • supports gill openings of pharynx

  • first visceral arch: palatoquadrate and mandibular arch

    • homologues are the quadrate and articular

    • mandibular arch is functionally replaced by dermal bones in bony fish/tetrapods

  • second: hyoid arch

  • hyomandibula links jaws to the rest of skull in fish

    • turns into stapes in Amphibia and Reptilia (tetrapods)

    • sits in spiracle, which is homologous to Eustachian tube in inner ear

  • in mammals, quadrate  incus and articularmalleus

  • angular, dermal bone of lower jaw  tympanic bulla

  • role of visceral skeleton changed with mammal evolution

    • visceral arches contribute to hyoid apparatus, larynx, and cartilage rings for trachea

Endochondral (replacement) bones of an early tetrapod



  • turbinate bones – form sinuses; between original and secondary palate in mammals

  • sphenethmoid

  • basisphenoid

  • quadrate

  • articular

  • occipitals

  • periotics

  • epipterygoid (alisphenoid) – part of the eye socket

Dermal bones of an early tetrapod



  • nasal, frontal, parietals, postparietals

  • premaxilla, maxilla, squamosal, pterygoid, parasphenoid, temporals

  • dentary, angular…

  • lots of bones get lost as tetrapod skull gets simpler

  • emargination accommodates stronger jaw muscles

  • beginning in synapsid reptiles – internal opening of nares moves back under 2° palate

Regional specialization of vertebrae






Atlas

Axis

Cervicals

Thoracics

Lumbars

Sacrals

Caudals

Fishes







X

X










Amphibia

X




1

X




1

X

Reptilia

X

X

X

X




2+

X

Aves

X

X

X

X




Synsacrum

Pygostyle

Mammalian

X

X

X

X

X

3+

X


Appendicular skeleton

Pectoral girdle



  • very stout in fish

  • major bone of teleosts is cleithrum (dermal bone), which is lost in tetrapods (except first Amphibia)

  • tetrapods have a glenoid fossa for articulation of humerus

  • two sides join ventrally via clavicle and interclavicle

    • fuse in Aves = furcula / wishbone

  • coracoid fuses to scapula  coracoid process

  • no clavicle in deer; connections are made by muscles in running mammals

  • in Aves, broad surfaces are needed for wing muscles to attach

Pelvic girdle



  • simpe and week in fish; is not attached to axial skeleton

  • composed of ilium, ischium, and pubis

  • synsacrum of Aves is broadly fused to axial skeleton

  • in humans, three bones are called inominate bone

  • fused pelvic girdle may be parallel adaptations for bipedalism

Limb modifications in cursorial tetrapods



Plantigrade = heel touches ground

Digitigrade = heel and metatarsals are elevated, run on the balls of the feet

Unguligrade = run on toenails, holding digits off ground; fibula lost

  • hindlimbs of running tetrapods = parallel evolution

- evolution in the horse forelimb – metacarpals fused  cannon bone



- forelimbs also adapted to flying / swimming


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