origin of bone and agnathans

Question 1

why are we missing pieces of the story?

Answer 1

• Puzzle - we don’t have all the pieces to confirm the exact origin - a lot of the earlier organisms could not be fossilized (soft bodied, easily decompose)

Question 2

what are the six types of mineralized tissue in vertebrates?

Answer 2

Mineralized cartilage
		b. Bone (mesoderm- mesodermal in origin)
Dentine
Enameloid
Cementum
Odontodes

Question 3

describe mineralized cartilage and its origin

Answer 3

a. Mineralized cartilage (mesoderm - mesodermal in origin, know the origin of these)
○ Approx. 70% mineralized
○ Most cartilage is not usually mineralized
○ Chondroblast will contain most of protein part - chondrin - matrix
○ Occurs in jawed vertebrates (ie. Internal skeleton of sharks)

Question 4

describe bone and its origin. osteocytes, blasts, and clasts?

Answer 4

○ Approx 70% mineralized
○ Internal skeleton of bonyfishes and tetrapods
○ Osteoblast - specialized bone cell that synthesized the bone matrix and coordinates the mineralization of the skeleton - the living cell which divides etc
○ Osteocytes - non dividing bone cells, make up 90-95% of bone cells, regulate + send signals
○ Osteoclasts - specialized cells in the bone matrix, secrete acids and enzymes - mediate bone remodeling - shedding function - clear away mineralized or calcified parts of the bone matrix that are aged or damaged

Question 5

describe enamel and its origin

Answer 5

c. Enamel (ectoderm- ectodermal in origin- outermost surface of the gastrula ) approx. 96% mineralized
Found in teeth and exoskeleton of some ancestral vertebrates

Question 6

describe dentine and its origin

Answer 6

d. Dentine (comes from neural-crest cells- mesoderm) approx. 90% mineralized
○ Found in teeth and exoskeleton of some ancestral vertebrates
○ Stronger than typical bone
○ Found beneath the enamel

Question 7

describe enameloid and its origin

Answer 7

e. Enameloid (mesodermal in origin) - 96% mineralized
○ Resembles enamel- teeth and dermal scales
○ Found in ancestral vertebrates and current cartilaginous fishes

Question 8

describe cementum and its origin

Answer 8

f. Cementum (mesoderm??? Believed to be mesodermal in origin but has yet to be confirmed)- bonelike material
○ Fastens teeth in their socket

Question 9

what does bone consist of?

Answer 9

○ Bone is full of osteoblasts which form the matrix which forms the basement/foundation - two components composing the matrix: sugar, and a protein in the form of collagen fibers
a. Organic matrix of complex proteins and carbohydrates
○ Network of collagen fibers - from fibroblast cells
○ Hydroxyapatite - produced by scleroblasts
§ A complex compound of calcium phosphate
§ Crystals on the collagen fibers that are arranged in alternating layers - lattice like structure

Question 10

what did ancient organisms have?

Answer 10

• Ancient organisms that had bones had bony like exoskeleton - different types of bone that originated from the ectoderm

Question 11

what are the two types of bone? describe them

Answer 11

a. Dermal bone - forms in dermis (true skin/living structure, living cells)
○ e.g. ostracoderms
○ Living vertebrates
§ Most skull bones
§ Protective case around the brain
Endochondral bone (replacement bone) - forms inside pre-existing cartilage - ie cartilage forming into a

Question 12

describe acellular and cellular bones and what groups theyre found in

Answer 12

i. Acellular (non living) - cells form bone on the margins then disappear
○ No living cells in bone
○ Also known as aspidin bone
○ Primitive condition
○ Early ostracoderms
ii. Cellular (living) - enclosed cells in the bone marrow
○ Gnathostomes, osteostracan ostracoderms, conodonts

Question 13

what are ondontodes? homology?

Answer 13

d. Odontodes - the basic unit of mineralized tissue in early vertebrates
○ Tooth like elements found in the skin - dermal bone
○ Dentine, covered sometimes with outer layer of enameloid; with a base of bone
○ Possible homology with teeth
○ Aggregates formed bony plates (armour) - ie. Ostracoderms
○ Similar to placoid scales in sharks

Question 14

what were the selective forces for bone? why?

Answer 14

a. Protection
§ Ie head shields
b. Storage of calcium and phosphorous - (important for energy and in a number of environments (especially aquatic) these are limited - become scarce, storage is important. If phosphorous is limited, atp production goes down)
§ For muscular activity
§ Energy reserves (P scarce)
§ Osmoregulation (reduces H2O permeability- hydroxyapatite osmeoregulates)
§ Early organisms had to keep moving in an aquatic environment - weren’t buoyant. Evolution brought about the development of the swim bladder)
c. Resistance to acidity
§ Hydroxyapatite - greater acidity resistance (buffers pH- lactic acid builds up in body (anaerobic reaction) - hydroxyapatite helps break down the lactic acid so that it becomes waste - a
d. Insulation for electroreceptors

Question 15

summary:

Answer 15

• Probably multiple origin
  
  • First appeared as dermal bone (exoskeleton)
  • As time passed size and locomotion increased - bone moved inward for support
  • Sources of earliest bone - dermal (ostracoderms) and as apatite( conodonts)
  • very ancient organisms did not have internal bone structure - some has external exoskeletons
  • Need for organisms to develop protection - come largely from calcium phosphate (appears in a specialized form - hydroxyapatite)