TEETH = consist of both hard and soft components

1. Hard Components (acellular)

a) Dentin = matrix mainly of collagen fibers and GAGs (20% of dentin),
becomes impregnated with hydroxyapatite crystals (80% of dentin) so similar to
bone but harder (bone consists of 65% hydroxyapatite, 35% organic matrix)

- Forms the bulk of the tooth volume

b) Enamel = matrix composed of protein (enamelin) but calcium phosphate
crystals make up 99% of enamel substance, so enamel is the hardest substance
within the body

- Forms the crown of the tooth
- Composed of units called enamel prisms (or rods) that lay perpendicular to the
surface of the dentin; between rods is interprismatic substance; both are formed
from apatite crystals

c) Cementum = histologically similar to bone; upper 1/3 is acellular, lower 2/3
is cellular (cells = cementocytes).

- Covers outside of root
- Cementum is a labile tissue, susceptible to resorption of hyperplasia under certain
conditions, but only in lower cellular portion

2. Soft Components

a) Dental Pulp = fills pulp cavity and root canals

- Consists of CT similar to mesenchymal CT with fine collagen fibrils
- Also contains many blood vessels and nerves

b) Periodontal Membrane = dense regular CT attaches tooth in socket of
underlying alveolar bone

- Sharpey's Fibers extend into both bone and cementum to anchor tooth to bone
Since hard components of the tooth are acellular (except for the lower portion of
cementum) and cells are responsible for remodeling in bone and cementum, there is
no capacity for remodeling in teeth. Consequently, there is no capacity for repair if
damaged (except, of course, for the dentist!).

HANDOUT - TOOTH STRUCTURE

DEVELOPMENT OF TEETH
1) Begins as epidermal invagination into dermis (as in all other skin derivatives)
2) Develop tooth bud - 5 in each half jaw in humans ("milk teeth")
3) Secondary tooth buds develop ventrolaterally (labial side) - 8 in each half jaw in
humans (permanent teeth)
4) Formation of enamel organ with outer and inner enamel epithelia investing
enamel pulp
5) Inner enamel epithelium ® Ameloblasts (ectodermal origin)
Adjacent mesenchyme cells (from neural crest) ® Odontoblasts
6) Ameloblasts secrete enamel; Odontoblasts secrete dentin



1. Avian Skin - typically bird skin is relatively thin compared with other
vertebrates. Consists of thin epidermis and thicker dermis. Epidermis has 4 layers
(SEE DIAGRAM BELOW). Dermis has two layers (both dense irregular CT): an upper stratum spongiosum that is less dense and quite vascular and a lower stratum compactum that plays an important role in fat storage.

- Bird skin is devoid of glands, except for uropygial; Cells of the epidermis
(sebokeratinocytes), however, accumulate and secrete oily lipid substances that serve
the same function as sebaceous gland secretion in mammals (maintenance of skin and
feathers). Epidermal cells also show a marked ability to become keratinized.



Feather Development proceeds in a manner similar to hair development.
- Dermis forms feather pulp which is responsible for nutrition and oxygen
- Str. corneum + str. transitivum produce feather sheath and outer layers of calamus (quill)
- Str. intermedium forms majority of feather + inner calamus
- Str. basale forms a germinating ring at feather base

SEE HANDOUT OF FEATHER STRUCTURE

2. Reptile Skin - similar to bird skin in structure, except no feathers

- Devoid of glands (except femoral glands)
- Contains scales : 2 Types

1) Epidermal Scale = hard keratin
2) Dermal Scale = formed from dermal bone

Sometimes both scale types are present in the same skin, other reptiles show only
epidermal scales

3. Amphibian Skin - adapted for terrestrial existence in that epidermis is
composed of a stratified squamous epithelium (relatively thin = 5-9 layers)
- Chromophores are abundant
1. Melanophores = brown/black; found in str. germinativum as well as dermis
2. Xantholeucophore = yellow/orange pigment
3. Irridophore = guanine crystals, colorless

- Simple alveolar glands present in dermis

1. Mucous Gland = functions to keep skin moist
2. Poison Gland = predator defense; varying chemicals secreted, some (e.g.,
tetrodotoxin) are very toxic

- Desquamation in stratum corneum is patchy rather than wholesale (as in reptiles)
- Dermis with stratum spongiosum (upper, less dense and vascular) and stratum
compactum

4. Aquatic Amphibians - generally are neotenic (sexual maturity during larval stage).
Since larvae are aquatic, neotenic adults retain larval aquatic skin which doesn't show
terrestrial adaptations. Epidermis is a thin stratified cuboidal epithelium of 2-3 layers.

- Lateral line system (= senses hydrostatic pressure) is present; some other cell type
differences as well (e.g., goblet cells are present)

5. Teleost Fish Skin - Epidermis is thin stratified cuboidal (2-3 layers)

- Fish scales are of dermal origin (bony)
- Goblet cells are prominent and provide a mucous covering which decreases friction
from water while swimming.
- Club/Flask cells are also present, although their function is unknown. It has been
suggested that these may serve a function like the chloride cells in the gills
(osmoregulation).
- Lateral Line System is well developed and senses hydrostatic pressure. 3 conditions
occur:

1. Pit Organ = isolated neuromasts (neuroepithelium) on head of fishes; common
state in aquatic amphibians and cyclostomes
2. Open Groove = Groove open to exterior with neuromasts arranged along the
groove; found in a few cartilagineous and bony fish
3. Closed Tube = Closed tube with neuromasts arranged along the length of the tube;
typical lateral line system in most fishes
6. Elasmobranch Skin - Epidermis is thin stratified cuboidal; there is a decrease in cell
size toward surface

- Goblet cells are present
- Possess placoid scales = homologous with teeth in other verts.; develop as
outpocketing of dermis and consist of enamel covering dentin which surrounds a
dermal pulp.
- Ampullae of Lorenzini = clusters of jelly-filled tubules scattered over head region
with sensory cells at bases; some may be sensitive to hydrostatic pressure, some to
temperature, and some to electrical charges

7. Cyclostome (Lampreys, Hagfish) Skin - Epidermis is thin stratified cuboidal;
numerous goblet cells are present; Dermis lacks stratum spongiosum


To Lecture 15