Reptiles: scales and the amniotic egg
Cornified scales of reptiles increased their capacity to live in terrestrial environments.
Generalized skin of vertebrates has two principal layers:
Superficial epidermis, derived from ectoderm
Deeper dermis, derived from mesoderm
Epidermis is stratified into two layers:
1. stratum germinativum - living inner layer
2. stratum corneum - dead outer layer of cells filled with a protein called keratin (horny material - horns, nails, etc)
Dermis - acellular bony scales embedded in this layer, derived from bony plates of ostracoderms
Amphibians lack bony plates, epidermis is very thin.
The stratum corneum is very thin, with little keratin.
Skin is used for gas exchange.
Thick keratin layer would restrict oxygen exchange and water absorption.
Amphibians have many mucus glands to reduce water loss.
As a result of this skin, they are restricted to moist, humid habitats.
In reptiles, the epidermis forms a complete body covering of keratinized scales.
Epidermis consists of stratum germinativum and outer stratum corneum
Five layers, from outside in:
First, a thick, dead, heavily keratinized layer formed by death of cells and deposition of keratin in its place
Below this, two more keratinized layers and two living layers
Several times a year, this epidermis is replaced during molting.
At the onset of molt, the stratum germativum proliferates to form a new set of inner epidermal layers
Outer epidermal layer separates at the separation zone
Outer skin is shed and upper cells die, dry out and form new scales.
Formation of keratinized scales
Scales are formed by folds of the integument.
Raised and depressed areas appear in the integument.
Each bump consists of epidermis with the dermis underneath it and is known as a papilla.
The papilli grow upward but become lopsided and flattened because the cells on the anterior outer surface multiply faster than those on the posterior inner surface.
The stratum corneum gets thicker and the flat scale is produced.
Function of scales:
Scales function for protection, to seal body from environment to reduce water loss.
Thick layers of keratin reduce water loss.
Animal is less dependent on moist environment.
Provides protection from abrasion, as reptile moves over land.
During the Carboniferous era, there were many amphibians; the first reptile evolved from labyrinthodont ancestor.
Amniotic egg appears very early in reptilian evolution.
Egg has several special extra-embryonic membranes, which allow it to develop on land independent of water.
Just as significant as evolution of jaws to the evolution of vertebrates
Structure of egg
Reptiles - large amount of yolk and small amount of protoplasm concentrated at germinal disc
Telolecithal egg - large amount of yolk, unevenly distributed
Cleavage only occurs in germinal disc (meroblastic cleavage).
The periphery of the disc spreads out over the yolk to eventually cover it, forming the yolk sac.
Yolk sac is then infiltrated with a system of blood vessels, carrying nutrients from the yolk back to the embryo.
Other extra-embryonic folds expand up over the embryo to form the amniotic folds.
2 layers with coelom between them
4 extra-embryonic membranes
1 - yolk sac
2 - amnion: envelops the embryo
3 - chorion: outer membrane
4 - allantois
The amnion and chorion develop together as upwardly projecting amniotic folds, eventually closing over the dorsal surface of the embryo.
Eventually the free edges of the folds fuse and completely enclose the body of the embryo in the amniotic cavity.
When the amniotic folds fuse, the two membranes are formed, with the amnion next to the embryo and the chorion next to the shell.
The space between the embryo and the amnion is called the amniotic cavity
filled with fluid that is secreted to float the embryo in the cavity
Embryo is still connected to the yolk sac and to the amnion and allantois by the umbilical cord
Advantages of extra-embryonic membanes:
1. provides embryos with fluid medium for development
private pond for development of embryo
embryo can float in pond, develop evenly without compression from
2. protects the embryo from dessication
3. Fluid acts as a shock absorber, providing even pressure on all sides of the embryo.
Does create problems for embryo:
Isolates the embryo from the environment (shell is also present)
1. the chorion and amnion prevent embryo from being near the surface & access to oxygen
problems with gas exchange
2. no means of disposing of nitrogenous wastes
allantois role - the wastes are stored in a sac which develops from the
hind gut: the allantois sac
like urinary bladder for embryo
Allantois expands very rapidly to fuse with chorion and develop blood vessels on surface which connect to umbilical cord
function in gas exchange for the embryo
Its also a respiratory organ for the embryo
Egg allowed reptiles to move into habitats far from water, lay eggs on land.
Success of reptiles due to their adaptation for life on land
amniotic egg, keratinized epidermis, better jaw operation (jaw muscle attachment)
evolution of better locomotion, changes to head articulation
Dominant vertebrate life on land during Mesozoic era