Development of the Integument

This page focuses on the formation of the different skin (integument) types found both within an individual and between species. The major comparison will be between frogs and mammals.

The integument is made up of two layers: the epidermis (an example of squamous epithelium) and the dermis (an example of connective tissue). The epidermis is derived from ectoderm while the dermis is derived mainly from mesoderm. While many animals have the same basic integument structure, the epidermis has becomes specialized in a number of ways, as outlined in the chart below:

 

Amphibians	-	mucous and poison glands
Reptiles	-	scales and claws
Birds	-	feather follicles and claws
Mammals	-	hair follicles and sebaceous glands, nails, claws, hooves, horns, sweat glands and mammary glands
NOTE	:	The scales of fish are dermal derivatives of bone and are of mesodermal, not ectodermal, origin.

This page shows micrograph sections of the integument of the frog and the mammal.

 

Frog Integument

Higher Power

In the frog, the dermis is subdivided into two layers: the stratum spongiosum and the stratum compactum. These names refer to the appearance of the connective tissue within these layers. In the top picture, note the large poison glands. These produce substances to that can be anything from mildly irritating to very toxic, depending on the species, which serves to ward off predators. The epithelium of poison glands is anything from simple squamous to low columnar. It is difficult to appreciate here, but each gland is surrounded by connective tissue that contains smooth muscle cells. These contract when stimulated to squeeze the poison out of the gland lumen and onto the surface of the frog's skin. Mucous glands, seen in the lower picture, are smaller than poison glands and serve to produce mucous to keep the frog's skin moist and permiable for gas exchange. The epithelium of this gland is generally cuboidal. Both poison and mucous glands form as invaginations from the epidermis which migrate down into the dermis. In the developed animal, the bulk of the gland is in the dermis, with only a duct to the surface travelling through the epidermis. Finally, the very dark staining "spots", seen especially well on the lower picture, are chromatophores. These cells have a neural crest origin and produce pigment which is picked by the epithelial cells neighbouring it.

The Stratified Epithelial Tissue Type

Stratified Squamous Epithelium:

Here we see the epidermis (outer part of the skin) of a frog showing typical stratified squamous epithelium and the dermis showing connective tissue. The cells of the basal layer of the epidermis (closest to the dermis) are cuboidal to columnar in shape. These cells are actively mitotic, producing new cells that get pushed upward into the overlying layers. As these cells are pushed up, they become flatter and longer (just as if you were to squash something), taking on the typical squamous shape. When the cells reach the top, they are sloughed off and replaced by cells from below. The dermis which underlies the epidermis is composed of a dense, irregular connective tissue, which we will see again later. Epidermis (Stratified Squamous Epithelium) Dermis

 

The photo to the right is a cross section of a frog's skin that has  been stained to show the cells. The outside of the frog would be upward. Note the outer orange layer about a cell or two thick and the dark pink layer which is three or four cells thick. The dark purple structures are nuclei. Cells at the base of the pink layers divide and produce cells that mature as they are pushed outward by new cells divisions. As cells reach they outer layer, they die and slough off. The structure to the right in the photo is a mucus gland. Its secretions keep the skin moist.

 

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Mammal (Human) Integument

	Outer Layer - Dead Cells   Epidermis       Dermis

Cross section of human skin. Compare it to the frog skin. Note the several layers peeling from the surface. The outer portion of human skin is composed of flattened cells that make a rather water impervious barrier. Humans, in fact mammals, are well adapted to terrestrial living and do not have to remain close to moist environments.

Skin Model

The skin - or to use the correct term, cutaneous membrane - is the largest organ in the human body. There are about two square metres of it covering an average adult and it accounts for about an eighth of the total body weight. It's the body's first line of defence against a hostile environment, being waterproof (from both directions) and germ-proof. It also acts as a temperature control mechanism and the main access to one of the senses - touch. There are two distinct parts of the skin, known as epidermis and the dermis and a third layer, the hypodermis or subcutaneous tissue, not strictly skin, but made up of fatty cells which lie below the skin. The bit we can see and touch is the epidermis. Most of the epidermis is made up of dead skin cells - the living cells at the bottom of the epidermis rise to the surface of the skin, hardening, flattening, and eventually being shed providing us with the outermost layer of protection. Interestingly, dead skin cells constitute over 90% of house dust. The lower, living levels of the epidermis also contain melanin - the pigment that gives skin its colour and protects it from the sun - and a chemical compound called ergosterol that produces vitamin D in sunlight. The bottom layer of skin, the dermis, is richly supplied with blood vessels; this is the part of skin in which hair follicles, sweat and sebaceous glands, and nerve endings are located. The nerve endings are in ridges that push into the epidermis in a slightly different pattern in everybody. This is the reason behind unique fingerprints.

 

Analysis and Conclusion Questions:

1. What is the integumentary system?
2. Name and explain the two main layers of the integument.
3. What is the function of the poison glands? How is the poison secreted on to the frog skin?
4. What are mucus glands? What are the primary functions of these glands?
5. What layer produces new cells that move up toward the outside layer? What happens to these cells as they move up?
6. How does the skin protect us from the outside environment?
7. How does human skin differ from frog skin? Give specific examples.
8. In what ways is the human skin similar to the frog skin? Give specific examples.
9. How are the human skin and the frog skin adapted to the specific environment?