Derivatives of Integument: Soft Derivatives

 Derivatives of Integument: Soft Derivatives 

Both layers of integument have given rise to various types of derivatives. The epidermis gives rise to integumentary glands, epidermal scales, horns, digital structures, different corneal structures, feathers, and hairs. The dermis forms dermal scales of fishes and of some reptiles, plates or scutes in reptiles, fin rays in fishes and antlers in mammals.

I. Epidermal Derivatives:

Epidermal derivatives are epidermal glands (unicellular and multicellular), epidermal scales and scutes, horns, digital structures (claws, nails and hoofs), feathers and hairs.

 1. Epidermal Glands

 Epidermal glands are formed from the Malpighian layer of the epidermis. They arise from the epidermis and often penetrate the dermis.

According to their structure they are

1. 1.     Unicellular 
2. 2.     Multicellular,
3. 3.     Tubular
4. 4.     Alveolar
5. 5.     Simple Or Compound (branched) glands.
6. These are lined by cuboidal or columnar cells.

 (a) Unicellular glands are single modified cells found among other epithelial cells, they are present in amphioxus, cyclostomes, fishes, and larvae of amphibians. Unicellular glands are known as mucous cells or goblet cells. They secrete protein mucin which combines with water to form the mucus which lubricates the surface of the body. Other unicellular glands are granular cells and large beaker cells of cyclostomes and fishes, they also secrete mucus.

 

(b) Multicellular glands are of two types:

 (i) Tubular glands:

These are multicellular tubes of uniform diameter formed as ingrowths of the Malpighian layer into the dermis, e.g., glands of Moll on the margin of the human eyelids. Tubular glands may become coiled at the base deep in the dermis, e.g., sweat or sudoriferous glands of mammals, or they may divide into many tubules which are then called compound tubular glands, e.g., mammary glands of females and of males in monotremes and primates, etc., and gastric glands in stomach.

 (ii) Alveolar or saccular glands:

 These are multicellular down growths of the Malpighian layer into the dermis, having a tubular duct whose terminal parts form a rounded expansion to become flask-shaped, e.g., mucous and poison glands of amphibians. Alveolar glands may branch into many lobules which finally open into a common duct, they are then called compound alveolar glands, e.g., mammary glands of eutherians, and salivary glands.

 

Different Types of Epidermal Glands

 Kinds of Epidermal Glands:

According to function, the epidermal glands of vertebrates are of the following types:

  i. Mucous Glands:

They may be unicellular or multicellular. The unicellular glands are mucous gland cells, granular cells and beaker cells of amphioxus, cyclostomes and fishes. They secrete mucus which keeps the skin moist and slippery, and also affords protection against bacteria and fungi. Mucous cells and granular cells lie near the surface, but the beaker cells lie more deeply and extend from the Malpighian layer to the surface.   

Multicelluar mucous glands are alveolar found in some fishes and amphibians. They occur all over the surface of the body and produce mucus for lubricating the skin and in amphibians they keep the skin moist to aid in respiration.

 

ii. Poison Glands:

Amphibians also have alveolar poison glands which are larger but less numerous than mucous glands. In toads masses of poison, glands from parotoid glands behind the head. The secretion of poison glands has a burning taste and is used as a defense. Caecilians have giant poison glands.

Some tubular glands are found on the feet and suctorial discs of tree frogs which aid in climbing. Tubular glands are also found on the swollen glandular thumb parts of male frogs and toads during the breeding season. They aid in clasping the female during amplexus.

 iii. Luminescent Organs or Photophores:

They are found in longitudinal rows near the ventral side of the body in those fishes which live in the deep sea where no light penetrates. Each photophore is a group of epidermal cells lying in the dermis.

Each photophore has a lower layer of luminous cells below which is a layer of reflecting pigment cells, and the upper layer of mucous cells forms a lens. The glandular cells produce phosphorescent light which is transmitted to the outside by other cells. The light helps to attract the prey of deep sea fishes.

 

iv. Femoral Glands:

Femoral glands are found in male lizards (e.g., Uromastix) below the thighs in a row from the knee to the cloaca. They secrete a sticky substance which hardens into short spines that are used for holding the female during copulation.

 v. Uropygial Glands:

These are the only glands in birds, and they are best developed in aquatic birds. Uropygial glands are branched alveolar glands located on the dorsal side at the base of the tail or uropygium in the form of swelling. They secrete an oil which is odoriferous and attracts the opposite sex. The oil contains pomatum which is picked up with the beak and used for preening and water­proofing the feathers.

 

vi. Sweat Glands:

The largest number and variety of epidermal glands are found in the skin of mammals. They are tubular or alveolar and multicellular. Sudorific or sweat glands are long, coiled tubular glands embedded deep in the dermis. Their upper part forms a duct which opens on the surface by a pore and the lower coiled part lies in the dermis surrounded by a network of blood capillaries.

Sweat glands are not uniformly distributed. In man they are more numerous on palms, soles, and arm pits. In cats, rats, and mice they are confined to the soles of the feet. In rabbit they are around the lips; in bats on the sides of head; in ruminants on the muzzle and the skin between the digits and in hippopotamus they are found only on the pinnae.

 Modified sudorific glands form glands of Moll in the margins of the human eye in connection with eye-lashes.

 

Ceruminous glands:

in the external ear passages of mammals are modified sweat glands and secrete a waxy substance which combines with the secretion of sebaceous glands to form earwax which catches dust. Oil glands form ceruminous glands in the external ears of some gallinaceous birds.

 

vii. Sebaceous Glands:

Sebaceous glands are alveolar glands opening in hair follicles containing hairs. They also independently open at the skin surface around the genital organs, tip of the nose, and edges of the lips. Sebaceous glands secrete an oil (sebum) to lubricate the hairs and also cover the skin with a film of oily coating. The oily secretion of sebaceous glands contains waxes, fatty acids, and cholesterol, which makes the skin pliable.

Sebaceous glands are absent in Manis (pangolin), and Sirenia, and Cetacea which practically have no hairs.

Meibomian glands

Modified sebaceous glands form Meibomian glands in the eyelids, each has a long straight duct into which separate alveoli open.

They produce an oily secretion which forms a film over the lacrimal fluid or tears holding them evenly on the surface of the eyeball for keeping the eye moist, in weeping the oily film are broken and tears flow out.

Ceruminous glands:

Ceruminous glands of external auditory meatus are modified sebaceous glands. Their greasy or waxy secretion, called the cerumen traps the insects and dust particles.

viii. Scent Glands:

Scent glands are modified sebaceous glands or sweat glands. Their secretion is an allurement to the opposite sex. Scent glands are located in the deer family on the head near the eyes. Skunks and carnivores have scent glands around the anus, and pigs and goats have scent glands between their toes.

x. Mammary Glands:

These are characteristic of mammals. They secrete milk generally in the females for nourishment of the young. In monotremes both sexes may secrete milk, this condition is called gynaecomastism which is not a normal condition. Mammary glands of monotremes are compound tubular glands, while in other mammals they are compound alveolar.  Mammary glands of monotremes have no nipples, they open into pits on the surface of the skin, and the young ones obtain milk by licking tufts of hairs. In others, the mammary glands open by their ducts separately into a nipple.

Mammary glands along with fat form integumentary swellings called mammae or breasts. The number and location of mammae varies in different mammals. The number ranges from two in many mammals to 25 in the opossum. Mammae may run along two ventral milk lines from the armpits to the groin, or they may be axillary, thoracic, abdominal or inguinal in position.