Hard Derivatives . Dr. Vidhin Kamble Department of Zoology. Sangola College, Sangola

Hard Derivatives of Epidermis.

B. Sc- III Zoology. Dr. Kamble V. S.

1. Following types of horns are known:
2. 1.     Hollow Horns
3. 2.     Pronghorn
4. 3.     Antlers
5. 4.     Hair Horn
6. 5.     Giraffe Horns
7. Hollow Horns

Horns are found in ungulate (even-toed hoofed) mammals only. True horns of the hollow type are found in pronghorn, cattle, antelope, sheep and goats consist of an inner core of bone which is an outgrowth from the frontal bone. It is encased in a keratinised, epidermal covering. True horns continuously grow throughout life and are not shed.

Pronghorn

Pronghorn is a true horn, consists of a permanent projection of the frontal bone covered by a hard, horny epidermal sheath. The sheath is forked bearing one to three prongs made only of horny sheath. The horny sheath is shed annually and is replaced by another which grows from the skin that surrounds the core. It is found in Russian antelope Antilocapra.

Antlers

Antlers are found in the males of deer family, but they are present in both sexes in reindeer and caribou. An antler consists of a branching solid outgrowth of the frontal bone formed of dense connective tissue. It is covered during growth by hairy, vascular skin called ‘velvet’. The velvet is shed exposing the antler naked when the antler reaches full growth.

Thus, the antler consists only of dermal bone. The bony antler is also shed annually after the breeding season, and a new antler develops. Antlers are solid mesodermal bone, but they are formed under the influence of the integument. Formation of antlers is controlled by the hormones of testes and anterior lobe of the pituitary.

Rhinoceros or Hair Horn 

horn has no skeletal element. It is made by keratinised cells of the epidermis and consists of matted keratin fibres bound together, but its fibres are not true hair. It is a permanent epidermal structure and if broken it grows again. There is one horn in the Indian rhinoceros and two in the African species.

 

Giraffe Horns:

They develop from cartilaginous protrusions which are present at birth. They ossify and fuse at the top of the skull, where they appear as knobs permanently covered with living skin and hair. Giraffe possesses three of these knobs, one is median and anterior to the other two. These horns are short, unbranched and are permanent, and are present in both sexes.

 

Digital tips

In amniota the distal ends of digits have claws, nails or hoofs formed from the horny layer of the epidermis. They grow parallel to the surface of the skin and are built on the same plan.

Claws:

Claws made their appearance first in the reptiles. A claw is made of a hard horny dorsal scale-like plate called unguis and a relatively soft ventral subunguis, both converge terminally and cover the terminal part of the last phalanx. Claws of reptiles and birds are similar but in mammals the subunguis is much reduced and is continuous with a pad at the end of a digit. In cat family claws are retractile.

ii. Nails:

They are found in primates. The dorsal unguis is large and flat and subunguis is soft and much reduced. Tip of the digit forms a sensitive and vascular pad over which the nail groove is present. It is formed by the invagination of epidermis. Growth of the unguis takes place from the nail root lying below the skin in the nail groove.

iii. Hoofs:

They are found in ungulates. The horny unguis is thick and around the end of the digit, and encloses the thickened subunguis which touches the ground. Subunguis surrounds the soft, horny cuneus. Tip of digit, thus, forms a pad containing a blunt phalanx. Nails and hoofs of mammals are modified claws. Whalebone plates of toothless whales are also the modification of stratum corneum.

 Hair:

Hair is found only in mammals. It projects at an acute angle from the skin. Hair covers the entire integument in most (furred mammals), but in others only traces are left, such as whales have only a few core hairs on the snout.

But during development the body of the embryos of all mammals is covered with a coating of fine hair called lanugo which is usually shed before birth and replaced by a new one. Hair is entirely epidermal in origin.

Hairs are not modified scales but are new outgrowths of the epidermis only. A hair has an upper projecting shaft and a lower root lying in a hair follicle which is a sunken pit in the dermis. The shaft is made of only dead, keratinised cells. The part of the hair protruding above the skin is dead.

The hair shaft has an external cuticle of transparent overlapping cells which have lost their nuclei, inside the cuticle is a cortex (middle part of hair) containing shrivelled cells and pigments, and a central core or medulla having air spaces.

Feathers in Bird

Feathers are found only in birds and are formed from the epidermis in which the stratum corneum is highly specialised. Feathers are light, strong, elastic, waterproof and show many colours due to pigments and structural arrangement. The pigments are carotenoids and melanins. Carotenoids are frequently called lipochromes which are soluble in fat solvents like methanol, ether or carbon disulphide, and insoluble in water.

A typical feather consists of following parts.

1.     Central axis or Scapus

2.     Vexillum or vane.

3.       Scapus (Axis): The scapus is divided into

1. 1.     Basal calamus
2. 2.     Upper shaft or rachis.