Hard derivatives of integument - by Dr. Vidhin Kamble

 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.                     

(a) Calamus:

The calamus is hollow, tubular, and semitransparent. The base of the calamus remains inserted into a pit or follicle of the skin, from which non-striated muscle fibers pass to the feather and provide movement to each contour feather. The calamus opens below by a small opening called inferior umbilicus, which receives a small, conical, nutritive dermal papilla from the dermis.

The nutrients and pigments are passed through the dermal papilla into the feather from the dermis, during the development of the feather. Another pore, called the superior umbilicus, occurs on the ventral side of the junction of calamus and rachis.

In some birds, a small tuft of a soft down feather, called aftershaft or hyporachis, occurs near the superior umbilicus and covers it.

(b) Rachis:

The part of the scapus above the calamus is a rachis. It forms the longitudinal axis of the vane. It is solid, opaque, roughly quadrangular in transverse section, and filled with a closely packed mass of pith cells. A longitudinal furrow, the umbilical groove, runs along the ventral or inner surface of the rachis throughout its length.

2. Vexillum or vane.

The rachis bears a fan-like, webbed, or expanded membranous part of the feather, the vexillum or vane. The vane is divided by rachis into two unequal lateral halves. Its proximal end is broader than the distal end. Each half of vane is composed of a series of numerous, parallel, closely spaced, delicate, lateral, thread-like structures called the barbs or rami. The barbs arise obliquely outwards from the two lateral sides of the rachis.

All the barbs and barbules are loosely held together, so that the vane forms a flexible, firm, wide, flat, and continuous surface for striking the air during flight. This interlocking mechanism can be broken down so that the barbs become separate, but can be joined again by “preening” the whole feather.

 

Kinds of Feathers:

In pigeon, the feathers are variously modified to serve different functions.

They may be of the following kinds:

1. Quill or Flight Feathers:

The quill feathers have a strong rachis or shaft having barbules with an interlocking arrangement.

They are classified into the following types:

a.       Remiges: (wing feather)

b.       Rectrices: (Tail feather)

c.       Coverts:

d.       Contours:

e.       Filoplumes (Hair Feathers or Pin Feathers):

f.       Down Feather or Plumule:

g.       Rictal Bristles:

 

Remiges:

The quill feathers occurring on the wings and serving the purpose of flight are called pinions, wing quills, or remiges (singular, remex). In remiges, the posterior half of the vane is slightly broader than the outer or anterior half.

Rectrices:

The quill feathers occurring around the uropygium to form the tail of pigeon are called tail-quills or rectrices.

Coverts:

The quill feathers covering the bases of wing quills and tail quills are called coverts. The bases of the wing-quills are covered by several rows of upper and underwing- coverts, and the bases of the tail quills by upper and under tail-coverts. They are of a smaller size than the quill feathers but both are structurally similar. They close the interstices between the calamuses (quills) of remiges and rectrices and, thus, presenting a continuous area to oppose the buoyancy of the air.

  Contours:

The quill feathers forming the general covering of the body are called contours or pennae. They are smaller and woolly feathers having poorly developed barbules due to which barbs can be easily separated. These provide warmth and smooth air flow, without turbulence.

 

 Filoplumes (Hair Feathers or Pin Feathers):

The filoplumes are small, delicate, hair-like feathers which remain sparsely distributed over the body among the contour feathers. A filoplume consists of a short calamus and a long thread-like rachis with a few weak terminal barbs, and barbules without any hamuli.

 Down Feather:

The down feathers are small, soft and woolly and lack the rachis but have a short calamus. The calamus bears a fluffy tuft of barbs which are long, flexible and with short barbules.  There are two types of down feather.

a.     Nestling down feathers.

b.    Powder-Down Feathers:

In a young one, the down feathers cover the body and are called nestling down feathers. 

Powder-Down Feathers:

These are specialised type of fea

thers, These feathers produce some powdery fragments for cleaning the plumage

 Rictal Bristles:

Some birds, such as flycatchers, goat-suckers and whippoorwills have stiff hair-like feathers called rictal bristles at the base of bill (rictus) and eyes. Each rictal bristle has a short calamus, and a slender rachis with a few rudimentary barbs at its base. They do not occur in pigeons.