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External Features of Chicken

Wael Khamas and Josep Rutllant

College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA, USA

1.1 Skin

The chicken skin is thin, loosely attached to the hypodermis, and in most bird species, it has pale pink or bluish pink color. In some species, it can be yellow and even blackish colored based on the presence of xanthin, carotenes, and melanocytes. When compared to mammals, the avian skin is, for most species, dry, often translucent, and inelastic over most of the body, which makes it prone to tears (Nett and Tully 2003). As an additional adaptation for flying, the bird loosely fits inside its skin over most of the body. In some regions, it is strongly attached to the underlying tissue with little or no modification with a very small dermis, like in the skull and the wing tips.

The integument is highly modified on the beak, feet, and certain parts of the bird body such as the ornaments (wattles and crest). The avian skin is the largest organ and acts as the first line of defense against pathogens. It is also involved in heat regulation and prevents the loss of body fluids. It is the largest sensory organ in the body, having receptors for temperature, pain, pressure, and tactile discriminations (Lucas and Stettenheim 1972, p. 1975, pp. 485-486).

The skin has two main types of keratins: (i) soft keratin, which is present in the body and ornaments (alpha keratin) and (ii) hard keratin, found on the scales, spurs, beak, and feathers (beta keratin) (Greenwold et al. 2014). Both male and female may develop thickened areas within the dermis on the ventral abdominal region corresponding with the brooding or incubation patches.

Most of the skin is covered with feathers and no glands are present except for the aural, third eyelid, vent, and uropygial regions. The absence of sweat glands indicates that birds thermoregulate by panting and gular fluttering, among other mechanisms and behaviors. The thick coat of feathers limits the exchange of heat with the environment, a significant difference with mammal's skin. In certain regions of the chicken body, the epidermal cells of the skin produce a holocrine lipidic secretion, such as in the rictus, interdigital web, and in the uropygial gland (Menon et al. 1981).

The chicken skin is divided into the external epidermis and the deeper dermis that is anchored to the underlying hypodermis.

1.1.1 Epidermis

The epidermis is thin in feathered regions and thick in bare regions. The epidermis is composed of stratified squamous epithelium; the number of layers varies depending on the body region. The epidermis, except in the comb and wattle, is mostly composed of different strata (singular stratum): germinativum or basale, spinosum, granulosum, lucidum, and corneum. Not all layers/strata described in mammals are present in the epidermis of the thin skin of the chicken like the stratum lucidum. The avian epidermis is composed of one to six layers of epithelial cells depending on the body region. This includes the basal (germinal) layer, composed of simple cuboidal cells, an intermediate section of several layers composed of cuboidal cells (stratum spinosum), and the stratum corneum, composed of flattened cells without nuclei. The stratum corneum varies in thickness from region to region (Figure 1.1A and B).

Figure 1.1 (A and B) Histology sections of abdominal chicken skin. This extremely thin skin layer shows (a) slightly keratinized stratified squamous epithelium, (b) dermis, (c) blood vessels. (A) H&E and (B) trichrome stains.

1.1.2 Dermis

The dermis consists of irregular connective tissue of variable thickness between the subepithelial region to the deep dermis close to the hypodermal layer and adipose tissue. Three layers can be identified, the stratum superficialis being the more superficial, and consisting of irregular connective tissue with low innervation and in close contact with the stratum basale of the epidermis. A deeper layer, the stratum profundum, on the contrary, is richly innervated, and can be subdivided into two distinct regions or sublayers, the stratum compactum and the stratum laxum. The stratum compactum by name is a thick irregular connective tissue rich in collagen and with a small number of elastic fibers. This layer is filled with blood vessels of variable sizes and with bundles of nerve fibers. The stratum laxum consists of loosely arranged connective tissue, smooth muscle, and abundant adipose tissue. Homberger and Silva (2000) reported that the fat deposits in the stratum compactum and stratum laxum of the avian skin act as a non-compressible hydraulic tissue for the movement of feathers inside the dermis. This region mostly harbors contour feathers which are seen surrounded by a layer of collagen fibers (Bharathi et al. 2018).

1.1.3 Hypodermis (Subcutis)

The hypodermis, subcutis, or subcutaneous layer underlays the dermis, but it is not considered part of the skin. It consists of a layer of connective tissue rich in collagen bundles but with few elastic fibers. It is highly vascularized, innervated, and has lymphatic drainage. In gross anatomy, this layer is the superficial fascia and can be divided into three different layers: (i) the superficial layer, which is immediately below the dermis, (ii) a middle layer that may have adipocytes and called panniculus adiposus in mammals which extends in between skeletal muscles (intermuscular fascia), and (iii) the deeper layer or fascia, located between the skeletal muscles and the bone, and blending with the periosteum.

1.1.4 Blood Supply of the Skin and Nerve Ending

The blood supply to the chicken skin derives from larger blood vessels present in the hypodermis that branch into smaller arteries and arterioles when they penetrate the dermis. Blood capillaries branch from them to reach the subepithelial region, directly supplying blood to the upper portion of the dermis and the epithelium through diffusion. Certain regions of the skin and its associated structures, such as the ornaments, comb, wattle, and ear lobes, are highly vascularized.

Arteries and veins are usually accompanied by nerve fibers. Mechanoreceptors in the skin correspond to the mammalian Vater Pacini (Pacinian) corpuscle, called Herbst corpuscles in birds. They are present in the feathered skin, beak, and distal leg (Hodges 1974, pp. 11-12). The small type Herbst corpuscles are usually superficially located closer to the feather bulbs while the larger ones are localized deep in the dermis. These encapsulated structures are composed of an inner bulb surrounded by several layers of loose connective tissue. Another corpuscle, described by Gottschaldt (1985) is the Grandry corpuscle, equivalent to the Meissner corpuscles in mammals, present in the beak skin of geese and ducks that respond rapidly to active movement in aquatic environments.

1.2 Structures Associated with the Skin

1.2.1 Patagium (pl. patagia)

The patagia are thin, feathered membranous folds of skin that connect the humeral bone and shoulder with the distal ulna/radius and the carpus that sits along the cranial border of the wing. The cervical extension of the patagium (referred to as parapatagium), present in front of the shoulder joint which is not part of the wing. The patagium is composed of four parts (prepatagium, metapatagium, postpatagium and alular patagium). The main patagium of the wing is the prepatagium (wing web), between the shoulder and the carpus. This prepatagium is rich in elastic connective tissue which enables the chicken to hold the wings close to the body without muscular effort. The free prepatagium is a triangular fold or thin double fold of skin laying between the radius and ulna and the humerus. The thin free edge of the prepatagium is directed anteriorly (Lucas and Stettenheim 1972, p. 57). The metapatagium is also triangular, a very small fold of skin located between the thorax and the wing. Its edge attached to the wing along the caudal border of the triceps humeralis muscle. The postpatagium is a chevron-shaped skin web that lies caudal to the forewing and the carpal/metacarpal bones, extends from the elbow to the longest digit, and bears the primary and the secondary remiges. The alular patagium is a small web that lies between the adductor indicis muscle (Hudson and Lanzillotti 1972, p. 43) and the tendon to digit III. Its free edge directed toward the tip of the wing (Lucas and Stettenheim 1972, p. 57).

1.2.2 Operculum

The operculum is a cover or a lid present dorsal to the anterior nares (nasal openings) of a bird at the base of the beak (Figure 1.2). The shape and structure may vary from one breed to another.

1.2.3 Ornaments

1.2.3.1 Comb

The most distinctive skin protuberance in the head of the chicken is the comb. Its shape varies among different breeds of chicken. It could be a single crested, rose, pea shaped, or strawberry type of comb depending on the breed. Rooster's combs are always bigger than that of a hen. In poultry production, the comb's size and color are used as features to determine the health and egg production status of the hens. The comb size...