Abstract on the species but both are functional.

Abstract

The male reproductive system of
birds is made up of a paired testes, epididymis, vas deferens and phallus
(rudimentary copulatory organ). The testis is light yellow in colour and elliptical in shape. The left testis
may be larger than the right depending on the species but both are functional.
In contrast, there are no differences in the weight of the right and left
testis in male broiler breeders. In seasonal breeders, testis size has shown to
have about 500 folds increase during the reproductively active season when
compared to those on the non breeding
season. The paired testes unlike in some mammalian species are situated
internally in the body cavity, and in poultry the spermatozoa remain viable at
body temperature which is harmful to the survival of sperm in mammals. The
paired testes are located anterior to the cranial lobe of the kidneys. They are
attached to the body wall by the peritoneal fold referred to as mesorchium and
are encapsulated by the tunica albuginea and tunica vaginalis. Each testis is an aggregate of anastomosing seminiferous
tubules, with associated interstitial tissue enveloped by a connective tissue
capsule. The seminiferous tubules consist of Sertoli cells and germinal
epithelial cells, whereas the Leydig cells are responsible for steroidogenesis
and androgen secretion. Each testis is connected with an epididymis via
deferent duct that enters into the cloaca and the phallus which exists in some poultry
species helps in the introduction of semen into the female tract

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Keywords: Poultry, male, reproductive system, anatomy,
physiology

 

1.1 Introduction

Poultry include all domesticated birds that are reared
for meat and eggs. The most commonly domesticated bird is the chicken (Gallus gallus domestica), followed by turkey (Meleagris gallopova)
and quails (Coturnix coturnix). Other
domesticated birds include ducks (Cairina
moschata), guinea fowl (Numida
meleagris), geese (Anser spp),
emu (Dromaius novachollandiae) and ostrich (Struthia camelus) 1. However,
apart from the domesticated avian species, studies have also been carried out
in many non-domesticated avian species, of which many exhibits reproductive patterns similar to those of domesticated
species.

The commercial
importance of poultry products has resulted in numerous studies on poultry with
the major aim of enhancing product output. Genetic and reproductive physiology
has attracted enormous attention in poultry production 2. Unequivocally, an
understanding of the genetic potential, anatomy,
and physiology of poultry reproduction is of paramount importance to enhanced
product output (eggs and meat). Therefore, this chapter focuses on the male reproductive system of poultry and some non domestic bird species.

The male reproductive system of poultry consists of a
pair of testes (elliptical shaped and light yellow in colour) located close to the cephalic side of the kidneys. Each
testis is connected with an epididymis which connects to a deferent duct that
enters into the cloaca. In the bird,
spermatozoa remain viable within the body temperature unlike in the mammal
where sperm is not viable at body temperature. This is the main reason the
testes in mammals are located outside the body. There is also a copulatory
organ called phallus which exists in some
species and helps in the introduction of semen into the female reproductive
tract of the hen. Accessory sex glands include the para cloacal vascular bodies, dorsal proctodeal and lymphatic folds
3. The prostate, vesicular and bulbourethral glands are absent in birds 1.

 

1.2          Anatomy and physiology of the male reproductive system

The testis

Testes of the male
poultry are paired, and unlike in mammals are located inside the body cavity,
ventral and toward the cephalic border of the kidneys 1 as shown in Figure
1.1. Therefore, spermatogenesis proceeds at the internal body temperature of 410C
in birds as opposed to the scrotal temperature of 24-260C in
mammals. The paired testes are functional in the male bird as opposed to only
the left functional ovary and oviduct in the female bird. A thin fibrous
capsule made up of connective tissues and contractile fibers surround the
testes of birds 4. They may be of a similar/same
size in some species, but show conspicuous size variation in many species,
especially after maturity 5. In young birds, the testis can appear flattened
and pointed when compared to the round shape of the matured testicle. The
weight of the paired testes in chickens
is about 1% of the total body weight 6. The paired reproductive tracts lie
along the dorsal body wall with each tract
consists of a testis, an epididymis, and deferent duct. One of the two testes may be larger, depending on the poultry species
7 and both are functional. In seasonal breeders, under normal hormonal
control, the testis can increase in size by 300 to 500 fold during the sexually
active season as compared to those in the non
breeding state 1, 2. Vizcarra et al. 8 reported that in male broiler
breeders, no significant differences in the weight of the right and left testis
from 2 to 50 weeks of age. Fontana et
al. 9 and Leeson and Summers 10 reported a positive
correlation between body weight and testes weight during the breeding season, an indication that males that
lost weight during breeding may begin testis regression. Amann 11 reported a
positive relationship between testis size and sperm production in birds. The
overall size of the testis scales allometrically to body mass; however, once
controlled for body mass, testes size is often larger in bird species that
experience a higher degree of sperm competition 12. Sperm competition occurs
when the sperm of two or more males is present in the genital tract of the female
and must compete for egg fertilization.

The testes are connected to the body wall by a
peritoneal fold referred to as mesorchium. This peritoneal fold also serves as
a conduit for nerves and blood vessels. Vascular supply increases during
reproduction, resulting in a more prominent pattern of blood vessels on the
testicular serosal surface. Each testis is an aggregate of anastomosing
convoluted seminiferous tubules, Leydig (interstitial) cells and connective
tissue capsule permeated by blood capillaries.

 

 

 

 

 

Fig. 1.1 Male reproductive
system of chicken.
Adapted from Nickel et al. 17 and www.poultryhub.org

 

The testicular capsule
is a vital component of the contractile mechanism of the testes. 
It made up of three layers; the tunica serosa, tunica albuginea, and
tunica vasculosa. Among these layers, the
tunica albuginea is the main tissue layer and it comprised of cellular elements
that alternate with thick bundles of collagen fibers. The testicular capsule is
larger in mammals and ratites but thinner in other bird species 4, 13. The
seminiferous tubules consist of Sertoli cells and germinal epithelial cells
(stem spermatogonia), whereas the Leydig cells are responsible for
steroidogenesis and androgen secretion.

There are two types of
parenchymal tissue in the testis which include: the interstitial tissue and the
seminiferous epithelium. The interstitial tissue contains blood and lymphatic
vessels, nerves, peritubular epithelial cells, and Leydig cells. The
seminiferous epithelium within the seminiferous tubules of matured males is
compartmentalized into basal and adluminal regions through tight junctions
between adjacent Sertoli cells 14. The seminiferous epithelium contains germ
cells that develop in stages and these are arranged sequentially in a helix
that extends along the length of the seminiferous tubule 15. The testicular
blood supply in bird species is very simple than that found in mammals 16.
The reason  may be partly attributed to
the fact that avian have no pampiniform plexus, which in mammalian such as
goats, sheep, rats among others is a countercurrent mechanism to maintain testes
temperature below body temperature. Blood moves to each testis from the abdominal
aorta, through a common trunk with the anterior renal artery, to the testicular
artery (Figure 1.2). Thereafter testicular artery branched to form numerous
smaller arteries that inter-connect among
the seminiferous tubules.

 

The excurrent ducts

The excurrent ducts are
ducts derive from the mesonephros and include the efferent ducts, epididymis, and deferent
ducts. The epididymis is located on the dorsomedial
aspect of the testis, which is referred to as the hilus. The epididymis contains a series of ducts which include; the
rete testis, efferent ducts, connecting ducts and the epididymal duct that
empties into the deferent ducts. Seminiferous tubules connect with the rete
testis at discrete sites along the testis–epididymal interface 19. In
domestic fowl, quail, guinea fowl, and ducks, the rete testis is lined with
simple cuboidal and simple squamous epithelial cells 20, whereas in ratite
birds, such as the ostrich, the epithelium contains cuboidal and columnar cell
types 21, 22. The rete testis of ratite birds makes up a much lower
volumetric proportion of the epididymal tissue and it travels a relatively
larger distance from the testicular capsule before entering the epididymis
23. The efferent ducts, unlike the rete testis,
are wide at their proximal ends and narrow at their distal ends in many avian
species 22, 24. Both efferent ducts and rete testis are similar in height and
diameter in ratite bird species which makes it difficult in distinguishing one
from another 21, 25. On a volumetric basis, the efferent ducts are the
principal excurrent duct within the epididymis 26. In the majority of the avian species, the efferent
duct mucosa is highly folded, mainly at the proximal end 24, 26. In ratite birds,
there is the absence of mucosal fold in
both the proximal and distal efferent ducts which differs from other avian
species 25. The efferent duct mucosa is made of pseudostratified columnar
epithelium that contains ciliated and nonciliated epithelial cells 24.

The tortuous epididymal
and deferent ducts have low mucosal folds covered with nonciliated
pseudostratified columnar epithelial cells 24, 26. The epididymal duct makes
up 2.5 to 10% of the total epididymal volume in gallinaceous birds (chicken,
pheasant, turkey etc.) 26, whereas in ratite birds it is about tenfold higher
23. The deferent duct is a continuation of the epididymal duct and according
to Tingari 19, there is a threefold increase in luminal diameter between
the cranial epididymal duct and the distal deferent duct. The distal deferent
duct widens at its juncture with the cloaca to form a structure called
receptacle of the deferent duct. The receptacle of the deferent duct has the
shape of a bean when it is filled with semen. Each deferent duct ends in the
cloacal urodeum as a papilla immediately
below the ostium of a ureter (Figure 1.3). The epididymis and the vas deferens aid the passage of sperm from the testes,
allowing for the expulsion of the stored
sperm from the vas deferens at ejaculation 27. The protruding phallic folds
of the ventral cloaca are filled with lymphatic fluid and thus allow the semen
to flow through the depression 28. There are about 0.06 – 3.5 × 1012
sperm per ejaculate and between 3 – 7 × 109 sperm per ml in chicken 30.

 

Fig. 1.2 Internal structure of
the domestic chicken testis

Source: Marshall 18

 

 

 

 

Fig. 1.3 Schematic of the excurrent ducts of the testis.  ST – seminiferous tubule; RT- rete testis; ED
– efferent duct; CD – connecting duct; ER – epididymal region; ED – epididymal
duct; DD – deferent duct, Source:
Lake 31

 

The accessory organs

The accessory sexual reproductive organs of the male
birds include the para cloacal vascular
bodies, dorsal proctodeal gland, ejaculatory groove, lymphatic folds and
phallus (penis) 3. An analogous of the mammalian penis known as the phallus is present in male birds and is of two
types; the intromittent type and the non-intromittent type. The phallus is
located on the ventral lip of the vent and is not to be confused with the
papilla. In species like the Psittacines (parrots, cockatoo etc), where the
phallus is lacking, copulation is done by eversion
of the cloacal wall exposing the slightly raised papilla for transfer of semen
to the everted orifice of the female oviduct during cloacal contact. Unlike the
mammalian penis, the erectile mechanism of the phallus is lymphatic and not
vascular. Avian have no organs comparable to the mammalian prostate gland and
bulbourethral gland; and the seminal plasma is derived from the efferent ducts
and seminiferous tubules 31. The accessory reproductive organs are either in
proximity to or are an integral part of the cloaca. The para-cloacal vascular body is found alongside the receptacle of the
deferent duct, and lymphatic folds exist within the wall of the proctodeum. An
intromittent phallus exists in only 3% of avian species 32 and these species
include the ratites (ostrich), the anseriforms
(swans, geese) and the waterfowl. In domestic chickens and turkey, the phallus
is non intromittent and semen is
ejaculated into the female tract via the “cloacal kiss” during mating 33. The
cloacal kiss involves the coming together of the male and female cloacae for a
few seconds. During copulation or in response to massage, a non intromittent
phallus forms a tumescent lymphatic tissue, which is everted through the vent
immediately before ejaculation. The para-cloacal
vascular bodies are essential for lymphatic tissue tumescence, as they are the
sites where lymph is formed by ultrafiltration of blood 3. In the majority of avian species including a rooster, the phallus is small and arises as a
modification of the cloaca and erection of the phallus occurs when folds within
the cloaca become engorged with lymph 34. Thus, phallus erection in birds is
lymphatic whereas in mammals, penile erection is blood-vascular and, during
ejaculation, semen is transported through the urethra. In avian species, semen
is transported by the sulcus spermaticus,
an external groove present in the phallus 35.

 

Conclusion

The male reproductive system of poultry is comprised of a
paired testes located close to the cephalic of the kidney unlike in the mammal
where they are located in the scrotal sac. They are surrounded by a thin
fibrous capsule made up of connective tissue and contractile fiber. Sperm
production proceeds at the internal body temperature of 41oC in
birds as opposed to the scrotal temperature of 24-26oC in mammals.
The paired testes are functional as opposed to the only the functional left
oviduct and ovary in the female bird. Paraclocal
vascular bodies, dorsal proctodeal glands, ejaculatory groove, lymphatic fold and phallus are sexual reproductive organs
in male poultry. Phallus, an analogue of mammalian penis is located on the ventral lip
of the vent and is of two types; the intromittent type and non intromittent type. Unlike in the mammalian
counterpart, the erectile mechanism of the phallus is lymphatic and not
vascular.

 

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