Abstract

The female reproductive system of the poultry
is comprised of unpaired ovary and oviduct. The ovary is dark brown to black in
colour with several ovarian follicles
concentrated on the ventral surface. The ovary is attached to the bodies of the
lumbar vertebrae by a thin ligamentous structure known as mesovarium. The
ventral surface of the ovary is covered by the developing follicles. In adult
birds, the left oviduct appeared as a long convoluted and highly vascular tube.
The infundibulum, the first portion of the oviduct is composed of a cranial funnel-shaped part and a caudal narrow tubular
part. The funnel-shaped part opened
towards the left ovary by a wide slit like structure. The magnum, the second
portion of the oviduct is the largest in size and diameter, and most convoluted
part in laying birds. The isthmus, the third region of the oviduct is a narrow
tube while the shell gland region which is the fourth portion is an expanded pouch-like structure where the eggshell is deposited. The vagina is a short
muscular s-shaped tube connecting the
uterus to the cloaca. The highly distensible nature of the vagina allows for the passage of hard-shelled egg during oviposition

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

 

2.1
Introduction

It is important to
understand the anatomy and physiology of the reproductive system of female
chicken so that its reproductive activities can be well appreciated. In
domestic chickens, reproduction is sexual, and it takes both the male and
female gametes to unite in order for a new life form to be produced. But egg
production in chicken, as in various avian species, is not dependent on the availability of male sexual gamete. This means
that a female chicken can lay eggs without being mated by a male 1.

A matured female
chicken is referred to as “Hen”, while the growing young is commercially
referred to as “Pullet”. Like other birds, a chicken
lays an egg
which later hatches (if fertilized) to
bring forth offspring, unlike mammals
which give birth to young ones alive. One striking difference between mammals
and avian species with respect to reproductive physiology is the fact that
mammalian embryos develop inside the body of the female in special organs adaptive
to the function of maintaining the embryo till “term”, and have the potentials
to deliver the young ones alive 1, 2. In this situation, the offspring
obtains its nutrients requirement from the dam’s circulatory system till it
matures and is delivered. The avian differs from this, in that the embryo
develops outside the parent and its nutritional requirements during and shortly
after incubation is fixed in the eggs
prior to lay.

The
anatomy of the female reproductive system of a chicken is divided into two
distinct parts viz: the ovary (site of sex steroid synthesis, gametogenesis, and yolk formation) and the
oviduct (organ receiving the egg yolk during ovulation and successful
depositing the egg white, shell membrane and the shell) 1. In the majority of
the avian species including domestic chicken, only the left ovary and oviduct
are functional. The right ovary and its associated oviduct are initially
present in the embryo but regress during development to adult birds. This
characteristic of the single left avian
reproductive tract (ovary and oviduct) dates back to enantiornithine birds from
the early cretaceous
period 2.

 

2.2 The Ovary

The normal reproductive tract of matured hen consists of a left ovary appearing as a bunch of grape as reported by Gilbert 3 in chicken
(Figure 2.1), Vijayakumar et al. 4 in
emu (Dromaius novaehollandiae) and Rao 5 in domestic duck (Carina moschata).
The right ovary and oviduct are present in the embryonic stages, but these
tissues normally regress in most avian species. The ovary is dark brown to
black in colour with several ovarian
follicles concentrated on the ventral surface 6 and in the young bird; the
ovary is flat with inverted L shape. The left ovary is located in the anterior
body cavity left to kidney and adjacent to the adrenal gland and weighs about
300 mg in newly hatched chicks. The lengths of the ovary in laying and non laying birds were 28.63 cm and 18.02 cm,
respectively as reported by Vijayakumar
et al. 4 in the emu. The mean
weight of the left ovary in laying emu was 349.66 gm which decreased to 103.66
gm in the non-laying emu. The ovary is
attached to the abdominal wall by the mesovarium ligament in birds. A large body of follicles is found in the ovary
which later matured serially and the ovary receives its blood supply from the
ovarian artery, which arises from the left reno
lumbar artery or from the dorsal aorta 4, 7. The ovary consists of a highly vascularized
inner area (medulla) and an outer area
(cortex) which presents granular aspects as earlier as 5 weeks of age due to
the progressive development of follicles.
It grows rapidly between 16 and 20 weeks of age and its
weight increases from 5 to 60 g during this period and can reach up to 120 to
150 g in breeder hens fed ad libitum
8. The development of ovary in chicken starts at hatching, and it grows to
attain full development at the time of sexual maturity. Some thousands of eggs
are formed in the ovary at the time of hatching, but most eggs do not develop
to ovulatory size, thus, fewer eggs are laid by a hen in its live time
comparatively.

According
to Kaspers 10, a pre-ovulatory egg, otherwise called ovum, develops from a
single cell covered by a vitelline membrane to which egg yolk is added. About 2
to 3 weeks before the onset of lay, small (<1 mm in diameter) white yolk follicles begin to accumulate yellow yolk, with some being recruited into a hierarchy of maturing yellow yolk follicles. At ovulation, the largest follicle often referred to as F1, is discharged for possible fertilization if male semen was introduced; although egg formation goes on normally in the absence of sperm. About 17 days is required for the 1 mm diameter white egg yolk follicle to mature to a pre-ovulatory 40 mm diameter yellow yolk follicle in chicken. After the F1 follicle is ovulated, the next largest follicle, formerly referred to as F2 follicle, becomes the F1 follicle and is ovulated at the beginning of the next daily ovulatory cycle in 24 – 26 hour (Figure 2.1). The typical numbers of each classification of follicle found at sexual maturity are presented in Table 2.1. In terms of nerve network, follicles are profusely innervated by both adrenergic and cholinergic fibers 3. Neurons are present within the thecal layers of the F1 follicles and provide the follicles with several neurochemicals such as catecholamines, neurotropins, vasoactive intestinal peptides, calcitonin gene-related peptides 11, 12.     Fig. 2.1 Ovary with a cluster of mature and immature follicles in the chicken hen. F1- F6 denotes pre-ovulatory follicles. Source: Johnson 9   Table 2.1 Classification of ovarian follicle Classification Colour Minimum diameter Maximum diameter Number of follicles per ovary in a breeder hen at sexual maturity Large yellow follicles Yellow 10 mm   7 – 9 Small yellow follicles Yellow 5 mm 10 mm 5 – 15 Large white follicles White 2-5 mm 5 mm 5 – 15 Small  white follicles White   1 mm 1000 to 1,200 or more Source: Robinson and Renema 13   2.3 The Oviduct The second major part of the hen's reproductive system is the oviduct 14 and is derived from the left paramesonephric Mullerian duct 15. Khokhlov and Kuznetcov 16 reported a well-developed oviduct of the hen at the left side and atrophied at the right side. In contrast, Rao 5 identified a slightly developed right oviduct measuring 4 cm in length in domestic duck. In immature pullets, the oviduct is visible as thin, pale narrow tube stretching from the ovary to the cloaca. Cellular differentiation in the oviduct occurs mainly during sexual maturity, 2 to 3 weeks before lay. Oviductal weight increases from <1.0 g to 40 g in 2 weeks and its length from 15-21 cm to 70 cm. At maturity, the oviduct appears as a long convoluted gray to pale pink tube with one of its ends in contact with the ovary, while the other end opens into the cloaca. The oviduct is vascularized at four levels from the general arterial system; innervation of the distal portion of the oviduct is highly developed 3. The oviduct is attached to the dorsal body wall by a ligamentous structure called mesovarium. In the growing or nonbreeding hen, the oviduct is an inconspicuous, narrow tube, straight and uniform in diameter, while in the reproductively active hens, the oviduct undergoes tremendous enlargement and occupies a large part of the abdominal cavity. The increase in length causes a folding of the oviduct upon itself. The oviduct consists of five functional microscopically distinguishable regions: infundibulum, magnum, isthmus, uterus (or shell gland) and vagina as described in Figure 2.2.   The Infundibulum This is the first part of the oviduct which is about 10 cm and receives ova from the ovary via a wide slit-like opening 17. Its main function is to engulf the oocyte released by the largest follicle and to move it to the next segment of the oviduct. The infundibulum activity appears to be under the control of a combination of muscles, the engorgement of blood vessels and the activity of the ligaments. It is located in the proximal part of the oviduct and has a funnel shape (Figure 2.2). It is about 8 - 10 cm long and can be subdivided into three morphological regions including the fimbriated region, the funnel region, and the chalaziferous region. The fimbriated region guides the ovulated ovum into the ostium of the infundibulum; the funnel region is the site where sperm contact an ovulated egg if semen was introduced; while the chalaziferous region, otherwise referred to as distal infundibulum is the "secondary" sperm storage site in the oviduct called sperm storage tubule (SST). Numerous cell types are present in the inner mucous membrane of the infundibulum with some having either a secretory function because of their role in outer vitelline membrane synthesis, or sperm storage. This is the site of fertilization which takes place before the oocyte is covered by albumen. The funnel region does not possess sub-epithelial tubular glands, while the distal infundibulum is characterized by sub-epithelial tubular glands which secrete albumen-like materials around the ovum referred to as a perivitelline layer. The name "funnel" will depict that the infundibulum waits to receive an ovulated egg from the ovary; but actually, the muscular infundibulum stretches to cover an ovulated ovum. Here the egg spends about 15-18 minutes and it is during this time that fertilization of the egg occurs if there were viable spermatozoa from a male, or through artificial insemination.   The Magnum Magnum (Figure 2.2) is the second and largest section of the oviduct which is about 35 cm long. Magnum is a thick, milky-white, distensible tube with a well-developed fold (4-5 mm in height) and the muscles layers are well developed to facilitate the transport of the egg 6. Sharaf et al. 19 observed extremely tortuous mucosal folds in laying ostrich. In domestic fowl, King and McLelland 20 observed a spiral pattern of mucosal folds in the magnum. It contains the largest number of secretory cells in both the epithelium and tubular glands. These secretory cells contain a large amount of proteinaceous material stored as granule which is secreted with the passage of the egg. The internal surface of the magnum is light gray in colour, more or less transparent depending on the elapsed time since the last egg passage and the associated protein secretion 21, 22, 23. It is in this segment that most (about 40 %) of the albumen is added to the egg 24. The proportion of albumen added by the magnum depends on the age of the hen and storage conditions 25.   Fig. 2.2 Oviduct of the hen showing infundibulum (10 cm), magnum (35 cm), isthmus (10 cm), uterus (10 cm), vagina and cloaca. Source: Bakst and Dymond 18   The Isthmus The third section of the oviduct, the isthmus is 10 - 15 cm long 1, 14 (Figure 2.2). The isthmus as the name implies is slightly constricted and has two parts i.e., upper white isthmus and lower red isthmus from where the outer and inner shell membrane is deposited, respectively. The boundary between the isthmus and magnum was described by King and McLelland 20 as zonula translucens in domestic fowl. This area was a glandless zone at the beginning of the magnum and was usually visible with the unaided eye 26. The zonula translucens is absent in some birds such as emu. The magnum-isthmus junction is involved in the secretion of the peri-albumen layer 27. The tubular gland is many in the isthmus than in any other region of the oviduct and is responsible for the formation of the membrane fiber core before the deposit of the mammillary knob in the tubular shell gland, a tube-shaped portion connected to the pouch-like section of the shell gland. Poultry Hub 25 puts the length of this section at about 12 cm and stated also that it functions in secreting some albumen as well as the shell membrane. Clauer 28 added that 10% of albumen is added in the isthmus and the shape of the egg is determined here too. Isthmus is regarded as the section of the oviduct where the tough outer membrane of the egg or where initial calcification of the eggshell takes place. Susan 24 stated that isthmus is the only portion of the oviduct that can produce sulfur-containing amino acids necessary for shell membrane production.   Shell Gland The next section of the oviduct is the shell gland or uterus (Figure 2.2) and is about 7 cm long, reddish in colour and pouch like in structure 14. The shell gland was divisible into the cranial narrow tubular part referred to as pars minor uteri and the main pouch-like part referred to as the pars major uteri 29, 30. It is involved in the hydration of the forming soft egg and in eggshell deposition. The uterine wall is thick and contains well developed muscular layer. The uterine mucosa is dark red in coloured and has internal leaf shaped fold. The uterine mucosa differed from the uterovaginal mucosa by the lack of direction of folds. Poultry Hub 25 estimated that the length of this section would be about 10 cm. But Jacob 1 stated that the length is about 10 to 13 cm and that this section is responsible for the formation of the eggshell. About 47 % of calcium used for eggshell formation is derived from the bone, while a great proportion is supplied from the ration. Shell formation starts with small clusters of calcium carbonate crystals depositing on the shell membrane whilst the egg was still in the isthmus. This then progresses into the shell gland in two layers including a mammillary layer (inner layer of the eggshell), and palisade layer (outer layer). Commonly, egg spends up to twenty hours in the uterus during which time eggshell colour is also formed. In the circumstance of deficiency in calcium, phosphorus, or vitamins, especially vitamin D, the shell-less egg can result. In some cases, eggs may return to the uterus from vagina, and obtain another application of shell, thus forming an egg within - an egg situation 31.   The Vagina In domestic fowl 26 and in domestic duck 5, the vagina was described as a short muscular s-shaped tube connecting the uterus to the cloaca. It leads from the uterus to the cloaca at a narrow opening in its middle left part. The vaginal mucosa is free of secretory glands and shows long and narrow longitudinal ridge with conspicuous secondary folds which thicken in the uterovaginal region. In this uterovaginal junction (1-2 cm), located at the cranial end of the vagina adjacent to the uterus, are the sperm storage tubules which play a vital role in the extended storage of sperm 32. The SSTs are branched tubular structures in the lamina propria of the mucosa folds with large diameter than the tubular gland of the uterus. The vagina is tightly bound to the uterus by a thick fibrous structure and has a flared shape that is narrower at its distal ends. The vaginal wall is made up of well developed circular muscle with few longitudinal muscles as well as a small band of connective tissue in contact with the mucosa. This particular arrangement allows the vagina to be highly distensible for the passage of the hard-shelled egg. This is the last part of the hen's oviduct which is just about 4 – 5 inches long 1. Poultry Hub 25 puts the length of this part of oviduct at about 12 cm and added that it secretes egg's outer cuticle and shell pigment. The muscular sphincter at the uterine – vaginal junction serves to expel the egg out of the hen's body.     Conclusion Egg production is the primary function of the female reproductive tract of chicken. The study of the anatomy and physiology of this very important system will allow the reader an opportunity to understand in detail how it works, what affects its functionality, how to remedy such defects and thus obtain maximum performance from the flock.   References 1.              Jacob J (2015) Avian reproductive system-female. The University of Kentucky. 2.      Johnson AL (2014) The avian ovary and follicle development: Some comparative and practical insight. Turk J Vet Anim Sci 38: 660-669 3.      Gilbert AB (1979) Female genital organs. In: AS King; J McLelland (Eds.), York: Academic Press. 4.      Vijayakumar K, Balasundaram K, Paramasivan S, Kumaravel A, Madhu N (2014) Microanatomy of female reproductive tract during laying and non-laying period in adult emu birds (Dromaius novaehollandiae). Asian J Sci and Tech 5(12): 793-795 5.      Rao TSC (1994) Microanatomical studies on the reproductive system of the domestic duck (Anas Boschas domesticus). Ph.D. Thesis, Tamil Nadu Veterinary and Animal Sciences University, Madras. 6.      Reed RB Jr., Cope LA, Blackford JT (2011) Macroscopic anatomy of the reproductive tract of the reproductively quiescent female emu (Dromaius novaehollandiae). Anat Histol Embryol 40:134 - 141 7.      Hodges RD (1965) The blood supply to the avian oviduct, with special reference to the shell gland. J Anat, 99:485-506 8.      Etches RJ (1996) Reproduction in poultry: Cab International ISBN 0-85198738-9 9.      Johnson AL (2015) Ovarian follicle selection and granulosa cell differentiation. Poult Sci 94: 781-785 10.    Kaspers B (2016) An egg a day- the physiology of egg formation. Lohmann Tierzucht. http://www.ltz.de/en/news/lohmann-information. 11.    Onagbesan O, Bruggeman V, Decuypere E (2009) Intraovarian growth factors regulating ovarian function in avian species. Anim Reprod Sci 111: 121-140 12.    Johnson AL, Whittow GC (2000) Reproduction in the female. Sturkie's Avian Physiology (5th Ed), San Diego, Academic Press. 13.    Robinson FE, Renema RA (2003) Female reproduction: Control of ovarian function. pp. 3-10. In: Optimizing chick production in broiler breeder. Robinson FE, Fasenko GM, Renema RA (ed.) Spotted Cow Press Alberta, Canada. 14.    Rahman A (2013) An introduction to the morphology of the reproductive system and anatomy of hen's egg. J Life Earth Sci 8:1-10 15.    Jacob M, Bakst MR (2007) Developmental anatomy of the female reproductive tract. In: Jamieson BGM (Ed.) Reproductive biology and phylogeny of birds. Enfield, Jersey, Plymouth. The University of Queensland, Science Publishers. 16.    Khokhlov RY, Kuznetcov SI (2007) Morphogenesis of a tunica mucosa of the oviduct of the hens. Int J Morphol 25: 329 -333 17.    Drenowatz C,  Elrod C (1995) The Ratite Encyclopedia. San Antonio: Ratite records 18.    Bakst MR, Dymond JS (2013) Artificial insemination in poultry,  success in artificial insemination – quality of semen and diagnostic  employed (Alemayehu L Ed), pp.175-195   19.    Sharaf A Eid W, Abuel-Atta AA (2012) Morphological aspects of the ostrich infundibulum and magnum. Bulgarian J Vet Med 15: 145-159 20.    King AS, McLelland J (1984) Birds their structure and function. London and Philadelphia, Bailliere Tindall. 21.    Chousalkar KK, Robert JR (2007) Ultrastructural study of infectious bronchitis virus infection in infundibulum and magnum of commercial laying hens. Vet Microbiol 122: 223-236 22.    Chousalkar KK (2008) Effects of infectious bronchitis virus on the oviduct of laying hens. Ph.D. Thesis, The University of New England, Australia. 23.    Chousalkar KK, Robert JR (2008) Ultrastructural changes in the oviduct of laying hen during laying cycle. Cell and Tissue Res 332: 349-358 24. Susan O (2013) The female reproductive system: Along comes to the egg! http://lafeber.com/pet-birds/the-female-reproductive-system-along-comes-the-egg/ 25.    Poultry Hub (2017) www.poultryhub.org/physiology/body-system/reproductivesystem/ 26.    Baumel JJ (1993) Handbook of Avian Anatomy. Nomina Anatomica Massachusetts: Club, Cambridge. 27.    Sultana F, Yokoe A, Ito Y, Mao KM (2003) The peri-albumen layer: A novel structure in the envelopes of an avian egg. American J Anat 203:115-122 28.    Clauer P (2017) The Hen' Peennstate Extension. The Pennsylvania State University. extension.psu.edu Form and function in birds. New 29.    Patak A, Baldwin J (1993) Structural and metabolic characterization of the muscles used to power running in the emu (Dromaius novaehollandiae), a giant flightless bird. J Exp Biol 175: 233-249 30.    Bronneberg RG, Taverne MA (2003) Ultrasonography of the female reproductive organs in farmed ostriches (Struthio camelus). Theriogenol 60: 617-633 31.    Melissa H (2015) The hen's reproductive system and issues- Chicken health issues. http:/www.tillysnets.com 32. Bakst MR, Wishart G, Brillard JP (1994) Oviducal sperm selection, transport, and storage in poultry. Poult Sci 5: 117-143          

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