AbstractThe female reproductive system of the poultryis comprised of unpaired ovary and oviduct. The ovary is dark brown to black incolour with several ovarian folliclesconcentrated on the ventral surface. The ovary is attached to the bodies of thelumbar vertebrae by a thin ligamentous structure known as mesovarium. Theventral surface of the ovary is covered by the developing follicles.

In adultbirds, 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 tubularpart. The funnel-shaped part openedtowards the left ovary by a wide slit like structure.

The magnum, the secondportion of the oviduct is the largest in size and diameter, and most convolutedpart in laying birds. The isthmus, the third region of the oviduct is a narrowtube 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 shortmuscular s-shaped tube connecting theuterus to the cloaca. The highly distensible nature of the vagina allows for the passage of hard-shelled egg during ovipositionKeywords:Poultry, female, reproductive system, anatomy, physiology 2.1Introduction It is important tounderstand the anatomy and physiology of the reproductive system of femalechicken so that its reproductive activities can be well appreciated.

Indomestic chickens, reproduction is sexual, and it takes both the male andfemale gametes to unite in order for a new life form to be produced. But eggproduction in chicken, as in various avian species, is not dependent on the availability of male sexual gamete. This meansthat a female chicken can lay eggs without being mated by a male 1. A matured femalechicken is referred to as “Hen”, while the growing young is commerciallyreferred to as “Pullet”. Like other birds, a chickenlays an eggwhich later hatches (if fertilized) tobring forth offspring, unlike mammalswhich give birth to young ones alive. One striking difference between mammalsand avian species with respect to reproductive physiology is the fact thatmammalian embryos develop inside the body of the female in special organs adaptiveto the function of maintaining the embryo till “term”, and have the potentialsto deliver the young ones alive 1, 2. In this situation, the offspringobtains its nutrients requirement from the dam’s circulatory system till itmatures and is delivered.

The avian differs from this, in that the embryodevelops outside the parent and its nutritional requirements during and shortlyafter incubation is fixed in the eggsprior to lay.Theanatomy of the female reproductive system of a chicken is divided into twodistinct parts viz: the ovary (site of sex steroid synthesis, gametogenesis, and yolk formation) and theoviduct (organ receiving the egg yolk during ovulation and successfuldepositing the egg white, shell membrane and the shell) 1. In the majority ofthe avian species including domestic chicken, only the left ovary and oviductare functional. The right ovary and its associated oviduct are initiallypresent in the embryo but regress during development to adult birds.

Thischaracteristic of the single left avianreproductive tract (ovary and oviduct) dates back to enantiornithine birds fromthe early cretaceousperiod 2. 2.2 The OvaryThe 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 inemu (Dromaius novaehollandiae) and Rao 5 in domestic duck (Carina moschata).The right ovary and oviduct are present in the embryonic stages, but thesetissues normally regress in most avian species. The ovary is dark brown toblack in colour with several ovarianfollicles concentrated on the ventral surface 6 and in the young bird; theovary is flat with inverted L shape. The left ovary is located in the anteriorbody cavity left to kidney and adjacent to the adrenal gland and weighs about300 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 Vijayakumaret al.

4 in the emu. The meanweight of the left ovary in laying emu was 349.66 gm which decreased to 103.66gm in the non-laying emu. The ovary isattached to the abdominal wall by the mesovarium ligament in birds. A large body of follicles is found in the ovarywhich later matured serially and the ovary receives its blood supply from theovarian artery, which arises from the left renolumbar artery or from the dorsal aorta 4, 7. The ovary consists of a highly vascularizedinner area (medulla) and an outer area(cortex) which presents granular aspects as earlier as 5 weeks of age due tothe progressive development of follicles.It grows rapidly between 16 and 20 weeks of age and itsweight increases from 5 to 60 g during this period and can reach up to 120 to150 g in breeder hens fed ad libitum8.

The development of ovary in chicken starts at hatching, and it grows toattain full development at the time of sexual maturity. Some thousands of eggsare formed in the ovary at the time of hatching, but most eggs do not developto ovulatory size, thus, fewer eggs are laid by a hen in its live timecomparatively. Accordingto Kaspers 10, a pre-ovulatory egg, otherwise called ovum, develops from asingle cell covered by a vitelline membrane to which egg yolk is added.

About 2to 3 weeks before the onset of lay, small (<1 mm in diameter) white yolkfollicles begin to accumulate yellow yolk, with some being recruited into ahierarchy of maturing yellow yolk follicles. At ovulation, the largest follicle often referred to as F1, is dischargedfor possible fertilization if male semenwas introduced; although egg formation goes on normally in the absence ofsperm. About 17 days is required for the 1 mm diameter white egg yolk follicleto mature to a pre-ovulatory 40 mm diameter yellow yolk follicle in chicken.After the F1 follicle is ovulated, the next largest follicle, formerly referredto as F2 follicle, becomes the F1 follicle and is ovulated at the beginning ofthe next daily ovulatory cycle in 24 – 26 hour (Figure 2.1). The typicalnumbers of each classification of follicle found at sexual maturity arepresented in Table 2.1.

In terms of nerve network, follicles are profuselyinnervated by both adrenergic and cholinergic fibers 3. Neurons are presentwithin the thecal layers of the F1 follicles and provide the follicles with several neurochemicals such ascatecholamines, neurotropins, vasoactiveintestinal peptides, calcitonin gene-relatedpeptides 11, 12.   Fig.

2.1 Ovary with a cluster of mature and immature follicles in the chicken hen. F1- F6 denotes pre-ovulatoryfollicles. Source: Johnson 9 Table 2.1 Classificationof 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 2.3The OviductThe second major part of the hen’sreproductive system is the oviduct 14 and is derived from the left paramesonephric Mullerian duct 15.

Khokhlovand Kuznetcov 16 reported a well-developed oviduct of the hen at the left side and atrophied at the rightside. In contrast, Rao 5 identified a slightly developed right oviduct measuring4 cm in length in domestic duck. In immature pullets, the oviduct is visible asthin, pale narrow tube stretching from the ovary to the cloaca. Cellulardifferentiation in the oviduct occurs mainly during sexual maturity, 2 to 3weeks before lay. Oviductal weight increases from <1.0 g to 40 g in 2 weeksand its length from 15-21 cm to 70 cm. At maturity, the oviduct appears as a long convoluted gray to palepink tube with one of its ends in contact with the ovary, while the other endopens into the cloaca. The oviduct is vascularized at four levels from thegeneral arterial system; innervation of the distal portion of the oviduct ishighly developed 3.

The oviduct is attached to the dorsal body wall by aligamentous structure called mesovarium.In the growing or nonbreeding hen, theoviduct is an inconspicuous, narrow tube, straight and uniform in diameter,while in the reproductively active hens, the oviduct undergoes tremendousenlargement and occupies a large part of the abdominal cavity. The increase inlength causes a folding of the oviduct upon itself. The oviduct consists offive functional microscopically distinguishable regions: infundibulum, magnum,isthmus, uterus (or shell gland) and vagina as described in Figure 2.2.

 TheInfundibulumThis is the firstpart of the oviduct which is about 10 cm and receives ova from the ovary via awide slit-like opening 17. Its main function is to engulf the oocyte releasedby the largest follicle and to move it to the next segment of the oviduct. Theinfundibulum activity appears to be under the control of a combination ofmuscles, the engorgement of blood vessels and the activity of the ligaments. Itis located in the proximal part of theoviduct and has a funnel shape (Figure 2.2). It is about 8 – 10 cm long and canbe subdivided into three morphologicalregions including the fimbriated region, the funnel region, and the chalaziferous region.

The fimbriated region guidesthe ovulated ovum into the ostium of the infundibulum; the funnel region is thesite where sperm contact an ovulated egg if semen was introduced; while thechalaziferous 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 theinner mucous membrane of the infundibulum with some having either a secretoryfunction because of their role in outer vitelline membrane synthesis, or spermstorage. This is the site of fertilization which takes place before the oocyteis covered by albumen. The funnel region does not possess sub-epithelialtubular glands, while the distal infundibulum is characterized by sub-epithelialtubular glands which secrete albumen-like materials around the ovum referred toas a perivitelline layer.

The name”funnel” will depict that theinfundibulum waits to receive an ovulatedegg from the ovary; but actually, the muscular infundibulum stretches to coveran ovulated ovum. Here the egg spends about 15-18 minutes and it is during thistime that fertilization of the egg occursif there were viable spermatozoa from a male, or through artificialinsemination.  The MagnumMagnum(Figure 2.2) is the second and largest section of the oviduct which is about 35cm long.

Magnum is a thick, milky-white, distensible tube with a well-developed fold (4-5 mm in height) and themuscles 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 mucosalfolds in the magnum. It contains the largest number of secretory cells in boththe epithelium and tubular glands. These secretory cells contain a large amount of proteinaceous material storedas granule which is secreted with the passage of the egg. The internal surfaceof the magnum is light gray in colour,more or less transparent depending on the elapsedtime 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 tothe egg 24. The proportion of albumenadded by the magnum depends on the age of the hen and storage conditions 25.  Fig. 2.2 Oviductof the hen showing infundibulum (10 cm),magnum (35 cm), isthmus (10 cm), uterus (10 cm), vagina and cloaca.

Source: Bakstand Dymond 18 TheIsthmusThethird section of the oviduct, the isthmus is 10 – 15 cm long 1, 14 (Figure2.2). The isthmus as the name implies isslightly constricted and has two parts i.e., upper white isthmus and lower redisthmus from where the outer and inner shell membrane is deposited, respectively. The boundary between the isthmus and magnum was described by King and McLelland 20as zonula translucens in domestic fowl.

This area was a glandless zone at the beginning of the magnum and was usuallyvisible with the unaided eye 26. Thezonula translucens is absent in somebirds such as emu. The magnum-isthmus junction is involved in the secretion ofthe peri-albumen layer 27. The tubular gland is many in the isthmus than inany other region of the oviduct and is responsible for the formation of themembrane fiber core before the deposit of the mammillary knob in the tubularshell gland, a tube-shaped portionconnected to the pouch-like section ofthe shell gland. Poultry Hub 25 puts the length of this section at about 12 cm and stated also that it functions insecreting 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 ofthe oviduct where the tough outer membrane of the egg or where initialcalcification of the eggshell takesplace. Susan 24 stated that isthmus is the only portion of the oviduct thatcan produce sulfur-containing amino acids necessary for shell membraneproduction.

 Shell GlandThe next section of the oviduct is the shellgland or uterus (Figure 2.2) and is about 7 cm long, reddish in colour and pouch like in structure 14. Theshell gland was divisible into the cranial narrow tubular part referred to aspars minor uteri and the main pouch-likepart referred to as the pars major uteri 29, 30. It is involved in thehydration of the forming soft egg and in eggshell deposition. The uterine wall is thickand contains well developed muscularlayer. The uterine mucosa is dark red in colouredand has internal leaf shaped fold. The uterine mucosa differed from the uterovaginal mucosa by the lack of direction offolds. Poultry Hub 25 estimated that the length of this section would beabout 10 cm.

But Jacob 1 stated that the length is about 10 to 13 cm and thatthis section is responsible for the formation of the eggshell. About 47 % of calcium used for eggshell formation is derived from the bone, while a greatproportion is supplied from the ration. Shell formation starts with smallclusters of calcium carbonate crystals depositing on the shell membrane whilstthe egg was still in the isthmus. Thisthen progresses into the shell gland intwo layers including a mammillary layer(inner layer of the eggshell), andpalisade layer (outer layer). Commonly, egg spends up to twenty hours in theuterus 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 theuterus from vagina, and obtain another application of shell, thus forming an egg within – an egg situation 31. TheVaginaIn domestic fowl 26 and in domestic duck 5,the vagina was described as a short muscular s-shaped tube connecting theuterus to the cloaca.

It leads from the uterus to the cloaca at a narrowopening in its middle left part. The vaginal mucosa is free of secretory glandsand shows long and narrow longitudinal ridge with conspicuous secondary foldswhich 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 spermstorage 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 mucosafolds with large diameter than the tubular gland of the uterus.

The vagina istightly bound to the uterus by a thick fibrous structure and has a flared shapethat is narrower at its distal ends. The vaginal wall is made up of welldeveloped circular muscle with few longitudinal muscles as well as a small band of connective tissue in contact withthe mucosa. This particular arrangement allows the vagina to be highlydistensible for the passage of the hard-shelledegg. This is the last part of the hen’s oviduct which is just about 4 – 5inches long 1. Poultry Hub 25 puts the length of this part of oviduct atabout 12 cm and added that it secretes egg’s outer cuticle and shell pigment.

The muscular sphincter at the uterine – vaginaljunction serves to expel the egg out of the hen’s body.   ConclusionEgg production is the primary function of the femalereproductive tract of chicken. The study of the anatomy and physiology of thisvery important system will allow the reader an opportunity to understand indetail how it works, what affects its functionality, how to remedy such defectsand thus obtain maximum performance from the flock.  References1.             Jacob J (2015)Avian reproductive system-female. The Universityof Kentucky.2.      Johnson AL (2014) The avian ovary and follicle development: Somecomparative and practical insight.

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Bakst MR,Wishart G, Brillard JP (1994) Oviducal sperm selection, transport, and storagein poultry. Poult Sci 5: 117-143     

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