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Author: LEITE-BROWNING PubID: UNP-0107 |
Title: | BIOLOGY OF REPRODUCTION OF GOATS |
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Status: WEB ONLY |
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Biology of Reproduction of Goats
Introduction
Reproduction is considered a primary trait
of economical importance in animal production systems. Within
this context, there is a need to adequately manage the reproduction
of bucks and does to increase reproductive efficiency and herd
production. Through reproductive management, goat herds may improve
production levels. In order to understand and manipulate the reproductive
processes of goats, it is important to be aware of their reproductive
systems and functions.
Doe's Reproductive System
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As seen in Figures 1 & 2, a doe's reproductive tract is composed of the following:
Ovaries
Ovaries are almond-shaped, paired, and located on each side of
the pelvic cavity. They produce the ova or female gametes and
sex steroid hormones such as estrogen and progesterone. Estrogens
are responsible for the development of the secondary sex characteristics
of does and the physical and behavioral changes that does display
during heat. Progesterone is responsible for changes in the uterine
environment for embryo implantation, as well as for maintaining
pregnancy and promoting mammary gland growth and development during
pregnancy.
Oviducts
Oviducts are tiny, convoluted tubes located on each side of the
uterus that connects the ovary to the uterine horns. The oviducts
are divided into three distinct segments that transport the ova
and spermatozoids in opposite directions. Once the ova are released
from the ovary during ovulation, they are captured in the oviduct.
The oviduct is the site where the ova are fertilized. In a segment
known as the ampulla. The oviduct is the site where further
capacitation of the spermatozoa occur.
Uterus
A doe's uterus is bicornuate, which means that it has two long
cornus or horns that connect the uterine body to the oviducts.
Does are known to have a small uterus, generally 3 cm in length
and 2 cm wide. The uterus is a smooth, muscular organ that stretches
during the pregnancy along with the growth of a fetus or fetuses.
The uterus protects and provides nourishment to the embryo and
fetus during pregnancy. The uterus is also the site where the
sperm cells reach maturation or capacitation, and where the embryo
migrates and develops throughout the pregnancy until parturition.
The endometrium, or internal layer of the uterus, is formed by glands that secrete endometrial milk that nourishes the embryo. The endometrial glands also secrete prostaglandin F2a or PGF2a, a hormone responsible for the luteolysis or degradation of the corpus cycle or days before parturition. The uterus separates itself from the vagina through a cartilaginous structure named the cervix.
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Cervix
The cervix is a fibro-cartilaginous-like structure composed of
three or four cartilage tissues named rings. The cervix
connects the uterus with the vagina.
The cervix has an anterior and a posterior opening or OS. The cervix remains closed; however, it opens during heat under the influence of thehormone estrogen to facilitate the penetration of the sperm cells. The cervix is also opened during parturition for the passage of the fetus. During pregnancy, the cervix enlarges like the uterus. The inner layer of the cervix has secretory cells that produces a thick mucus, or "plug," that accumulates during pregnancy to protect the uterine environment against pathogens or infectious agents and foreign bodies.
Vagina
The vagina is a large and tubular elastic structure 9 to 15 cm
in length. It is located between the cervix and vulva. The vagina
is the copulation organ of the female, receiving the penis during
mating, and it expands during birth.
Vulva
The vulva is the external genitalia consisting of the vestibule
and the labia. The vestibule, generally 3 cm in length, is common
to the urinary and genital tract. The vestibule joins the vagina
with the urethral orifice.
The labia consists of the labia majora and minora, the outer and inner folds of skin outside the vagina. The labia majora is homologous to the scrotum in males, and it is the visible external portion of the female tract.
Clitoris
The clitoris is located in the lower portion of the vulva. It
is the excitatory organ of a doe's reproductive tract.
| Organ | Functions |
| Ovaries |
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| Oviducts |
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| Uterus |
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| Cervix |
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| Vagina |
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| Vulva |
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| Clitoris |
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Supporting Structures
The female reproductive tract is located in the pelvic cavity, and is supported by the broad ligament. This ligament supports the ovaries, oviducts, and uterus. Blood vessels and nerves pass through the broad ligament to supply blood to the female tract.
Buck's Reproductive Tract
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The buck's reproductive system (see Figures 4 and 5) consists of testes (testicles), scrotum, spermatic cords, accessory glands, penis prepuce, and the male duct system.
Testes
The testes or testicles are oval-shaped
and paired. They are the main reproductive organs of a buck. The
testicles are housed in the scrotum. They are symmetrical in shape
and size, elastic to firm in consistency, and mobile in the scrotal
sac. The primary functions of testicles are to produce spermatozoa
or male germ cells in the seminiferous tubes, and to secrete steroid
hormones (testosterone) by the Leydig cells. A buck within 8-14
months of age should have 25 cm of scrotal circumference.
Scrotum
The scrotum is the sack-like pouch formed by the skin that is
responsible for protecting the testicles and the epididymis that
is located in the inguinal region between the legs. The scrotum
also aids in the thermoregulation of the testicles.
Epididymis
This is the first external duct of the testicles that is divided
into three parts: the head, body, and tail. The epididymis transports,
matures, nourishes, and stores spermatozoa produced in the testes.
It is the site where the spermatozoa acquire motility.
Spermatic Cord
The spermatic cord is composed of muscles and fiber tissues and
a portion of the vas deferens. The cord connects the testicles
to veins and arteries that irrigate the testicles in conjunction
with the scrotum to position the testicles outside the body, and
to help regulate the temperature of the testicles.
Vas Deferens
The vas deferens, or ductus deferens, are a pair of ducts tied
to the tail or cauda of the epididymis. They pass along the spermatic
cord to the pelvic cavity where they merge with the urethra. The
vasa deferentia transport spermatozoa from the epididymis to the
urethra.
Urethra
The urethra is a duct common to the urinary and reproductive tracts.
A distal portion of the urethra is connected to the urinary bladder.
The distal portion is inserted in the penis and serves in discharging
urine and semen.
Accessory Glands
The accessory glands are located next to the urethra and consist
of the vesicular, prostate, and bulbouretrals glands. The accessory
glands are responsible for producing seminal fluid that nourishes,
serves as a buffer, and provides other substances needed for the
motility and fertility of spermatozoa. The spermatic fluid and
spermatozoa combined form the semen. Accessory gland fluids empty
into the urethra.
Penis
This organ is responsible for male copulation and deposits semen
in the female tract. The corpus spongiosum and cavernosum are
expandable tissues that enlarge and fill with blood when arousal
occurs. In this excitatory state, the penis is erect, facilitating
copula and the ejaculation of semen in the female genital tract.
Prepuce
The prepuce is the sheath or foreskin that protects the penis.
Thermoregulation
The temperature inside a testicle is 2°C below body temperature.
In cold weather, the testicles rise near the abdominal cavity;
in hot weather, the muscular relaxation permits testes to swing
and hang down from the body. Structures responsible for thermoregulation
are the muscles cremaster, dartus and the plexus pampiniform.
This relaxed state maintains optimum temperature for the spermatogenesis
to process and the spermatozoa to survive. A buck's low fertility
rate is attributed to environmental conditions and the incapacity
to regulate the optimum testicular temperature.
| Organ | Major Functions |
| Testis or Testicles |
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| Scrotum |
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| Spermatic Cord |
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| Epipidymis |
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| Vas deferens |
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| Urethra |
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| Accessory Glands |
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| Penis |
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| Prepuce |
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Hormonal Control of Reproductive Process and Doe Cyclicity
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The reproductive process is complex and regulated by hormones. Hormones are chemicals produced by endocrine glands and secreted and released in the bloodstream that act directly on target organs, or indirectly through the regulation of other hormones. These hormones can be classified as peptides and proteins or steroid hormones.
In temperate regions such as in the United
States and Canada, goat breeds are seasonal or "short-day
breeders." This means that the period of cyclicity is regulated
by the photoperiod. Thus, the decrease in day length triggers
neuro-endocrine and ovarian interaction that occurs during late
summer and continues on through the fall and winter months. Seasonality
can be a limiting factor in the reproductive process because it
reduces the annualbreeding season and limits the opportunity for
producers to market their kids year-round.
The phenomenon of photoperiod is regulated by melatonin, a hormone produced by the pineal gland. The pineal is an endocrine gland located in the brain. The reduction in daylight exposure stimulates the optic nerve of the retina, which, in response, stimulates the cells of the pineal gland to secret melatonin. Higher levels of melatonin trigger a sequence of positive and negative feedback involving several endocrine glands and the gonads.
The increased levels of the gonadotropin-releasing hormone (GnRH) generated from the hypothalamus, a gland located in the brain, triggers doe cyclicity. The GnRH stimulates the cells of the anterior pituitary, or hypophysis, located at the base of the brain to secrete the follicle stimulating hormones (FSH) and the luteinizing hormone (LH). The FSH supports the development and growth of primordial follicles into secondary and tertiary follicles. These follicles produce estrogens. The increase in hormone estrogens will induce the production of LH surges, triggering ovulation or the release of the ova from the graffian follicles. After ovulation, the ova are captured in the oviduct, where in the presence of spermatozoa, they will be fertilized to generate an embryo.
After ovulation, the cells of the ruptured follicle will be transformed by luteinization to form a new ovarian structure called the corpus luteum (CL). The luteinization process is promoted by the action of LH. During the fifth day of formation, the CL is active, secreting progesterone to maintain a possible pregnancy. The maintenance of the CL is determined by the presence of an embryo. In case of pregnancy, the CL remains active, secreting progesterone to maintain the appropriate uterine environment for fetal development during pregnancy. In the case of a nonpregnancy, the CL will suffer luteolysis or regression. Luteolysis is caused by the action of prostaglandins F2a secreted by uterine glands. The prostaglandin PGF2a is transported from the uterus to the ovaries through the arteries and uterine vein connections to promote luteolysis or regression of the CL. The regressed CL will allow the ovary and other endocrine glands to prepare for another cycle.
In bucks, the main class of androgens is the testosterone that is produced in the testicles by Leydig cells. Testos-terone is responsible for a buck's secondary sex characteristics, to maintain the libido, and to promote the function of the accessory glands and spermatogenesis among others. Testosterone regulates the release of hypothalamic and anterior pituitary hormones like progesterone in does.
The process of spermatogenesis in a buck
takes 49 to 63 days, while spermatozoa are formed daily during
the life of a buck. However, the daily sperm production and quality
of ejaculation decrease according to day length.
References
Chemineau, P., Berthelot, X., Daveau, A., Maurice. F, Viguié, C., and Malpaux, B. (1993, October). [Can melatonin be used in out-of-season reproduction in domestic mammals?] Contracepion Fetilité Sexualité, 21(10), 733738.
Chemineau, P., Guillaume, D., Migaud, M., Thiéry, J. C., Pellicer-Rubio, M. T., and Malpaux, B. (2008, July). Seasonality of reproduction in mammals: Intimate regulatory mechanisms and practical implications. Reproduction in Domestic Animals, 43 Supplement 2, 4047.
Gonzalez-Stagnaro, C. (1983). Comportamiento reproductivo de las razas locales de rumiantes en el Tropico Americano. In: Chemineau, P., Gauthier, D., Thimonier, J. (Eds.), Reproduction des Ruminants en Zone Tropicale (pp. 180). France: Institut National de la Recherche Agronomique.
Hafez, E. S. E., and Hafex, B. (2000). Reproduction in Farm Animals (7th ed). New Jersey: Blackwell.
Hawken, P. A., Beard, A. P., Esmaili, T., Kadokawa, H., Evans, A. C., Blache, .D, and Martin, G. B. (2007, July). The introduction of rams induces an increase in pulsatile LH secretion in cyclic ewes during the breeding season. Theriogenology, 68(1), 56-66.
Morrow, D. A. (1986). Current therapy in theriogenology. Toronto: W. B. Saunders.
UNP-0107, May 2009, Maria Lenira Leite-Browning, DVM, Extension Animal Scientist, Alabama A&M University
For more information, contact your county Extension office. Visit http://www.aces.edu/counties or look in your telephone directory under your county's name to find contact information.
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