COURSE CODE: ANS 581
THE
DEPARTMENT OF ANIMAL SCIENCE
FACULTY
OF AGRICULTURE AND NATURAL RESOURCES MANAGEMENT
1.0 INTRODUCTION
Puberty
is the point of sexual development at which the animal becomes capable of
reproduction (first ovulation in the female and first spermatozoa in the
ejaculate of the male), but animals are not yet fully sexually mature at this
stage. Sexual maturity is the time when the animal expresses it’s full
reproductive capacity. In the ruminants like sheep and goat, puberty may often
be reached without adequate physical growth to support reproduction and in the
females the first ovulation may not necessarily coincide with first estrus.
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In
males, puberty is the time when complete separation of the prepuce and the
penis occurs and motile spermatozoa are first detected in the ejaculate. In
immature rams and bucks the penis has adhesions that prevent it from being
fully extended. At puberty, these adhesions dissolve under the influence of testosterone
and the penis can be fully extended. However, fully reproductive competence may
not occur until 15 months of age. Spermatogenesis has been found to begin as
early as 84 days of age, with spermatozoa present in the epididymis at 140 days
of age.
The pubertal process is characterized by an activation of physiological events within the hypothalamic-adeno hypophyseal-gonadal axis which culminate in reproductive competence. Excessive weight gain and adiposity during the juvenile period is associated with accelerated on set of puberty in females.
The pubertal process is characterized by an activation of physiological events within the hypothalamic-adeno hypophyseal-gonadal axis which culminate in reproductive competence. Excessive weight gain and adiposity during the juvenile period is associated with accelerated on set of puberty in females.
The
mechanisms and pathways by which excess energy balance advances puberty are
unclear, but appear to involve an early escape from estradiol negative feedback
and early initiation of high frequency episodic gonadotropin releasing hormone
(GnRH) secretion.
Hypothalamic
neurons, particularly neuropetide Y and proopiomelanocortin neurons are likely
important components of the pathways sensing and transmitting metabolic information
to the control of GnRH secretion.
Recent
studies indicate that leptin-responsive neurons within the ventral
pre-mammillary nucleus play a critical role in pubertal progression and
challenge the relevance of kissipeptin neurons in this process.
Pubertal
development involves physical and behavioural changes that are linked to the
activation of the hypothalamic –adenohypohyseal-gonadal axis (Sisk and Foster,
2004). The progression of events is controlled largely by genetic and
environmental factors, among which nutrition has a major influence. However,
evidence that the weight gain and adiposity during early stage are associated
with early onset of puberty in females (Lee et
al.,2007; Jasik and Lustig, 2008; Rosenfield et al., 2009). And indications that precocious puberty is
associated with increased risks for development of polycystic ovarian syndrome,
reproductive cancers, and psychological distress (Golub et al., 2008) has renewed interest in understanding the mechanism
by which nutrient sufficiency supports reproduction maturation.
A
decrease in sensitivity to negative feedback of gonadal steroids-allowing
higher levels of gonadotropin (FSH and LH) required for cyclicity and
spermatogenesis result in the initiation of puberty.
The
first heat of the pubertal animal is often “silent”, that is the reproductive
system is ready for mating to occur, with ovulation, yet there is no external
manifestation of sexual receptivity. It is believed that the central nervous
system requires to be primed with progesterone before it will respond and the
animal show behavioral signs of heat.
2.0 ONSET OF PUBERTY
Puberty is
controlled by sexual hormones in bucklings and doelings, this process generally
occurs when they reach 60-70lbs or 30-50 percent of adult body weight.
Testosterone is the bucklings and estrogen is doelings. These hormones cause
physiological and behavioural changes in the both sexes leading up to puberty.
The
ovaries are essentially inactive from birth until puberty. Because of hormonal
support from the hypothalamus and anterior pituitary gland is insufficient. The
ovaries become active as the hypothalamus and anterior pituitary gland begin to
produce stimulating hormones such as the gonadotropin-releasing hormone
(GnHR), the follicle-stimulating
hormone (FSH) and the luteinizing hormone (LH).
Photo period, an environmental
factor, also greatly affects the onset of puberty especially in does. In does,
puberty and estrus cyclicity in general depend on photoperiodic changes. The
onset of puberty typically occurs during the short days of autumn. Spring-born
animals are exposed to long days and achieve puberty during the short days of
autumn in the same year. In contrast the October-born animals are exposed to
short days thereby decreasing days to reach puberty during the winter of the following
year. However, puberty can be manipulated through management techniques such as
permanent exposure to the buck, enhanced nutrition and breeding management
practices in the females.
In
the male, puberty contrast, it involves testicular growth and spermatogenesis
(spermatozoa production). Photoperiodic changes, does not affect male in
attaining puberty. The season of birth also does not appear to significantly
affect the onset of puberty in bucks.
Figure 1: Factors Affecting Female Puberty.
Source: Alabama Cooperative Extension System, 2009
Fig 2: Factors
Affecting Male Puberty.
Source: Alabama Cooperative Extension System, 2009
4.0 FACTORS AFFECTING
PUBERTY
There
are several factors that affect puberty which are; nutrition, body weight,
breed, season of birth and growth rate. Nutrition is among the most significant
factors influencing reproductive development and the onset of puberty.
·
A low plane of nutrition delays first estrus and
reduces uterine and ovarian development in ruminant animals.
·
Increasing the overall plane of nutrition
generally advances the on set of puberty.
·
Overfeeding will decrease subsequent fertility
and impair mammary gland development and, therefore care must be taken to avoid
overfeeding.
·
In most sheep and goats breeds, attainment of
puberty is dependent on achieving satisfactory body weight, usually between
40-70% of the mature body weight. A late-maturing breed like the Somali goat is
known to attain puberty at a later age.
·
Energy and protein restriction influences age at
puberty, with energy restriction having a greater influence on delaying on set
of puberty than protein restriction.
·
Age, weight and growth rate seem to be
interacting in determining the process of on set of puberty. In general, faster
growth rates resulting from higher planes of nutrition enables kids to attain
puberty at a younger age and heavier body weight than kids reared on low planes
of nutrition
·
Complete separation of males and females during
the early growth period may delay the on set of puberty.
Different
investigations on the reproductive performance of Ethiopian sheep and goats
have shown the following: ewe lambs of the Menz breed attain puberty at 10
months of age and 16.2kg mean weight or 56% of mature body weight. The onset of
puberty was earlier in animals with higher weaning weights. Somali kids were 19
months and weighed 26kg at puberty, indicative of a late maturing breed.
In Horro sheep,
kept under low-to high nutritional regime, age at first maturing was reported
to be 206 to 285 days while weight was 18-21kg. From on-farm monitoring work on
Ada District, weight and age at first successful mating for sheep was reported
to be 20kg and 8.7 months, respectively, while for goats it was 17kkg and 7.4
months.
3.0 BREEDING AGE AND/ OR
WEIGHT
Puberty
is not synonymous with sexual maturity. Dairy heifers and bulls may not produce
ova and spermatozoa at 6-8 months of age, conception at this early age can be
disastrous because the animal has not attained physical maturity. Puberty is
more closely related to body weight that age in domestic animals, except in
swine. Diary cattle usually reach puberty when their body weight is 30-40
percent and sheep 40-60 percent adult weight. Small breeds usually reach puberty
earlier than large breeds. Based on these finding, it has been recommended that
Holstein and Brown Swiss heifers should not be bred until they weight 750lbs,Aryshires
about 650lbs, Guernseys about 550lbs and Jerseys about 500lbs. For beef cattle,
it is recommended that Charolais heifers should not be bred until they weight
725lbs, her, Brahman and Brahman crosses about 700lbs, Hereford about 650 lbs,
Angus 600lbs and Angus and Hereford crosses at about 600lbs.
In
ewes and does, there are two schools of thought regarding their appropriate age
of mating. One school of thought advocates early mating regardless of what is
called critical body weight. It is argued that this will increase the life time
productivity of the females, despite the fact that some abortions are evident.
However the concept of “critical body weight” is favoured. According to this
concept, animals should attain a minimum body weight in order to avoid growth
retardation (in most cases two-third of mature body weight) before they are
mated. This reduces reproductive wastage (abortion) that could result from
mating small-sized and sexually immature animals.
5.0 THE ESTROUS CYCLE
Following
the onset of puberty in ruminant animals there is the development of physiological
rhythm of the reproductive system, called the estrous cycle. This pattern is
characterized by periods of sexual receptivity of the male during estrus. In
cow estrus activity may occur throughout the year and is said to be
polyestrous. In ewes, nannies, estrus activity occurs during certain seasons of
the year and they are referred to as being seasonally polyestrous. The estrous
cycle is divided into four phases or periods which blend one into the other
they are; proestrus, estrus, metestrus, diestrus.
But some
theriogenologists also divide the estrus cycle into two periods based on the
dominant secretary structure on the ovary-the estrogenic or follicular phase comprising
proestrus and estrus, and the progestational or luteal phase comprising of
metestrus and diestrus.
5.1 PROESTRUS
This
phase immediately preceding estrus, there is follicular growth and increased
secretion of estrogen and regression of the corpus luteum of the previous cycle
(in polyestrous species) and decreasing levels of progesterone. The principal distinguishing
feature of proestrus is the occurrence of rapid follicular growth and late in
this period the effect of estrogen on the tubular tract or diet system and the
behavioural symptoms of approaching heat can be felt. The proestrus period
directly proceeded that heat period which is a period of increasing estrogenic
activity and all the symptoms associated with it.
5.2 ESTRUS
This
is the period of acceptance of the male. The onset and end of this phase are
the only accurately measurable points in the estrus cycle and hence are used as
the baseline for determining cycle length. Ovulation occurs during this phase
in ruminant animals except in cow. Ovulation is a spontaneous process in all
species of ruminant animals.
There
are certain psychological and physiological changes in the brain which
manifests in some characteristic behaviour by the female largely described as
behavioural estrus. In the cattle the animal will stop grazing, become
restless, refuse to rest while others are resting and attempt to mount other
females and at the late stage of this period, it will stand for the male to
mount and mate with her and this point the 1st meiotic division will
have taken place in the secondary oocyte within the Graafian follicle which is
now large and matured. Estrogen secretion is more profound and the tubular
tract is made ready to accept the ovum/ova at ovulation judging by the increasing
contact between the fabriae of infundibulum and ovary as well as the increase
in the erectile nature of the cilia. There is also corpus mucus secretion by
the oviduct and other parts of the tubular tract and this time is thin and
copious. In the cow and goat, the mucus can be seen draining from the vulva.
5.3 METESTRUS
This
is the phase succeeding estrus. The granulosa cells of the ovulated follicle
give rise to lutein cells which are responsible for the formation of the corpus
luteum and increasing amounts of progesterone ovulation occurs at this stage in
cows and does. Also a phenomenon known as metestrus bleeding occurs in the
cows. This bleeding should not be confused with the menstruation bleeding in
humans but it is simply the bleeding that resulted from the rupturing of blood
vessels when the estrogen sensitized tubular tract are trying to return to
normal. It is just noticed as a patch of blood at the upper part of the tail
adjacent to the vulva and occurs in about 90% of all metestrus period in
heifers and 45% in matures cows this period lasts 4-8 days in the cow, 3-4 days
in the sheep and goat.
5.4 DIESTRUS
This
is the period of corpus luteum when it is fully functional in the cow it starts
at about day 5 of the cycle when an increase in blood concentration of
progesterone can first be detected and ends with the regression of corpus
luteum on day 16 or 17 of the cycle. But ewe extends from about day 4 through
days 13, 14 and 15. At the end of third phase development of another set of
follicle that is if pregnancy did not occur but where it occurs, the corpus
luteum persists ad the influence of progesterone will continue until the end of
the pregnancy.
Table 1.0 different phases of the
estrus cycle in does
Stage
|
Day of cycle
|
Characteristics
|
Estrus
|
1 - 2
|
Final growth and ovulation of
1-2 pre ovulatory cycles. Sexual receptivity of does. Cervical mucus thinner
and more corpious, cloudy towards ovulation
|
Metestrus
|
3-4
|
Functional organization of
corpus luteum
|
Diestrus
|
5-18
|
Luteal phase
|
Proestrus
|
19-21
|
Growth of pre-ovulatory
follicles.
|
Source: Ethiopia sheep and goat
productivity improvement, 2003
6.0 HORMONAL CONTROL OF THE
ESTRUS CYCLE
Estrus
cycle is controlled by Gonadotrophic releasing hormone (GnRH) released by the
hypothalamus. Just before the onset of estrus, the pituitary gland, under the
control of the phypothalamus in the brain, releases an increasing amount of
luteinizing hormone, (LH) and follicular stimulating hormone (FSH) (Ulfina, et al., 2000).
The
growth of the follicles being regulated by pituitary hormone (luteinizing
hormone and follicle stimulating hormone). The luteinizing hormone stimulates
the final maturation of the follicle containing the eggs and stimulates
follicle to produce the hormone estrogen (Ugwu, 2012). The estrogen brings the
animal into behavioural estrus or heat (Ugwu, 2012).
The
rising concentration of estrogen stimulates a surge in luteinizing hormone that
stops further secretion of estrogen by the follicle (Ugwu, 2012). Once the egg
has been released, luteinizing hormone transforms the follicle into a corpus
luteum (Mukasa, 1986).
The corpus luteum produces
progesterone which in turn suppresses pituitary activity (Ugwu, 2012).
Therefore if pregnancy does not occur, lysis (destruction) of the corpus luteum
occurs due to endogenous release of prostaglandin from the uterus, thus causing
a fall in the progesterone level, and the cycle starts again (Ugwu, 2012)
7.0 PUBERTY AS A NEUROENDOCRINE EVENT
The
onset of puberty in animals is characterized by an activation of the
hypothalamic- adeno-hypophyseal-gonadal axis that precedes the establishment of
cyclic ovarian activity. The peripubertal increase in pulsatile release of
gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH) supports
final maturation of ovarian follicles and enhances ovarian steroidogenesis
(Kinder et al., 1987). Elevated
circulating concentration of estradiol includes the pre-ovulatory surge of
GnRH/LH, which leads to first ovulation (Kinder et al., 1987). During juvenile period, the hypothalamic-adenohypophyseal-gonadal
axis remains relatively quiescent and the frequency of LH release is low (Foster
and Jackson, 2006; Plant and Witchel, 2006). A major limiting factor for
increased secretion of LH and Development of Pre-ovulatory follicles during the
juvenile period is the lack of appropriate stimulation of the gonadotropes by
GnRH. This assertion is supported by the observation that ovarian function is
stimulated in immature female monkeys treated with GnRH (Wildt et al., 1980). And in lambs treated with
Luteinizing hormone (Foster et al.,
1984). Because estradiol can lead to a surge-like release of LH in Pre-pubertal
female (Foster and Karsch, 1975; Andrews and Ojeda, 1977), it is believed that
the estradiol positive feedback is functional before reproductive maturation is
established. However, the lack of an appropriate stimulatory signals that restrain
GnRH neural activity, may explain the infrequent release of GnRH characteristic
of the Pre-pubertal period increased sensitivity to estradiol negative feedback
contributes to the inhibition of GnRH release in ewe, lambs and heifers (Foster
ad Ryan, 1979; Day et al., 1987). The
ability of low circulating concentrations of estradiol to inhibit the pulsatile
release of LH is diminished during maturation, and frequency of LH pulses increase
(Ebling et al., 1990)
8.0 METABOLIC-SENSING
PATHWAYS MEDIATING THE NUTRITIONAL CONTROL OF PUBERTAL DEVELOPMENT
Adequate
growth and adiposity are critical for normal progression of puberty in ruminant
animals. Growth restriction (Foster and Olster, 1985, Suttie et al., 1991) and excessive exercise
(Manning and Bronson 1989, Malina, 1994) during the juvenile period delay
puberty, likely by decreasing the release of GnRh (I’Ason et al., 2008) in association with heightened negative feedback
sensitivity to estrodiol (Foster and Olster, 1985). In contrast, increased
adiposity seems to facilitate reproductive maturation and advance the onset of
puberty (Kaplowitz et al., 2001; lee et al., 2007; Rosen field et al.,2009). In cattle, a high
proportion of heifers fed to gain weight at high rates during the juvenile
period exhibit precocious puberty (Gasser et
al., 2006 a,b). This occurrence is associated with attenuation of estradiol
negative feedback and increased pulsatile release of LH (Gasser et al., 2006 a). Therefore, nutritional
cues interact with gonadal steroid feedback to time the onset of puberty in
females.
Studies
investigating adiposity and adipocyte derived hormones as essential factors for
the initiation of puberty have revealed that leptin; a hormone secreted
predominantly by adipocytes has a critical role for the progression of puberty
in various species, including ruminants (Zieba et al., 2005). Although leptin does not affect secretion of LH in
adequately fed ewes (Henry et al.,
1999) and cows (Amstalden et al.,
2002) leptin prevents fasting-induced reduction in LH pulsatility in pre-pubertal
heifers (Maciel et al., 2004),
leptin’s action on GnRH/LH release in ruminants are likely mediated by
intermediate pathways. In addition to leptin, information from other hormones
like (insulin, and ghrelin) and nutrient (glucose, fatty acids, and amino
acids) is also likely to be integrated in a complex neural network that
perceive and signal availability of metabolic fuels to the control of
reproductive functions (Schneider, 2004)
Critical
neuronal pathways mediating signals of nutrient sufficiency and insufficiency
have been identified. Hypothalamic neuropeptide Y (NPY)/agouti-related protein
(AgRP) neurons, and proo-piomelanocortin (POMC) neurons are considered major
pathways by which nutritional signals are affected (Crown et al., 2007). These populations of neurons in the arcuate nucleus
expressed the leptin receptor and are responsive to changes in nutritional
status (Kalra and Kalra, 2003). Specifically, NPY has been shown to mediate the
inhibitory effects of under nutrition on reproductive function. (Kalra and
Crowley, 1984). Contrary to rats in which NPY has both stimulatory and
Inhibitory effect on LH release depending on gonadal steroid milieu (Sahu et al., 1987), NPY has a predominant
inhibitory action on the release of LH in ruminants in the presence and absence
of estradiol (Gazal et al., 1998;
Estrada et al., 2003; Morrison et al., 2003). This effect of NPY has
been shown to largely due to inhibition of GnRH release (Gazal et al., 1998) and may be mediated by
direct NPY actions on GnRH neurons (Klenke
et al., 2010). An animal with elevated body weight at a high rate between 4
and 6.5 months of age, the expression of NPY In the arcuate nucleus was
decreased compared to heifers that gained weight at lower rates (Allen et al., 2009). Using a similar dietary
treatment, we also observed that the proportion of GnRH neuron is close
proximity of NPY fibers in the preoptic area and hypothalamus was reduced in
heifers gaining body weight at high rate (Alves et al., 2011). Therefore structural and functional changes involving
hypothalamic NPY circuitry during the early juvenile period may involve in the
mechanisms by which excessive nutrition and adiposity support early on set of
puberty. In a study during hypothalamic tissue from non-human primates and rats
(Rot et al., 2007) suggested that a
network of genes involved in a range of cellular functions, including control
of transcription and cellular metabolism, is activated at the time of puberty.
In a recent study using microarray technology to investigate changes in gene expression
in the arcuate nucleus to pre-pubertal heifers fed to gain weight at high or
low rates, we observed that genes involved in a variety of biological functions
are responsive to nutritional input during the juvenile period (Allen et al., Unpublished). Differentially-
regulated genes included those associated with regulation of cellular metabolic
processes, receptor and intracellular signaling and neuronal communication.
Therefore, the pre-pubertal, growing female seems exquisitely sensitive to
nutrient inputs because changes in the regulation of metabolic-sensing and effectors
pathways are in tune for the fine control of neuroendocrine functions.
Mechanism involved may include regulation of gene expression, control of
cellular functions, and plasticity of functional structures within the
hypothalamus.
9.0 FACTORS
AFFECTING DEVELOPMENT OF THE REPRODUCTIVE ORGANS IN MALE
8.1 AGE
It
has been found that in bulls, age and testicular development have a curvilinear
relationship (Hahn et al., 1969;
Coulter and Foots, 1976; Elmore et al.,
1976; Lunstra et al., 1978 and
Osinowo et al., 1981). Work done by
Coulter et al., (1975) showed that
increase in scrotal circumference with age is up to a certain limit. Correlation
between age and scrotal circumference in young Bunaji Bulls was 0.88 (Osinowo et al., 1981). In ram, Watson et al., (1956) reported a general
increase in testicular weight and mean cross-sectional area of the seminiferous
tubules, with increase in age. Testicular size increased steadily in young
bulls and declines in the old bulls (Coulter, 1979; Coulter and Foote 1979).
Although spermatogenesis was found to have started by 13 months of age in Bunaji
bulls, consistently large gonadal and epididymal sperm reserves were only found
in bulls age 17 months and above (Osinowo et
al., 1981 Wildeus and Entwistle, 1983).
8.2 NUTRITION
Under
nourishment has been reported to delay growth and puberty in rams and bulls,
and age of selecting Holstein bulls for artificial
insemination by many months (Foots and Trimberger, 1968., Leathern 1975).
Nutrition affects age at puberty there is a correlation between body weight and
testicular weight in males or age at the first oestrus in the dams. If
nutrition is maintained at a normal level, puberty occurs when body weight
reaches 60% of the adult body weight in sheep and 45% of the adult weight in
cattle (Thinbault and Levasseur, 1974). If growth is accelerated by overfeeding
body weight at the time of puberty is higher that normal and the animal reaches
maturity at a younger age. On the otherhand, if growth is slowed down by
underfeeding, puberty is delayed and body weight does not reach the level of
that of normally fed animals (Thibault and Levasseur, 1974(. Dietary protein
deficiency results in a loss of testes mass, spermotogenic arrest and lowering
of the secretary ability of the interstitial cells of the secretary ability of
the interstitial cells of leydig (Tripathi
et al., 1968). Lambs reared on a high plane of nutrition reach puberty
earlier that those on a low plane (Ward, 1980). This is partly a reflection of
the tendency to reach puberty only above a certain body weight, and partly of a
direct effect of nutrition on the reproductive system. In rams testis size, which
is highly correlated with spermatogenesis activity, responds to nutrition, and
sperm production can be increased by flushing with high protein supplement for
about 8 weeks before mating states (Pryot, 1980). This is especially impotent
both under natural mating conditions and when semen is to be collected for
artificial insemination Vitamin A deficiency it could lead to testicular
malformation and aspemia (Barones et al.,
1969).
Reports
indicate that amino acid, carbohydrates, a few essential fatty acid and mineral
are necessary for the production of the fluid medium, produced by the cellular
environment of the testes, which is necessary for sperm transport (Leatherm,
1975). The number of motile spermatozoa in the ejaculate of underfed ruminants
was found to be 50% of normal, if the underfeeding was pre-pubertal (Flipse and
Almguist, 1961). However, low protein intake has been found to be produced more
adverse effect on immature animals than mature ones (Leathem, 1975)
8.3 GENETIC FACTORS
Age
at sexual maturity in sheep varies form breed to breed (Terrill, 1974). The
rate of testicular development is found to be higher for fast growing like
merino (Chag and Rae, 1970).
Lunstra et al., (1978) found the scrotal
circumference of various breeds of cattle to be almost the same (22.9±0.2cm) at
puberty, but the age body weight and testosterone content of the plasma at the
time of puberty varied from breed to breed. In Nigeria however, Osinowo et al.,
(1981) reported a scrotal circumference of 19-24 cm for Bunaji bulls at
puberty. The difference could be due to breed, level of nutrition and
management practice.
Work on Angus
and Holstein bulls by Coulter et al
(1975) showed that the Angus had a larger testis up to 3 years of age when they
were surpassed by the Holstein. The Angus
breed thus reached maximum testicular size at a younger age than the Holstein breed. The Heritability estimate of scrotal
circumference in bulls was reported to be about 0.67 (coulter et al., 1976).
8.4 SEASONAL EFFECTS
It
has been shown that changes in season have profound effect on sexual activities
and reproduction in general. Seasonal changes in the spermatogenic and androgenic
activities of the testis occur in a wide range of ruminants (Lincoln, 1976). Weight and scrotal circumference
measurements taken on some rams by Motimer and Lincoln (1982) during the
non-breeding season showed a decrease in their testicular weight and size, with
about one third decrease in weight recorded. Seasonal variation has been
observed in the episodic secretion of LH and testosterone in rams, with
increase from the non-breeding to the breeding season (Lincoln 1976, Pelletier et al., 1982). This shows that seasonal
changes affect testicular development in the young animals but affect the
weight of the testes in mature animals.
8.5 HORMONAL EFFECTS
The
sex differentiation of the male sex organs is guided under the influence of a
masculinizing mechanism that emanates from the secretion of androgen if these
testicular hormones are absent, the mullerian ducts and the urogenital sinus
develop into female sex organ. If the testicular androgens are present, the
Wolfian ducts and the urogenital sinus develop into male sex organs. The regression
or involution of the Mullerian ducts seems to be affected by the testes (Josso et al., 1977) it has been shown that the
proliferation of gonocytes is stimulated by the pituitary gonadotropin. The gonadototrophin
releasing hormone from the hypothalamus stimulates the anterior pituitary to
release Luteinizing hormone (LH) and follicle stimulating hormones (FSH)
(White, 1974). The LH stimulates the leydig cells to produce testosterone which
in turn stimulates the germinal cells, and maintains spermatogenesis (Gemzell
and Roose, 1966). Androgen also helps in the development and maintainance of
the accessory sex glands, stimulation of sexual behaviour and spermatogenesis
(White, 1974).
The
final differentiation of permuted to spermatozoa requires FSH and probably,
testosterone. The FSH stimulates the testes and causes a significant increase
in the diameter of the seminiferous tubules (White, 1974). Lunstra et al., (1978) found that testosterone
production in bulls was related to growth which was in turn related to
testicular development. Androgen stimulates protein anabolism causing increased
nitrogen retention which accounts for the more rapid growth and greater
ultimate weight of makes in many animals (Frandson, 1974; Nalbandoo, 1976).
8.6 EFFECT OF DISEASES
Epididymitis
has been reported as one of the diseases affecting testicular development
(Konig, 1964). It may result to testicular degeneration, which has been found
to be the most frequent cause of infertility in rams and bulls (Konig, 1964;
Smith et al. 1970).
Orchitis
is another disease which may lead to degeneration of the testes. It is usually
due to hermatogenous mataslasis of infection, and less frequently through
spread of the infection from the genitor- urinary passage (Konig, 1964. Jubb
and Kenedy, 1970). Orchitis may be interstitial, intratubular or necrotic (Konig,
1964). Infections which lead to epididymilis and orchilis have been found to
cause testicular degeneration in bulls (Kumi-Diaka, 1978). In rams epididymilis
produces by Actinobacilus seminus
hinders testicular development and result in a decrease in semen quality and
quantity (smith et al., 1970). Similarly, testicular hypoplasia effects
testicular development but may not be observed until after puberty (Jubb and
Kennedy, 1970). Some testicular disorders like cryptorchidism (White, 1961;
Swift, 1968; Smith et al., 1970; Bishop 1972; Jumi-Diaa et al, 1981) and testicular hypoplasia (Erickson, 1943; Laing and
Young, 1956; Bishop, 1972; Lundgren, 1972) have been reported to be of genetic
origin. Many infectious agents that interfere with the general well being of
the animal may adversely affect the function of the testis depending on the
duration and severity of infection.
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early onset of puberty in heifers alter gene expression in the nucleus. Proc.
Soc”. Study Reprod. 81,abstr., 489.
Alves,
B.R.C., Liu, S., Stevenson, E., Thorson, J.F., Cardoso, R.D., Tendeschi, L.O.,
Keisler, D.H., Williams G.L., and Amstalden, M. (2011). “Accelerated body
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