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Ova spot trichrom

Sex in gradina povești

I1 I David Noakes Endogenous and exogenous control of ovarian cyclicity In nature, it is th e general rule that animals breed once annually and pa nurition occurs in the spring, the time most favourable to the progeny, since the early neonatal period of their life will be during the period of increasing light and warmth, and also at the tim e when food fo r the mother is most abundant to ensure adeq uate lactation.

Under the conditions of feeding a ova spot trichrom h ousing provided by domestication the breeding season tends to be lengthened, and some of our species, panicularly cattle, may breed at any time during th e ova spot trichrom all do mesticated animals, however, show a co nstant tendency to revert to the natural breeding season, as evidenced by reduced fertility during summer and early autumn in sows.

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For an ani mal to breed, it must be mated and hence must attract th e male and be sexually receptive in heat or in oestrus. All domestic species show recurring periods of sexual receptivity, or oestrous cycles, which are associated with the ripening ova spot trichrom the ovaries of one or more graafian follicles Fig.

If a fertile mating occurs then pregnancy may ensue. The onset of such changes when the female becomes sexually mature and able to reproduce is referred ova spot trichrom as puberty. Among females of the domestic species, puberty precedes the development of physical maturity a nd, although they become capable of reproducing, their efficiency, panicularly with respect to their fecundity, has not reached its maximum.

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The initiation of puberty is largely a function of the animal's age and maturity since the female is born with a genetic potential for cyclic reproductive activity. Provided the environmental influences are favourable at this time, then once the 'biological ova spot trichrom is started it will continue for as long as the environ ment remains favo urable.

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In none of our domesti c species is there a physiological change comparable with the m enopause of women. Among non-seasonal polycyclic animals, such as the cow and sow, the recurring cyclic activity is intermpted by pregnancy, lactation and pathological conditio ns. In those species that are seasonally polycycl ic, the mare, ewe, doe or nanny goat and cat, or monocyclic, such as the bitch, there are periods of sexual quiescence or a noestms.

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When the fe male reaches puberty the genital ova spot trichrom increase in size. Lasley The changes that occur at puberty depend directly upon the activity of the ovaries, which have two functions: the production of the female gametes and ova spot trichrom synthesis of hormones. Let us consider the changes that occur in the ovary of the young heifer calf.

At birth, each ovary may contain up to primary or primordial follicles; each consists of an oocyte surrounded by a single layer of epithelial cells but with no thecal cells. Soon after birth, the ovaries start to develop and produce growing follicles, which consist of an oocyte with two or more layers of granulosa cells and a basement membrane.

The stimulus for the development of these follicles is intraovarian and, until the heifer reaches the age of puberty, they will develop only to the stage where they have a theca interna and then start to undergo atresia. Further development of these follicles to produce mature graafian or antral ova spot trichrom, of which there are about growing follicles at puberty in the heifer, is dependent upon the stimulus of gonadotrophic hormones Fig.

Despite the absence of oestrous cycles, there is follicular growth, as has been shown using transrectal ultrasonography in calves from 2 weeks of age.

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It was noted that 4 there were follicular waves in response to folliclestimulating hormone FSI-1 secretion that were similar to those of--tft'e adult, and that individual follicular development was characterized by growing, static and regressing phases Adams The sheep has been used extensively for studying many of ova spot trichrom mechanisms involved in the initiation of puberty; however, it must be stressed that seasonality will exert an overriding influence in this species see below.

The onset of puberty is signalled by either the occurrence of ova spot trichrom first oestrus or the first ovulation; in the ewe lamb these do not occur simultaneously because the first ovulation is not preceded by behavioural oestrus. A similar response is seen in sexually mature ewes at the onset of the normal breeding season.

The hormone that is primarily responsible for the onset of ovarian activity, and hence puberty, is luteinizing hormone LI In adult ewes during the normal breeding season, basal Ll-1 concentrations increase together with the LH pulse frequency to one per hour during the period of maximum follicular growth. This results in the development of follicles to the preovulatory stage, and their secretion of oestradiol, which activates the LI-1 surge causing ovulation and corpus luteum formation.

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In the prepubertal ewe Iamb, LI-1 pulses occur at similar amplitudes but much lower frequencies one every hours. As a consequence, follicular growth is insufficient to activate the LI-I surge necessary for final follicular maturation and ovulation.

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Experimental evidence in prepubertal ewe lambs has shown that ovarian follicles are capable of responding to exogenous gonadotrophin stimulation and the pituitary is capable of secreting Ll-1 at a frequency to stimulate ovulation. The failure of the prepubertal ewe lamb to undergo ovulation and exhibit oestrus is due to the high threshold for the positive-feedback effect of oestradiol, and thus there is no LJ-1 surge. At puberty, the threshold is lowered, thus allowing the pituitary to respond.

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This is sometimes referred to as the 'gonadostat' theory. Other factors are also involved. The frequency of LJ-1 secretion is dependent upon gonadotrophinreleasing hormone CnRH from the hypothalamus, which is controlled by an area in the hypothalamus referred to as the neural GnRI-I pulse generator. Age-related changes in brain morphology and neuronal cytoarchitecture may also ova spot trichrom important, since extrapolation from studies performed in rats, for example, has shown an increase in the number Chapter I 1 I Endogenous and exogenous control of ovarian cyclicity ofGnRH cells with spine-like processes on the soma and dendrites.

The reason for the 'silent' first oestrus of the pubertal animal is believed to be because the central nervous system requires to be primed with progesterone before it will respond and the animal will show be havioural signs of heat. The first ovulatoJy cycle has been shown to be short in pubertal heifers 7. One explanation for this is that the domi nant follicle, from which the first ovulation arises, has a lready entered the static phase of growth. The su bsequent interovulatory interval is normal Adam s As will be discussed later in this chapter, o nce puberty has occurred, in most domestic species there are waves of follicul ar growth and regression.

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External ova spot trichrom influencing the time of onset of puberty The time of o nset of puberty is determined by the individual's genotype, with smaller breeds of animal tending to be slightly more precocious. However, this inherent timing is influenced b y a number o f external facto rs. Nutrition There is good evide nce that in most domestic species, the onset of puberty is closely linked to the attainment of a critical body weight as well as a minimum percentage body fat.

Animals that are well fed with good growth ra tes reach puberty before those that are poorly fed with slow growth rates. Hmvever, unl ess the a n imal is severely malnourished, cyclical activity will eventually occur. The protei n lepti n see belowsecreted by white fat cells in adipose tissue, may well provide the link between metabolic status and the n euroendocrine axis.

Season of the year In those species that are seasonal breeders, such as the ewe, mare and queen cat, the age at which puberty occurs will be influenced by the effect of season of the year. For instance, a filly born ea rly in the year, i. January or February, m ay have her first oestrus in the May or June of the following year, i.

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A filly foal born late in the year, July o r August, may not have h er first oestrus until she is 21 or 22 months old. The same is true of ewes, which, depending u pon the time of year at which they are born, may reach puberty as early as 6 months or as late as 18 months o ld.

Proximity of the male Studies in sheep and pigs have shown that exposure to the male of the species will advance the timing of the o nset of puberty. This so-called 'ram or boar effect' is probably mediated by phero mo nal and other sensory cues influencing hypoth ala mic GnRH secretion.

Climate An thropomorphic extrapolation has assumed that animals living in the tropics reach ova spot trichrom at an earlier age than those in temperate climates.

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Studies ca rried out in Zambia have shown that in cattle this is not true. The phase s~ding oestrus. The granulosa cells of the ovulated follicle give rise to lutein cells which are responsible for the formation of the corpus luteum.

There is a reduction in the amount of secretion from the uterine, cervical and vaginal glands. I The oestrous cycle and its phases Traditionally, the oestrous cycle is divided into a number of phases.

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Pro-oestrus The phase immediately preceding oestrus. It is characterized by a marked increase in activity of the reproductive system.

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There is follicular growth and regression of the corpus luteum of the previous cycle in polycyclic species.