Heat Stress & Heat Stress & Animal Animal ReproductionReproduction

Dr. Bhaskar GangulyDoctoral FellowVeterinary Physiology & BiochemistryCollege of Veterinary & Animal SciencesPantnagar - 263145 (INDIA). E-m

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• Endotherms maintain high core body temperatures• The ‘set-point’ varies diurnally and/ or seasonally• Low body temperatures are better tolerated

HIBERNATING MAMMALS MAY MAINTAIN CORE BODY TEMPERATURES AS LOW AS 6 - 10 °C!

• High temperatures disrupt membranes & proteins, &cause loss of fluids & electrolytes

• Homoeothermy is a priority over several otherphysiological functions

Mechanisms by which Heat Stress affects Reproduction

• Re-distribution of blood flow from core to periphery• Reduced feed intake

• Effects on cellular functions viz. oxidative stress, membrane integrity, etc.

Heat Stress & Male Heat Stress & Male ReproductionReproduction

a.a. SpermatogenesisSpermatogenesis• Most mammalian testes & epididymis are suspended

outside the abdomen in scrotal sacs• Tunica dartos regulates surface area, Cremaster regulates

distance from abdomen, Pampiniform plexus allowscounter-current heat exchange

• Heat stress: Environmental heat, Fever, Cryptorchidism• Susceptibility: spermatocyte > spermatid > B-

spermatogonia• Initially concentrated semen is produced which becomes

dilute with increasing heat stress• Thermal damage is associated with oxidative stress,

resulting in apoptosis & DNA breakage• Effects of cryptorchidism are enhanced in Superoxide

dismutase knockout mice

Spermatogenesis…Spermatogenesis…• Fertilization by spermatozoon exposed to heat stress results inpoor developmental competence of embryos; the conceptusesare small & have lower placental weights• Semen characteristics are not immediately affected becausedamaged spermatogenic cells do not enter ejaculates for sometime after heat stress• In the bull, where spermatogenesis takes 61 days, alterations insemen occur about two weeks after heat stress & persist up toeight weeks• Ejaculated spermatozoa do not show apoptosis or loss offertilizing capacity when cultured at temperatures characteristicof heat stress

• However, pre- & post-implantation survival of resultingembryos is compromised• X & Y spermatozoa are affected differentially by elevatedtemperature; ratio is skewed towards female embryoswhen males experiencing scrotal heat treatment are usedformating

b.b. Hormone secretionHormone secretion

• Pulsed heat stress causes decline in circulatingconcentrations of testosterone lasting for about twoweeks

• Concentrations are restored in the face of continuedheat stress

• Severe heat stress may compromise LH secretion inmales as well

Heat Stress & Female Heat Stress & Female ReproductionReproduction

• The lactating dairy cow & laying hen are particularlysensitive to heat stress because of the metabolicdemands of lactation & oviposition, respectively

• The magnitude of the summer decline in fertility ismuch less for non-lactating heifers or cows producinglow amounts of milk than for highmilk yielders

Heat Stress & Female ReproductionHeat Stress & Female Reproduction

a.a. The oocyteThe oocyte• Heat stress compromises the oocyte & the follicle in

which it is encased• High air temperatures 10 days before estrus were

associated with low fertility• Steroid production by cultured granulosa & theca cells

was low when cells were obtained from cows exposedto heat stress 20–26 days previously• Follicular changes due to heat stress are near-

permanent; the resumption of fertility seen inlactating dairy cows in the autumn could behastened by removing follicles formed in thesummer

The oocyte…The oocyte…• Alterations in follicular function involve those of

follicular growth, steroid secretion & gene expression• In goats, heat stress reduces plasma concentrations of

estradiol & lowers follicular estradiol concentration,aromatase activity & LH receptor level, & delaysovulation

• In rats, heat stress reduces the levels of gonadotropinreceptors & aromatase activity of granulosa cells & thefollicular fluid concentrations of estradiol• Heat stress can reduce LH secretion

• Number of small & medium follicles increases dueto a decrease in circulating concentrations ofinhibin & increased FSH secretion

The oocyte…The oocyte…• Heat stress coincident with ovulation & oocyte

maturation may not have an effect on their capacity tobe fertilized but the resultant embryos develop slowlyor abnormally

• Damage to the oocyte during the pre-ovulatory periodby heat shock seems to involve the generation ofreactive oxygen species, as the effects are reduced byadministration of antioxidants• Approx. 15–30% of oocytes exposed to elevated

temperature undergo apoptosis & inhibitors ofapoptosis can reduce the effect of elevated culturetemperature on oocyte competence for fertilization& subsequent development

b.b. Embryonic DevelopmentEmbryonic Development

• The pre-implantation embryo is susceptible;susceptibility declines as development proceeds

• Exposure of lactating cows to Heat stress at day 1 afterestrus, when embryos are one to two cells, reduces theproportion of embryos that develop to the blastocyststage at day 8 after estrus

• Heat stress at days 3 (8–16 cells), 5 (morula) and 7(blastocysts) has no effect on the proportion ofembryos that are blastocysts at day 8

Embryonic Development …Embryonic Development …

• The adverse effect of heat shock on development ofpre-implantation bovine embryos is less for breeds ofcattle that evolved in hot climates (Brahman,Romosinuano, Nelore) than for breeds that evolved incooler climates (Angus, Holstein)

• The fertility of Holstein cows inseminated during heatstress is greater if semen is from bulls of the Gir breed,than if semen is from Holstein bulls

Embryonic Development …Embryonic Development …

• Actions of elevated temperature on the pre-implantation embryo probably involve increasedproduction of reactive oxygen species

• In the mouse, maternal heat stress results in increasedreactive oxygen species activity in oviducts andembryos, and reduced glutathione content in recoveredembryos

• Treatment of female mice with either melatonin orvitamin E reduces the effects of heat stress onembryonic development• Female embryos are better able to survive than

male embryos due to reduced reactive oxygenspecies production in females

Embryonic Development …Embryonic Development …• Generation of reactive oxygen species in response to heatshock declines as bovine embryos advance in developmentwhile intracellular concentrations of the cytoplasmicantioxidant glutathione increase• Induced thermo-tolerance response develops (day 4 incattle; 8 cell stage in mice): exposure to a mild elevation intemperature makes cells more resistant to a subsequentsevere elevation in temperature• Induced thermo-tolerance involves synthesis of heat shockprotein 70 (HSP70); glutathione is required in mice• Apoptosis inhibitors increase the magnitude of thereduction in development caused by elevatedtemperatures; apoptosis of the damaged cells of theembryo allow the embryo to develop normally

• Heat stress may reduce circulating concentrations ofprogesterone

c.c. Fetal DevelopmentFetal Development

• Heat stress during gestation causes reduced fetalgrowth

• Exposure of pregnant ewes to heat stress causesreduced fetal and placental weights andconcentrations of placental hormones in the blood

• Effects on growth are greater when occurring duringmid gestation than when occurring during latergestation

Fetal Development …Fetal Development …• Reduced fetal weights are not caused due to reduced

perfusion to the placenta; placental blood flow per gram offetus is similar between heat-stressed and control ewes

• Rather, the cause is an increase in placental vascularresistance due to altered angiogenesis as reflected byaberrant expression of genes such as vascular endothelialgrowth factor and its receptors and placental growth factor

• Heat stress probably has more effects during mid-gestation than late gestation because angiogenesis is more extensive in the former period• Glucose transport across the placenta is also reduced by

maternal heat stress due to reduced expression of GLUT8in cotyledonary placenta

Fetal Development …Fetal Development …• Reduced secretion of placental hormones as a result of

heat stress can cause reduced milk yield: inadequatenutrition for the neonate

• Maternal hyperthermia can also increase the incidenceof teratologies

• Fetal stress caused by hyperthermia results in changesin physiological functions in adulthood

• In guinea pigs, heat stress in utero reduces learningactivity in adulthood

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