Reproductive Physiology

Reproductive PhysiologyWhat in the World is Going on in

There!

Located at the base of the forebrain Receives and interprets both sensory

and hormonal cues Secretes gonadotropin-releasing

hormone (GnRH) into the portal vascular system of the median eminence.

Two modes of GnRH release; pulsatile and surge

Hypothalamus

Gonadotropin-releasing Hormone (GnRH)Actions Pulsatile release stimulates similar pulses

of pituitary hormones which supports the growth and endocrine function of both follicles and the corpus luteum.

A surge of GnRH stimulates a surge release of pituitary hormones that induces rupture of a follicle (ovulation) and subsequent formation of a CL.

Endocrinology

GnRH

Response organ to the hypothalamus and ovaries

Secretes luteinizing hormone (LH) and follicle stimulating hormone (FSH) in response to GnRH from the hypothalamus

Pituitary

Luteinizing Hormone (LH) [yellowing hormone]Origin Gonadotrophic cells of the anterior pituitary GnRH release from the hypothalamus stimulates

release of LHActions Two modes of release for LH; pulsatile and surge. Pulses of LH stimulates follicle and CL growth and

endocrine function A surge of LH stimulates follicle rupture and

release of the oocyte (ovulation)

Endocrinology

Luteinizing Hormone (LH)Ho

rmon

e Co

ncen

tr atio

n

Days Relative to EstrusEstrus

Estrus

5 10 15

GnRH Pituitary LHOvulation

Ovulation

Follicle Stimulating Hormone (FSH)Origin Gonadotrophic cells of the anterior pituitary GnRH release from the hypothalamus

stimulates production of FSH. Release controlled by hormones from follicle.

Action FSH is released in small surges that

precede the onset of follicle growth.

Endocrinology

Follicle Stimulating Hormone (FSH)

Horm

one

Conc

entr a

tion

Days Relative to EstrusEstrus

Estrus

5 10 15

Ovulation

Ovulation

GnRH

LH and FSH

+

Follicles Balloon-like structures Contain the oocyte (egg) Produces estrogen

Corpus Luteum (CL) Solid structure Produces progesterone

Ovarian Structures

0 1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

21

Day of the Estrous Cycle

Hor

mon

e Le

vels

Estrus

Estrus

The Ovarian Cycle – Growth of the CL

Waves of Follicular Development

Horm

one

Conc

entr a

tion

Days Relative to EstrusEstrus

Estrus

5 10 15

1st

Wave

2ndWave

Ovulation

Ovulation

Estrogen (estrus generating hormone)Origin Produced by growing follicles on the ovary As follicle increases in size, its production

of estrogen increasesActions Stimulates behavioral estrus Stimulates LH surge which results in follicle

rupture and release of the oocyte (ovulation)

Endocrinology

Pattern of EstrogenHo

rmon

e Co

ncen

tr atio

n

Days Relative to EstrusEstrus

Estrus

5 10 15

Follicles Estrogen

Ovulation

Ovulation

Follicle Growth and Estrogen Concentrations

Horm

one

Conc

entr a

tion

Days Relative to EstrusEstrus

Estrus

5 10 15

1st

Wave

2ndWaveOvulatio

n

Estrogen

Progesterone (Pro gestational hormone)Origin Produced by the corpus luteum (CL) As CL size increases its production of

progesterone increasesActions Controls LH pulse frequency; 4-6 per day Inhibits the final development of

dominant follicles and thus the expression of estrus

Vital for maintenance of pregnancy

Endocrinology

Production of Progesterone by the CL

Horm

one

Conc

entr a

tion

Days Relative to EstrusEstrus

Estrus

5 10 15

CL Progesterone

Ovulation

Ovulation

The Ovarian CycleHo

rmon

e Co

ncen

tr atio

n

Days Relative to EstrusEstrus

Estrus

5 10 15

1st

Wave

2ndWaveOvulatio

n

Estrogen

GnRH

LH and FSH

+Estrogen +

Progesterone

-

Steroid Effects on LH Secretion

++

Horm

one

Conc

entr a

tion

Days Relative to EstrusEstrus

Estrus

5 10

15

Estrogen

Progesterone

GnRH Pituitary LH

Increased

Decreased

Ovulation

Ovulation

Prostaglandin F2α

Origin Produced by the uterus; released in

pulses Action Stimulates the regression of the CL

resulting in the expression of estrus and/or ovulation.

Endocrinology

Prostaglandin F2Ho

rmon

e Co

ncen

tr atio

n

Days Relative to EstrusEstrus

Estrus

5 10 15

Progesterone

Uterus Prostaglandin

Ovulation

Ovulation

GnRH

LH and FSH

+

Estrogen +

Progesterone

- PGF -

The Estrous Cycle in CattleHo

rmon

e Co

ncen

tr atio

n

Days Relative to EstrusEstrus

Estrus

5 10

15

1st

Wave

2ndWaveProgesterone

EstrogenFSH

LH

Expression of estrus; behavior Breakdown of the hypothalamic-

gonadotropic axis. Typically results in altered LH secretion, premature luteinization of follicular cells, premature resumption of meiosis and the oocyte is compromised.

2 vs 3 waves of follicle growth Luteal size and progesterone

production

Critical Control Points for Fertility

ANESTRUS! Length of the

postpartum or prepubertal anestrus period dramatically impacts the ability of a female to conceive in a controlled breeding season.

Critical Control Points for Fertility

JanAug Sep Oct Nov Dec DecJan Feb

Months

Impact of Anestrus

Gestating CowsNovMar Apr May Jun Jul Aug Sep

Calving

Season

Breeding

Season

HEAT STRESS

Conc Rate =

30%

Oct

Reproductive Classes Within the Cow Herd

Random stages ofthe cycle

AnestrousCows Deep

Anestrous

Shallow Anestrou

s CyclicCows

50%50%

01234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677

Postpartum Anestrus in Beef CowsFo

llicle

Di a

me t

er

Days Relative to CalvingCalving

First Estrus

Progesterone

* *

Anestrus25 to 100+ days in US Beef Cows 1st

DF

Uterine involution and pituitary replenishment

Factors Regulating the Length of Anestrus Presence of the calf

Initiates anestrus Body Condition Score (BCS) Age Days Since Calving Others effectors include dystocia,

health, and calving season

Presence of the Calf Presence of the calf initiates anestrus

Mechanism unknown but both the physical contact of the calf (nursing) and the mere presence of the calf at side are involved.

Short-term calf removal (48 h) has been shown to be an effective method to induce estrus.

Body Condition Score (BCS)

Body Condition Score is an estimate of the degree of fatness of an animal 1 = emaciated 9 = extremely obese

Estimate of available energy stores

Influence of BCS on Anestrus

8071

5949

2542

0%

20%

40%

60%

80%

100%

<3.5 4 4.5 5 5.5 >6

Body Condition Score

% C

yclin

g

3371,002726481157 188

Source: Stevenson et al., 2003

Age Young cows (2 year old suckled cows) simply

require 20-30 more days resume estrous cycles.

Recommendation has always been to calve first calf heifers 20-30 days before the mature cow herd. Stevenson et al., 2003

Age Days PP Percent Cyclic2 86 553+ 68 64

Influence of Days Since Calving on Anestrus

9

25

44 4555

6270

62

01020304050607080

<30 31-40 41-50 51-60 61-70 71-80 81-90 >90

Days Postpartum

% C

yclin

g

Source: Stevenson et al., 2003

Reproductive physiology……clear as mud!

Critical control points for fertility: Expression of estrus Disruption of the HPG axis Number of follicle waves Luteal size and progesterone production Length of the postpartum anestrous

period.

Summary

Anatomy and Physiology of the Pregnancy and the

Calving Process

Fertilization

Occurs in the oviduct at the ampulla-isthmus junction

Zygote is the term for a newly fertilized oocyte; only has 1 cell

An embryo is the term used after the zygote has begun cellular divisions.

Embryos migrate from the oviduct about 5 days after fertilization.

Embryo Development8 cell - D 4 Morula - D 6 Early Blastocyst – D 6-7

Blastocyst – D 6-7 Hatching Blastocyst – D7-8

Pregnancy Stages:

Embryo - fertilization to app. D 30 Fetus - Day 30 of pregnancy to parturition

Components of a pregnancy: Fetus/embryo Placenta

membranes responsible for the protection and nourishment of the fetus

Amnion directly surrounds the fetus; Chorio-allantoic extends throughout the uterus

Pregnancy Placenta

Membranes Amnion directly surrounds the fetus to protect

it Chorion (chorio-allantoic) extends throughout

the uterus and its primary role is nutrient transfer to the fetus.

Nutrient transfer occurs at specialized structures called placentomes. The maternal side is called a caruncle, the fetal side is called the cotyledon.

Anatomy of Pregnancy

Parturition (birthing) Stimulated by the fetus fetus cortisol

placenta Prostaglandin F

ovary CL regresses, releases oxytocin, relaxinUterus contractions begin

Pregnancy

General Repro Statistics Estrous Cycle Length = 21 days;

range 17-24 Length of estrus = 12 hours; range 6-

24 Ovulation = 10-12 hours after the

end of estrus or 30 hours after beginning of estrus; range 18-48

Gestation length = 283; range 260-300