Equine Reproduction

 

LESSON TWO

 

 

Reproductive Anatomy and Physiology of the Mare

 

http://www.pugsly.com/A-EQ-09.jpg

 

Anatomical Structure                          Function

Ovary  (paired)

Follicle growth (secretion of estrogen)

Corpus Luteum (CL) Secretion of progesterone

Ovary produces ovum (egg)

 

Oviduct (paired), also called fallopian  tubes or uterine tubules

Where fertilization occurs in the mare. Connects ovary with the uterus.

 

Uterus (consists of two horns and a body)

Secretion of hormones. Site of embryonic and fetal growth and maturization.

 

Cervix

Connects uterus to vagina. Closed during pregnancy, diestrus and anestrous. Open during estrus to allow for sperm introduction and parturition to allow for foal delivery.

 

Vagina

Connects external genitalia with internal genitalia.

Accepts penis during copulation.

 

Vulva {consists of vulvar lips (labia) and clitoris}

Entrance to the vaginal tract.

 

 

http://www.wisc.edu/ansci_repro/101equinelab/reproduction/mare_tract.jpg

 

          Understanding the anatomy and physiology of the mare reproductive system is the key to the operation of a successful Equine breeding operation. It is not only mandatory for the Veterinarian, but the breeding farm manager as well, to be well versed in the details and peculiarities of the mare’s estrous cycle.

 

 

          Reading assignment: Chapter 1 and 2 in required text (Manual of Equine Reproduction).

 

Understanding and being able to visualize the anatomy of the mare’s reproductive system is mandatory for a successful breeding farm operation.

 

A veterinarian using a technique called rectal palpation often examines mares at a breeding farm daily or at least every other day.

 

This procedure is performed in the following manner:

With the mare in a chute or restrained in a way to prevent excessive movement, the veterinarian will take a plastic sleeve covered with a lubricating material (K-Y jelly) and enter the rectum of the mare with this sleeved hand and arm.  Manure is removed until there is no fecal matter interfering with the veterinarian’s ability to actually feel through the wall of the rectum and grasp the left or right ovary.  The veterinarian can actually determine follicular structures, their size and consistency, during this process.  He/she will continue to examine the mare’s entire reproductive tract in the same manner and make notes as to what was found.

 

An ultrasound machine can be used in the same manner by taking the ultrasound probe into the palpating hand of the examiner. As you can see form the diagram, the rectum is located above the reproductive tract, making the palpation a fairly simple procedure. If you wish to participate in the equine breeding industry, knowledge of these intricate examinations should be able to be understood and this can be done by the study of the anatomy and physiology of the mare’s reproductive tract.        

It will be up to you to have a working knowledge of the anatomy of the mare’s reproductive tract seen in the diagrams above and the pictures found in the text.

 

http://www.animalherbcompany.com/images/mareexam.jpg

Palpating a mare

 

In reading of Chapter 1 and 2 you will learn the major hormones that control the mare’s estrous cycle.  The key features to consider in these chapters will be detailed below. Use this as a guide for study and review purposes.

 

 

The Mare's Estrous Cycle

Terminology

"Estrous refers to the entire reproductive cycle of the mare (usually considered to be 21 days).


"Estrus" refers to the "heat" stage of that cycle when the mare is receptive to the stallion, shows interest in being bred by the stallion. It is considered to be 5 days of the 21-day estrous cycle on average.


"Diestrus" refers to the period in the 21-day estrous cycle when the mare is not receptive to the stallion, an average of 16 days, normally.


"Anestrus” refers to the compete absence of estrus; winter anestrous refers to the short daylight days of the winter months when the mare will often not go through any estrous cycles.


         
The mare is “seasonally polyestrus”, meaning that she undergoes regular estrous cycles during a portion of the year (late spring, summer and early fall), and usually during the winter months she is in winter anestrous.  This is nature's way of preventing the arrival of a foal during bad weather.

These cycles are controlled by the mare's endocrine system (hormones produced by endocrine glands that control the cycle), which in turn respond to an increase or decrease in daylight duration with the onset of spring or fall, which affects the pineal gland. This study of hormonal activity is known as endocrinology.

It is important to understand there is a closely linked feedback system between many of the reproductive hormones present in the mare which will alter the level or presence of some hormones as levels of other different hormones increase or decrease. This means that artificially altering a single hormone will be likely to have an effect on one or more of the other hormones. The same can be said of natural hormonal changes - whether they are happening in a correct manner or not. Many of these hormonal changes do occur naturally, but when something becomes unbalanced either naturally or artificially, we can see estrous cycle problems develop in the mare.

http://www.ava.com.au/content/private/ava_upload/theriogenology/images/Teasing.jpg

A mare “breaking down” during the estrus (heat)

Hormonal feedback in the mare

Hormones Active during the Estrous Cycle of the Mare

 

o   FSH levels are found to increase just before the beginning of estrus and continue in an elevated level as the primary follicle (ovulatory follicle) gets closer to maturation.  As this primary follicle grows on the ovary it produces estrogen which is responsible for the behavioral characteristics shown by the mare in heat (heat period or estrus).  High levels of estrogen feedback on the pituitary to cause the greater secretion of LH and a slowing down of the production of FSH. In this way the cycle repeats itself unless pregnancy is encountered.

 

 

 

 

A Brief Sequential Overview of the Regular Estrous Cycle

The Hormones of the Mares Estrous Cycle

 

          The following will help you understand the use of hormones in a breeding farm operation.

Hormones Associated with Reproduction

 

Hormone

Primary Production site

Target organ

Effect

Therapeutic use

Estrogen - Heat hormone

Ovary (ovarian follicle)

Uterus, cervix, and external genetalia of mare

Expression of estrus. Female sex characteristics, genital tract changes

Enhances estrus (heat) but may interfere with normal cycling.

Progesterone -Pregnancy Hormone

Corpus luteum of ovary (CL) also the placenta during pregnancy

Uterus

Mammary gland

Maintains pregnancy

Prevent abortion. Synchronize or suppress estrus (regumate)

 

Prostaglandin

Uterus

Corpus Luteum (CL)

Luteolysis.(CL regression)

Synchronize estrus. Treat persistent CL.

Gonadotropin releasing hormone (GnRH)

Hypothalamus

Pituitary gland (master gland)

LH and FSH release

 

Follicle Stimulating Hormone(FSH)

Pituitary gland

 Ovary-mares

Seminiferous tubules-male

Growth of follicle-mare spermatogenesis-male

Same as effect

Luteinizing Hormone (LH)

Pituitary gland

Mature follicle–mare

Leydig cells –male

Follicle maturation and ovulation

Stimulates testosterone production

Same as effect. (gonadotropin(HCG)

Oxytocin

Pituitary gland

Uterus , mammary gland

Uterine contraction, milk let down

Retained placenta elimination, induce abortion or foaling

Prolactin

Pituitary gland

Mammary gland

Milk production

 

Relaxin

Ovary, placenta

Pelvic tissues

Relaxation of pelvic area for foaling

 

 

 

 

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