THE REPRODUCTIVE TRACT
UTERUS
The uterus is a muscle. It is the "Y" shaped female reproductive organ that includes the two UTERINE HORNS, with connecting FOLLOPIAN TUBES, and the UTERINE BODY, separated from the vagina by the cervix. The uterine wall has three layers;
1. The outer layer (Serous layer); which continues on to form the BROAD LIGAMENT.
2. The middle layer (Myometrium); which is the muscular layer.
3. The inner layer (Endometrium); a mucus membrane that contains glands.
  UTERINE HORNS: The uterine horns are approximately 25 cm in length.
  UTERINE BODY: The uterine body is approximately 20 cm in length, and 10 cm in diameter.
BROAD LIGAMENT
The broad ligament suspends the OVARIES, uterine horns and uterine body, from the abdominal wall. It also encases the ovarian artery, the uterine artery and vaginal artery, and nerves.
FALLOPIAN TUBES
The fallopian tube is between each the ovary and the uterine horn.  It is where the OVUM (egg) awaits fertilisation, and once fertilised the fallopian tube will transport the OVUM to the uterus. The fallopian tube is highly twisted, measuring approximately 25 cm in length. Its width varies from 5 - 6 mm at the ovarian end, and 2 - 3 mm at the uterine end.
OVARY
The ovaries are a pair of female glands of a fibrous nature, from which FOLLICLES develop and rupture shedding the OVUM to the fallopian tube (OVULATION). Typically the ovaries are bean shaped, varying in size and shape depending on the age of the female, and the stage of the estrous cycle.
FOLLICLE
Small fluid filled sac or gland, which contains and liberates the OVA. The female is born with her life supply of follicles. There are thousands of them, but only a few reach maturation and ovulate. Usually in a cycle only one, sometimes two follicles in the same ovary OVULATE, with the rest degenerating or regressing. Follicle growth and development is totally dependant on the cyclic status of the mare. Follicle growth is instigated by the stimulation of FSH. The release of the OVUM is instigated by LH.
OVUM
Plural term is OVA. The ovum is the female egg, gamete or sex cell. It is one of the largest cells in the body.
CERVIX
The cervix is the muscular neck of the uterus. It forms the "barrier" between the uterus and the vagina. The cervix is 10 - 20cm" long, and much like a sphincter muscle. It projects into the vaginal cavity. When a mare is in season the cervix will relax, and when she is between cycles or pregnant it will be tight and constricted it is blocked by a mucus plug.
VAGINA
The vagina is the area extending from the cervix to the VULVA. Its walls are made up of muscle and mucus. The vagina is about 20 cm long, and although about 8 cm wide, its walls are touching. The vagina may include the VESTIBULE, which is just inside the vulva lips, extending 2 - 3 cm's.
VESTIBULAR MUSCLE
The vestibule muscle or constrictor muscle surrounds the vestibule.
URETHRA
The urethra is the duct that conveys the urine from the bladder to outside.
VULVA
The vulva is the external opening of the genital tract. It consists of two muscular lips (Labiae) which protect the clitoris.
THE PERINEUM
Or the perineal region. The area between the anus and the vagina


DIAGRAM OF THE REPRODUCTIVE TRACT



THE REPRODUCTIVE CYCLE & HORMONES
INTERPRETING THE TERMS – THE REPRODUCTIVE CYCLE
ANESTROUS
Anestrous is when a mare is not sexually active. She is not cycling. This inactivity is usually present in dry mares during the winter months.
TRANSITIONAL HEAT
Transitional heat is the period between anoestrous and the first ovulation of the season. This usually coincides with the onset of spring. The transitional heat varies in length from mare to mare, and season to season.
ESTROUS CYCLE
The estrous cycle refers to the sexual cycle of the mare.
ESTROUS
Estrus, or in oestrus is when the mare is in season, i.e. she will accept a stallion. The mare’s reproductive tract and behaviour will alter to accommodate the stallion, and allow fertilisation.  During this period a follicle(s) will mature and ovulate. Estrus finishes when the ovum is shed, however the mare may still "show", or accept a stallion for a short period after ovulation. Typically this is a period of 5 days, but is variable.
DIESTROUS
Diestrous is the period between heats. She will not accept a stallion, and her uterus prepares for the fertilised egg. If she is not pregnant, the CL will cease to function and she will come back into season. Typically this period extends to 15 days, but is variable.
EMBRYO
The embryo is the conceptus, or fertilized egg. Terminology ends when embryo is 40 days old post ovulation.
FOETUS
The foetus is the unborn foal. Referral as "foetus" begins at 41 days from ovulation.


INTERPRETING THE REPRODUCTIVE HORMONES
GONADOTROPHIN RELEASING HORMONE (GnRH)
GnRH is a hormone produced in the Hypothalamus (part of the brain) causing the production of LH (Luteinizing hormone) and FSH (follicle stimulating hormone) by the pituitary gland.
PITURITY GLAND
The pituitary gland is situated at the base of the brain. There are two lobes of the pituitary gland – posterior (rear) and anterior (front). The posterior area will among other hormones produces oxytocin, and the anterior produces LH and FSH.
LUTEINISING HORMONE (LH)
Luteinizing hormone is produced by a lobe at the front of the pituitary gland (anterior of  peturity gland). It will target the ovaries to cause the ovulation of a ripe follicle. In colts and stallions it will target the testes, stimulating testosterone production.
FOLLICLE STIMULATING HORMONE (FSH)
FSH is produced by the anterior lobe of the pituitary gland. It targets the ovaries causing the development of follicles. In the male it targets the testis to produce spermatozoa's production.
OVULATION (TO OVULATE)
Ovulation is the rupturing of the follicle, and shedding of the ovum into the fallopian tube. A dominating, mature follicle will rupture. This will allow the egg to escape through the OVULATION FOSSA. (A weak point in the capsule surrounding the ovary.)
CORPUS LUTEUM (CL OR YELLOW BODY)
The CL is formed in the ovary as a result of follicle rupture and the shedding of the ovum. Luteal cells (from LUTEINISING HORMONE - LH) from the blood will fill the ruptured follicle, form a clot and organise themselves to form the CL. It will secrete progesterone that will cause Diestrous and prepare the uterus for pregnancy.
PROGESTERONE
 The CL secretes progesterone in the non pregnant mare, targeting the uterus to prepare for the fertilised egg. Should the mare conceive the CL will continue to produce progesterone for about 120 days. Up to this period it is present in the bloodstream. After this, the placenta takes over progesterone production.         
PROSTAGLANDIN
Prostaglandin is produced by the uterus if the mare is not pregnant 12 - 14 after Estrus. It is directly responsible for ceasing CL function and existence and instigating the re-commencement of Estrus. Embryo movement is thought to inhibit prostaglandin production; hence the CL will continue to function.
ESTROGEN
Estrogens are a group of hormones produced by the lining in the follicle. In the sexual cycle they are responsible for the behavioural and genital tract changes in the mare, to display heat High levels of estrogens in the blood will trigger LH production.


DEPICTING THE CYCLE - SOURCE AND ACTION OF HORMONES IN THE CYCLING MARE




PUTTING TOGETHER THE HORMONE ACTIVITY, PHYSICAL AND BEHAVIOURAL EVENTS
I will call the first day of the cycle when the corpus luteum has formed. The Corpus Luteums function is to produce progesterone, targeting the uterus, resulting in the mare going out of season in both a behavioural manner and physical changes to the uterine environment. Most mares cease displaying estrous (heat) symptoms 12 - 36 hours after the ovulation takes place. Her uterus will start preparing for a possible pregnancy. It will become turgid. Mucus secretion will cease, and there will be little blood flow to the uterine lining. Estrogen levels will drop and Luteinizing Hormone production will also be reduced. She is now in true Diestrous.

Should the mare is not in foal or not been exposed to a stallion then 12 - 14 days after ovulation the uterus will commence producing prostaglandin. (Prostaglandin production would not take place if the mare was in foal, as the movement of the embryo would inhibit prostaglandin production.) The prostaglandin will target the CL, causing it to cease functioning, in turn reducing progesterone production.
 
With the lowered levels of progesterone, the pituitary gland is stimulated to produce Follicle Stimulating Hormone. The FSH targets the ovary, causing the follicles to develop. From the maturing follicle lining, estrogens is secreted, changing the mares' behaviour so that she would "take" a stallion, and altering her genital tract to prepare for service and conception. As the mare is coming back in season, the uterine walls will become flaccid, the uterine glands will secrete mucus, and the uterine surface will become inundated with blood. The cervix will also relax.

Estrogen levels will peak, coinciding with maturing follicles, triggering the pituitary to release Luteinizing Hormone. The LH will cause the mature, ripe follicle/s to rupture and the ovum is released into the fallopian tube. There is a weak point in the capsule surrounding the ovary. This is referred to as the Ovulation fossa (the indented part of the bean shaped ovary). This is the point where the ovum will escape through, and enter the fallopian tube. Ideally the service would take place 12 - 24 hours prior to ovulation, so that the semen is in the fallopian tube awaiting the ovum. At the rupture site, blood will fill the vacant space. The luteal cells will organise themselves and form a clot, creating a gland - the Corpus Luteum.  The CL will start producing progesterone, and the whole cycle will re-establish.