BROOMARE AND FOAL MANAGEMENT

Robin DeRue

LESSON ONE: FEMALE REPRODUCTIVE ANATOMY

This lesson is an overview of basic female equine reproductive anatomy.

The best way to learn anatomy is by dissection or hands on experience. A working knowledge of the terminology and relative aspects of the reproductive tract in two-dimensional form (drawings) is a good point from which to begin.

Scientific terminology is very important when learning biological structure and function. This terminology will serve as the “toolbox” from which you will be able to communicate your ideas with colleagues and clients. Each lesson will have a list of terms. This lesson will begin with an extra set of terms that will help you navigate pertinent equine structures.

ANATOMICAL TERMINOLOGY denoting direction. Each term denotes a position in the body relative to another organ.

1. CRANIAL or ANTERIOR – toward the head.

2. CAUDAL or POSTERIOR – toward the rear.

3. LATERAL – toward the outside or farther from the median.

4. MEDIAL – toward the inside or closer to the median.

5. DORSAL – toward the back or upper surface.

6. VENTRAL – situated near the undersurface, or toward the belly.

7. SAGITTAL – (in direction or location) from front to back in the median plane or in a plane parallel to the median.

8. PROXIMAL – near.

9. DISTAL – far.

We can categorize the female reproductive organs (also known as “tubular genitalia”) by placement in the body as follows:

I. Abdominal Organs

II. Pelvic Organs

Click Here To See THE REPRODUCTIVE TRACT OF THE MARE – Dorsal view ^{^[i]}

I. ABDOMINAL ORGANS

The ovaries, uterine body, uterine horns and the anterior portion of the cervix.

A. LIGAMENTS – suspend the reproductive organs.

1. The BROAD LIGAMENTS

a. Each consisting of two layers of sheet-like tissue with arteries, veins and smooth muscle in between. These arteries, veins and lymph vessels within supply the genital tract.

b. The broad ligament is divided into 3 areas:

i. Mesometrium - attaches uterus

ii. Mesovarium - attaches ovaries

iii. Mesosalpinx - attaches oviduct

c. The broad ligament arises embryonically from a fold of peritoneum. The peritoneum generally is the serous sac that lines the abdominal cavity and covers the viscera within. Specifically, the parietal layer of peritoneum lines the abdominal cavity.

d. The outermost covering of the uterus then, is a continuation of the broad ligament. To refer specifically to this outer covering of the uterus, one would refer to the perimetrium.

e. The broad ligaments attach to the reproductive organs beginning dorsally from lumbar and sacral vertebra and laterally from the pelvic walls along the dorsal aspect. The broad ligaments are long, thus allowing support by the abdominal floor of the gravid uterus. [ii] There is also adipose (fatty) tissue interspersed throughout the reproductive tract.

f. The cranial edge of the broad ligament contains the ovarian artery, vein, lymph vessels and nerve, which attach to each reproductive organ on the dorsal aspect.

2. It should be mentioned that the uterus, fallopian tubes and ovaries are in direct contact with the remainder of the mare’s viscera. Thus they will intermingle with other organs which can at times change the overall shape of the reproductive tract making palpations difficult.

3. Skeletal muscles and tendons which support the abdomen are important later in pregnancy for overall support of the unborn fetus.

a. Internal and external abdominal oblique muscles are located on the upper portion of each side of the abdomen, and form a continuous sling that passes under the abdomen and passively supports the abdominal contents when relaxed, or compresses them when the muscle is tensed. [iii]

b. Rectus abdominus muscle is located ventrally and runs cranio-caudally, originating from the cartilage of the ribs and inserting on the cranial end of the pelvis. Wide tendons pass over and under the rectus abdominus.

c. Linea Alba – While not a separate structure per say, it is formed from the aponeuroses (where muscle becomes tendon) of all of the above muscles on the midline of the abdomen. The linea alba is a tendinous thickening of the midline of the abdomen that passes from the caudal end of the sternum to the cranial end of the pelvis..[iv] You will also hear the term “pre-pubic tendon.” Specifically, the caudal aspect of the linea alba combines with insertion muscles of rectus abdominus to form a broad transverse plate called the prepubic tendon (PPT). The PPT has two branches that fasten at iliopubic eminence. These muscles and tendons have been known to rupture during pregnancy so that only the ventral abdominal wall supports the entire weight of the fetus.

B. OVARIES

1. Structure

a. The ovaries are kidney bean shaped approximately 2" x 1 1/2" or 5 cm x 3.5 cm. The outer covering of the ovary is comprised of epithelial cells.

b. The caudoventral side is concave (a form like the underneath side of an arch) and is where the “ovulation fossa” is located. This is the point from which oocytes depart into the oviduct or fallopian tube by passing first through the infundibulum.

c. The dorsal side is convex (curved like the top surface of an arch) and has an attachment of mesovarium, vessels and nerves. The convex surface, therefore, is the hilus of the ovary. The term hilus is defined as the part of an organ where nerves and vessels enter and leave. [v]

d. The medial and lateral sides, cranial (tubal) pole and caudal (uterine) pole are flattened surfaces.

e. Ends are rounded: The cranial end is attached by some of the fimbrae of oviduct; the uterine pole is caudal and is attached by the ovarian ligament.

f. The medulla is a soft vascularized area inside the ovary, but is superficial to the cortex in the mare. This is reversed in other species.

g. The cortex is the center containing oocytes and follicles. The cortex joins surface at the ovulation fossa.

2. The function of the ovaries is twofold:

a. Gametogenic – producing gametes. A gamete is a reproductive cell having the haploid (half) number of chromosomes, especially a mature sperm or egg capable of fusing with a gamete of the opposite sex to produce a fertilized egg.

b. Endocrine – producing hormones.

3. Dynamic anatomy of the ovary.

a. FOLLICLES – Upon dissection, these would be the outwardly visible structures on the surface of the ovary which house the oocyte. These can be palpated (felt) rectally by a veterinarian.

i. Follicles appear as rounded fluid filled “blister” like structures.

ii. The size of follicles can be anywhere from approximately 10mm in diameter to 70mm. The “feel” of a follicle is firm to soft depending on the stage of maturity.

iii. Follicle types: The follicular maturation process.

b. PRIMORDIAL FOLLICLES: The structure of this follicle consists of an oocyte surrounded by a simple layer of squamous cells. The oocyte within this follicle has entered a prolonged meiotic or resting stage. These follicles are present at birth, built in stockpile and depleted progressively as the mare cycles, ovulates and ages. The oogonia within the primordial follicles have transformed into primary oocytes from the first stage of meiosis just before or soon after birth.

c. PRIMARY FOLLICLES: When the primordial follicles leave the reserve pool to mature, their squamous cells are converted to cuboidal cells and this structure is known as a “primary follicle.” The cuboidal cells eventually become granulosa cells, important in hormone production.

Click Here To See FOLLICULAR ACTIVIY THROUGHOUT THE ESTROUS CYCLE

d. SECONDARY FOLLICLES: As the follicle progresses towards ovulation, a chamber appears within the follicle. The oocyte is housed in this chamber or “antrum.” These secondary follicles are the “pre-antral” follicles.

e . TERTIARY FOLLICLES: This follicle has formed an antrum and the oogonium is pushed to the inner part of the follicle. It takes 4 to 5 months for a primary follicle to become a tertiary follicle. There are always all stages of follicles maturing within the ovary.

f. GRAAFIAN FOLLICLE: This is the ovulatory follicle containing the oocyte.

i. The Graafian follicle has thecal and granulosa cells which function producing hormones as well as structural support for the follicle. Thecal cells produce androgens that are converted to estrogen (E) by the granulosa cells.[vi] At ovulation the oocyte detaches, but keeps the cumulus cells known as the corona radiata. These cells help the fimbrai of the distal end of the fallopian tube pick up the embryo and they also help with the ciliary transport inside the fallopian tube. Oocytes have an outer covering that is non-cellular. This covering or capsule is called the zona pellucida and is located between the oocyte and cells known as cumulus or corona radiata.

Click Here To See CROSS SECTION OF A GRAAFIAN FOLLICLE

g. CORPUS HEMHORRAGICUM: When ovulation occurs the oocyte departs the ovary and the follicle fills with blood, becoming a corpus hemhorragicum or CH. The follicle fills with blood in the first 24 hours. Granulosa cells, or luteal tissue, increases. Granulosa cells produce progesterone (P) as follows: > 1 ng/ml of serum by 2 to 3 days post ovulation. Peak levels of progesterone (P) are reached by 6 days post ovulation.

h CORPUS LUTEUM, CL or “yellow body”: This is the next progressive stage of the ovulated follicle. The blood in the CH is absorbed and the granulosa cells predominate, producing progesterone (P). At this time, progesterone (P) should be > or equal to 4 ng/ml of serum. This structure is now an endocrine producing body. The CL will begin as a large structure on the ovary and then progressively shrinks with age.

i. CORPUS ALBICANS, CA: This is an old regressing CL. Characteristically, these are small and white in color.

C. FALLOPIAN TUBES (FT) OR OVIDUCTS:

These are a tourtuous small diameter tube located in mesosalpinx. Each FT extends from the ovary to the tip of the uterine horn. The FT is not connected directly to the ovary, but is connected to the tip of the uterine horn as one continuous tube. The FT is divided into the following parts:

1. Infundibulum – A funnel shaped “catchers mitt” ventral to the ovary, specifically ventral to the ovulation fossa. The infundibulum has irregular edges called fimbriae or “fingers”. The cranial portion of the fimbriae are attached to the ovary. Upon ovulation, the infundibulum “catches” the oocyte exiting the ovulation fossa. Inside the infundibulum are ciliated cells to help move the oocyte into the FT. The opening of the FT is at the center of the infundibulum.

2. Ampulla – The expanded portion of the FT is (like an inverted funnel) between the infundibulum and the isthmus. The inside of the ampulla has mucosal folds and a thin layer of smooth muscle. The mucous produced is thought to nourish and help move the fertilized ovum into the isthmus and thus into the uterus. The ampulla is thought to be the place where fertilization occurs. In that regard, the thickness of the muscularis increases from the ovarian end to the uterine end of the FT. Conversely, mucosal tissue decreases from the ovarian end to the uterine end. The thinner musculature in the ampulla (as compared to the uterine end of the oviduct) suggests a more quiescent area for fertilization. [vii] Most unfertilized ovum are thought to be resorbed here because few are collected from the uterus.

3. Isthmus – The narrowed portion of tubule connecting ampulla and uterine horn. The connection of the isthmus and the uterine horn is called the utero-tubal junction or UTJ. The isthmus has a well-developed layer of smooth muscle.

4. The walls of oviducts are complex and consist of three layers:

a. The mucosal layer – Is inner most with ciliated cells that are more numerous at the ovarian than uterine end.

b. The muscularis layer – On the outside is longitudinal and on the inside is circular smooth muscle.

c. The serosal layer is on the outside and consists of connective tissue and the broad ligaments.

D. UTERUS:

The uterus is “Y” shaped and serves as the organ that will attach, nourish and house the developing fetus during pregnancy. It consists of two uterine horns at the cranial end and one uterine body at the caudal end.

1. Histology - The inner lining of both of the horns and body consist of prominent folds arranged longitudinally. The lumen of the non-gravid (non-pregnant) uterus is not readily apparent as the folds are in close apposition. There are glandular and cellular changes during the reproductive cycle.

a. The endometrium. This is the innermost layer of tissue (next to the lumen). The high columnar cells are secretory and can have cilia. Notice on the diagram the branched nature of the glands in the wall of the endometrium. These glands are deep and complex.

b. The myometrium is the muscular layer. The inner part of the layer is thick smooth muscle in a circular pattern, and the thin outer layer is again smooth muscle but in a thinner longitudinal pattern. There is a vascular layer in between the two muscular layers. Contractile action of this musculature serves to move sperm through the UTJ into the oviducts. Note: The spermatozoa do not swim through the uterus into the fallopian tubes!! It is muscular contractions that move the spermatozoa. This musculature also helps to clear the uterus of any unwanted fluid or debris.

c. The serosa or perimetrium is the outer wall and is continuous with the broad ligament.

d. The thickness and vascularity of these cell layers are influenced by hormones from the ovary during the estrous cycle and from the placenta during pregnancy. The “tone” or feel of the uterus changes during differing stages of the mare’s reproductive cycle. The uterus can change shape with handling.

Click Here To See THE STRUCTURE OF THE EQUINE UTERINE WALL [viii]

2. Cyclic changes in the uterus. The uterus changes shape, both grossly and histologically according to the stage of the mare’s estrous cycle.

a. Proestrus – just prior to the mare showing signs of estrus. The tissues in the uterus are under the influence of estrogen (E) secreted from follicles. There is an increase in vascularity, glands show some growth, while surface epithelium is still simple columnar.

b. Estrus and metestrus – The mare is “in season” or “in heat,” more correctly “in estrus.” Post ovulation hormonal changes move from estrogen to progesterone (P). This transition is metestrus.

c. Diestrus – The mare is not in estrus. The endometrium increases in thickness, uterine glands become larger, more coiled and branched. The glands actively secrete uterine “milk” (glycogen and nitrogenous products believed to sustain embryo in early stage of pregnancy); the surface (luminal) epithelium becomes high columnar. In late diestrus, the thickened endometrium shrinks, the glands decrease secretory activity and become smaller.

d. Comparative to other species, the cyclic changes in endometrium are minor in the mare.

e. There can be luminal fluid in the uterus which is believed to be an infiltrate of blood plasma supplemented by secretions of uterine glands. However, most mares will actively expel this fluid. Standing fluid in the uterus is not normal.

3. Vasculature. The uterus is supplied on each side by three arteries and their corresponding veins. There is also a vast network of lymph vessels.

II. PELVIC ORGANS consist of a portion of the cervix, the vagina and the vulva.

A. CERVIX

1. The cervix is thick walled and muscular with longitudinal folds continuous with endometrial folds.

2. The cellular lining is mucosal, or, it produces mucus that can act as a lubricant or sealant.

3. The muscularis is rich in collagenous and elastic tissues and smooth muscle.

4. During estrus the cervix is edematous (the tissues hold fluid). The tone should be (not always in maiden mares) softer and more relaxed than in diestrus, i.e., the cervix can lose its regular shape during estrus, which we will call “relaxed.” In diestrus it is whitish in color and firm with a more pronounced tubular shape.

5. Vasculature. The uterine branch of the vaginal artery passes along the cervix laterally.

B. VAGINA

1. The vagina is located from the cervix and (moving caudally) to the transverse fold over urethral orifice (bladder opening). The urethral orifice is on the ventral floor of the vagina not very far from the opening of the vagina. There is a small flap closure over the urethral orifice.

2. Just cranial to the urethral orifice is a transverse fold of tissue that forms the hymen in young mares. This serves as a physical barrier to debris and infectious organisms. The hymen can rupture spontaneously with exercise.

3. Vaginal tissue is fibrous and intramuscular connective tissue.

4. The fornix is the cranial annular cavity surrounding the cervix.

5. There are no glands in the vagina.

6. Vasculature – The interpudenal vessels have many branches and supply much of the caudal reproductive tract.

C. VESTIBULE AND VULVA

1. The vestibule is the area located from urethral orifice to vulvar opening.

2. It is rich in elastic fibers, contains lymph patches and leukocytes can be found in the epithelium of this area. The lymph and leukocytes may be present as defense mechanisms to aid in combating environmental bacteria and bacteria contained in fecal debris from the mare herself.

3. The vestibular glands produce mucous secretions that serve as a lubricant during copulation.

4. The vulva consists of two prominent external labia that keep the entrance into the reproductive tract closed. The vulva is lined with stratified squamous epithelium. Externally, a small ventral portion hangs over the ischial arch. This is important when using a vaginal speculum – the speculum would need to be inserted at an upward angle to pass over the ischial arch and into the vagina and is the natural angle during copulation.

5. The clitoris in located in the ventral commisure or fossa clitoris of the vulva. It is homologous to the penis in the stallion as it has nerves and erectile tissue. When a mare is in estrus, she everts the clitoris which is called “winking.”

6. Only the caudal portion of reproductive tract (vulva) contains sensory fibers, the uterus does not.

D. ARTERIES, VEINS AND LYMPH VESSELS

1. These vessels serve to supply essential nutrients to the organs, connecting these organs to the remainder of the body. They have been referenced above under the appropriate organ. They also serve to carry hormones from the reproductive organs to the remainder of the body.

MAMMARY GLANDS or UDDER

1. These are skin glands not directly connected to the abdominal cavity. There are two, left and right, divided by a septum (a thin dividing wall) located ventrally between the mare’s rear legs. Each gland has a laterally flattened teat and at the apex (the point) of each teat there are two openings.

2. Blood vessels, specifically the external pudenal artery (with corresponding veins), supply these glands. The vessels descend through the inguinal ring (located internally in the flank area between the hind limb and the abdomen) and entering caudally into the udder. There is also a well-developed lymph system. Nerves, like the blood vessels, descend through the inguinal ring.

3. Within each mammary gland, grape like structures called alveoli produce & collect milk. Myoepithelial cells surround alveoli. These are important for milk letdown. Any pleasurable sensation from the foal such as touch or sight, etc. lets milk down. The hormone, oxytocin is involved in milk letdown.

4. Streak canal – This is the muscular walled canal that begins at the opening of the teat and connects to the lactiferous cisterns (small chambers which hold milk). It acts as the final passageway for the milk to the outside. It must also prevent milk from escaping and bacteria from gaining entrance. There is not an organized sphincter at the apex of the streak canal. Thus, in some instances, colostrum and milk can leak out and be lost. There are two streak canals per teat.

5. The streak canal then connects internally to the milk collection system. This includes the teat cistern, gland cistern and collecting duct. There are two collection systems per teat.

6. The skin of the udder is sparsely haired and slightly thicker than average for equine skin. There are numerous sebacious glands and sweat glands present on the surface. The sebaceous glands exude “sebum” which collects along with dirt and debris between the teats. The sebum is thought to act as a lubricant between the teats during motion. The sebum should be cleaned away and the udder washed prior to foaling.

7. The udder enlarges progressively as the mare approaches parturition.

Click Here To See EQUINE UDDER – SAGGITAL SECTION [ix]

TERMINOLOGY

You should familiarize yourself with these terms

A. Antrum - any nearly closed cavity

B. Apposition – the act of placing together or bringinging into proximity.

C. Cilia (plural) cilium (singular) – A motile extension of a cell surface. It appears as a hair-like projection that moves back and forth either to propel a cell or to move contents along the surface of a cell.

D. Fimbrae – Finger like.

E. Fossa – A depression.

F. Hilus - depression or slitlike opening in an organ where nerves and vessels enter and leave.

G. Homologus – Corresponding or alike in certain critical attributes.

H. Leukocyte – A white blood cell or an infection fighting cell.

I. Ligament – A band or sheet of tissue connecting two or more bones, cartilages, or other structures, or serving as support for fasciae or muscles. In our case, a fold of peritoneum supporting abdominal viscera.

J. Lumen – The space in the interior of a tubular structure.

K. Metrium – Having to do with the uterus.

L. Oocyte – The immature ovum.

M. Os - opening into a hollow organ or canal.

N. Parenchyma - Distinguishing or specific cells of a gland or organ contained in and supported by the connective tissue framework or stroma.

O. In situ - In position.

P. Peduncle - A stalk or stem.

Q. Peri – Around.

R. Peritoneum – The serous sac that lines the abdominal cavity.

S. Serosa – A thin membrane lining the closed cavities of the body.

T. Squamous – Resembling scales, thin and flat like a scale.

U. Tortuous – Characterized by twists and turns.

V. Viscera – The organs in the abdominal cavity.

Click Here To Take Quiz

[i] Ginther, O. J. 1992 Reproductive Anatomy. In: Reproductive Biology of the Mare, 2^nd edition. Equiservices, Cross, Plains, Wisconsin 53528, p 3

[ii] Ginther, O. J., 1992 Reproductive Anatomy. In: Reproductive Biology of the Mare, 2^nd edition. Equiservices, Cross Plains, Wisconsin 53528, pp 6-9

[iii] Goldfinger, Eliot. Animal Anatomy for Artists, pp 54-56.

[iv] Goldfinger, Eliot. Animal Anatomy for Artists, pp 54-56.

[v] Ginther, O. J., 1992, Reproductive Anatomy. In: Reproductive Biology of the Mare, 2^nd edition. Equiservices, Cross Plains, Wisconsin 53528, pp15

[vi] Ginther, O. J., 1992, Reproductive Hormones. In: Reproductive Biology of the Mare, 2^nd edition. Equiservices, Cross Plains, Wisconsin 53528, pp 63.

[vii] Ginther, O. J., 1992, Reproductive Anatomy. In: Reproductive Biology of the Mare, 2^nd edition. Equiservices, Cross Plains, Wisconsin 53528, p 26.

[viii] Ginther, O.J., 1991, Reproductive Anatomy. In: Reproductive Biology of the Mare, 2^nd edition. Equiservices,s Cross Plains, Wisconsin 53528, p. 28.

[ix] McKinnon, Angus O., Voss, James L., 1993, Reproductive Organs of the Mare, In: Equine Reproduction. Lea and Febiger, Malvern, Penna., 19355 pp. 3-19.