Mare Reproduction (Week 3 ASCI 333)

Neuroendocrine control components

Eye, pineal gland, hypothalamus, and pituitary gland regulate mare reproduction

GnRH

Gonadotropin-releasing hormone produced by hypothalamus that stimulates LH and FSH release from pituitary

LH and FSH

Luteinizing hormone and follicle-stimulating hormone; gonadotropins that regulate follicular development and steroidogenesis in ovaries

CRH and ACTH

Corticotropin-releasing hormone from hypothalamus triggers ACTH from pituitary, which stimulates adrenal glucocorticoid production

Seasonal polyestrous

Mares are reproductive only during certain seasons, influenced by photoperiod (day length)

Photoperiod effect on reproduction

Lengthening spring photoperiods increase GnRH, FSH, and LH, triggering breeding season; winter darkness increases melatonin and suppresses reproduction

The vulva

External opening to vagina located ventral to anus; consists of pigmented skin externally and mucous membrane internally; serves as mechanical barrier

Caslick's vulvoplasty

Surgical procedure closing upper portion of vulva to prevent air aspiration (windsucking) and bacterial contamination in mares with poor perineal conformation

The clitoris location and structure

Located at ventral commissure of vulva; has three dorsal clitoral sinuses and one ventral clitoral fossa

Clitoral sinuses disease importance

Can harbor pathogenic bacteria like those causing contagious equine metritis (CEM); used for disease screening

Three protective barriers of mare reproductive tract

Vulvar, vestibular, and cervical seals that prevent bacterial contamination

The vestibule

Caudal portion of vagina with vestibular seal formed by transverse folds

Urethral opening location

Located caudal to transverse folds to prevent urine contamination of cranial vagina and cervix

Vaginal dimensions

Approximately 20 cm long and 15 cm in diameter

Vaginal environment

Acidic, providing antimicrobial protection

External cervical os

Visible opening of cervix on vaginal floor

The cervix

Muscular sphincter about 8 cm long serving as entrance to uterus and third protective seal

Cervical changes during estrous cycle

During estrus: relaxed, edematous, reddened with clear mucus; During diestrus: tightly closed, pale, and dry

Uterine shape and dimensions

T-shaped organ with single tubular body (20 cm long) and two tubular horns (25 cm long each)

Myometrium

Middle muscular layer of uterus with smooth muscle in longitudinal and circular orientations; responsible for contractions during estrus and parturition

Endometrium

Inner glandular mucous membrane with extensive folding; provides surface for embryonic attachment and nutrient secretion

Perimetrium

Outer layer of uterus continuous with broad ligament

Middle uterine artery

Blood supply to uterus entering along dorsal aspect; rupture is critical peripartum emergency causing potentially fatal hemorrhage

Utero-tubular junction (UTJ)

Muscular sphincter separating uterine horns from oviducts; appears as papilla in endometrium with ciliated, highly folded epithelium

Equine UTJ uniqueness

Structure and function unique among domestic species; serves as significant barrier regulating embryo descent into uterus

Infundibulum

Funnel-shaped portion of oviduct nearest ovary with ciliated epithelium that captures ovulated oocyte

Ampulla

Expanded middle portion of oviduct where fertilization occurs

Isthmus

Narrowed portion of oviduct connecting to uterine horn

The broad ligament

Ligamentous sheet from lumbosacral region providing major support for reproductive tract; continuous with uterine smooth muscle; contains blood vessels and nerves

Mesovarium

Portion of broad ligament attaching to ovaries

Mesosalpinx

Portion of broad ligament attaching to uterine tubes/oviducts

Mesometrium

Portion of broad ligament attaching to uterus

Mare ovarian shape

Bean-shaped structures

Ovarian functions

Endocrine (hormone production: estrogen and progesterone) and cytogenic (gamete/oocyte production)

Mare ovary "inside-out" arrangement

Unique inverted structure with inner cortex and outer medulla, opposite to all other mammals

Ovulation fossa

Unique wedge-shaped area on concave ventral border of mare ovary; exclusive site where ovulation occurs

Ovulation fossa uniqueness

Found only in mares; ovulation happens internally through this fossa rather than on external ovarian surface

Ovarian bursa

Membranous pouch containing ovary, defined laterally by mesosalpinx and medially by broad ligament attachment to proper ovarian ligament

Proper ovarian ligament

Attaches ovary to tip of uterine horn, maintaining anatomical position

Oogonia

Present at birth, surrounded by epithelial cells forming primordial follicles

Primary oocytes

Develop from primordial follicles at puberty onset due to hormonal stimulation

Oogenesis process

Diploid oogonia divide by mitosis inside ovary, then enter meiosis as primary oocytes

Graafian follicle

Mature, fluid-filled follicle containing oocyte surrounded by cumulus oophorus, granulosa cells, basement membrane, theca interna, and theca externa

Follicular antrum

Fluid-filled cavity that expands as follicle matures

Follicular fate

Follicles either ovulate or degenerate (atresia)

Corpus hemorrhagicum

Post-ovulation structure formed 1-3 days after follicle collapse; contains granulosa cells, theca cells, blood clot, and epithelial cells

Corpus luteum (CL)

Develops from corpus hemorrhagicum; secretes progesterone to maintain pregnancy or luteal phase; regresses after 14-16 days if mare not pregnant

Corpus albicans

"Old" regressed corpus luteum that appears white