Читать книгу Bovine Reproduction - Группа авторов - Страница 354

22 Anatomy of the Reproductive System of the Cow

Ben Nabors

Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Starkville, MS, USA

Introduction

The anatomy of the reproductive system in the cow is functionally grouped into the components associated with oocyte production and transport and those involved with gestation and copulation.

Production

The cellular machinery for oogenesis and steroid production is found in the ovary (Figure 22.1). The ovary consists of a cortex and medulla. The medulla is composed of connective tissue, lymphatic vessels, blood vessels, and nerves. Surrounding the medulla is the cortex. The cortex contains the ova surrounded by follicular cells within the connective tissue stroma [1]. Exterior to the cortex, the ovary is covered by the dense fibrous tunica albuginea and a superficial epithelium [2].

Figure 22.1 (a) Ov, ovary, Uh, uterine horn, F, follicle, 22.1 (b) Corpus luteum, Image courtesy Dr. Fuller Bazer, Texas A & M, (c) Corpus lutea (image from pregnant cow), Image courtesy Dr. John Roberts, College of Veterinary Medicine, University of Florida.

Because the ovary in the cow descends further from its embryologic origin near the kidney than other species, it is positioned closer to the pelvis. The consequence of this ovarian location and the attachment of the short mesovarium is that the uterine horns bend ventrally and caudally [3] (Figure 22.2). (Editor's note: for additional histological images of the ovary see Chapter 25.)

Figure 22.2 The internal reproductive tract from oblique angle. V, Vagina; C, cervix; Uh, uterine horn; Ov, ovary; Ub, urinary bladder.

Transport and Gestation

The reproductive system of the cow is designed to transport spermatozoa toward the ovary and to transport an ovum toward the spermatozoa (Figure 22.3). The parts of this tubular system include the vestibule, vagina, cervix, uterine horns, and uterine tubes (oviduct).

Figure 22.3 The internal reproductive tract from dorsal view. V, Vagina; C, cervix; U, uterus body; Ov, ovary; Uh, uterine horn.

Uterine Tube

The uterine tube is arranged like a funnel near the ovary. The funnel‐shaped end, or infundibulum, contains processes, the fimbriae, which collect the ovum on ovulation (Figure 22.4). The ovum is then transported through the abdominal opening of the uterine tube located at the base of the infundibulum [4]. The ampulla of the uterine tube is the region adjacent to the infundibulum where fertilization takes place. The isthmus, the continuation of the uterine tube from the ampulla toward the uterus, is relatively long due to the meandering course it takes before ending at the uterine opening where it releases the ovum into the uterine horn [4].

Figure 22.4 Structures of the ovary. Ov, Ovary; Inf, infundibulum; Ut, uterine tube; Ovb, ovarian bursa; Uh, uterine horn.

Uterus

The uterus consists of a body and two horns (Figure 22.5). The body is short, beginning immediately after the cervix ends. The horns branch from the body but are joined together by the peritoneum, giving the appearance that the body is longer than it truly is. As the horns progress craniad they divide at the intercornual ligaments, each turning abruptly ventrally, then proceeding caudally, and finally ending dorsal to the ovary [3] (Figure 22.6).

Figure 22.5 Dorsal view of reproductive tract. U, Uterus body; Ov, ovary; Uh, uterine horn.

Figure 22.6 Uterine horns positioned to better view the intercornual ligament. Uh, Uterine horn; Dl, dorsal intercornual ligament; Vl, ventral intercornual ligament; Ovb, ovarian bursa; Ov, ovary.

From external to internal, the uterus can be divided into three layers: the perimetrium, the myometrium, and the endometrium [3]. The perimetrium is the continuation of the abdominal peritoneum onto the uterus. The myometrium constitutes the muscular layers, which can undergo substantial hypertrophy [5]. The endometrium is the internal epithelial lining of the uterus and is arranged into two distinct regions, caruncular and intercaruncular [6]. The caruncles are raised mucosal regions of the endometrium that are highly vascularized (Figure 22.7). The caruncles of the uterus join with the cotyledons of the fetal placental membranes to form the placentomes of a cotyledonary placenta [4].

Figure 22.7 The uterine lumen. Pm, Perimetrium; Mm, myometrium; Em, endometrium; Cr, caruncle.

Cervix

The cervix is located between the body of the uterus cranially and the vagina caudally (Figure 22.8). It is a firm, muscular, sphincter‐like structure that acts as a barrier separating the external genitalia from the internal genitalia [5]. Characteristic of the cervix are the three to four circular folds projecting into its lumen.

Figure 22.8 Vaginal and cervical lumen. V, Vagina; C, cervix; U, body uterus; Uh, uterine horn; Ov, ovary.

The arrangement of the cervical musculature and mucosa is responsible for this characteristic architecture (Figure 22.9). The superficial mucosa is arranged in longitudinal folds punctuated by circular folds that interdigitate to form a series of ridges and interlocking notches when the cervix is closed [4]. This arrangement effectively seals the external environment from the internal uterine environment.

Figure 22.9 Close‐up of vaginal and cervical lumen allowing visualization of the external cervical os. V, Vagina; C, cervix; U, uterus; Cf, circular folds; Lf, longitudinal folds.

Vagina

The vagina is positioned between the caudal extent of the cervix and border of the vestibule at the external urethral orifice. The cervix projects into the lumen of the vagina caudoventrally, causing the dorsal vaginal fornix to form a deeper recess than the ventral fornix [4].

Vestibule

The vestibule is a small area in the cow that originates at the urethral opening and ends caudally to blend with the labia of the vulva.

Vulva

The labia of the vulva are located on either side of the labial fissure [3] (Figure 22.10). The labia meet dorsally, forming the dorsal commissure, and again ventrally to form the ventral commissure. The clitoris is found just cranial to the ventral commissure [3].

Figure 22.10 External view of perineum. A, Anus; Vs, vestibule; Uo, urethral orifice; V, vulva.

Blood Supply

The arterial supply to the reproductive system of the cow is provided by the ovarian, umbilical, vaginal, and internal pudendal arteries with their associated branches (Figure 22.11). The ovarian artery leaves the abdominal aorta, passing within the mesovarium to perfuse the ovary. The ovarian artery gives off branches to the uterine tube and the uterine horn [4]. The umbilical artery arises from the internal iliac artery near the pelvic inlet and sends out a uterine branch that supplies the cervix, body, and horns of the uterus. The vaginal artery arises directly from the internal iliac artery. The vaginal artery gives off a uterine branch that anastomoses with the uterine branch of the umbilical artery. The uterine artery formed by this anastomosis in turn anastomoses with the uterine branch of the ovarian artery, completing an arterial loop that supplies all the internal components of the reproductive system [4]. The internal pudendal artery is a direct continuation of the internal iliac artery. It courses caudally after the vaginal artery branches off within the ischiorectal fossa. The internal pudendal artery terminates as ventral perineal and dorsal labial branches [4].

Figure 22.11 Arterial blood supply to the reproductive tract. Aa, Abdominal aorta; Ov, ovarian artery; Um, umbilical artery; Ii, internal iliac artery; V, vaginal artery; Uv, uterine branch of vaginal artery; Uu, uterine branch of the umbilical artery; Uo, uterine branch of ovarian artery.

The innervation of the external genitalia of the cow consists of the pudendal nerve and its branches. The pudendal nerve carries motor, sensory, and parasympathetic nerve fibers [2]. The pudendal nerve passes through the pelvic cavity medial to the sacrosciatic ligament and divides as it approaches the lesser ischiatic notch of the pelvis into proximal and distal cutaneous branches that supply the skin of the caudal hip and thigh [2, 3]. The pudendal nerve continues through the ischiorectal fossa and terminates as the dorsal nerve of the clitoris and a mammary branch [3]. Parasympathetic components of the pudendal nerve come by way of the pelvic nerve, which originates as a coalescence of branches of the sacral spinal nerves at the sacral plexus [4]. Sympathetic components of the pudendal nerve arise from the paired hypogastric nerves, which contribute sympathetic fibers from the caudal mesenteric plexus to the genital system [4].

Placenta

The cotyledonary bovine placenta is composed of the fetal cotyledons and the maternal caruncles that fuse and form the placentomes [4]. The placentomes are the sites for the transfer of maternal nutrients to the fetal circulation. The fetal membranes consist of two fluid‐filled sacs. The innermost sac is the amnion, which surrounds the developing embryo; the outermost sac is the chorioallantois, which encircles the amnion and is composed of two separate tissues that fuse as the embryo develops. The outer layer is the chorion that develops from the trophoblast layer, which is the outer layer of the blastocyst of the embryo. As the embryo develops, the allantois, a sac that arises from the embryo hindgut, expands and eventually fuses with the chorion to form the chorioallantois.

References

1 1 Ross, M., Kaye, G., and Pawlina, W. (2003). Histology: A Text and Atlas, 4e, 875. Philadelphia: Lippincott Williams & Wilkins.

2 2 Schaller, O. and Constantinescu, G. (1992). Illustrated Veterinary Anatomical Nomenclature, 614. Stuttgart: F. Enke Verlag.

3 3 Budras, K.‐D. (2003). Bovine Anatomy: An Illustrated Text, 138. Hannover: Schlütersche.

4 4 Nickel, R., Schummer, A., Seiferle, E., and Sack, W. (1973). The Viscera of the Domestic Mammals, 401. Berlin: Verlag Paul Parey; New York: Springer‐Verlag.

5 5 Pineda, M. and Dooley, M. (2003). McDonald's Veterinary Endocrinology and Reproduction, 5e, 597. Ames, IA: Iowa State Press.

6 6 Mullins, K. and Saacke, R. (2003). Illustrated Anatomy of the Bovine Male and Female Reproductive Tracts: From Gross to Microscopic, 79. Blacksburg, VA: Germinal Dimensions.