Reproductive Physiology PDF

Introduction  The ovary is an ovoid relatively dense structure, the primary functions of which are to produce female gametes (ova) and the hormones estrogen and progesterone.  After the animal is born, it must grow and achieve puberty by acquiring the ability to produce fertile ga...

Introduction  The ovary is an ovoid relatively dense structure, the primary functions of which are to produce female gametes (ova) and the hormones estrogen and progesterone.  After the animal is born, it must grow and achieve puberty by acquiring the ability to produce fertile gametes - must be accompanied by reproductive behavior and copulation.  After copulation, fertilization occurs and development of the pre-attachment embryo follows.  The conceptus attaches to the uterus by a specialized organ called the placenta. It allows the conceptus to grow and develop to term.  The fully developed fetus is born and the female giving birth to it must lactate to provide nourishment for the neonate.  During or after lactation the dam must reestablish cyclicity before she can become pregnant again.  Knowledge and understanding of the reproductive process will become increasingly important as the human population continues to grow and resources become increasingly scarce.  Among the many scientific subjects in the natural sciences, knowledge about reproductive physiology commands interest even among those who have no scientific inclination at all.  The subject of reproductive physiology carries with it interest, imagination, expectation, emotion and an intrinsic desire to know more.  Almost without exception, everyone wants to know more about the reproductive process, whether it relates to humans, food-producing animals, their pet or just for the sake of having more knowledge.  Gynecology is a reproductive branch of physiology and medicine that deals specifically with reproductive issues in women.  Theriogenology is a branch of veterinary medicine that focuses on the reproductive system in animals.  Obstetrics is a branch of reproductive physiology, veterinary medicine and/or human medicine that specializes in the female before, during and after parturition. Historical Development of Reproductive Physiology  The study of reproductive physiology started with Aristotle around 2350 years ago.  Aristotle provided the first recorded information on how he thought the reproductive system functioned in his book entitled Generation of Animals.  Aristotle believed that the fetus arose from menstrual blood.  He also proposed that the conversion of menstrual blood to a fetus was initiated by seminal fluid deposited in the female during copulation.  A scientist named Dumas collected bodies about 1 mm in diameter from rabbit follicles. This discovery led Dumas to conclude that the "animalcules," now called spermatozoa, were responsible for uniting with the ovum and producing an embryo.  Using rabbits, he demonstrated in 1825 that spermatozoa were the fertilizing agents. This early description of fertilization marked the beginning of modern reproductive physiology.  Over 2000 years elapsed from the original conjectures that spermatozoa were required to fertilize the ova in females.  The era of modern physiology that followed can be characterized as an explosion of knowledge.  Once a certain fundamental level of understanding had been achieved reproductive physiologists began to develop ways to perturb or to manipulate reproductive events within the animal.  Such manipulations are a major goal in reproductive physiology research today.  Techniques for enhancing reproduction are important when one considers that animal-derived food products are based on the ability of the species to reproduce. Overview  The ovum, also known as the egg cell, is the female reproductive cell or gamete in animals and humans. It is typically large and spherical in shape and, when fertilized by a sperm cell, can develop into a new organism. The ovum carries half of the genetic material necessary for reproduction, with the other half provided by the sperm.  The female reproductive tract includes the ovaries, oviducts, uterus, cervix, vagina and the external genitalia.  The ovaries produce gametes and a variety of hormones that act upon other parts of the reproductive tract.  The oviducts provide the optimal environment for fertilization and preattachment development of the embryo.  The uterus provides the environment for sperm transport, early embryogenesis and the site for attachment of the conceptus.  The cervix is a barrier that secretes mucus during estrus and produces a cervical seal during pregnancy.  The vagina is the copulatory organ and produces lubricating mucus during the time of estrus.  Each tubular part of the tract has an outer serosal layer that is continuous with the peritoneum, a muscularis consisting of a longitudinal and circular layer of smooth muscle, a submucosal layer and a mucosal layer lining the lumen of each organ, that secretes substances vital to the function of each region.  The female tract is a series of tubes. Each tube is organized in concentric layers called the: -serosa (onter) -muscularis -submucosa -mucosa (inner)  The outer serosal coating is a single-cell layer of squamous (flattened) cells that simply cover the surface of the reproductive tract.  The muscularis is usually a double layer of smooth muscle consisting of an outer longitudinal layer and an inner circular layer. The purpose of the muscularis is to provide the tubular components with the ability to contract important for the transport of secretory products, gametes (spermatozoa and ova).  The submucosa is a layer of varying thickness - houses blood vessels, nerves and lymphatics.  The lumen in all the parts of the reproductive tract is lined with a secretory layer of epithelium known as the mucosa oviduct (ciliated and non-ciliated simple columnar epithelium to produce fluids and move materials along the oviduct; posterior vagina (stratified squamous epithelium) to provide protection during copulation.  The reproductive tract develops in a retroperitoneal position (behind the peritoneum) in the conceptus.   The peritoneum is the connective tissue lining of the abdominal cavity and completely surrounds or covers the reproductive tract.  During embryonic development the tract grows and begins to push against the peritoneum.  As the tract continues to grow it becomes completely surrounded by the peritoneum.  A portion of the peritoneum eventually fuses to form a double layered connective tissue sheet, called the broad ligament, which supports and suspends the ovaries, oviduct, uterus, cervix and the anterior vagina. Embryonic Development of the Broad Ligament A. The uterine horns (UH) and the rectum (R) develop dorsal to the peritoneum. Devel- opment "behind the peritoneum is called retroperitoneal. B. As development advances. the uterine horn and rectum push into the body cavity (arrows in B) and eventu ally become completely surrounded by a layer of peritoneum (C). C. The broad ligament conses of two layers of peritoneum that "sandwich" the between them. Each of peritoneum is continuous with the peritone peritoneal lining of the body cavity. The Broad Ligament  The reproductive tract is surrounded by the peritoneum that is continuous with the broad ligament.  It supports the various organs of the female reproductive tract.  The broad ligament houses the vascular supply, the lymphatic drainage and nerves.  Components of the Broad Ligament ➤ Mesovarium ➤ Mesosalpinx ➤ Mesometrium Mesovarium  The anterior (cranial) portion of the broad ligament that attaches to and supports the ovary.  The mesovarium houses the blood and lymphatic vessels and nerves that supply the ovary and forms the hilus of the ovary.  An additional supportive ligament for the ovary is also present in most species. This ligament is the utero-ovarian ligament (ovarian ligament) and, as the name implies, it attaches the ovary to the uterus.  The utero-ovarian ligament is sometimes called the proper ligament of the ovary and is not actually part of the broad ligament. Mesosalpinx  The mesosalpinx is a serious part of the broad ligament that surrounds and supports the oviduct (salpinx).  A serous membrane is a smooth transparent surface that either covers, lines, or attaches to an organ.  This delicate subdivision of the broad ligament not only supports the oviducts but serves as a bursa like pouch that surrounds the ovary.  The mesosalpinx helps to orient the infundibulum so that ova released at ovulation have a high probability of being directed into the oviduct. Mesometrium  The mesometrium is the largest and most conspicuous part of the broad ligament.  It supports the uterine horns (cornua) and the body of the uterus.  The dorsal portion of the mesometrium is continuous with the dorsal peritoneum and thus the uterus literally "hangs" from the dorsal body wall. FEMALE REPRODUCTIVE TRACT  Broad Ligament  Cervix  Intercornual Ligament  Ovary  Oviduct  Rectum  Rectogenital Pouch  Uterine Horn The Structures on the Ovary Undergo Constant Change  The ovary is an ovoid relatively dense structure, the primary functions of which are to produce female gametes (ova) and the hormones estrogen and progesterone.  The corpus luteum also produces oxytocin, relaxin, inhibin and activin.  The ovary is composed of an outer connective tissue surface called the tunica albuginea.  The tunica albuginea is covered by a single layer of cuboidal cells called the germinal epithelium. This layer has no function relating to production of the germinal cells and is thus erroneously named.  Beneath the tunica albuginea is a zone referred to as the ovarian cortex.  Generally (the mare is the exception), the ovarian cortex houses the population of oocytes. Cells surrounding oocytes will develop and produce follicles that will mature and eventually ovulate.  The ovarian cortex also houses the functional corpus luteum, abbreviated CL (plural = corpora lutea), and the degenerating corpora lutea known as corpora albicantia (singular = corpus albicans).  Corpora lutea ("yellow bodies") are relatively large, conspicuous structures that produce progesterone.  Corpora albicantia can readily be observed on ovaries of most species. The word "albicans" is derived from the word "albino," that implies a white color.  The central part of the ovary is called the ovarian medulla.  The medulla houses the vasculature, nerves and the lymphatics and is composed of relatively dense connective tissue.  Morphologically, the ovaries of the mare present several important exceptions to the information presented previously. 1. the ovarian medulla and cortex are reversed (cortex inside, medulla outside) when compared to other species. 2. ovulation occurs at only one location in the mare's ovary (ovulation fossa), while it occurs at random locations in the ovaries of the other mammals. 3. follicles can be palpated per rectum in the mare, but corpora lutea cannot. This is because corpora lutea do not protrude significantly from the ovarian surface but tend to penetrate into the ovarian tissue.  The ovaries of most females are relatively dense, turgid structures that can be distinguished tactilely from other tissues in the immediate anatomical vicinity in some species using palpation per rectum.  By inserting the arm into the rectum (cow, mare, camel), the ovaries can be palpated by carefully manipulating the cranial portion of the tract. Primary Ovarian Structures  Within any region of the ovarian cortex, one can encounter several different types of ovarian follicles.  The various types of ovarian follicles represent different stages of follicular development and maturity.  The process whereby immature follicles develop into more advanced follicles and become candidates for ovulation is referred to as folliculogenesis.  There are four types of follicles present within the ovary.  (1) Primordial follicles that are microscopic, are the most immature and are the smallest encountered in the ovarian cortex.  The oocyte (egg) within the primordial follicle is surrounded by a single layer of flattened (squamous) cells.  The primordial follicle will develop into a slight aslightly more advanced follicle called the (2) primary follicle, which is characterized by having an oocyte that is surrounded by a single layer of cuboidal (cube-like) epithelium or follicular cells.  Females are born with a lifetime's supply of primordial and primary follicles Primary follicles do not divide into other primary follicles. Instead, they either develop into a more advanced secondary follicle or they degenerate.  A (3) secondary follicle, also microscopic, is characterized as having two or more layers of follicle cells, but without an antrum or cavity.  The oocyte within a secondary follicle is characterized as being surrounded by a relatively thick translucent layer called the zona pellucida.  An antral follicle (sometimes referred to as a tertiary follicle) is characterized by a fluid-filled cavity called the antrum.  The fluid within the antrum is called follicular fluid.  When the tertiary follicle becomes a dominant preovulatory follicle, it is sometimes called a Graafian follicle.  Some antral follicles can be observed with the naked eye on the surface of the ovaries. They appear as blister-like structures that vary in size from less than 1 mm to several centimeters.  The sizes of the antral follicles vary depending on their stage of development or regression and upon species.  Antral follicles consist of three distinct layers namely, theca externa, the theca interna and the granulosal cell layer.  The theca externa is composed primarily of loose connective tissue that completely surrounds and supports the follicle.  The cells of the theca interna express receptors for luteinizing hormone (LH) to produce androstenedione, which via a few steps, gives the granulosa the precursor for estrogen production.  The granulosa cells produce a variety of materials and have follicle-stimulating hormone (FSH) receptors. The most important products of these cells are estrogen, inhibin and follicular fluid.  Granulosal cells are also believed to govern the maturation of the oocyte.  When dominant antral follicles ovulate, small blood vessels rupture, causing local hemorrhage.  Also, the loss of fluid from the antrum of the follicle causes the follicle to collapse into many folds yielding a structure called the corpus hemorrhagicum.  The formation of the corpus hemorrhagicum ("bloody body'), the cells of the theca interna and the granulosal cells differentiate into luteal cells to form a CL  The CL produces progesterone and is essential for the maintenance of pregnancy.  In general, all types of follicles are present within the ovary at any point in time.  Developing and functional CL may or may not be present depending on the stage of the estrous cycle.  With the exception of the mare, development (and regression) of all ovarian structures occurs at random locations within the ovary. The Oviduct  The oviduct consists of the infundibulum, ampulla, and isthmus.  The infundibulum is the terminal end (cranial or ovarian end) of the oviduct and consists of a funnel-shaped opening. This funnel like opening forms a pocket that "captures" the newly ovulated oocyte.  The surface of the infundibulum is covered with many velvety, finger-like projections called fimbriae, which greatly increase the surface area of the infundibulum and cause it to glide or slip over the entire surface of the ovary near the time of ovulation.  The surface area of the infundibulum ranges from 6-10 cm in sheep to 20-30 cm in cattle.  The infundibulum leads directly into a thick portion of oviduct called the ampulla.  The ampulla occupies one-half or more of the oviductal length and merges with the isthmus of the oviduct.  The ampulla has a relatively large diameter, with the internal portions characterized by many fern-like mucosal folds with ciliated epithelium.  The junction between the ampulla and the isthmus (ampullary isthmic junction) is generally ill-defined.  In the mare, the ampullary-isthmic junction serves as a control point that allows only fertilized oocytes to pass into the isthmus and eventually into the uterus.  The isthmus is smaller in diameter than the ampulla. It is connected directly to the uterus and the point of juncture is called the uterotubal junction.  The primary function of the smooth muscle layer (muscularis) of the oviduct is to transport newly ovulated oocytes and spermatozoa to the site of fertilization (the ampulla).  Gamete transport by the oviduct requires that spermatozoa and ova move in opposite directions so that they encounter each other in the ampulla.  The mechanisms controlling gamete transport by the oviduct are not well understood. The Uterus  The uterus is the organ of pregnancy.  The uterus connects the oviducts to the cervix.  In most mammals, the uterus consists of two uterine horns or cornua.  The degree to which the uterine horns are developed constitutes the basis for classification of mammalian uteri.  Among mammals there are three distinct anatomical types of uteri.  The first of these is a duplex uterus, characterized as having two cervical canals that separate each uterine horn into distinct compartments.  There are two types of duplex uteri.  The first is characterized by having a single vaginal canal opening to the exterior. On the interior it bifurcates (splits) into two vaginas and two cervices. Marsupials have this type of uterus.  In the opossum, this interesting female anatomical configuration is accommodated by the forked penis of the male.  It is believed that after intromission, the male opossum deposits semen in each of the two sides of the reproductive tract simultaneously.  The second less complex type of duplex uterus is found in the rabbit.  In this type of duplex uterus, there are two uterine horns and two distinct cervical canals connected to a single vaginal canal.  In species like the rabbit, it is possible to artificially inseminate the female into one horn with sperm from one male and to artificially inseminate the contralateral (opposite) horn with semen from another male; the offspring will represent two genetic types.  The rabbit is an excellent animal to use for the study of various experimental seminal or embryo treatments, because transuterine migration of the gametes or embryos is not likely to occur.  The bicornuate uterus is characterized by having two uterine horns and a small uterine body.  The length of the uterine horns is dependent on the degree of fusion between the paramesonephric ducts in the developing female fetus.  In species where there is a high degree of fusion (mare) there are short uterine horns and a relatively large uterine body.  When a moderate degree of fusion occurs, uterine horns of intermediate length result (cow, ewe and goat).  When little fusion takes place between adjacent paramesonephric ducts, long uterine horns result (sow,bitch and queen).  In all types of bicornuate uteri, the uterus opens into the vagina through a single cervical canal.  The term "bipartite" was once used to describe bicornuate uteri with short (mare) to moderate length (cow) uterine horns.  Simplex (No uterine horns)  In an attempt to simplify the classification of mammalian uteri, the suggestion has been made that the term "bipartite" be dropped from the uterine classification nomenclature.  The simplex uterus is characterized as having a single uterine body. Only small rudiments of a structure resembling a uterine horn may be apparent.  Fusion of the paramesonephric ducts is almost complete, resulting in a single- chambered uterus without horns.  The simplex uterus is found in primates, including humans. Components of the Uterus  The uterus consists of a serosal layer called the perimetrium (serosa) that is part of the peritoneum. It is continuous with the serosal layer covering the mesosalpinx.  Beneath the serosal layer is a longitudinal layer of smooth muscle. The outer longitudinal layer and the inner circular muscle layer are referred to as the myometrium (muscularis).  The myometrium provides motility (a form of contraction) for the uterus. During parturition, the myometrium becomes a major driving force for expulsion of the fetus and fetal membranes.  The inner portion of the uterus is composed of the endometrium (mucosa + submucosa) - responsible for secreting materials into the lumen of the uterus that enhance embryo development and sperm viability.  The primary functions of the uterus are: -sperm transport -luteolysis and control of cyclicity -environment for preattachment embryo -maternal contribution to the placenta -expulsion of the fetus and fetal placenta  Uterine glands develop from the endometrial mucosa penetrate into the submucosa and become colled.  They secrete material into the lumen of the uterus arrows.  The uterus consists of a serosal layer called the perimetrium (serosa) that is part of the peritoneum. It is continuous with the serosal layer covering the mesosalpinx.  Beneath the serosal layer is a longitudinal layer of smooth muscle. The outer longitudinal layer and the inner circular muscle layer are referred to as the myometrium (muscularis).  The myometrium provides motility (a form of contraction) for the uterus. During parturition, the myometrium becomes a major driving force for expulsion of the fetus and fetal membranes.  The inner portion of the uterus is composed of the endometrium (mucosa + submucosa) - responsible for secreting materials into the lumen of the uterus that enhance embryo development and sperm viability.  Uterine glands produce materials that are believed to be important to the survival and function of the preimplantation embryo.  The submucosa is predominantly connective and supporting tissue and houses the uterine glands.  A distinct difference between lower mammals and primates, particularly humans, is that the endometrium of the uterus in the human is sloughed to the exterior. The endometrial glands in domestic mammals are not sloughed.  The functionality of the uterine glands changes during the estrous cycle.  In other words, secretory activity of the uterine glands changes as a function of the stage of the estrous cycle. The mechanisms whereby uterine glands may be lost (or replenished) in domestic animals remains undefined.  At a critical time during the estrous cycle the cells of the uterine endometrium produce prostaglandin F2a.  Prostaglandin F2a causes luteolysis or regression of the corpus luteum if the animal is not pregnant.  In ruminants, the surface of the endometrium is characterized as having small, non-glandular areas that protrude from the surface of the endometrium. These small protuberances are referred to as caruncles.  The caruncular regions are highly vascularized and will give rise to the maternal portion of the placenta if attachment of the embryo occurs.  In contrast to the cow and ewe, the endometrium of the sow and mare have no caruncles. Their endometrium is characterized by having many endometrial folds instead - the folds provide the uterine surface for the development of the placenta.