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Chapter 3: Organization and structure of mammalian reproductive systems.

CHAPTER OBJECTIVES

* Describe the major anatomic components of the reproductive system.

* Discuss how the major components of the reproductive system are related functionally.

* Review basic anatomic terminology.

* Review the basic principles of gross and microscopic anatomy that are pertinent to reproductive physiology.

THE REPRODUCTIVE SYSTEMS OF MAMMALS

The reproductive systems of mammals can be analyzed into five major components:

* the gonads,

* the genital ducts,

* the external genitalia,

* the pituitary gland, and

* the hypothalamus.

The gonads are the gamete-producing organs; that is, testes in males and ovaries in females. The genital ducts are involved with the transport of gametes and/or the development of offspring. The genital ducts of males include the epididymis and the ductus deferens, whereas the female genital ducts include the oviducts, uterus, and innermost portion of the vagina. The external genitalia are the organs of copulation (sexual connection between a male and female).

The scrotum and penis make up the male external genitalia, whereas the vulva (consisting of the major and minor labia), clitoris, and outermost portion of the vagina (vestibule) make up the female external genitalia. The pituitary gland is located at the ventral surface of the forebrain and consists of anterior and posterior lobes. The hypothalamus is a small region of forebrain located above the pituitary gland. A more detailed description of the sexual anatomy of males and females is provided in subsequent chapters. Figure 3-1 provides a schematic representation of the major components of the mammalian reproductive system as well as the functional relationships among these tissues. A few general concepts concerning each of the major subdivisions are included in the following sections.

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The Hypothalamic-Pituitary Axis

It is wellknown that various environmental stimuli influence reproduction in animals. For example, the reproductive activity of many animals is restricted to a particular time of year and/or set of environmental conditions. Thus it is readily apparent that animals have the ability to sense environmental conditions, such as day length, and convert this information into internal signals that affect the activity of reproductive tissues. Animals also have the ability to sense changes in their internal environments (e.g., concentrations of gases, ions, and metabolites in extracellular fluids), and these too can affect reproductive activity. It is obvious that the central nervous system (brain and spinal cord) provides the ability to sense and respond to such environmental changes. Various sense organs (e.g., the eyes, olfactory bulb, chemical receptors, and so on) respond to changes in the environment and convey this information to the brain via afferent (flowing into the central nervous system) neuronal pathways. Diverse neuronal inputs are integrated (blended) and transformed into humoral (blood-borne) signals within the hypothalamic-pituitary axis.

Numerous afferent neurons converge on the hypothalamus and impinge upon neurosecretory cells that respond to these inputs by releasing hypothalamic hormones into the blood. Some of these hormones travel directly to the anterior pituitary gland to induce the release of other hormones that enter the general circulation and regulate other organs. Gonadotropin-releasing hormone (GnRH) is a hypothalamic hormone that plays a pivotal role in regulating reproductive activity. GnRH stimulates the release of two gonadotrophic hormones (gonadotropins) from the anterior pituitary gland: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The gonadotropins support gametogenesis in the testes and ovaries and stimulate production and secretion of gonadal hormones, which regulate other reproductive tissues, including the brain. With respect to the latter action, hormones produced by the gonads feed back on the brain and pituitary gland to regulate release of hypothalamic and pituitary hormones. Prolactin is another anterior pituitary hormone that plays a role in reproduction. This hormone is best known for its role in promoting synthesis of milk in the mammary glands. Prolactin also influences ovarian function in some species (e.g., rodents). The anterior pituitary gland is involved with much more than reproduction. It also produces several hormones that regulate a variety of physiologic processes including growth, thyroid activity, and adrenal activity.

The posterior pituitary gland is also involved with the regulation of reproduction. Unlike the anterior pituitary gland, this lobe does not produce hormones. Rather, it stores hormones that are produced by neurons located in the hypothalamus. One of these hormones is oxytocin. It acts on the female genital ducts and mammary glands to regulate the birthing process and milk secretion.

Gonads, Gonadal Hormones, and Genital Ducts

The testes and ovaries have two major functions: 1) production of gametes (spermatozoa and oocytes) and 2) production of gonadal hormones. As noted earlier, these processes are dependent on gonadotropins. Thus any disruption in release of GnRH by the hypothalamus and/or release of LH and FSH by the anterior pituitary gland results in lowered fertility and reduced production of gonadal hormones.

Gonadal hormones act on the genital ducts, external genitalia, mammary glands, brain, anterior pituitary gland, and other tissues not directly involved with reproduction. In the embryo, gonadal hormones regulate development of the genital ducts and external genitalia. At sexual maturity these hormones stimulate development of secondary sex traits; that is, specific characteristics that distinguish males from females, but are not directly involved with reproduction. In adults, gonadal hormones are important in maintaining proper function of the genital ducts as well as in regulating gonadotropin secretion.

Physiologic relationships between the ovaries and female reproductive tract are of particular importance. Hormones produced by the ovaries (e.g., estrogen, testosterone, and progesterone) promote changes in the vagina, uterus, and oviducts to support copulation and pregnancy. Moreover, the uterus produces hormones (e.g., prostaglandins) that affect ovarian function.

Gonadal hormones act on the hypothalamus and anterior pituitary gland to regulate release of LH and FSH. Hormonal relationships between the gonads and the hypothalamic-pituitary unit can be negative or positive. For example, estradiol, progesterone, and testosterone can feed back on the hypothalamus and pituitary gland to suppress release of LH and FSH. This is commonly known as a negative feedback loop. Negative feedback loops serve the purpose of keeping hormone levels within a narrow range. On the other hand, gonadal hormones can also stimulate release of GnRH, LH, and FSH. Such positive feedback loops are not as common as negative feedback loops and serve the purpose of inducing rapid increases in concentrations of a hormone. We will examine these types of feedback relationships in later chapters dealing with the hormonal control reproduction.

REVIEW OF GROSS ANATOMY

Before you can begin learning how a physiologic system functions it is necessary to develop an appreciation for the structures of tissues that make up the system. The following sections provide a brief review of general anatomic principles which will prepare you for chapters that provide anatomic descriptions of the male and female reproductive organs.

Directional Terms and Planes

An object is identified by its relationships with other objects. Anatomy requires the ability to visualize and describe such relationships in three dimensions. Thus it is necessary to have terms to establish a frame of reference. Figure 3-2 illustrates terms that are commonly used to describe locations of anatomic structures in four-footed animals. These are particularly useful in studying gross (visible to the naked eye) anatomy.

The first task is to establish direction, like the points on a compass. Cranial or anterior refers to the front, or toward the head, whereas caudal or posterior refers to the rear, or toward the tail. Within the head, forward is called rostral (toward the nose). The upper (back) part of the body is referred to as dorsal, whereas the undersurface is referred to as ventral. These terms alone would be sufficient if we lived in a two-dimensional world. However, our world consists of three dimensions, so we require terminology to divide the body into different planes.

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Differentiating between left and right allows us to describe anatomy in three dimensions. Two terms are important in this regard. The median plane passes through the body from head to tail and divides it into equal left and right halves. A sagittal plane is any plane parallel to the median plane; that is, dividing the body into unequal left and right halves. You may run across the term "mid-sagittal plane," which is another term for median plane.

The body can be further divided using terms to describe additional planes. A transverse plane occurs at a right angle to the median plane and divides the body into front and rear halves. Finally, a dorsal plane is at right angles to the median and transverse planes and divides the body into upper and lower halves.

Meaningful anatomic descriptions require more than identifying location by plane and direction. Although it is accurate to state that the uterus is caudal to the transverse plane and dorsal to the dorsal plane, this doesn't tell us very much because many other structures (kidneys, bladder, intestines, and so on) are also located in this general location. A more precise description of the uterus' location requires terminology that can describe spatial relationships among structures. The term medial refers to the middle or center. To say that the uterus is medial to something is to say it is in the center of it; for example, the uterus is medial to the ovaries. Lateral, on the other hand, means away from the center; that is, the ovaries lie laterally to the uterus. Deep and superficial refer to depth relative to the body surface. For example, the skin is superficial to muscle, whereas the kidney is deep relative to the skin. Finally, proximal and distal refer to the relative distance between structures. For example, the oviducts are proximal to the ovaries, whereas the uterus is distal to the ovaries.

Body Cavities

Additional precision in locating anatomic structures can be gained by dividing the body into major cavities. The dorsal cavity contains the brain in its cranial cavity and the spinal cord which lies in the vertebral cavity. The ventral cavity includes the thoracic cavity, located cranially, and the abdominal and pelvic cavities (or abdominopelvic cavity), located caudally. The thoracic and abdominopelvic cavities are separated by the diaphragm.

Much of our attention will focus on the abdominopelvic cavity; the location of the reproductive tracts of males and females. Therefore it is worthwhile to elaborate on the anatomy this region. Figure 3-3 is a schematic illustration of the abdominopelvic cavity showing the position of the female reproductive tract relative to some other familiar organs. This cavity is surrounded by the body wall, which consists of an outer layer of skin followed by a double layer of fascia (sheet of fibrous tissue), a musculoskeletal layer, and an inner layer of fascia. The entire abdominopelvic cavity is lined by a thin, translucent membrane; that is, the peritoneum. The peritoneum forms a sac that encases the organs located in this region. During development, abdominal-pelvic organs form outside the peritoneum, near the body wall. As they develop they move into the cavity carrying the peritoneum with them. Thus, each organ is embedded in a fold of peritoneum, which acts to suspend the organs from the body wall. These folds are called omenta (a fold passing from the stomach to other viscera), mesentery (a fold that attaches to the intestines to the dorsal wall of the abdominal cavity), and ligaments (folds that connects viscera other than the digestive organs to the abdominal wall. The reproductive organs are suspended in the abdominopelvic cavity by various ligaments, the names of which we will encounter in a later chapter. The peritoneum that surrounds an organ is called the visceral peritoneum, whereas the peritoneum that lines the body cavity is called the parietal peritoneum.

[FIGURE 3-3 OMITTED]

MICROSCOPIC ANATOMY

In order to fully appreciate how reproductive tissues interact, it is necessary to develop an understanding of how these tissues are organized at the cellular level. Body tissues fall into one of four categories: epithelial tissue, connective tissue, nervous tissue, and muscle tissue. Each of these tissue types plays an important role in the structure and function of the organs involved with reproduction.

Origin of Major Tissue Types

The major body tissues develop from one of three layers of germ cells in the embryo. We will study embryogenesis later in the book. At this point, it is only necessary that you understand the organization of the early embryo as it relates to development of body tissues.

Soon after its formation, the zygote undergoes a series of mitotic divisions giving rise to numerous identical cells. Within 5 to 12 days, the embryo becomes a blastocyst (Figure 3-4), which consists of an inner cell mass, a group of cells clumped together at one end of the embryo, and the trophectoderm, a layer of thin cells (trophoblasts) forming a cyst-like cavity called the blastocoele. The inner cell mass will develop into the body of the embryo, whereas the trophectoderm will form extra-embryonic membranes that will ultimately form the fetal part of the placenta. Three distinct layers of cells will develop from the inner cell mass. The cells closest to the trophoblast become the endoderm, which will eventually give rise to the linings of the gastrointestinal and respiratory tracts, as well as endocrine glands. The outermost layer (ectoderm) will give rise to the nervous system, skin, and hair. The layer of cells between the inner and outer layers is called the mesoderm and provides the precursors for the muscular, skeletal, cardiovascular, and reproductive systems.

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As the embryo develops it takes on the physical characteristics of the species of which it is a member and the various organ systems, including the reproductive organs, assume their familiar structures. In general, organs are composed of several different types of tissues. The general anatomic features of the major tissue types found in reproductive tissues are described in the following sections.

Epithelial Tissue

Epithelial tissue (epithelium) accounts for a large portion of the mass of reproductive organs. Epithelial cells exist in aggregates and typically form sheets that serve as coverings, linings, or organ surfaces. The cells of epithelial tissues are in close association with each other. Therefore, the intercellular space is small and not penetrated by blood vessels. In almost every case, these aggregates of cells lie on a supporting basement membrane, a noncellular layer of connective tissue that is produced and secreted by epithelial cells.

Epithelial tissue is classified based on the shape and arrangement of cells (Figure 3-5). With respect to shape, epithelial cells can be squamous (thin and flat), cuboidal (equal height and width), or columnar (height is greater than width). Simple epithelium consists of only one layer of cells, whereas stratified epithelium consists of two or more layers of cells. Some common types of epithelial tissue include: simple squamous, stratified squamous, simple cuboidal, stratified cuboidal, simple columnar, stratified columnar, pseudost-ratified, and transitional.

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Simple squamous epithelium forms thin sheets of tissue. Examples of this type of epithelial tissue are the endothelium (inner lining of the heart and blood and lymph vessels), the mesothelium (inner lining of the body cavities), and the mesenchyme (embryonic connective tissue). One of the most common types of epithelium is the simple cuboidal. This type of tissue covers the surface of ovary. Simple columnar epithelium forms the linings of the digestive tract as well as other tubular organs such as the uterus and cranial vagina. The cells of this tissue may have cilia, projections that extend into the lumen, or cavity, of the tubular organ.

Pseudostratified epithelium consists of long and short cells that overlap, giving the impression of a stratified organization. Closer examination reveals that this tissue is made up of only a single layer. This type of arrangement is seen in the trachea of the respiratory system. Transitional epithelium is made up of layers of cells of varying shapes. The urinary bladder provides an excellent example of this type of tissue. Areas of the body that require protection typically have stratified squamous epithelium. In this type of tissue, only the outermost layers of cells are usually squamous. The layer resting on the basement membrane is frequently columnar. The skin and tubular organs that are prone to trauma have this type of epithelial tissue; for example, caudal vagina.

Epithelial cells form various types of glands. These can be either exocrine or endocrine glands (Figure 3-6). In both cases epithelial cells have secretory functions. In the case of exocrine glands, cells secrete their products into ducts which carry secretions to a free surface (either internal or external). In contrast, endocrine glands are ductless; cells secrete their products into the surrounding extracellular space, adjacent to capillaries. Both types of glands are found in reproductive tissues.

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Glands can be unicellular or multicellular. In the former case, specialized cells are scattered within an epithelial lining; for example, endocrine cells in the epithelial lining of the digestive tract. Multicellular glands consist of sheets of specialized epithelial cells that, together, serve some secretory function. Multi-cellular glands are organized in different ways. Simple, multicellular glands consist of one or more secretory portions and an unbranched excretory duct. The secretory end may take on the shape of a straight tube, a coiled tube, or alveolus (pear shape). Examples of simple, multicellular glands can be found in the intestine and skin (sweat glands). Multicellular glands with a compound structure consist of secretory units organized around branches of a duct, each of which drains into the main excretory duct. The shape of the secretory unit can be tubular, alveolar, or tubuloalveolar (Fig ure 3-7). Most of these glands are lobulated; that is, subdivided into smaller sections called lobules. The testis is an excellent example of this type of gland. Within each lobule there is a network of small ducts that drain into lobular ducts which drain into several main excretory ducts.

Connective Tissue

Connective tissue is ubiquitous and exists in a variety of structures. All types of connective tissues are derived from a portion of the embryonic mesoderm called the mesenchyme. Mesenchymal cells are stem cells that give rise to each type of connective tissue found in adults. The major adult connective tissues include the blood (cells and plasma), supportive connective tissues (cartilage and bone), and proper connective tissue. Our major concern will be with proper connective tissues, the type that connects organs to one another and suspends organs from the body wall. This type of tissue contains different types of cells that are separated by an extracellular material called intercellular substance. This is composed of both fibrous and amorphous components. The latter component is called ground substance and consists of mucopolysaccharides. One of the more common types of cells seen in connective tissue is the fibroblast.

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There are two types of proper connective tissue; loose and dense. Loose connective tissue (Figure 3-8) is distributed throughout the body forming the superficial fascia; filling spaces between organs and binding organs together. The intercellular substance of loose connective tissue is produced by long, flat fibroblasts and consists of collagenous and elastic fibers embedded in the ground substance. Dense connective tissue contains the same types of fibers as loose connective tissue but the fibers are arranged in tight, parallel bundles to form tendons and ligaments, or in tightly interwoven networks to form dense matting (e.g., the dermis of the skin).

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Circulatory Tissue

The circulatory system provides the primary means of communication among the reproductive tissues. In order to fully appreciate how this communication system works, it is necessary to review some basic principles of circulatory anatomy. Many of the reproductive organs contain endocrine cells that produce various regulatory chemicals in response to particular blood-borne signals. These regulatory substances are secreted into the extracellular fluids and then diffuse locally to affect adjacent cells and/or enter the blood to affect other, more distant cell types. Endocrine tissues are richly supplied with blood. Arterial blood flows into this area via an arteriole (small artery), which supplies a capillary plexus or network (Figure 3-9). A capillary is an extremely small and delicate blood vessel. The wall of capillary is formed by a single layer of endothelial cells and associated pericytes (undifferentiated cells that can transform into cells such as fibroblasts and smooth muscle cells). Both cell types are embedded in a basement membrane. Small slits (pores) form at the borders between adjacent endothelial cells. This allows substances of low molecular size to pass between the blood and surrounding extracellular fluid. There are five types of capillaries. Continuous capillaries consist of endothelial cells that lack pores. This prevents exchange of fluids between the blood and surrounding extracellular fluid. This type of capillary is commonly found in nervous tissues and forms the so-called blood-brain barrier. In contrast, the endothelial cells of fenestrated capillaries contain pores through which small amounts of fluid can be transported. These exist primarily in endocrine glands, intestines, and kidneys. There are areas in the central nervous system with this type of capillaries. These regions seem to be important in the transduction of humoral signals into neuronal signals. For examples, certain regions detect blood concentrations of sodium in order to help the brain regulate circulating concentrations of this ion. These areas might also be involved with mediating the effects of metabolic state on reproductive activity. The third type capillary is the sinusoidal capillary. These capillaries are larger and more irregular in shape than the former types and have an inconspicuous basement membrane. They are found in endocrine organs as well as the carotid and aortic bodies. Sinusoids are larger than sinusoidal capillaries and they frequently lack a basement membrane. Venous sinuses are the largest type of capillary and consist of only endothelial cells and a discontinuous basement membrane. These provide the means for blood to drain from the brain. Regardless of the type of capillary, branches of the capillary plexus converge to form a venule (small vein), which returns blood to the venous circulation. Blood flow into capillary beds is regulated by smooth muscle cells called precapillary sphincters.

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Although the transition between arterial and venous blood typically occurs in a capillary bed, direct connections (arteriovenous anastomoses) between a small artery and vein permit a short-circuiting of capillary beds (Figure 3-9). These arrangements allow blood to be shunted away from tissues during times of low activity (e.g., the digestive tract between meals), or to assist with temperature regulation (shunting blood to chilled areas or shunting blood to an appendage and/or surface to promote cooling). Arteriovenous anastomoses also permit a counter current exchange of substance; that is, direct transfer of material between blood vessels. These structures are not necessarily required for counter current exchange. For example, heat or molecules that will diffuse across the walls of blood vessels can be transferred between arteries and veins that lie in very close proximity to each other. You will encounter this latter type of vascular arrangement in both the male and female reproductive tracts.

Erectile tissue is a unique type of vascular tissue found primarily in the reproductive system (e.g., penis and clitoris), but also exists in nasal tissue. Such tissue is characterized by numerous, tightly packed spaces which are lined by endothelial cells. These areas are supplied by arterioles which are innervated by neurons which keep vessels constricted (closed). During periods of stimulation, the arterioles relax and allow the spaces to engorge with blood.

Nervous Tissue

Nerve cells provide another means for communication among reproductive tissues. As noted earlier, the activity of the reproductive system proper (i.e., the gonads, genital ducts, and external genitalia) is regulated by the pituitary gland, which is regulated by the brain. A good deal of our later discussions will focus on how the hypothalamus regulates pituitary function as well as how reproductive organs communicate with the nervous system. Therefore, it is worthwhile to review some basic information about how information is transmitted within the nervous system. A schematic drawing of a nerve cell, also called a neuron, is shown in Figure 3-10. The neuron consists of a cell body, dendrites (processes emanating from the cell body), an axon, and terminal branches of the axon. Neurons transmit information in the following way. Upon stimulation, the cell body becomes depolarized. This initiates a wave of depolarization along the axon away from the cell body and into the axon terminals. Depolarization of the axon terminals induces the release of chemical messengers. In cases where the axon terminals end in close proximity to another cell, the chemical messengers are called neurotransmitters. The area in which an axon terminal makes contact with another cells is called a synapse (Figure 3-10). Neurotransmitters released from the presynaptic neuron can either stimulate or inhibit the postsynaptic cell. When the chemical messenger is released into the extracellular space and enters a nearby capillary, the chemical is called a neurohormone. Neurons that conduct impulses to other cells typically have myelin sheaths, which enhance speed of transmission. Nerve cells that secrete neurohormones are not myelinated.

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Finally, it is important to clarify some terminology regarding the organization of neurons. A nerve fiber is another name for axon. A bundle of parallel nerve fibers within the central nervous system is called a tract. A similar bundle of fibers in the peripheral nervous system is called a nerve. A grouping of nerve cell bodies is called a nucleus in the central nervous system, and a ganglion outside the brain and spinal cord.

Muscle Tissue

Both smooth muscle and skeletal muscle play important roles in regulation of reproductive activity. Obviously, the skeletal muscles that permit voluntary movement play are essential in sexual behaviors. You are undoubtedly familiar with the basic structure and function of skeletal muscle. The structure and function of smooth muscle (Figure 3-11) may be less familiar.

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The genital ducts of males and females are lined with a layer of smooth muscle, which is primarily concerned with transport. Rhythmic contractions of smooth muscle in these tissues facilitate movement of fluids and gametes along the lumens of these organs. The smooth muscle cells of these tissues are arranged in two ways; longitudinally (along the length of the duct) and circular (as a concentric ring around the lumen of the duct). Smooth muscle cells are identified by their spindle shape and a centrally-located nucleus. They lack visible striations due to the lack of an ordered arrangement of contractile fibers. These cells are regulated by the autonomic nervous system which evokes either contraction or relaxation of smooth muscle cells. Microscopic examination reveals that axons of autonomic nerve cells terminate near smooth muscle cells, but do not form the distinctive neuromuscular junctions characteristic of skeletal muscle tissue. In the genital ducts, contraction of the longitudinal layer causes a shortening of the tube, whereas contraction of the circular layer causes the diameter of the tube to shrink. Humoral factors also affect activity of smooth muscle cells.

ORGANIZATION OF ORGANS

There are three major types of organs: tubular, solid, and membranous. In each case, two general types of tissues can be distinguished. The cell types that give the organ the ability to perform a particular function make up the parenchyma. These are usually epithelial cells. For example, the gametes and cells that support gamete development make up the parenchyma of the testis and ovary. Likewise, neurons are the parenchymal cells of the central nervous system. The remaining tissues (i.e., connective tissues) are known as the stroma.

Structure of Tubular Organs

Tubular organs are made up of concentric layers of tissues (tunics). Organs such as those found in the digestive tract have four tunics: tunica mucosa, tela submucosa, tunica muscularis, and tunica adventitia or tunica serosa (Figure 3-12). There is significant variation in the arrangement of these layers. Depending on the organ, particular layers may be missing or altered.

The tunica serosa, or outermost layer, faces the body cavity and consists of a surface layer of mesothelium reinforced by irregular fibroblastic connective tissue. Its major function is to provide support as well as the tissue through which nerves, blood vessels, and lymphatics enter an organ. Tubular organs not closely associated with the celomic cavities (e.g., urethra) have a different type of outer covering; tunica adventitia. This is a loose connective tissue devoid of mesothelium. The tunica muscularis lies beneath the serosa. In its most organized state, this layer consists of two layers of smooth muscle; an outer longitudinal layer (running the length of the tube) and an inner circular layer (concentric to the circumference of the tube). These layers provide the means for tubular organs to contract. The next layer of tissue is the tela submucosa, a loosely organized layer of connective tissue containing large blood vessels, nerves, nerve plexes, and autonomic ganglia. The nerves in this layer regulate motility of the tunica muscularis. The tunica mucosa is the innermost layer of tissue, bordering on the lumen of the organ. Three sublayers can be distinguished within this layer. The lamina epithelialis mucosa consists of a layer of one or more types of epithelial cells. This outer layer is supported by the lamina propria mucosa, a connective tissue layer that contains small blood vessels and nerves that regulate and nourish the epithelial cells. In some cases, a lamina muscularis mucosa underlies the lamina propria mucosae. This thin layer of smooth muscle is less developed than the tunica mucosa, sometimes consisting of only a single layer of cells. It is not uncommon for the mucosal epithelium to evaginate into the lumen to form villous projections (villi). This layer might also invaginate into the lamina propria mucosa to form mucosal glands.

[FIGURE 3-12 OMITTED]

Structure of Solid Organs

Solid organs (e.g., testis, liver, and kidney) are enveloped by a capsule, or a dense collagenous connective tissue. Strands of looser connective tissue project from the capsule into the parenchyma and divide it into smaller units called lobules. These projections are called trabeculae. The capsule and trabeculae provide structural support for blood vessels and nerves to enter and leave the organ.

Structure of Membranous Organs

There are two types of membranes: mucous and serous. We have already encountered the former type; that is, the tunica mucosa of tubular organs is a mucous membrane. Mucous membranes consist of a layer of moist epithelial cells that rests on a layer of connective tissue. Secretions from the epithelial cells of the lamina epithelialis mucosa and/or glands within the lamina propria mucosa keep the outer lining moist.

Serous membranes are made up of a layer of mesothelium and surrounding connective tissue. These membranes line spaces within the body cavities and are moistened with the serous fluids found in these areas. The peritoneum is one familiar example of this organ type.

SUMMARY OF MAJOR CONCEPTS

* The reproductive system includes portions of the central nervous system (i.e., hypothalamus), pituitary gland, gonads, genital ducts, and external genitalia.

* Communication among reproductive organs occurs via humoral and neuronal mechanisms.

* Anatomic descriptions are facilitated by terminology that identifies direction and major body cavities.

* Each of the major tissue types (epithelial, connective, nervous, and muscle) is involved with reproductive function.

* Organs are identified by the types of cells making up their parenchymal tissue as well as the organization of the parenchyma and stroma.

DISCUSSION

1. Imagine that you are consulted for an opinion regarding the infertility of a 1-year-old dog. The semen from dog does not contain sperm cells and the testes appear atrophied. Overall, the dog's appearance resembles that of a puppy; that is, the animal doesn't have the masculine traits of normal adult males of a similar age. On hunch, you decide to give the dog daily injections of GnRH. Within a month, the dog's testes enlarge and start producing sperm cells. Moreover, the dog looks more like an adult. The owner is pleased and asks you what sort of miracle you performed. How would you explain your treatment? What would you conclude if your treatment failed to produce noticeable effects?

2. What organs would you expect to find in the following locations: dorsal cavity, caudal portion of the abdominal cavity, thoracic cavity, lateral to the heart, superficial to the abdominal muscles?

3. Why wouldn't you expect to see stratified squamous epithelium forming the small intestine, or simple cuboidal epithelium forming the external genitalia?

4. Suppose you take simultaneous blood samples from an artery supplying an organ and a vein that drains the organ. Also assume that this organ produces and secretes a hormone that exerts its effect somewhere else in the body. How would the concentrations of this hormone compare in the artery and vein from which you sampled? Explain your answer.

5. In general terms explain why some spinal injuries result in erectile dysfunction in males.

REFERENCES

Banks, W.J. 1993. Applied Veterinary Histology (Third Edition). St. Louis: Mosby Year Book.

Cochran, P.E. 2004. Laboratory Manual for Comparative Veterinary Anatomy and Physiology. Clifton Park, New York: Thomson Delmar Learning.

Frandson, R.D., W.L. Wilke, and A.D. Fails. 2003. Anatomy and Physiology of Farm Animals, 6th edition. Philadelphia: Lippincott, Williams & Wilkins.
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Author:Schillo, Keith K.
Publication:Reproductive Physiology of Mammals, From Farm to Field and Beyond
Geographic Code:1USA
Date:Jan 1, 2009
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Previous Article:Chapter 2: Life, reproduction, and sex.
Next Article:Chapter 4: Sexual differentiation.
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