3.CELLS AND HOMEOSTASIS INTRODUCTION

3.CELLS AND HOMEOSTASIS 



INTRODUCTION

• About 200 different types of specialized cells carry out a multitude of functions that help each system contribute to the homeostasis of the entire body. 
• At the same time, all cells share key structures and functions that support their intense activity.

• Atoms and molecules  comprise the alphabet of the language of the human body. 
• These are combined into about 200 different types of words called cells—living structural and functional units enclosed by a membrane. 

• All cells arise from existing cells by the process of cell division, in which one cell divides into two identical cells. 
• Different types of cells fulfill unique roles that support homeostasis and contribute to the many functional capabilities of the human organism. 

• Cell biology is the study of cellular structure and function. 

• As you study the various parts of a cell and their relationships to one another, you will learn that cell structure and function are intimately related. 

• In this chapter, you will learn that cells carry out a dazzling array of chemical reactions to create and maintain life processes—in part, by isolating specific types of chemical reactions within specialized cellular structures.

THE FEMALE REPRODUCTIVE CYCLE

Oogenesis and Follicular Development

• The formation of gametes in the ovaries is termed oogenesis
• (o¯ -o¯ -JEN-e-sis; oo- egg). 

• In contrast to spermatogenesis, which begins in males at puberty, oogenesis begins in females before they are even born. 

• Oogenesis occurs in essentially the same manner as spermatogenesis; 
• meiosis  takes place and the resulting germ cells undergo maturation.

• During early fetal development, primordial (primitive) germ cells migrate from the yolk sac to the ovaries. 
• There, germ cells differentiate within the ovaries into oogonia (o¯ -o¯ -GO¯ -ne¯ -a; singular is oogonium). 
• Oogonia are diploid (2n) stem cells that divide mitotically to produce millions of germ cells.
• Even before birth, most of these germ cells degenerate in a process known as atresia (a-TRE¯ -ze¯ -a). 

• A few, however, develop into larger cells called primary oocytes (O¯ -o¯-sı¯tz) that enter prophase of meiosis I during fetal development but do not complete that phase until after puberty. 
• During this arrested stage of development, each primary oocyte is surrounded by a single layer of flat follicular cells, and the entire structure is called a primordial follicle.

•  The ovarian cortex surrounding the primordial follicles consists of collagen fibers and fibroblast-like stromal cells. 

• At birth, approximately 200,000 to 2,000,000 primary oocytes remain in each ovary. 
• Of these, about 40,000 are still present at puberty, 
• and around 400 will mature and ovulate during a woman’s reproductive lifetime. 
• The remainder of the primary oocytes undergo atresia.

• Each month after puberty until menopause, gonadotropins (FSH and LH) secreted by the anterior pituitary further stimulate the development of several primordial follicles, although only one will typically reach the maturity needed for ovulation. 

• A few primordial follicles start to grow, developing into primary follicles. 
• Each primary follicle consists of a primary oocyte that is surrounded in a later stage of development by several layers of cuboidal and low-columnar cells called granulosa cells. 
• The outermost granulosa cells rest on a basement membrane.

• As the primary follicle grows, it forms a clear glycoprotein layer called the zona pellucida (pe-LOO-is-da) between the primary oocyte and the granulosa cells. 

• In addition, stromal cells surrounding the basement membrane begin to form an organized layer called the theca folliculi (THE¯ -ka fo-LIK-u¯ -li).

• With continuing maturation, a primary follicle develops into a secondary follicle . 
• In a secondary follicle, the theca differentiates into two layers: 

(1) the theca interna, 


     - a highly vascularized internal layer of cuboidal secretory cells that secrete estrogens and

 (2) the theca externa, 

     - an outer layer of stromal cells and collagen fibers. 

• In addition, the granulosa cells begin to secrete follicular fluid, which builds up in a cavity called the antrum in the center of the secondary follicle. 

• The innermost layer of granulosa cells becomes firmly attached to the zona pellucida 
• and is now called the corona radiata (corona = crown; radiata = radiation) 

• The secondary follicle eventually becomes larger, turning into a mature (graafian) follicle . 

• While in this follicle, and just before ovulation, the diploid primary oocyte completes meiosis I, producing two haploid (n) cells of unequal size—each with 23 chromosomes . 
• The smaller cell produced by meiosis I, called the first polar body,

            -  is essentially a packet of discarded nuclear material. 

• The larger cell, known as the secondary oocyte,              - receives most of the cytoplasm.

• Once a secondary oocyte is formed, it begins meiosis II but then stops in metaphase. 
• The mature (graafian) follicle soon ruptures and releases its secondary oocyte, a process known as ovulation.

• At ovulation, the secondary oocyte is expelled into the pelvic cavity together with the first polar body and corona radiata.

• Normally these cells are swept into the uterine tube. 

• If fertilization does not occur, the cells degenerate. 
• If sperm are present in the uterine tube and one penetrates the secondary oocyte, however, meiosis II resumes. 
• The secondary oocyte splits into two haploid cells, again of unequal size. 

• The larger cell is the ovum, or mature egg; 
•  the smaller one is the second polar body. 

• The nuclei of the sperm cell and the ovum then unite, forming a diploid zygote. 

• If the first polar body undergoes another division to produce two polar bodies, then the primary oocyte ultimately gives rise to three haploid polar bodies, which all degenerate, and a single haploid ovum. 

• Thus, one primary oocyte gives rise to a single gamete (an ovum). 

• By contrast, in males one primary spermatocyte produces four gametes (sperm).

THE FEMALE REPRODUCTIVE CYCLE

During their reproductive years, 

• nonpregnant females normally exhibit cyclical changes in the ovaries and uterus. 

• Each cycle takes about a month 
• and involves both 

                      - oogenesis and

                      -  preparation of the uterus to receive a fertilized ovum.

• Hormones secreted by

1. the hypothalamus, 
2. anterior pituitary, and 
3. ovaries 

         - control the main events. 

• The ovarian cycle is a series of events in the ovaries that occur during and after the maturation of an oocyte

• The uterine (menstrual) cycle is a concurrent series of changes in the endometrium of the uterus to prepare it for the arrival of a fertilized ovum that will develop there until birth.

•  If fertilization does not occur, ovarian hormones wane, 

          - which causes the stratum functionalis of the endometrium to slough off. 

• The general term female reproductive cycle encompasses 

a. the ovarian and uterine cycles, 

b. the hormonal changes that regulate them, 

c. and the related cyclical changes in the breasts and cervix.

Hormonal Regulation of the Female Reproductive Cycle

1. Gonadotropin-releasing hormone (GnRH) 

• secreted by the hypothalamus

• controls the ovarian and uterine cycles 

• stimulates the release of

a.  follicle-stimulating hormone (FSH) and 
b.  luteinizing hormone (LH) from the anterior pituitary. 

FSH 

• initiates follicular growth, 
• Under the influence of FSH, the androgens are taken up by the granulosa cells of the follicle and then converted into estrogens.

while LH 

• stimulates further development of the ovarian follicles. 
• LH stimulates the theca cells of a developing follicle to produce androgens.
• At midcycle, LH triggers ovulation 
• and then promotes formation of the corpus luteum, 
• the reason for the name luteinizing hormone.
• Stimulated by LH, the corpus luteum produces and secretes estrogens, progesterone, relaxin, and inhibin.

In addition, both FSH and LH 

• stimulate the ovarian follicles to secrete estrogens.

  

• At least six different estrogens have been isolated from the plasma of human females, 
• but only three are present in significant quantities: 

1. beta ( )-estradiol, 
2. estrone, and 
3. estriol. 

• In a nonpregnant woman, the most abundant estrogen is -estradiol,
• which is synthesized from cholesterol in the ovaries.

Estrogens secreted by ovarian follicles have several important functions

A.  Estrogens promote the development and maintenance of

1. female reproductive structures, 
2. secondary sex characteristics,
3. and the breasts. 

The secondary sex characteristics include

• distribution of adipose tissue in the breasts, abdomen, monspubis, and hips;
• voice pitch; 
• a broad pelvis; 
• and pattern of hair growth on the head and body.

 B. Estrogens increase protein anabolism, including the building of strong bones. 

• In this regard, estrogens are synergistic with human growth hormone (hGH).

C.  Estrogens lower blood cholesterol level, 

• which is probably the reason that women under age 50 have a much lower risk of coronary artery disease than do men of comparable age.

D.  Moderate levels of estrogens in the blood inhibit 

• both the release of GnRH by the hypothalamus 
• and secretion of LH and FSH by the anterior pituitary.

 Progesterone, 

• secreted mainly by cells of the corpus luteum,

1. cooperates with estrogens to prepare and maintain the endometrium 

• for implantation of a fertilized ovum 
• and to prepare the mammary glands for milk secretion. 

2. High levels of progesterone

• also inhibit secretion of GnRH and LH.

Relaxin 

• The small quantity of relaxin produced by the corpus luteum during each monthly cycle                           - relaxes the uterus by inhibiting contractions of the myometrium. 

• Presumably, implantation of a fertilized ovum occurs more readily in a “quiet” uterus. 

During pregnancy,

•  the placenta produces much more relaxin, 
• and it continues to relax uterine smooth muscle. 

At the end of pregnancy,

• relaxin also increases the flexibility of the pubic symphysis and
• may help dilate the uterine cervix, both of which ease delivery of the baby.

Inhibin

• is secreted by granulosa cells of growing follicles and by the corpus luteum after ovulation. 
• It inhibits secretion of FSH and, to a lesser extent, LH.

Phases of the Female Reproductive Cycle

• The duration of the female reproductive cycle typically ranges from 24 to 35 days.
• we assume a duration of 28 days and 
• divide it into four phases: 

1. the menstrual phase, 
2. the preovulatory phase, 
3. ovulation, and
4. the postovulatory phase

1. Menstrual Phase

• The menstrual phase (MEN-stroo-al), also called menstruation (men -stroo-A¯ -shun) or menses (MEN-se¯z month), 
• lasts for roughly the first 5 days of the cycle.
•  (By convention, the first day of menstruation is day one of a new cycle.)

a. EVENTS IN THE OVARIES 

Under the influence of FSH, 

• several primordial follicles develop into primary follicles 
• and then into secondary follicles. 

           - This developmental process may take several months to occur. 

• Therefore, a follicle that begins to develop at the beginning of a particular menstrual cycle may not reach maturity and ovulate until several menstrual cycles later.

b. EVENTS IN THE UTERUS 

Menstrual flow from the uterus consists
of 

• 50–150 mL of blood, 
• tissue fluid, 
• mucus, and
•  epithelial cells shed from the endometrium. 

This discharge occurs because

• the declining levels of progesterone and estrogens stimulate release of prostaglandins 

                    - that cause the uterine spiral arterioles to constrict. 

• As a result, the cells they supply become oxygen-deprived and start to die. 
• Eventually, the entire stratum functionalis sloughs off. 

• At this time the endometrium is very thin, about 2–5 mm, because only the stratum basalis remains. 
• The menstrual flow passes from the uterine cavity through the cervix and vagina to the exterior.

2. Preovulatory Phase

The preovulatory phase 

• is the time between the end of menstruation and ovulation. 

• The preovulatory phase of the cycle is more variable in length than the other phases and 
• accounts for most of the differences in length of the cycle.
•  It lasts from days 6 to 13 in a 28-day cycle.

a. EVENTS IN THE OVARIES 

• Some of the secondary follicles in the ovaries begin to secrete estrogens and inhibin. 
• By about day 6, a single secondary follicle in one of the two ovaries has outgrown all the others to become the dominant follicle. 

Estrogens and inhibin

• secreted by the dominant follicle 
• decrease the secretion of FSH, 
• which causes other, less well-developed follicles to stop growing and undergo atresia.

 Fraternal (nonidentical) twins or triplets 

• result when two or three secondary follicles become codominant 
• and later are ovulated and fertilized at about the same time.

Normally, 

• the one dominant secondary follicle becomes the mature (graafian) follicle,
•  which continues to enlarge 
• until it is more than 20 mm in diameter and ready for ovulation . 

• This follicle forms a blisterlike bulge due to the swelling antrum on the surface of the ovary. 

During the final maturation process,

•  the mature follicle continues to increase its production of estrogens .

With reference to the ovarian cycle,

•  the menstrual and preovulatory phases together are termed the follicular phase (fo¯- LIK-u¯-lar) 
• because ovarian follicles are growing and developing.

b. EVENTS IN THE UTERUS 

Estrogens liberated into the blood by growing ovarian follicles 

• stimulate the repair of the endometrium;
• cells of the stratum basalis undergo mitosis and produce a new stratum functionalis. 

As the endometrium thickens,

• the short, straight endometrial glands develop, 
• and the arterioles coil and lengthen as they penetrate the stratum functionalis. 
• The thickness of the endometrium approximately doubles, to about 4–10 mm. 

With reference to the uterine cycle, the preovulatory phase is also termed the proliferative phase 

• because the endometrium is proliferating.

Ovulation

•  the rupture of the mature (graafian) follicle and the release of the secondary oocyte into the pelvic cavity, 
• usually occurs on day 14 in a 28-day cycle. 

During ovulation, 

• the secondary oocyte remains surrounded by its zona pellucida and corona radiata.

• The high levels of estrogens during the last part of the preovulatory phase exert a positive feedback effect on the cells that secrete LH and gonadotropin-releasing hormone (GnRH) and cause ovulation, as follows 

1. A high concentration of estrogens 

• stimulates more frequent release of GnRH from the hypothalamus.
•  It also directly stimulates gonadotrophs in the anterior pituitary to secrete LH.

2.  GnRH 

• promotes the release of FSH and additional LH by the anterior pituitary.

3. LH

•  causes rupture of the mature (graafian) follicle 
•  and expulsion of a secondary oocyte about 9 hours after the peak of the LH surge.
•  The ovulated oocyte and its corona radiata cells are usually swept into the uterine tube.

• From time to time, an oocyte is lost into the pelvic cavity,
• where it later disintegrates. 

• The small amount of blood that sometimes leaks into the pelvic cavity from the ruptured follicle can cause pain, known as mittelschmerz (MIT-el-shma¯rts pain in the middle), at the time of ovulation.

• An over-the-counter home test that detects a rising level of LH can be used to predict ovulation a day in advance.

3. Postovulatory Phase


The postovulatory phase of the female reproductive cycle 

• is the time between ovulation and onset of the next menses. 

In duration,

• it is the most constant part of the female reproductive cycle
• It lasts for 14 days in a 28-day cycle, 
• from day 15 to day 28 .

a. EVENTS IN ONE OVARY 

After ovulation, 

• the mature follicle collapses,
•  and the basement membrane between the granulosa cells and theca interna breaks down. 

Once a blood clot forms from minor bleeding of the ruptured follicle, 

• the follicle becomes the corpus hemorrhagicum (hemo- blood; rrhagic-  bursting forth) . 

• Theca interna cells mix with the granulosa cells as they all become transformed into corpus luteum cells under the influence of LH. 
• Stimulated by LH, the corpus luteum secretes progesterone, estrogen, relaxin, and inhibin.

• The luteal cells also absorb the blood clot. 

• With reference to the ovarian cycle, this phase is also called the luteal phase.

• Later events in an ovary that has ovulated an oocyte depend on whether the oocyte is fertilized.

 If the oocyte is not fertilized,

• the corpus luteum has a lifespan of only 2 weeks. 
• Then, its secretory activity declines, 
• and it degenerates into a corpus albicans. 
• As the levels of progesterone, estrogens, and inhibin decrease, release of GnRH, FSH, and LH rises due to loss of negative feedback suppression by the ovarian hormones.

• Follicular growth resumes and a new ovarian cycle begins.

If the secondary oocyte is fertilized and begins to divide,

• the corpus luteum persists past its normal 2-week lifespan.
•  It is “rescued” from degeneration by human chorionic gonadotropin (hCG). 
• This hormone is produced by the chorion of the embryo beginning about 8 days after fertilization.
• Like LH, hCG stimulates the secretory activity of the corpus luteum.

• The presence of hCG in maternal blood or urine is an indicator of pregnancy and
•  is the hormone detected by home pregnancy tests.

b. EVENTS IN THE UTERUS 


Progesterone and estrogens produced by the corpus luteum 

• promote growth and coiling of the endometrial glands, 
• vascularization of the superficial endometrium, and 
• thickening of the endometrium to 12–18 mm (0.48–0.72 in.).

Because of the secretory activity of the endometrial glands, which begin to secrete glycogen, 

• this period is called the secretory phase of the uterine cycle. 

• These preparatory changes peak about one week after ovulation, at the time a fertilized ovum might arrive in the uterus. 

If fertilization does not occur,

• the levels of progesterone and estrogens decline due to degeneration of the corpus luteum. 
• Withdrawal of progesterone and estrogens causes menstruation.

28. ANATOMY OF FEMALE REPRODUCTIVE SYSTEM B. FEMALE INTERNAL REPRODUCTIVE ORGANS

B. FEMALE INTERNAL REPRODUCTIVE ORGANS


include 

1. Ovaries (female gonads);
2. Uterine (fallopian) tubes, or oviducts; 
3. Uterus; 
4. Vagina; 




1. Ovaries

• The ovaries ( egg receptacles), 
• which are the female gonads,
• are paired glands 
• that resemble unshelled almonds in size and shape; 

• they are homologous to the testes. (Here homologous means that two organs have the same embryonic origin.) 

• The ovaries produce 

        (1) gametes, 

                    - secondary oocytes that develop into mature ova (eggs) after fertilization, and 

        (2) hormones, including
               - progesterone 
               - and estrogens (the female sex hormones),
               - inhibin, 
               - and relaxin.

• The ovaries, one on either side of the uterus,

• descend to the brim of the superior portion of the pelvic cavity during the third month of development. 

• A series of ligaments holds them in position. 

1. The broad ligament of the uterus , which is itself part of the parietal peritoneum, attaches to the ovaries by a double-layered fold of peritoneum called the mesovarium. 


2. The ovarian ligament anchors the ovaries to the uterus, and 

3. The suspensory ligament attaches them to the pelvic wall. 

• Each ovary contains a hilum, 

           - the point of entrance and exit for blood vessels and nerves along which the mesovarium is attached.



Histology of the Ovary


Each ovary consists of the following parts :


1.  The germinal epithelium (germen sprout or bud)

•  is a layer of simple epithelium (low cuboidal or squamous) that covers the surface of the ovary. 

(We now know that the term germinal epithelium in humans is not accurate because it does not give rise to ova; the name came about because, at one time, people believed that it did. We have since learned

that the cells that produce ova arise from the yolk sac and migrate to the ovaries during embryonic development.)


2.  The tunica albuginea 

• is a whitish capsule of dense irregular connective tissue 
• located immediately deep to the germinal epithelium.

3.  The ovarian cortex 

• is a region just deep to the tunica albuginea.
• It consists of ovarian follicles 
• surrounded by dense irregular connective tissue that contains

            - collagen fibers and 
            - fibroblast-like cells called stromal cells.



4. The ovarian medulla 

• is deep to the ovarian cortex. 
• The border between the cortex and medulla is indistinct, 
• but the medulla consists of more loosely arranged connective tissue 
• and contains

• blood vessels,
• lymphatic vessels, and 
• nerves.

5.  Ovarian follicles 

• (folliculus little bag) 
• are in the cortex 
• and consist of oocytes in various stages of development, 
• plus the cells surrounding them. 

• When the surrounding cells form a single layer, they are called follicular cells; 
• later in development, when they form several layers, they are referred to as granulosa cells. 

• The surrounding cells nourish the developing oocyte and 
• begin to secrete estrogens as the follicle grows larger.

6.  A mature (graafian) follicle 

• is a large, fluid-filled follicle
• that is ready to rupture and expel its secondary oocyte, a process known as ovulation.

7.  A corpus luteum 

• ( yellow body) 
• contains the remnants of a mature follicle after ovulation. 

• The corpus luteum produces

             - progesterone, 

             - estrogens, 
             - relaxin, and 
             - inhibin 

• until it degenerates into fibrous scar tissue called the corpus albicans ( white body).

2. Uterine Tubes

• Females have two uterine (fallopian) tubes, or oviducts, 
• that extend laterally from the uterus . 

• The tubes, which measure about 10 cm (4 in.) long, 
• lie between the folds of the broad ligaments of the uterus. 

•  They provide a route for sperm to reach an ovum and transport secondary oocytes and fertilized ova from the ovaries to the uterus. 

• The funnel-shaped portion of each tube, called the infundibulum, 
• is close to the ovary 
• but is open to the pelvic cavity. 

• From the infundibulum the uterine tube extends medially and eventually inferiorly and attaches to the superior lateral angle of the uterus.

 

• It ends in a fringe of fingerlike projections called fimbriae (FIM-bre¯ -e¯ fringe), 

• one of which is attached to the lateral end of the ovary. 

The ampulla of the uterine tube 

• is the widest, longest portion, 
• making up about the lateral two-thirds of its length. 

The isthmus of the uterine tube 

• is the more medial, 
• short, narrow, thick-walled portion that joins the uterus.

HISTOLOGY 


Histologically, 

The uterine tubes are composed of three layers:

1. mucosa, 
2. muscularis, and 
3. serosa. 

The mucosa 

•  consists of epithelium and lamina propria (areolar connective tissue). 
• The epithelium contains 

a. ciliated simple columnar cells, 

          - which function as a “ciliary conveyor belt” to help move a fertilized ovum (or secondary
oocyte) within the uterine tube toward the uterus, and

b. nonciliated cells called peg cells, 
     
             - which have microvilli and
            -  secrete a fluid that provides nutrition for the ovum . 


The middle layer, the muscularis,

•  is composed of 

      a. an inner,thick,circular ring of smooth muscle 

      b. an outer, thin region of longitudinal smooth muscle. 

• 1. Peristaltic contractions of the muscularis and 2. the ciliary action of the mucosa 

                     - help move the oocyte or fertilized ovum toward the uterus. 

• The outer layer of the uterine tubes is a serous membrane, the serosa. 
• Local currents produced by movements of the fimbriae,

              - which surround the ovary during ovulation, 
             - sweep the ovulated secondary oocyte from the pelvic cavity into the uterine tube.

•  A sperm cell usually encounters and fertilizes a secondary oocyte in the ampulla of the uterine tube, although fertilization in the pelvic cavity is not uncommon. 

• Fertilization can occur at any time up to about 24 hours after ovulation. 
• Some hours after fertilization, the nuclear materials of the haploid ovum and sperm unite.

•  The diploid fertilized ovum is now called a zygote and 
• begins to undergo cell divisions while moving toward the uterus.
•  It arrives at the uterus 6 to 7 days after ovulation.

3. Uterus

• The uterus (womb) serves as part of the pathway for sperm deposited in the vagina to reach the uterine tubes. 
• It is also the site of implantation of a fertilized ovum, development of the fetus during pregnancy, and labor. 

• During reproductive cycles when implantation does not occur, the uterus is the source of menstrual flow.

Anatomy of the Uterus

• Situated between the urinary bladder and the rectum, 

• the uterus is the size and shape of an inverted pear .

•  In females who have never been pregnant, it is about 
• 7.5 cm (3 in.) long, 
• 5 cm (2 in.) wide, and 
• 2.5 cm (1 in.) thick. 

• The uterus is larger in females who have recently been pregnant, 
• and smaller (atrophied) when sex hormone levels are low, as occurs after menopause.

Anatomical subdivisions of the uterus 

include: 

(1) a domeshaped portion superior to the uterine tubes called the fundus,
(2) a tapering central portion called the body, and (3) an inferior narrow portion called the cervix that opens into the vagina.

• Between the body of the uterus and the cervix is the isthmus
• (IS-mus), 
• a constricted region 
• about 1 cm (0.5 in.) long. 

• The interior of the body of the uterus is called the uterine cavity, and
• the interior of the cervix is called the cervical canal. 

The cervical canal 

• opens into the uterine cavity at the internal os (os mouthlike opening) 
• and into the vagina at the external os.

Normally, 

• the body of the uterus projects anteriorly and superiorly over the urinary bladder in a position called anteflexion.

• The cervix projects inferiorly and posteriorly and enters the anterior wall of the vagina at nearly a right angle.

•  Several ligaments that are either extensions of the parietal peritoneum or fibromuscular cords maintain the position of the uterus. 

1. The paired broad ligaments are double folds of peritoneum attaching the uterus to either side of the pelvic cavity. 

2. The paired uterosacral ligaments, also
peritoneal extensions, lie on either side of the rectum and connect the uterus to the sacrum. 

3. The cardinal (lateral cervical) ligaments 

• are located inferior to the bases of the broad ligaments and 
• extend from the pelvic wall to the cervix and vagina.

4. The round ligaments 

• are bands of fibrous connective tissue between the layers of the broad ligament; 
• they extend from a point on the uterus just inferior to the uterine tubes to a portion of the labia majora of the external genitalia. 

• Although the ligaments normally maintain the anteflexed position of the uterus,
•  they also allow the uterine body enough movement such that the uterus may become malpositioned. 

• A posterior tilting of the uterus, called retroflexion (retro- backward or behind), 
• is a harmless variation of the normal position of the uterus. 
• There is often no cause for the condition, but it may occur after childbirth.

Histology of the Uterus


Histologically, 

• the uterus consists of three layers of tissue:

1. perimetrium, 
2. myometrium, and
3. endometrium 


The outer layer—the perimetrium (peri- around; -metrium uterus) or serosa—

• is part of the visceral peritoneum;
• it is composed of simple squamous epithelium and areolar connective tissue.

• Laterally, it becomes the broad ligament.

• Anteriorly, it covers the urinary bladder and forms a shallow pouch, the vesicouterine pouch (ves -i-ko¯-U¯ -ter-in; vesico- bladder; . 

• Posteriorly, it covers the rectum and forms a deep pouch between the uterus and urinary bladder, the rectouterine pouch (rek-to¯-U¯ -ter-in; recto- rectum) or pouch of Douglas
• — the most inferior point in the pelvic cavity.

The middle layer of the uterus, the myometrium (myo- muscle), 

• consists of three layers of smooth muscle fibers that are

1. thickest in the fundus and 
2. thinnest in the cervix. 

• The thicker middle layer is circular; 
• the inner and outer layers are longitudinal or oblique. 

• During labor and childbirth, coordinated contractions of the myometrium in response to oxytocin from the posterior pituitary help expel the fetus from the uterus.

• The inner layer of the uterus, the endometrium (endo- within), 
• is highly vascularized 

• and has three components: 

• (1) An innermost layer composed of simple columnar epithelium (ciliated secretory cells) lines the lumen.
• (2) An underlying endometrial stroma is a very thick region of lamina propria (areolar connective tissue). 
• (3) Endometrial (uterine) glands develop as invaginations of the luminal epithelium and extend almost to the myometrium. 

• The endometrium is divided into two layers.

A. The stratum functionalis (functional layer) 

• lines the uterine cavity 
• and sloughs off during menstruation. 

B. The deeper layer, the stratum basalis (basal layer), 

• is permanent 
• and gives rise to a new stratum functionalis after each menstruation.

Blood supply 

• Branches of the internal iliac artery called uterine arteries supply blood to the uterus. 

• Uterine arteries give off branches called arcuate arteries ( shaped like a bow) that
•  are arranged in a circular fashion in the myometrium. 

• These arteries branch into radial arteries that penetrate deeply into the myometrium. 

• Just before the branches enter the endometrium, they divide into two kinds of arterioles: 

a. Straight arterioles 

• supply the stratum basalis with the materials needed to regenerate the stratum functionalis; 

b. Spiral arterioles 

• supply the stratum functionalis 
• and change markedly during the menstrual cycle.

Venous drainage

Blood leaving the uterus is drained 

• by the uterine veins into the internal iliac veins. 

• The extensive blood supply of the uterus is essential to support regrowth of a new stratum functionalis after
• menstruation,
•  implantation of a fertilized ovum, and 
• development of the placenta.

Cervical Mucus

• The secretory cells of the mucosa of the cervix produce a secretion called cervical mucus, 
• a mixture of 

                 - water, 
                 - glycoproteins,
                 - lipids,
                 - enzymes, and
                 - inorganic salts.

• During their reproductive years, females secrete 20–60 mL of cervical mucus per day.

• Cervical mucus is more hospitable to sperm at or near the time of ovulation because 

a. it is then less viscous and
b.  more alkaline (pH 8.5). 

• At other times, a more viscous mucus forms a cervical plug that physically impedes sperm penetration. 

1. Cervical mucus

• supplements the energy needs of sperm, and

2.  both the cervix and cervical mucus 

• protect sperm from phagocytes and the hostile environment of the vagina and uterus. 

3. Cervical mucus may also play a role in capacitation

•    —a series of functional changes that sperm undergo in the female reproductive tract before they are able to fertilize a secondary oocyte. 

• Capacitation causes a sperm cell’s tail to beat even more vigorously, and it prepares the sperm cell’s plasma membrane to fuse with the oocyte’s plasma membrane.

4. Vagina


The vagina ( sheath) is 

• a tubular, 
• 10-cm (4-in.) 
• long fibromuscular canal 
• lined with mucous membrane 
• that extends from the exterior of the body to the uterine cervix . 

• It is the receptacle for the penis during sexual intercourse,
• the outlet for menstrual flow, 
• and the passageway for childbirth.

•  Situated between the urinary bladder and the rectum,
• the vagina is directed superiorly and posteriorly, where it attaches to the uterus. 

• A recess called the fornix ( arch or vault) surrounds the vaginal attachment to the cervix. 
• When properly inserted, a contraceptive diaphragm rests in the fornix, where it is held in place as it covers the cervix.

• The mucosa of the vagina is continuous with that of the uterus.

HISTOLOGY

 Histologically,

•  it consists of

•  nonkeratinized stratified squamous epithelium and 
• areolar connective tissue

            -that lies in a series of transverse folds called rugae (ROO-ge¯). 

• Dendritic cells in the mucosa are antigen-presenting cells . 
• Unfortunately, they also participate in the transmission of viruses—for example, HIV (the virus that causes AIDS)—to a female during intercourse with an infected male. 

• The mucosa of he vagina contains large stores of glycogen, the decomposition of which produces organic acids. 
• The resulting acidic environment retards microbial growth, but it also is harmful to sperm.

• Alkaline components of semen, mainly from the seminal vesicles,

                - raise the pH of fluid in the vagina and                     - increase viability of the sperm.


The muscularis is composed of 

A. an outer circular layer and
B. an inner longitudinal layer of smooth muscle
     - that can stretch considerably 

• to accommodate the penis during sexual intercourse and
•  a child during birth.

The adventitia, the superficial layer of the vagina, consists of
                - areolar connective tissue. 
It anchors the vagina to adjacent organs 

• such as the urethra and urinary bladder anteriorly and 
• the rectum and anal canal posteriorly.

• A thin fold of vascularized mucous membrane, called the hymen ( membrane), 
• forms a border around 
• and partially closes the inferior end of the vaginal opening to the exterior, the vaginal orifice . 

• After its rupture, usually following the first sexual intercourse, only remnants of the hymen remain. 
• Sometimes the hymen completely covers the orifice, a condition called imperforate hymen (im-PER-fo¯-ra¯t). 
• Surgery may be needed to open the orifice and permit the discharge of menstrual flow.