Objectives
You should be able to recognize the following:
| Ovary | Oviduct | ||||
| tunica albuginea | fimbriae | ||||
| germinal epithelium | ciliated columnar epithelium | ||||
| stroma | peg cells | ||||
| follicles | |||||
| oocyte | Uterus | ||||
| zona pellucida | endometrium | ||||
| granulosa layer | basal layer | ||||
| theca interna | functional layer | ||||
| theca externa | follicular stage | ||||
| antrum | luteal stage | ||||
| cumulus oophorus | endometrial glands | ||||
| follicular stages | spiral arteries | ||||
| primordial | myometrium | ||||
| primary | cervix | ||||
| secondary | vaginal surface epithelium | ||||
| mature = Graafian follicle | cervical glands | ||||
| atretic follicles | |||||
| early | Vagina | ||||
| late | stratified squamous epithelium | ||||
| glassy membrane | vascular laminar propria | ||||
| corpus luteum | |||||
| theca lutein cells | |||||
| granulosa lutein cells | |||||
| corpus albicans | |||||
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Blue Histology |
Slides
| D-51 | Uterus, follicular phase (H&E) | |
| D-52 | Uterus (H&E) | |
| D-53 | Uterus, luteal phase (H&E) | |
| D-54 | Corpus luteum (H&E) | |
| D-55 | Ovary (H&E) | |
| D-56, D-58 | Ovaries monkey (H&E) | |
| D-59 | Cervix (H&E) | |
| D-60 | Oviduct, child (H&E) | |
| D-61 | Vagina (H&E) | |
| D-72, D-73 | Ovaries monkey (H&E) | |
| D-81 | Oviduct and fimbriae, monkey (H&E) |
Optional
slide
| D-56 | Ovary and oviduct, monkey (H&E) | |
| D-62 | Clitoris (H&E) | |
| D-63 | Mammary gland (H&E) | |
| D-64 | Lactating Breast (H&E) |
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Prologue
The eggs in the ovary are organized into structures called follicles. These are embedded individually in the stroma of the outer portion (cortex) of the ovary. They undergo a complex growth process which produces both the mature female gametes (i.e. fertilizable eggs) and female sex hormones.
The smallest follicles, called primordial follicles, are dispersed in the cortex of the ovary under the tunica albuginea. Each contains a primary oocyte with a large round, pale nucleus, prominent nucleolus, and abundant pale-staining cytoplasm. The oocyte is surrounded by a single layer of flattened follicular cells. Primordial follicles form before birth. They then remain unchanged throughout childhood and into adult hood until some unknown mechanism triggers a particular follicle to start to grow and mature.
The early growing follicle is called a primary follicle. It is a histological unit consisting of a primary oocyte, one to several layers of cuboidal follicular cells, now called granulosa cells, and a basement membrane surrounding the follicle. While the follicle still has a single layer of granulosa cells it is termed unilaminar. Later it becomes a multilaminar primary follicle. A zona pellucida forms around the oocyte at about this transitional stage. Also, at this stage follicular development becomes dependent on FSH.
Primary follicles differentiate into secondary follicles with the following histological features:
| 1. | Fluid filled spaces open up within the mass of granulosa cells. These coalesce into a single antrum cavity. | |
| 2. | The granulosa cells form a stratified cuboidal epithelium. | |
| 3. | The egg (primary oocyte) is embedded in a thickened region of granulosa epithelium, called the cumulus oophorus. A strong acellular membrane, the zona pellucida, surrounds the egg and separates it from the surrounding granulosa cells. | |
| 4. | The stromal cells around the follicle, just outside of the basement membrane, specialize to form a theca interna and theca externa. |
A large follicle matures to the point that it will ovulate in response to a surge of LH from the pituitary. It bulges at the periphery of the ovary as a mature or Graafian follicle (see slide D-73). Its large cavity or antrum pushes the stratified granulosa cells to the periphery of the follicle. The oocyte is located on one end of the follicle and, if visible, appears buried in a mound of granulosa cells. This accumulation of cells is called the cumulus oophorus. The corona radiata is derived from these cells.
The distinction between a secondary and Graafian follicle is a physiological definition, not morphological. What a follicle looks like when it matures depends on the species. A Graafian follicle of an animal such as a small monkey might look like a secondary follicle of a human. Our Graafian follicles get huge - maybe as much as two centimeters across.
One potentially confusing aspect of terminology are the terms "primary" and "secondary". Applied to follicles, they stand for visible morphological stages of development. For the oocyte they indicate the stage of meiosis that the cell is in. An egg or ovum is called an oocyte while it is in the process of meiosis. It is a primary oocyte up until the first meiotic division and then a secondary oocyte until it completes the second division. Female germ cells enter meiosis before birth and the oocytes remain in arrested prophase of meiosis I for decades. Primary oocytes complete the first meiotic division just before ovulation. The released secondary oocyte will complete the second meiotic division shortly after fertilization occurs. After these two events, the (diploid) cell is called a zygote. Be aware, therefore, that secondary follicles contain primary oocytes.
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Slide descriptions
D-58 Ovary, monkey (H&E)
Four slides, D-56 D-58, D-73, D-74, contain sections of ovaries from monkeys. Begin with D-58 and use the other slides as auxiliary resources for observing all of the various stages of follicular development.
The sections of ovaries are ovoid in shape and have a smooth external contour except at the hilum where blood vessels enter. Microscopically, a layer of cuboidal cells called the germinal epithelium covers the external surface of the ovary (illustration). The name is a poor one in that it has nothing to do with the germinal cells. Actually, this epithelium is just a mesothelial layer which, as a curiosity, is cuboidal instead of squamous. If this covering is not visible, it was lost during histological processing. A dense connective tissue capsule, called the tunica albuginea, underlies this germinal epithelium. Deep to it the stroma of the ovary consists of a very cellular connective tissue. Its fibroblasts are fusiform with basophilic nuclei. Situated within the stroma are small, medium and large follicles, each size representing a stage of development. Blood vessels of various size and planes of section are also present. Pools of red blood cells in the stroma are artifacts.
Scan your slide under medium power for primary and secondary follicles in various stages of development and examine the large Graafian follicle if present. Verify the characteristics listed above. As a follicle gets bigger the chances that a random section through it will show the ovum decreases, even though the egg is growing as well. So do not be surprised that most large secondary follicles seem to lack an ovum and cumulus oophorus (illustration) .
Your slides also show many examples of atretic follicles. These are follicles which developed partway and then degenerated. Many primary follicles begin to develop during each ovarian cycle. Only one (from the two ovaries) will complete maturation and ovulate. The others die off along the way. These atretic follicles take a long time to fully degenerate. They present a diversity of appearances for two reasons: 1) follicles that have developed further before beginning to degenerate will give larger and more complex atretic forms. For example, secondary atretic follicles will have remains of the zona pellucida and perhaps thecal remnants, whereas follicles that began to degenerate as early primary follicles will not; 2) As atresia proceeds the follicles become more and more degenerate. In very old atretic follicles only a zona pellucida and maybe remains of the glassy membrane (which is formed from the basement membrane) is left. Various progressive stages of atresia are seen in example-1, example-2 and example-3, example-4. Most of the mottling and texture that you see in the stroma of the ovary represent remains of old follicles (illustration).
D-54, Corpus luteum
After the ovum is expelled from a Graafian follicle the remaining wall of the follicle collapses into a wrinkled oval with an internal cavity. LH stimulates the granulosa cells and the theca interna cells to proliferate and hypertrophy, a process called leuteinization. The granulosa cells become granulosa lutein cells and the theca internal cells become theca lutein cells. The cavity fills with loose connective tissue, the amount of which varies with the age of the corpus luteum (more connective tissue forms with time). In a young corpus luteum, red blood cells and fibrin deposits can be seen in this cavity resulting from hemorrhage occurring at ovulation.
The wall of the corpus luteum consists of three layers (illustration):
1) an inner, thick stratified layer of granulosa lutein cells. This layer is noticeably puckered because when these cells hypertrophied the layer had to wrinkle up. The granulosa lutein cells are large, polygonal, pale-staining cells and rich in accumulated lipid. They stain palely because alcohol dissolves out the lipid during histological processing (illustration). After ovulation the basement membrane between the granulosa epithelium and the theca degenerates. Capillaries then enter between the granulosa lutein cells.
2) a thinner irregular layer of theca lutein cells. These cells are smaller and more deeply stained than granulosa lutein cells. Most have gotten pushed up into columns of cells under the folds of the granulosa lutein cells.
3) an exterior capsule containing blood vessels. It immediately surrounds the theca lutein cells and contains flattened fibroblasts and vascular channels. Small branches of these blood vessels, accompanied by some connective tissue, run from the capsule up through the columns of thecal lutein cells.
D-55 Ovary, Human.
This slide gives you a view of the human ovary from an older woman. Examine it carefully by eye before jamming it under the microscope. Several sections through corpora albicans are present. These bodies are scars that replaced the tissue of the corpus luteums when they degenerated. They appear as irregular puffy pink bodies (locate them by eye before looking at them at low power). They are composed of a mass of delicate collagen fibers. At high magnification you can see these fibers and the embedded nuclei of the fibrocytes that made them (illustration). In one place you can see the partial remains of the corpus luteum as it is being replaced by scar tissue (illustration).
You can also see a smattering of large secondary follicles, a few primary follicles and some primordial follicles.
Slide D-81 Fallopian tube and fimbriae, monkey (H&E)
The oviduct ( = ovarian tube = Fallopian tube) is a long trumpet-shaped organ. An entire one was dissected out and plopped into formaldehyde. It ended up twisted around so that when it was sectioned the blade cut through the tube several times. Hold the slide up to the light. You should see four or five sections through the tube. Most of the tissue is part of the infundibulum, the large funnel-shaped opening into the abdomen. The margins of the infundibulum are drawn out into numerous tapering, finger-like processes, the fimbriae.
Under medium and high powers, look at the elaborate mucosal folds that become more elaborate towards the trumpet-shaped opening. Trace out one of these folds in a cross section to appreciate how complex it is, remembering that what you are looking at is actually just a section across an amazingly elaborate longitudinal fold. It makes the mucosal folds of the seminal vesicle which you will look at in the next laboratory session runts by comparison.
Focus in on the simple epithelium of the folded mucous membrane. You see a beautiful interspersion of broad pale ciliated cells and narrow secretory cells ("peg cells") (illustration). In some places the cilia show up superbly. Are basal cells present? Glance at the underlying lamina propria and check out the amount of smooth muscle in the wall.
D-60 Oviduct (child).
This slide gives you a chance to examine a section of human oviduct, although, as is typical of your slides, the animal tissues are better preserved. Examine D-60 as you did D-81, but you will have to choose an area carefully to make the epithelium worth studying. Decide how you would describe the orientation of the muscle layers (illustration) Are there smooth muscle cells in the folds of lamina propria? (don't ask an instructor, LOOK. I ask the question to encourage you to look and interpret what you see. In the grand scheme of things it probably does not matter one hoot whether smooth muscle does or does not extend into the mucosal folds of the oviduct of our species. But it does matter that you realize that you are able to recognize smooth muscle wherever you see it).
In contrast to the sorry state of the mucosa, the serosa is pretty well preserved in many places.
D-52 Uterus (H&E)
Under low power, determine the limits of the myometrium and endometrium. The perimetrium is shabby, so it is a good thing that you looked at the serous layer of the oviduct. Verify that the myometrium is comprised of bundles of smooth muscle cells separated by seams of connective tissue. The larger uterine arteries run in the middle part of this muscle layer.
Study the endometrium in detail. It is a mucosa although the epithelium is missing over much of the surface due to harsh treatment when the organ was collected. The boundary of the endometrium with the myometrium is quite obvious but at this stage it is very difficult to draw the line between the endometrium's basal and functional layers (illustration). The basalis is more densely cellular as seen at low power and perhaps has more sections through glands (because the glands twist around in this layer). Note that the endometrial glands are simple tubular structures which empty to the surface without intervening ducts. Here the glands are narrow because they are not active yet. Their epithelium is tall, orderly, and columnar.
Find some of the remnants of the epithelium along the endometrial surface and note that it is similar to the epithelium of the glands. The reason is that the surface epithelium comes from the glands. After the functional layer of the endometrium is shed during menstruation the surface is bare connective tissue. The ends of the glands remain in the basalis of the endometrium. Their cells divide rapidly and migrate out of the glands and over the surface of the endometrium. If you look carefully you may see a continuity of the two epithelia and imagine the sheet of cells being pushed out of the glands (illustration). Remembering back, the glands of the GI tract also have a cytogenic function. Even the sweat glands can serve as a reserve of stem cells for regenerating the epidermis if, for example, you "skin your knee" on a skateboard and take all of the epidermis off. (I do not know why anyone would want to risk his or her life on a skateboard, but that is a different matter. Please keep your attention on histology). Can you find the coiled arteries of the endometrium? (Hint, think how corkscrew shaped arterioles should appear when sectioned).
D-51 Uterus, follicular phase (H&E)
The follicular phase of the endometrium can also be called the proliferative or estrogenic phase. This slide represents an even earlier stage in the menstrual cycle than D-52. The endometrium is very shallow. You can see that the basal part is more cellular than farther up but the boundary is not sharp (illustration). Also the glands twist in various directions in the basalis but run straight towards the surface higher up. Towards the top their profiles are round because the section is more or less parallel to the surface there (this orientation makes the endometrium appear to have a greater thickness than it actually does). In this slide the surface epithelium has been entirely lost during section preparation.
Some blood has hemorrhaged into regions of the endometrium. If you look around at high power you can find macrophages with bright orange, half-digested red blood cells in them (illustration). Also, you can see the remnant of the perimetrium.
D-53, Uterus luteal phase (H&E)
This slide demonstrates the uterine endometrium in the middle of the secretory (progesterone) phase. Notice its lacy appearance due to the development of the glands (illustration). Their cells are actively secreting mucus and much of it remains inside the glands, enlarging them. Now you can tell the basal layer of the endometrium from the functional layer without trouble. (illustration). The former is much more cellular. Also the glands there have not become enlarged and tortuous. They remain much as they were during the follicular phase.
Zoom in on the epithelium of the glands. It is quite gaudy (illustration). Compare it with the epithelium lining the lumen of the uterus, pretty similar, huh? If you look carefully you can see lymphocytes, neutrophils and eosinophils in the endometrium near the surface (with lymphocytes invading into the epithelium). Locate the coiled arteries near the basal layer. .
Why is this phase of endometrial development also called the luteal phase?
D-59 Cervix (H&E)
Figure out the orientation of this tissue with the aid of the diagram at the beginning of this section. The slide is cut in the plane of that drawing. The large tongue-like projection, which can be observed with the naked eye, is the inferior lip of the cervix. A small segment of the vagina has been left attached (the wall of the posterior fornix of the vagina, to be more exact). The other lip of the cervix is missing. Note that the stratified squamous epithelium of the vagina extends over the outer, exposed wall of the cervix. Keep following along the surface until it abruptly changes to a simple epithelium (illustration). The point of transition between stratified squamous and simple columnar epithelium marks the entrance to the cervical canal. The transition is abrupt and it irritates the liver out of me that this exact point of interest is damaged on a bunch of your slides *!$#&%!.
Now examine the tissue under the l0X objective. This mucosa of the cervical canal is considered to be endometrium but it does not undergo menstrual shedding. The endometrium has numerous mucous glands (illustration). They are permanent, although the consistency of their mucous secretion varies over the menstrual cycle. It is particularly watery in midcycle. This is a good thing. Otherwise you would have had to be conceived parthenogenically because sperm could not get through. At other times in the cycle the mucus is thicker to discourage pathogens from wandering in.
The cervical glands also provide fluid to lubricate the vagina (together with glands in the labia) because, as we shall see in the next slide, the vagina has no glands of its own.
It would have been nice to have used a trichrome stain on the cervix to distinguish collagen from smooth muscle. The main job of the cervix is to form a sphincter to hold the fetus in the uterus until term. Both smooth muscle and collagen fibers help to carry out this function. At birth the smooth muscle relaxes but the fibers of collagen have to be digested with enzymes.
D-61 Vagina (H&E)
The vagina is basically a tube of smooth muscle lined with thick mucosa. By eye you can discern the stratified squamous unkeratinized epithelium underlying connective tissue and surrounding smooth muscle (illustration). The mucosa of the vagina actually is thrown into prominent transverse ridges, but these do not show up in this transverse section.
Examine these layers at 100X and 400X. The flattened epithelial cells on the surface of the epithelium look swollen and empty (illustration). This is because they are filled with glycogen which leeches out during fixation. When the cells are desquamated (shed) from the surface, resident bacteria in the lumen ferment the glycogen in them. This keeps the pH low inside the vagina. I suppose you could call this holocrine secretion, although your pathology teacher would call you eccentric if you did so.
Look at the connective tissue. Just below the epithelium it is quite cellular, as you would expect for a lamina propria. However there is no sharp boundary between this layer and the markedly less cellular submucosa farther down (illustration). Maybe we should consider the vagina not to have a submucosal layer. Did you notice how vascular the connective tissue is, whatever we call it? Good for you! The underlying muscle layer has two or three sheets running in alternative orientations.
The histological appearance of the vagina is, at first glance, similar to that of the esophagus (compare with slide D-101)*. However, the wall of the vagina has no glands, and the muscular portion is less well organized than that of the esophagus. There is no muscularis mucosa, nor striated muscle anywhere along its length.
*In the bad old days of medical teaching histology profs. would ask questions that elicited rude wrong answers from students who mistook the esophagus for the vagina.
* * * * *
Optional slides
The breast
You have already looked at the mammary gland of the breast in the gland laboratory. In case you are interested you can repeat your examinations of D-63 and D-64 here.
D-63, Mammary gland (H&E)
This slide is from a female who had never given birth to a child. At this stage of development the gland consists almost entirely of ducts. True secretory units are not present. Nevertheless, the lobular organization is easy to see by eye and at low power. The ducts are embedded in intralobular connective tissue that is quite cellular. The lobules are surrounded by less cellular interlobular connective tissue. It contains numerous adipose cells. The prepubertal breast tissue does not differ markedly from the postpubertal, nulliparous breast with regard to its glandular tissue. The duct system is more rudimentary in the child but the main difference is that the breast is much, much less filled out with fat.
Slide D-64, Lactating breast
Compare the glandular tissue in this section with that in slide D-63. Note that the lobules are greatly expanded due to the addition of more ducts and alveoli (illustration). The interlobular septa are thinner than in the resting breast.
It is difficult to tell the intralobular ducts from the alveoli because both are
lined with secretory cells that secrete milk (illustration).
Do not worry about distinguishing them. The ducts need larger lumens because
they also store the milk between breast feedings. Observe the jagged lumenal surfaces
of the secretory cells.
These cells secrete in an apocrine manner, by pinching off vesicles of their apical
cytoplasm containing fat droplets. The material inside the expanded ducts is milk.
The alveoli and ducts are provided with many myoepithelial cells. Their nuclei
are elongated instead of spherical. Another abundant cell type is the plasma cell.
These cells make antibodies that are secreted into the milk. You deferred looking
at them way back in the lab on glands but are able to pick them out now. What
do you think their function is?
The clitoris
D-62, Clitoris (H&E)
The clitoris is also an important organ in the female reproductive system. It is the homologue of the male penis. We will examine both structures during the lab on the male reproductive system because emphasizing the similarities between the two organs make it is most informative way to understand them. If you wish to preview the clitoris now as an important organ of the female reproductive system, you have but to click here.
Ovary with oviduct
D-56 Ovary and oviduct, monkey (H&E)
You have probably already examined this ovary for its follicles. The slide also happens to show a well preserved section through the ampulla of an oviduct. Note that it is connected to the ovary. The infundibulum of the oviduct would swing around to be right up against the ovary, especially at the time of ovulation. At low magnification note the labyrinthine appearance of its folded mucous membrane. The epithelium is simple with two cell types, but do not bother examining it here as it showed up much better in the human slide D-81. The smooth muscle under the lamina propria is well developed and does deserve a peep. It is generally described as composed of two layers, inner circular and outer longitudinal. However you can see that the muscle is actually not well organized, with several vague layers running in various directions (illustration). Here is a good place to practice distinguishing areas of smooth muscle from those of connective tissue. A serosa with relatively plump epithelial cells surrounds the organ. Check it out.
I presume that the scattered large follicles are in a late secondary stage, but they might be mature. As mentioned earlier, maturity depends on the physiological characteristics of the follicle (especially whether its cells have the numbers of receptors for LH to be triggered to rupture by a surge of that hormone) instead of its particular shape. In the human, mature follicles are one or two centimeters across and bulge from the surface. Anything much smaller is still secondary. However, the follicles of a smaller animal mature at a smaller size. This is why some atlases show follicles labeled mature although they look like pretty young secondary follicles of a woman (always read the fine print, they have photographed a rat ovary). Because the D-56 ovary is much smaller than a human counterpart its follicles may be mature. If you are still fretting about this, the best thing to do is talk with Dr. Lu, since he really is the world specialist on the ovary. His particular expertise is on the mechanisms of aging of the ovary and their ramifications for human reproduction and menopause.