Seen here as pink connective tissue strands are the outer splenic capsule and its continuation into a trabecula. There are some sinusoids, filled with blood, to the left. The red pulp tissue between splenic sinusoids is often called cords of Billroth. The sinusoids of spleen are comparable structurally to the sinusoids of bone marrow.

Splenic phagocytes (macrophages) loaded with yellowish-brown hemosiderin particles ingested during the breakdown of dead red blood cells. The spleen filters and "cleans" blood in the same way that the lymph nodes filter lymph.

Panoramic view of the infant thymus, showing its lobulated structure. Within each lobule is a dark cortex and pale medulla. There are no round nodules or germinal centers in the cortex, just diffusely and densely packed lymphocytes. The thymus is seeded by lymphocytic stem cells very early in life and is particularly active in the production of lymphocytes in the young person. Those lymphocytes which resided in the thymic cortex early in life are forever after "thymus dependent"-cells , no matter where they reside in the body later on.

Detail of Hassall's corpuscle, with concentric layers of keratinizing epithelial cells. Although the significance of the corpuscles is uncertain, they are apparently formed from the epithelioid stromal cells.

Panoramic view of adult thymus, largely replaced with adipose tissue. There are recognizable remnants of thymic lymphatic tissue, however, and Hassall's corpuscles are still present in the medulla.

Parathyroid at higher power, showing that the cells are actually tightly packed epithelial cords. The larger, pale pink cells in the middle and to the right are oxyphil cells; they have a smaller, darker nucleus and relatively larger amount of cytoplasm than the majority of cells, which are called chief cells (to the left in the photo). The chie f cells secrete parathormone; the significance of the oxyphil cells is not clear.

Pars distalis stained to show the 3 major types of secretory epithelial cells there:
a = pale chromophobes
b = blue basophil
c = red acidophilNotice that these cells are characteristically in cord-like clumps. The clumps are separated by a fine reticular fiber stroma (thin blue lines), where the blood capillaries lie. All endocrine glands are highly vascularized, since secretions go directly into capillaries (depending on the organ.) In endocrine glands the endothelium of these vessel s is typically continuous and fenestrated.

Detail of sebaceous gland. Cells look foamy because of loss of lipid droplets during tissue fixation. This gland exhibits holocrine secretion, in which whole cells swell up, degenerate, and are desquamated as part of the oily secretion (sebum). The secretion is emptied into the hair follicles and eventually reaches the surface of the skin.

Higher magnification of taste buds (from the foliate papillae of rabbit in this case). In the lower buds note surface pores through which salivary fluids in the lumen of the furrow reach sensory nerve endings within the taste bud capsule. The cell bodies for these dendritic endings are pseudounipolar and lie within the sensory ganglion of a cran ial nerve (such as Nerve VII).

Detail of previous wall, showing the large chondrocytes of the cartilage. The epithelium lining the lumen looks pseudostratified still. A layer of pink smooth muscle lies between it and the cartilage. Considerable elastic tissue lies in the respiratory wall as well, but is not distinguishable in H&E stain.

Lung:
a = alveoli, with thin interalveolar septa between them
b = smooth muscle in its wall
c = blood vessel, filled with r.b.c.'s
d = bronchiole (again, no cartilage in its wall)

Another view of terminal branches of respiratory tree in the lung. In the middle of the field is a small terminal bronchiole branching off into three or four alveolar ducts to the right. The bronchiole has a definite, pink epithelial lining, while the walls of the alveolar ducts consist mainly of alveoli. The bronchiole is called terminal simpl y because it's the last generation of bronchiole before alveoli start to appear in the wall. Just before the alveolar ducts branch off, you can see a couple of small alveolar outpocketings in the bronchiole wall, thus making this short segment a respiratory bronchiole. (Note: along the top of this field is a wall of a bronchus, with two very sma ll, basophilic patches of cartilage just under the pink layer of smooth muscle.)

The outer reaches of two lung lobules, with a connective tissue septum running vertically between them. The lower edge of the tissue here is the visceral pleura. Lymphatic vessels (a) and veins (b) run in the septa at the periphery of each lobule. Arteries, as we've noted previously, typically follow the branchings of the respiratory tree itsel f. At (c) there is a bronchiole. The mucosa lining its lumen is typically thrown into folds or scallops because of contraction of smooth muscle and elastic fibers in the wall.

Detail of sebaceous gland. Cells look foamy because of loss of lipid droplets during tissue fixation. This gland exhibits holocrine secretion, in which whole cells swell up, degenerate, and are desquamated as part of the oily secretion (sebum). The secretion is emptied into the hair follicles and eventually reaches the surface of the skin.

Detail of Meissner's corpuscle lying in dermal papilla. The arrows point to nuclei of the specialized connective tissue sheath that surrounds the dendritic ending that is twining around inside, among the sheath cells. Silver stain would make the ending visible. You get the feeling that this corpuscle has some substance to it, i.e. that you coul d shell it out as a more or less solid unit, from the surrounding loose areolar connective tissue.

Detail of myoepithelial cell processes as seen in H&E section. Look at the large tubule on the left for pink "hoops" that seem to be extending from the basement membrane of the upper row of epithelium toward the secretory cell nuclei which lie near the lumen. These "hoops" are the cytoplasmic extensions of the stellate myoepithelial cells. Thes e cells lie within the basal lamina of the tubule.

Detail of sweat gland. The darker circles in the lower part of the field are ducts; the lighter cross-cuts above are the secretory portions.

Detail of sweat duct coiling throughout the stratum corneum to the surface of the skin. Since cells are dead here, there is no longer a living duct lining; just the tunneling of its lumen remains.

Detail of sweat duct originating (at lower left) from the basal layer of the epidermal epithelium.

Higher power of valve made of a core of fine c.t. with endothelium covering both surfaces. Valves in veins are constructed similarly.

Early compact bone, decalcified so it can be stained. This has been cut so that the Haversian systems are cut in cross section. Vascular channels cut longitudinally are parts of Volkmann's canals.

Vascular elements from bone marrow (on the left) are continuous with vascular spaces within the bone. The endosteum lining the marrow cavity is therefore continuous with the endosteal linning of Haversian canals.

Detail of bone-forming osteoblasts lined up along the inner (endosteal) edge of bone next to the marrow cavity. In young bones growth continues in width, constantly laying down bone and resorbing it and laying down more. Real width, of course, increases by the laying down of periosteal bone on the outside of the bone, but activity continues on the endosteal surface also. Notice osteocytes inside the bony substance, lying in lacunae.

Detail of osteocytes in lacunae. The collagenous fibers of the decalcified matrix are quite acidophilic, as always. Osteocytes like these are present in both compact and spongy bone; their arrangement, however, is in concentric lamellae in compact bone and in randomly arranged lamellae in spongy bone. Remember, too, that osteocytes have processes which extend out into canaliculi in both kinds of bone.

Detail of the appearance of crypts of Lieberkuhn as they appear in both the colon and appendix. The goblet cells are particularly numerous because of the need for lubrication of the gut contents. (The appendix, of course, is a vestigeal structure.) Notice also the numerous lymphocytes in the c.t. lamina propria.

In the GI tract, the main differences from one part to the next are in the mucosa, and occasionally in the submucosa. The muscularis externa is pretty much the same in its basic pattern all along the way. Here you see the typical outermost layers, with a relatively thick inner circular band of smooth muscle and a usually thinner coat of outer lo nizitudinal smooth muscle. These directions are so consistent that you can determine the plane of section of the gut just by looking at the external muscle. In this particular case, the gut was cut in cross-section. This section also shows a serosa, with its "finished" edge of mesothelium. Any part of the gut that is slung in a mesentery has a serosa; any part that is retroperitoneal has an adventitia on the deep, buried surface and a serosa on the surface facing the peritoneal cavity. [Note: at the top of this structure you see an accumulation of lymphocytes in the submucosa.]

Another view of outer gut wall. At the extreme right there seems to be the beginning of some fat, so that is presumably the adventitia or serosa. This wall, then, like the previous one, has been cut transversely, as we can judge by the direction of cells in the musde layers. Between the two muscle layers is a pale, tangled-looking Auerbach's mye nteric p1exus. No cell bodies of neurons are visible, however.

Another view of outer gut wall. At the extreme right there seems to be the beginning of some fat, so that is presumably the adventitia or serosa. This wall, then, like the previous one, has been cut transversely, as we can judge by the direction of cells in the musde layers. Between the two muscle layers is a pale, tangled-looking Auerbach's mye nteric p1exus. No cell bodies of neurons are visible, however.

Autonomic (parasympathetic) ganglion cells lying between the two muscle layers of the muscularis externa (in middle of picture). There are two general locations where such ganglion cells are found in the gut: Meissner's (submucosal) plexus in the submucosa, and Auerbach's (myenteric) plexus between the two muscle layers. Both plexuses are parasy mpathetic, these being the postganglionic (or second) neurons in the basic two-cell autonomic chain. The preganglionic cells which synapse with most of the postganglionics of the gut lie in the dorsal motor nucleus of the vagus in the brain. (For the lowest portions of the gut, the preganglionics originate in the intermediate gray matter of the sacral spinal cord.)

Higher power of neuronal cell bodies of Auerbach's plexus in the middle of the picture. They are larger than surrounding cells and have the vesicular nuclei and dark prominent nucleoli typical of so many nerve cells.

In the lower left is a taenia coli, one of 3 strips of outer longitudinal muscle peculiar to this part of gut (the colon). Inner circular coat is continuous here above it, but the outer muscle coat is very thin at the right and thick at lower left where the taenia is.

Simple cuboidal epithelium in Mallory stain (longitudinal cut). Note the dark chromatin clumps in the nuclei. Underneath the epithelium lies a small blood vessel filled with orange-colored blood cells.

Low power view of larger vessels, showing endothelial nuclei lining the lumen. The yellowish cells filling each vessel's lumen are blood cells.

High power view of endothelial cells lining a small blood vessel cut in cross-section. (You see just the nuclei - the cytoplasm between them is extremely flat.) Endothelium = the simple squamous epithelium lining blood vessels.

Mesothelium seen as if looking down on a surface view to see "pavement" effect of the lining cells. Silver stains the intercellular cement dark between adjacent cells. Notice how corrugated the cell membranes are. Mesothelium = the simple squamous epithelium lining body cavities and mesenteries.

The heart wall, like blood vessels in general, has three main layers, though they are not called intima, media, and adventitia. As in vessels, however, the innermost and outermost layers are primarily connective tissue; the middle one is muscle --- in this case, cardiac muscle. From left to right, then, in this picture of ventricle wall, there is first a very thin endocardium, which consists primarily of an endothelial lining and a very small amount of connective tissue underneath it. The muscle layer, or myocardium is next and is by far the thickest layer and constitutes the bulk of the heart. To the far right is the epicardium, which contains considerable fat. In gross anatomy the epicardium is called the visceral layer of the serous pericardium; it has an outermost lining layer of mesothelium.

A high magnification reminder of the appearance of cardiac muscle cut longitudinally, with central nucleus, branching fibers, and cross-striations. Muscle fibers spiral around the heart in all directions and can thus exert the necessary squeezing action as the heart contracts. Remember that these muscle cells are attached end to end by junctions at the intercalated disc. Axon terminals of autonomic neurons innervate some of the muscle cells, and the stimulus is spread to neighboring muscle cells by the intercalated discs and by gap junctions along the side walls of the cells.

Cardiac muscle in cross-section. Note also the many cross cut capillaries in the connective tissue endomysium between muscle fibers. As you might expect from the constant work the heart performs, it is a highly vascularized organ. Capillaries in this (or any) muscle have endothelium that is continuous and non-fenestrated.

The surface of the ventricular lumen is very irregular because of the presence of papillary muscles in the wall. These irregularities are, of course, lined with endothelium.

Low power of a Mallory-stained heart, showing two channels (above) that are continuous with the lumen of the left ventricle (below). The left-hand channel is the aorta, with some blue connective tissue in its wall. There is also one cusp of the semilunar valve, with its blue core of dense collagen. Remember that valves are lined over their entire surface by endothelium which is continuous with aortic endothelium above and the ventricular endothelium below. To the right in this picture is the atrioventricular channel, with chordae tendinae extending down from the mitral valve and attaching to the papillary muscles of the ventricle. Like valves, the chordae tendinae are also composed of dense collagenous connective tissue covered by an endothelial lining.

A capillary lying in the endomysium between skeletal muscle fibers. This one shows very dark endothelial nuclei and has 3 pink r.b.c.'s lined up in a row inside.

In the middle of the field is a sinusoid (filled with orange-colored r.b.c.'s) in the marrow cavity of spongy bone. (The larger empty circles are fat cells.)

This sinusoid, like a capillary, has only an endothelial wall, but its lumen is characteristically considerably wider. Also, in some locations in the body (such as bone marrow, liver, and spleen) the endothelial cells of sinusoids are rather loosely joined together, thus permitting passage of blood cells between them.

This capillary running through embryonic mesenchyme has a wall consisting solely of a single layer of endothelium. Notice that the lumen of the vessel is only slightly larger than the diameter of the r.b.c.'s within.

Another section of esophagus, showing again the characteristic stratified squamous epithelium, the rather thick dense muscularis mucosae, and the very thick muscularis externa, with some patches of large-fibered skeletal muscle mixed with bands of smooth muscle. Notice that the muscle layers look more "solid" here than the lighter staining connec tive tissue layers. Blood vessels are evident in both the lamina propria and the submucosa. A piece of submucosal gland shows to the right.

Detail of mesothelium, looking down on its surface. The simple squamous cells fit together in a "pavement" effect. The intercellular boundaries have been silvered here.

Histological section of the GI tract, showing the basic layers clearly. The glandular mucosa (to the right) is quite dark because of all the epithelial and connective tissue nuclei it contains. A thin strip of pink marks the muscularis mucosae. Next comes the very dark pink submucosa which is mainly dense collagen fibers. Further left are the two, paler pink layers of the muscularis externa: a wide band of inner circular smooth muscle and a narrower band of outer longitudinal smooth muscle. Furthest left comes the serosa, so recognized because it has a "finished" edge of mesothelium. This is probably quite near the mesenteric attachment because there is so much adipose tissue and some fairly large blood vessels within the serosa.

The GI tract typically has a glandular mucosa (looks dark and thick here). Characteristics of the mucosa depend upon the function of the particular part of the tract concerned. This particular section is of colon, with lots of intestinal glands (crypts of Lieberkuhn) but no villi.

A capillary lying in the endomysium between skeletal muscle fibers. This one shows very dark endothelial nuclei and has 3 pink r.b.c.'s lined up in a row inside.

This sinusoid, like a capillary, has only an endothelial wall, but its lumen is characteristically considerably wider. Also, in some locations in the body (such as bone marrow, liver, and spleen) the endothelial cells of sinusoids are rather loosely joined together, thus permitting passage of blood cells between them.

Cross-cut of skeletal muscle to show connective tissue partitioning of muscle into groups or bundles of fibers. Endomysium is very delicate and lies between individual fibers, while perimysium is more visible and lies around a group of fibers. Epimysium is not seen here but ensheaths a whole muscle. In this picture notice the presence of small blood vessels in both perimysium and endomysium. Notice also the cross-cuts of myofibrils within the muscle cells, making them look grainy.

Smooth muscle - with cells more separated so as to see their extent and shape better, and the central position of their nuclei. A loose, irregular connective tissue (endomysium) lies between the cells. Nuclei seen in this c.t. belong to fibroblasts mainly.

Smooth muscle - long, slender central nuclei, lying within narrow, fusiform cells that lie parallel to each other in a smooth arrangement. (Muscle cells are often referred to as muscle fibers because of their narrowness and length.)

Female Reproducive Tract

Ovary, Oviduct, and Uterus




Ovary with surface cuboidal epithelium. (Really a modified mesothelium.)

Cortex of ovary. A thick connective tissue capsule, the tunica albuginea underlies the surface epithelium. Somewhat deeper lie several small, primary (primordial) follicles. (All egg cells have reached the primary oocyte stage by birth and are held in this "suspended animation", in very early prophase, until such time as they may ovulate or undergo atresia.)



Primary follicles with one single layer of flat follicle cells surrounding an oocyte. Although an oocyte is a giant compared with its neighbors, this early stage is small for an oocyte, and the cell will grow considerably in size when it begins to mature, under the influence of FSH. The nucleus looks lightly granular, and the dark nucleolus is prominent. Cytoplasm is very pale. Note the "swirly" interstitial tissue of the ovarian stroma.






Early maturation stage of follicle with beginning proliferation of follicle cells around an enlarging oocyte. The nucleolus shows clearly inside the nucleus. As the oocyte enlarges, its chromosomes prepare further for the first meiotic division, which will occur at ovulation.



Further developed follicle
a = with antrum beginning at arrows. The homogenous gray-blue line immediately surrounding the egg cell itself is the zona pellucida.
b and c = primary follicles, containing oocytes which are still small.



A group of follicles in various stages of early development in the cortex of a rat ovary. Blood vessels of the ovarian medulla are seen in the center of the field. Development of follicles is regulated by FSH from the anterior pituitary.



Maturing follicle, so called because it contains a definite antrum (or fluid-filled space) and many layers of granulosa cells. The egg is still a primary oocyte and sits to one side of the follicle on a mound of cells called the egg hillock or cumulus oophorus. The cells closest to the oocyte will be expelled with it at ovulation as the corona radiata. Surrounding the granulosa cells of the follicle is the theca interna, a rather cellular and vascular connective tissue layer, which secretes estrogen. Outside of this is the theca externa a more fibrous connective tissue layer, not well defined here. Note that several follicles may start to develop in any one monthly cycle, but in the human only one will mature, unless there are to be multiple ovulations and therefore possible multiple births. All follicles that don't complete their maturation undergo atresia (i.e., degenerate). The egg dies, the granulosa layer breaks up, and the whole follicle collapses and undergoes fibrotic change.




Large ruptured follicle, just after ovulation.
Arrow = stigma, the point of rupture where oocyte was expelled. The reduction division (or first meiotic division) takes place at the time of ovulation.
a = granulosa cells that will now proliferate under the stimulus of pituitary LH and enlarge to become granulosa lutein cells, filling in the follicular cavity and becoming the major portion of the new corpus luteum.
b = corpus albicans -- old scar of an earlier corpus luteum.

Detail of corpus luteum showing the rounded foldings of large, pale granulosa lutein cells in the lower half of the picture; these secrete progesterone. Pushing down between the folds is a wedge of smaller, darker theca lutein ; these secrete estrogen.

Heart with metastatic calcification

Heart with metastatic calcification -- Calcified myocardial cells

Hemosiderosis (kidney)

Hemosiderosis (kidney) -- Same section stained with Prussian blue to determine the nature of the brown pigment

Pancreas in hemochromatosis

Pancreas in hemochromatosis -- Detail of islets. Hemosiderin in macrophages and some epithelial cells

Gaucher's disease

Gaucher's disease -- Masses of Gaucher's cells with dark-staining hepatocytes

Steatosis of the liver

Steatosis of the liver-- Topographic view

Xanthelasma

Xanthelasma -- Topographic view. Clusters of foam cells are visible.

Tattoo

Tattoo -- Topographic view

Phagocytosis by Kupffer cells

Phagocytosis by Kupffer cells -- Medium power: blue pigment (Monastral blue) injected i.v. is contained only in Kupffer cells

Prostate: apoptosis

Prostate: apoptosis in atrophy (ApopTag) -- Note the dark brown nuclei of the apoptotic cells in the glandular epithelium and in some of the lumens of the glands. Mast cells filled with blue granules stand out in the interstitium

Kidney atrophy (hydronephrosis)

Kidney atrophy (hydronephrosis) -- Topographic view of cortex

Atrophy of the testis

Atrophy of the testis -- Tubules at various stages of atrophy.

Note Leydig cells

Atrophy of the testis

Atrophy of the testis -- Topographic view