Thymus

The gland where T-cells are nurtured and matured. (Lymphocytes-to-be come from the bone marrow; the transformation of some cells into T-cells in the thymus begins early in the unborn child.) "Thymos" is Greek for both "warty-bumpy" and "feel-good", both of which makes sense. The gland is largest at puberty, and in sick children, it shrinks by depletion of its cells -- hence the myth that a large thymus caused "unexplained death in healthy children" (many of whom were smothered) and the whole stupid 1950's racket of radiating the gland in babies.

The first organ to become populated with lymphocytes in the unborn child. It's largest relative to body size right at birth. A holocrine gland (i.e., its secretion is whole cells; the other holocrine glands are the other lymphoid organs, the sebaceous glands, and the gonads.) In the front of the chest, two lobes, a real fibrous capsule from which extend septa to divide the gland into lobules, a blood supply composed of little arteries that run down the septa and break into arterioles that run between cortex and medulla, and only a few little lymphatics which follow the arteries. At puberty, the gland weighs around 40 gm.

Thymic epithelium (reticular cells, from endoderm) starts as cuboidal as in an endocrine gland, but when the gland is infiltrated by lymphocytes, they stretch the cells into star-shapes connected only at their (few) desmosomes. This forms the structural framework (reticular network, "cytoreticulum", what the thymus uses as stroma) of the gland, instead of the mesenchyme and reticulin of other lymphoid organs. The epithelium contains tonofilaments and secretory granules (probably thymosin and other stuff).

The cortex in the functioning gland is almost all lymphocytes, mostly small-resting types, packed tight. New arrivals from the marrow are larger and appear mostly under the capsule. If you see a germinal center (lymphoid nodule) in the thymus, the person is sick. Don't expect to be able to distinguish the epithelial cells, endothelial cells, and macrophages (both major types) from each other. The capillaries here are surrounded by a space which may contain a mix of white cells, and which is in turn surrounded by a thin layer of collagen and a complete covering of thymic epithelial cell processes, so that blood does not contact the T-lymphocytes of the cortex. This is the famous "blood-thymus barrier", which macromolecules do not cross.

The medulla, away from the capsule, forms a continuous unit. In addition to epithelium and small lymphocytes as in the cortex, there's more of a mix of cells, with some fibrous tissue extending from the vessels, and a variable number of macrophages (both major types), eosinophils, plasma cells. This is why it stains paler. In the centers of the medullary lobules, you're likely to see thymic corpuscles ("Hassall's corpuscles"), areas where the interconnected thymic epithelial cells have decided to grow in whorls, mimicking hair (no I am not making this up). These can vary in size (up to 100 ), and show a host of other changes that mean nothing (cysts, microvilli, granules, mucus, grunge, etc., etc., etc.) The capillaries are not specially shielded here, and lymphocytes leave the thymus by entering them.

By the time you're fully grown, most of your T-cells have left your thymus and gone to live elsewhere. The epithelial cells remain, maybe in reduced numbers, and the gland becomes a storage depot for far. The best way to be sure you're looking at thymus in an adult is to spot Hassall's corpuscles, which stay forever.

Heart

You're already acquainted with the basics about cardiac muscle. The histology of the heart reflects its function.

The fibrous ("parietal") pericardium (the tough sack surrounding the heart) is tough connective tissue, with mesothelium lining its inner surface. If you want, you're allowed to call the epicardium (with its mesothelium, fat, coronary arteries and veins, loose fibrous tissue, lymphatic vessels, nerves, macrophages, lymphocytes, etc., etc.) the "visceral pericardium".

The fibrous skeleton of the heart includes the connective tissue of the septum (including most of the interatrial septum, and the little "membranous" spot at the top of the interventricular septum), and the annuli around the valves. It's dense connective tissue in various orientations (less regular than tendon, more regular than dermis).

The myocardium consists of cardiac muscle cells arranged mostly parallel, connected to the fibrous skeleton of the heart which you may or may not see in any particular section. You will probably see lipofuscin in granules near the poles of the nuclei. Some muscle cells in the right atrium, and maybe the left, contain neurosecretory granules visible by electron microscopy; this is where atrial natriuretic peptide comes from. You're familiar with the anatomy of the pectinate muscles and trabeculae carnae, which are just cardiac muscle. Some anatomists have studied the orientations of the cardiac muscles in exquisite detail ("bulbospiral muscle", "sinospiral muscle", etc., etc.). I'll trust them.

The endocardium is thickest in the left atrium, thinner in the right atrium, and very thin in the ventricles. Under the single layer of endothelium is a bit of connective tissue with lots of collagen and elastin, nd some little nerves and vessels and sometimes some smooth muscle for some reason.

The vascular supply of the heart includes a variety of vessel types. The coronary arteries give rise to arteries which penetrate the wall, branch to supply the epicardium, myocardium, and endocardium, and may empty into the cardiac chambers. Branches of various large arteries anastomose with each other; you can build these up (and build up your myocytes, too) by aerobic exercise. In addition, "myocardial sinusoids" are channels that allow chamber blood to pass into the myocardium; myocardial capillaries may empty into these instead of returning to the coronary veins. The thebesian veins are an interconnecting system which empty directly into the chamber; they anastomose with the venules that feed the coronary veins.

Valve leaflets are composed of three layers. The fibrosa is dense collagen that extends up from the valve ring to the free edges, and in the case of the atrioventricular valves, into the chordae tendineae. The spongiosa is underneath (toward the atrium on the atrioventricular valves, toward the lumen of the semilunar valves), and is composed of strands of collagen and lots of proteoglycan. It serves as the shock absorber. The ventricularis is the part closest to the blood stream; of course it's covered with endothelium, and it's largely made up of elastic fibers. All layers contain some fibroblasts. There are nerves and lymphatics, but no blood vessels, in the leaflets. The chordae tendineae are tough fibrous bands, with cores composed sometimes of cardiac muscle, sometimes of fibrous tissue. The outside extends into the fibrous cap on the tips of the papillary muscles.

The conduction system is myocardial fibers designed for rhythm rather than contraction. You can spot the sinoatrial node where the superior vena cave joins the crest of the right atrial appendage, just under where the fibrous pericardium takes off. It's a ring of tiny muscle fibers around a central artery; they have only a few fibers and very poor sarcomere formation. There are normally three little tracts of conduction-type muscle fibers; these meet at the atrioventricular node, on the right side of the interatrial septum, just under the endocardium, just in front of where the coronary sinus opens, just in back of the membranous spot on the interventricular septum, and just above the insertion of the tricuspid valve. From here, the impulse follows the bundle of His, parallel conduction-type muscle fibers that pierce the fibrous septum between the mitral and aortic rings, then divided into the right branch and left branch which run down the interventricular wall under the endocardium. These supply impulses to the Purkinje cells, pale-staining cardiac muscle fibers which on electron microscopy lack tubules.

There are abundant lymphatics in the endocardium and epicardium, but the channels in the myocardium are small and probably just drain the endocardium. On reaching the epicardium, they follow the coronary arteries, then supposedly exit the heart and join the bronchial lymphatics.

03467 heart, normal histology, composite

14622 cardiac muscle, normal

14623 cardiac muscle, normal

14624 cardiac muscle intercalated disks

14625 cardiac muscle intercalated disks

14753 thymus, fetal, normal

14754 thymus, fetal, normal

14755 thymus, juvenile, normal

14756 thymus, juvenile, normal

14757 thymus, adult, normal

14758 thymus, adult, normal

14759 thymus, juvenile, normal

14760 thymus, juvenile, normal

14765 thymus (septum)

14766 thymus (septum)

15110 muscle, cardiac, normal

15199 thymus, normal

15200 thymus, cortex, medulla, normal

15201 thymus, Hassall's corpuscle, normal

15202 thymus, medulla, normal

15203 thymus, epithelial reticular cell, normal

15205 thymus, adult, normal

15209 Purkinje fibers, heart, normal

16563 thymus involution early

20728 muscle, cardiac, normal

20729 muscle, cardiac, intercalated disks

20812 thymus, overview

20813 thymus, medulla

20814 thymus, cortex

20815 thymus, Hassall's corpuscle

20816 Purkinje fiber, heart

20821 atrium OF heart, Endocardial surface

20822 atrium OF heart, Endocardial surface

20823 atrium OF heart, epicardial surface

25725 heart, embryo

25726 Endocardial cushing -- embryo

26263 epicardium, ventricle of heart

26295 Purkinje fibers, heart

26296 Purkinje fibers, heart

26298 Purkinje fibers, heart

26305 thymus

26307 thymus

37643 heart, normal

44360 muscle, cardiac, branching cells, intercalated disks

44361 muscle, cardiac

44362 muscle, cardiac

44363 muscle, cardiac

44433 Purkinje fibers, cardiac muscle heart

44434 Purkinje fibers, cardiac muscle heart

50441 thymus, lymphoid

50442 thymus, lymphoid, Hassall's corpuscle

50443 thymus, lymphoid

50444 thymus, lymphoid

50448 heart, endocardium

50449 heart, atrial endocardium

50450 heart, normal histology

50452 heart, Purkinje fibers

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