Week 10

Histology pre-lab and lab help : Click Here.

LABORATORY CLASS - WEEK 10

Gross Anatomy - Mock Test!

Histology - Lymphatic System

Histology - Lymphatic System

Having seen the spleen, appendix, lymph nodes and the remnant of the thymus, this is an appropriate time to take a look at the microscopic organisation of these organs. To complete the picture, we also include tonsillar tissue.

Lymph node (H&E) : Slide 56

Examine the slide with the unaided eye. In a well-stained section, you should be able to see the outer dark area (cortex) and the inner pale medulla. Under low magnification you can see the outer part of the cortex studded with tiny blue areas, the lymphatic nodules. Deeper inside, closer to the medulla is the part of the cortex known as paracortex – the so-called thymus-dependent area. In many of the nodules in the cortex, you may see a pale centre, called the germinal centre. The darker part of the nodule is often somewhat crescent-shaped and is called the corona. Do not confuse the germinal centre (part of a nodule) with the medulla (part of the entire lymph node).

The entire node is enclosed in a connective tissue capsule. Afferent lymph vessels open into the lymph node at multiple points through the capsule and empty into a subcapsular sinus. The afferent lymphatics are rarely seen (except in a ‘lucky’ section), but subcapsular sinus is clearly seen in most. Lymph trickles down through the cortex to the medulla. The medulla is a mass of loosely arranged network of cellular columns (cords), among which are spaces called medullary sinusoids. From the sinusoids lymph is collected and taken out by an efferent lymphatic. Blood vessels enter and leave the lymph node close to the efferent lymphatic, the area of entry and exit is the hilum of the node. Unless a section passes exactly through the hilum, it will not be seen. Think of 5 micrometre sections cut through a bean!

Tonsil (H&E) : Slide 94
The ‘tonsil’ we commonly refer to is more accurately known as the palatine tonsil located between the palate and the tongue. It is a part of a lymphoid tissue ring around the entire pharynx. Though all tonsillar tissue is structurally similar, the palatine tonsil is the largest aggregate mass.

It is important to realise the tonsil as a part of lymphoid tissue under epithelium. in this case it is, anatomically, an integral part of the wall of the pharynx. In a section of the tonsil, therefore, components of the pharyngeal wall like glands and muscle are often seen, and the tonsil has a covering of stratified squamous epithelium. on the outer side it has an incomplete connective tissue capsule.

The epithelium dips down as deep infoldings into the underlying connective tissue. These crevices, called crypts, are the primary organizational feature of the tonsil. They accumulate bacteria and other antigenic debris and leak antigens into the surrounding connective tissue. Numerous nodules lie along them, especially in children.

Tonsils are structures with huge numbers of lymphocytes. These cells are drawn in from the blood through tall postcapillary venules. The lymphocytes wander around here waiting to be stimulated by antigens that diffuse in from the decaying debris in the crypts. Many invade the epithelium; so much so that in some places you have trouble recognizing it as an epithelium at all. Since the tonsils are directly exposed to antigenic material in the environment, they have no afferent lymphatics, only efferent lymphatics. The tonsil  has no differentiation such as cortex and medulla.

Appendix (H&E) : Slide 4.

The appendix is often called the tonsil of the abdomen. It is a blind structure off of the large intestine. Note the similarity in structure between appendix and tonsil. Both contain enormous depots of lymphoid tissue separated from a dirty, stagnant space by an epithelium infiltrated with lymphocytes.

Spleen (H&E) : Slide 82.

The spleen is a complex organ. Besides being a lymphoid structure, it is also the graveyard of aging red blood cells.

Hold your slide up to the light and note the blue dots on a red background. The blue dots (called the white pulp) are so coloured after staining, in the fresh state they are actually white. The red areas (red pulp) are red in any case.  View the slide under low magnification. You can make out a fibrous capsule. The capsule also has some smooth muscle, but the two cannot be differentiated well with H&E staining. The capsule sends incomplete partitions into the splenic substance. These are the trabeculae. Branches of splenic artery and vein are seen in the trabeculae. At low power distinguish red pulp from white pulp. The white pulp looks like small islands of dense lymphatic tissue always accompanied by a small artery, the central artery. They are purple because the predominant cell types are small lymphocytes with dark nuclei. Actually the white pulp is an aggregation of lymphocytes around the artery. The central arteries (really, arterioles) are characteristic of the spleen.

The red pulp is composed of a lattice of reticular cells infiltrated by enormous quantities of blood cells. Many venous sinusoids traverse it. The areas of tissue between sinusoids are called "cords" (a poor term), so that red pulp is a mixture of cords and sinusoids.

Thymus, active (H&E) : Slide 88.
The thymus has extremely sophisticated functions but they are not reflected in structure visible on routine histological slides, nor do these slideshow many of the structural features.  You will be able to distinguish only two types of tissue, cortical and medullary, and two classes of cells, T cells and epithelial reticular cells. Examine this slide with the unaided eye and note that it is divided into lobules, each with a darker-staining cortex and lighter-staining medulla. Do not confuse lobules with nodules! The thymus has no nodules.

Look at a cortical region under the microscope. The cells are predominantly T-lymphocytes and their immediate precursors. T-lymphoblasts actively divide here. The only blood vessels in the cortex are capillaries. There is a blood-thymus barrier in the cortex.

The medullary regions are paler because they have far fewer thymocytes. The epithelial reticular cells are abundant. Some of them aggregate into easily seen pink structures called Hassall's corpuscles (thymic corpuscles).  The function and significance of these structures are unknown (likely they have none). They are composed of swirls of epithelial reticular cells around a keratinized core. The blood-thymus barrier is interrupted in the medulla, and here one can find blood vessels larger than capillaries. This allows T-lymphocytes to escape from the medulla into the circulation.

You can summarise the essential features of these organs thus :

Cortex and medulla :

Lymph node OR thymus.

            Lobulated structure, no nodules, corpuscles in the medulla : thymus

            Nodules in the cortex, medullary sinusoids : lymph node.

No cortex/medulla :

            Stratified squamous epithelium with crypts : tonsil

            Capsule, trabeculae, red pulp, white pulp with central arteries : spleen.