The document discusses various lymphatic tissues in the body, including the thymus, tonsils, mucosa-associated lymphoid tissue (MALT), and tertiary lymphoid organs. It describes the microanatomy and functions of the thymus, tonsils, Peyer's patches in the small intestine, and nasal-associated lymphoid tissue. It also explains that tertiary lymphoid organs can develop in non-lymphoid tissues affected by chronic conditions to provide immune responses.

1. Lymphatic System: 2
Dr. Komal Parmar

2. Previously we discussed:
 The microanatomy of:
 Lymphatic Channels
 Lymph nodes
 Spleen

3. Today’s Objectives:
 Microanatomy of
 Thymus
 Tonsils
 Lymphoid tissue associated with epithelial surface
 Tertiary lymphoid organs

5. Thymus
 One of the two primary
lymphoid organs.
 It is an encapsulated soft,
bilobed organ, the two parts
being joined in the midline by
connective tissue that merges
with the capsule of each lobe.
 The thymus is largest in the
early part of life, particularly
around puberty, and persists
actively into old age despite
considerable fibrofatty
degeneration which sometimes
hides the existence of thymic
tissue.

6.  Site of mature T
lymphocyte
production.
 Acquiring immune
tolerance to the
body’s own
components.
 Also part of the
neuroimmunologic
al and
neuroendocrine
axes of the body

7. General Architecture
 Embryological
Consideration:
 The thymus gland is
formed from the ventral
part of the third pharyngeal
pouch on each side.
 Cardiac Neural Crest
mesenchyme forms
connective tissue septa
which produce the
lobulated architecture of
the gland.
 Angiogenic mesenchyme,
including lymphoid stem
cells, invades this local
mesenchyme

9. Capsule
 Both thymic lobes have a loose fibrous connective tissue capsule, from which
septa penetrate to the junction of the cortex and medulla, and partially separate
the irregular lobules, which are each 0.5–2.0 mm in diameter.
 The connective tissue septa form a route of entry and exit for blood vessels and
nerves and carry efferent lymphatics. Most migrant cells enter or leave the
thymus by this route.

10. Cortex
 Mainly composed of
lymphocytes, supported by a
network of finely-branched
epithelial reticular cells.
 Rich Capillary plexus
Medulla
 Network of reticular cells is coarser
than in the cortex,
 The lymphoid cells are relatively
fewer in number
 Hassall's corpuscles
 Few vessels

11.  In each lobule, the cortex is composed of a superficial subcapsular
cortex (a narrow band of cells immediately beneath the capsule), and
the main cortex, which is much more extensive. The central medulla of
both thymic lobes is continuous from one lobule to the next.

12. Epithelial framework
 Unlike other lymphoid structures, in which the supportive framework is
chiefly collagenous reticular tissue, the thymus contains a network of
interconnected epithelial cells.
 They create an appropriate microenvironment, by cell–cell contact and
the release of paracrine factors, in which thymic lymphocytes (T cells)
develop and mature.
 common origin from pharyngeal endoderm

13.  They vary in size and shape
according to their positions
within the thymus.
 Typically they have pale, oval
nuclei, a rather eosinophilic
cytoplasm and intercellular
desmosomal attachments.
Intermediate filament bundles
of cytokeratin lie within their
cytoplasm.
 Subcapsular cells: Blood-
Thymus Barrier
 Medullary epithelial cells tend
to form more solid cords as
well as thymic or Hassall’s
corpuscles.
 Thymic nurse cells

14. Hassall’s corpuscles
 whorls of fl attened, concentrically layered medullary epithelial cells
 30 to 100 μm in diameter
 characteristic features of the thymic medulla.
 contain keratohyaline granules

15. Thymic Nurse Cells
 Large epithelial cells may be associated with around 50 or more
thymocytes.
 Found in the cortex of thymus and also in cortico-medullary junction,
Myoid Cells
 Thymic myoid cells correspond to a muscle-like cell population present in the
thymic medulla.
 They are large, rounded cells, and possess a central nucleus surrounded by
irregularly arranged bundles of myofilaments.
 Their functions are unknown, although it has been suggested that their
contractions might aid the movement of lymphoid cells across or out of the
thymus.

17. Epithelioreticular Cells
 Epithelial reticular cells (or epithelioreticular cells) are a structure in
both the cortex and medulla of the thymus. However, histologically, they
are more easily identified in the medulla. These cells contain secretory
granules which are thought to contain the thymic hormones.
 There are six different types: Types 1-3 are in the cortex, and types 4-6
are in the medulla

18. Thymocytes
 The cortex is densely packed with small thymocytes (thymic
lymphocytes, presumptive T cells).
 Distinct subcapsular zone houses the thymic stem cells and
lymphoblasts undergoing mitotic division.
 The processes of thymocyte development and maturation to generate T
cells depend on the microenvironment provided by epithelial cells,
dendritic cells, macrophages and fibroblasts.

19. Microcirculation
 Major blood vessels enter
the gland at the
corticomedullary junction
and pass within each lobe,
giving off small capillaries
to the cortex and larger
vessels to the medulla.
 Capillary Loops
 Cortical Capillaries: Blood
Thymus Barrier
 Medullary blood vessels
are not as well protected
by epithelial cells

20. Thymic changes during life

22. Mucosa Associated Lymphatic
Tissue (MALT)

23.  unencapsulated lymphoid tissue
 exist in the walls of the alimentary, respiratory, reproductive
and urinary tracts, and in the skin.
 the main subclasses are gut-associated lymphoid tissue
(GALT) and bronchus-associated lymphoid tissue (BALT).
 located in the lamina propria and in the submucosa as
discrete follicles or nodules
 More scattered cells, derived from these follicles, are found
throughout the lamina propria and in the base of the
epithelium.
 Macroscopically visible lymphoid masses, notably the
peripharyngeal lymphoid (Waldeyer’s) ring of tonsillar tissue
(palatine, nasopharyngeal, tubal and lingual), and the
Peyer’s patches of the small intestine.
 Lymphocyte populations are supported mechanically by a fi
ne network of fine type III collagen (reticulin) fibres and
associated fi broblasts, as they are in lymph nodes.

25. Follicles and parafollicular
Zones
The close proximity of lymphocytes
within the MALT to an epithelial
surface facilitates their access to
pathogens.
Lymphocytes migrate into MALT
through its HEV and leave mainly
via its efferent lymphatics, which
drain interstitial fluid as lymph.
MALT lacks afferent lymphatic
vessels.

26. Changes in the overlying
epithelium
 The main function of B lymphocytes in MALT is to produce
IgA for secretion into the lumen of the tracts which they line.
 Epithelium samples and transfers these antigens to
antigen-presenting cells.
 In the small and large intestine these specialized epithelial
cells have characteristic short microvilli on their luminal
surface and are known as microfold (M) cells.
 In the palatine tonsils they include modified stratifi ed
squamous reticulated epithelial cells.

27. Gut Associated Lymphoid
Tissue (GALT)
 The following comprise lymphoid tissue in the gut:
 Waldeyer's tonsillar ring
 Peyer's patches
 Lymphoid aggregates in the appendix and large
intestine
 Lymphoid tissue accumulating with age in
the stomach
 Small lymphoid aggregates in the esophagus
 Diffusely distributed lymphoid cells and plasma cells
in the lamina propria of the gut

28. Tonsils: MALT of Oropharynx

29. Palatine Tonsils
 stratified squamous nonkeratinized epithelium
 deep grooves called tonsillar crypts
 Lymphatic nodules are distributed along the lengths of the tonsillar crypts.
 A dense connective tissue underlies the palatine tonsil and forms its capsule.
Sends trabeculae from the lower aspect (hemicapsule)

31. Pharyngeal tonsil
 pseudostratified ciliated epithelium
 located in the upper posterior part of the throat

32. Lingual Tonsils
 small, individual tonsils, each with its own tonsillar crypt
 nonkeratinized stratified squamous epithelium
 excretory ducts

35. Peyer’s Patches
 characteristic feature of the ileum
 located in the wall of the ileum opposite the mesenteric attachment
 diffuse lymphatic tissue of the lamina propria
 Villi are absent in the area of the intestinal lumen where the nodules reach the
surface of the mucosa.
 Disrupt the muscularis mucosae

43. Cross-section from the chicken's nasal cavity. (A) Panoramic scanning of the
section, (b) concha nasalis media, (c) meatus nasi, (d) optic nerve of nervi
trigeminus, (e) nasal septum, (f) inferior nasal meatus, (g) infraorbital sinus, (i)
choanal cleft. (B) Diffuse lymphoid follicle covered by FAE located on the concha
nasalis media. (C) NALT located on the dorsal side of choanal cleft. (D) NALT
located on the nasal septum.

45. Tertiary Lymphoid Organs
 Tertiary lymphoid organs
(TLOs) are induced
postnatally in non-lymphoid
tissues such as those
affected by chronic
infections, autoimmune
diseases, and chronic
allograft rejection, and also
in cancer tissues. TLOs are
thought to provide important
lymphocytic functional
environments for both
cellular and humoral
immunity, similar to lymph
nodes or Peyer’s patches.