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1. BY- DR. VIBHASH KUMAR VAIDYA
DEPARTMENT OF ANATOMY
HISTOLOGY OF LYMPHATIC
SYSTEM

2. Histology of lymphatic system
 The lymphoid system includes lymphatic vessels and
lymphoid organs.
 Lymphatic vessels are closely associated with the blood
vascular system, and they drain into the systemic
circulation.
 Large aggregation of lymphocytes form lymphoid organs
( thymus, spleen, lymph nodes and tonsils).

3. Lymphocytes
 All lymphocytes originate in the bone marrow. They are
present in blood, lymph, tissue spaces and lymphoid
tissues.
 There are three types of lymphocytes:
1. T lymphocytes ( responsible for cell- mediated
immunity)
2. B lymphocytes ( responsible for humoral immunity)
3. Natural killer ( they kill tumour cells and virus-infected
cells)

4. Development of T and B cells

5. Thymus
 Thymus is a bilobed gland located in the superior
mediastinum.
Microscopic features:-
 Thymus has a capsule of connective tissue covering both the
lobes.
 From the capsule, numerous septa containing blood vessels
extend into the substance of the organ and divided into
incomplete lobules.
 Each lobule consists of peripheral cortex and central medulla.
 Since the septa do not divide the organ completely, the central
part of each lobule is continuous with the medulla of the
neighboring lobules.

6. Histology of Thymus
Cortex
 Peripheral dark zone.
 Mainly composed of densely packed T lymphocytes.
 Apart from lymphocytes, cortex also contains epithelial reticular
cells and macrophages.
Medulla
 Medulla is the central lighter zone of each lobule.
 Lymphocytes in medulla are fewer while epithelial reticular cells
are more in number.
 Prominent feature in medulla is hassall’s corpuscles, also known
as thymic corpuscles.
 They consist of concentrically arranged epitheliocytes.

7. H&E drawing Section of Thymus

8. Blood – thymus barrier
 The antigen circulating in the blood does not come in
contact with thymic lymphocytes due to the presence of
blood- thymus barrier.
 This barrier consists of an endothelium of capillaries, a
thick basement membrane and a layer of reticular
epithelial cells.

9. Lymph node
 Lymph nodes are bean-shaped structures present in the course of
the lymphatic vessels.
 Lymph node have a capsule, cortex, medulla and various sinuses.
Capsule
 The capsule is made up of connective tissue.
 Numerous trabeculae arise from the capsule and enter the
substance of the node.

10. Cortex
 Cortex is the darkly stained peripheral part of the lymph node, laying
underneath the capsule.
 It is packed with lymphocytes.
 Plasma cells and macrophages are also present.
 It divided into the outer cortex and inner cortex.
Outer cortex
 lies underneath the capsule.
 Contains mainly B lymphocytes with spherical lymphoid nodule.
 Some of these nodules show a light-stained zone in the central called
germinal center, which is the site of B lymphocyte proliferation.
Lymph node

11. Lymph node
Inner cortex
 lies underneath the outer cortex and surrounds the medulla.
 Also known as paracortex.
 It has mainly T lymphocytes; hence , it is aslo called thymus-
dependent cortex.
 No lymphoid nodule is seen.
Medulla
 Light- stained central part of the lymph node.
 It consists of medullary cord and sinuses.
 Medullary cords are extensions of the inner cortex separatesd by
medullary sinuses.

12. Lymph node
Sinuses
 Lymph passes through various sinuses of lymph nodes and gets
filtered in them.
a) Subcapsular sinus: present between the capsule and and the
outer cortex.
b) Intermediate sinus: present on both sides of the trabeculae and
connects subcapsular sinus to medullary sinus.
c) Medullary sinuses: these are anastomosing sinuses present in
between the medullary cord in the medulla.

13. H&E drawing Section Lymph node

14. Spleen
 It is the largest lymphoid organ.
 Spleen is covered by a capsule.
 The parenchyma of the spleen consist of white pulp, red pulp
and a marginal zone.
Capsule
 It has a capsule of dense connective tissue from which
trabeculae arise and enter the substance of the organ.
 Blood vessels present in trabeculae also enter the organ along
with it.

15. Spleen
White pulp
 Terms white pulp are based on the appearance of the unstained
sections of fresh specimen.
 In the unstained sections of fresh specimen, there are grey-
coloured circular regions( white pulp) surround by dark red
tissue (red pulp).
 Arteries in trabeculae give rise to arterioles.
 These arterioles , called central arteries.
 Lymphoid tissue surrounds the central arteries.

16. Spleen
Red pulp
 Most of the spleen consist of red pulp.
 Terms red pulp are based on the appearance of the unstained
sections of fresh specimen.
 It has two components- red pulp cords and sinusoids.
 Red pulp cords are also called cords of billroth.
 They are irregular anastomosing cords surrounding the
sinusoids.
 Sinusoids have wide lumen; the endothelial cells of sinusoids are
elongated and lie parallel.

17. H&E drawing Section of spleen

18. Palatine tonsil
 It is a pair of lymphoid tissue located in the lateral wall
of oropharynx.
 The medial surface ( facing the lumen of oropharynx)
and crypts of the palatine tonsil are lined by stratified
squamous non- keratinised epithelium.
 Underneath the crypts, there are numerous lymphoid
follicles.
 The lateral surface of the palatine tonsil is separated
from the underlying tissues by a hemicapsule.

19. H&E drawing Section palatine tonsil