J. Anat. (2001) 198, pp. 491–495, with 5 figures Printed in the United Kingdom 491

Short Report

Intrathymic lymph nodes in humans

A. TANEGASHIMA1, I . USHIYAMA2, Y. KIKUI1 AND H. YAMAMOTO1

"Department of Forensic Medicine and Sciences, Mie University School of Medicine, Mie, and

#Department of Legal Medicine, Shiga University of Medical Science, Shiga, Japan

(Accepted 14 November 2000)

An unusual lymph node exists in the centre of the human thymus. This lymph node, which we call an

intrathymic lymph node (ITLN), possesses some interesting morphological characteristics. In ontogeny, this

node seems to appear at the latter half of fetal period. The function of the ITLN is still unknown, but it is

assumed that it may play a different role in the immune system than other peripheral lymph nodes by

its characteristics.

Key words : Lymphatic system; thymus.

The human thymus is located in the anterior medias-

tinum and overlies the pericardium. It comprised of

right and left lobes which are completely invested by

fibrous capsules, and each lobe is subdivided by septa

from the capsule into many lobuli. The upper portion

(cervical thymus) and the lower portion (thoracic

thymus) are clearly divided by septa containing large

blood vessels. We have previously reported the

existence of several lymph nodes, called parathymic

lymph nodes (PTLNs), located at the apex of the

cervical lobes or between the cervical and thoracic

lobes, and some interesting characteristics of those

lymph nodes (Tanegashima et al. 1999). In this study,

we made sequential sections of the whole thymus, and

discovered the existence of the intrathymic lymph

node (ITLN) located in the centre of the thymus.

All thymus samples were obtained from cadavers

during autopsy in the Osaka Medical Examiner’s

Office, Department of Forensic Medicine and

Sciences, Mie University School of Medicine, and the

Department of Legal Medicine, Shiga University of

Medical Science. Age, sex and cause of death of each

donor in this study were summarised in the Table.

For Cases 1–5 in the Table, the whole thymus was

Correspondence to Dr Akio Tanegashima, Department of Forensic Medicine and Sciences, Mie University School of Medicine, 2-174,

Edobashi, Tsu, Mie 514-8507, Japan. Tel. : ­81-59-232-1111, ex: 6383; fax: ­81-59-231-5014; e-mail : tane!doc.medic.mie-u.ac.jp

fixed in 10% phosphate buffered formalin, embedded

in celloidin and 25 µm serial sections were cut. All

sections were stained with haematoxylin and eosin.

For Cases 6–8 in the Table, the whole thymus was

fixed in 10% phosphate buffered formalin, embedded

in paraffin and serial 10 µm sections were cut (Cases 7,

8) or at 15 µm (Case 6). In addition to haematoxylin

and eosin, immunohistochemical staining was per-

formed; deparaffinised sections were treated with

methanol containing 0±3% H#O

#to block endogenous

peroxidase, hydrated and treated with PBS (0±1

phosphate buffered saline) 3 times. They were then

incubated in PBS containing 1% bovine serum

albumin (BSA) for 30 min. After blotting dry with

paper filter, the sections were incubated with mono-

clonal antibody (1:100) at room temperature of 2 h.

Tissue sites reactive with the antibody were visualised

with the streptavidin-biotin-peroxidase complex

method, using the Histofine SAB-PO kit (Nichirei,

Tokyo, Japan). Finally, the sections were counter-

stained with haematoxylin. Anti-CD20 mouse mono-

clonal antibody was perchased from DAKO A}S

(Glostrup, Denmark).

An ITLN was present in all 7 cases except for a fetus

of 5 mo gestational age (Table, Case 7) and was

Table. Age, sex and cause of death of each donor

Case Age Sex Cause of death

1 3 mo Fe Sudden infant death syndrome

2 4 mo M Pneumonia

3 8 mo M Subdural haemorrhage, trauma

4 11 mo Fe Asphyxia

5 15 y Fe Sudden death*

6 31 y Fe Subarachnoid haemorrhage, moyamoya disease

7 Fetus** M Stillbirth

8 Fetus*** Fe The mother’s sudden death

* Unexplained sudden death; cause of death could not be determined at autopsy; ** fetus at 5 mo gestational age; *** fetus at 6 mo

gestational age.

Fig. 1. Thymus obtained from a 3-mo-old infant (Table, Case 1). (a) Macroscopic view of section 275 of 598 sequential sections. The yellow

arrow shows ITLN located in the centre of the thymus, and the red arrow PTLNs located between left cervical and thoracic lobes. (b)

Microscopic view of PTLNs. (c) Microscopic view of ITLN. A large part of this node is dominated by the paracortical area and, in contrast,

only a thin cortex with a few follicles is poorly developed. Medullary cords and sinus are scarce. (d ) High magnification view of the paracortical

area adjacent to the hilus (surrounded by a yellow box in Fig. c). Many blood vessels (yellow arrows) spread radially from the hilus, yielding

HEVs. Medullary cords and sinus are lacking in this area.

492 A. Tanegashima and others

Fig. 2. Thymus obtained from an 11-mo-old infant (Table, Case

3). (a) Macroscopic view of section 220 of 336 sequential sections.

The yellow arrow shows ITLN located in the centre of the thymus,

and the red arrows show PTLNs located at the apex of the cervical

thymus and between left cervical and thoracic lobe. (b) Microscopic

view of ITLN. Germinal centres are relatively more developed

compared with Case 1. Similar to Case 1, a large part of this node

is dominated by the paracortical area, and medullary cords and

sinus are scarce.

always located to the right of the septum dividing the

thymus into left and right lobes. The ITLNs in

postnatal individuals (Table, Cases 1–6) possessed

some morphological characteristics in common (Figs

1c, d, e, 2b, 3a), as all had a well-developed para-

cortical area rich in high endothelial venules (HEVs)

which occupied most parts of this node, and only a

thin cortex with a few follicles. The marginal sinus

showed many macrophage-like cells. It is of interest

that there were few trabeculae, medullary cords or

medullary sinuses. Immunohistochemical staining

with anti-CD20 antibody revealed that the superficial

cortex possessed small-sized follicles containing B

cells and CD20-positive B cells were also present in

the perivascular area (Fig. 3b). These unusual his-

tological findings are similar to those of the human

PTLNs which we have reported previously.

Fig. 3. Thymus obtained from a 4-mo-old infant (Table, Case 2).

(a) Microscopic view of ITLN. A large part of this node is

dominated by the paracortical area and, in contrast, only a thin

cortex with a few follicles are seen. Medullary cord and sinus are

scarce. Haematoxylin and eosin. (b) Immunohistochemical staining

with anti-CD20 antibody. The subcapsular area and small primary

follicles (Fol) are stained. CD20-positive B cells are also present

generally in the perivascular area.

The ITLN was barely detectable in the thymus of a

fetus at 5 mo gestational age in this study (Table,

Case 7), which may suggest that at this fetal age the

ITLN is still in early stages of development and does

not possess sufficient cell density to be detectable. In

the thymus of a fetus at 6 mo gestational age (Table,

Case 8), the ITLN was detected clearly (Fig. 4). At

this fetal age, the cellular density of this node was still

low, but HEVs were well-developed (Fig. 4b). Im-

munohistochemical staining with anti-CD20 antibody

showed that only a few B cells were detected in the

perivascular area but the follicles were not formed in

the ITLN at this fetal age (data not shown). Lymph

nodes in various stage of development can be observed

in a single fetus (Bailey & Weiss, 1975), and therefore,

it is thought that the development of the ITLN may

occur relatively later in ontogeny.

The phylogenesis of the lymphoid tissues starts in

the lower vertebrates (Jonsson & Christensen, 1978),

Intrathymic lymph nodes in humans 493

Fig. 4. Thymus obtained from a fetus at 6 mo gestational age

(Table, Case 8). (a) Macroscopic view of the thymus. The yellow

arrow shows the ITLN located in the centre of the thymus, and the

red arrow a PTLN located between left cervical and thoracic lobes.

(b) Microscopic view of ITLN. HEVs are well-developed, but the

cellular density of this node is much less than in postnatal

individuals (see Fig. 1). The developing capsular sinus can be seen.

The nodal capsule is still thin.

but the definitive forms of lymph node-like structures

are first present in the lower mammals. Furthermore,

the ITLN is thought to be first present in the higher

mammals in phylogeny because such lymph nodes

are absent at least in the thymus of rats and mice.

In adults, the ITLN may gradually involute

following thymic involution. It is interesting to

consider that the fatty degeneration of the ITLN may

start from the outer side (Fig. 5), which for peripheral

lymph nodes usually occurs by replacement of the

parenchyma with adipose tissue (Luscieti et al. 1980).

As shown in Figure 5b, replacement with adipose

tissue is seen in the capsule ; the boundary between

ITLN and thymic parenchyma thus becomes less

definite as involution progresses.

It seems likely that the human ITLN and PTLNs

may be located downstream from the thymus, and

that some newborn T cells produced in the thymus

may enter these nodes because it is generally con-

sidered that the human thymus has no afferent

lymphatic vessels (Goldstein & Mackay, 1969;

Fig. 5. Thymus obtained from a 34-y-old adult (Table, Case 6). (a)

Microscopic view of ITLN. (b) High magnification view (sur-

rounded by a green box in a). There is some replacement by adipose

tissue in the capsule and subcapsular area. Arrows show the capsule

infiltrated with adipose tissue. The marginal sinus (*) is filled with

many macrophage-like cells.

Bloodworth et al. 1975). Moreover, self antigens and

some antigen-presenting cells (e.g. macrophages and}or dendritic cells) may also enter these nodes from the

thymus through the lymphatics. On the other hand,

some peripheral lymphocytes may also enter these

nodes through the HEV. Thus these nodes are likely

to be the place for interaction between peripheral

lymphocytes and cells from the thymus.

The role of intra- and parathymic lymph nodes is

still unknown. It is suggested, however, that these

nodes may play a particular role in the immune

system which differs from that of other peripheral

lymph nodes on the basis of the morphological

characteristics of these nodes. Moreover, it is

suggested that the ITLN may be related to some

autoimmune diseases (e.g. myasthenia gravis) with the

development of germinal centres in the thymus.

This work was supported in part by a Grant-in-Aid

for Scientific Research (B)(2) from the Japanese

Ministry of Education, Science, Sports, and Culture.

494 A. Tanegashima and others

We thank Miss Naoko Mizuhara and Miss Misako

Okuno for technical assistance and Dr Steven Rand

for valuable comments and English corrections.

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Intrathymic lymph nodes in humans 495