Marginal zone and germinal center development in the spleens of neonatally thymectomized and nonthymectomized young rats

Effect of alternate-day hydrocortisone therapy on the immunologically immature rat. II: Changes in T- and B-cell areas in spleen

The histologic effect of 400 mg/M2/day of hydrocortisone, given alternatively from day 7 to day 19 after birth, was studied on the spleen of the immunologically immature rat. Two days after the cessation of treatment, the periarteriolar lymphatic sheaths were found to be largely depleted of small lymphocytes. Immunoperoxidase studies confirmed a depletion of T lymphocytes. The effects of hydrocortisone on the thymus seem to be more important than its direct lymphocytolytic effect in producing this splenic lesion. In contrast, no apparent change in the number of medium-sized B lymphocytes in the marginal zone was detected. Medium-sized B lymphocytes of the spleen, reported to be responsible for IgM synthesis, appeared to be subjected to a different mechanism of hydrocortisone action, other than lysis, resulting in a decrease in antibody production. Primary follicles were not seen in spleens of hydrocortisone-treated rats. Twenty-three days after treatment, spleens had a histologically normal appearance.

Immunohistochemical observation of recovery of the periarteriolar lymphoid sheath, lymph follicle and marginal zone in the 60Co-irradiated spleen of rats

After 60Co irradiation of rat spleen, the most severe damage was found initially in the lymph follicles (LFs); lymphocytes were decreased in number in the periarteriolar lymphoid sheath (PALS), and almost completely depleted in the marginal zone (MZ). Recovery was observed first in the PALS, followed by the LFs, and finally the MZ. From the results of immunohistochemistry, the process of recovery was assumed to involve migration of T and B lymphocytes along the arterioles from the area surrounding the terminal arteriole at the end of the PALS into the central part of the PALS and into the LF. Peripheral blood IgM+, IgD- B lymphocytes, known to be MZ precursor cells, flowed from the follicular artery to settle in the MZ. Furthermore, recovery of the PALS prior to the LFs was suggested to be essential for lymphocyte migration to the LFs.

Electron-microscopic study of the development of the periarteriolar zone in splenic white pulp of rats

To obtain more information concerning the origin of interdigitating cells, the postnatal development and morphology of the periarteriolar lymphatic sheath in splenic white pulp of rats was investigated by light- and electron-microscopy. Special attention was paid to the ontogeny of interdigitating cells. The spleens of the animals were studied in the age range from 1 h to 28 days after birth. The splenic white pulp of neonatal rats consists only of a few reticuloblasts, which are concentrically arranged around central arterioles. After 21 h an increase in promonocytes and monocytes was noted. Between the fifth and seventh postnatal day monocytogenic cells with a light and almost translucent cytoplasm appear, which display long cytoplasmic projections between the adjacent cells. Neighbouring lymphocytes often insert finger-like processes into the invaginated cellular membrane of these transitional forms. This intimate cellular contact is supported by zonulae occludentes. These cells represent transitional forms between monocytes and interdigitating cells. From seven days of age onwards typical interdigitating cells were present as in adult animals. After the differentiation into an inner and outer periarteriolar lymphatic sheath, the T-cell-dependent area of splenic white pulp has attained its adult appearance and further changes are not to be expected. On the basis of these findings, it is highly probable that interdigitating cells develop via transformation of monocytes.

Marginal zones: the major B cell compartment of rat spleens

The marginal zones of rat and human spleens are a broad band of cells, which surround the small lymphocyte area of the white pulp. Staining for acid phosphatase and alpha-naphthyl acetate esterase activity indicates that the majority of cells in this compartment are intermediate-sized lymphocytes. Macrophages only form a minor component. Using quantitative histology, it is shown that the marginal zones are the largest white pulp compartment in terms of volume. They contain approximately as many lymphocytes as the small lymphocyte zones. The cells of the marginal zone are strongly C3- and Fc receptor-positive. While the cells of this zone have demonstratable surface membrane Ig, it is difficult to exclude that this is not passively absorbed. Other means were therefore sought to determine the lineage of lymphocytes in this area. First, the cells in this zone do not bear appreciable quantities of the rat T cell selective antigen detected by the monoclonal antibody W313. Second, marginal zone cells are present in normal numbers in congenitally athymic "nude" rats. Finally, marginal zones do not become populated with lymphocytes in rats treated from birth with anti-mu antibody. Further experiments showed that marginal zone lymphocytes do not recirculate.

Development of T and B cell areas in peripheral lymphoid organs of the rat

In the present study the early development of peripheral lymphoid organs (spleen, popliteal lymph node, mesenteric lymph node and Peyer's patches) is described in terms of homing patterns of T and B cells, demonstrated with immunohistoperoxidatic detection of characteristic membrane antigen in normal rats and with routine histology in neonatally thymectomized rats. In the first days after birth the peripheral lymphoid organs are almost exclusively populated by T cells. After neonatal thymectomy lymphocytes appear in the dome areas of Peyer's patches from four to six days after birth, in mesenteric and popliteal lymph nodes lymphocytes are found in the outer cortex from day 6 and day 8 respectively and in the marginal zone of the spleen from eight days onwards. These lymphocytes showed no membrane staining when reacted for T antigen with immunohistoperoxidatic techniques. The morphological evidence for considering Peyer's patches of rats as central inductive sites for the generation of B cells is poor. The discrepancy in the order of appearance of T and B cell (sub)populations in spleen compartments in normal ontogenetic development and lethally irradiated, stem cell reconstituted animals is discussed.

Germinal centers in the spleens of neonates and stillbirths

Sections of spleens from 235 stillbirths and 227 babies who died between the ages of 1 and 35 days were examined. None of the stillbirths showed any germinal centres, neither did the spleens of any of the babies who died between the ages of 1 and 14 days. From the age of 15 days onwards, the germinal centres started to appear in varying numbers. The appearance of germinal centres in the spleen is further delayed in infants born before 37 wk gestation. Germinal centres in the spleen in neonates do not appear to be related to the presence of inflammatory changes elsewhere in the infant.

Development of germinal centres in the spleen in infants related to birth and unexpected death

The development of germinal centres in the spleen has been estimated in a series of 266 children dying in hospital and 205 children presenting as unexpected death in infancy. The prevalence of germinal centres in the spleen increases rapidly after birth. The percentage of Malpighian bodies showing germinal centres in the spleen is greater in all categories of our classification of death in children dying unexpectedly at home when these are compared with hospital deaths. It is concluded that the children presenting as unexpected death in infancy show no evidence of any deficiency in cellular immunological response and may well have been subjected to increased or precocious antigenic stimulation.