IMMUNE TISSUES
Monotreme and marsupial lymphoid tissues are anatomically and functionally similar to those of eutherian mammals. One point of difference is the timing of immune tissue development, which in eutherians occurs in the sterile uterine environment.
Monotreme and marsupial immune tissues are underdeveloped at birth and undergo significant cellular and structural changes while young are in the pouch or burrow (Borthwick et al. 2014). Immune tissues have not been studied in Australian bats.1.1 Thymus
The thymus is the first lymphoid tissue to mature in monotremes and marsupials, as it is essential for the development of T lymphocytes. Thymus location and number differs between mammals. Most eutherians have a single thoracic thymus, as do polyprotodont marsupials and monotremes (Diener and Ealey 1965; Yadav et al. 1972; Connolly et al. 1999). Diprotodont marsupials have a cervical and a thoracic thymus (Yadav et al. 1972), except wombats, which have only a cervical thymus (Yadav 1973). Both the cervical and thoracic thymus are involved in the development and maturation of T lymphocytes (Wong et al. 2011). The cervical thymus is the first to mature, ~20 d postpartum, followed by the thoracic thymus at ~30 d (Yadav et al. 1972). Thymus maturity is defined by a distinct cortex and medulla containing Hassall’s corpuscles (Yadav et al. 1972; Borthwick et al. 2014).
1.2 Liver and bone marrow
The liver is not directly involved in immunity in adults, but is the major site of haematopoiesis at birth in marsupials. Eutherians differ in that the liver has ceased haema- topoiesis before birth. In marsupials, high levels of granulocytopoiesis and erythropoiesis are evident in the liver at birth, with lymphocytopoiesis beginning shortly after. Hepatic haematopoiesis reaches a maximum within the first 2 wk of life, then begins to decline (Old 2016).
This coincides with an increase in haematopoietic activity in the bone marrow and within 1 mo of birth the bone marrow is the major haematopoietic organ (Borthwick et al. 2014). As PY age, bone marrow is infiltrated and replaced by adipose tissue, with a resulting decline in haematopoiesis (Borthwick et al. 2014). Haematopoiesis in monotremes is not well studied, but one study describes activity in adult platypus bone marrow. It is unknown if the liver plays a role in haematopoiesis in hatchlings (Tanaka et al. 1988).1.3 Lymph nodes
The lymph nodes are a primary site of lymphocyte production and where lymph is filtered for foreign antigens. Lymph nodes have been described in numerous marsupial species and are similar to those in eutherian mammals (Borthwick et al. 2014). Marsupial lymph nodes first appear at ~5 d after birth and are histologically mature by 3 mo in the quokka (Setonix brachyurus) (Borthwick et al. 2014). Their structure is similar to that of eutherian mammals: a capsule surrounding an outer cortex containing numerous primary and secondary follicles with germinal centres and an inner medulla with cords and sinuses (Borthwick et al. 2014). Monotremes differ in that their lymph nodules are not encapsulated and contain only a single primary or secondary follicle (Diener and Ealey 1965; Connolly et al. 1999).
1.4 Spleen
The structure and function of the spleen in monotremes and marsupials is similar to that of eutherian mammals. The spleen is an important secondary immune tissue involved in antigen recognition and antibody production. Lymphocytes populate the spleen within 12 d of birth and the tissue is mature by ~3 mo in the tammar wallaby (Borthwick et al. 2014). The spleen is surrounded by a connective tissue capsule and contains areas of white and red pulp. White pulp constitutes lymphoid follicles with germinal centres and periarterial lymphoid sheaths. The red pulp is the site of haematopoiesis and contains numerous granulocytes and megakaryocytes (Diener and Ealey 1965; Connolly et al. 1999).
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