Haematopoiesis in Marsupials

• Haematopoiesis occurs in the marsupial liver before birth and after birth.
• No bone marrow or spleen haematopoiesis occurs prior to birth in marsupials.
• Most studies on marsupial haematopoiesis have been histologically based.
• Some factors involved in eutherian haematopoiesis have been isolated in marsupials.
• Additional research will provide further insights into the evolution of haematopoiesis.

Marsupials are a group of mammals that give birth to immature young lacking mature immune tissues at birth, and are unable to mount their own specific immune defence. Their immune tissues develop in a non-sterile ex-utero environment unlike that of eutherian mammals such as ourselves. Marsupials are therefore ideal models for studying the development of immune tissues, in particular haematopoiesis, yet relatively little has been investigated. Most studies have been restricted to histological or immunohistological studies, however some factors likely to be involved, based on eutherian studies in haematopoiesis, have been isolated and characterised, including a few key markers, and some cell signaling and regulation molecules, mostly involved in lymphocytopoiesis. However the role of many molecules in haematopoiesis is largely presumed. We currently lack much of the rudimentary information regarding time of appearance and expression levels of these molecules, and no functional studies have been conducted. This paper reviews our knowledge of marsupial haematopoiesis to date, and highlights the need for future research in marsupials to gain further insights into the evolution of haematopoiesis.