Ontogeny of the extraembryonic membranes of the oviparous lizard, Eumeces fasciatus (Squamata: scincidae)

Oviposited eggs of Eumeces fasciatus contain embryos in the limb bud stage. Amniogenesis is complete and two yolk sac membranes, vascular trilaminar omphalopleure (choriovitelline membrane) and bilaminar omphalopleure, enclose the yolk vesicle. A small allantoic vesicle contacts the chorion. The choriovitelline membrane is the primary vascular system. Blood islands, sites of hematopoiesis, are associated with omphalomesenteric vessels of the choriovitelline membrane. The bilaminar omphalopleure, which contacts the eggshell over the abembryonic hemisphere of the egg, lies external to an isolated yolk mass and yolk cleft and is not vascularized. The definitive yolk sac (splanchnopleure) is formed when the extraembryonic coelom and allantoic vesicle intrude into the choriovitelline membrane. Omphalomesenteric vessels are retained with the yolk sac splanchnopleure and the associated hematopoietic sites are present throughout incubation. The chorioallantoic membrane reaches the equator of the egg, entirely supplanting the choriovitelline membrane, after 25% of incubation is completed. Further growth of the allantois is stalled until 65% of incubation is completed when rapid expansion of the allantoic vesicle, in conjunction with resorption of the isolated yolk mass, supplants the bilaminar omphalopleure. As a result, the chorioallantoic membrane completely envelopes the egg for the final 35% of incubation. This developmental event is coincident with published reports for the timing of increased growth and metabolism of embryos. As the isolated yolk mass regresses, intravitelline cells associated with the yolk cleft invade and resorb the yolk to form a large cavity. The wall of this cavity is a germinal epithelium that produces cells that fill the cavity. This structure appears to be a site of hematopoiesis previously undescribed in vertebrates.

Ontogeny of IgM and IgM-bearing cells in rainbow trout

We have studied the ontogenic development of immunoglobulin M (IgM) and of IgM-bearing cells in the rainbow trout, Oncorhynchus mykiss. Lymphocytes showing cytoplasmic IgM were first observed in embryos at 12 days before hatch (14 degrees C). At this stage, no cells positive for surface IgM were present. Lymphocytes bearing surface IgM were observed at 8 days before hatch (14 degrees C). Unfertilized trout eggs contained detectable amounts of IgM (11.2 +/- 2.6 micrograms/g of egg weight), indicating that transfer of IgM from mother to embryo can occur in salmonids. The levels of IgM from whole fish increase slowly after the appearance of intraembryonic cells that express surface IgM. The amount of IgM/g of tissue peaks around hatch, but this parameter shows lower values up to 2 months after hatch.

Immunological changes during pregnancy in the viviparous lizard, Chalcides ocellatus

Splenic cells from pregnant and non-pregnant viviparous lizards (Chalcides ocellatus) were stimulated in vitro with the mitogens, concanavalin A (Con A), phytohemagglutinin (PHA) and Escherichia coli lipopolysaccharide (LPS). Cell cultures from pregnant animals were significantly less responsive to Con A and PHA than comparable cultures from non-pregnant animals. The response was depressed during the first period of pregnancy and remained low in magnitude until parturition. By contrast, the response of maternal splenic cells to LPS was reduced in pregnant lizards only during advanced pregnancy. The drastic decrease in mitogenic responsiveness was associated with marked involution of the maternal spleen. These findings strongly suggest that pregnancy impairs the immunoreactivity of viviparous lizards. Possible mechanisms for this impairment and the relationship to circulating levels of sex hormones are discussed.