Differences between human and mouse alpha-fetoprotein expression during early development.

Alpha-fetoprotein (AFP) is the major serum protein during development. AFP is one of the earliest proteins to be synthesised by the embryonic liver. The synthesis of AFP decreases dramatically after birth and only trace amounts are expressed in the adult liver. The tissue distribution of AFP in early human embryogenesis has not been defined. We have studied the expression pattern of AFP mRNA in human and mouse embryos by in situ hybridisation. In humans, AFP is expressed in the hepatic diverticulum at 26 d postovulation as it differentiates from the foregut endoderm (i.e. in the most primitive hepatocytes). It is also expressed in the endoderm of the gastrointestinal tract and in the yolk sac at this age. AFP is subsequently expressed in the mesonephros and transiently in the developing pancreas. In the mouse, no expression of AFP was observed in the mesonephros but other sites of expression were similar. Thus AFP has a distinct temporospatial expression pattern during the embryonic period and this differs between human and mouse species. It is interesting that AFP is expressed by tumours such as primitive gastrointestinal, renal cell and pancreatic tumours as well as those of hepatocyte origin. This distribution reflects the sites of AFP expression during development.

JAGGED1 expression in human embryos: correlation with the Alagille syndrome phenotype.

Alagille syndrome (AGS, MIM 118450) is an autosomal dominant disorder with a variable phenotype characterised by hepatic, eye, cardiac, and skeletal malformations and a characteristic facial appearance. Mutations within the gene JAGGED1 (JAG1), which encodes a ligand for NOTCH receptor(s), has been shown to cause Alagille syndrome. Interactions of NOTCH receptors and their ligands influence cell fate decisions in several developmental pathways. We report the tissue expression of JAG1 in human embryos. We have performed tissue in situ hybridisation on human embryos aged 32-52 days using (35)S labelled riboprobes for JAG1. JAG1 is expressed in the distal cardiac outflow tract and pulmonary artery, major arteries, portal vein, optic vesicle, otocyst, branchial arches, metanephros, pancreas, mesocardium, around the major bronchial branches, and in the neural tube. We conclude that JAG1 is expressed in the structures affected in Alagille syndrome, such as the pulmonary artery, anterior chamber of the eye, and face.

Changing pattern of expression of parvalbumin immunoreactivity during human fetal spinal cord development.

Expression of the calcium binding protein parvalbumin (PV) by different classes of spinal neuron has been shown to be developmentally regulated in both rat and monkey. From postmortem studies of eight human cervical spinal cords ranging in age from 11 to 35 weeks postconceptional age, we report that parvalbumin immunoreactivity is similarly plastic in human lower cervical spinal cord development, with many changes occurring prenatally. At 11-14 weeks postconceptional age, there was prominent immunostaining of primary sensory afferents that could be seen coursing through the dorsal horn and extensively innervating the motoneuron pools. Motoneurons were also found to be clearly immunoreactive for choline acetyltransferase by this age. A few ventral horn neurons that were not motoneurons were also parvalbumin immunoreactive. By 24-27 weeks postconceptional age, sensory afferents were still immunoreactive, as were many other axons throughout the white matter. In addition, many ventral horn neurons were now immunoreactive as well as a few dorsal horn neurons. By 31-35 weeks postconceptional age, there was extensive immunostaining of neurons throughout the spinal cord, including a few moderately immunoreactive motoneurons. There were many immunopositive axons in all the white matter tracts except the corticospinal tracts; however, staining of sensory axons traversing the grey matter was less prominent by this age. In the rat, expression of PV by primary sensory neurons coincides with the onset of fetal limb movement. The onset of expression of PV in ventral horn neurons coincides with later developmental events after the arrival of corticospinal inputs, whereas widespread PV immunoreactivity in dorsal horn neurons marks the attainment of a mature pattern of PV expression. The extent to which expression of PV immunoreactivity can be taken to indicate landmarks in human development will be discussed.

The short stature homeobox gene SHOX is involved in skeletal abnormalities in Turner syndrome.

Turner syndrome is characterized by short stature and is frequently associated with a variable spectrum of somatic features including ovarian failure, heart and renal abnormalities, micrognathia, cubitus valgus, high-arched palate, short metacarpals and Madelung deformity. Madelung deformity is also a key feature of Leri-Weill syndrome. Defects of the pseudoautosomal homeobox gene SHOX were previously shown to lead to short stature and Leri-Weill syndrome, and haploinsufficiency of SHOX was implicated to cause the short stature phenotype in Turner syndrome. Despite exhaustive searches, no direct murine orthologue of SHOX is evident. SHOX is, however, closely related to the SHOX2 homeobox gene on 3q, which has a murine counterpart, Og12x. We analysed SHOX and SHOX2 expression during human embryonic development, and referenced the expression patterns against those of Og12x. The SHOX expression pattern in the limb and first and second pharyngeal arches not only explains SHOX -related short stature phenotypes, but also for the first time provides evidence for the involvement of this gene in the development of additional Turner stigmata. This is strongly supported by the presence of Turner-characteristic dysmorphic skeletal features in patients with SHOX nonsense mutations.

SRY, SOX9, and DAX1 expression patterns during human sex determination and gonadal development.

SRY, SOX9, and DAX1 are key genes in human sex determination, by virtue of their associated male-to-female sex reversal phenotypes when mutated (SRY, SOX9) or over-expressed (DAX1). During human sex determination, SRY is expressed in 46,XY gonads coincident with sex cord formation, but also persists as nuclear protein within Sertoli cells at 18 weeks gestation. High-level SOX9 expression in the sex cords of the testis parallels that seen during mouse development, however in humans, SOX9 transcripts also are detected in the developing ovary. Low-level DAX1 expression predates peak SRY expression by at least 10 days, and persists in Sertoli cells throughout the entire sex determination period. In Dosage Sensitive Sex reversal, the anti-testis properties of DAX1 over-expression could act prior to the peak effects of SRY and continue during the period of SOX9 expression. These findings highlight expression differences for the SRY, SOX9, and DAX1 genes during sex determination in humans and mice. These results provide a direct framework for future investigation into the mechanisms underlying normal and abnormal human sex determination.

Expression of steroidogenic factor 1 and Wilms' tumour 1 during early human gonadal development and sex determination.

The transcription factors SF-1 and WT1 play pivotal roles in mammalian gonadal development and sexual differentiation. In human embryos, both SF-1 and WT1 are expressed when the indifferent gonadal ridge first forms at 32 days post-ovulation. As the sex cords develop - providing morphological evidence of testis differentiation - SF-1 localises predominantly to developing Sertoli cells in the sex cords, whereas WT1 retains a broader pattern of expression. Later, SF-1 localises predominantly to steroidogenic Leydig cells, and WT1 localises to the sex cords. In the ovary, SF-1 and WT1 transcripts persist in the gonadal ridge from the earliest developmental stages throughout the critical period of sex determination. These studies, which delineate for the first time the sequential expression profiles of SF-1 and WT1 during human gonadal development, provide a framework for understanding human sex reversal phenotypes associated with their mutations.