Ultrastructural study of the development of the pars distalis (anterior pituitary) in the foal.

The study examines the development of the pars distalis (anterior pituitary) in horse embryos and fetal foals. It suggests that the secretory cells in this region may be active in hormone production and secretion throughout most of the pregnancy.

Research Methodology

• The researchers studied the pituitary glands of 4 horse embryos aged between 41 and 55 days using light microscopy.
• They also examined the pars distalis from 10 fetal foals ranging between 75 and 300 days through electron microscopy. Electron microscopy allows for a higher level of detail than light microscopy, suitable for observing at a cellular level.

Key Findings

• They detected the development of Rathke’s pouch and the saccus infundibuli at Day 41 of gestation. Rathke’s pouch is the precursor to the anterior pituitary and the saccus infundibuli is part of the posterior pituitary’s development.
• By Day 43, Rathke’s pouch was entirely separate from the stomodaeum (an embryonic feature that later forms the mouth and oral cavity), and the pouch’s wall was starting to grow unevenly.
• The most growth was observed in the ventral wall of the pouch, which eventually forms the pars distalis (a part of the anterior pituitary gland).
• From Day 75, the epithelial cells of the pars distalis started to arrange into clusters of secretory (hormone-producing) and non-secretory cells.
• As pregnancy progressed, these clusters became larger and granulation within individual secretory cells increased. Granulation refers to the appearance of small grains like structures within the cells, which are often hormone-containing granules.
• Mammotrophs (cells that secrete the hormone prolactin) were the most abundant type of secretory cell and were intensely active throughout pregnancy.
• Other hormone-secreting cells like corticotrophs were also detected in the earliest fetus examined. However, somatotrophs and gonadotrophs (cells that secrete growth hormone and reproductive hormones respectively) were not identifiable before Day 250 of gestation.

Research Implications

• Based on their findings, the researchers suggest that the secretory cells of the fetal foal pars distalis might be actively producing and secreting hormones for the majority of the pregnancy period.
• This research gives a deeper understanding of the development of the pituitary gland in horse embryos and fetal foals, which could have implications for studies relating to horse reproduction and developmental biology.