Early Organogenesis and Cell Contacts in the Proliferating Hypophysis of the Developing Mouse

Abstract

Summary: The mouse foetal hypophysis which contains the diencephalic downgrowth (Dd) and Rathke's pouch (Rp) was examined using morphological and immunocyto/histo-chemical methods to study the cell-cell and cell-extracellular matrix (ECM) interactions that occur during early organogenesis. While many studies have described that the differentiation of ACTH cells precedes the proliferation of other endocrine cells in the Rp, light and conventional transmission electron microscopic (TEM) investigations have failed to differentiate between the various endocrine cell types in the proliferating distal lobe at the late mid-foetal stage. Conversely, TEM studies have shown that the occurrence of dense secretory vesicles in glandular cells of the presumptive pars intermedia in dose apposition with the presumptive neural lobe by a basal lamina, is the earliest sign of endocrine activity in the foetal hypophysis. However, a confocal laser scanning microscopy (CLSM) and fine structure study did not observe direct cell-to-cell contacts between the Dd, Rp and their associated mesenchyme at the late mid-foetal stage. Using CLSM and TEM, we detected active cell turnover with programmed cell death in the proliferating Rp, Dd and their associated cephalic mesenchyme. Morphological findings indicated that apoptosis in the cephalic mesenchyme subsequently brings neurohypophyseal pituicytes and adprecursor stem cells into closer proximity, and alternations in the boundary cell surfaces might initiate signal transduction mediated via the intervening ECM at the proliferation stage.

Immunohistochemical Localisation of Epidermal Growth Factor, Transforming Growth Factor a and EGF Receptor during Organogenesis of the Murine Hypophysis in vivo

“…EGFR expression in the cytoplasm and nucleus seemed to indicate downregulation by an internalisation mechanism of EGFR and EGF-receptor complexes in the actively proliferating NP and AP cells. The results of the present study are consistent with those of our previous histological study, which indicated that the mid-foetal murine pituitary is at the active proliferation stage (Han et al, 1998).…”

“…In a previous TEM study, we observed that the proliferating AP was heavily lobulated, but did not differentiate into endocrine cell types in the mid-foetal stage. In addition, we identified the first appearance of MSH cells in the presumptive pars intermedia (Han et al, 1998).…”

“…The transmission electron microscopic (TEM) study suggested that signal transduction between the AP and NP via the extracellular matrix (ECM) might initiate the differentiation of melanocyte-stimulating hormone (MSH) cells in the presumptive pars intermedia at the late mid-foetal stage (Han et al, 1998).…”

Immunohistochemical Localization of Certain Inducers Related to the Proliferation of the Pituitary Gland and a Study on Cell Contacts between the Presumptive Neurohypophysis and the Adenohypophysis in the Foetal Mouse

“…We previously detected active cell turnover with apoptosis in the developing Dd, Rp and the intervening diencephalic mesenchyme thus brought the presumptive NL and the IL into close approximation. Invasion of the mesenchyme enhanced the lobulation of the AL (Han et al, 1998). In the present study, we examine the localization of TGFa, EGF, EGFR, basic fibroblast growth factor (bFGF; FGF-2) and FGF receptors (FGFR-1 , FGFR-2 and FGFR-3) using optic (light) microscopy (LM) and CLSM to elucidate the temporo-spatial expression of these pro-teins during the morphogenetic/differentiative events of the developing murine hypophysis.…”

“…In contrast, transmission electron microscopy (TEM) studies have enabled the earliest signs of endocrine activity to be detected in IL cells, which are in close apposition with the NL via a layer of electron-dense intercellular junctions (Han et al, 1998).…”