Lame basale | EPFL Graph Search

The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often incorrectly referred to as the basement membrane, though it does constitute a portion of the basement membrane. The basal lamina is visible only with the electron microscope, where it appears as an electron-dense layer that is 20–100 nm thick (with some exceptions that are thicker, such as basal lamina in lung alveoli and renal glomeruli). The layers of the basal lamina ("BL") and those of the basement membrane ("BM") are described below: Anchoring fibrils composed of type VII collagen extend from the basal lamina into the underlying reticular lamina and loop around collagen bundles. Although found beneath all basal laminae, they are especially numerous in stratified squamous cells of the skin. These layers should not be confused with the lamina propria, which is found outside the basal lamina. Basement membrane The basement membrane is visible under light microscopy. Electron microscopy shows that the basement membrane consists of three layers: the lamina lucida (electron-lucent), lamina densa (electron-dense), and lamina fibro-reticularis (electron-lucent). The lamina densa was formerly called the “basal lamina”. The terms “basal lamina” and “basement membrane” were often used interchangeably, until it was realised that all three layers seen with the electron microscope constituted the single layer seen with the light microscope. This has led to considerable terminological confusion; if used, the term “basal lamina” should be confined to its meaning as lamina densa. Some theorize that the lamina lucida is an artifact created when preparing the tissue, and that the lamina lucida is therefore equal to the lamina densa in vivo. The term "basal lamina" is usually used with electron microscopy, while the term "basement membrane" is usually used with light microscopy.

Asymmetric Cell Division in the Skin

Wan-Hung Fan

An asymmetric cell division (ACD) is defined as a division that gives rise to two daughter cells with different fates. It is the most common type of division during development. Analogous to Drosophila neuroblasts, proteins involved in ACD such as Insc and LGN are found to be asymmetrically localized during perpendicular cell divisions in mammalian skin, suggesting that the cutaneous tissue homeostasis may be controlled by using similar mechanisms. Most cell divisions in the basal layer of the skin are perpendicular to the basal membrane (BM), but when a skin tumor is formed, most cell divisions would become parallel to the BM. Previous studies in our lab have shown that regressing skin tumor caused by β-catenin depletion have increased perpendicular cell divisions accompanied by reduced stem cell numbers and enhanced mTrim2 expression. Furthermore, induced expression of mTRIM2 in skin tumors led to tumor regression. Taken together, this suggests that progenitor cells in the basal layer of the epidermis may use vertical ACDs to maintain progenitor cell number while using horizontal SCDs to expand progenitor cell number and mTrim2 may function as a cell fate determinant in skin. To validate the hypothesis that vertical divisions of basal keratinocytes to the BM are ACDs, I utilized light sheet based fluorescence microscopy (LSFM) to perform the time lapse recording of murine embryonic skin development between E14.5 and E17.5 and to monitor individual dividing cells and to track their fates after division. For visualizing the dividing cells, protein localization and differentiating cells, I have generated several transgenic mouse lines and combined them together. My study showed that ACD of basal keratinocytes can be either vertical or horizontal to the BM and divisions in the suprabasal layer can be both ACDs and SCDs. To study the potential function of mTRIMs in skin, I generated the knock-out (KO) mice. Neither mTrim2 KO, mTrim3 KO or mTrim32 KO mice showed any phenotype in skin, therefore I combined these mice to generate mTrim2/3 double KO and mTrim2/3/32 triple KO mice. Still, no skin phenotype was observed in these mice, indicating that mTrim2/3/32 might not function as a cell fate determinant in skin.

EPFL2012

Neurogenin3 Initiates Stepwise Delamination of Differentiating Endocrine Cells During Pancreas Development

Anne Grapin-Botton, Mathieu Gouzi, Yung Hae Kim, Keiichi Katsumoto

During development, pancreatic endocrine cells are specified within the pancreatic epithelium. They subsequently delaminate out of the epithelium and cluster in the mesenchyme to form the islets of Langerhans. Neurogenin3 (Ngn3) is a transcription factor required for the differentiation of all endocrine cells and we investigated its role in their delamination. We observed in the mouse pancreas that most Ngn3-positive cells have lost contact with the lumen of the epithelium, showing that the delamination from the progenitor layer is initiated in endocrine progenitors. Subsequently, in both mouse and chick newly born endocrine cells at the periphery of the epithelium strongly decrease E-cadherin, break-down the basal lamina and cluster into islets of Langerhans. Repression of E-cadherin is sufficient to promote delamination from the epithelium. We further demonstrate that Ngn3 indirectly controls Snail2 protein expression post-transcriptionally to repress E-cadherin. In the chick embryo, Ngn3 independently controls epithelium delamination and differentiation programs. Developmental Dynamics 240:589-604, 2011. (C) 2011 Wiley-Liss, Inc.

2011

Role of Neurogenin3 in migration of pancreatic endocrine cells during development

Mathieu Gouzi

The pancreas is one of the major organs of the digestive tract. Its endocrine function, mainly regulating glucose blood level, is achieved by endocrine cells. These cells are organized in clusters, the so-called islets of Langerhans. During development, these cells are specified within the pancreatic epithelium, and will subsequently delaminate and migrate out of the epithelium in order to form the islets. Neurogenin3 (Ngn3) is a bHLH transcription factor that is responsible for the differentiation of all endocrine cell types, but whether or not it has a role in their migration is still an open question. By using the chicken embryo model organism, we found that the differentiation and migration programs are two different processes that are induced by Ngn3 and that can be uncoupled. Therefore, we tried to unravel the mechanisms by which Ngn3 can induce endocrine cell migration. We found that, in both chick and mouse models, over-expression of Ngn3 induces a loss of apico-basal polarity, a breakdown of basal lamina, and more importantly, a loss of the epithelial marker E-cadherin (E-cad). It seems that this is not a direct effect mediated by direct binding to E-boxes in the E-cad promoter. Therefore, we are currently trying to find targets of Ngn3 that could mediate repression of E-cad, focusing on the zinc-finger transcription factors Snail1 and Snail2. Moreover, we also used a pancreatic explant culture method, developed in the laboratory of Pr. Jonathan Slack, which allows us to do time-lapse imaging and describe precisely the migration of endocrine cells in a system similar to in vivo conditions. Taking advantage of our Pdx1::Ngn3ERTM transgenic line, we followed Ngn3 misexpressing cells to understand in a more physiological manner how they migrate in the developing pancreas. We found that endocrine cell migrate at an average speed of 11.35 µm/h and migrate through an average length of 132.30 µm in 12 hours. We also found that endocrine cells migrate in the three compartments we defined: in the mesenchyme, inside the epithelium and at the edge of the epithelium. Their average velocity significantly changes depending on the compartment cells are migrating in. We found that the fastest cells are the ones migrating at the edge of the epithelium. RNA profiling comparing Ngn3 knockouts rescued or not by the Pdx1::Ngn3ERTM transgene was also used to identify the genes that function in endocrine cell migration downstream of Ngn3. We identified new Ngn3 target genes, some of which are implicated in migration such as the Doublecortin family.

EPFL2010