On the nature of pharyngeal muscle cells in the Tardigrada

As part of a transmission electron microscopic study of the embryological development in tardigrades, the ultrastructure of the pharynx was examined. The intent was to establish whether the pharyngeal muscle cells constitute an eclodermal myoepithelium (as in many aschelminth pharynges) or whether they are mesodermal (as in certain other aschelminth groups and among articulates). In the latter case the cuticle would be produced solely by specialized epithelial cells. The eutardigrade species Halobiotus crispae Kristensen, 1982, was investigated in four embryological stages, as a newly hatched juvenile, in the active adult stage, and in the hibernation stage pseudosimplex I. A comparison was made with the arthrolardigrade Actinarctus doryphorus Schulz, 1935, in the active adult stage and in the simplex stage. The results indicate that the tardigrade pharynx is an ectodermal myoepithelium. The muscles appear to be truly cross-striated and monosarcomerial. The phylogenetic implications of these findings are discussed briefly.     

Heart and the peritoneal cover of the gut sinus in the polychaete Arenicola marina: an ultrastructural and autoradiographic study

To elucidate the cellular mechanism underlying the growth of the peritoneal cover of the gut sinus and the heart in the polychaete Arenicola marina, cellular organization of these structures and proliferative potential of their cells were investigated using electron microscopy and electron microscopic autoradiography. Arenicola has a pair of dorsolaterally situated hearts connected to the gut sinus via a short duct and composed of two muscular layers separated by a layer of the extracellular matrix (ECM). The peritoneal cover of the gut sinus and the outer muscular layer of the heart present a myoepithelial layer resting on the ECM. The inner muscular layer of the heart is composed of myofibril-containing cells lacking well-defined polarity in arrangement of organelles. However, their persistent connection to branches of the ECM and the adherens-like intercellular junctions allow for considering the inner layer a modified myoepithelium. In the peritoneal cover of the gut sinus and in both myoepithelial layers of the heart, noncontractile epithelial cells have been observed. As determined by thymidine labeling, these epithelial cells are capable of DNA synthesis, while myoepithelial cells are not. Some suggestions are made about the myogenic nature of the epithelial cells in the investigated structures of A. marina.     

Microcinematographic demonstration of synchronous and asynchronous myoepithelial contractions in mouse submandibular gland rudiments in organotypic culture

Summary Time-lapse phase-contrast cinematography revealed contractile activity within mouse submandibular salivary gland rudiments in organotypic culture. Three types of contraction were distinguishable. In type I (voiding contractions), all portions of the gland contracted synchronously, and the active state ranged from 30 min to 2 hr. In type II (priming contractions), all portions of the gland contracted synchronously, but the active state was shorter, ranging from 4 to 10 min. In type III (churning contractions), isolated foci in lobules or secretory units throughout the gland contracted asynchronously and had very short active states of about 1 min. By electron microscopy, myoepithelial cells could first be demonstrated in submandibular glands developing either in vitro or in vivo, at 21 days postconception. Contractions in the cultured rudiments began as early as 18 days postconception. Since neither smooth nor striated muscle could be identified in these glands by electron microscopy, the contractions are believed to result from myoepithelial activity that apparently may begin before ultrastructural evidence of myoepithelial differentiation is contractile function and indirect evidence has lent ample support to this presumption, the present study represents the first direct cinematographic demonstration and characterization of myoepithelial contractions, under conditions in vitro.     

Control of mammary myoepithelial cell contractile function by α3β1 integrin signalling

In the functionally differentiated mammary gland, basal myoepithelial cells contract to eject the milk produced by luminal epithelial cells from the body. We report that conditional deletion of a laminin receptor, α3β1 integrin, from myoepithelial cells leads to low rates of milk ejection due to a contractility defect but does not interfere with the integrity or functional differentiation of the mammary epithelium. In lactating mammary gland, in the absence of α3β1, focal adhesion kinase phosphorylation is impaired, the Rho/Rac balance is altered and myosin light-chain (MLC) phosphorylation is sustained. Cultured mammary myoepithelial cells depleted of α3β1 contract in response to oxytocin, but are unable to maintain the state of post-contractile relaxation. The expression of constitutively active Rac or its effector p21-activated kinase (PAK), or treatment with MLC kinase (MLCK) inhibitor, rescues the relaxation capacity of mutant cells, strongly suggesting that α3β1-mediated stimulation of the Rac/PAK pathway is required for the inhibition of MLCK activity, permitting completion of the myoepithelial cell contraction/relaxation cycle and successful lactation. This is the first study highlighting the impact of α3β1 integrin signalling on mammary gland function.     

Progesterone and Wnt4 control mammary stem cells via myoepithelial crosstalk

Ovarian hormones increase breast cancer risk by poorly understood mechanisms. We assess the role of progesterone on global stem cell function by serially transplanting mouse mammary epithelia. Progesterone receptor (PR) deletion severely reduces the regeneration capacity of the mammary epithelium. The PR target, receptor activator of Nf‐κB ligand (RANKL), is not required for this function, and the deletion of Wnt4 reduces the mammary regeneration capacity even more than PR ablation. A fluorescent reporter reveals so far undetected perinatal Wnt4 expression that is independent of hormone signaling. Pubertal and adult Wnt4 expression is specific to PR⁺ luminal cells and requires intact PR signaling. Conditional deletion of Wnt4 reveals that this early, previously unappreciated, Wnt4 expression is functionally important. We provide genetic evidence that canonical Wnt signaling in the myoepithelium required PR and Wnt4, whereas the canonical Wnt signaling activities observed in the embryonic mammary bud and in the stroma around terminal end buds are independent of Wnt4. Thus, progesterone and Wnt4 control stem cell function through a luminal–myoepithelial crosstalk with Wnt4 acting independent of PR perinatally.