Myoepithelial cells

Four different kinds of muscle are found in animals (Figure 17.10). They are skeletal muscle, cardiac (heart) muscle, smooth muscle, and myoepithelial cells. 

FIGURE 17.10 The four classes of muscle cells iu mammals. Skeletal muscle and cardiac muscle are striated. Cardiac muscle, smooth muscle, and myoepithelial cells are mononucleate, whereas skeletal muscle is multinucleate. 

AVP plays a central role in water homeostasis of terrestrial mammals, leading to water conservation by the kidney. OT is primarily involved in milk ejection, parturition and in sexual and maternal behaviour. Both hormones are pqDtides secreted by the neurohypophysis, and both act also as neurotransmitters in the central nervous system (CNS). The major hormonal targets for AVP are the renal tubules and vascular myocytes. The hormonal targets for OT are the myoepithelial cells... 

OTRs are mainly expressed in myoepithelial cells of the galactiferus channels and the myometrium. The OTRs in vascular endothelial cells, renal epithelial cells (macula densa, proximal tubule) and cardiomyocytes induce the production of NO (vasodilation), natriuresis and release of ANP, respectively. The endometrium, ovary, amnion, testis, epididymis, prostate and thymus also express the OTR supporting a paracrine role of this peptide. Osteoblasts, osteoclasts, pancreatic islets cells, adipocytes, and several types of cancer cells also express OTRs. More over, expression of the OTR... 

Moll et al. 1982a, 1983 Troyanovsky et al. 1989, 1992). Thus, K17 may also be regarded as a basal/myoepithelial cell keratin which probably is mainly expressed in progenitor cells. 

Myoepithelial cells are contractile elements found in salivary, sweat, and mammary glands that show a combined smooth muscle and epithelial phenotype (Foschini et al. 2000). In the normal breast, the ductal and acinar units are lined by two cell layers the inner layer lining the lumen and an outer layer of myoepithelial cells. An intact myoepithelial cells layer is seen in both benign and in situ lesions, whereas loss of the myoepithelial cells layer is considered the rule for the diagnosis of invasive cancer (Kalof et al. 2004 and citations therein). 

Because myoepithelial cells are not always readily identifiable on routine hematoxylin- and eosin-stained sections, many immunohistochemical methods have been used to highlight an intact myoepithelial cells layer. Recent studies have reported CD10 and smooth muscle myosin heavy chain (SMMHC) expression in... 

Fig. 13.6 Triple staining of an adenoid cystic carcinoma for K14 (green a), (b) for K8/18 (red b) and for SMA (pink c). Note that the k 4+ cells differentiate to glandular and myoepithelial cells...

Fig. 13.8 Triple staining of a usual ductal hyperplasia for K5 (red), K8/18 (green) and SMA (pink). Note that the proliferating cells are only glandular cells. Myoepithelial cells are seen only in the outer layer (pink) and do not participate in the proliferation process...

Bocker W, Moll R, Poremba C, Holland R, Van Diest PJ, Dervan P, Burger H, Wai D, Ina DR, Brandt B, Herbst H, Schmidt A, Lerch MM, Buchwalow IB (2002) Common adult stem cells in the human breast give rise to glandular and myoepithelial cell lineages a new cell biological concept. Lab Invest 82(6) 737 746... 

Kalof AN, Tam D, Beatty B, Cooper K (2004) Immunostaining patterns of myoepithelial cells in breast lesions a comparison of CD10 and smooth muscle myosin heavy chain. J Clin Pathol 57 625 629... 

Fig. 2. Combined miR-205 ISH and sm-a-actin IHC for delineation of the cellular origin of miR-205 in normal breast gland. MiR-205 was detected with TSA-FITC substrate greerf) and sm-a-actin with Cy3 (red). Section was counterstained with DARI (blud). MiR-205 ISFI signal is seen in sm-a-actin-positive myoepithelial cells surrounding the unstained luminal ductal epithelial cells.

The obvious differential expression of miR-205 and sm-a-actin is noteworthy. MiR-205 is likely not associated with smooth muscle differentiation in general since no expression is seen in the VSMC and myofibroblasts. MiR-205 is more likely related to the basal cell characteristics also seen in skin (20). The specific targets and role of miR-205 in differentiation of basal cells are not known (45). The differential expression of miR-21 and sm-a-actin is also interesting but is not as drastic as for miR-205. In addition to myofibroblasts, both smooth muscle cells in colon (46) and myoepithelial cells in breast (44) have been found to express miR-21. 

Oxytocin (Pitocin, Syntocinon) causes milk release (letdown) by stimulating contraction of the myoepithelial cells of the milk ducts in lactating mammary glands this forces milk from the alveoli of the breast. Oxytocin release is stimulated by suckling and by auditory and visual stimuli, such as a baby s cry. Oxytocin is available as a nasal spray, which is used as an aid to lactation when milk ejection is impaired. 

It promotes contraction of the uterine muscle. It also causes contraction of the myoepithelial cells of the lactating breast and squeezing milk into the large ducts situated behind the nipple of the mammary gland. 

Oxytocin also causes contraction of myoepithelial cells surrounding mammary alveoli, which leads to milk ejection. 

The mammary gland consists of the external nipple and the system of ducts from the alveoli to the skin surface. The alveoli do not fully develop until a female experiences pregnancy and lactation. Cortisol, insulin and placental lactogen contribute to alveolar development, but estrogen and progesterone are most important. The cells that line the alveoli synthesize milk, and they are surrounded by a layer of myoepithelial cells that eject the milk. 

The substrates for milk synthesis enter the secretory cell across the basal membrane (outside), are utilized, converted and interchanged as they pass inwards through the cell and the finished milk constituents are excreted into the lumen across the lumenal or apical membrane. Myoepithelial cells (spindle shaped) form a basket around each alveolus and are capable of contracting on receiving an electrical, hormonally mediated, stimulus, thereby causing ejection of milk from the lumen into the ducts. 

Oxytocin alters transmembrane ionic currents in myometrial smooth muscle cells to produce sustained uterine contraction. The sensitivity of the uterus to oxytocin increases during pregnancy. Oxytocin-induced myometrial contractions can be inhibited by 13-adrenoceptor agonists, magnesium sulfate, or inhalation anesthetics. Oxytocin also causes contraction of myoepithelial cells surrounding mammary alveoli, which leads to milk ejection. Without oxytocin-induced contraction, normal lactation cannot occur. Oxytocin has weak antidiuretic and pressor activity. 

Eccrine (merocrine) sweat glands are simple tubular glands that open directly onto the skin surface (Figure 35.1). In humans, they are found over the entire body surface except for the lips, external ear canal, clitoris, and labia minora. Myoepithelial cells located in the secretory portion of these glands are specialized smooth muscle cells, which, upon contraction, aid in moving the secretions toward the duct. The eccrine sweat gland duct is comprised of two layers of cuboidal epithelium... 

Oxytocin Uterus, mammary gland ducts Contraction of smooth muscle of uterus and myoepithelial cells lining mammary ducts... 

Oxytocin may be used to facilitate the contraction of the mammary alveolar myoepithelial cells and thus promote milk ejection. After the mammary gland has been emptied manually, i.v. oxytocin (20 lU) will allow the additional collection of milk from lactating mares. 

A. Oxytocin causes uterine contractions during labor and contraction of mammary myoepithelial cells during lactation. 

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