Veterinary Histology and Embryology

SMOOTH MUSCLE

• Consists of fusiform or spindle shaped cell 15 to 500 microns in length with abundant cytoplasm in whose central thickest position the nucleus lies.
• The nucleus is oval elongated or rod-shaped. The nuclei of contracted muscle cells usually have a folded outline.
• The cytoplasm contains the usual organoids and appears more or less homogenous in routine sections. By treatment with dilute nitric acid, longitudinally arranged myofibrils can be made out.
• The cytoplasm in routine preparations takes the eosin stain. A plasma membrane, similar to those in other cells is present.
• As a rule the cells are gathered into dense sheets or bands, but they may occur as isolated units scattered among the connective fibres in a tissue like the dartos tunic of the scrotum.
• Within the bands the cells are parallel to each other but are packed together so that the narrower position of one lies against the wider portions of its neighbors.
• Delicate reticular and elastic fibres surround individual cells in a bundle. Between larger bundles there is collagenous tissue.
• The smooth muscle cells increase in the pregnant uterus where the length may exceed half m.m. (500 microns).
• Smooth muscle is not richly supplied with blood vessels. Capillaries are seen in between groups of cells. There are nerve endings in smooth muscle but motor termination for every cell is not seen.
• The contraction of smooth muscle is slow and sustained, in contrast to the skeletal muscle which contracts rapidly.
• Smooth muscle is found in the wall of the alimentary canal from the stomach to anus, gall bladders, trachea bronchial tree, ureter, bladder, prostate and Cowper’s glands; oviduct; uterus; vagina; blood vessels and larger lymphatic, spleen and lymph nodes.
• Smooth muscle occurs in skin in connection with hair follicles. In the eye, it is found in the Iris and ciliary body.