The concept that the demarcation membrane system delineates platelets within the cytoplasm of megakaryocytes has been examined. In short-term culture of mouse bone marrow, mature megakaryocytes extended long, attenuated processes that were found by electron microscopy to have a limited amount of invaginated membrane. When such megakaryocytes were exposed to microtubule depolymerizing agents, the attenuated processes retracted, became thicker, and an extensive demarcation membrane reappeared. It is suggested from the results that the demarcation membrane system functions to provide a membrane reserve that undergoes evagination during the formation of attenuated processes and thereby envelops putative platelets, rather than to demarcate platelets in the maturing megakaryocyte. The term “invaginated membrane system” is considered more appropriate than “demarcation membrane system.”

Summary In the foot of the horse, arteriovenous anastomoses (AVAs) of epithelioid type occurred in the dermis of the coronary band, in the coronary and terminal papillae, in neurovascular bundles and at the entrance to and along the length of the dermal laminae. A particular feature of the epithelioid segment of AVAs in the horse, compared with that of other species, was the height and surface complexity of many of the endothelial cells. They extended into the lumen, forming undercut and tunnel‐like areas which correlated with the characteristic surface marking of AVAs observed in vascular casts. The number of cell organelles, including the concentration of vesicles in the luminal cytoplasm, suggested cells with a high metabolic activity. The luminal surface possessed numerous microvilli and long cytoplasmic cell processes which appeared to surround material in the lumen. The innervation of AVAs was more dense than that of the arteries and consisted of adrenergic and peptidergic nerves. Noradrenaline‐ and neuropeptide Y‐containing nerves were identified as the vasoconstrictor components of the nerve supply and occurred along arteries and formed dense plexuses around AVAs. Calcitonin gene‐related peptide, substance P and vasoactive intestinal polypeptide are vasodilators and were present in single nerve fibres which accompanied arteries and AVAs along the length of the dermal laminae. In this study the distribution, density and innervation of AVAs in the equine foot are correlated with their proposed role in the development of acute laminitis. The release of vasoactive peptides from diseased organs remote from the foot may induce inappropriate prolonged dilatation of AVAs and thus contribute to the laminar ischaemia of acute laminitis.

Neuroepithelial bodies (NEB) were identified in the lung of Bufo marinus. The characteristics of the cells and their innervation were studied with electron and fluorescence microscopy before and after close vagosympathetic denervation. The bodies consist of low columnar cells which rest on the epithelial basal lamina. The majority of the cells do not reach the lumen of the lung (basal cells); the few which do (apical cells) are bordered by microvilli and possess a single cilium. The neuroepithelial cell cytoplasm contains a variety of organelles the most characteristic of which are dense cored vesicles. Microspectrofluorometry and electron microscopic cytochemistry indicate significant quantities of 5-hydroxytryptamine in these cells. The neuroepithelial bodies could be divided into three groups on the basis of their innervation: 1) About 60% of the NEBs are innervated solely by nerve fibers containing agranular vesicles which form reciprocal synapses; 2) about 20% are innervated solely by adrenergic nerve fibres which from distinct synaptic contacts; and 3) the remaining 20% are innervated by both types of nerve fibres. It is proposed that the NEBs are receptors monitoring intrapulmonary P CO2 and so leading to modulation of activity in afferent nerve fibres (type containing a granular vesicles). The presence of NEBs soley with an adrenergic (efferent) innervation poses a problem with this interpretation.

Degenerating senescent megakaryocytes have been identified in mouse bone marrow by light and electron microscopy. The ultrastructural changes which occur as degeneration proceeds are characteristic of death by apoptosis, although most cells appear to round up rather than undergo fragmentation. A hitherto unreported finding in degenerating cells was the presence of bundles of approximately 7 nm diameter parallel filaments in nuclei and membrane-bound nuclear fragments. Structurally, they resembled bundles of filaments induced in nuclei with dimethyl sulphoxide and identified as actin. Often a bundle appeared to terminate at the inner membrane. In the cytoplasm the presence of microtubules and centrioles indicates that not all the latter organelles are lost by the megakaryocyte during platelet release. Degenerating senescent megakaryocytes are rare in the marrow of normal mice but increase in frequency during 5-fluorouracil stimulated thrombocytosis. The dying cells are eventually phagocytosed by macrophages, a process that can occur extravascularly, showing that entry of senescent megakaryocytes into the circulation is not necessary for their disposal.

The cytological features and membrane specialisations of neuroepithelial cells (apical cells) in direct contact with the lumen of the lung were studied with transmission and scanning electron microscopy. The luminal surface of the apical cell is characterised by microvilli, a cilium with an 8 + 1 microtubular pattern and numerous coated vesicles. The cytoplasmic region immediately beneath the luminal plasma membrane contains numerous smooth-walled vesicles, tubules and microtubules, a few microfilaments and dense granules (15-20 nm in diameter). The luminal pole of the cell is marked off from the basal or vascular pole by a well-defined terminal web associated with junctional complexes. Protrusion of the luminal pole occurs as a transient phenomenon and is accompanied by a pinching in of the cell at the terminal web. It is proposed that the distinctive features of the luminal pole of the apical cell are comparable to those of recognised chemoreceptor cells. It is also proposed that in view of the common features of apical and basal cells the apical cell functions as a receptor/transducer and the basal cells served as an accessory source of peptides/5-hydroxytryptamine to be released on stimulation of the apical cell. Furthermore, we have drawn attention to the structural heterogeneity of the neuroepithelial bodies in various vertebrate classes.