Promonocytes differentiate from bone marrow stem cells (monoblasts) and multiply to give rise to monocytes.
Monocytes spend only a short period of time in the marrow before being released into the bloodstream.
Monocytes are transported in the blood but are also found in connective tissues, body cavities and organs.
Outside the blood vessel wall, they are transformed into macrophages of the mononuclear phagocyte system.
Thrombopoiesis, or the formation of platelets, occurs in the red bone marrow.
Megakaryoblast is a large basophilic cell that contains a U—shaped or ovoid nucleus with prominent nucleoli. It is the last cell that undergoes mitosis.
Megakaryocytes are the largest of bone marrow cells, with diameters of 50 mm or greater. They undergo 4–5 nuclear divi sions without concomitant cytopla—smic division. As a result, the megakaryocyte is a cell with polylobulated, polyploid nucleus and abundant granules in its cytoplasm. As megakaryocyte maturation proceeds, «curtains» of platelet demarcation vesicles form in the cytoplasm. These vesicles coalesce, become tubular, and eventually form platelet demarcation membranes. These membranes fuse to give rise to the membranes of the platelets.
A single megakaryocyte can shed (i. e., produce) up to 3,500 platelets.
New words
capable – способный
spherical – сферический
indented – зазубренный
chromatin – хроматин
23. Arteries
Arteries are classified according to their size, the appearance of their tunica media, or their major function.
Large elastic conducting arteries include the aorta and its large branches. Unstained, they appear yellow due to their high con tent of elastin.
The tunica intima is composed of endothelium and a thin sub jacent connective tissue layer. An internal elastic membrane marks the boundary between the intima and media.
The tunica media is extremely thick in large arteries and con sists of circularly organized, fenestrated sheets of elastic tissue with interspersed smooth muscle cells. These cells are responsi ble for producing elastin and other extracellular matrix com ponents. The outermost elastin sheet is considered as the external elastic membrane, which marks the boundary between the media and the tunica adventitia.
The tunica adventitia is a longitudinally oriented collection of collagenous bundles and delicate elastic fibers with associated fibroblasts. Large blood vessels have their own blood supply (vasa vasorum), which consists of small vessels that branch profusely in the walls of larger arteries and veins. Muscular distributing arteries are medium—sized vessels that are characterized by their predominance of circularly arranged smooth muscle cells in the media interspersed with a few elastin compo nents. Up to 40 layers of smooth muscle may occur. Both internal and external elastic limiting membranes are clearly demonstrated. The intima is thinner than that of the large arteries.
Arterioles are the smallest components of the arterial tree. Generally, any artery less than 0,5 mm in diameter is considered to be a small artery or arteriole. A suben—dothelial layer and the inter nal elastic membrane may be present in the largest of these vessels but are absent in the smaller ones. The media is composed of sev eral smooth muscle cell layers, and the adventitia is poorly devel oped. An external elastic membrane is absent.
New words
endothelium – эндотелий
media – средняя
arteries – артерии
to be classified – классифицированный
according – соответственно
their – их
size – размер
appearance – вид
tunica – оболочка
major – главный
elastic – эластичный
conducting – проведение
arteries – артерии
to include – включать
aorta – аорта
branches – ветви
up to – до
layers – слои
smooth – гладкий
may – может
infima – внутренняя полость артерии
24. Capillaries
Capillaries are thin—walled, narrow—diameter, low—pressure vessels that generally permit easy diffusion across their walls. Most capillar ies have a cross—sectional diameter of 7 —12 mm. They are composed of a simple layer of endothelium, which is the lining of the entire vas cular system, and an underlying basal lamina. They are attached to the surrounding tissues by a delicate reticu—lum of collagen. Associated with these vessels at various points along their length are specialized cells called pe—ricytes. These cells, enclosed within their own basal lamina, which is continuous with that of the endothelium, contain contractile proteins and thus may be involved in the control of capillary dynamics. They may also serve as stem cells at times of vascular repair. Capillaries are generally divided into three types, according to the structure of their endothelial cell walls.
Continuous (muscular, somatic) capillaries are formed by a single uninterrupted layer of endothelial cells rolled up into the shape of a tube and can be found in locations such as connective tissue, muscle, and nerve.
Fenestrated (visceral) capillaries are characterized by the presence of pores in the endothelial cell wall. The pores are covered by a thin diaphragm (except in the glome—ruli of the kidney) and are usually encountered in tissues where rapid substance interchange occurs (e. g., kidney, intestine, endocrine glands).
Sinusoidal capillaries can be found in the liver, hemato—poietic and lymphopoietic organs, and in certain endocrine glands. These tubes with discontinuous endothelial walls have a larger diame ter than other capillaries (up to 40 mm), exhibit irregular cross—sec tional profiles, have more tortuous paths, and often lack a con tinuous basal lamina. Cells with phagocytic activity (macropha—ges) are present within, or just subjacent to, the en— dothelium.
New words
capillaries – капилляры
to thin—walled – окруженный тонкой стеной
narrow—diameter – узкий диаметр
low—pressure – низкое давление
that – тот
generally – главным образом
permit – разрешение easy – легкий
diffusion – распространение
cross—sectional – поперечный
to be composed – быть сложным
simple – простой
endothelium – эндотелий
lining – выравнивание
