Histology

Question 1

Cardiac muscle and Heart

Answer 1

• Striated with T-tubule system in close proximity to sarcoplasmic reticulum
• Myocytes are short, branched and joined to other myocytes to be arranged in whorled manner, optimises wringing of blood from ventricles to pulmonary trunk and aorta during ventricular contraction
• Intercalated discs between individual cells that contain gap, adherens and desmosomal junctions
• Not supplied by somatic NS so no neuromuscular junctions, supplied by autonomic that influences rate and strength of contraction
• Structures of heart:
  1. Endocardium = forms luminal surface, composed of simple squamous epithelium, deeper layers contains subendocardial tissue that has loose vascularised connective tissue, nerves, and Purkinje fibres, where myocardium = thin, endocardium = thick
  2. Myocardium = middle layer, large quantity of muscle cells, cardiomyocytes arranged in branched, linear manner, highly vascularised, and cardiomyocytes contain glycogen granules for energy
  3. Epicardium = pericardium is fibrous doubled layered connective sheath, visceral portion of pericardium is epicardium (mesothelium derivative) rich in adipocytes and neurovascular tissue, well lubricated layer that supports smooth movement of heart against parietal pericardium in contraction
  4. Purkyne fibres carry impulse through ventricles, cause contraction, extend from AV node and divide into left/right ventricles travelling under endocardium (paler = more glycogen but less microfibrils with no intercalated discs)

Question 2

Blood vessels

Answer 2

• Myocytes have direction communication with adjacent cells via gap junctions, allowing passage of small ions
• Blood vessels move blood to and from capillaries, capillaries allow exchange of metabolic substrates by rapid diffusion
• Fluid leaves at capillaries, cells leave at postcapillary venules (remainder of vascular tree is impermeable)
• Lymphatic vessels carry excess water to pre-venular capillary, vessels contain more valves but less developed adventitia
• Most vessels composed of: tunica intima, tunica media, and tunica externa
• Innermost intima: endothelial cells, basement membrane, and thin layer of connective tissue (particularly fibres)
• Media: smooth muscle dominated, arranged circumferentially, has outer adventitia (collagen fibre dominated, simple squamous epithelium on outside)
• Arteries + arterioles: thick walls to slow diffusion allows perfusion, vasa vasorum supply cells forming wall
• Arteries: elastic allow for expansion-recoil to maintain arterial pressure in diastole, muscular have smooth muscles in intima, aging → lose elastic tissue, increase BP + resistance and arteriosclerosis
• Arterioles: control blood flow to capillaries by smooth muscle contraction in media, control resistance/pressure at periphery, internal elastic lamina only in large arterioles
• Capillaries: connect arterioles to venules, have supportive pericytes, can be continuous, fenestrated or sinusoid
• Venules: allow blood return, thin adventitia
• Veins: valves, post capillary veins form venules → larger veins that return deoxygenated blood, posses all 3 tunics in arteries but thinner media
• Arteriosclerosis = broader term for condition where arteries narrow and harden → poor circulation of blood throughout the body, atherosclerosis = build of plaque, specific kind of arteriosclerosis, terms used interchangeably