Anatomy of sphenoid

Anatomy of sphenoid 

 Anatomical points

1. Body lies centrally
2. superior surface of body lies horizontally
3. Two greater wings & two lesser wings project laterally from the sides of the body
4. Superior surfaces of greater wings are deeply concave
5. Two pterygoid processes are directed downwards from adjoining parts of the body and greater wings Morphological type of bone: pneumatic irregular bone

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Parts of sphenoid bone 

Body 

Two greater wings 

Two lesser wings 

Two pterygoid processes 

Body:  It has six surfaces : superior , inferior , anterior , posterior and two lateral surfaces 

Shape : cuboid 

It contains two large air sinuses which are separated from each other by a septum 

Superior or cerebral surface 

Articulates with ethmoid bone anteriorly and basilar part of occipital bone posteriorly. It shows: 

 Ethmoidal spine : it is articulate with posterior border of cribriform plate of ethmoid bon

  Jugum sphenoidale: smooth area which is the part of anterior cranial fossa . this part is related with gyri recti of cerebrum and olfactory tracts  

Sulcus chiasmaticus: it is a groove which connects both optic canal . it lodges optic chiasm

  Tuberculum sellae: round elevation it bears middle clinoid processe

 Sella turcica: deep depression behind the tuberculum sellae . deepest part of sella turcica is 

known as hypophyseal fossa

 Dorsum sellae: square shape bone which form posterior boundary of sella turcica . it bear

 posterior clinoid processes 

 Clivus: slopping area behind dorsum sellae . it continue with basilar part of occipital bone this part is related with pons 

Inferior surface 

Rostrum of sphenoid 
 Sphenoidal conchae 
Vaginal processes of medial pterygoid plate 

Anterior surface 

Sphenoidal crest articulates with the perpendicular plate of ethmoid leading to formation of a part of the septum of nose. 

Posterior surface: Basilar part of occipital bone 

Lateral surface

• It is united with the greater wing and medial pterygoid plate ,
  Carotid sulcus is present in the lateral surface which is extend from superior orbital fissure to foramen lacerum s lodging cavernous sinus and internal carotid
  
• Development:
• up to rostral half of sella turcica is developed from neural crest
• caudal half of sella turcica is developed from paraxial mesoderm

 

 

Difference between arteriole and venule

Arteriole	Venule
Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues.	Blood of capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.
·         The lumina of arteriole are much smaller than corresponding venule	·         The lumina of venules are much larger  than corresponding arterioles.
·         It is also known as resistance vessel	It is also known as capacitance vessel
·         The walls of the arteriole are much thicker  than venule	·         The walls of the veins are much thinner than arterioles
Arteriole provides 50% peripheral resistance	·         Veins and venules can withstand a much lower pressure from the blood flowing through them. Their lumens are correspondingly larger in diameter to allow more blood to flow with less vessel resistance.

Arteriole : lecture note

 

Arteriole

The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries. An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries

Arteriole is also known as resistance vessels . Most arterioles can dilate 60% to 100% from their resting diameter, and they can maintain as much as 40% constriction for a long time. Therefore, a large decrease or increase in vascular resistance has a direct effect on distribution of blood flow and systemic arterial pressure.

Arteriole About 10 percent of the total blood volume is in the systemic arterial system at any given

If all the arteries, veins, and capillaries were laid end to end, the total length would be  100,000 km

The diameter of arterioles  are in between  100  to 300 µm.

Arteriole: Like any other blood vessels arteriole is made by three concentric layers (within outward )

Tunica intima ,Tunica media and Tunica adventitia

Tunica intima of arteriole  contain

1. endothelium & its basement membrane

2. Subendothelial connective tissue is formed by loose connective tissue which contain smooth muscle cells

3. Large arteriole contain internal elastic lamina but small arteriole contain no  internal elastic lamina

Tunica media of arteriole contain Circularly arrange smooth muscle cells layers(1-2 cells layer)  with variable amounts of elastin, reticular fibers, and proteoglycans are interposed between the smooth muscle cells

Arteriole  has tunica adventitia which  is composed of collagenous & elastic connective tissues, whose thickness approaches that of the tunica media.

Large arteriole contain external elastic lamina but small arteriole contain no  external elastic lamina  

Arterioles control blood flow to capillary networks by contraction of the smooth muscle cells. Arterioles serve as flow regulators for the capillary beds. In the normal relationship between an arteriole and a capillary network, contraction of the smooth muscle in the wall of an arteriole increases the vascular resistance and reduces or shuts off the blood going to the capillaries.

Arteriole is also known as resistance vessels . Most arterioles can dilate 60% to 100% from their resting diameter, and they can maintain as much as 40% constriction for a long time. Therefore, a large decrease or increase in vascular resistance has a direct effect on distribution of blood flow and systemic arterial pressure.

 

 

                                               Tunica intima of blood vessel in general

Tunica intima: the innermost layer of the blood vessel.

Tunica intima consists of three components:

1.      Endothelium with its basement membrane

2.      the sub-endothelial connective tissue layer

3.      Internal elastic membrane

Endothelium with its basement membrane: 

1.     A single layer of squamous epithelial cells and the basal lamina of the endothelial cells (a thin extracellular layer composed chiefly of collagen, proteoglycans, and glycoproteins). The squamous cells of endothelium are polygonal, oval, or fusiform, and have very distinct round or oval nuclei.

This endothelium is brought into view most distinctly by staining with silver nitrate.

Several well-characterized markers are employed to identify ECs, including vascular endothelial cadherin, platelet endothelial cell adhesion molecule 1, vascular endothelial growth factor receptors (VEGFRs), and isolectinB4.

During development, most Endothelial Cells derive from the lateral plate mesoderm, and through the process of vasculogenesis, primitive Endothelial Cells coalesce into the initial blood vessel tubes . Subsequently, these initial Endothelial Cell  tubes give rise to further vessels through angiogenesis, a multistep process consisting of Endothelial Cells proliferation, migration, invasion, lumen formation, and tube stabilization. 

                                                                                                    

The sub-endothelial connective tissue layer

2.  It is consisting of loose connective tissue. Occasional smooth muscle cells are found in the loose connective tissue. In arteries of less than 2 mm in diameter. The thickness is depend on the size of blood vessel.

3.     An elastic or fenestrated layer, which consists of a membrane containing a network of elastic fibers, having principally a longitudinal direction, and in which, under the microscope, small elongated apertures or perforations may be seen, giving it a fenestrated appearance. It was therefore called by Henle the fenestrated membrane. This membrane forms the chief thickness of the inner coat, and can be separated into several layers, some of which present the appearance of a network of longitudinal elastic fibers, and others a more membranous character, marked by pale lines having a longitudinal direction. In minute arteries the fenestrated membrane is a very thin layer; but in the larger arteries, and especially in the aorta, it has a considerable thickness.

 

Internal elastic membrane:

It is  a sheetlike layer or lamella of fenestrated elastic material present within the subendothelial layer of the intima in arteries and arterioles,  is  called the internal elastic membrane. Fenestrations enable substances to diffuse readily through the layer and reach cells deep within the wall of the vessel

Presence of internal elastic lamina  :

Internal elastic lamina is present  in medium sized artery, large sized arterioles , and occationally present in large and medium sized vein

In elastic or large artery it is present but cannot seen clearly.

Absence of internal elastic lamina:

It is absent in small sized arteriole, most of the veins  

 

In dissection, the inner coat (tunica intima) can be separated from the middle (tunica media) by a little maceration, or it may be stripped off in small pieces; but, because of its friability, it cannot be separated as a complete membrane. It is a fine, transparent, colorless structure which is highly elastic, and, after death, is commonly corrugated into longitudinal wrinkles.

                     Difference between tunica intima of large artery and vein 

Tunica intima of large artery	Tunica intima of large vein
Lined by short, polygonal endothelial cells.  The subendothelial connective tissue is fi broelastic connective tissue which is thicker than the large vein and houses some longitudinally disposed smooth muscle cells.	The endothelium with relatively thicker amount of subendothelial connective tissue are always present.
Internal elastic lamina is present but not clearly defined.	Occasionally, a thin internal elastic lamina is observed
Valves are absent	Valves are present