Histology of Blood Vessels & The Blood Film

Question 1

Blood Vessel Layers

Answer 1

Tunica intima - single squamous epithelial cell layer (endothelial cells), supported with basal lamina and thin CT

Tunica media - mostly smooth muscle

Tunica adventitia - supporting CT

Internal Elastic Membrane - in between TI and TM

External Elastic membrane - in between TM and TA

Question 2

Elastic Arteries

Answer 2

TM contains more elastic fibres than smooth muscle - elastic recoil
Largest arteries

Vasa vasorum - obtain nutrients as inner layer can only receive this

Question 3

Arteriole

Answer 3

Smallest division of muscular arteries

1-2 layers of smooth muscle
Almost no TA
30-200um diameter
Important in controlling blood flow in a tissue

Terminal Arteriole - no internal lamina, covered by smooth muscle coat

Meta-Arteriole - pericytes instead of smooth muscle, terminates into capillaries

Question 4

Capillaries

Answer 4

Endothelial cells and basal lamina

4-8um diameter

Absent in specific body regions such as epithelial cells resting on BM, epidermis of skin/hair and nails, cornea of eye, hyaline cartilage

Continuous - no pores, muscle/nerve/lung/skin

Fenestrated - small pores (50nm), continuous basal lamina, found in gut mucosa, endocrine glands, kidney

Discontinuous - large gaps, no continuous basal lamina, found in liver/spleen/bone marrow

Question 5

Microvasculature

Answer 5

Meta-arteriole - short microvessel that links arterioles and capillaries
Thoroughfare channels - tail end of the metarteriole that connects to the venule, lacks smooth muscle
Capillaries - normally in a network where exchange occurs, endothelial cells and basal lamina
Precapillary sphincters - segments of smooth muscle that help direct blood flow into capillaries

Question 6

Venules

Answer 6

Capillaries drain into Post-capillary venule (endothelial cells and thin CT)

Once the vessel gains more smooth muscle cells in TM –> Venule

Question 7

Veins

Answer 7

Thin continuous TM
Few layers of smooth muscle
No obvious TA

Largest veins - thick TA with longitudinally oriented smooth muscle

Small veins - have valves that prevent the backflow of blood

Question 8

Lymph Vascular System

Answer 8

Thin walled vessels that drain tissue fluid into blood stream

Transports lymph to lymph nodes for immunological surveillance

Smooth muscle in walls, hydrostatic pressure in tissue and compression of vessels by voluntary muscle combined with vessel valves, produces flow

Question 9

Blood

Answer 9

4.6-6L

Where? - mostly peripheral veins, then the heart/lungs, peripheral arteries and capillaries

Components
- Formed elements –> Red cells, White cells, Platelets –> Granulocytes & Agranulocytes –> Neutrophils, Eosinophils & Basophils, Lymphocytes, Monocytes
- Plasma –> Water, Proteins, Nutrients & Salts

Separating Blood
- RBC are the densest - bottom
- White cells are the next densest
- Plasma is the liquid portion of blood, top
- Serum - remove clotting factors

Question 10

Erythrocytes

Answer 10

RBCs - Biconcave, 7um diameter
Mature RBCs - no nucleus, 1/3 volume taken up by Hb
Contain cytoskeletal elements - allow to change shape easily
4 months max in circulation - removed by spleen and liver

Question 11

Hematocrit

Answer 11

Proportion of blood that is RBCs
Hematocrit of 40 –> 40% of blood is cells

Question 12

Leukocytes

Answer 12

WBCs part of the immune system

Question 13

Neutrophils

Answer 13

40-75%
Granulocytes
Most common
Cytoplasm contains granules
Multi-lobed nucleus
Inactive in circulation
Stimulated, they enter the tissue
Abundant and short-lived
Bone marrow production

Question 14

Eosinophils

Answer 14

5%
Bilobed nucleus
Granules - hydrolytic enzymes (inducing/maintaining inflammation)
Fight parasitic infection
Granules have affinity for red acidic eosin
8-12hrs circulate and then move into the tissue
Larger than neutrophils

Question 15

Basophils

Answer 15

0.5%
Rarest granulocyte
Granules - affinity for basic dyes
Bilobed nucleus - can’t see due to granules
Granules contain histamine, heparin and other inflammatory mediators
Effector cells in allergic reactions
Possess high affinity IgE receptors to specific allergen - release granules

Question 16

Lymphocytes

Answer 16

20-50%
Round nucleus surrounded by thin-moderate cytoplasm
No visible granules
B cells - give rise to antibody secreting plasma cells
T cells - CD4+ and CD8+, differentiate in thymus, defence functions

Question 17

Monocytes

Answer 17

1-5%
Precursors of tissue macrophages –> mononuclear phagocyte system
Macrophages are widely distributed in body, especially loose CT
Monocytes have numerous small lysosomal granules in cytoplasm
Largest cell in circulation
Non-lobulated nucleus - kidney shaped
Members of mononuclear phagocytic system are wanderers but some remain resident in specific tissues

Question 18

Platelets

Answer 18

Small cell fragments
2um diameter
Haemostasis - prevention of blood loss
Developed cytoskeleton - participates in extrusion of granules and clot retraction
Some organelles but no nucleus
Include coagulation factors in granules

Question 19

Haemopoiesis

Answer 19

Development
- Earliest site for erythrocyte formation is outside the embryo in the yolk sac beginning at about 3 weeks gestation.
- Later, the liver and to some extent the spleen is colonized by hemopoietic stem cells. Thus during the second trimester the liver is the principal site of blood formation.
- Birth - bone marrow is the main site of blood formation
- Bone enlarge - haemopoesis shuts down in many bones
Marrow can revert to blood formation in emergency

Question 20

Bone Marrow

Answer 20

Megakaryocytes - no nuclear division, nucleus large
Platelets fromed from extensions

Question 21

Lineage of Blood Cells

Answer 21

In Bone marrow, a very small number of self renewing pluripotent stem cells which are capable of giving rise to all blood cell types, including lymphocytes

The committed progenitor cells are often termed colony-forming units (CFU) of the lineage

Development of granulocytes and monocytes. Haemopoietic stem cells give rise to colony-forming units (CFUs). CFU-GEMMs have the potential to give rise to all blood cells except lymphocytes. These are granulocytes (G), erythrocytes (E), monocytes (M) and megakaryocytes (M), which each develop in response to specific cytokines and hormones. Intermediate CFUs or, for erythrocytes, burst-forming units (BFUs),