HEMA 2_Prelims_Lec: Week 1- Platelet Production, Structure, and Function

Megakaryocytopoiesis

Also called megakaryopoiesis; the process of formation and development of megakaryocytes and ultimately platelets.

Thrombopoietin (TPO)

One of the major regulatory factors needed for platelet production; the most important hormone regulating megakaryopoiesis.

TPO molecular weight

Approximately 70,000 daltons.

TPO homology with EPO

Approximately 23% homologous with erythropoietin (EPO).

Major source of TPO

Liver; Trivia: TPO mRNA is also found in the kidney and smooth muscle cells.

Main function of TPO

Stimulates platelet production, proliferation, differentiation, maturation, and platelet release.

Platelets

Anucleated blood cells that are responsible for major functions in primary hemostasis.

Why platelets are anucleated

They do not have a nucleus and therefore do not undergo normal mitosis.

Normal platelet concentration

150–400 × 10^9/L.

Platelet count in women

Slightly higher than in men according to the lecture.

Platelet count with aging

Both sexes have slightly lower platelet counts around age 25; platelet count decreases in people older than 65 years.

Primary hemostasis

Involves platelets and blood vessels and results primarily in formation of the platelet plug.

Secondary hemostasis

Involves coagulation factors and platelets and results in formation/stabilization of the fibrin clot.

Megakaryocyte

The unique bone marrow cell from which platelets arise.

Platelets produced by one mature megakaryocyte

Approximately 2,000–4,000 platelets.

Platelet life span

8–9 days.

Megakaryocytes

Largest cells in the bone marrow.

Megakaryocyte size

Approximately 30–50 µm in diameter.

Megakaryocyte proportion of bone marrow cells

Approximately 0.5% of all bone marrow cells.

Megakaryocyte ploidy

They possess multiple chromosome copies and are described as polyploid in the lecture.

Unique maturation feature of megakaryocytes

The only cell line in which cells become larger as they mature.

Megakaryocyte mitosis

Megakaryocytes do not undergo normal mitosis during maturation.

Endomitosis

A partially characterized form of mitosis unique to megakaryocytes in which DNA replication continues but the cell loses the capacity to divide.

Purpose of endomitosis

Allows DNA replication and increasing ploidy without actual cell division.

3 stages of megakaryocyte progenitors

BFU-Meg, CFU-Meg, and LD-CFU-Meg.

BFU-Meg

Burst-Forming Unit–Megakaryocyte; least mature megakaryocyte progenitor and undergoes mitosis.

CFU-Meg (Colony-Forming Unit–Megakaryocyte)

; intermediate progenitor and undergoes mitosis.

LD-CFU-Meg

(Light-Density Colony-Forming Unit–Megakaryocyte)

; more mature progenitor that undergoes endomitosis instead of mitosis.

BFU-Meg colony production

Approximately hundreds of colonies.

CFU-Meg colony production

Approximately dozens of colonies.

LD-CFU-Meg division capacity

Has no capacity to divide but retains DNA replication through endomitosis.

Terminal megakaryocyte differentiation stages

Megakaryoblast (MK-I), Promegakaryocyte (MK-II), and Megakaryocyte (MK-III).

Megakaryoblast (MK-I)

Least differentiated megakaryocyte precursor.

Promegakaryocyte (MK-II)

Intermediate stage that develops more cytoplasmic ultrastructure, granules, and demarcation membrane system.

Megakaryocyte (MK-III)

Most mature and abundant stage; largest bone marrow cell; releases mature platelets.

Megakaryoblast + TPO

TPO helps stimulate the megakaryoblast toward platelet-producing maturation.

Thrombopoiesis

The stage involving shedding/release of platelets from mature megakaryocytes.

Megakaryoblast precursor percentage

Approximately 20% of the listed megakaryocyte precursors.

Promegakaryocyte precursor percentage

Approximately 25% of the listed megakaryocyte precursors.

Megakaryocyte precursor percentage

Approximately 55% of the listed megakaryocyte precursors.

Megakaryoblast diameter

14–18 µm.

Promegakaryocyte diameter

15–40 µm.

Megakaryocyte diameter

30–50 µm.

Megakaryoblast nucleolus

Rounded.

Promegakaryocyte nucleolus

Indented.

Megakaryocyte nucleolus

Multilobed.

Megakaryoblast nucleoli

2–6 nucleoli.

Promegakaryocyte nucleoli

Variable.

Megakaryocyte nucleoli

Not visible.

Megakaryoblast chromatin

Homogeneous.

Promegakaryocyte chromatin

Condensed.

Megakaryocyte chromatin

Deeply condensed.

Megakaryoblast N:C ratio

3:1.

Promegakaryocyte N:C ratio

1:2.

Megakaryocyte N:C ratio

1:4.

Megakaryoblast cytoplasm

Basophilic.

Promegakaryocyte cytoplasm

Basophilic and granular.

Megakaryocyte cytoplasm

Azurophilic and granular.

Megakaryoblast mitosis

Absent.

Promegakaryocyte mitosis

Absent.

Megakaryocyte mitosis

Absent.

Megakaryoblast alpha granules

Present.

Promegakaryocyte alpha granules

Present.

Megakaryocyte alpha granules

Present.

Megakaryoblast dense granules

Present.

Promegakaryocyte dense granules

Present.

Megakaryocyte dense granules

Present.

DMS (Demarcation Membrane System)

membrane-lined channels that develop from the megakaryocyte plasma membrane and subdivide the cytoplasm into future platelet territories.

Origin of DMS

Identical to the megakaryocyte plasma membrane.

Function of DMS

Provides future platelet sites and helps subdivide megakaryocyte cytoplasm for platelet formation.

Megakaryoblast blebs

Pseudopod-like structures used for locomotion.

Megakaryoblast (MK-I)

Develops cytoplasmic ultrastructure, alpha granules, dense granules, and DMS; commits to platelet production.

Promegakaryocyte nuclear change

Nuclear lobularity/indentation develops.

Megakaryocyte major feature

Most abundant and largest bone marrow cell; its indented nucleus and mature cytoplasm allow recognition.

Platelet activation

An activated platelet develops thorn-like projections/pseudopods and releases its granule contents.

Contents released during platelet activation

Alpha granules, dense/delta granules, ATP, ADP, and lysosomal granules.

Reticulated platelets

Also called “stress platelets”; larger platelets that appear as compensation for thrombocytopenia.

Reticulated platelet diameter

6 µm.

Reticulated platelet MPV

12–14 fL.

Normal platelet MPV

8–10 fL.

Reticulated platelet contents

Free ribosomes and fragments of rough endoplasmic reticulum.

Reticulated platelets and thrombocytopenia

They appear as a compensatory response to thrombocytopenia.

Reticulated platelets and cardiovascular disease

The lecture states that increased reticulated platelets are associated with high cardiovascular disease risk because platelets are prothrombotic.

EDTA effect on reticulated platelets

Round.

Citrate effect on reticulated platelets

Cylindrical; may be mistaken for megakaryocytes.

Normal platelet size

2–4 µm.

Platelet color

Pale blue cells.

Platelet shape

Disk-shaped or circular to irregular.

Platelet appearance in Wright-stained wedge preparation

Lavender and granular.

Mature platelet nucleus

Absent.

Platelet azurophilic granules

Present as part of platelet granule content.

Platelets in circulation

Approximately 70%.

Platelets in spleen

Approximately 30%; used in inflammation and trauma according to the lecture.

Platelet function in hemostasis

Responsible for formation of the platelet plug and contributes to formation of a stable fibrin clot.

IL-3 (Interleukin-3)

acts synergistically with TPO to induce early differentiation of stem cells and is important for platelet production.

IL-6

Interleukin-6; enhances later endomitosis, megakaryocyte maturation, and platelet release.

IL-11

Interleukin-11; enhances later megakaryocyte maturation and platelet release and has therapeutic applications.

Stem Cell Factor

One of the additional cytokines/factors involved in megakaryopoiesis.

GM-CSF

Granulocyte-macrophage colony-stimulating factor; listed as an additional factor involved in megakaryopoiesis.

Granulocyte-CSF

One of the additional cytokines/factors listed in the lecture.