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HEMOCITOPOYESIS
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HEMOCITOPOYESIS

Overview of Connective Tissue Origins

Classification and Function of Hematopoietic Tissue

  • The classification chart serves to remind us of the origins of connective tissue and its specialized variants, particularly highlighting hematopoietic tissue represented by bone marrow.
  • Bone marrow is a highly vascularized organ rich in stem cells and precursors for blood cell formation, supported by a cellular stroma.

Blood Supply Mechanisms in Bone

  • Blood supply to long bones involves vessels entering through Haversian canals, which are horizontal conduits transporting medium-caliber blood vessels.
  • These vessels connect perpendicularly with Volkmann's canals that carry smaller caliber blood vessels like capillaries, facilitating nutrient delivery to osteocytes.

Nutritional Arteries and Their Pathways

  • The nutrient artery travels longitudinally through the nutrient canal, branching into ascending and descending branches that reach the bone marrow near trabecular surfaces.
  • In some long bones, large blood vessels accompany nerve trunks that branch within the medullary cavity; delicate nerve fibers often go unnoticed during routine functions.

Structural Composition of Bone

Layers and Types of Bone Tissue

  • The outer surface of bone is covered by periosteum except at articular surfaces; beneath it lies cortical bone which is thicker at the diaphysis than at epiphyseal regions.
  • Trabecular (spongy) bone contains hematopoietic tissue (bone marrow), forming a network known as vascular compartments filled with stem cells and blood cell precursors.

Vascular Compartments in Bone Marrow

  • A schematic representation shows two compartments: vascular sinusoids (1) and hematopoietic compartment (2), both supported by fibrocellular stroma essential for normal development.

Importance of Reticular Framework

Role of Reticular Fibers in Bone Marrow Functionality

  • The reticular framework supports both vascular structures and endosteum while surrounding adipocytes; this structure is crucial for maintaining normal bone marrow function.

Activity Levels in Bone Marrow

Classification Based on Activity Level

  • Active or red bone marrow is present throughout all bones in newborns but only found in specific locations such as femur, humerus, vertebrae, sternum, and ilium in adults.
  • Inactive or yellow bone marrow predominates with adipocytes; histological examination reveals progenitor cells including megakaryocytes involved in platelet production.

Characteristics of Vascular Sinusoids

Structure and Functionality

  • Vascular sinusoids measure 50 to 75 micrometers in diameter; they are highly branched within a reticular framework featuring continuous endothelial cells with junction complexes.

Hematopoiesis and Endothelial Cell Interaction

Structure and Function of Endothelial Cells

  • The fusion of endothelial cell membranes creates transient pores, allowing cells to enter circulation while restoring endothelial continuity.
  • Adventitial reticular cells, similar to fibroblasts, form a discontinuous outer layer; they are difficult to identify due to low affinity for basic and acidic stains.
  • These reticular cells contribute structurally to the stroma and leave uncovered areas for cellular migration, particularly at the ends of endothelial cells where the basal lamina is interrupted.

Role of Reticular Cells in Hematopoiesis

  • Reticular cells provide structural support and synthesize extracellular matrix components, including collagen fibers that anchor hematopoietic cells within the bone marrow stroma.
  • They also produce growth factors like interleukin 7 and thrombopoietin, essential for blood cell precursor maturation observed in bone marrow cultures.

Stages of Hematopoiesis

  • Hematopoiesis occurs in stages: mesoblastic (first month), hepatic (from week six), and myeloid (from third month onward), with each stage having distinct roles in blood cell development.
  • The final outcomes include erythropoiesis (red blood cell formation), leukopoiesis (white blood cell formation), and thrombocytopenia (platelet production).

Growth Factors Influencing Hematopoietic Development

  • The endodermal layer influences hematopoietic stem cell proliferation through growth factors such as FGF-2 during early embryonic development.
  • Vascular endothelial growth factor (VEGF) plays a crucial role by transforming central island cells into hematopoietic stem cells during prenatal hematopoiesis.

Microscopic Observations in Bone Marrow

  • At three months gestation, fetal liver shows large megakaryocytes as precursors for platelets alongside normoblast clusters indicating active erythropoiesis.
  • Postnatal hematopoiesis is exclusively conducted by bone marrow with lymphatic organs assisting in lymphocyte production.

Characteristics of Stem Cells

  • Stem cells are identified by their surface receptors for cytokines that guide their proliferation towards specific lineages; these include various colony-stimulating factors essential for differentiation.

Hematopoiesis and Stem Cell Differentiation

Overview of Hematopoietic Stem Cells

  • The pluripotent stem cell is the origin of all blood cells, generating both myeloid and lymphoid lineages.
  • Myeloid lineage includes colony-forming units (CFU) for erythrocytes, granulocytes, monocytes, and megakaryocytes; while lymphoid lineage involves CFUs for lymphocytes.

Multipotentiality of Stem Cells

  • Two key multipotential stem cells are highlighted: CFU-E (for erythropoiesis) and CFU-GM (for granulocyte formation).
  • These stem cells undergo maturation influenced by specific cytokines to produce various blood cell types including erythrocytes and leukocytes.

Erythropoiesis Process

  • The process begins with multipotential stem cells differentiating into unipotential progenitors that lead to platelet formation.
  • Lymphoid multipotent stem cells differentiate into three types of unipotential progenitor cells: B lymphocytes, T lymphocytes, and natural killer (NK) cells.

Stages of Erythrocyte Development

  • Erythropoiesis involves several stages from precursor cells to mature red blood cells; these include proerythroblasts which are identifiable in bone marrow samples.
  • Proerythroblasts are characterized by a large nucleus and basophilic cytoplasm due to ribosomal content; they proliferate slowly under the influence of erythropoietin (EPO).

Role of Erythropoietin in Red Blood Cell Formation

  • Hypoxemia stimulates increased production of EPO from renal interstitial cells, promoting the proliferation of CFU-E leading to early erythroid precursors.
  • Early precursors measure 12–20 micrometers in diameter with a pale basophilic cytoplasm rich in ribosomes; their nuclei occupy most of the cell volume.

Transition Through Precursor Stages

  • Proerythroblasts require iron transport via transferrin for hemoglobin synthesis; they also synthesize globin proteins necessary for hemoglobin formation.
  • As development progresses through different stages like basophilic erythroblasts, there is an increase in hemoglobin synthesis reflected by changes in cytoplasmic coloration.

Final Stages Before Maturation

  • Polychromatic erythroblasts represent a later stage where hemoglobin presence alters cytoplasmic staining properties; they continue mitotic division until reaching normoblast status.
  • Normoblasts lose their nuclei as they mature into reticulocytes before entering circulation as fully functional red blood cells.

Erythropoiesis and Thrombopoiesis Overview

Erythropoiesis Process

  • Reticulocytes, also known as polychromatic erythrocytes, are immature red blood cells characterized by the presence of hemoglobin and remnants of ribonucleoprotein organized in a reticular structure.
  • Reticulocytes constitute about 12% of circulating erythrocytes; their count reflects the bone marrow's response rate. They mature into erythrocytes within 24 to 48 hours after leaving the bone marrow.
  • The entire process of erythropoiesis, from stimulation by erythropoietin to the formation of mature red blood cells, takes approximately one week. The colony-forming unit (CFU) is sensitive to erythropoietin.
  • During maturation, there is a progressive decrease in cell volume starting from proerythroblasts, with surface receptors for transferrin facilitating iron incorporation necessary for hemoglobin synthesis.
  • Mature erythrocytes lack storage reserves in the bone marrow and immediately enter circulation upon formation. Microscopic examination reveals pale erythrocytes due to ruptured small vessels during aspiration.

Characteristics of Erythroid Cells

  • A typical normoblast has an eccentric dark nucleus with condensed chromatin and minimal cytoplasm. Size ranges from 10 to 12 micrometers in diameter.
  • In a bone marrow smear, precursor cells are visible alongside myeloid lineage cells; these include normoblast stages that exhibit distinct nuclear characteristics.

Thrombopoiesis Process

Megakaryocyte Development

  • The multipotential stem cell CFU-GM differentiates into megakaryocyte progenitors under growth factor stimulation like interleukin-3 (IL-3), leading to megakaryoblast development.
  • Megakaryoblasts measure between 15 to 30 micrometers and are identifiable in bone marrow smears; they have large nuclei and abundant cytoplasm but lack granules at this stage.
  • Megakaryocytes undergo endomitosis without cytokinesis, resulting in polyploidy. Their size can reach up to 50–70 micrometers with multiple lobulated nuclei.

Platelet Formation

  • Each megakaryocyte produces around 10,000 platelets through fragmentation of its cytoplasmic extensions that penetrate vascular sinusoids in the bone marrow.
  • The life cycle of megakaryocytes involves complex interactions with endothelial structures as their extensions release platelets into circulation through transendothelial pores.

Summary Insights on Thrombocytopenia

  • Thrombocytopenia results from differentiation processes where multipotential stem cells develop into specific lineages leading ultimately to platelet production via megakaryocyte maturation stages.

Overview of Myeloid Cell Development

Formation and Characteristics of Precursor Cells

  • The unit forming colonies, known as "gránulos y tica," proliferates and matures into the first precursor cell, identifiable in a bone marrow smear, measuring 12 to 15 micrometers in diameter, sometimes up to 20 micrometers with a pale nucleus.
  • This precursor cell has 3 to 5 nuclei and scant cytoplasm that is moderately basophilic with few nonspecific granules; it proliferates through mitosis.

Promyelocytic Stage

  • The promyelocyte is a recognizable precursor cell larger than its predecessor, measuring between 18 to 25 micrometers. It contains rough endoplasmic reticulum with abundant cisternae and numerous mitochondria.
  • The cytoplasm is eosinophilic due to developed lysosomes; the nucleus is eccentric and small, characterized by dense chromatin.

Maturation of Myeloblasts

  • In this stage, myeloblasts acquire specific granules and chemotactic capabilities along with receptors for complement and immunoglobulin fractions. They differentiate into metamyelocytes which are immediate precursors in the bone marrow.
  • Metamyelocytes can be neutrophils or eosinophils based on their specific granule content; they measure between 10 to 12 micrometers in diameter.

Basophil Development

  • Basophils have an irregularly shaped nucleus resembling a rod or hook; they contain more specific granules compared to neutrophils. These cells remain in the bone marrow for up to five days before migrating into connective tissue.
  • After circulating for about 6 to 9 hours, basophils can survive for another 2 to 5 days unless destroyed during phagocytosis.

Bone Marrow Smear Analysis

  • A bone marrow smear reveals various precursor cells including large myeloblasts (18–25 micrometers), smaller cells like metamyelocytes (10–12 micrometers), and mature segmented neutrophils.
  • The smear shows erythrocytes alongside myeloid lineage precursors arranged centrally; notable features include deep nuclear indentations in metamyelocytes.

Neutrophil Maturation Stages

  • Immature forms of neutrophils appear prior to nuclear segmentation while mature segmented neutrophils exhibit lobulated nuclei.
  • At high magnification (1000x), central cells show eccentrically located nuclei filled with granules indicative of maturation stages from myeloblast through metamyelocyte.

Granulopoiesis Process

  • Granulopoiesis involves differentiation from multipotential stem cells through various stages until reaching mature granulocytes such as neutrophils, eosinophils, and basophils.

Hematopoiesis and Cell Differentiation

Monocyte Development

  • The process of monocyte development begins with a specific progenitor cell, which differentiates into colony-forming units (CFUs) under the influence of granulocyte-macrophage colony-stimulating factor (GM-CSF).
  • The initial precursor in monocyte differentiation is the monoblast, which is challenging to identify in smears. After mitosis, it matures into the promonocyte stage.
  • Promonocytes measure 10 to 15 micrometers in diameter and have a slightly indented nucleus with one or two nucleoli; they are the last cells to divide before entering circulation.
  • Mature monocytes can reach up to 20 micrometers in diameter, characterized by a kidney-shaped nucleus and fine cytoplasmic granules. They have a lifespan of up to 72 hours in blood.
  • Monocytes migrate from blood vessels into connective tissue where they differentiate into macrophages; there is no reserve pool for mature monocytes.

Cytokines and Hematopoiesis Regulation

  • The differentiation of hematopoietic stem cells involves various cytokines that regulate cellular stages during hematopoiesis, including interleukins and erythropoietin.
  • A diagram illustrates how multipotent stem cells generate lineage-specific progenitors through local factors produced by reticular cells or via endocrine mechanisms from other tissues.
  • Cytokines play crucial roles at different stages of hematopoiesis, influencing both proliferation and differentiation processes within the bone marrow environment.

Lymphocyte Development

  • Lymphocytopoiesis involves the differentiation of specific lineage stem cells into lymphocytes through stages involving lymphoblast precursors.
  • T lymphocytes mature in the thymus while B lymphocytes develop in lymph nodes; natural killer (NK) cells also arise from similar precursors within bone marrow.
  • Recognizable medullary precursors include lymphoblastic and prelymphocytic stages leading to rapid maturation as they enter circulation.

Bone Marrow Structure

  • Microscopic examination reveals megakaryocytes as large precursors for platelets within myeloid tissue alongside adipose reticular cells storing lipids.
  • Observations show erythrocytes present among reticular cell structures, indicating active hematopoietic activity within bone marrow niches.

Stem Cell Characteristics

  • Stem cells possess dual capabilities: self-renewal (producing more stem cells) and differentiation into specialized cell types such as neurons or hepatocytes.

The Role of Umbilical Cord Blood in Medical Treatments

Overview of Umbilical Cord Blood

  • Umbilical cord blood from newborns is identified as an alternative source of progenitor cells for treating oncohematological and immunological diseases. The first public umbilical cord blood bank in the country was inaugurated at Hospital Garrahan in 2005.
  • The purpose of this bank is to collect, process, cryopreserve, and validate cord blood for use in patients needing a bone marrow transplant, especially when no compatible donor is available within the family.

Collection and Processing

  • There are private autologous banks where pregnant women can allow the collection of their baby's umbilical cord blood at birth. This blood contains stem cells that can be preserved for future use through cryopreservation.
  • After childbirth, the blood from the umbilical cord and placenta is collected using a sterile closed circuit into a bag. This procedure takes about 15 minutes before being transported to a processing laboratory.
  • The collected unit of blood is then cryopreserved with liquid nitrogen at temperatures below -196 degrees Celsius, allowing stem cells to remain viable indefinitely.

Unique Properties and Applications

  • Stem cells from umbilical cord blood possess unique biological qualities; they have greater proliferative capacity than those from bone marrow and are immunologically immature, meaning they haven't aged or been exposed to external viruses.