Histology Summary

Red Bone Marrow (Active)

• Function: Blood cell formation, destruction of old RBCs, iron & fat storage.
• Location (Adults): Flat bones (skull, sternum), ribs, vertebrae.
• Stroma: Reticular fiber network with CT cells, pericytes, bone cells.
• Blood Sinusoids: Wide channels, lined by endothelium, no basement membrane, surrounded by macrophages.
• Free Cells: Immature blood cells in various stages of development.

Yellow Bone Marrow (Inactive)

• Function: Fat storage. Can convert to red marrow on demand (e.g., hemorrhage).
• Structure: Stroma with a large number of fat cells, no free cells.

1. Proerythroblast: Large cell, pale nucleus with 2 nucleoli, basophilic cytoplasm.
2. Basophilic Erythroblast: Smaller, dark nucleus, intensely basophilic cytoplasm (due to many ribosomes).
3. Polychromatophilic Erythroblast: Smaller, condensed nucleus. Cytoplasm shows acidophilia (Hb) and basophilia (ribosomes).
4. Normoblast: Pyknotic (condensed) nucleus which is then lost. Cytoplasm is mostly acidophilic (more Hb).
5. Reticulocyte: Immature RBC, nucleus is lost, cytoplasm has remnants of polyribosomes. >2% in blood indicates high RBC production.
6. Mature Erythrocyte (RBC).

1. Myeloblast: Large, pale nucleus, non-granular cytoplasm.
2. Promyelocyte: Largest cell in the series. Contains non-specific (azurophilic) granules.
3. Myelocyte: Indented nucleus. Appearance of specific granules (neutrophilic, eosinophilic, or basophilic).
4. Metamyelocyte: Kidney-shaped nucleus. More specific granules.
5. Mature Granulocyte.

• Megakaryoblast: Large cell, indented nucleus, basophilic cytoplasm.
• Promegakaryocyte: Larger, lobulated nucleus, azurophilic granules appear.
• Megakaryocyte: Very large cell (40µm) with a large, dark, lobulated nucleus.
• Platelet Formation: Cytoplasm is shed in fragments, either through membrane invaginations or from pseudopodia extending into sinusoids.

Lymphatic Follicle (Nodule)

• Primary Follicle: Uniformly dark, no germinal center.
• Secondary Follicle: Has a pale germinal center due to proliferating B-lymphocytes responding to an antigen.

Lymph Node Structure

• Stroma: Capsule, Trabeculae, Reticular network.
• Parenchyma:
  • Cortex: Outer region with lymphatic follicles. The deeper part is the thymus-dependent zone (rich in T-cells).
  • Medulla: Inner region with medullary cords and medullary sinuses.
• Lymph Flow: Afferent vessels → Subcapsular sinus → Cortical sinuses → Medullary sinuses → Efferent vessel (at hilum).
• Functions: Lymph filtration, humoral immunity (B-cells), cell-mediated immunity (T-cells).

Tonsils

• Palatine: Covered by stratified squamous non-keratinized epithelium with deep crypts.
• Pharyngeal (Adenoid): Covered by pseudo-stratified columnar ciliated epithelium.
• Function: Quick defense against inhaled/ingested pathogens.

Spleen

• Function: Filters blood, not lymph. No afferent lymphatics.
• White Pulp: Lymphatic follicles (Malpighian corpuscles) for immune surveillance.
• Red Pulp: Splenic cords and blood sinusoids for blood filtration and RBC destruction.

Thymus Gland

• Function: Site of T-lymphocyte maturation. Involutes after puberty.
• Cortex: Outer dark area, packed with developing T-lymphocytes. Site of the blood-thymus barrier.
• Medulla: Inner pale area with fewer T-cells and unique Hassall's corpuscles (concentric layers of reticular epithelial cells).
• Blood-Thymus Barrier: Prevents antigens from reaching developing T-cells in the cortex.

Key Properties

• Self-Renewal: Can divide to produce more stem cells.
• Differentiation: Can give rise to specialized cells.
• Unspecialized: No tissue-specific functions.
• Plasticity: Potential for a stem cell from one tissue to become a cell type of another tissue.

Classification by Potency

• Totipotent: Can form an entire organism (embryonic + extra-embryonic tissues). E.g., zygote.
• Pluripotent: Can form all cell types of an adult organism (all 3 germ layers). E.g., Embryonic Stem Cells.
• Multipotent: Can form a limited range of cell types. E.g., Adult Stem Cells.
• Oligopotent: Can form a few related cell types (e.g., myeloid stem cell).
• Unipotent: Can form only one cell type.

Embryonic Stem Cells (ESCs)

• Source: Isolated from the inner cell mass of a blastocyst.
• Sources of Embryos: In Vitro Fertilization (IVF) clinics, Somatic Cell Nuclear Transfer (SCNT).
• Advantages: Pluripotent (flexible), immortal (endless supply).
• Disadvantages: Immunogenic (rejection), Tumorigenic (can form tumors), ethical concerns.

Adult (Somatic) Stem Cells

• Role: Maintain and repair their native tissue.
• Sources: Fetal tissue, amniotic fluid, umbilical cord blood, and various adult tissues (bone marrow, skin, fat, brain).
• Advantages: Not immunogenic if from the patient, non-tumorigenic, no ethical issues with donor.
• Disadvantages: Multipotent (less flexible), limited quantity, finite lifespan in culture.
• Cord Blood Advantage: Very low immunogenicity, less chance of graft-versus-host disease. Rich in hematopoietic stem cells.