Classification principles
The organs of definitive (final) hematopoiesis in adults are divided into two main groups:
- Central organs: red bone marrow (RBM) and thymus.
- Peripheral lymphoid system: spleen, lymph nodes, and mucosa-associated lymphoid tissue.
The division of functions in the body occurs in stages. In early stages, the liver completely performs the hematopoietic function. Later, it passes the "baton" to specialized organs (the medullary stage begins). During this period, the red bone marrow retains the creation of blood cells, the thymus begins to train T cells, and the peripheral system becomes the site of direct operation for immune cells.
Central organs: RBM and thymus
Red bone marrow (RBM) is the main hematopoietic organ with a mass of up to 3.5 kg. In adults, it is localized in the cells of the spongy substance of flat and short bones, as well as in the epiphyses of tubular bones.
Age-related dynamics: in children up to 12–18 years of age, RBM also fills the bone marrow cavities of the diaphyses of tubular bones. With age, the hematopoietic tissue there is replaced by adipose tissue, forming yellow bone marrow. RBM tissue has a semi-fluid consistency, so smears or sections of decalcified bones are used for histological study.
Thymus is located behind the sternum. Its main functional role is the maturation (proliferation) of T lymphocytes and strict selection: the elimination of autoreactive clones capable of attacking self-antigens. The organ undergoes age-related involution: after reaching peak mass (35–40 g) at 14–15 years of age, the thymus tissue gradually undergoes replacement.
Peripheral lymphoid system
Peripheral organs are the site where the immune response is realized, where antigen-dependent proliferation of lymphocytes and their differentiation into effector cells take place. The total mass of this system is about 1.5 kg.
It includes:
- Lymph nodes (located along the course of vessels).
- Spleen.
- Mucosa-associated lymphoid tissue.
The latter group includes Waldeyer's tonsillar ring (lingual, pharyngeal, two palatine, and two tubal tonsils), intestinal follicles (solitary follicles, Peyer's patches of the small intestine, clusters in the appendix), and bronchus-associated lymphoid tissue (BALT). Histologically, these organs are organized as nodules (follicles) and interfollicular tissue.
Histological composition and origin
Regardless of localization, any hematopoietic tissue includes four mandatory components:
- Stromal — the supporting framework. In RBM, spleen, and lymph nodes, this is reticular tissue. In mucous membranes, it is loose fibrous connective tissue. In the thymus, it is reticuloepithelial tissue.
- Hemal — the hematopoietic cells themselves, which together with the stroma form the functional microenvironment.
- Macrophagic — cells of the macrophagic lineage.
- Vascular — blood and lymphatic vessels.
Embryogenesis: As a general rule, hematopoietic cells, vessels, and stroma develop from mesenchyme. The only exception is the stroma of the thymus, which is of epithelial origin (from the epithelium of the foregut).
Classification of hematopoietic tissues
Hematopoiesis is divided into two lines determining the tissue type:
- Myeloid tissue (localized in RBM). Provides myelopoiesis — the formation of erythrocytes, granulocytes, monocytes, and platelets. The initial stages of T-lymphocyte maturation and the full development of B-lymphocytes also take place here.
- Lymphoid tissue (localized in the thymus and peripheral system). Responsible for lymphopoiesis — the maturation and active functioning of lymphocytes.
Briefly: answers to topical questions
From which germ layer does the stroma of the thymus develop?
The stroma of the thymus develops from the endoderm of the foregut with the participation of neural crest cells. The thymic epithelium forms directly within the endoderm of the 3rd pharyngeal pouch. Unlike the stromal component of other hematopoietic organs, which are of mesenchymal origin, the thymus stroma is formed by specific reticuloepithelial tissue.
What zones are distinguished in the structure of a lymph node?
Three main zones, differing in architectonics and functions, are distinguished in a lymph node:
- Cortex — a B-dependent zone where lymphoid tissue forms rounded clusters (lymphatic nodules or follicles).
- Paracortical zone — a T-dependent zone in which cellular elements are arranged diffusely (disorderly, without the formation of clear structures).
- Medulla — the plasmacytogenesis zone (effector zone), where lymphoid tissue forms specific cords (medullary cords).
What stages of embryonic hematopoiesis does the fetus go through?
During intrauterine development, the fetus goes through three successive stages of hematopoiesis:
- Mesoblastic stage (extraembryonic) — takes place from the 9th day to the 9th week in the mesenchyme of the yolk sac, chorion, and body stalk. Primary blood cells and 1st-generation hematopoietic stem cells are formed here.
- Hepatic stage (hepatosplenic) — begins at the 6th week in the embryonic liver, where all formed elements of blood and 2nd-generation HSCs are formed.
- Medullary and lymphoid stage (bone marrow stage) — the stage when the red bone marrow becomes the central organ of hematopoiesis containing 3rd-generation HSCs.
First the liver does everything, then it passes the baton: the bone marrow keeps cell stamping for itself, the thymus teaches T lymphocytes, and the spleen and lymph nodes become their "office".
Class and exam questions
What is the difference between a child's and an adult's bone marrow?
In children under 12–18 years of age, red bone marrow is found even in the diaphyses of tubular bones. In adults, hematopoietic tissue there is replaced by adipose tissue, turning into yellow bone marrow.
What belongs to mucosa-associated lymphoid tissue?
This is Waldeyer's tonsillar ring (6 tonsils), solitary follicles and Peyer's patches of the intestine, clusters in the appendix, as well as the bronchus-associated tissue of the respiratory tract.
Where do stromal cells of hematopoietic organs come from?
The stroma of most organs (RBM, lymph nodes, spleen) develops from mesenchyme. The exception is the thymus — its stroma is formed from the epithelium of the primitive gut.
What else to study on this topic
- Methodology for studying histological sections of decalcified bones
- Mechanisms of elimination of autoreactive T lymphocytes in the thymus
- Structure of a lymphoid nodule (follicle)
- Interaction of the hemal component and the microenvironment
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