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Hematopoietic Organs

For medical students2 min readUpdated 2026-10-10

Hematopoietic and lymphoid organs ensure the continuous renewal of blood cells and coordinate immune defenses. Structurally and functionally, they are divided into primary (central) organs, where basic cell production and maturation occur, and secondary (peripheral) organs, which serve as sites for active immune responses.

Bone marrow massApproximately 3.0–3.5 kg in an adult

Age-related involutionThe thymus reaches its maximum development at puberty (14–15 years, weighing 35–40 g)

Specific stromaThe thymic framework is formed by epithelium rather than connective tissue

Peripheral systemThe total mass of the organs of the immune response reaches up to 1.5 kg

Principles of Classification

The definitive hematopoietic organs of an adult are divided into two main groups:

Primary (Central) Organs: Red bone marrow and the thymus.
Secondary (Peripheral) Lymphoid System: Spleen, lymph nodes, and mucosa-associated lymphoid tissue (MALT).

Functional division in the body occurs sequentially. In early embryonic development, the liver performs hematopoietic functions entirely. Later, it transfers this role to specialized organs (the medullary stage). During this period, red bone marrow assumes the production of blood cells, the thymus begins T-cell education, and the peripheral system becomes the site for direct immune cell effector functions.

Primary Organs: Red Bone Marrow and Thymus

Red bone marrow is the primary hematopoietic organ, weighing up to 3.5 kg. In adults, it is located in the spongy spaces of flat bones, short bones, and the epiphyses of long bones.

Age-related dynamics: In children up to 12–18 years of age, red bone marrow also occupies the medullary cavities of the diaphyses of long bones. With age, the hematopoietic tissue there is replaced by adipose tissue, forming yellow bone marrow. Red bone marrow has a semi-liquid consistency, which is why smears or sections of decalcified bones are used for histological study.

The thymus is located in the superior mediastinum behind the sternum. Its primary 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 its peak mass (35–40 g) at 14–15 years of age, thymic tissue is progressively replaced by adipose tissue.

Secondary Lymphoid System

Secondary (peripheral) organs are the sites where immune responses take place, involving antigen-dependent lymphocyte proliferation and differentiation into effector cells. The total mass of this system is about 1.5 kg.

It includes:

Lymph nodes (located along lymphatic vessels).
The spleen.
Mucosa-associated lymphoid tissue (MALT).

The latter group includes Waldeyer's ring (lingual, pharyngeal, two palatal, and two tubal tonsils), intestinal follicles (solitary follicles, Peyer's patches in the small intestine, clusters in the appendix), and bronchus-associated lymphoid tissue (BALT). Histologically, these organs are organized into lymphoid follicles and interfollicular tissue.

Histological Composition and Origin

Regardless of location, any hematopoietic tissue includes four essential components:

Stromal component — the supportive framework. In red bone marrow, spleen, and lymph nodes, this is reticular connective tissue. In mucous membranes, it is loose fibrous connective tissue. In the thymus, it is reticuloepithelial tissue.
Hemopoietic component — the hematopoietic cells themselves, which together with the stroma form the functional microenvironment.
Macrophage component — cells of the mononuclear phagocyte system.
Vascular component — blood and lymphatic vessels.

Embryogenesis: As a general rule, hematopoietic cells, vessels, and stroma develop from mesenchyme. The single exception is the thymic stroma, which has an epithelial origin (derived from the epithelium of the pharyngeal pouches of the foregut).

Classification of Hematopoietic Tissues

Hemopoiesis is divided into two primary lineages defining tissue type:

Myeloid tissue (localized in red bone marrow). Drives myelopoiesis — the formation of erythrocytes, granulocytes, monocytes, and platelets. It also supports the early stages of T-lymphocyte maturation and the complete development of B-lymphocytes.
Lymphoid tissue (localized in the thymus and peripheral system). Responsible for lymphopoiesis — the maturation and active functioning of lymphocytes.

Mnemonic

The liver does everything first, then passes the baton: the bone marrow stamps out cells, the thymus schools the T-cells, and the spleen and lymph nodes serve as their headquarters.

Frequently asked questions

From which germ layer does the thymic stroma develop?

The thymic stroma develops from the endoderm of the anterior foregut with the participation of neural crest cells. The thymic epithelial primordium originates directly from the endoderm of the 3rd pharyngeal pouch. Unlike the stromal components of other hematopoietic organs, which are of mesenchymal origin, the thymic stroma is formed by specialized reticuloepithelial tissue.

What are the main structural zones of a lymph node?

A lymph node is divided into three main zones, which differ in cytoarchitecture and function:

Cortex (superficial cortex) — a B-cell-dependent zone where lymphoid tissue forms rounded clusters (lymphoid follicles).
Paracortex (paracortical zone) — a T-cell-dependent zone where cellular elements are arranged diffusely without forming distinct follicles.
Medulla — an effector zone of plasmacytogenesis where lymphoid tissue forms specific branching cords (medullary cords).

What stages of embryonic hematopoiesis does the fetus undergo?

During intrauterine development, the fetus undergoes three sequential stages of hematopoiesis:

Mesoblastic stage (extraembryonic) — occurs from day 9 to week 9 in the mesenchyme of the yolk sac, chorion, and body stalk, producing primitive blood cells and 1st-generation hematopoietic stem cells (HSCs).
Hepatic stage (hepatosplenic) — begins at week 6 in the embryonic liver, generating all formed blood elements and 2nd-generation HSCs.
Medullary and lymphoid stage (bone marrow stage) — the definitive stage when red bone marrow becomes the central organ of hemopoiesis, harboring 3rd-generation HSCs.

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 even found in the diaphyses of long bones. In adults, the hematopoietic tissue there is replaced by fat, turning into yellow bone marrow.

What structures comprise mucosa-associated lymphoid tissue (MALT)?

It includes Waldeyer's pharyngeal ring (6 tonsils), solitary follicles and Peyer's patches of the intestine, lymphoid aggregates in the appendix, and bronchus-associated lymphoid tissue (BALT) of the respiratory tract.

Where do the stromal cells of hematopoietic organs originate from?

The stroma of most organs (red bone marrow, lymph nodes, spleen) develops from mesenchyme. The exception is the thymus, whose stroma forms from the epithelium of the embryonic foregut.

Go deeper

Histological techniques for studying decalcified bone sections
Mechanisms of negative selection and elimination of autoreactive T-lymphocytes in the thymus
Fine structure of the lymphoid follicle (nodule)
Interactions between the hemopoietic component and the bone marrow microenvironment