How the Medicinal Leech Extracts Blood

The skin is a multifunctional organ that responds with precision to environmental influences. It is intimately connected with all internal organs through neural, vascular, and endocrine pathways (Chernukh & Frolov, 1982). The skin comprises three structural components — the epidermis, dermis, and subcutaneous adipose tissue — that exist in multifunctional unity. Understanding this architecture is essential because each layer presents distinct challenges that the leech’s SGS must overcome.

1.1 Epidermis

The epidermis receives no direct blood supply. It consists of five cell layers that differ in morphology, cytologic characteristics, and functional role. Deep within the epidermis, in the cells of the basal layer, synthesis of proteins, polysaccharides, and lipids takes place. The epidermis serves as the outermost barrier that the leech’s jaws must penetrate to access the vascular bed beneath. The epidermal epithelium also produces vascular endothelial growth factor (VEGF), which plays an important role in skin regeneration following injury — a process that SGS actively suppresses to maintain wound patency.

1.2 Dermis

The dermis constitutes the bulk of the skin volume. Separated from the epidermis by the basement membrane, it transitions gradually into the subcutaneous adipose tissue. The dermis contains a rich cellular population and tissue infrastructure that the leech must contend with:

• Fibroblasts — synthesize collagen and extracellular matrix components; express membrane-bound SCF that retains mast cells in perivascular locations
• Mast cells — sentinel innate immune cells; highest density immediately sub-epidermally; cluster around blood vessels (see Section 3)
• Histiocytes/macrophages — phagocytic defense; tissue remodeling; antigen presentation
• Melanocytes — pigment production; UV protection
• Leukocytes — recruited during inflammatory response; source of proteases and inflammatory mediators
• Nerves — sensory endings branch through dermis; bidirectional communication with mast cells via neuropeptides
• Blood and lymphatic vessels — the ultimate target of leech feeding; organized as microvascular bed (see Section 2)
• Sweat and sebaceous glands, hair follicles — contribute to local microenvironment

1.3 Extracellular Matrix — The Primary SGS Target

The intercellular matrix of the dermis is the first major barrier that SGS must overcome to reach the microvascular bed. It is a complex hydrated gel composed of structural proteins, adhesive glycoproteins, and proteoglycans — each with specific relevance to the leech’s feeding strategy.

The deep layer of the dermis is permeated by a network of small vessels that constitutes the ultimate target of leech feeding. Understanding the architecture and physiology of this microvascular bed explains why SGS achieves its greatest pharmacological impact in specific vascular compartments.

2.1 Vascular Architecture

The cutaneous microvascular system follows a consistent organizational pattern:

• Arterioles ascend from the deep dermal network and penetrate through the dermis to form the subpapillary plexus beneath the epidermis
• The subpapillary arterial plexus gives rise to progressively branching arterioles that ultimately become capillary loops
• Capillaries converge to form venules, which constitute the capacitance portion of the venous subpapillary plexus
• The venous plexus lies superficial to the arterial plexus
• Arteriolovenular anastomoses serve as functional shunts, allowing direct arterial-to-venous blood transfer (Chernukh & Frolov, 1982)

2.2 The Capillary-Venular Permeability Gradient

2.3 Microlymphatic Circulation

Microcirculation is directly linked to the lymphatic drainage system. Approximately 60% of plasma and 45% of plasma proteins daily pass from the microcirculatory system into the tissues and thence into the lymphatic system (Chernukh & Frolov, 1982). Disruption of this balance following skin injury immediately leads to fluid accumulation and edema.

Two SGS components directly modulate lymphatic function:

• Hyaluronidase enhances lymphatic drainage by reducing interstitial viscosity through depolymerization of hyaluronic acid
• Eglins and bdellins limit inflammatory edema that would otherwise impair lymphatic function

The lymphatic system also participates importantly in delivering SGS components to the systemic circulation. Organs and systems situated closest to the sites of leech application experience the most immediate effects.

2.4 Cutaneous Blood Distribution and Regulation

Approximately 60% of the blood in the skin is venous. At any point within the cutaneous microcirculation, blood composition and vessel diameter are determined by the local profile of physiologically active substances. The epidermal epithelium produces VEGF, which plays an important role in skin regeneration following injury.

Autoregulation of cutaneous microvessels is mediated by both neural and humoral pathways. The histamine-like component of SGS exploits the existing regulatory architecture, producing sustained vasodilation that enhances blood flow to the feeding site and augments blood delivery to the leech. This pharmacological hijacking of normal vascular regulation is one of the leech’s most elegant evolutionary adaptations.

Mast cells occupy a central position in the host’s response to leech feeding. They are the first cellular effectors activated by tissue injury, and the leech must overcome their defense functions to ensure uninterrupted blood extraction.

3.1 Discovery and Origin

The metachromatic staining of mast cell granules was first identified in 1878 by Paul Ehrlich while still a medical student. In his doctoral dissertation, he designated these cells “Mastzellen” (mast cells). Mast cells are large, round or oval cells with a small nucleus and long, slender cytoplasmic processes. Their hallmark is the presence of numerous large membrane-bound granules that vary in appearance — resembling scrolls, crystals, fine particles, or inclusions of uniform density.

Mast cells are multifunctional hematopoietic effector cells. They arise from hematopoietic precursors, but unlike other hematopoietic lineages, mast cell differentiation and maturation occur in circulating progenitors that subsequently migrate into tissues. This process is regulated by local tissue factors, particularly stem cell factor (SCF) — also known as mast cell growth factor — whose receptor (KIT/CD117) is expressed on the mast cell surface (Valent, 1994).

SCF is primarily expressed by fibroblasts and endothelial cells of blood vessels, in both a soluble form (released by thrombin-activated endothelial cells) and a membrane-bound form. This dual expression creates a microenvironmental niche that retains mast cells in perivascular locations through adhesion of KIT-positive mast cells to SCF-bearing stromal cells (Zsebo et al., 1990).

3.2 Tissue Distribution

The distribution of mast cells across the skin is mosaic in character. Their density is highest immediately beneath the epidermis and decreases toward the base of the dermis. Mast cells form extensive clusters around blood vessels, capillaries, and postcapillary venules — underscoring the importance of mast cell-endothelial interactions. This perivascular positioning means the leech inevitably encounters a dense population of mast cells at its feeding site.

3.3 Mast Cell Subtypes: MC_T vs MC_TC

Human mast cells are classified into two principal subtypes based on their granule protease content. The subtype encountered at the leech bite site has direct implications for the composition of released mediators and the SGS compounds required to counteract them.

The MC_TC phenotype is particularly relevant because chymase converts angiotensin I to angiotensin II (independent of ACE), activates transforming growth factor-β1, and degrades lipoproteins — all functions that affect vascular homeostasis at the feeding site.

3.4 “Unicellular Endocrine Glands”

The multifunctionality of mast cells — their capacity to synthesize, accumulate, and release regulatory physiologically active compounds, and to repeat this process multiple times — has given rise to the concept of mast cells as “unicellular endocrine glands.” To this may be added their capacity to accumulate heparin from the bloodstream (Umarova et al., 1989). In the context of hirudotherapy, the leech effectively co-opts these unicellular endocrine glands, redirecting their secretory output to serve the leech’s feeding requirements. The isolation of 26 novel cDNA clones from stimulated murine mast cells (Cho et al., 1998), including genes responsible for allergic inflammatory protein synthesis, has revealed the full scope of mast cell molecular complexity.

The physiological significance of mast cells derives from the biologically active compounds stored in their cytoplasmic granules and synthesized upon activation. These mediators are released in two temporal waves: preformed mediators are discharged within seconds, while newly synthesized mediators require minutes to hours.

4.1 Preformed Mediators (Released in Seconds)

4.2 Newly Synthesized Mediators (Minutes to Hours)

4.3 Surface Receptors

Mast cell surface receptors mediate both activation signals and functional interactions with surrounding tissue:

• FcεRI — high-affinity IgE receptor; triggers degranulation upon antigen-IgE crosslinking
• uPAR — urokinase-type plasminogen activator receptor; upregulated by SCF; links mast cells to fibrinolysis
• KIT (CD117) — SCF receptor; mediates mast cell chemotaxis, survival, and perivascular retention
• C3a and C5a receptors — complement anaphylatoxin receptors; activate degranulation independently of IgE
• Growth factor receptors — respond to FGF, HGF, and other growth factors in the tissue microenvironment

Mast cells regulate vascular homeostasis, tissue injury and repair, and inflammatory responses. This regulatory role is of particular relevance in the context of overcoming host defense mechanisms during leech feeding.

5.1 Histamine and Vascular Permeability

Histamine released from mast cells induces hemodynamic shifts in the cutaneous vascular system and increases microvessel permeability, particularly in venules. Stimulation of endothelial H1-histamine receptors produces a time-limited increase in permeability (over 10–15 minutes) through the formation of broad intercellular channels — predominantly in postcapillary venules. These “histamine gaps” involve a contractile response of endothelial cells that permits rapid delivery of protective leukocytes and antibodies to the damage site (Chernukh & Frolov, 1982).

A clear correlation exists between vascular network density, mast cell concentration, and histamine content in specific regions of human skin. Histamine also participates in hemostasis regulation:

• Modulates thrombomodulin activity on endothelial cells, enhancing the anticoagulant potential of the vascular wall (Hirokawa & Aoki, 1991)
• Induces endothelial cell secretion of tissue plasminogen activator (tPA) (Hanss & Collen, 1987)

5.2 Heparin: The Functional Antagonist of Histamine

5.3 Mast Cells and Fibrinolysis

A compelling hypothesis holds that mast cells sustain endogenous fibrinolysis in venous vessels near which they reside (Valent et al., 2002; Bankl & Valent, 2002). The proposed mechanism operates through a precisely orchestrated cellular cascade:

1. Thrombin activates endothelial cells
2. Activated endothelial cells express and release soluble SCF
3. SCF induces mast cell chemotaxis and local accumulation around thrombosed vessels via KIT receptor
4. SCF upregulates urokinase receptor expression on the mast cell surface
5. Mast cells secrete heparin and tPA, which penetrate the vessel wall and directly influence thrombus state

5.4 Nerve-Mast Cell Interactions

The influence of mast cells on the peripheral nervous system operates bidirectionally. Multiple neuropeptides serve as inducers of mast cell activation and histamine release:

• Substance P — potent mast cell degranulation trigger
• Bradykinin — inflammatory pain mediator
• Endothelin-1 — vasoconstrictor peptide
• Somatostatin, VIP — neuropeptide modulators
• Morphine — opioid receptor-mediated activation

A correlation exists between histamine-releasing capacity and peptide structural features: the presence of positively charged amino acid residues in the N-terminal fragment and hydrophobicity of the C-terminal fragment. Such amphipathic peptides also confer pronounced antimicrobial activity (Giangaspero et al., 2001; Konno et al., 2001). Notably, destabilase-lysozyme from SGS — whose C-terminal region contains three positively charged α-helical segments (Zavalova, Baskova, Yudina, Akopov, Snezhkov, unpublished data) — may be one of the initiators of host mast cell degranulation, linking the leech’s antimicrobial defense to its strategy for overcoming mast cell barriers.

5.5 Mast Cells and Angiogenesis

Mast cells, localized in close proximity to capillary endothelial cells, stimulate endothelial proliferation upon activation (Metcalfe et al., 1997). The effect of histamine on angiogenesis, mediated through activation of H1 and H2 receptors, has been demonstrated. TNF-α, tryptase, and other mast cell enzymes further influence endothelial processes by participating in the regulation of the angiotensin-converting enzyme system. Increased capillary density following hirudotherapy (Zhuravsky, 2000) is consistent with these mast cell-mediated angiogenic effects.

Analysis of SGS components reveals a systematic array of antagonists directed against specific mast cell products. The evolutionary strategy is selective: the leech blocks mast cell products that would impede feeding (proteases, platelet activators) while co-opting those that enhance blood flow (histamine, heparin).

*Specific data on the presence of LCI (leech carboxypeptidase inhibitor) in native SGS, as opposed to whole leech extracts, are not yet definitive.

Any disruption of the skin is accompanied by an inflammatory response — a protective reaction characterized by complex changes in microcirculation and connective tissue. Cellular and tissue damage activates the protective blood systems of the microvasculature: coagulation, fibrinolytic, kallikrein-kinin, and complement systems. The inflammatory reaction proceeds through five sequential stages (Chernukh & Frolov, 1982), each of which SGS modulates for the leech’s benefit.

7.1 Wound Healing Sequence

In normal wound healing, epidermal cells are the first to respond — they rapidly migrate from wound edges to fill the defect. This newly formed epithelial layer thickens, lysosomal proteases are activated within keratinocytes, and fibrin and other proteins are cleared, creating conditions for dermal restoration. The initial dermal response consists of vasodilation and leukocyte diapedesis; leukocytes lyse damaged tissue, after which fibroblasts synthesize collagen to fill the defect. Consequently, the epidermal response to injury precedes the dermal response.

SGS deliberately disrupts this healing sequence. By maintaining anticoagulation, vasodilation, and protease inhibition at the wound site, SGS sustains an environment incompatible with normal reparative processes — ensuring continued blood flow long after the leech has detached.

To appreciate the tissue-disrupting effect of the leech bite, it is instructive to compare it with the host’s response to skin puncture by an acupuncture needle. The contrast dramatically illustrates that mechanical tissue injury alone, no matter how extensive, is insufficient to produce the sustained hemostatic blockade achieved by the leech.

The contrast is striking: a fingertip puncture for blood sampling inflicts considerably greater mechanical trauma than a leech bite, yet bleeding ceases within minutes. The leech bite produces sustained bleeding lasting hours to a full day — underscoring the unique pharmacological potency of SGS. During hirudotherapy, a tendency toward increased mean capillary density per unit area compared with untreated animals has been observed (Zhuravsky, 2000), consistent with the angiogenic effects of mast cell histamine and SGS components acting in concert.

The host’s microvascular response to leech-induced tissue injury, described above through classical inflammation physiology, has been substantially reframed by the convergent model of coagulation proposed by Yong and Toh (2023). This model integrates the traditional coagulation cascade, the cell-based model of hemostasis (Hoffman & Monroe, 2001), and the concept of immunothrombosis (Engelmann & Massberg, 2013) into a unified framework.

9.1 Why SGS Needs BOTH Anticoagulant AND Anti-Inflammatory Components

The convergent model provides a more complete understanding of why the leech’s SGS contains both anticoagulant and anti-inflammatory components. Blocking coagulation alone would be insufficient because the innate immune arm of the response would continue to promote thrombus formation through NET-dependent and DAMP-mediated pathways. The leech’s multi-target SGS simultaneously addresses both arms:

• Hirudin — blocks thrombin (coagulation arm)
• Calin, saratin — block platelet adhesion (vascular arm)
• Eglins, bdellins, LDTI — neutralize inflammatory proteases (innate immune arm)
• C1s inhibitor — modulates complement activation (complement arm)

This evolutionary solution anticipates by millennia the modern recognition that effective antithrombotic therapy in many clinical settings requires combined anticoagulant, antiplatelet, and anti-inflammatory approaches.

A leading role in the dissemination of SGS through host tissues belongs to hyaluronidase (orgelase). The enzyme was first noted by Heidenhein in 1891 in the context of accelerated lymph flow following injection of crude leech extract. Nobel laureate Albert Claude (1937) recognized this as a “spreading” effect and performed one of the most dramatic comparative experiments in leech biology.

10.1 Bond Specificity — Why Leech Hyaluronidase Is Superior

In 1960, Linker, Meyer, and Hoffman demonstrated the molecular basis for orgelase’s superiority: leech hyaluronidase hydrolyzes β(1→4) glucuronidic bonds in hyaluronic acid, in contrast to mammalian hyaluronidases that catalyze hydrolysis of β(1→3) bonds. The enzyme was purified in 1963 (Yuki & Fishman), and its inability to hydrolyze chondroitin and its derivatives was confirmed — unlike mammalian β-hyaluronidases.

10.2 Properties of Orgelase

• Molecular mass: 28,500 Da
• Substrate: Hyaluronic acid exclusively (β(1→4) glucuronidic bonds only)
• pH stability: Active across a wide pH range
• Thermal stability: Withstands 1 hour at 50°C
• Critical advantage: NOT inhibited by heparin (unlike testicular hyaluronidase, which is potently inhibited by heparin). This ensures that orgelase remains fully active even when mast cell degranulation releases heparin into the tissue

10.3 Commercial Development

In 1988, Biopharm (Roy Sawyer, director) established production of orgelase from the medicinal leech and obtained a patent for its development as a pharmaceutical agent for cardiovascular and ophthalmological diseases. The anti-ischemic properties of orgelase and its compatibility with heparin support its potential as a tissue-penetrating agent for drug delivery applications.

11.1 SGS Volume and Protein Content

The SGS constitutes an optimally balanced complex of bioactive compounds. Key pharmacokinetic parameters:

• Maximum SGS volume: ~20 μL per leech
• Approximately half enters the intestinal tract with ingested blood
• 10 μL is sufficient to ensure blood non-coagulability during feeding and to prolong this effect after leech removal
• Total peptides/proteins per leech: 0.07–0.09 mg

11.2 GPCR Signaling

A primary pathway for SGS biological activity involves interaction with seven-transmembrane G-protein coupled receptors (GPCRs) on target cells. These receptors — the most important and best-characterized receptor type in pharmacology — mediate the actions of signaling molecules of diverse chemical nature:

• Small-molecule transmitters: histamine, catecholamines, acetylcholine
• Lipid mediators: prostaglandins, PAF, leukotrienes
• Neuropeptides: opioids, neuropeptide Y, tachykinins
• Peptide hormones: angiotensin, glucagon, calcitonin

Multiple SGS components — the histamine-like vasodilator, prostacyclin analogs, and kininases — either activate or modulate GPCR signaling pathways in the host tissue.

11.3 Proteinase-Activated Receptors (PARs) — The Hirudin Connection

11.4 Intrinsic vs. Extrinsic Pathway Limitation

The leech bite creates a small skin incision — a wound sufficient to activate only the intrinsic coagulation pathway. Under these conditions, tissue factor does not enter the microcirculation in quantities that would trigger the avalanche-like thrombin generation of the extrinsic pathway.

The phenomenon of prolonged wound bleeding following leech detachment — which may persist for up to 24 hours — has been discussed for as long as the medicinal leech has been in clinical use. It represents one of the most intriguing molecular paradoxes in hirudotherapy.

12.1 The Molecular Paradox

12.2 The Abuladze Technique

The technique proposed by Dr. Abuladze provides indirect evidence: removing the attached leech several minutes after the onset of feeding results in relatively quick cessation of bleeding, even though the leech has already released a substantial portion of its SGS during this brief interval. This suggests that the duration of SGS exposure to the microcirculatory bed, not merely the quantity delivered, is the critical determinant.

12.3 Factors Affecting Bleeding Duration

• Physiological state of the individual receiving treatment
• Anatomical site of leech application
• Session frequency — with repeated application to the same area, bleeding time may decrease
• Strictly localized effect — bleeding time from a needle puncture 1–1.5 cm from the leech bite does not exceed the normal physiological level

12.4 Proboscis Leech Comparison

The proboscis leech (Haementeria ghiliani) provides an instructive comparison: its bite does not cause prolonged bleeding, because its SGS does not enter the wound but rather remains within the proboscis where it contacts the host’s blood (Sawyer, 1986). This confirms that SGS delivery into the wound tissue, not merely contact with blood, is required for the prolonged bleeding effect.

12.5 Current Hypothesis

The phenomenon of post-leech bleeding, while often viewed as a complication in general hirudotherapy, becomes a therapeutic asset in reconstructive microsurgery. When a severed digit, ear, or tissue flap is replanted, restored arterial inflow may function adequately while venous return remains compromised due to the technical difficulty of venous microanastomosis. The resulting venous congestion causes blood to stagnate in the capillary and postcapillary venular beds, where it rapidly coagulates, occluding the microvasculature and threatening tissue viability.

Despite the extensive molecular characterization of SGS-host tissue interactions described in this section, several critical questions remain unresolved. These represent priority research areas for advancing both the basic science and clinical application of hirudotherapy.