User:Christina Berotte/Invertebrate

Open vs. Closed Circulatory System
Commonly, invertebrates have an open circulatory system. Which is defined as the tissues bathing in pseudo-coelomic or coelomic fluid (such as hemolymph) which circulates throughout the body via the coelom. Or a dorsally located heart in a hemocoel, pumping hemolymph through anterior and/or posterior aortic vessels. These vessels end at the coelom where its contents move into and where gas, nutrient, and waste exchange take place. The former definition is the most primitive followed by the production of hemoglobins, iron-based, oxygen-binding pigments. The latter definition describes the more modern circulatory system of the phyla Annelida. Annelids have evolved into different mechanisms for the transportation of materials. In larger and more active worms, such as the Annelid class the Polychaeta, they have a more complex open circulatory system and a defined heart.

In the class Polychaeta, a dorsal vessel runs above the digestive tract. Blood flows anteriorly where the dorsal vessel connects with a ventral vessel. The ventral vessel is under the digestive tract which carries the blood posteriorly. They have parapodial vessels (vessels of the protrusions of the body) that supply the rest of the body and give rise to intestinal vessels. Blood moves from the ventral vessel through the parapodial vessels and returns to a pair of dorsal parapodial vessels into the dorsal vessel. The Polychaeta circulatory system does not have endothelial layer; instead, it has a basal layer of overlying cells.

Invertebrates can also have a closed circulatory system, like their vertebrate counterparts. Described when a centralized pump, or heart, delivers blood through a system of arteries, veins, and capillaries. And it has a functional barrier that separates the circulating blood from the tissues. The cephalopods, members of the molluscan class, have a closed circulatory system. The molluscan cardiovascular system has extensive vascular systems with "true hearts" and its blood is circulated at high pressures. Because of its high oxygen levels, paired branchial hearts pump venous blood through their gills then its arterial blood flows to the ventricle in a systemic circuit. Cephalopods have a multichambered heart which separates the venous and the arterial blood as well as regulating branchial and systemic circulations. Despite having an endothelial-like lining, it is permeable and not an effective barrier like in vertebrates; which makes it a more open system. However, in terms of function and physiology, it's classified as a closed system.

Endothelial Cells
The only major difference between closed circulatory systems and open circulatory systems is that closed systems have an endothelium lining the vessels and open systems do not. An endothelium is basically a continuous layer of interconnected cells lining near the lumen. Its main function is to be a barrier separating the blood from the body tissues. Two different hypotheses have been suggested on where the endothelial cells came from. The first hypothesis is that primitive endothelium originates from hemangioblast of the mesoderm. Hemangioblasts are the multipotent precursor cells that can differentiate into hematopoietic and endothelial cells. The second hypothesis is that they originated from amoebocytes, a specialized form of blood cells that adhere and circulate over the vascular basement membrane. The transition form amoebocytes to endothelial cells require an epithelial phenotype.

The transport of oxygen, nutrients, and waste in invertebrates are performed by the coelom and the hemal system. The coelomic cavities are lined by the mesodermal epithelium (mesothelium) and the hemal cavity are lined by the basement membrane of the epithelia. The hemal cavity is found in between the endodermal epithelium and the mesothelium. Or is surrounded by a single layer of myoepithelial cells; cells that are a muscular differentiation of mesothelial cells that compose the vascular wall of micro vessels in invertebrates with more complex circulatory systems like cephalopods. Another characteristic of the endothelium is that it has blood cells migrating through it. The main cells in the cavities are coelomocytes and hemocytes, respectively. Hemoglobin is found only in hemocytes of invertebrates, such as annelids and mollusks. The aforementioned amoebocytes are a type of hemocyte found in annelids, mollusks, echinoderms, and cephalochordates. In some invertebrates, the presence of amoebocytes are so abundant that they adhere together to form an endothelium. As is the case of holothurians and cephalopods.

Decapod crustaceans
The definitions of "open" and "closed" circulatory systems are not as black and white as we are led to believe. Just because an organism have hemolymph does not explicitly characterize them with an "open" system. Likewise, the absence of hemolymph does not mean they have a "closed" system. Comparative physiologists define hemolymph as an replacement of red blood cells, platelets, and leukocytes.

Decapod crustaceans were once known to have an open circulatory system until recent research explained that they actually fall into this middle ground. Its circulatory system functions as a closed system but also contains hemolymph. It consists of a globular heart that delivers hemolymph at high pressures to capillary-like vessels that supplies active tissues. Thus, it's an "incomplete" closed system. Its heart rate and stroke volume are controlled independently via neurons input from cardiac ganglion and CNS or directly by the neurohormones on the cardiac muscles. This allows rapid modulation of the amount of blood the heart pumps through the system in a minute. Usually, blood flow in a closed vertebrate system is regulated by the contraction of smooth vascular muscles. Decapod crustaceans do not have these smooth muscles; instead, the contraction of a pair of muscular cardioarterial valves located at the base of each arterial system controls hemolymph flowing through the arteries. Decapod crustaceans collects hemolymph in sinuses before flowing back into the heart as there is no venous system. The major sinuses are bordered by fibrous connective tissue and the lacunae by the basal lamina on the organ they bathe. The presence of the sinuses was why the crustacean's circulatory system was defined as open in the past.

Crustaceans lack an immune system, so they rely the cellular and humoral responses from antimicrobial peptides and proteins in the hemolymph that become immune mechanisms. For example, the function of phenol oxidase (a copper-containing enzyme) is the melanizes pathogens and damaged tissues as a major innate defense mechanism. That in turn is regulated to avoid unnecessary production of highly toxic and reactive compounds. Another example is lipids, which is a major source of energy. Also, they are involved in processes for their growth, molting, and reproduction purposes as their energy storage. Also, Glycoproteins (proteins that contain glycans covalently attached to polypeptide side-chains) are integral to cell-cell interactions. In copepods, glycoproteins bind to the exoskeleton surfaces. Specifically, the antennule, genital pore, and caudal ramus. This is so possible mates can recognize the species and the sex.

Coagulation
Coagulation is essentially the clotting system for invertebrates. Clotting usually happens when proteins join together in plasma triggered by factors from the damaged tissue. In animals that are low in plasma proteins, these triggers do not occur. Instead, blood cells stick to each other to form a strong cellular clot. Transglutaminases (TGase) are the main enzymes involved in the clotting process. Hemocyte-driven factors, sometimes the coagulation proteins themselves, are released from hemocytes during the activation of the coagulation reaction.

Horseshoe Crabs
Horseshoe crabs are not actual crabs; they are more closely related to spiders and scorpions. Hence, they are not decapods as they possess an open circulatory system.

Firstly, the exocytosis (the process of moving materials within a cell to its exterior) of clotting components begins by binding LPS (Lipopolysaccharide) to plasma membrane-associated form of factor C. Factor C is the LPS-responsive serine proteinase zymogen; it's present in large atoms of hemocytes. This also activates platelets by thrombin, enzyme of hemostasis, through proteinase-activated receptors. Then, Factor C cleaves Factor B, which is responsible for cleaving the proclotting enzyme. Converting it to its active form, clotting enzyme. Finally, the clotting enzyme transforms the soluble protein coagulogen into the insoluble coagulin gel. In an alternate pathway, Factor G activates the proclotting enzyme; Factor G is a dimeric protein with glucanase and xylanase-like domains and a smaller-subunit possessing the serine proteinase domain. TGase is not needed for coaulgin formation but is still utilized in the clotting process as it cross-links caraxins (chitin-binding proteins) to the subcuticular epidermis. With the clot, TGase binds to the chitin via another protein called stablin; stablin is important for the immobilization of bacteria to the clot by binding itself to LPS. Thereby, stabilizing the clot and sealing the wound.

Other Crustaceans
The plasma contains a large clotting lipoprotein that structurally resembles vitellogenin even through they are two different proteins. Crustaceans only have 1 or 2 TGase genes. Firstly, when TGase is released from hemocytes, the polymerization of the lipoprotein begins. The lipoprotein forms long vitro chains with 60 or more other clotting proteins in under a minute. In crayfish, shrimp, and other crustaceans, the clotting protein contains a von Willebrand factor domain. Which is a large multimeric glycoprotein in the plasma.

In P. leniusculus, or signal crayfish, TGase affects hematopoiesis as well as coagulation. Hematopoiesis is the formation of blood cell componets. The enzyme is present at high levels of hematopoietic tissue, meaning it's important for blood cell formation. The process of astakine, a group of cytokines, production and hematopoiesis are under circadian control. Meaning the process happens daily and is light-dependent. So the efficiency of the blood clots will depend on weather.