Human Histology - O. D. Lutsyk 2003

Special Histology

Special Histology studies the microscopic and ultramicroscopic Structure, as well as the relationship between the Structure and function of Organs. An organ is a distinct part of the body composed of various Tissues that performs specific Functions. The fine and ultrastructure of organs, including the characteristics of their vascularization and innervation, ensures the performance of their respective functions. Based on the General Principles of their Organization, the organs of The Human Body are divided into tubular-hollow and parenchymal organs.

Tubular-hollow organs include those of The Cardiovascular system, the digestive tract, the respiratory and urinary tracts, and most Organs of the male and female reproductive systems. Parenchymal organs include the Hematopoietic organs, endocrine and exocrine glands, Lungs, Kidneys, and organs of The Nervous system.

The wall of tubular-hollow organs typically consists of three layers, the inner layer being the primary one in terms of function. If these organs communicate with the external environment (such as the digestive tract, respiratory tract, and urogenital organs), their inner layer is called the mucous membrane (mucosa). If the hollow organs are not directly connected to the external environment (such as the organs of the cardiovascular system), their inner layer is called the intima (in Arteries, Veins, and lymphatic vessels) or the endocardium (in The Heart). In most cases, the Middle layer of tubular-hollow organs is muscular, while the outer layer is composed of Connective Tissue (adventitia) or is serous (serosa).

Parenchymal organs consist of two main structural components: the tissue (or tissues) that performs the specific function, known as the parenchyma, and the supporting and trophic component, called the stroma. In the vast majority of parenchymal organs, the stroma is formed by connective tissue.

Organs are grouped into Organ Systems, which comprise organs sharing a common origin and function. For example, the Digestive System includes the tubular-hollow organs of the alimentary canal (Esophagus, Stomach, small and large intestines), as well as parenchymal organs (major Salivary Glands, Liver, and Pancreas).

Some authors (P.K. Anokhin) also distinguish functional systems of the body, which include a complex of nervous structures and corresponding effector organs that ensure the performance of a specific function beneficial to the given system or the Organism as a whole. Organs belonging to different anatomical systems can participate in The formation of functional systems. For example, In addition to the organs that perform conducting and respiratory functions proper (providing gas exchange), the functional Respiratory system includes nerve endings, afferent and efferent nerve fibers, the respiratory center, as well as Respiratory Muscles, Ribs, and relevant PARTS OF THE cardiovascular system.

Groups of organs that have different origins but perform a common function are called apparatuses. For example, the Locomotor Apparatus includes the Skeletal System, bone junctions, and the skeletal Muscular System.