Medical School Histology
Understanding the components of the cell is essential to microscopic biology. The nucleus and organelle of the animal cell vary in organization and function, and differ depending on the purpose of that specific cell.
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The epithelial cells come in several forms and functions depending on their placement in the body. These cells form the lining of all internal and external body structures. They are characterized by their shapes, sizes, and additional appendages such as cilia.
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Connective tissue is the connecting point and structural support of other tissues and organs. It is one of the four animal tissues (along with nervous, muscular, and epithelium) and includes blood, adipose, tendons, and more. CT is grouped by two types of dense CT (regular and irregular) as well as loose CT (areolar).
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This section focuses on the brain, spinal cord, and the peripheral nervous system. Myelinated white matter covers axons of the central nervous system (via oligodendrocytes) and peripheral nervous system (via Schwann cells). Three forms of meninges are also present in the nervous system; the pia, the arachnoid, and the dura matter.
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There are three categories of muscle tissue: smooth, skeletal, and cardiac. These types are each histologically and functionally distinct, though many organs may have a mixture of muscle tissues. The basic muscle unit is called a sarcomere, and consists of actin and myosin myofilaments.
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Cartilage & Bone
Most of the body was supported by cartilage in the fetal life, which converted to bone by birth. Cartilage can be categorized as hyaline, elastic, and fibrocartilage; some of which persist in the adult in select areas of the body. Bone is divided into cancellous (spongy) bone and dense (compact) bone. Chondroblasts and osteoblasts form the cartilage and bone, respectively.
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Blood is a form of connective tissue that varies greatly. Erythrocytes (red blood cells) are one of the few cells that shed their nucleus upon maturing, and leukocytes (white blood cells) come in five subcategories. These include the agranular leukocytes and monocytes, as well as the granular neutrophils, basophils, and eosinophils. Leukocytes and erythrocytes have several embryologic origins that must be understood to understand their development.
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The immune system consists of several types of lymph tissue. Primary lymph tissue includes bone for B lymphocyte (B cell) maturation and the thymus for T lymphocyte maturation. Secondary tissues include the spleen, lymph nodes, and associated lymphoid tissue (MALT, GALT, BALT, etc.). The spleen also acts as the bodies main filter for old and damaged erythrocytes.
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The respiratory tract is separated into two sections. The conducting system consists of the nostrils, larynx, trachea, primary bronchi, and terminal bronchioles. The respiratory part, the most distal end of the entire system, consists of respiratory bronchioles, alveolar ducts and sacs, and the alveoli themselves. These tissues vary greatly histologically with the more proximal end containing cartilage and the more distal end with club (Clara) cells.
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The digestive system begins at the oral cavity and continues to the rectum. It includes tissues from the esophagus, stomach, small intestines (duodenum, ileum, and jejunum), and the large intestine (cecum, ascending, transverse, descending, and sigmoid colon). The complex network of tissues, specialized cells, and glands make this one of the most intricate systems for histological purposes.
The gastrointestinal tract also consists of several organs that add to the digestive process. These include the pancreas, liver, gallbladder and appendix. The liver and pancreas produce substances that aid digestion, while the gallbladder stores bile created from the liver. The appendix is involved in the immune actions of the GI Tract, and consists of many lymphoid nodules.
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The renal system is composed of the kidneys, ureters, urinary bladder, and the urethra. The kidneys primary function come from the nephrons (convoluted tubules and collecting ducts) which drain into the calyx of the kidney. The nephrons are composed of several parts, and also play a role in the production of renin and erythropoietin, which regulate blood pressure and red blood cell production, respectively.
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Male Reproductive Tissue
The male reproductive system includes the testis (seminiferous tubules, rete testis, efferent tubules), epididymis, ductus/vas deferens, seminal vesicles, prostate, and urethra. The urethra is divided into the prostatic, membranous, and penile sections. Spermatogenesis, or the creation and development of sperm, begins in the seminiferous tubules with the aid of testosterone supplying Leydig cells and nutrient supplying Sertoli cells.
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Female Reproductive Tissue
Female germ cells begin in the ovaries where they go several differentiations before a select few are expelled during ovulation. Around 400-450 ovum (eggs) are expelled into the peritoneal space between the ovary and the uterine/fallopian tubes. Before ovulation, an ovum matures from a primordial follicle with a primary oocyte (which are all developed before birth) to become a primary follicle, then secondary follicle. At the same time, the oocyte is maturing from a primary to a secondary oocyte within the protective covering of the outer follicular layer.
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Skin and its related appendages (hair, glands, nails) constitute the integument system. Thick skin is limited to the palms and soles of the feet and lacks hair. The skin has two layers; the dermis and the epidermis. The dermis has a deep layer, the reticular dermis, and a more superficial layer that contacts the epidermis, the papillary dermis. The epidermis has five layers and several associated cells. From deep to superficial, this consists of the stratum basale (germinativum), stratum spinosum, stratum granulosum, stratum lucidum (thick skin only), and stratum corneum.
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The eye serves a complex function, and is suited with a complex system to do so. Composed of 3 main layers (fibrous, vascular, and sensory), the eye can further subdivided into many parts of each layer; including the sclera, cornea, choroid, iris, retina, lense, and eyelid itself. Phototransduction, or light sensing, is transmitted by rods and cones through the retina, which is actually a direct connection of the brain. The ear has two systems; audition and balance. The cochlea is the transducer of sound waves via the Organ of Corti. When sound travels through the ear canal, vibrations are detected by the fine "hairs" and nerves in this organ. The vestibular, or balance, system is sensed by the 3 semicircular canals. Positioned at angles to each other, they detect motions of the head from all three dimensions, working similar to a carpenter's leveler tool.
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Many of the endocrine organs come from the endoderm cells. However, much of the body has endocrine cells individually or in small clusters that do not constitute their own organ. Unlike exocrine organs, endocrine do not have ducts with with to share their hormones, and must do so via the bloodstream. The hypothalamus sends hormones and signals to the two divisions (anterior and posterior) of the pituitary, which send hormones to many other parts of the body. The adrenal glands also have two division, each supplying different hormones, while the thyroid, parathyroids, pancreas, and pineal do not have the medulla-cortex separation seen in the adrenals.
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