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Chapter 1 Introduction to Anatomy Anatomy Human anatomy is the study and organization of the structures which make up the human body. There are many ways to study Anatomy. Regional anatomy considers the body as organized into segments or parts....

Chapter 1 Introduction to Anatomy Anatomy Human anatomy is the study and organization of the structures which make up the human body. There are many ways to study Anatomy. Regional anatomy considers the body as organized into segments or parts. Systemic anatomy sees the body as organized into organ systems. Surface anatomy provides information about structures that may be observed or palpated beneath the skin. Radiographic, sectional, and endoscopic anatomy allows appreciation of structures in the living, as they are affected by muscle tone, body fluids and pressures, and gravity. Clinical anatomy emphasizes application of anatomical knowledge to the practice of medicine. 1 The levels of structural complexity Anatomy encompasses various levels of complexity, from the microscopic level of cells to the macroscopic level of organs and organ systems. 1.1 The Cytoplasm Cells and extracellular material together comprise all the tissues that make up the organs of multicellular animals. In all tissues, cells themselves are the basic structural and functional units, the smallest living parts of the body. Animal cells are eukaryotic (Gr. eu, good, + karyon, nucleus), with distinct membrane-limited nuclei surrounded by cytoplasm containing many different organelles. 2 The main cellular functions performed by specialized cells in the body are listed in Table below: Table 2–1. Cellular functions in some specialized cells Function Specialized Cell(s) Movement Muscle and other contractile cells Form adhesive and tight junctions between cells Epithelial cells Synthesize and secrete components of the Fibroblasts, cells of bone and extracellular matrix cartilage Convert physical and chemical stimuli into action Neurons and sensory cells potentials Synthesis and secretion of enzymes Cells of digestive glands Synthesis and secretion of mucous substances Mucous-gland cells Synthesis and secretion of steroids Some adrenal gland, testis, and ovary cells Ion transport Cells of the kidney and salivary gland ducts Intracellular digestion Macrophages and some white blood cells Lipid storage Fat cells Metabolite absorption Cells lining the intestine Junqueira Basic Histology Cytoplasmic Organelles The cell is composed of two basic parts: cytoplasm and nucleus. The outermost component of the cell, separating the cytoplasm from its extracellular environment, is the plasma membrane (plasmalemma). The plasma, or cell, membrane functions as a selective barrier that regulates the passage of certain materials into and out of the cell and facilitates the transport of specific molecules. The cytoplasm is the part of the cell located outside the nucleus. It contains organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and ribosomes. Nucleus The nucleus is a membrane-bound organelle found in eukaryotic cells (cells with a defined nucleus). It contains genetic material, including DNA, and is often referred to as the control center of the cell. The nucleus regulates cellular activities and houses the nucleolus, involved in ribosome production. 3 DNA (Deoxyribonucleic Acid): DNA is a molecule that carries genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. It consists of two long strands forming a double helix, composed of nucleotides with adenine (A), thymine (T), cytosine (C), and guanine (G) bases. DNA is organized into structures called genes, which encode specific proteins. RNA (Ribonucleic Acid): RNA is another type of nucleic acid involved in protein synthesis and various cellular functions. Unlike DNA, RNA is usually single-stranded and contains uracil (U) instead of thymine. There are different types of RNA, including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Chromosomes: Chromosomes are structures composed of DNA and proteins found in the cell nucleus. They contain genes and carry hereditary information. Humans typically have 23 pairs of chromosomes (46 in total), with one set inherited from each parent. Mitochondria: Mitochondria are double-membraned organelles found in the cells of most living organisms, including humans. They are often referred to as the "powerhouses of the cell" due to their primary role in energy production. Mitochondria generate adenosine triphosphate (ATP), a molecule that provides energy for various cellular activities. The Endoplasmic reticulum: The endoplasmic reticulum (ER) is a cellular organelle involved in the synthesis, folding, modification, and transport of proteins and lipids. It consists of a network of membranes within the cell and comes in two forms: rough ER, studded with ribosomes on its surface, and smooth ER, lacking ribosomes. The Golgi apparatus: The Golgi apparatus, or Golgi complex, is a cellular organelle responsible for processing, packaging, and distributing molecules within or outside the cell. It consists of flattened membrane-bound sacs called cisternae. Key functions of the Golgi apparatus include modifying and sorting proteins and lipids synthesized in the endoplasmic reticulum (ER). During protein processing, the Golgi apparatus adds molecular tags, such as carbohydrates (glycosylation), and further refines the structure of these molecules. After modification, the Golgi sorts and packages them into vesicles for transport to their final destinations, which could be other cellular organelles or the cell membrane for secretion. 4 Lysosomes: Lysosomes are membrane-bound organelles within cells that contain enzymes responsible for breaking down and digesting cellular waste, damaged organelles, and foreign substances. These enzymes are acidic and work optimally in the acidic environment within lysosomes. Additionally, lysosomes are involved in the degradation of cellular organelles through a process known as autophagy, where damaged or obsolete organelles are engulfed and digested. 1.2 Cell Cycle The cell cycle represents a self-regulated sequence of events that controls cell growth and cell division The development of a single, fertilized egg cell to form a complex, multicellular organism involves cellular replication, growth and progressive specialization (differentiation) for a variety of functions. The fertilized egg (zygote) divides by a process known as mitosis to produce two genetically identical daughter cells, each of which divides to produce two more daughter cells and so on. Some of these daughter cells progressively specialize and eventually produce the terminally differentiated cells of mature tissues, such as muscle or skin cells. Most tissues however retain a population of relatively undifferentiated cells (stem cells) that are able to divide and replace the differentiated cell population as required. The interval between mitotic divisions is known as the cell cycle. All body cells divide by mitosis except for male and female germ cells, which divide by meiosis to produce gametes. Mitosis Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell. Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During this replication phase chromosomes are extremely long, they are spread diffusely through the nucleus, and they cannot be recognized with the light microscope. With the onset of mitosis, the chromosomes begin to coil, contract, and condense; these events mark the beginning of prophase. Each chromosome now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere. Throughout prophase, the chromosomes continue to condense, shorten, and thicken but only at prometaphase do the chromatids become distinguishable. During metaphase, the chromosomes line up in the equatorial plane, and their doubled structure is clearly visible. Each is attached by microtubules extending from the centromere to the centriole, forming the mitotic spindle. 5  Soon, the centromere of each chromosome divides, marking the beginning of anaphase, followed by migration of chromatids to opposite poles of the spindle. Finally, during telophase, chromosomes uncoil and lengthen, the nuclear envelope reforms, and the cytoplasm divides. Each daughter cell receives half of all doubled chromosome material and thus maintains the same number of chromosomes as the mother cell. (fig, 2.3). Meiosis Meiosis is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells, respectively. Meiosis requires two cell divisions, meiosis I and meiosis II, to reduce the number of chromosomes to the haploid number of 23 (Fig. 2.4). As in mitosis, male and female germ cells (spermatocytes and primary oocytes) at the beginning of meiosis I replicate their DNA so that each of the 46 chromosomes is duplicated into sister chromatids. In contrast to mitosis, however, homologous chromosomes then align themselves in pairs, a process called synapsis. The pairing is exact and point for point except for the XY combination. Homologous pairs then separate into two daughter cells, thereby reducing the chromosome number from diploid to haploid. 6  Shortly thereafter, meiosis II separates sister chromatids. Each gamete then contains 23 chromosomes. Difference between Mitosis and Meiosis: Mitosis is a cell division process that results in two identical daughter cells with the same number of chromosomes as the parent cell. It is involved in growth, repair, and maintenance of body tissues. Meiosis, on the other hand, is a specialized cell division process that produces gametes (sperm and egg cells) with half the number of chromosomes. Meiosis involves two rounds of division, resulting in four non-identical daughter cells, each with a unique combination of genetic material 1.3 Tissue: Introduction A human tissue is a group of cells with similar structure and specialized function. Tissues combine to form organs, and organs work together in organ systems. For instance, muscle tissue is composed of muscle cells, and the heart is an organ made up of various tissues, including muscle tissue, connective tissue, and nerve tissue. The human body is composed of only four basic types of tissue: epithelial, connective, muscular, and nervous. These tissues, which are formed by cells and molecules of the extracellular matrix, exist not as isolated units but rather in association with one another and 7 in variable proportions, forming different organs and systems of the body. The main characteristics of these basic types of tissue are shown in Table 4–1. Junqueira Basic Histology Epithelial Tissue: Covers body surfaces and lines organs, serving as a protective barrier. Examples include skin epithelium and the lining of the digestive tract. Here are different epithelial tissue types. Simple Squamous Epithelium: Thin and flat cells that form a single layer, found in areas where diffusion or filtration occurs, such as the lining of blood vessels and air sacs of the lungs. Simple Cuboidal Epithelium: Single layer of cube-shaped cells, often involved in secretion and absorption. Found in kidney tubules and various glands. Simple Columnar Epithelium: Single layer of elongated cells, often with microvilli on the surface, found in the lining of the digestive tract, where absorption and secretion occur. Stratified Squamous Epithelium: Multiple layers of flat cells, providing protection. Found in the skin (epidermis) and lining of the mouth, esophagus, and vagina. Stratified Cuboidal Epithelium: Two or more layers of cube-shaped cells, relatively rare in the human body but found in certain ducts, like in the mammary glands. Stratified Columnar Epithelium: Multiple layers of elongated cells, found in the male urethra and parts of the pharynx. Pseudostratified Columnar Epithelium: Appears stratified as all cells touch the basement membrane but level of nuclei are different. Found in the respiratory tract, where it helps move mucus with cilia. 8 Transitional Epithelium: Specialized for stretching, found in the lining of the urinary bladder, ureters, and urethra. 9 10 Connective Tissue: provides a matrix that supports and physically connects other tissues and cells together to form the organs of the body. Unlike the other tissue types (epithelium, muscle, and nerve), which consist mainly of cells, the major component of connective tissue is the extracellular matrix (ECM). Extracellular matrices consist of different combinations of protein fibers (collagen and elastic fibers) and ground substance Specialized Connective Tissue Adipose Tissue, Cartilage, Bone & Blood 1. Adipose tissue Connective tissue in which fat-storing cells or adipocytes predominates is called adipose tissue. Adipose tissue normally represents 15%-20% of the body weight in men, somewhat more in women, serving as storage depots for neutral fats 2. Cartilage 11 A firm, flexible tissue found in areas like the nose and joints. Types include hyaline, elastic, and fibrocartilage. 3. Bone (Osseous Tissue) The main component of the adult skeleton, bone tissue provides solid support for the body, protects vital organs such as those in the cranial and thoracic cavities, and encloses internal (medullary) cavities containing bone marrow where blood cells are formed. Bone (or osseous) tissue also serves as a reservoir of calcium, phosphate, and other ions that can be released or stored in a controlled fashion to maintain constant concentrations in body fluid Hematopoietic Tissue: Found in the bone marrow and responsible for blood cell formation 4. Blood: Consists of red and white blood cells in a liquid matrix (plasma). It plays a crucial role in transportation and defense. Muscle Tissue: One of the basic tissues, responsible for movement by contracting and relaxing. Three types of muscle tissue can be distinguished on the basis of morphologic and functional characteristics, with the structure of each adapted to its physiologic role. Skeletal muscle contains bundles of very long, multinucleated cells with cross-striations. Their contraction is quick, forceful, and usually under voluntary control. Location: Attached to bones by tendons. Control: Voluntary control. Appearance: Striated (striped) appearance under a microscope. Function: Responsible for body movement, posture, and voluntary actions. 12  Cardiac muscle also has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs that are unique to cardiac muscle. Contraction is involuntary, vigorous, and rhythmic. Location: Exclusive to the heart. Control: Involuntary control. Appearance: Striated like skeletal muscle but with branching cells. Function: Contracts to pump blood, exhibits a rhythmic and synchronized contraction Smooth muscle consists of collections of fusiform cells that lack striations and have slow, involuntary contractions. In all types of muscle, contraction is caused by the sliding interaction of thick myosin filaments along thin actin filaments. Location: Found in the walls of internal organs (e.g., stomach, intestines). Control: Involuntary control. Appearance: Non-striated, with spindle-shaped cells. Function: Contracts slowly and rhythmically, regulating the movement of substances within organs. Nervous Tissue: The human nervous system, by far the most complex system in the body, is formed by a network of many billion nerve cells (neurons), all assisted by many more supporting cells called glial cells. Each neuron has hundreds of interconnections with other neurons, forming a very complex system for processing information and generating responses. Nerve tissue is distributed throughout the body as an integrated communications network. Anatomically, the general organization of the nervous system has two major divisions: Central nervous system (CNS), consisting of the brain and spinal cord Peripheral nervous system (PNS), composed of the cranial, spinal, and peripheral nerves conducting impulses to and from the CNS (sensory and motor nerves, respectively) and ganglia that are small aggregates of nerve cells outside the CNS. 13