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Posts Tagged ‘DNA’

TESTES AND EPIDIDYMIS TESTES The testes are the primary reproductive organs or gonads in the male. They are ovoid, reproductive and endocrine organs responsible for sperm production and testosterone production. They are suspended in the scrotum by scrotal tissues including the dartos muscle and spermatic cords. Average testicular dimensions are 4–5 cm in length, 2.5 […]

IDENTIFICATION FROM THE SKULL There are many ways of identifying an individual: in physical and forensic anthropology the most important concern biological and personal identity. Biological identity pertains to those features that allow an individual to be classified in relation to features present in other individuals e.g. sex, age, race and stature, whereas personal identity […]

MECHANISMS OF NEURAL DEVELOPMENT For more than a century the mechanisms that operate during the development of the nervous system have been studied experimentally. While much has been established, answers to many fundamental questions still remain obscure. In recent years, significant advances in our understanding of the development of vertebrates have come from work on […]

INDUCTION AND PATTERNING OF THE BRAIN AND SPINAL CORD The generation of neural tissue involves an inductive signal from the underlying chordamesoderm (notochord), termed the ‘organizer’. The observation by Spemann in 1925 that, in intact amphibian embryos, the presence of an organizer caused ectodermal cells to form nervous tissue, whereas in its absence they formed […]

MICROSTRUCTURE OF SKIN AND SKIN APPENDAGES EPIDERMIS The epidermis (Fig. 7.2, Fig. 7.3) is a compound tissue consisting mainly of a continuously self-renewing, keratinized, stratified squamous epithelium: the principal cells are called keratinocytes. Nonkeratinocytes within the mature epidermis include melanocytes (pigment-forming cells from the embryonic neural crest), Langerhans cells (immature antigen-presenting dendritic cells derived from […]

SKELETAL MUSCLE Shape and fibre architecture It is possible to classify muscles based on their general shape and the predominant orientation of their fibres relative to the direction of pull (Fig. 5.36). Muscles with fibres that are largely parallel to the line of pull vary in form from flat, short and quadrilateral (e.g. thyrohyoid) to […]

DENDRITIC CELLS There are two distinct groups of dendritic cell, myeloid dendritic cells (also known as conventional dendritic cells) and plasmacytoid dendritic cells. Both groups of cells are derived from haematopoietic stem cells. Until recently it was thought that plasmacytoid dendritic cells were derived from the lymphoid precursor cells, while the myeloid dendritic cells were […]

CELL LINEAGES Haemopoietic stem cells Within the adult marrow there is a very small number (0.05% of haemopoietic cells) of self-renewing, pluripotent stem cells which are capable of giving rise to all blood cell types, including lymphocytes (Fig. 4.12). Although they cannot be identified morphologically in the marrow, they can be recognized in aspirates by […]

CELL DIFFERENTIATION As the embryo develops, its cells pass through a series of changes in gene expression, reflected in alterations of cell structure and behaviour. They begin to diversify, separating first into two main tissue arrangements, epithelium and embryonic mesenchyme, then into more restricted subtypes of tissue, until finally they mature into cells of their […]

CYTOPLASM Compartments and functional organization The cytoplasm is highly concentrated, with about 200 mg/ml of proteins (about twice the concentration in blood) that must be precisely organized for correct molecular interactions to occur. It normally has extremely low levels of Ca2+, high K+ and low Na+ ions in comparison to extracellular fluid, differences which are […]

NUCLEUS The nucleus (Figs 1.1, 1.2) is generally the largest intracellular structure and is usually spherical or ellipsoid in shape, with a diameter of 3–10 μm. Histological stains used to identify nuclei in tissue sections mainly detect the acidic molecules of deoxyribonucleic acid (DNA), which are largely confined to the nucleus. Nuclear membrane The nucleus […]

CELL DIVISION AND THE CELL CYCLE During prenatal development, most cells undergo repeated division as the body grows in size and complexity. As cells mature, they differentiate structurally and functionally. Some cells, such as neurones, lose the ability to divide. Others may persist throughout the lifetime of the individual as replication-competent stem cells, e.g. cells […]