histology

• Squamous epithelium is a type of epithelial tissue that consists of flat, scale-like cells and is involved in diffusion and filtration.
• A tissue is a combination of cells and their extracellular matrix that have common embryonal origin, morphological characteristics and functions.
• Tissue is the cellular organisational intermediate between cells and a complete organ.
• Organs are formed by the functional grouping together of multiple tissues.
• In humans, there are four basic types of tissue: epithelial, connective, muscle and nervous tissue.
• Epithelia are sheets of epithelial cells arranged in single or multiple layers (simple and stratified epithelia).
• Epithelial tissues line the cavities and surfaces of blood vessels and covers hollow organs throughout the body.
• There is a minimal amount of intercellular substance between the epithelial cells.
• When the menstrual cycle begins, one, or maybe even a few, primary oocytes begin to grow larger and the follicle cells increase in number and cause the follicle to grow larger too.
• The secondary oocyte still has two copies of each chromosome, ovum, it must undergo the second meiotic division in order to produce haploids.
• The stages of growth of the oocytes during the menstrual cycle occurs as follows: primordial follicle, primary follicle, secondary follicle, tertiary follicle and graafian follicle.
• Growing follicle contains the primary oocyte surrounded by zona pellucida, theca interna, theca granulosa and theca externa.
• In this way, the ovum achieves its haploid state while retaining as much cytoplasm as possible.
• When primary follicles survive, secondary follicles with follicular epithelium surrounding multiple rows are created.
• Even after this meiosis is still not completed yet as in females this only occurs after fertilisation.
• Secondary follicle contains theca interna and cumulus oophorus which consists of the oocyte and corona radiata.
• If the secondary follicles survive, tertiary follicles are produced.
• If the tertiary follicle survives it gives rise tothe graafian follicle which is used during ovulation-the follicle ruptures releasing the secondary oocyte.
• Oocyte development occurs in the ovaries.
• They give rise to the primary follicles.
• This division is also uneven, like the first one, with half of the chromosomes going to another very small degenerate polar body and half of the chromosomes being retained by the ovum along with almost all of the cytoplasm.
• At the time of birth all the surviving primary oocytes are surrounded by thin, single layers of primordial follicles.
• The control of oogenesis is regulated by three main hormones: FSH and LH which are produced by the pituitary gland and oestrogen which is secreted by the ovaries.
• Each oocyte is surrounded by a number of follicle cells (female germ cells and satellite cells) to create a follicle.
• The sheets of epithelial cells are found upon basal lamina that separates epithelium and underlying connective tissue.
• The functions of epithelia include protection, secretion and resorption.
• Examples of type C fibers are postganglionic autonomic efferent and afferent fibers to skin.
• Neurolemmal sheath, representing the plasma membrane of the Schwann cell, surrounds the myelin sheath and interdigitates at the nodes of Ranvier.
• Neurolemmal sheath is necessary for regeneration of a damaged nerve.
• A nerve fibre is a threadlike extension of a nerve cell and consists of an axon and (in some cases) a myelin sheath.
• Type A fibres are the thickest and fastest conducting, they are myelinated, have a diameter of 1.5-20 micron, and their speed of conduction is 4-120 m/sec.
• Examples of type B fibers are preganglionic autonomic efferents.
• Non-myelinated fibers comprise the smaller axons of the CNS, in addition to peripheral several types of fine sensory fibers (C fibers of skin).
• Myelin sheath acts as an insulator for the nerve fibers.
• Myelinated fibers form the bulk of the somatic nerves and are made up of an axis cylinder that forms the central core of the fiber, consisting of axoplasm covered by axolemma, with a myelin sheath, derived from Schwann cells, surrounding the axis cylinder.
• There are nerve fibres in the central nervous system and peripheral nervous system.
• In peripheral nerves, both the myelin and neurolemmal sheaths are derived from Schwann cells in the PNS and in the CNS it is made by the oligodendrocytes.
• There three main types of nerve fibres, A, B and C.
• Type B fibers are medium in size, i.e. they are smaller than type A fibers but larger than type C fibers, they are myelinated, have a diameter of 1.5-3.5 microns, and their speed of conduction is 3-15 m/sec.
• Type C fibers are the smallest and thinnest, they are non-myelinated, have a diameter of 0.1-2 microns, and their speed of conduction is 0.5-4 m/sec.