Histology Lecture 4 PDF

Nucleus & Nucleolus Nucleus & Nucleolus By Dr. Mona Soliman Assistant prof. of Histology Faculty of medicine Menoufia university Intended learning outcome ✓Describe the structure of nucleus. ✓Describe structure of NPCs. ✓Identify types of chromatin and their sites....

Nucleus & Nucleolus Nucleus & Nucleolus By Dr. Mona Soliman Assistant prof. of Histology Faculty of medicine Menoufia university Intended learning outcome ✓Describe the structure of nucleus. ✓Describe structure of NPCs. ✓Identify types of chromatin and their sites. ✓Define life cycle of cell, List its stages. ✓Differentiate between mitosis and meiosis. ✓Outline steps of mitosis and meiosis.  General features Site: It may be central, eccentric or peripheral. ▪ Number: Cells may be mononucleated,binucleated(liver cells)or multinucleated (osteoclast). ▪ Size: It may be large or small. ▪ Shape: It may be flat, round, oval or kidney-shaped. It may be lobulated as in neutrophils. ▪ Appearance: It may be either vesicular nuclei pale with few chromatin as in nerve cells or condensed nuclei dark as in lymphocytes.  Structure of nucleus The nucleus of interphase cell (non-dividing cell) consists of: 1.Nuclear envelope. 2. Chromatin. 3.Nucleolus. 4. Nuclear sap.  Nuclear envelope With the LM: It appears surrounding the nucleus as a single basophilic line (due to the presence of peripheral chromatin on its inner side). With the EM: It is formed of two thin membranes (8 nm thick) separated by a perinuclear space (25 nm wide). -The outer membrane is continuous with the membranes of the endoplasmic reticulum in the cytoplasm. -The inner membrane shows chromatin granules (peripheral chromatin).  Nuclear envelope - Nuclear pores (about 30-100 nm in diameter) interrupt the nuclear membrane at intervals. At the pore, the two layers of the membrane fuse together, forming a nuclear pore complex (NPCS) which allows the exchange of chemical molecules between the nucleus and the cytoplasm.  Nuclear pore complex Each pore complex is formed of tubular structure, the wall of which is anchored to the nuclear envelope and has on each of its ends a ring-shaped structure, an outer ring attached to the outer envelope membrane and an inner ring attached to the inner envelope membrane. The NPC has an octagonal shape, as its wall contain eight subunits.  Nuclear pore complex In the center of the NPC, a central granule or plug is present, projecting inwards from the subunits are eight radially arranged spokes that converge at the central granule or plug. The central granule and radial spokes form the principal barrier to movement through pores (NPCS) and are responsible for the false appearance of pore diaphragm in micrographs of low magnification. Nuclear pore complex  Nuclear pore complex barrier NPCS barriers may have a central channel of 10 nm that allows passive diffusion of molecules less than l0nm in diameter in the two- way traffic between cytoplasm and nucleoplasm.  Nuclear pore complex barrier The macromolecules that have a diameter more than 10 nm as ribosomes (that leave nucleus) and cytoplasmic proteins (that enter nucleus) require energy dependent mechanism (active transport) that widen the opening of the channels from 10 nm to 20 nm or more to permit the transport of such large molecules as ribosomes and cytoplasmic proteins, leaving or entering the nucleus.  Chromatin LM: It appears as basophilic granules which may be coarse or fine according to their size. E/M: There Are Two Types Of Chromatin: Euchromatin and Heterochromatin. - The Euchromatin or Extended Chromatin or Active Chromatin: ▪ They represent the extended (uncoiled) parts of the chromosomal thread. ▪ They are the most active chromatin. ▪ They control protein synthesis.  Condensed chromatin 2. Condensed chromatin (Heterochromatin or inactive type): It appears as aggregations of electron-dense granules (coiled parts). It is distributed within the nucleus as: (a) Peripheral chromatin aggregated close to the inner nuclear membrane. (b) Nucleolus-associated chromatin aggregated around the nucleolus. (c) Chromatin islands aggregated into clumps in the nuclear sap..  Function of chromatin 1.It directs and guides the synthesis of protein required by the cell to perform specialized function. 2. It contains the genetic material, which directs all cellular activities and regulates cellular structure.  Nucleosomes When the chromatin threads are well extended (each thread becomes several centimetres in length) and examined using EM, biochemical and X ray analysis, each chromatin thread, was found to consist of a string of beads, each beadlike particle is called, nucleosome.  Nucleosomes The core of each nucleosome is composed of two molecules. Each consists of four histones (H2A, H2B, H3 and H4) i.e. 8 molecules (octomer). DNA double helix forms two and half turns around each nucleosome. The nucleosomes are attached to each other by segments of DNA molecule called, linker DNA Nucleosomes are found in condensed and extended chromatins; they help in shortening the chromatin threads thousands of times. Nucleosomes  Nucleolus Round, dense, well-defined structure, it has no limiting membrane. There are l-4 nucleoli per cell nucleus; more active cells have more nucleoli. By EM, nucleolus consists of: I. Nucleolar organizer DNA: consists of DNA coding for rRNA. II. Pars fibrosa: are the sites of rRNA synthesis. III. Pars granulose: consists of maturing subunits of ribosomes. Functions of nucleolus: Nucleolus is the site of synthesis of rRNA and ribosomes.  Nuclear sap Nuclear matrix, karyoplasms, or nucleoplasm, is a semifluid substance occupying the space between chromatin particles and nucleolus. Fibrous lamina lies immediately beneath and connected to inner nuclear membrane. It consists of three proteins Lamins A, B and C. Degenerative changes in nucleus indicating cell death Three signs may appear in the nucleus indicating that the cell is dying: 1. Pyknosis: nucleus becomes smaller in size and deeply stained. 2. Karyorrhexis: nucleus breaks down into smaller fragments. 3. Karyolysis: nucleus dissolves and finally disappears. A kidney biopsy is examined by electron microscopy. The nucleus exhibits a peripheral ring of dark-stained chromatin (arrow, shown in the image). Which of the following best describes the functional significance of the dark-stained ring of marginal chromatin observed in this electron micrograph? a) DNA replication center b) Kinetochore complex assembly c) Nucleosome assembly d) Organization of inactive chromatin e) Ribosomal RNA biosynthesis  Life cycle of the cell ▪ It is the period that includes an interphase and a complete cell division. ▪ The interphase: is the period between two consecutive cell divisions. ▪ It is divided into three phases: ✓ Gl-phase (Gap or presynthesis):is the phase in which the cell restores its normal size after mitosis, forms RNA and protein, needed for its growth. ✓ S-phase(DNA synthesis): is the phase during which duplication of DNA and centrioles takes place. ✓ G2-phase(Post DNA duplication): is the phase in which the cell produces the energy needed for the next cell division  Cell cycle ❖ Cells vary according to their ability to divide and renew into: 1- Cells that cannot divide------Nerve cells. 2- Cells unable to divide but after degeneration are replaced by stem cells of the same family----- Cells of small intestine. 3-Cells that divide only at time of need-- Cells of liver and endocrine gland cells.  Mitosis It is an cell division that occurs in somatic cells. The result is two daughter cells with the same number of chromosomes as the mother cell (46 chromosomes). It includes prophase, metaphase, anaphase and telophase.  Stages of mitosis 1- Prophase: ✓ Chromatin changes to chromosomes (thick & rod- like). ✓ Each chromosome consists of two chromatids, connected at the centromeres (92 chromatids). ✓ The centrioles (duplicate in S-phase) move to the opposite poles of the cell. ✓ Microtubules start to appear between the centrioles, forming the mitotic spindle. ✓ The nuclear membrane and the nucleoli disappear. (Marks the end of prophase and the beginning of metaphase).  Stages of mitosis 2- Metaphase: ✓ Chromosomes arrange themselves in the equatorial plane. The spindle becomes well developed and its microtubules attach to the centromeres of the chromosomes (Kinetochore). ✓ At the end of metaphase , there is duplication of centromere DNA after which the centromere splits. ✓ The two chromatids separate and move toward the centrioles of the opposite poles of the cell.  Stages of mitosis 3- Anaphase: The chromatids separate and move towards the corresponding centrioles, (46 chromatids move towards each pole). 4- Telophase: ✓ Chromatids elongate and form chromatin of the nucleus. ✓ The nuclear membrane and the nucleoli reappear. ✓ A constriction develops at the equatorial plane of the parent cell and progresses until the cytoplasm and its organelles are divided into two. ✓ Thus, two daughter cells are produced. Each has a diploid number of chromosomes (46chromosome). Stages of mitosis Mitotic spindle  Meiosis It is a reductional cell division that occurs in the germ cells of testis and ovary. It consists of two divisions, but only one DNA replication (duplication of DNA). The mother cell gives rise to four daughter cells with haploid number of chromosomes (s-chromosome=23 chromosome). In testis four sperms are produced, while in ovary, it gives one ovum and three Polar bodies (degenerate).  First meiotic division 1-Prophase I: It is the longest phase of meiosis, in which the following stages are seen. Leptotene stage: Chromosomes appear as thin threads as in early prophase of mitosis. Zygotene stage: Chromosomes are arranged in homologous pairs. Each pair forms a bivalent (one chromosome is of paternal origin from the father, while the other is its maternal homologue from the mother). Pachytene stage: Each chromosome splits lengthwise into 2 chromatids, which remain attached only at their centromeres. The bivalent therefore consists of four chromatids and is called a tetrad.  First meiotic division Diplotene stage: Homologous chromatids move slightly a part and the tetrad formation becomes more obvious. At points along their length the homologous chromatids form chiasma (X-shaped crossing over). These chiasmata are the points where crossing over of genetic material takes place. Diakinesis: The nuclear membrane and the nucleolus disappear, and the tetrads migrate to the equator of the cell. Stages of first meiotic division  First meiotic division 2- Metaphase I: It is similar to the metaphase of mitotic division except that pairs of chromosomes (bivalents), not single chromosomes are arranged in the equatorial plate and attach to the microtubules of the spindle.  First meiotic division 3- Anaphase 1: Homologous chromosomes each consisting of 2 chromatids are separated (centromeres have not split) and move towards the corresponding poles of the cell. 23 chromosomes (d- chromosome) move towards each pole. Each chromosome contains a double amount of DNA so that the total amount of DNA within the resulting cells remains diploid (d-chromosome). 4-Telophase 1: Chromosomes form the chromatin of the new nucleus, and the cytoplasm separates into two parts.  2nd meiotic division ❖ Prophase II: It is similar to mitosis without a period of DNA replication. ❖ Metaphase II: ▪ The 23 d-chromosomes arrange themselves in the equatorial plane. ▪ There is duplication of centromere DNA after which the centromere splits.  2nd meiotic division ❖ Anaphase II: The two chromatids, of each chromosome, separate and move towards the corresponding poles of the cell. ❖ Telophase II: Chromosomes form the chromatin of the new nucleus, and the cytoplasm separates into two parts, resulting in cells with haploid number of chromosomes and haploid amount of DNA. Unlike mitosis the sister chromatids are not identical due to crossing over that occurred during the first meiotic prophase. 2nd meiotic division Mitosis versus Meiosis A mouse embryo is examined by microscopy (shown in the image).The embryo exhibits a cleavage furrow and appears to be undergoing cytokinesis. These events take place during what phase of mitosis? (A) Anaphase. (B) Interphase. (C) Metaphase. (D) Prophase. (E) Telophase.