Week 7 - Nucleus.pdf

Week 7 HISTOPATHOLOGY (BMS4470A) THE NUCLEUS Dr. Merin Thomas [email protected] Office hours : Tuesday & Thursday – 1.00pm to 3.00pm Learning Objectives Nuclear envelope Chromatin Nucleolus The Cell cycle Apoptosis THE NUCLEUS Containing the code for a cell’s enzymes and other proteins, the...

Week 7 HISTOPATHOLOGY (BMS4470A) THE NUCLEUS Dr. Merin Thomas [email protected] Office hours : Tuesday & Thursday – 1.00pm to 3.00pm Learning Objectives Nuclear envelope Chromatin Nucleolus The Cell cycle Apoptosis THE NUCLEUS Containing the code for a cell’s enzymes and other proteins, the nucleus is the command center of the cell. The nucleus also contains the molecular machinery to replicate the DNA and to synthesize and process all types of RNA. THE NUCLEUS - COMPONENTS The nucleus usually appears as a large rounded or oval structure, often near the cell’s center. Typically, the largest structure within a cell, it consists of a nuclear envelope containing chromatin, the mass of DNA and its associated proteins, with specialized regions of chromatin called nucleolus. THE NUCLEUS – COMPONENTS – Nuclear Envelope Forms a selectively barrier between permeable the nuclear and cytoplasmic compartments. Has two concentric membranes separated by a narrow (30-50 nm) perinuclear space. This space and the outer nuclear membrane are continuous with the extensive cytoplasmic network of the rough endoplasmic reticulum (RER). THE NUCLEUS – COMPONENTS – Nuclear Envelope Closely associated with the inner nuclear membrane, is a highly organized meshwork of proteins called the nuclear lamina which stabilizes the nuclear envelope. Major components of this layer are the class of intermediate filament proteins called lamins that bind to membrane proteins and associate with chromatin in nondividing cells. THE NUCLEUS – COMPONENTS – Nuclear Envelope The inner and outer nuclear membranes are bridged at nuclear pore complexes. Various core proteins of a nuclear pore complex, called nucleoporins display eightfold symmetry Ions and small solutes pass through the lumen by simple diffusion, but the nucleoporin complex regulates the movement of macromolecules between nucleus and cytoplasm. THE NUCLEUS – COMPONENTS – Nuclear Envelope A growing cell has 3000-4000 such channels, each providing passage for up to 1000 macromolecules per second. Individual pores permit molecular transfer in both directions simultaneously. Macromolecules shipped out of the nucleus include ribosomal subunits and other RNA THE NUCLEUS – COMPONENTS – Chromatin Chromatin consists of DNA and all the associated proteins involved in the organization and function of DNA. In humans, each cell’s chromatin (except that of eggs and sperm) is divided among 46 chromosomes (23 pairs). After DNA replication but before cell division, each chromosome consists of two identical chromatin units called chromatids held together by complexes of cohesion proteins. THE NUCLEUS – COMPONENTS – Chromatin THE NUCLEUS – COMPONENTS – Chromatin DNA of each human cell is approximately 2m long, with 3.2 billion base pairs (bp), and therefore must be extensively packaged within the nucleus. This occurs initially by the DNA associating with sets of small basic proteins called histones. The structural unit of DNA and histones is called the nucleosome which has a core of eight histones, around which is wrapped about 150 bp of DNA. THE NUCLEUS – COMPONENTS – Chromatin Microscopically two categories of chromatin can be distinguished in nuclei of most nondividing cells Euchromatin is visible as finely dispersed granular material in the electron microscope and as lightly stained basophilic areas in the light microscope. Heterochromatin appears as coarse, electron-dense material in the electron microscope. Two types Constitutive heterochromatin Facultative heterochromatin THE NUCLEUS – COMPONENTS – Chromatin The ratio of heterochromatin to euchromatin seen with nuclear staining can provide a rough indicator of a cell’s metabolic and biosynthetic activity. Euchromatin predominates in active cells such as large neurons, while heterochromatin is more abundant in cells with little synthetic activity such as circulating lymphocytes. Facultative heterochromatin also occurs in the small, dense “sex chromatin” or Barr body, which is one of the two large X chromosomes present in human females but not males. THE NUCLEUS – COMPONENTS – Chromatin The Barr body remains tightly coiled, while the other X chromosome is uncoiled, transcriptionally active, and not visible. Cells of males have one X chromosome and one Y chromosome; like the other chromosomes, the single X chromosome remains largely euchromatic. THE NUCLEUS – COMPONENTS – Nucleolus The nucleolus is a spherical structure found in the cell's nucleus whose primary function is to produce and assemble the cell's ribosomes. It is best known as the site of ribosome biogenesis. The intense density of nucleoli is due not to heterochromatin but to the presence of densely concentrated ribosomal RNA (rRNA) that is transcribed, processed, and assembled into ribosomal subunits. THE NUCLEUS – COMPONENTS – Nucleolus Regions of euchromatin and heterochromatin display variable electron densities with the transmission electron microscope (TEM). An active nucleus typically has much diffuse, light- staining euchromatin and smaller subdomains of electron-dense heterochromatin (H), with many of these associated at the periphery associated with the nuclear lamina. The more heterogeneous electrondense subdomain is the nucleolus (N), the site of rRNA synthesis, and ribosomal subunit assembly. (X25,000) THE NUCLEUS – COMPONENTS – Nucleolus Molecules of rRNA are processed in the nucleolus and very quickly associate with the ribosomal proteins imported from the cytoplasm via nuclear pores. The newly organized small and large ribosomal subunits are then exported back to the cytoplasm through those same nuclear pores. THE CELL CYCLE Before differentiation, most cells undergo repeated cycles of macromolecular synthesis (growth) and division (mitosis). The regular sequence of events that produce new cells is termed the cell cycle The cell cycle has four distinct phases: 1. Mitosis and periods termed 2. G1 (the time gap between mitosis and the beginning of DNA replication), 3. S (the period of DNA synthesis), and 4. G2 (the gap between DNA duplication and the next mitosis). THE CELL CYCLE THE CELL CYCLE - MITOSIS The period of cell division, or mitosis (Gr. mitos, a thread), is the only cell cycle phase that can be routinely distinguished with the light microscope. During mitosis, a parent cell divides and each of the two daughter cells receives a chromosomal set identical to that of the parent cell. The chromosomes replicated during the preceding S phase are distributed to the daughter cells. The long period between mitoses (the G1, S, and G2 phases) is also commonly called interphase. The events of mitosis are subdivided into four major stages THE CELL CYCLE - MITOSIS THE CELL CYCLE - MITOSIS THE CELL CYCLE - MITOSIS CYTOGENETIC ANALYSIS/KARYOTYPING In humans, each cell’s chromatin (except that of eggs and sperm) is divided among 46 chromosomes (23 pairs). After DNA replication but chromosome consists of two before cell division, each identical chromatin units called chromatids held together by complexes of cohesion proteins. CYTOGENETIC ANALYSIS/KARYOTYPING Any tissue with living nucleated cells that undergo division can be used for studying human chromosomes. Most commonly, circulating lymphocytes from peripheral blood are used, although samples for chromosomal analysis can be prepared using skin, bone marrow, chorionic villi or cells from amniotic fluid. The cells are cultured; after processing and staining, the condensed chromosomes of one nucleus are photographed by light microscopy and rearranged digitally to produce a karyotype in which stained chromosomes can be analyzed. Photographic representation of all the chromosomes. It reveals how many chromosomes are found within an actively dividing somatic cell APOPTOSIS Less evident, but no less important than cell proliferation for body functions, is the process of cell suicide called apoptosis (Gr. apo, off + ptosis, a falling). Apoptosis is a rapid, highly regulated cellular activity that shrinks and eliminates defective and unneeded cells. Apoptosis is controlled by cytoplasmic proteins in the Bcl-2 family, which regulate the release of death-promoting factors from mitochondria. APOPTOSIS Nuclear morphology suggestive of tumor cell death Karyolysis: nuclear fading caused by dissolution of the chromatin Pyknosis: irreversible condensation of the chromatin causing nuclei to shrink in size Karyorrhexis: destructive fragmentation of a pyknotic nucleus Apoptotic bodies: late-stage apoptosis with fragmented nuclei. Naipal, K. A., Verkaik, N. S., Sánchez, H., Van Deurzen, C. H., Bakker, M. a. D., Hoeijmakers, J. H., Kanaar, R., Vreeswijk, M. P., Jager, A., & Van Gent, D. C. (2016). Tumor slice culture system to assess drug response of primary breast cancer. BMC Cancer, 16(1). https://doi.org/10.1186/s12885016-2119-2 Apoptotic cells in adult tissues are rare because the process is completed very rapidly. Moreover, with their highly condensed chromatin in pyknotic nuclei and rounded shape, cells early in apoptosis may superficially resemble some mitotic cells APOPTOSIS REFERENCES Mescher, A. L. (2018). Junqueira’s Basic Histology Text and Atlas, FIFTEENTH. In McGrawHill Education eBooks. http://125.212.201.8:6008/handle/DHKTYTHD_123/5904