secondary cell

Examples of secondary cell in the following topics:

• Primary and Secondary Growth in Stems

  • It is caused by cell division in the lateral meristem .
  • The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside and secondary phloem (sieve elements and companion cells) to the outside.
  • The cells of the secondary xylem contain lignin, which provides hardiness and strength.
  • During the spring growing season, cells of the secondary xylem have a large internal diameter; their primary cell walls are not extensively thickened.
  • During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood.

• Secondary Active Transport

  • Both antiporters and symporters are used in secondary active transport.
  • Co-transporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions across the cell membrane.
  • Secondary active transport brings sodium ions, and possibly other compounds, into the cell.
  • Many amino acids, as well as glucose, enter a cell this way.
  • This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP.

• Gametogenesis (Spermatogenesis and Oogenesis)

  • The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell.
  • A secondary meiotic arrest occurs, this time at the metaphase II stage.
  • At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte.
  • This results in a secondary oocyte, which will complete meiosis if it is fertilized.
  • During spermatogenesis, four sperm result from each primary spermatocyte, which divides into two haploid secondary spermatocytes; these cells will go through a second meiotic division to produce four spermatids.

• Maturation of B Cells

  • Immature B cells are produced in the bone marrow and migrate to secondary lymphoid tissues where some develop into mature B cells.
  • After reaching the IgM+ immature stage in the bone marrow, these immature B cells migrate to secondary lymphoid tissues (such as the spleen, lymph nodes, Peyer's patches, etc.) where they are called transitional B cells, and some of these cells differentiate into mature B lymphocytes.
  • Once a B cell encounters its cognate antigen and receives an additional signal from a T helper cell, it can further differentiate into either plasma B cells or memory B cells.
  • This subsequent amplification with improved specificity of immune response is known as secondary immune response.
  • B cells that encounter antigen for the first time are known as naive B cells.

• Immunological Memory

  • As B cells and T cells mature into effector cells, a subset of the naïve populations differentiates into B and T memory cells with the same antigen specificities .
  • This is known as the secondary immune response.
  • When exposed to the corresponding pathogen to which an individual was vaccinated, the reaction is similar to a secondary exposure.
  • Vaccines, often delivered by injection into the arm, result in a secondary immune response if the vaccinated individual is later exposed to that pathogen.
  • As a result, memory B cells and plasma cells are made.

• Specific T-Cell Roles

  • T helper cells assist the maturation of B cells and memory B cells while activating cytotoxic T cells and macrophages.
  • Cytotoxic T cells (TC cells, or CTLs) destroy virus-infected cells and tumor cells, and cause much of the damage in in transplant rejection and autoimmune diseases.
  • The secondary immune response mediated by memory T cells is much faster and more effective at eliminating pathogens compared to the initial immune response.
  • Memory T cells comprise two subtypes: central memory T cells (TCM cells) and effector memory T cells (TEM cells), which have different properties and release different cytokines.
  • Effector memory cells may be either CD4+ or CD8+, and produce either helper or cytotoxic T cells in a secondary immune response.

• Lymphoid Tissue

  • The lymphoid tissue may be primary or secondary depending upon its stage of lymphocyte development and maturation.
  • T cells mature in the thymus, while B cells mature in the bone marrow.
  • T cells born in bone marrow travel to the thymus gland to mature.
  • During antigen presentation, such as from the dendritic cells, lymphocytes migrate to germinal centers of the secondary lymphoid tissues, where they undergo clonal expansion and affinity maturation.
  • Secondary lymphoid tissue provides the environment for the antigens to interact with the lymphocytes.

• The Evolution of Plastids

  • Plastids may derive from cyanobacteria engulfed via endosymbiosis by early eukaryotes, giving cells the ability to conduct photosynthesis.
  • Endosymbiosis involves one cell engulfing another to produce, over time, a co-evolved relationship in which neither cell could survive alone.
  • This leads to the question of the possibility of a cell containing an endosymbiont to become engulfed itself, resulting in a secondary endosymbiosis .
  • The process of secondary endosymbiosis is not unique to chlorarachniophytes.
  • In a secondary endosymbiotic event, the cell resulting from primary endosymbiosis was consumed by a second cell.

• Natural Killer Cells

  • Natural killer cells (or NK cells) are a type of cytotoxic lymphocyte critical to the innate immune system.
  • Natural killer cells (or NK cells) are a type of cytotoxic lymphocyte critical to the innate immune system.
  • Often NKT cell activity promotes NK cell activity by secreting IFNγ.
  • Numerous experiments have demonstrated their ability to adjust to the immediate environment and formulate antigen-specific immunological memory, which is fundamental for responding to secondary infections with the same antigen.
  • Functions of NK cells include: Cytolytic Granule Mediated Cell Apoptosis; Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC); Cytokine induced NK and CTL activation; Missing 'self' hypothesis; Tumor cell surveillance; NK cell function in adaptive response; NK cell function in pregnancy; and NK cell evasion by tumor cells .

• Maturation of T Cells

  • They are distinguished from other lymphocytes, such as B cells and natural killer cells (NK cells), by the presence of a T cell receptor (TCR) on the cell surface.
  • T cells can be either helper T cells or cytoxic T cells based on whether they express CD4 (helper) or CD8 (cytotoxic) glycoprotein.
  • They travel to sites that contain secondary lymphoid tissue, such as the lymph nodes and tonsils, where antigen presentation.
  • A T cell is then signaled by the thymus to become a CD4+ cell by reducing expression of its CD8 cell surface receptors.
  • The remaining cells exit the thymus as mature naive T cells.