AioftheStorm

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Hippocrates

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Aristotle

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Galen holds that respiration for the fetus occurs through the umbilical chord.

1724

Hermann Boerhaave

1850s

Robert Remak, cell theory, fertilized egg division

1876

Oscar Hertwig publishes his findings on sea urchins where he observed that fertilization occurs from the fusion of a male spermatozoon with a female egg.^[6]

1888

Wilhelm Roux published the results of a series of defect experiments in which he took 2- and 4-cell frog embryos, and killed half of the cells of each embryo with a hot needle. He then reported that they grew into half-embryos and surmised that the separate functions of the two cells had already been determined after they split. This provided support to the mosaic theory of epigenesis which held that the hereditary material of cells was passed down unequally during development. Although flaws in the experiment's design led Roux to incorrect conclusions it influenced other embryologists by pioneering a detailed, interventionist methodology that Roux called Entwicklungsmechanik.

1892

Hans Driesch performs experiments similar to Roux using sea urchin eggs instead of frog embryos. Importantly though he separated the cells instead of killing them with a hot needle yielding different results. His separated cells formed complete, although smaller, embryos giving support to the idea that all the hereditary information was present in each cell.

1902

Theodor Boveri publishes a paper on sea urchins that demonstrates that all chromosomes are required for normal embryonic development.

1930-50s

E. E. Just and Johannes Holtfreter theorize differences in embryonic cell membranes could give rise to different organs

• • Place 3 germ layers of amphibian into alkaline solution, allowing them to dissociate into individual cells, and mixed these different cell types together. Cells of different species were used to be able to visually observe and follow their movements. Cells of similar types migrated to their correct location and reaggregated to form germ layers in their correct positions.
  • Holtfreter develops selective affinity concept based on results. Theorized that well-timed changes to selective affinity throughout development guided morphogenesis.

1964

Malcolm Steinberg introduces Differential adhesion hypothesis that uses thermodynamic principles to describe and explain patterns of cell sorting.

1968

Elizabeth Hay introduces the term Epithelial–mesenchymal transition, previously referred to as Epithelial–mesenchymal transformations.

1996

Foty et. all show that cells arrange themselves based on surface tension.

The concept of cell adhesion arose from the work of Friedrich Daniel von Recklinghausen in 1862 while working on squamous epithelium.^[7]

Egg jelly is a gelatinous layer that surrounds oocytes and releases species-specific chemoattractants that activate and guide sperm to the oocyte.^[8] abbr

Developmental biology is the study of the processes and phenomena that give rise to and maintain mature organisms. Processes of particular interest include cell growth, cell differentiation, and morphogenesis.

While originally embryogenesis occupied most of the study of developmental biology, the past several decades have seen the scope of developmental biology broaden to include a variety of biological phenomena such as aging, regeneration, cancer, and metamorphosis.

One of the central processes that developmental biology explores is cell growth. Cell growth includes the division of cells to make more cells(cell proliferation), the enlargement of cells, the production of additional genetic material, as well as development of the extracellular matrix. In addition to stimulation and inhibition of these processes, regulation of cell growth also includes apoptosis, or programmed cell death. Inquiries into these processes underlie many fields of study. One such field is the emerging discipline of regenerative medicine which commonly studies the mechanisms by which certain organisms regrow lost body parts in hopes that application of this knowledge will lead to the development of techniques to regenerate lost or damaged organs in humans.

Probably the most fundamental question that developmental biologists seek to answer, however, is how does the body so precisely regulate cell growth? During human embryogenesis the fingers of the hands grow at opposite sides of the body, yet they end up nearly equal in length, and while the cells in the fingers are instructed to grow and divide, the cells that make up the webbing between the fingers are instead signaled to undergo apoptosis. While especially critical during embryogenesis, the regulation of cell growth and death remains important throughout an organism's life. Following birth, the outer layer of skin, the inner lining of the digestive tract, and even taste buds regularly slough off and get replaced by new cells. In its first year of life a human infant will replace nearly 100% of its skeleton with new skeletal tissue.^[9] For a child between the ages of 8 and 14, approximately 20 billion to 30 billion cells undergo apoptosis daily, and for adults that number grows to 50-70 billion.^[10]

If this tight balance of cell growth and cell death is disrupted then serious health issues can arise. A major example of this is cancer, which refers to a broad collection of diseases characterized by uncontrolled cell growth. Additionally, failure of the body to efficiently induce apoptosis in dying cells and subsequently remove them contributes to various autoimmune diseases by allowing the autoantigens of the malfunctioning cells to persist for longer periods of time.^[11]

Morphogenesis is the coordination of cell adhesion, migration, and signaling to create structure and form in an organism.

Cell-to-cell communication

• Cell adhesion
  • cadherins: calcium-dependent adhesion molecules, held by catenins. Cadherin-catenin complex binds to cytoskeleton.
    • Governs strength of cell adhesion which allows proper cell sorting.
    • Aid cytoskeleton
    • Function in gene expression

• Cell migration
  • First polarization:
  • Second protrusion:
  • Third adhesion:
    • Integrins
    • Focal adhesions
  • Fourth Release of adhesion in back
  • Chemotactic factors: chemoattractants and chemorepulsive factors

• Cell signaling
  • Induction
    • Inducer: releases paracrine factors to induce a change in adjacent cells
      • Fibroblast growth factor (FGF) family, attaches to Fibroblast growth factor receptors (FGFR)
      • Hedgehog family
      • Wnt family
      • TGF-ß superfamily
      • juxtracrine factors, cell membrane to cell membrane, autocrine factors, within the cell
      • signal transduction cascades
        • receptor tyrosine kinase: affects either transcription factors or cytoskeleton, RTK Pathway
    • Responder: that which is affected by the inducer, if it responds to the paracrine factors then the responder is competent.
    • Reciprocal inductions
    • Epithelial-mesenchymal interactions
  • Instructive vs. permissive interactions
  • Regional specificity
  • Genetic specificity

Embryonic Tissues

• Epithelial cells attach to one another to form tubes and sheets, close together, gives shape and form to embryo as well as define inside and outside
• Mesenchymal cells travel individually and forms large extracellular matrices, far apart,

Cancer refers to a group of diseases caused by

A cell becomes cancerous when its mechanisms for controlling

Cancer thus only occurs when both the mechanisms regulating cell growth and the mechanisms for destroying cancerous cells do not function properly.

this can cause harm by sequestering nutrients, rerouting bloods flow, applying pressure Cancer drugs commonly work by selectively harming cells that undergo rapid growth