History of Embryology

Historical Background

• Aristotle secured information about embryos, marking a shift from superstition to observation
• Aristotle attempted to solve ontogeny, distinguishing "substance" from "form," implying the necessity of fertilization
• Galen learned about advanced fetuses' structure
• de Graaf first described the ovarian follicle
• Leeuwenhoek invented the microscope
• Hamm and Leeuwenhoek first saw human sperm
• Bonnet discovered some insect eggs develop parthenogenetically
• Spallanzani's work brought experimental methods to embryology, proving both sex products are needed for development
• Wolff argued for embryological development through progressive differentiation
• von Baer demonstrated germ layers' existence in embryos
• Matthias Schleiden and Theodore Schwann stated the adult body comprises cells/cell products. They realised the new individuals body develops from a single cell through the union of germ cells during fertilization

Prevailing Theories on Ontogenetic Development

Theory of Preformation

• Claims that if something is developing from an egg, it was always present but invisible
  • Leaves can be found inside flowers way before the bud starts growing
  • Parts of a butterfly can be seen before a butterfly emerges from its chrysalis
• All embryo parts were thought to be in the egg but were transparent, folded and small
• When the embryo develops, the parts grow, unfold, stretch, and become visible
• The embryo, and the future animal, is preformed in the egg

Theory of Epigenesis by Wolff

• States the early egg lacks a preformed embryo, containing only the materials to build it
• The material is granular and forms layers (germinal layers) , and the embryo forms via layer transformation
• A "vital force" acts as the architect, similar to Aristotle's "creative principle"

Phases of Ontogenetic Development

Gametogenesis

• Production of sex cells (gametes) in sexually reproducing organisms via meiosis
  • spermatogenesis produces spermatozoa
  • oogenesis produces ova.
• Creates conditions for embryogenesis to start
• Initial cells giving rise to gametes are similar in both sexes, with the exception that the cells aren't involved in differentiation for parent life support
• Mitosis causes first rapid proliferation of cells - spermatogonia in testes, oogonia in ovaries
• Once proliferation ceases, the cells are called spermatocytes and oocytes and growth/maturation occurs
• Spermatozoon cytoplasmic differentiations enable it to reach and fertilize the egg, and the substances in the egg are later used for development
• Meiosis discards half of the chromosomes, singles out the set of genes that operate in the development of a particular individual

Fertilization

• Involves independent processes:
  • Spermatozoa proximity with the eggs relies on parental adaptations
  • Spermatozoa must find/fuse with the egg, requiring morphological and physiochemical mechanisms
• The egg is activated after spermatozoon contact and starts developing
• Formation of organ forming substances occurs in the egg after fertilization

Cleavage

• The fertilized egg undergoes mitotic cell divisions in rapid succession
• Blastomeres get smaller with each division, increasing cell number, but the embryo size remains
• Changes in the developing embryos substances aren't discovered, as if the embryo focuses on cell number increase
• The whole process is dominated by the cytoplasmic organoids of the cells, the centrosomes and achromatic figures
• Nuclei multiply, but don't interfere with cytoplasm processes
• The result is a compact heap of cells arranged in a hollow spherical body - blastula with a blastoderm

Gastrulation

• Single blastoderm layer gives rise to germinal layers, the complex rudiments the animals organs are derived from
  • Animals consist of tissue and organ layers
• The ectoderm gives rise to the skin epidermis/nervous system
• Mesoderm is the source of muscles, blood vascular system, body lining, and sex organs
  • The excretory system/skeleton are derived from the mesoderm
• The innermost endoderm layer forms the alimentary canal and digestive glands
• The germinal layers are produced by the disappearance of a part of the blastoderm from the surface and it becoming enclosed by the remainder of the blastoderm
  • Remaining surface becomes ectoderm; the interior becomes endoderm/mesoderm
  • Disappearance of the endoderm/mesoderm converts a simple body into a double-walled cup

Organogenesis

• The process subdivides the embryo into specific parts
• Cells of the 3 germinal layers split into smaller groups that create an organ or part of the animal
  • Each organ starts as a segregated group of cells called the rudiment
  • Rudiments that subdivide germinal layers are called primary organ rudiments that contain cells to to produce entire systems.
  • Complex primary organ rudiments further subdivide into secondary ones
• With primary/secondary rudiments, the embryo resembles the adult animal, or the larva

Growth and Histological Differentiation

• Organ rudiments increase in volume to achieve its parent's size
• Cells in each rudiment become histologically differentiated, allowing them to perform physiological functions
• Cells in organs that are vitally important become capable of performing their physiological processes, enabling them to embark upon an independent existence
  • The animal is a miniature copy
  • The animal differs to a greater extent and may not be differentiated fully, and is called a larva
  • During metamorphosis the animal becomes similar to the adult
• Many animals repair injuries sustained, meaning the developmental processes may sometimes be repeated
• Asexual reproduction involves developing new parts and organs in animals that have already achieved the adult stage

Cellular and Genetic Background of Development

Cell Processes

Intracellular Synthesis and Its Regulation

• Development aspects include regulatory mechanisms restricting protein synthesis

• Portions of nuclear DNA lack proteins, directing the synthesis of mRNA in a transcription step

• RNA molecules commonly contain exons and introns and in mRNA processing, the introns are cut out

• Final mRNA molecules migrate to the cytoplasm and follow pathways:

  • Protein molecules function in the cell, the mRNA molecules link with ribosomes to form polyribosomes
  • Protein molecules secreted from the cell form complexes with the rough endoplasmic reticulum

• Regulation processes include strictly intracellular and are extracellular influences

• Receptor molecules mediate extracellular influences and are located at the cell surface.

• Adenyl cyclase catalyses ATP into cAMP and controls synthesis rate and cAMP influences intracellular processes

• Protein kinases (enzymes that phosphorylate proteins) stimulate pathways like glycogen breakdown

• Increasing cAMP levels are stimulated by gene transcription, energy metabolism and calcium influx.

Gene Activation

• Genes aren't typically active in the zygote, where they are highly complexed with histones
• Chromosomal DNA with histones is called chromatin, and densely staining chromatin represents repressed genetic material
• As development starts, certain gene groups are activated by being freed from its histones and the first genes become being involved with metabolic activity of the cell.
• As cleavage progresses and the embryo enters gastrulation, the first tissue-specific genes become activated
• It's estimated that at any given development stage, not more than 5 to 10% of genes are active, the rest remain repressed

Restriction and Determination

• In the fertilized ovum capacity to form an entire organism is present and resulting cells from the first divisions retain this capacity and are described as totipotent

• As development goes on cells gradually lose ability to form all the types of cells

• Reduction is called restriction

• Early cleavage cells remain totipotent and germ layers are locked into separate developmental channels

  • Brain, spinal cord, glands, skin, hair etc are all examples of ectodermal channels

• Developmental events see the ectoderm further subdivide

• Determination of occurs once restriction has proceeded to a single developmental fate

• Tissue interactions called induction shortly precede the process of determination

Differentiation

• Differentiation signifies the progressive expression of the genome that remains available to a particular group of cells to become specialized
  • Biochemistry - a cell chooses synthetic pathways
    • Hemoglobin
    • Crystalline proteins
  • Functional - represented the the development of conductivity along a nerve
  • Morphology - specific cell shapes and structures
    • Two pathways
      • results in specialized cells that have lost their nuclei
        • Platelets and eryhtocrytes
      • Synthesis of highly specialized intracellular molecules
        • Contractile proteins in muscles
    • Secretion of extracellular substances such as hormones and collagen fibrils

Regulation and Regeneration

• Vertebrate embryos have the ability to detect when a structure is damaged
• The body reconstitutes and sets processes into motion which is called regluation
• Twinning in mammals from subdivisions of embryos during early cleavage stages.
• Reconstitution can occur and is called morphogenetic fileds
• The processes are capable of forming overal natures
• If differentiation has already occurred its regenation

Induction

• Embryonic development involves developmental signals being generated and transmitted

• Most important effect is the process of one embryonic tissue on another so that the developmental course of the responding tissue is qualitatively changed

  • The inductive action of the optic cup forms the eye on the overlying ectoderm

• Inductive interactions span to postnatal life

  • The nervous system is induced during primary induction and induces other structures

• Internal organs are produces through induction -Natochord - neural tube; the optic vesicle then instructs the head to become a lens and cornea

Classification of Inductive Interactions

• Permissive - inductive signal brings about development and the responding tissue will form no other structure
  • ex. cell division (embryonic epithelial cells)
• Instructive - the responding tissue can from more than one type of tissue - embryonic lens is absent the cornea differentiates into
  e.g. without the proper induction, cells that would have normally formed mesoderm in the early amphibian embryo go on to form ectoderm. Ex without an embryonic lens, cells differentiate into epidermis
• Diffussion, direct contact, secretion can all induce these changes

Cell Division and The Cell Cycle

• Indispensable importance and living properties are cell division which take either mitosis or meiosis form
• One component is mitosis where the cell divdes into four periods
  • G1 - cell carries out its normal activites
  • synthesis of DNA in the S phase
  • The G2 period is where the gap gets filled before M phase commences
  • M phase - Mitosis itself occurs
• Mitosis occurs because of proteins leading up to conclusions
• Cyclin is synthesized from the G1 before Mitosis where the cell can operate
• Cyclin levels drop for it bring end to the cell, active cells help to break phosphorylating cell
• Dephosphorylating allows reassociation, starting a changes for the next cycle