EARLY EMBRYOLOGY Why do we need knowledge of embryology to study anatomy? Show � The fact is that all of those structures in all organisms are derived from a single cell that was formed by the union of two gametes.Reproduction starts with two cells, the sperm and egg - haploid cells formed through the process of meiosis which are specially designed for their own specific purpose. Sperm are:At fertilization, enzymes in the acrosome of the sperm help to penetrate the egg � requires that the sperm break through the plasma and vitelline membrane surrounding the eggJust after fertilization the zygote (fertilized egg) undergoes cleavage (mitotic cell divisions) and becomes subdivided into smaller cells - the gross arrangement of cells differs greatly among vertebrates, depending on the amount of yolk in the egg: Holoblastic cleavage occurs when the cleavage furrows pass through the entire eggGastrulation is characterized by cell movement and reorganization within the embryo (morphogenetic movements) to the interior of the embryo, forming three primary germ layers: ectoderm, mesoderm, and endoderm. The cells migrate inward at the blastopore, which forms, or is close to, the location of the anus
in the adult This blastopore-to-anus developmental pathway is found in Chordata, Hemichordata, Echinodermata (starfish, sea urchins, sea cucumbers, etc.), uniting these groups into a monophyletic group called the Deuterostomes. The plesiomorphic condition, found in the Protostomes, is for the blastopore to become the mouth. Gastrulation in Amphioxus *Note: One important thing to realize at this point is that in your text, as well as in other diagrams of embryonic development, cells are color-coded to help you
recognize where they are from: Gastrulation in amphibians Gastrulation in birds Neurulation During neurulation One derived characteristic found in vertebrates is the formation of neural crest cells Differentiation and derivation - Organogenesis:
� dermatome - forms the dermis of the mid-dorsal skin- Mesomere: gives rise to the kidney tubules, excretory organs, and reproductive ducts. - Hypomere: lateral-plate mesoderm is confined to the trunk and is divided into somatic mesoderm (parietal peritoneum) and splanchnic mesoderm (visceral peritoneum, mesenteries, heart and associated structures, lymphatic system, gonads and visceral muscles) * Ectoderm - gives rise to: � Neural tube � Epidermis and associated glands � Neural crest and its derivatives: migrate through the embryo, giving rise to a diversity of structures � Ectodermal placodes: localized thickenings that sink below the surface and give rise to sensory neurons and sensory structures: olfactory placodes, forming the olfactory sacs; lens placodes, for the lens of the eye; otic placodes, to become the membranous labyrinth; a group of placodes that contributes neurons to the sensory ganglia of cranial nerves V, VII, VIII, IX, and X; and last, placodes that form the neuromasts of the cephalic and lateral line canals Sources of energy during development and extraembryonic membranes Extraembryonic membranes vary in complexity among the vertebrates: Mammalian developmental modifications Ontogeny and Phylogeny Carl Von Baer: made a number of general conclusions about development called Von Baer's laws: 1. In development from the egg the general characters appear before the special characters.In other words, a chick embryo would be recognizable at an early stage as Vertebrata, but not any particular subtaxon. Later, it would be recognizable as Aves, and finally, it would be recognizable as a Gallus domesticus. Therefore, the ontogenetic stages do not run parallel to the sequence of taxa on the scale of being. Ernst Haeckel: believed that the adult stages of the chain of ancestors are repeated during the ontogeny of the descendants, but that these stages are crowded back into the earlier stages of ontogeny. Thus, ontogeny is an abbreviated version of phylogeny. Haeckel claimed that the gill slits of human embryos were literally the same structures of ancestral adult fishes, that were pushed back into the early ontogeny of humans by an acceleration of development in lineages. In other words, the sequence of ontogenies was condensed, and new features were added by terminal addition. Von Baer, in contrast, argued that the gills slits are not the adult stages of ancestors; rather they are simply a stage common to the early ontogeny of all vertebrates. That is, evolution proceeds from "undifferentiated homogeneity to differentiated heterogeneity"; from the general to the specific. Von Baer's theory requires only that organisms differentiate; Haeckel's requires a change in developmental timing. Biogenetic Law The biogenetic law eventually lost popularity with the rise of experimental embryology and Mendelian genetics. Embryology showed that many varieties of change in developmental timing were possible, and that different parts of the organism might differ in rates of development; Mendelian genetics showed that genes could effect changes at any stage of development, and that terminal addition was not the only possibility. Allantois - extraembryonic membrane that develops as an outgrowth of the hindgut. Serves for respiration and excretion in reptile and bird embryos, contributes to the placenta in eutherians, and forms the urinary bladder and part of the urethra in adult amniotes Archenteron - the embryonic gut cavity that is lined with endoderm Blastocoel - a cavity of the blastula that becomes obliterated during gastrulation and mesoderm formation Blastodisk - the disk of cells formed during cleavage that lies on top of the yolk of large-yolked eggs of fishes, reptiles and birds, and on the top of the yolk sac of mammals Blastopore - external opening of the gastrula Blastula - ball of cells formed during cleavage, usually containing a blastocoel Chordamesoderm - the longitudinal middorsal group of meosdermal cells that moves into the roof of the archenteron during gastrulation and gives rise to the notochord Delamination - downward movement of cells to form a new layer near the yolk Dermatome - the lateral portion of a somite which will form the dermis of the skin Fate map - shows the cell areas of blastulas that will subsequently give rise to particular kinds of cells Holoblastic cleavage - cleavage furrows pass through the entire egg Ingression - the longitudinal movement of cells along the surface of the yolk Mesenchyme - an embryonic tissue that consists of star-shaped wandering cells that gives rise to most adult tissues Myotome - a muscle segment, usually applied to embryonic segments Nephric ridge - the region of the mesoderm between the somite and lateral plate that gives rise to the kidneys and gonads Neural crest - a pair of ridges of ectodermal cells that develop along the top of the neural tube as the neural folds close; this derived character of vertebrates gives rise to many of their distinctive features, including visceral skeleton, pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, and certain bony scales Neural tube - the tube formed in the embryo by the joining of the pair of neural folds; the precursor of the brain and spinal cord Neuroectoderm - the portion of the ectoderm that gives rise to the neural tube and neural crest. Primitive streak - a longitudinal thickening of cells along the blastoderm of large-yolked eggs through which prospective chordamesoderm and mesoderm cells move inward Sclerotome - the medial portion of a somite that forms the vertebrae Somatic - descriptive of structures that develop into the body wall or appendages as opposed to those in the gut tube, such as the somatic muscles, somatic skeleton Splanchnic - descriptive of structures that supply the gut Trophoblast - outer layer of the mammalian blastocyst; initiates placenta formation What are the 3 stages of prenatal development quizlet?This period is generally divided into three stages: the germinal stage, the embryonic stage, and the fetal stage.
What exact cells are localized to the neural plate of the embryo during the 3rd week of prenatal development?Third Week of Human Development
As the notochord develops, it induces the overlying embryonic ectoderm, located at or adjacent to the midline, to thicken and form an elongatedneural plate of thickened epithelial cells (seeFig. 4.8C andD). The neuroectoderm of the plate gives rise to theCNS, the brain and spinal cord.
What is connective tissue derived from during prenatal development?The mesoderm gives rise to the muscle cells and connective tissue in the body.
What period starts 3 weeks after conception quizlet?Which period begins three weeks after conception? the embryonic period.
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