Reviewer - EmbryoLec Chapter 1 Part II

Reviewer: Embryo (Lec) - Chapter 1 [Part II] (continuation) Fundamental Fundamen tal Processes and Concepts in Development Morphogenesis  Entire group of processes that mold the external and internal configuration of an embryo  Pattern formation VS morphogenesis o pattern formation  laying down of the morphogenetic blueprint o morphogenesis  realization of the plans  Drosophila  molecular basis of pattern formation  Tight genetic control in Drosophila – Genetic Hierarchy is as follows: 1) Maternal effect genes  establish the anteroposterior and dorsoventral axes of the early embryo 2) Zygotic gap genes  define broad regions in the egg 3) Pair rule genes  define 7 segments 4) Segment polarity genes  define 14 segments 5) Homeotic genes  determine the regional characteristics of each segment  Positional information means that a given cell is able to: 1) Recognize its position in a coordinate system that is set up within the primordium of a structure and; 2) differentiate according to its position

Intercellular Communicat Communication ion  Ability of the components of a biological community to generate signals and to respond in turn to signals from other members of that community  Gap junctions  where the membrane of one cell is in intimate contact with that of another Cell Movements  At numerous periods during embryonic life, cells or groups of cells move from one part of the embryo to another  Individual cells in embryos commonly migrate by means of amoeboid of amoeboid movements o In amoeboid movement, the cell continually tests its surroundings and its activity is characterized by the presence of a ruffled membrane along the leading surface  A unique form of individual cell movement occurs in early avian embryos wherein the primordial germ cells move from the wall of the yolk sac into the bloodstream and are carried via the blood to the gonads

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Note: neural crest = ectoderm; primary germ cells = endoderm  Movement as a sheet   principally a property of  cells, particularly those of the ectodermal germ layer

Cell Death (Apoptosis)  A necessary component of many phases of  development  Hormones sometimes play an important role in stimulating the death of cells  In the case of the CNS, death is the fate of motor nerve cells that fail to make functional contact with a muscle fiber The Clonal Mode of Development  Clone  a group of cells arising from a single precursor  Basis of the “clonal selection” theory (Burnet) theory (Burnet)  After introduction of a foreign antigen into the body, a single immunologically competent cell undergoes a massive proliferative response and subsequently produces antibody against the antigen Regulation and Regeneration  If part of a structure is lost, the loss is recognized and reparative processes are set in motion  Regulation  restoration of the missing material if  loss occurs before differentiation of the structure has set in  Regulation is the basis for the development of  identical twins  Morphogenetic fields o Areas of the body that are able to reconstitute lost portion o A region of the body in which the cells as a group are somehow cognizant of the overall nature of  the structure to be formed  Regeneration  process of replacement if  differentiation of recognizable structures has already occurred  One of the main features of a regenerating system is the formation of a mass of primitive-appearing cells  – the regeneration blastema – that demonstrate many of the properties of the embryonic primordium of the structure Growth  Simplest definition: increase in mass  Differential growth  commonly all parts of the embryo do not grow at the same rate

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Striking feature of young embryos: rapid growth of  the head region  Two major patterns of growth in animals: 1) Determinate growth o The body grows to a certain point that is characteristic of the species and sex, then the growth ceases o Common mode in mammals 2) Indeterminate growth o Growth continues throughout the life span, although at a reduced rate in later life o More common in more ancestral vertebrates, such as fishes

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Recapitulation  Biogenetic law of Müller and Haeckel  – the story of  individual development sketches for us an approximate outline of the evolutionary changes passed through by our forebears   An animal in its individual development passes through a series of constructive stages like those in the evolutionary development of the race to which it  belongs  Ontogeny is an abbreviated recapitulation of   phylogeny   Ontogenetic recapitulation  a conservative process which retains the basic ontogenetic stages of more primitive forms Heredity and Environment  Heredity  establishes the inherent potentialities of a developmental system or an individual  Environment  determines how far an individual can go toward a full realization of this inheritance  Note: There are conditions in which heredity and  environment interact  Methods Used in the Study of Embryonic Development Direct Observation of Living Embryos              

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