Medical brain notes 33

  Developmental Biology   The process of sexual reproduction ensures the formation of a diploid zygote which could constitute the next generation. A zygote is a single celled structure. By an ontogenetic process the zygote undergoes various develop-mental phases resulting in multicellular embryonic organisation. These phases include  cleavage ,  gastrulation , neurulation ,  organogenesis  and the pe-riod of  growth  and  histological differentiation . Inspite of the fact that or-ganisms vary in their structure, form and mode of life, the processes of embryogenesis, development and differention are remarakably similar in all metazoans. Till later stages of development a fundamental uniform pattern in development can be observed. The ontogenetic stages also reflect the historical development of species or  phylogenetic development .   Realising the mode of formation of a young individual of the next gen-eration has always interested ...

  Types of eggs   For the embryo to develop inside a fertilised egg nutrition is needed. The amount of food needed varies for different organisms. It normally depends on the duration of development. Food is provided in the form of  yolk . It may be '  fatty yolk ' or  protein yolk '. It is provided by the ovary during differentiation of the egg. Due to accumulation of yolk a maturing egg rapidly increases in size. In amphibian eggs yolk occurs in the form of large granules, called  yolk platelets . Chemically, the yolk platelets contain two main pro-teinaceous substances namely  phosvitin  and  lipovitellin .   The amount of yolk is an important determining factor for further pat-terns in embryological stages. The amount of yolk influences cleavage and gastrulation methods. The eggs can be classified based on amount and distribution of yolk. Amount of yolk and egg types :   In certain animals the developmental stages are not very el...

  Cleavage and types - Frog's egg.   The process of cleavage reamains one of the earliest mechanical ac-tivity in the conversion of a single celled egg into a multicellular embryo. It is initiated by the sperm during fertilization. However in parthenogenetic eggs cleavage can commence without the influence of fertilization.   The process of cleavage or  cellulation  happens through repeated mi-totic divisions. These divisions result in cells called  blastomeres . In later stages of development the blastomeres occupy different regions and differen-tiate into several types of body cells.   The first cleavage of frog's egg was observed by  Swammerdam  in 1738. The entire process of cleavage in frog's egg was studied by  Prevost  and  Dumas  in 1824. With the development of microscopes cleavages and further stages were observed in the eggs of sea urchin, star fishes, amphioxus and hen's eggs.   From all these studies it h...

  Gastrulation in frog embryo   The process of gastrulation is a continuous activity succeeding, cleav-age. During this process the blastodermal cells begin to move. They wander and occupy their prospective organ forming zones. During this movement at one region on the blastula, the cells wander inside and occupy the blastocoelic cavity.   At a specific region below the equator the blastoderm cells assume an elongated bottle like shape. They move toward the interior of the blastula. As the cells move further inside, an invagination happens. A deepening of the invagination results in a cavity called the  archenteron  or  gastrocoel .The opening of the archenteron on the surface of the blastula is called the  blastopore .   The blastopore gradually assumes a crescentic shape. Finally it be-comes circular. The region dorsal to the blastoporal opening is called the ' dor-sal lip '. The lower edge may be called the '  ventral lip '. The surface ce...

  Organogenesis of Frog   The primary organ rudiments from ectoderm, mesoderm and endo-derm get well established during the processes of gastrulation and neurula-tion. In the next stage the primary organ rudiments subdivide into  secondary   organ rudiments . These rudiments get differentiated into various organs and   organ systems.   The development of ectodermal organs   The neurula of frog has three kinds of ectodermal tissues namely, epi-dermal ectoderm, neural ectoderm and neural crest cells.   Epidermal ectoderm   The epidermal derivatives are the skin, olfactory sense organs, ear, lateral line sense organs, median fins, external gills and lining of mouth and anus.   Neural ectoderm   This layer of cells form the central nervous system and peripheral nervous systems.     The development of mesodermal organs   The mesodermal derivatives are the limbs, endoskeleton, heart, blood vessels, kidney, coelom and reprod...