Topics: Developmental biology, Embryo, Embryology Pages: 6 (863 words) Published: June 29, 2013

* Sequential expression of the genes
* Organisms proceeds from cell fertilized egg to adulthood to eventual death

Successful development
* Has to follow the prescribe sequence in normal pattern

Three Fundamental Processes in Embryology:
1. Morphogenesis
* (morph- = shape, + genesis = origin)
* Origin of shape, form

2. Differentiation
* Process of cells becoming different, and specializing for different functions 3. Growth
* Increase in size, requires input of matter, food

Two Modes of Reproduction

Indirect development
* Exhibited by the eggs of most marine invertebrates and those of many freshwater vertebrates * feeding larval forms that look and behave differently from mature adult * Eggs often contain rather limited food reserves and programmed for rapid differentiation Direct development

* Exhibited by eggs of mammals, birds and reptiles
* Develops without passing through a highly specialized larval phase *
Oviparity and Viviparity
Oviparous - Animals that spawn (lay their eggs)
* Viviparous – animals that give birth
* Ovoviviparity - which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch * Euviparity – designates the viviparous condition

* Converts diploid (2N) into haploid cells (N), sperm or ova * Originated from stem cell which grow into primary oocyte and primary spermatocyte * Undergo meiosis

Stages of Development
* Fertilization
* Cleavage
* Blastulation
* Gastrulation
* Neurulation
* Organogenesis

* Sperm release enzymes that pierce the egg’s coat.
* Receptor protein of eggs – surface protein of sperm, plasma membrane fuse and nuclei combine * Change in polarization – only 1 sperm enters
* Fertilization restores the 46 number chromosomes in the zygote

Egg types
Vary in the amount of yolk they contain
* Microlecithal – little amount of yolk
* Mesolecithal- moderate amount of yolk
* Macrolecithal – large amount of yolk
Yolk distribution
* Isolecithal – equal distribution
* Telocithal – unequal distribution
2 kinds of fertilization
External fertilization
* Aquatic animals which usually produce a large number of eggs and sperms * Outside of the mother’s body

Internal fertilization
* Copulatory organs
* Inside the mother’s body

* Early cell division of the zygote initiated by the fertilization of the egg  segmentation * Zygote undergoes a series of active mitosis turning it into morula * No growth period between divisions, zygote is turned into a ball of tiny cells * Blastomeres

* Each cell of the blastula

MORULA - 32 cell
* Blastoderm
* Further division result in the form of a hollow ball with wall consisting of a single layer of cells * Blastocoel
Fluid-filled cavity

In humans - Blastocyst
* Burrows into the uterine wall
* Chorion and amnion
* Vital for proper development
* Trophoblast – outer layer
* Blastocyst consists of a surface layer of cells

amnion containsthe pond of amniotic egg with 3 membranesamniotic fluid surrounding the embryo. chorion forms a protective membrane around the egg.
allantois extends out to contact the chorion for gas exchange and stores metabolic wastes. GASTRULATION (“gut formation”
* Formation of the germ layers – ectoderm, mesoderm, endoderm Three distinct processes
* Epiboly – surface of blastula move rapidly downward and end up by engulfing the yolk cells * Involution – process of rolling or curving inward
* Invagination – indentation and sinking in of the vegetal cells; accounts for the formation of a gastrocoel

Archenteron or primitive gut
* endodermal tube that is formed at the...
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