Development

	DEVELOPMENT * occurs after both the male and female gametes (sperm and eggs) fuse-- FERTILIZATION * the process by which the fertilized egg, the ZYGOTE develops from a single cell into an adult multicellular organism I. ANIMAL DEVELOPMENT A) Embryonic Development * the changes a zygote undergoes as it changes into an embryo 1) cleavage-- * a series of mitotic cell divisions that increase the number of cells but not the size of the cells * actually, the cell size becomes smaller * leads to the formation of a hollow ball called the BLASTULA 2) gastrulation-- * in certain animals, the blastula becomes indented, forming a GASTRULA * the gastrula has 3 layers, sometimes called germ layers: * ENDODERM-the inner layer * ECTODERM-the outer layer * MESODERM-the layer in between the endoderm and the ectoderm 3) differentiation-- * the different germ layers in the gastrula develop into special cells, tissues, and organs of multicellular animals ECTODERM: nervous system and the outer skin MESODERM: muscles, skeleton, circulatory and excretory systems, gonads ENDODERM: linings of the digestive and respiratory systems, as well as parts of the liver and pancreas * these cells are called embryonic stem cells and can become all types of cells * during differentiation, the stem cells become more specialized for their function * scientists are trying to revert mature, specialized cells back to the stem cell phase (dedifferentiation--see pic below) so they can help out individuals by turning them into a needed type of tissue! examples: healing spinal cord injuries growing new heart tissues forming new muscles for trauma patients 4) growth-- * an increase in size or cell number as differentiation continues * early development consists chiefly of the differentiation and growth of cells and tissues The above embryonic development can occur either outside (externally) or inside (internally) the mother.
Development



		stem cells--what are they and why are they so darn cool



a great look at the early stages of embryonic development		regentsprep.org has a nice walkthrough of development
		regentsprep.org has a nice walkthrough of development


awesome site from Univeristy of Utah's Genetic Science Center on stem cells; look at their main page too!

		Cold Spring Harbor Laboratory has a nice overview of stem cells


Tim and Moby discuss stem cells.



		see these insects in their different stages of life

B) External Development
* the development of the embryo outside the mother

1) in water

* in fish and amphibians, the eggs are fertilized externally and then develop externally as well
* the embryo's food source is the yolk in the egg
* gas exchange for the embryo occurs by diffusion through the thin egg with the watery environment
* water also acts as a shock absorber for protection

2) on land

* after being fertilized internally, the eggs of birds, reptiles, and a few mammals (like the duck billed platypus) lay their eggs to develop on land

ADAPTATIONS OF EGGS

a) shell-outermost layer which provides protection and a favorable environment

b) yolk-food source of egg; produced by uneven cytoplasmic division during oogenesis (meiosis)

c) different membranes-

i) chorion- the outermost membrane which is under the shell; contains the other membranes

ii) amnion- contains AMNIOTIC FLUID which cushions the embryo and prevents cells from sticking to the shell

iii) yolk sac- surrounds the yolk; blood vessels penetrate the yolk sac to carry food to the developing embryo

iv) allantois- functions as a respiratory surface (exchange of gases); also acts as a storage site for the nitrogenous waste produced, uric acid

C) Internal Development
* the development of an embryo inside the mother

1) placental mammals

* mammal embryos develop within the UTERUS after internal fertilization
* the PLACENTA is a special organ made of both embryonic and maternal (mother) tissues

* the placenta is the place where the exchange of nutrients, wastes, and respiratory gases occurs by diffusion
* an UMBILICAL CORD with blood vessels attaches the developing embryo to the placenta
* the bloodstreams of the embryo and the mother never mix!

2) marsupial mammals

* these mammals have eggs that are fertilized internally
* they develop internally for a short period of time without a placenta
* the embryo is then born VERY EARLY IN ITS DEVELOPMENT!
* then, the born embryo continues its development externally in a pouch which contains the mammary glandssupplies the nutrients to the young " kangaroos, opossum, and koala bears (not shown) are examples of marsupials

Development

	II. PLANT DEVELOPMENT * both meiosis (the production of sex cells- gametes) and fertilization (the fusion of gametes) occur in the flower, a plant structure specialized for reproduction A) The Flower * may contain both male (stamen) and female (pistil) reproductive organs... * some species might have only one or the other * flowers might have other accessory structures such as: petals- used to attract insects sepals- the leaf of the flower 1) the stamen-- * composed of the anther and filament (which looks like a wire) * the diploid (2n) cells of the anther produce the monoploid (n) pollen grains, which is the male gamete * MEIOSIS OCCURS IN THE STAMEN 2) the pistil-- * composed of the stigma, style, and the ovary a) stigma- catches pollen (sticky-stigma) is the upper part of the pistil b) style- connects the stigma to the... c) ovary- monoploid (n) eggs are produced here in ovules; MEIOSIS OCCURS IN THIS PART OF THE PISTIL B) Pollination * the transfer of pollen grains from the anther (part of the stamen) to the stigma (part of the pistil) * pollination may be accomplished by wind, insects, or birds that carry the pollen * colored petals and nectar can attract different organisms to help pollination occur 1) Self-Pollination * the transfer of pollen on the same plant * can occur in the same flower or from flower to flower of the same plant * uses the genetic information from that same plant but is still *sexual reproduction! (two cells are involved) 2) Cross-Pollination* * the transfer of pollen of one plant to another plant entirely * allows for the combination of characteristics from both "parent plants" * only one problem...the environment is usually very dry * pollen grains have thick walls to help prevent water loss (dehydration) of the nucleus (found inside the pollen grain)
Development



		plant sperm--I mean pollen!



pollination--so that is why they talk about the 'birds and the bees'...they mean plant sex!

	C) Fertilization and Development * after pollination, the pollen grain germinates (grows) on the stigma * as it grows, it forms a pollen tube which extends into the ovary of the pistil * the sperm now forms from the nucleus of the pollen and travels down the pollen tube 1) Fertilization in the Flower * when the sperm fertilizes the egg in the ovule (part of the ovary) a zygote is formed * the zygote develops by mitosis into an embryo * the embryo and now matured ovule is called a SEED a) seed coat- a tough protective outer layer that develops from the walls of the ovule b) fruit- the entire ripened ovary 2) Development of Embryos * the seed has 3 different parts: a) hypocotyl- becomes roots and lower stem of adult plant b) epicotyl- becomes upper stem and leaves of adult plant c) cotyledon- stored food for germinating plant 3) Germination and Growth seeds grow under suitable environmental conditions such as: sufficient moisture * proper temperature * sufficient oxygen for aerobic respiration--NRG! * seeds will grow into mature plants * growth occurs at regions called MERISTEMS
Development



		all about seeds