Early development
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Early Development. Chapter 21. Early Development. Highly variable among different organisms Common genetic and cellular mechanisms in development Begins with gametogenesis Proceeds in ordered phases. Phases of Human Development. Gametogenesis and Fertilization. Gametogenesis.

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Early development l.jpg

Early Development

Chapter 21

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Early Development

  • Highly variable among different organisms

  • Common genetic and cellular mechanisms in development

  • Begins with gametogenesis

  • Proceeds in ordered phases

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  • Formation of gametes, sperm and egg

  • In the reproductive organs of adult organisms

  • Sperm and egg contribute an equal number of chromosomes to the offspring

    • Egg is 100x larger, contributes more cytoplasm

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Sperm Structure and Function

  • Animal sperm has four major compartments:

    • the head (the acrosome)

    • the neck (a centriole)

    • a midpiece packed with mitochondria

    • a tail (a flagellum)

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Sperm Structure and Function

  • In plants, sperm develop from pollen grains

  • Pollen are several haploid cells from meiosis

  • Pollen cell comes into contact with the stigma and divides by mitosis to produce two sperm nuclei

  • Move down the pollen tube to the egg cell

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Egg Structure and Function

  • Large, contain the nutrients required for the embryo’s early development

  • In species that lay eggs in the environment, stores in the egg are the only source of nutrients until it hatches

  • In mammals need stores until the egg implants in the placenta

  • Plants also provide endosperm to nourish the embryo

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Egg Structure and Function

  • Corticalgranules- vesicles filled with enzymes that assist fertilization in egg-laying animals

  • Vitelline envelope- fibrous, matlike sheet of glycoproteins that surrounds the egg

  • Jelly layer (a large, gelatinous mass that also encloses the egg

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  • Fusing of a haploid sperm cell with a haploid egg cell to form a diploid zygote

  • Can be internal or external

  • Requires exact timing and recognition

  • Must start development

  • Starts with gamete release

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  • Enzymes from the acrosome digest through the egg's jelly layer

  • Acrosomal process contacts the vitelline envelope

  • Plasma membranes fuse

  • Sperm nucleus, mitochondria, and centriole enter the egg

  • Sperm and egg nucleus fuse

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Species Recognition

  • Fertilizin is a compound on the surface of sea urchin egg cells

  • Binds to bindin, a protein on the head of sea urchin sperm

  • Binding occurs in a species-specific manner

  • Fertilizin from the eggs of one species binds to sperm of its own species but does not bind to sperm of different species

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Blocking Polyspermy

  • Wide variety of mechanisms to block polyspermy

  • In sea urchins, fertilization results in erection of a physical barrier

  • Generates a fertilization envelope as cortical granules fuse with the plasma membrane

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Fertilization in Mammals

  • Internal fertilization, so species recognition is generally not an issue

  • Acrosome still breaks down zona pellucida

  • Egg cells have a binding site for sperm

  • Glycoprotein ZP3 in the zonabinds to the head of sperm

  • Enzymes released from cortical granules modify ZP3 to prevent binding by additional sperm

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Fertilization in Flowering Plants

  • Takes place inside ovule

  • Double fertilization-one sperm nucleus fuses with an egg to form a zygote, and the other sperm nucleus fuses with two polar nuclei to form the triploid endosperm

  • Interaction between the pollen grains and the ovule involve species recognition

  • May also prevent self-fertilization

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  • Endosperm provides nutrients for the embryotic development, germination and early seedling growth

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  • Cleavage is the set of rapid cell divisions (without growth) that follows fertilization

  • Divides up the cytoplasm into cells, no growth

  • Cells are called blastomeres

  • Forms a blastula, sphere of cells

  • Pattern of cleavage varies among species

  • Sometimes makes cells around yolk

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  • Cells can divide at right angles to one another, forming tiers in a pattern called radial cleavage

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  • Divide at oblique angles so that they pile up in a pattern called spiral cleavage

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What Determines Cleavage and Development?

  • Cytoplasmic determinant is a molecule found in the egg that helps direct early development

  • Affect development independently of sperm or zygote genotype

  • Involved in differentiation—the generation of different cell types from a single cell

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Activating the Zygotic Genome

  • Zygotic Genome not active during cleavage

  • In most animals it is not transcribed until after cleavage is well under way

  • Mammals are the exception

    • Transcribe from the zygotic genome at the two-cell stage

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Embryonic Tissues

  • Animal embryos develop three types of tissues, called germ layers:

    • Ectoderm forms the outer covering and nervous system

    • Mesoderm gives rise to muscle, internal organs, and connective tissues such as blood and cartilage

    • Endoderm produces the lining of the digestive tract or gut, along with some of the associated organs

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  • After cleavage is complete a bastula is the result

    • Hollow ball of cells

  • Gastrulation rearranges cells

  • Results in the gastrula that contains the three embryonic tissue types

    • Each gives rise to different tissue types

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Only one cleavage

stage–the eight-cell

embryo–is shown here.

In most animals, cleavage results in the

formation of a multicellular stage called a blastula.

The blastula of many animals is a hollow ball of cells.

The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage.







The endoderm of

the archenteron de-

velops into the tissue

lining the animal’s

digestive tract.


Cross section of blastula

Eight-cell stage





The blind pouch

formed by gastru-

lation, called

the archenteron,

opens to the outside

via the blastopore.






Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer).



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  • At the end of gastrulation, the three embryonic tissues are arranged in layers, the gut has formed, and the major body axes have become visible

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  • First division is asymmetric

    • The large basal cell generates the suspensor structure;

    • Apical cell will give rise to the shoot apical meristem and the root apical meristem

  • Meristem is area of rapidly dividing cells

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  • An embryo contains an:

    • Epidermis- an outer covering of cells that protect the individual.

    • Ground tissue- a mass of tissue that may later differentiate into cells for specialized functions

    • Vascular tissue- that will differentiate into specialized cells that transport food and water between root and shoot.

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Embryogenesis in Fruit Fly

  • Fertilized egg undergoes mitoses without cytokinesis

    • Produces a multinucleate cell with a cytoplasm filled with nutrient-rich yolk

  • Each nucleus migrates to the outside of the embryo cell and receives a plasma membrane

  • Embryo becomes an outer sheet of cells surrounding the original cytoplasm

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Embryogenesis in Fruit Fly

  • Gastrulation starts with the formation of a cleft or furrow, followed by formation of furrows that define the head region and the series of body regions called segments

  • Embryo hatches to from larva

  • Larva forms pupa after a few days

  • Pupa goes through metamorphosis to become fly

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Embryogenesis in Frog

  • Frog embryo goes from one large cell to a ball of cells (blastula) through a series of cleavage events

    • Does not increase in size

  • Neural tube, which becomes the spinal cord and brain, forms at the end of gastrulation

  • Embryo hatches into tadpole

  • Tadpole goes through metamorphosis

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Early Development in Humans

  • Human eggs are released into the fallopian tube by the ovary

  • After fertilization zygote goes through cleavage as it goes down the fallopian tubes

  • Embryo undergoes implantation into the wall of the uterus, and the placenta forms

    • Placenta carries nutrients to and waste from the developing fetus