1. Early Development
Chapter 21

2. Early Development Highly variable among different organisms
Common genetic and cellular mechanisms in development
Begins with gametogenesis
Proceeds in ordered phases

3. Phases of Human Development

4. Gametogenesis and Fertilization

5. Gametogenesis 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

6. 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)

7. 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

8. Sperm Structure and Function

9. 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

10. Egg Structure and Function
Cortical granules- 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

12. Fertilization
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

14. Fertilization
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

15. 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

17. 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

19. 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

20. 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

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

23. Animal Cleavage and Gastrulation

24. Cleavage
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

25. Cleavage

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

27. Cleavage
Divide at oblique angles so that they pile up in a pattern called spiral cleavage

28. 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

29. 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

30. 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

32. Gastrulation 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

33. Cross section of blastula
Gastrulation
Zygote
Cleavage
Eight-cell stage
Blastula
Cross section of blastula
Blastocoel
Gastrula
Gastrulation
Endoderm
Ectoderm
Blastopore
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. 3
The endoderm of
the archenteron de-
velops into the tissue
lining the animal’s
digestive tract. 6
The blind pouch
formed by gastru-
lation, called
the archenteron,
opens to the outside
via the blastopore. 5
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). 4
Only one cleavage
stage–the eight-cell
embryo–is shown here. 2
The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage. 1

34. Gastrulation
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

35. Plant Development

36. Plant Life Cycle

38. Embryogenesis 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

39. Embryogenesis 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.

40. Invertebrate Development

41. 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

42. Embryogenesis in Fruit Fly

43. 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

45. Vertebrate Development

46. 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

47. Embryogenesis in Frog

48. 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

49. Early Development in Humans