DevelopmentDevelopment

Egg (female gamete) (n) +

Sperm (male gamete) (n)

fertilization

Zygote (2n)

FertilizationFertilization

Embryo DevelopmentEmbryo DevelopmentThe single cell zygote must go through a vast number of

changes to become a multicellular organism with differentiated cells.

Cleavage Cleavage A series of rapid mitotic divisions

occur without growth or gene expression

This forms a solid ball

of cells called a morula

The cells form a ball with a fluid filled center

called a blastula

16 – 32 cells

Day 1 Day 2

Day 3

Day 4 Day 5

Blastula (has no ICM)Morula

Blastomeres (prechorion)

•Does not have an opening •Blastocoel – fluid filled center

•Blastomeres – outer cells to become chorion •Inner cell mass will become the embryo

BlastulaBlastula

blastocoel

Inner Cell Mass in

Blastocyst

Human DevelopmentHuman DevelopmentFertilization and morula formation occur in the oviduct.

The blastula forms as the developing embryo enters the uterus

Blastocyst secretes enzymes allowing it to imbedin the uterine lining (day 7 - 13)

Implantation:

ImplantationImplantation

Human Chorionic Gonadotropin (HGC) (secreted by the embryo) maintains the corpus luteum and production

of progesterone The placenta will secrete its own progesterone

later. (takes over in weeks 10-12)

ImplantationImplantation

GastrulationGastrulation starts when cells on one side pulled

into the embryo (invagination)

ends with the formation of a gastrula

Blastopore

(opening)Gastrula: three tissue layers (ectoderm mesoderm and endoderm) and a primitive digestive tract called the archenteron

Day 16

invagination

ectodermendoderm

mesoderm

archenteron

GastrulationGastrulationAll tissues and organs will develop from these three

layers (called germ layers). Cell fate has been determined. Gene expression begins as

differentiation starts.

blastopore

Gastrulation in HumansGastrulation in Humans

Inner Cell Mass produces an embryonic disc Primitive streak - Site of invagination

Neurulation Neurulation Initiating the Nervous System

•Ectoderm becomes nerve chord and brain

•Induction by notochord

•Cell migration forms peripheral nerve tissue

•Begins development of body segmentation

•Somites from mesoderm form precursors to vertebrae (~ 21 days)

Embryonic Germ LayersEmbryonic Germ Layers

Cell differentiation begins to occur at the blastula stage. Different types of cells use (express) different

parts of the DNA code

Extra Embryonic TissuesExtra Embryonic Tissues

PlacentaPlacentaSite of exchange of nutrients and waste

Embryo part forms from chorion

Maternal part forms from uterine tissues

Blood of mother and embryo or fetus never mix

Umbilical CordUmbilical CordStretches between placenta and fetus

Carries oxygen and nutrients to fetus

Carries carbon dioxide and wastes to placenta

Embryo DevelopmentEmbryo DevelopmentFifth Week

Limb buds form, head (brain) enlarges, sensory organs noticeable

Embryo DevelopmentEmbryo Development

Embryo DevelopmentEmbryo Development

Human

Pig

Chick

Fish

Embryonic stages occur in the first eight weeks.

All organs are initiated in the embryonic stage

Fetal development occurs from week nine through week 40

Marked by a great increase in size and weight

Fetal Fetal DevelopmentDevelopment

12 – 16 weeks

Figure 29.24Figure 29.24

24 weeks

Parturition Parturition

Three stages: 1. Expansion of the cervix 2. Delivery of the fetus

3. Expulsion of the placenta (“afterbirth”)