Developmental BioBTHS

Introduction to Life Cycles

Today’s Objectives

• Identify 3 model organisms used in Developmental Biology and the advantages and disadvantages of each.

• Describe the basics of a mammalian life-cycle including:– the diploid state and the haploid state, cleavage, implantation,

embryo and fetus.

• Identify the location of human embryonic development, and fertilization.

• Define several stages of animal development including: – fertilization, embryogenesis, cleavage, blastula, gastrulation,

organogenesis, metamorphosis, larva, gamete, somatic cell, gametogenesis

Let’s take a brief look at humans first

• Important points:– Diploid vs. haploid

• Somatic cells vs. gametes

– Where does fertilization take place?– Terms: Uterus, fallopian tube, morula,

blastocyst, implantation, placenta

Figure 2.9(1) Summary of Meiosis

Figure 2.9(2) Summary of Meiosis

Figure 2.9(3) Summary of Meiosis

Figure 11.27 - Early Human Development

Stages of Animal Development

• The mere coordination to create offspring is a major consideration in the life cycles

• Let’s take a look at stages within a typical animal life cycle– Can be generalized - most of the same steps, but

with variation

Basic Stages, in order

• Fertilization - > makes a zygote• Embryogenesis

– Cleavage– Blastula– Gastrulation– Organogenesis– Larva -> metamorphosis -> adult– Gametogenesis

Fertilization

• Joining of gametes

• Haploid to diploid– Restore the diploid number (2n)

• Different from species to species!

Cleavage

• Period of rapid cell divisions – Really fast! 1 per hour in or faster in some

cases– What has to happen before every cell

division?

• Cytoplasm of zygote is divided into many small cells – Blastomeres– This then becomes the blastula stage

Gastrulation

• Massive cell rearrangements of blastula to form gastrula– Set up axis (Anterior-posterior, dorsal-

ventral, proximal-distal)– Forms 3 germ layers

• Ectoderm (outer coating)• Mesoderm (middle layer)• Endoderm (inner lining)

Organogenesis

• Cells interact with each other, rearrange to produce organs and tissues– Cells can migrate great distances!

• (Ex. Blood, gametes, pigment cells, bones of face)

Maturation

• Some species undergo metamorphosis – Emerge from the egg as a larva– Mature within the pupa to become an adult– The larva can look very different from the

adult

Gametogenesis

• In some species the cells that are going give rise to sperm and eggs are set aside immediately after fertilization– Germ line

• Migrate to the gonad, undergo meiosis

• Eventually -> if fertilization occurs, start all over!

Frog Life Cycle (Xenopus)

• Let’s examine a specific example of a life cycle, and observe the various stages

• Xenopus laevis or Rana pipiens

• Seasonal mating

Figure 2.2(1) Early Development of the Frog Xenopus laevis

Figure 2.2(2) Early Development of the Frog Xenopus laevis

Figure 2.3(1) Continued Development of Xenopus laevis

Figure 2.3(2) Continued Development of Xenopus laevis

Figure 2.4 Metamorphosis of the Frog

Eggs of Rana and Xenopus

• 2 hemispheres - one is yolky, one is not – (vegetal and animal - we’ll learn more later)

• At fertilization, egg completes meiosis II

• Then sperm and egg nuclei fuse to become one nucleus

• Cleavage begins– What does this mean?

• As Cleavage occurs– Fertilized egg (zygote) stays the same size– Tens of thousands of cells are made from

the large egg cytoplasm

– On one side of the Blastula, a hollow cavity forms called a blastocoel

• Then gastrulation begins– 3 germ layers begin to form

• What are they?

– Can you see a head or tail end yet?

• Next the nervous system begins to form in a process called Neurulation– We’ll discuss in detail . . . . Neural tube

• The organism becomes segmented into repeating segments– Seen in SOMITES– We’ll discuss more

• Mouth and anus develop• Neurons start to make connections• Finally larva hatches, undergoes

metamorphosis

• Quiz on Life Cycles - FRIDAY