Embryological Development

• The sequence of events converting organic and inorganic molecules into a living organism is an amazing phenomena
• When cracking a chicken egg, the yellow yolk and clear egg white consist of carbohydrates, lipids, proteins and nucleic acids
• Through gene expression, the egg turns into a chicken in 27 days

Embryological Development

• Focuses on embryological development in starfish and chickens
• The basic animal design begins to take shape in early stages

Sexual Reproduction and Development

• Involves the production of sex cells
• Involves the fertilization after the fusion of male and female gametes
• Involves production of numerous cells from a fertilized egg via mitosis
• Involves cell organization and movement into specialized tissues
• Involves layering of tissues to form organs

Fertilization

• Ovum and sperm fuse
• Sperm cell with enzyme-filled acrosome contacts the ovum's jelly coat
• The acrosome releases enzymes during the acrosomal reaction, creating a hole in the jelly coat
• Sperm cell and ovum membranes fuse, releasing the sperm nucleus into the ovum's cytoplasm
• Fusion of haploid nuclei is called karyogamy

Cortical Reaction

• Prevents additional sperm cells from entering the ovum
• An impenetrable fertilization membrane forms
• A fast block changes the egg surface's polarity, involving electrical charge

Mitosis

• The fertilized egg divides rapidly through mitosis
• The egg divides into two cells during primary cleavage
• These two cells divide, producing four cells during secondary cleavage
• Each of the four cells divides again, producing eight cells

Morula

• Cell divisions result in a solid ball of cells known as a morula

Blastocoel

• Cells continue to divide within the morula
• A cavity begins to form in the morula's center

Blastula

• The blastocoel grows, turning the morula into a one-cell layer thick balloon
• The shape of the mass varies from animal to animal

Gastrulation

• Cells migrate inward, forming a pocket
• This pocket, called the archenteron, compresses the blastocoel
• Results in the formation of the gastrula

Gastrula

• Comprised of 2 cell layers called primary germ layers
• All tissue and organs develop from these layers
• The outer cell layer is called the ectoderm, which forms the skin and nervous system
• The inner layer is called the endoderm, that becomes the digestive organs
• The mesoderm is the third layer and forms between the first two germ layers
• The mesoderm develops into muscle, the skeleton, kidneys and gonads

Organ Rudiments and Cell Differentiation

• Primary tissue layers replicate and arrange into adult organ shapes
• Cells of the ectoderm form the beginnings of the nervous system in a process called neurulation
• The formation of organs is called organogenesis
• The individual cells must learn to perform their functions in a process termed cell differentiation

Development Similarities

• Development is well studied in major groups of animals, including humans
• Sequence of events is essentially the same for major groups of complex multicellular animals
• There is, however, considerable variation among various animal groups

Asterias (Starfish)

• Early development sequence in Asterias is similar to humans
• This is much easier to visualize and understand

Chicken

• Study of human development can be done using chicken eggs
• Occurs because development through the embryonic stage in chicks and humans is almost identical
• Because of this, it can be difficult to distinguish a chicken embryo from a 56-day old human embryo at a similar stage of development

Asteria Embryos

• Suitable for study because eggs are of the isolecithal type
• Yolk material is evenly distributed throughout the egg
• Eggs divide completely during cleavage, producing cells of equal size
• Cell layers are more visible

Observation

• Shells and opaque membranes are absent, enhancing observation
• Focus is on the most readily observable events
• Development also involves complex biochemical changes and cell movements that are not visible

Starfish Cleavage

• The starfish process of cleavage is simple
• The egg divides into two cells by mitotic division, producing a two-cell stage (primary cleavage)
• These cells produce four cells, each of which divides again, producing eight cells etc.
• These divisions produce the 4-cell, 8-cell, 16-cell stages of cleavage

Late Cleavage: Morula

• These divisions of early cleavage produce a solid ball of cells called the morula
• This occurs at around the 32-cell stage

The Blastula

• A cavity, the blastocoel, forms within the center of the morula
• The result is a blastula composed of a single layer of cells surrounding a hollow, fluid-filled center

Gastrulation Details

• The next development stage involves organizing cells into ectoderm, mesoderm, and endoderm
• As organization occurs, cells become differentiated
• The primitive gut cavity of the animal also forms
• These series of events composes the process gastrulation in starfish
• Begins when a few select cells of the blastula begin to divide and migrate into the center of the blastula

Gastrulation Analogy

• The cells of the blastula grow inward, producing a cup with an outer layer of cells (the ectoderm)
• The cup is lined with a layer of cells (the endoderm) and forming a narrow mouth (the blastopore)
• The hollow interior is called archenteron or primitive gut

Chick Embryo Development

• The life cycle begins at conception (fertilization of the egg)
• Proceeds through intrauterine development
• Continues through birth, growth and development to adulthood
• Continues through preparation for conception of the next generation
• The fertilized egg is a single cell with no clear features, and must undergo drastic change

Chick Cleavage

• After fertilization, the egg begins to divide
• Mass of cells needed to begin construction of the embryo are formed
• Rapid divisions following fertilization produce a morula then a hollow ball of cells

Trophoblast

• Outer layer of cells forming membranes such as the placenta
• The placenta is needed by the developing embryo to obtain nutrients
• On the inside of hollow ball is a mass of cells that separates and forms a two-layered disc in the trophoblast interior

Blastodisc

• This disc is essentially a flattened blastula
• Originates from flattened blastula and is beginning of the embryo
• Development to this stage takes 14 days and includes the implantation of the trophoblast into the uterine wall, called the germinal period

Chick Blastodisc

• Chick embryo produces a blastodisc
• Chicks do not produce extra embryonic membranes such as the placenta
• Events leading to blastodisc production are different
• The blastodisc consists of a bilaminar layer of cells comprised of embryonic endoderm and ectoderm
• The blastodisc divides rapidly and moves over the yolk

Gastrulation of Chick Embryos

• The gastrulation process is similar
• A long groove begins to form on the upper surface of the blastodisc
• This is called the primitive streak, where cells then migrate from the disc surface toward the groove

Cell Layer Formation and Interaction

• Cell migrate into groove, then into the space between upper and lower layers
• The interaction between migrating cells and cell layers result in the formation of 3 cell groups
• The upper cell layer becomes the ectoderm
• The lower layer of cells forms the endoderm
• The cells in between form the mesoderm

Primitive Streak

• The primitive streak and primitive pit will become the anus
• The developing embryo develops anterior to the streak
• As the ectoderm, endoderm, and mesoderm spread, they rise up and form a headfold
• The headfold contains an endoderm-lined cavity that represents the beginning of the tubular gut

Organogenesis with Somite and Neural Tube Formation

• The first event is the formation of the neural tube, the precursor to the nervous system
• Neural groove begins to form in the same location as the primitive streak nears completion
• Ectoderm cells migrate, forming a crest of cells on each side
• As the crests build, begin to lap over groove and come in contact with each other

Somites

• At the site where the neural crests first make contact, mesoderm gathers into small groups
• Form paired structures called somites
• Paired groups of mesoderm which give rise to segmentation, expressed in the vertebral column and ribs