Developmental Anatomy

Question 1

Define embryogenesis.

Answer 1

Embryogenesis refers to the stages of development between fertilisation and hatching/birth.

Question 2

What are the 6 fundamental processes of embryogenesis?

Answer 2

1. Fertalisation: fusion of mature gametes.
2. Cleavage: series of extremely rapid mitotic divisions that tale place directly after fertilisation. Cells divide into smaller blastomeres, makes up the blastula.
3. Gastrulation: mitotic division slows and the cells change position, forms germ layers.
4. Organogenesis: cells interact with one another and rearrange into tissues and organs.
5. Metamorphosis: stages involved in becoming a sexually mature adult.
6. Gametogenesis: development of gametes.

Question 3

What two poles are formed in a newly formed zygote?

Answer 3

1. Animal pole - rapidly dividing cells
2. Vegetal pole - slowly dividing cells

Question 4

What is a blastocoel?

Answer 4

A cavity that forms in the animal pole during cleavage that is important in allowing cell movments in gastrulation.

Question 5

Explain how the germ layers arise.

Answer 5

• Gastrulation begins roughly 180º opposite the point of sperm entry, which forms a dimple- blastopore. This expands to become a ring, and the cells which migrate through the blastopore become the mesoderm.
• The cells remaining on the outside become the ectoderm, which expands to enclose the entire embryo.
• The large yolky cells that remain in the vegetal hemisphere become endoderm.

Question 6

How is organogenesis initiated?

Answer 6

Organogenesis begins when the notochord signals the ectodermal cells above it that they are not going to become epidermis. Instead, these dorsal ectodermal cells become the nervous system. At this stage the embryo is called a neurula. The neural precursor cells stretch, elongate, and fold into the embryo, forming the neural tube; the future eidermal cells of the back cover the neural tube.

Question 7

What are somites?

Answer 7

Mesodermal tissue adjacent to the neural tube that has become segmented. These give rise to back muscles, spinal verterbrae, and dermis.

Question 8

What do the different germ layers give rise to?

Answer 8

1. Ectoderm: epidermis, brain and nervous system.
2. Mesoderm: blood, heart, kidneys, bone, gonads, muscles, connective tissue.
3. Endoderm: eithelium of digestive tube and associated organs (including the lungs).

Question 9

What is the difference between tripoblastic and diploblastic organisms?

Answer 9

• Triploblastic animals have embryos containing all three germ layers (ectoderm, mesoderm, and endoderm).
• Diploblastic animals have embryos containing only the ectoderm and endoderm, such as sponges and comb jellies.

Question 10

What are the two major cell types in an embryo?

Answer 10

1. Epithelial cells: tightly connected to one another in sheets or tubes.
2. Mesenchymal cells: unconnected to one another and operate as independent units.

Question 11

What changes in the cells brings about morphogenesis? [6]

Answer 11

1. Cell shape changes
2. Direction and number of cell divisions
3. Cell movement
4. Cell growth
5. Cell death
6. Changes in the compostion of the cell membrane or secreted products

Question 12

What is the difference between an analogous and a homologous structure?

Answer 12

Homologous structures are those organs whose underlying similariy arises from them being derived from a common ancestral structure.

Analogous structures are those whose similarity comes from their performing a similar function rather than their arising from a common ancestor.

Question 13

Define malformation.

Answer 13

A malformation is an abnormality caused by genetic events (gene mutations, chromosomal aneuploidies, and translocations), and often appear as syndromes, e.g. piebaldism.

Question 14

Outline the stages in development.

Answer 14

This example uses the leopard frog;

1. Fertilisation
2. Cleavage
3. Gastrulation
4. Organogenesis
5. [Larval Stages]
6. Maturity
7. Gametogenesis