Developmental Biology and Genetic Disorders

Cellular Development Stages

The early stages of development occur at the cellular, tissue, and organ levels of organization. At the cellular level, development includes cleavage, resulting in a multicellular embryo and the formation of a blastula.

Cleavage, which encompasses DNA replication and mitotic stages, is cell division without growth and results in a morula, a ball of cells. The next cellular stage is the formation of a blastula, a hollow ball of cells with a fluid-filled cavity called a blastocoel. The blastocoel forms when the cells of the morula pump Na+ into extracellular spaces, and water follows by osmosis.

Tissue Development Stages

The tissue stages of development are the early gastrula and the neurula. The early gastrula stage begins when certain cells invaginate into the blastocoel, creating a double layer of cells that will later develop into organs.

• The outer layer of cells is the ectoderm.
• The inner layer of cells is the endoderm.

The endoderm borders the gut, called the archenteron or primitive gut. The pore created by invagination is the blastopore, which will later become the anus.

Neurula Formation

The next stage is the neurula, which includes a middle layer of cells, the mesoderm. The formation of the mesoderm begins as outpocketings from the archenteron. These outpocketings grow until they meet and fuse, forming two layers of mesoderm. The space between them is the coelom, which will later become the thoracic and abdominal cavities.

Embryonic Germ Layers

Ectoderm, mesoderm, and endoderm are called the embryonic germ layers.

Genetic Disorders

Autosomal Recessive Disorders

Inheritance of two recessive alleles is required for an autosomal recessive disorder to appear. Tay-Sachs disease results from a lack of the enzyme Hex A and the subsequent storage of its substrate in lysosomes. Brain cells are the primary storage sites, leading to the onset of symptoms and progressive deterioration of psychomotor functions.

Tay-Sachs Symptoms

Symptoms are apparent between 4-8 months of age. The child gradually becomes blind, develops seizures, and eventually becomes paralyzed.

Autosomal Dominant Disorders

Inheritance of one dominant allele is required for an autosomal dominant disorder to appear. Marfan syndrome is caused by a defect in fibrillin, an elastic connective tissue protein abundant in the lens of the eye, bones of the limbs, fingers, and the aorta wall.

Marfan Syndrome Characteristics

• Dislocated lens
• Long limbs and fingers
• Caved-in chest

Affected individuals should avoid overexertion due to a weak aorta wall.

Cancer Cells

Cancer cells are genetically unstable, do not regulate the cell cycle correctly, escape apoptosis, cause angiogenesis, and can metastasize. Many mutations (e.g., p53 gene mutation) and chromosomal aberrations are present.

Carcinogenesis

The loss of tumor suppressor gene activity and the gain of oncogene activity result in carcinogenesis. Cancer cells that do not exhibit contact inhibition form benign tumors. Malignant tumors invade surrounding tissue.