In bacteria, the single cell itself functions as an organism performing all the functions like feeding, movement, reproduction etc. But think of a blue whale or ourselves, trillions of cells are present, all interacting with each other harmoniously to keep us alive. This journey towards this wonderful, giant structure had a humble beginning, which is from single cell or what we call, the zygote. In multi-celled organism, each task of life is carried out by different types of cells say nerve cell, cardiac cell, muscle cell, epithelial cells all differ in their appearance and function, but originated from the same parental cell, the zygote. These specialized cells are formed by a process called differentiation.
Definition: Cell differentiation?
Cell differentiation is the process by which genetically identical cells of an embryo become specialized or the process by which stable differences arise between cells of the embryo.
Let us have a look at human ontogeny.
From a fertilized egg, at least 250 distinct types of differentiated cells are present in adults. Is it amazing? Let us have a closer look.
Day 1: Zygote or fertilised egg cleaves into two.
Day 2: Then cleaves to 4
Day 4: Morula stage 16-32 cell stage
Day 5: Blastula stage with inner cell mass
Day 7-10: Gastrulation. The formation of three germinal layers. Ectoderm, endoderm and mesoderm
Ectoderm: cells differentiate to form neurons, skin cells etc.
Endoderm: internal organs
Mesoderm: bone, heart and muscles
Week 3-8: Development of organs from these specialised cells
By 9th week, development is complete except brain and lungs. Then the foetus increase in size by cell division and this process continues ~280 days. That is why our grandmothers often insist to take extra care during first three months of pregnancy.
Do you know the difference between cleavage and division?
After fertilization, certain group of cells in the embryo are determined or fated to become a particular organ or tissue. The process is called determination. The ‘fated’ cell finally differentiate to a fully functional state like nerve cell, muscle cell, fibroblast etc.
Characteristics of cell differentiation:
- Cell differentiation is a stable phenomenon. Take the example of neurons, once differentiated persist throughout lifetime.
- Differentiation is induced by specific stimuli.
- But once differentiated or established, these cells are stimulus independent.
- Differentiation is accomplished by morphological difference of that fated or determined cell. Look at the figure different shapes or morphology of various types of cells in our body.
- Ultimately differentiation is controlled by genetic factors.
Some important terms related to the topic
- Ontogeny refers to the development of an individual from zygote to maturity.
- Embryology is the study of early ontogenic levels.
- Epigenesis refers to the development of differentiated cells, tissues and organs that occurs by cells acquiring new structures and function while increasing in size and complexity.
- Cleavage and division: cleavage refers to series of mitotic division that increase the number of cells but does not change the size of the original mass. See the figure
- Division generally refers to mitotic division that result in the formation of two identical cells.
- Homeodomain: is a conserved protein encoding segment of 180 nucleotides present in Drosophila. The genes in this region encode transcription factors involved in controlling the early differentiation of embryo.
- Homeobox: a conserved DNA sequence found within the genes involved in the regulation of morphogenesis (pattern of anatomical development) in plants, animals and fungi. Homeobox genes encode transcription factors that switch on cascades of other genes involved in morphogenesis.
- Totipotent : capable of forming all differentiated cells of adult, example: embryonic stem cell
- Pluripotent : Capable of forming more than one differentiated cell type