Cell specialization

📚The biology

From zygote to specialized cells

After fertilization, the zygote divides repeatedly by mitosis to produce a ball of unspecialized cells. These cells then develop along different pathways to become specialized cells with particular structures and functions. This process is differentiation.

Every cell in the embryo has the same genes. Differentiation happens because different cells express different genes. What tells a cell which genes to express? In the early embryo, signalling chemicals called morphogens are produced in particular places and diffuse outwards, forming concentration gradients. A cell’s position in the gradient determines the concentration it is exposed to, and different concentrations switch on different sets of genes. In this way cells in different parts of the embryo — head or tail, front or back — take on different fates.

Stem cells

Stem cells are defined by two properties:

Stem cell niches in adults

Adults keep populations of stem cells in specific locations called niches. The niche is the microenvironment around the stem cells — neighbouring cells, signalling molecules, extracellular matrix — and it controls their behaviour. It can maintain the stem cells in an undivided, unspecialized state, or promote their proliferation and differentiation when new cells are needed.

• Bone marrow. Haematopoietic stem cells in the marrow of large bones produce all types of blood cell — red blood cells, white blood cells and platelets — replacing the billions lost each day.
• Hair follicles. Stem cells in a region of the follicle called the bulge regenerate the hair follicle during each hair growth cycle and can also help repair the skin after a wound.

Totipotent, pluripotent and multipotent

Cells in early animal embryos are totipotent but soon become pluripotent, and potency narrows further as development proceeds.

Cell size as a specialization

Human cells vary enormously in size, and each size suits the cell’s function:

Surface area to volume ratio

A cell exchanges materials — oxygen in, carbon dioxide and waste out, nutrients in — across its surface. So:

• the rate of exchange depends on surface area;
• the need for exchange depends on volume, since that is how much cytoplasm is carrying out metabolism.

As a cell grows, its surface area and volume do not increase at the same rate. For a cube of side length \( L \):

Doubling the side length multiplies the surface area by 4 but the volume by 8, so the ratio halves. As size increases, surface area to volume ratio decreases. A large cell would have too little surface for its volume: it could not take in materials or remove waste quickly enough, and substances would take too long to diffuse to the centre. That is why most cells are small, and why a growing cell divides.

Cubes are a model. Real cells are not cubes, but the same scale factors apply to any shape: if a shape is scaled up by a factor \( k \), its surface area rises by \( k^{2} \) and its volume by \( k^{3} \). The model is simpler than reality but predicts the same relationship.

Increasing surface area to volume ratio AHL

Cells whose function depends on rapid exchange have adaptations that increase their surface area relative to volume:

• Flattening. Red blood cells are flattened biconcave discs — thin in the middle and thicker at the rim. Compared with a sphere of the same volume, this shape has more surface for oxygen to diffuse across, and no part of the cytoplasm is far from the surface. Lacking a nucleus helps keep them thin.
• Microvilli. Tiny finger-like projections of the plasma membrane greatly increase surface area without much increase in volume.
• Invagination. Infolding of the plasma membrane into the cell also increases surface area.

Proximal convoluted tubule cells in the nephron use both. The surface facing the filtrate is covered in microvilli, forming a brush border that increases the area for reabsorbing glucose, amino acids, ions and water. The opposite surface, facing the blood, has deep invaginations, increasing the area for pumping reabsorbed substances out into tissue fluid. Many mitochondria sit close to these membranes to supply ATP for active transport (D3.3).

Type I and type II pneumocytes AHL

The wall of each alveolus in the lung is lined by an epithelium containing two types of cell, each adapted for a different part of the tissue’s overall function.

Alveolar epithelium is therefore an example of a tissue in which more than one cell type is needed, because no single set of adaptations could achieve both a very thin exchange surface and the secretion of surfactant (B3.1).

Cardiac muscle and striated muscle fibres AHL

Both types of muscle contain contractile myofibrils with repeating sarcomeres of actin and myosin filaments, which give them their striped (striated) appearance (B3.3). They differ in structure:

Hypotheses for these differences relate to function. Branching lets cardiac cells form a network through which the electrical signal spreads rapidly, so the heart wall contracts in a coordinated wave. A skeletal muscle fibre must contract along the whole length of a muscle at once, so a single long unit is effective, and many nuclei are needed to supply enough mRNA for proteins throughout such a large volume of cytoplasm.

Is a striated muscle fibre a cell? It is surrounded by a single plasma membrane and functions as one unit, which suggests yes. But it has many nuclei and forms by the fusion of many cells, and it is far larger than a typical cell, which suggests it does not fit the usual definition. It is best described as an atypical cell, or a syncytium — a multinucleate mass of cytoplasm formed by cell fusion.

Sperm and egg cells AHL

The contrast in size between the gametes follows from their roles: the sperm must travel, so it is small and mobile; the egg does not move and provides the resources, so it is large (D3.1).