The relationship of growth of cells and growth of the body is complex. Growth of cells in tissue culture (in vitro) with appropriate medium is exponential like that of unicellular organism, but within the body (in vivo) it is not exponential.
Pattern of growth in a multicellular animal is of 4 types :
Auxetic growth : Body size increases due to increase in the size of cells and not their number, a rare type of growth, found only in nematodes, rotifers and tunicates.
Multiplicative growth: Body grows by increase in the number of cells mitotically. Size of cells remains same; a characteristic of prenatal and postnatal growth in many higher animals.
Accretionary growth : In post embryonic stage most cells become differentiated and do not contribute to the growth, repair or reformation. But, some cells among these remain undifferentiated to reinforce or replace the worn out cells by differentiating into that particular cell type.
Appositional growth : In some tissues growth occurs by adding new cell layer successively, increase of bones and cartilage thickness or birth. Eg. RBC production by bone marrow.