An adult human body contains roughly 30–37 trillion cells (≈84 % of them erythrocytes by number) belonging to a few hundred morphologically distinct cell types. Despite their diversity — a 7.5 µm biconcave erythrocyte, a 120 µm oocyte, a neuron with an axon a metre long — almost all share the same eukaryotic plan: a plasma membrane enclosing cytoplasm (cytosol plus organelles) and a nucleus that holds the genome. Differences between cell types arise not from different DNA but from differential gene expression.
Eukaryotic cells are compartmentalised. Membrane-bound organelles create chemically distinct spaces: the nucleus separates transcription from translation; the endoplasmic reticulum and Golgi apparatus form the secretory (biosynthetic) pathway that makes, modifies and sorts membrane and secreted proteins; endosomes and lysosomes form the endocytic/degradative pathway; mitochondria generate most ATP by oxidative phosphorylation; peroxisomes handle very-long-chain fatty-acid oxidation and H₂O₂. A protein cytoskeleton (microfilaments, intermediate filaments, microtubules) gives shape, resists tension, powers movement and acts as a railway for motor proteins.
The plasma membrane is a selectively permeable fluid mosaic of phospholipids, cholesterol and proteins. Ion pumps — above all the Na⁺/K⁺-ATPase — build concentration gradients that drive secondary active transport, set the resting membrane potential and control cell volume. Receptors in the membrane (and inside the cell for lipid-soluble hormones) let cells respond to hormones, neurotransmitters and growth factors through second-messenger cascades.
Information flows from DNA → RNA → protein (the central dogma): DNA is replicated in S phase, transcribed in the nucleus into pre-mRNA that is capped, spliced and polyadenylated, exported through nuclear pores and translated by ribosomes. Cell number is controlled by the cell cycle (G1–S–G2–M) with checkpoints policed by cyclin–CDK complexes, p53 and Rb, and by programmed cell death (apoptosis). Loss of these controls is the cellular basis of cancer.
Cells assemble into tissues — groups of similar cells and their extracellular matrix performing a common function. Epithelium covers surfaces and forms glands; connective tissue binds, supports and protects (including cartilage, bone, adipose tissue and blood); muscle tissue contracts; nervous tissue conducts signals. Organs combine all four, typically as a functional parenchyma supported by a connective-tissue stroma.
Development
After fertilisation the zygote is totipotent. Cleavage produces a morula and then a blastocyst whose inner cell mass (embryoblast) is pluripotent and whose outer trophoblast forms the fetal part of the placenta. During gastrulation (week 3) the epiblast generates the three germ layers. Ectoderm gives rise to the epidermis and its appendages, the nervous system (neural tube) and neural crest; mesoderm to muscle, most connective tissues (via mesenchyme), cartilage, bone, blood, the cardiovascular system, kidneys, gonads and the mesothelium; endoderm to the epithelial lining of the gut and respiratory tract and the parenchyma of the liver, pancreas, thyroid and other derived glands.
Epithelia therefore arise from all three germ layers, whereas connective tissue and most muscle derive from mesoderm (head connective tissue largely from neural-crest "ectomesenchyme"). Differentiation proceeds by progressive restriction of potency under the control of morphogens (Wnt, Hedgehog, BMP/TGF-β, FGF, retinoic acid, Notch) and master transcription factors, stabilised by epigenetic marks. Apoptosis sculpts the embryo — for example, removing the webs between the digits.