What is development (ontogenesis)?
Ontogenesis is the whole life of one organism, from the zygote to death. It has two big periods:
- Embryonic period: from the zygote until birth or hatching.
- Post-embryonic period: growth, becoming adult, reproduction, ageing and death.
Development needs three things at the same time: cell division (more cells), growth (bigger cells and body) and differentiation (cells becoming special types). A fourth process, morphogenesis, gives the body its shape.
Early embryo: cleavage, blastula and gastrulation
Cleavage: fast mitotic divisions of the zygote. There is no growth between divisions, so the cells (blastomeres) get smaller and smaller. A solid ball of about 16–32 cells is called a morula (it looks like a mulberry).
Blastula: the cells move to the outside and leave a fluid-filled cavity, the blastocoel. In mammals this stage is the blastocyst, which implants in the uterus.
Gastrulation: cells move inwards through an opening (the blastopore). The embryo now has three germ layers:
- Ectoderm (outer): skin, hair, nails, brain, spinal cord, nerves, eye lens.
- Mesoderm (middle): muscles, bones, heart, blood, kidneys, reproductive organs.
- Endoderm (inner): lining of the gut, lungs, liver, pancreas.
Sponges and cnidarians (like jellyfish) have only two layers; most other animals have three.
Making tissues and organs: neurulation, histogenesis, organogenesis
In chordates the next stage is the neurula. Ectoderm on the back folds into a tube: the neural tube, which becomes the brain and spinal cord. Under it a rod, the notochord, forms from mesoderm.
Histogenesis is the making of tissues (like muscle tissue or nerve tissue). Organogenesis is the making of organs from these tissues. In humans most organs start in the first 8 weeks, which is why harmful drugs, alcohol and some infections are most dangerous early in pregnancy.
Differentiation, stem cells, induction and gene control
Almost every cell in your body has the same DNA. A nerve cell and a muscle cell differ because they switch on different genes. This is differentiation, and it is controlled by gene regulation (transcription factors turning genes on and off).
Stem cells are cells that can divide and still become other types:
- Totipotent: can make a whole organism, even the placenta (zygote, first few cells).
- Pluripotent: can make any body cell (inner cell mass of the blastocyst; embryonic stem cells).
- Multipotent: a few related types (blood stem cells in bone marrow).
Embryonic induction: one group of cells sends chemical signals that tell nearby cells what to become. In a classic experiment, a small piece of an amphibian embryo (the organiser) grafted onto another embryo made a second body axis.
Homeotic (Hox) genes are master genes that give each body segment its identity (head, chest, abdomen). A mutation can put a body part in the wrong place, for example legs growing where a fruit fly's antennae should be. Similar Hox genes are found in flies, fish and humans.
After the embryo: growth, metamorphosis, ageing and regeneration
- Direct development: the young looks like a small adult (humans, birds, reptiles).
- Indirect development (metamorphosis): a larva looks very different. Complete metamorphosis: egg → larva → pupa → adult (butterfly). Incomplete: egg → nymph → adult (grasshopper). Frogs change from tadpole to frog.
Life cycles can be simple (one form) or complex, with several forms or hosts. Plants show alternation of generations between a spore-making and a gamete-making form.
Growth is controlled by genes, hormones (growth hormone, thyroid hormones; in insects ecdysone controls moulting) and food. Some animals grow all life (fish); others stop (mammals).
Ageing: cells collect damage to DNA and proteins, chromosome ends (telomeres) shorten with each division, and repair slows. Scientists see ageing as a mix of these causes.
Regeneration: a lizard regrows a tail, a planarian a whole body, and the human liver can regrow lost parts. Medicine uses this: transplantation, stem-cell therapy, tissue engineering and 3D bioprinting of skin or cartilage.
Plant embryos and reproductive medicine
In flowering plants the zygote inside the seed divides to form an embryo with a radicle (future root), plumule (future shoot) and one or two cotyledons (seed leaves). Plant cells keep stem-cell zones called meristems all their life, so plants keep growing.
Reproductive medicine helps when natural conception is hard: IVF (fertilisation outside the body, embryo placed in the uterus), freezing of eggs or embryos, and screening for genetic disease. These raise ethical questions, which we discuss in bioethics.
Try it: the 3D and at home
In the 3D: after 6 divisions, how many cells will there be? Predict, then move the slider. (2⁶ = 64.)
At home: soak a few moong or chana seeds overnight, then open one. Find the radicle, plumule and the two cotyledons. Keep the rest on wet cotton and draw them each day for 5 days: you are watching post-embryonic growth.
Key formulas and definitions
- Cells after n cleavage divisions = 2ⁿ
- Order: zygote → cleavage → morula → blastula → gastrula → neurula → organogenesis
- Ectoderm → skin, nervous system; Mesoderm → muscle, bone, blood, heart, kidney; Endoderm → gut lining, lungs, liver, pancreas
- Totipotent > pluripotent > multipotent (range of cell types a stem cell can make)
- Complete metamorphosis: egg → larva → pupa → adult; incomplete: egg → nymph → adult
Worked examples
1. How many cells are present after 5 cleavage divisions, if every cell divides each time?
2⁵ = 32 cells. Each division doubles the number.
2. A baby is born with a problem in the spinal cord. From which germ layer did this tissue come?
The spinal cord forms from the neural tube, which comes from the ectoderm.
3. Why do a liver cell and a skin cell from the same person look so different?
They have the same DNA but different genes are switched on. The liver cell makes liver proteins; the skin cell makes keratin. This is differentiation by gene regulation.
4. Sort these: butterfly, human, grasshopper, frog into direct or indirect development.
Direct: human. Indirect: butterfly (complete metamorphosis), grasshopper (incomplete), frog (tadpole to adult).
5. In an experiment a piece of tissue from one early embryo is moved to the belly side of another embryo. A second nervous system forms there. What does this show?
Embryonic induction: the transplanted organiser cells sent signals that told the host's ectoderm to become nervous tissue.
Common mistakes
- Thinking the embryo grows during cleavage. The cell number rises but the total size stays about the same.
- Mixing up the germ layers: the brain and nerves come from the ectoderm, not the mesoderm.
- Saying different cell types have different DNA. They share the same DNA; they use different genes.
- Confusing blastula and gastrula. The blastula is a hollow ball; the gastrula has layers formed by inward movement.