LECTURE                                              DEVELOPMENTAL BIOLOGY

In this lecture you will learn about:

1. All life starts as a single cell.

2. Each cell has a GENOME that directs cell division.

3. The genome has different groups of genes that turn on and off in a specific sequence to produce that species.

4. So one cell continues to divide in pre-programmed stages until it reaches reproductive MATURITY.

5. Eggs come in ALL sizes.

DIFFERENTIATION: "the process by which a less specialized cell becomes a more specialized cell type."
Without differentitation cells would divide and just end up as a clump of cells.  Instead, as cells divide they get different sets of instructions.  This occurs very early in cell division.

It starts with the single cell dividing.
early cleavage click here
and here

Division stops when a pre-determined number of cells are reached.

At a certain point during the early cell divisions genes are turned on in certain cells, but not in others, while different genes turn on in other cells.

These cells are now destined to become different from each other that will each serve different purposes.

As groups of cells differentiate and undergo cell division they MIGRATE (move) to a different areas of the embryo. 
Gastrulation click here
Sea urchin click here

As they continue to differentiate they begin to now "LOOK" different and form
i. Structures like the backbone,
ii. Organs like liver,
iii. Eye sockets and eyes,
iv. The mouth, etc. ...
g. All of this occurs in a PRECISE SEQUENCE.

Complete Xenopus development 
click here

As additional groups of genes are turned on and off and further differentiation occurs, the morphology is changed yet again, cells and structures molded by the genetic program until the stage at which the species of the embryo is clearly set.

Even after external morphology is set, further differentiation occurs internally.


The DNA in a fertilized egg is like the conductors score seen at right.  It has all the instructions for all the parts of a symphony. But after the first few "equal" divisions of the egg the cells begin to play only "their" parts, they specialize and only play a specific part, sometimes playing sometimes going quiet.  Each cell has all the DNA but specialization turns off the "conductors" score and turns on the individual parts. 

And like any well played symphony each of the parts must be played at exactly the right time and must be played correctly or the whole can go horribly wrong. 

Example: Simple differentiation of mutlicellular organism

"Volvox is one of the best-known chlorophytes and is the most developed in a series of genera that form spherical colonies.[1] Each mature Volvox colony is composed of numerous flagellate cells similar to Chlamydomonas, up to 50,000 in total[2], and embedded in the surface of a hollow sphere or coenobium containing an extracellular matrix[2] made of a gelatinous glycoprotein.[3] The cells swim in a coordinated fashion, with distinct anterior and posterior poles. The cells have eyespots, more developed near the anterior, which enable the colony to swim towards light. The individual algae in some species are interconnected by thin strands of cytoplasm, called protoplasmates.[4]

An asexual colony includes both somatic (vegetative) cells, which do not reproduce, and gonidia near the posterior, which produce new colonies through repeated division. The daughter colonies are initially held within the parent coenobium and have their flagella directed inwards. Later, the parent disintegrates and the daughters invert. In sexual reproduction two types of gametes are produced. Volvox species can be monoecious or dioecious. Male colonies release numerous microgametes, or sperm, while in female colonies single cells enlarge to become oogametes, or eggs.[1][5]"

Stages of development
Development is not done at birth. A newborn may or may not "look" like a little version of the adult. 

I. Egg to birth
II. Birth to puberty
III. Puberty to death

I. Egg to birth
- the changes from a single egg to birth are profound. 

II. Birth to puberty
- the young grow larger and at the end assume the physical morphology of the adult of the species

- At PUBERTY the last set of genes kicks in to complete the changes resulting in reproductive maturity.

1. Sex organs
2. Hormones

1. Breast development,
2. Menses (Menstrual cycle),
3. Hair growth,
4. Deepening of the voice, etc.

III. Puberty to death
- The changes are subtle with increasing loss of funtion.  Only a few species live beyond their ability to reproduce.  Humans and elephants are two of them. 


I. An egg DEVELOPS and hatches into a caterpillar
II. It eats until it gets big enough, then it makes a cocoon and pupates
- In the cocoon the cells once again begin differentiating and completely changes structures.
III. It turns emerges from the pupa as a butterfly or moth and is sexually mature and can reproduce

From egg to butterfly click here 
and here



i. An egg develops, and hatches into a tadpole
ii. Tadpole eats until it has stored enough energy and is large enough
iii. Then its body undergoes metamorphosis into a frog/toad.
- Tadpole doesn’t hibernate or stop eating during this period, but it loses its gills and the tail is gradually reabsorbed into the body. When metamorphosis is complete it is sexually mature.   

click here to see metamorphosis


1. Of the 5,000 best-known human genes 75 percent have matches in the C. Elegans, a tiny worm.

2. It has only 959 cells yet reproduces, grows into a mature adult, eats, excretes and dies with many of the cellular interactions of other animals.

3. The genes that are shared are “core” genes like those in mitochondria that make ATP, Cytochrome C, Ubiquinone, ATPase, etc.

4. The genes that direct DNA synthesis, DNA replication, cell division, all the “housekeeping” duties that EVERY cell has to perform.

5. What is most interesting is APOPTOSIS or programmed cell death is found in C. Elegans.  In other words, some cells are genetically programmed to die off.