Contents
Learning Objectives
By the end of this section you should be able to:
- Describe how animals develop from fertilization to full grown offsprings.
- Differentiate between growth and development.
- Explain the factors that affect growth and development in animals.
Introduction
In our previous blog post we discussed the male and female reproductive systems. Here, we shall discuss how an embryo is formed, grows and develops.
Growth and Development
Growth is the physical increase in size, mass and weight of an organism. Growth involves structural changes in an animal's body.
Development on the other hand involves the changes in overall qualities, functionalities or physiology of an organism. Many people often use growth and development interchangeably, but these are two different terms. Growth involves quantitative structural changes, such as the changes in the skeletal system, muscles, height, etc; while Development involves qualitative functional changes. Growth can stop at a certain age, but development continues until death; this means that there can be development without growth.
However, both growth and development highly depend on genetic, nutritional, and environmental factors.
Stages of Growth and Development in animals are divided into two:
- Pre-natal stage - stage before birth or hatching.
- Post-natal stage - stage after birth of hatching.
Before we discuss these, how did we arrive at the pre-natal stage of growth and development?
We arrive at the pre-natal stage of an offspring's development when a male gamete (sperm) has fused with a female gamete (egg) to give rise to a zygote. This process is called fertilization.
Processes of fertilization
We have previously discussed that sperm must undergo activation (known as Capacitation) before it can fertilise an egg. The first stage of sperm capacitation that we earlier discussed, was only to make sperm motile (able to move); Now, sperm must undergo a second stage of Capacitation, which gives sperm the ability to fertilize an egg; and this process takes place in the female reproductive tract.
In the female reproductive tract, seminal fluids (in the semen) become dysfunctional, and then sperm must swim through the vagina, into the cervix.
The cervical canal is a much more welcoming environment and sperm that make it there find themselves awash in a sea of cervical mucus. The mucus is especially designed to transport sperm efficiently during fertility stage of the female animal.
Timing is also important. Sperm must reach their destination within the right frame for when the egg is there.
Another thing is destination. In mammals, there are two branched uterine (or fallopian) tubes on either sides of the uterus, and egg can be present in only one in any given month. Pick the wrong tube, and the sperm end up hanging out, partying together with no guest of honor in sight.
PRO TIP: egg has 2 layers that sperm need to get through – The outer Corona Radiata which is an irregular surface, and then the Zona Pellucida which is a transparent but thicker layer.
Sperm that picked the correct tube attach to the egg, to begin penetration. You should know that it is not the first sperm that reaches an egg that eventually fertilizes it. Rather hundreds of sperm must undergo acrosomal reaction.
Acrosomal reaction involves the enzymatic burrowing of the zona pellucida by sperm, using the zona-digesting enzymes that are available in its acrosome.
Acrosome (of sperm) is derived from the golgi body of the Spermatid; these specialize to form a structure that cover the top half of the sperm head (acrosome).
During enzymatic burrowing process, acrosomal contents (hydrolytic enzymes) are getting used up, and by the time the sperm successfully gets through the zona pellucida, there's usually no more enzymes, and only the nuclear contents of the sperm is exposed. The nuclear contents (which are chromosomes) are then released into the egg cell. Upon this action, the egg becomes activated.
Recall from the previous blog post, we discussed that before Fertilisation, an oocyte (egg) is arrested in metaphase 2 of its second meiotic division. When fertilization occurs and egg becomes activated, it completes its second meiotic division, and also secretes enzymes that prevent other sperm from entering (=prevention of polyspermy). These processes are called Egg Activation or Zona Reaction.
PRO TIP: When a sperm cell penetrates and fertilizes an egg cell, their genetic information combine. A sperm cell contains half number of chromosomes, which merge together with the half number of chromosomes present in the egg cell. The egg cell completes Meiosis II to yield a fertilised egg and a polar body. The polar body ends up donating its cytoplasm to the fertilised egg.
After fertilization, the zygote that is formed will begin its prenatal stage of growth and development.
Pre-Natal stage of growth and development
Pre-Natal stage of growth and development takes place in two phases:
- Embryogenesis - formation of embryo.
- Organogenesis - formation, growth, and development of organs.
Embryogenesis
Embryogenesis starts immediately after fertilization.
The zygote that resulted from fertilization begins to divide by mitosis.
It first divides into two cells, then each of these two cells divide to form another two cells, making a total of four cells, these four cells divide again to form a total of 8 cells, then again all eight cells divide to give rise to 16 cells.
The zygote at this stage is referred to as a Morula (Latin name for mulberry, because it looks like one) and each of the individual small cells is called a blastomere. This process of division is known as cleavage.
You should know that as these cells are dividing, they do not separate, and the bigger mass containing all the dividing cells is the same size as the original (first) cell. This process is likened to dividing an orange into two halves, then into four, then eight, then 16... The resultant is still the same size as the initial orange. I believe you got that.
During cleavage process, cells continue to divide, hence they become smaller and smaller and smaller until they reach the size of the normal adult animal's cells. By the time the cells have finally become microscopic (the size of an adult animal's cell) they have formed a hollow sphere (or ball) that has fluid inside. The fluid inside is called blastocoel. The embryo at this stage is referred to as a Blastula (or blastocyst in some other mammals).
Now, Blastula must undergo a process known as gastrulation to become a gastrula (processes to be explained in higher classes).
Gastrulation forms the ectoderm (that will form the outer skin and nervous system), mesoderm (that will form the muscular structures, bones, vascular system and connective tissues) and Endoderm (that will form the organs of the digestive system and epithelium of several tracts like digestive and respiratory tracts etc).
PRO TIP: Early in development at the time of gastrulation a small group of cells are "put aside" to later form oocytes (eggs) and sperm. These cells are described as primordial germ cells (PGCs) and are a type of stem cell.
Organogenesis
Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth or hatch.
During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal organs of the organism.
The cells of each of the three germ layers undergo differentiation (a process where less-specialized cells become more-specialized through the expression of a specific set of genes).
PRO TIP: Neurulation initiates the process of organogenesis, where the brain, nerves and spinal cord (in chordates) are formed from the ectoderm, and an embryo at this stage is called a neurula.
When all organs have finally been formed, embryo is referred to as a foetus (or fetus). Foetus then continues to grow until its birth/hatch.
Post-natal stage of growth and development
The period of post-natal growth and development starts at birth or hatch and continues until death. Muscle, tissue and fat are the main types of tissues which undergo changes that lead to growth of an animal.
Development occur especially during puberty and maturity.
Factors that affect growth and development in animals
Nutrition
Animals need food to survive, function and grow. Nutrition of the embryo or foetus is provided by the mother or yolk in the egg. Post natal nutrition must be adequate for normal growth of the animal. Poor nutrition is caused by undernourishment and its effect depend on three factors which are:
1. Nutrient deficiency that the animal is subjected to: lack of nutrients in the diet of animals lead to poor nutrition and redundant physical growth.
2. The length of time the animal was subjected to poor nutrition: the longer the length of poor nutrition, the worse its effect.
3. The age at which poor nutrition occur: poor nutrition at a younger age is a bit more easier to correct than poor nutrition at an older age. However, younger ones that are subjected to poor nutrition may not be able to withstand and may eventually die off.
Consequences of poor nutrition in animals
- Stunted or irregular growth.
- Malformed organs.
- Diseases.
- Soft bone or brittle skeletons.
- Poor immune system.
- Poor reproductive performance.
- Poor agility and inactive social engagement.
- Death.
Maintenance of Homeostasis
This is concerned with how an animal is able to maintain its internal environment (within its body). This is mostly involuntary and is controlled by nervous and endocrine systems. Normal growth patterns are affected if homeostasis is not maintained at all times.
Hormonal regulation
Several hormones that are secreted by the endocrine system play major roles in affecting homeostasis, growth and secondary development in animals. Hormones like somatotropin and somatostatin control the growth of muscles and bone tissue and can lead to disorder if hyper (over) secreted or hypo (under) secreted. Other hormones like testosterone, oestrogen and progesterone play major functions in developing secondary sexual characteristics displayed, in puberty and maturity. Read more on hormones and how they regulate the body in Endocrine system.
Immune Responses
Accumulation of bacteria and other pathogens in the body are fought against by the immune system. Poor immune response will lead to pathological dominance and the bad ones among them will negatively affect growth. Diseases will come in as a result which may make the animal regress in its growth/development.
Metabolism of the Animal
The main purposes of metabolism are:
- conversion of the energy in food to energy available to run cellular processes.
- conversion of food to body building blocks.
Metabolism is involuntary and is run by the autonomic Nervous system; while the Rate of metabolism (= how fast or how slow metabolism occurs) is affected by thyroxine from the thyroid glands, amongst other factors.
Metabolism directly affects the growth and development of an animal.
Gene
Genes play major roles in the physical appearance of an animal. The growth and development of an animal is directly and principally governed by the type of genes that an offspring inherits from its parents. Dominant genes determine how the animal will look/appear.
Environment
Environment also play significant roles in the growth and development of an animal. Animals in conducive environment generally tend to grow faster and better than those in poor environment.
Aging in Animals
Aging refers to a series of changes that lead to physical deterioration and final death in animals.
An age is reached in which each species reaches the peak of its productive life.
Most physiological functions of animals deteriorate as animals get older after maturity.
The reproductive organs secrete lower level of hormones.
Muscular strength and agility declines.
Collagens or proteins in the skin and blood vessels become less elastic with age, thus wrinkles form and blood vessels collapse or burst.
Reproductive and lactating abilities of females decrease with age, lowering their productivity.
Many factors including genetics and environment affect the lifespan of animals.
Longevity of an animal is an inheritable trait and it can be estimated by knowing the lifespan of the animal's parents or siblings.
Inadequate or excessive nutrition also hastens the aging process.
Lifespan is decreased if an animal is required to reproduce at higher than normal levels during a given period of time.
Higher (than normal) environmental temperatures shorten lifespan.
Females most times outlive males which make people deduce that sex may also be involved in longevity.
Key Terms
Pre-Natal • Post-Natal • Zygote • Fertilization • Capacitation • Acrosomal Reaction • Egg activation • Zona reaction • Embryogenesis • Organogenesis • Cleavage • Morula • Blastomere • Blastocoel • Blastula • Embryo • Gastrulation • Gastrula • Neurulation • Neurula • Foetus
Watch these videos for better understanding
Test Questions
TEST QUESTIONS: GROWTH AND DEVELOPMENT
please fill above required data
TEST QUESTIONS: GROWTH AND DEVELOPMENT
Total Questions: 20
you'll have 28 second to answer each question.
Quiz Result
Total Questions:
Attempt:
Correct:
Wrong:
Percentage:
Quiz Answers
1. Fusion of two haploid cells to give rise to a diploid cell is termed...
Fertilisation
2. Sperm must penetrate a thick membrane in the egg before it can fertilise the egg. The thick membrane is known as...
Zona pellucida
3. _ contain zona-digesting enzymes that is used to burrow the zona pellucida
Acrosome
4. Egg cells divide after fertilization by a process called?
Cleavage
5. The 16-32 celled stage of a Mammalian embryo is called...
Morula
6. The fluid inside a Blastula is called?
Blastocoel
7. Embryo development follow which of these trends?
Zygote > Morula > Blastula > Gastrula
8. Ectoderm of a gastrula will form...
Epidermis of skin and nervous system
9. Mesoderm of a gastrula will form...
Muscles, bones, heart and connective tissue
10. Endoderm of a gastrula will form...
Digestive organs and internal tracts
11. Poor nutrition does not affect growth and development of an animal
False
12. An animal becomes more efficient as it ages after maturity
False
13. All animals' lifespan are approximately the same
False
14. The first sperm to reach an egg is the one to fertilise it
False
15. An embryo that has undergone Organogenesis is now called?
Foetus
16. Cleavage does not occur in the eggs of oviparous animals
False
17. The individual cells in a morula or Blastula are called
Blastomeres
18. One of these is not a factor affecting growth and development of animals
Blood group
19. Dwarf animals exhibit limited growth due to which of these factors
Hormonal regulation
20. Environment + Genotype make up _ of an animal
Phenotype
