Name the organ where growth hormone is produced in humans

 GROWTH

Growth is one of the basic characteristics of life. All organisms grow from a young stage to an adult stage. Growth is defined as an irreversible increase in the mass or overall size of a tissue or an organism. The increase in body size of a growing organism is associated with some concurrent changes in their cells, tissues and organs. Growth is accomplished by three processes namely:

 (a) cell proliferation or cell multiplication as in lens cells, 

(b) cell enlargement as in cardiac cells, muscle cells and neurons and 

(c) by accretion or by the secretion of a large amount of extracellular materials.

Bone and cartilage cells grow by the secretion of the extracellular matrix. Generally, growth involves the synthesis of protoplasm (cytoplasm and nucleus) or apoplasmic material (substances present outside the cell like a matrix of bone marrow or fibres of connective tissues or even water). Growth occurs when anabolism (synthetic activities) of cells exceeds their catabolism (destructive activities). There will be no growth when anabolism equals catabolism. Prolonged starvation may lead to the increased catabolism of reserve food (such as fat in the adipose tissue). In extreme cases when the reserve fat is exhausted, energy is produced at the expense of the protein of the protoplasm. This leads to a decrease in the mass of living matter which is called degrowth.

Types of Growth :

There are three basic ways in which the growth of a multicellular animal is achieved through the growth of its cells. These are 

(i) auxetic growth,

(ii) multiplicative growth and 

(iii) accretionary growth.

1. Auxetic Growth: In auxetic growth the volume of the animal's body increases due to the growth of the individual cells without an increase in their numbers. Thus there is no division or proliferation of cells. This is found in nematodes, rotiters and larvaceae among the tunicates.

2. Multiplicative growth : The growth of the animal results due to an increase in the number of its constituent cells. The increase in number is brought about by the mitotic division of all the cells, while the average size of the cell remains the same or nearly so. This type of growth is found quite commonly in the embryos and it is especially characteristic of the prenatal growth of the higher vertebrates including humans.

3. Accretionary growth: In this type, the growth of the animal is due to a greater or lesser degree of the activity of the special cells, which retain their ability to divide mitotically, while other cells have lost this ability more or less completely and can not proliferate. These later cells are the differentiated cells of the body performing various physiological functions necessary for the animal. The former cells are called reserve cells because they supply new cells capable of reinforcing and replacing the functional differentiated cells. For example, the erythrocytes are differentiated non-dividing cells and the proliferating erythopoietic tissues constantly supply new erythrocytes. Secretion of extracellular matrix by chondrocytes and bone osteocytes which do not divide are other examples of accretionary growth.

Growth Rate :

All higher animals grow at a specific rate and rhythm. But the rate of growth is not uniform. Animals grow at different rates during different periods of their life. Human growth provides a good model for the study of growth rate. The human embryo is about 150 um at the time of implantation in the uterus. It grows to about 50 cm during the gestation period covering nine months. During the first two months after implantation, the basic human body plan is laid down, but the embryo does not show considerable growth. Starting from nearly the fourth month of implantation, the embryo grows at the rate of 10 cm per month. During the first year after birth, growth occurs at a rate of 2 cm per month. Then there is a steady decline in growth rate. After that, the growth rate shoots up at puberty. After puberty, the physical growth of the human body starts declining. It almost stops after 20-30 years of age.

The growth rates of different body parts such as the head, neck, thorax, limbs, endoskeletons and internal organs are different. This can be observed by examining a human child or its photographs from birth to adulthood. Such an examination gives a clear indication of the external appearance, shapes and sizes of morphological parts of the body (Fig 10.3). The head is proportionately much larger in early development than in adults. This forms about half the length of the body in a 2-month-old foetus. The arms attain their proportionate size shortly after birth but the legs attain their proportionate size only after 10 years of age. Reproductive organs do not grow rapidly until 12-14 years after the birth of a child. These organs grow very slowly until the age of 12 years. After this, they grow rapidly at puberty. The maximum rate of linear growth occurs before birth, at the 4th month of foetal life. A final growth maximum reaches its peak at an age of 12 in girls and about 14 in boys. The brain and spinal cord grow rapidly in early childhood and nearly reach their adult size by the age of 9. The growth curve of the skeleton follows that of the body as a whole. Lymphoid tissue including the thymus reaches its maximum at the age of 12. It attains adult values at the age of 20.

Hormonal Control of Growth :

Growth factors play a significant role in the embryonic growth of an individual. Insulin-like growth factors (IGFs) play an important part in both the embryonic and post-embryonic growth of mammals. Growth hormone (GH) produced in the anterior lobe of the pituitary gland is important for normal embryonic growth and the post-embryonic growth of humans as well as other mammals. Production of growth hormone starts within the first year after birth. The hypothalamus produces and releases somatotropin releasing hormone (SRH) and somatostatin (growth-inhibiting hormone or GIH). SRH promotes growth hormone synthesis while GIH inhibits the production of growth hormones. The growth hormone also controls its release through negative feedback signals to the hypothalamus. It works by stimulating the production of IGH, which is mainly released from the liver and promotes the synthesis of growth hormones. The Thymosin hormone secreted by the thymus gland also controls the growth between 10-13 years of age. The Thyroxin hormone secreted by the thyroid gland promotes metabolism and therefore, this hormone also influences growth.   Sex hormones (testosterone in males and estrogen and progesterone in females) are considered to control growth as well as secondary sexual characters. It has been established that cells must receive signals from growth factors for their division and survival.