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- In human embryology, weeks 6 through 8 are characterized by the growth and differentiation of tissues into organs. This process is known as organogenesis and occurs from weeks 3 through 8, the embryonic period. During week 3, gastrulation occurs, establishing 3 distinct cell layers: the mesoderm, endoderm, and ectoderm.
www.ncbi.nlm.nih.gov/books/NBK563181/
Human organs and organ systems develop in a process known as organogenesis. This begins in the third week of embryonic development, when the gastrula forms three distinct germ layers , the ectoderm , mesoderm and endoderm .
Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development.
- Overview
- Basic form and development
- Effects of aging
- Change incident to environmental factors
1.the outer enclosing epidermal membrane (in the embryo called ectoderm)
2.the dorsal neural tube
3.the supporting notochord
4.the ventral alimentary tube, which becomes the lining of the stomach and intestine (in the embryo called endoderm)
5.the intermediate mass (in the embryo called mesoderm)
6.a rather fluid tissue that fills the interspaces, derived from the mesoderm and in the embryo called mesenchyme
1.the outer enclosing epidermal membrane (in the embryo called ectoderm)
2.the dorsal neural tube
3.the supporting notochord
4.the ventral alimentary tube, which becomes the lining of the stomach and intestine (in the embryo called endoderm)
5.the intermediate mass (in the embryo called mesoderm)
6.a rather fluid tissue that fills the interspaces, derived from the mesoderm and in the embryo called mesenchyme
As the human body ages it undergoes various changes, which are experienced at different times and at varying rates among individuals.
The skin is one of the most accurate registers of aging. It becomes thin and dry and loses elasticity. Patches of darker pigmentation appear, commonly called liver spots, though they have no relation to that organ. Hair grays and thins. Wounds take longer to heal; some reparations take five times as long at 60 as at 10 years of age. Sensory fibres in spinal nerves become fewer; the ganglion cells become pigmented and some of them die. In the auditory apparatus some nerve cells and fibres are lost, and the ability to hear high notes diminishes. In the eye the lens loses its elasticity.
Organs such as the liver and kidneys lose mass with age and decline in efficiency. The brain is somewhat smaller after the age of 40 and shrinks markedly after age 75, especially in the frontal and occipital lobes. This shrinkage is not, however, correlated with declines in mental capacity. Intellectual declines in the elderly are the consequence of underlying disease conditions, such as Alzheimer disease or cerebrovascular disease.
The bones become lighter and more brittle because of a loss of calcium. This loss in bone mass is greater in women than men after the fifth decade. In joints the cartilage covering the ends of bone becomes thinner and sometimes disappears in spots, so bone meets bone directly and the old joints creak. Compression of the spinal column can lead to a loss of height. Muscular strength decreases but with marked individual variability.
The arteries become fibrous and sclerosed. Because of decreasing elasticity, they tend to become rigid tubes. Fatty spots, which appear in their lining even in youth, are always present in old age.
In vitro experiments indicate that the body’s cells are programmed to undergo a finite number of divisions, after which time they lose their reproductive capacity. Thus, the potential longevity of the human body—about 100 years—seems to be encoded within the very cells of the body.
Although the basic form of the human body was established in human anthropoid ancestors, evolutionary adaptations to different environments are apparent among various human populations. For example, physical adaptations in humans are seen in response to extreme cold, humid heat, and high altitudes.
Extreme cold favours short, round persons with short arms and legs, flat faces with fat pads over the sinuses, narrow noses, and a heavier than average layer of body fat. These adaptations provide minimum surface area in relation to body mass for minimum heat loss, minimum heat loss in the extremities (which allows manual dexterity during exposure to cold and guards against frostbite), and protection of the lungs and base of the brain against cold air in the nasal passages.
In hot climates the problem is not in maintaining body heat but in dissipating it. Ordinarily the body rids itself of excess heat by sweating. In conditions of humid heat, however, the humidity of the surrounding air prevents the evaporation of perspiration to some extent, and overheating may result. Hence, the heat-adapted person in humid climates is characteristically tall and thin, so that there is maximum surface area for heat radiation. The person living in hot climates has little body fat; often a wide nose, since warming of the air in the nasal passages is not desirable; and, usually, dark skin, which provides a shield from harmful solar radiation.
High altitudes demand a degree of cold adaptation, as well as adaptation for low air pressure and the consequent low oxygen. This adaptation is accomplished by an increase in lung tissue generally.
- The Editors of Encyclopaedia Britannica
Nov 8, 2024 · human development, the process of growth and change that takes place between birth and maturity. Human growth is far from being a simple and uniform process of becoming taller or larger. As a child gets bigger, there are changes in shape and in tissue composition and distribution.
Jul 17, 2024 · The reproductive system is a collection of internal and external sex organs which work together for the purpose of sexual reproduction. The development of these reproductive organs begins at an early stage in the embryo. There is a close link throughout with the development of the urinary system.
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3 days ago · Introduction. Following the first 3 weeks of human development embryonic systems begin to be established. Over the next 5 weeks these embryonic systems lay down a template for the majority of body structures. As much as possible the known human data is shown on the individual systems notes, though there is also reference to animal models of ...
Nov 12, 2024 · The nine major organ systems in the human body are the integumentary system, the musculoskeletal system, the respiratory system, the circulatory system, the digestive system, the excretory system, the nervous system, the endocrine system, and the reproductive system.
