Control del desarrollo de la inflorescencia en Arabidopsis

The interaction between two proteins, actin and myosin, is responsible for muscular contraction. In skeletal muscles, actin and myosin are arranged into units known as sarcomeres.

Long filaments of actin extend from each end of the sarcomere toward the middle, almost meeting, but not actually touching. In between these actin filaments are short, fat filaments of myosin that are arranged longitudinally. The myosin does not connect to the ends of the sarcomere. The ends of the sarcomere to which the actin filaments are attached are called the Z lines. When the muscle is stimulated to contract by a neuron, an influx of calcium ions (Ca++₎ causes the myosin to pull on the actin filaments by means of tiny connections known as cross bridges. Neither the myosin nor the actin filaments change in length, but when the myosin pulls the actin together, the actin pulls the Z lines together and the whole sarcomere contracts. Sarcomeres are arranged end to end into long fibers known as myofibrils, which bundle to form the primary muscle fibers that make up the muscle. Contracting sarcomeres cause muscle fibers to contract, which, in turn, cause the whole muscle to contract.

The Skin

It’s easy to think of the skin as just a thin covering for the important internal organs. But skin itself is an organ, with a multitude of functions:

• Protects against infection, abrasion, and water loss

• Contains nerve endings vital for sensation, such as touch, pain, heat, and cold

• Excretes water to maintain water and salt balance in the body

• Produces vitamin D on exposure to the UV rays in sunlight

• Regulates body temperature (thermoregulation)

topmost layer, which touches the outside environment. Active cell division occurs in the lower region of the epidermis. As new cells are created, old cells are pushed toward the surface, where they form a hardened, dead layer that is constantly shed. The dermis is living tissue that contains many blood vessels, sweat glands, and sebaceous glands, which produce oils that keep the skin from drying out. The dermis also contains nerve endings that are responsive to touch, pressure, heat, cold, and pain. Hair follicles originate in the inner portions of the dermis as well. The hypodermis, or subcutaneous layer of the skin, is mainly composed of loose connective tissue and fat cells.

Skin and Thermoregulation

The skin helps warm-blooded animals maintain constant body temperature in varying

environments. When the body becomes too warm, blood vessels in the skin dilate, allowing heat to escape through the surface of the skin. Special glands called sweat glands produce a salty

secretion called perspiration that evaporates off the surface of the skin, taking heat with it. When the body becomes too cold, the opposite processes occur. Sweat glands are shut down, and blood vessels in the skin constrict, keeping the blood away from the surface of the body, where heat could be lost. In addition, the muscles begin to contract rapidly and shiver, which generates significant heat.

The Reproductive System

As dictated by evolution, an organism’s purpose is to reproduce and ensure the survival of the species. All of the other organ systems exist just to keep the animal alive long enough to mate and pass its genetic makeup down to its progeny.

Animals can reproduce either asexually or sexually. Asexual reproduction usually occurs among less highly evolved animals. Budding, which can occur in certain cnidarians like the hydra, is a process by which the offspring literally grow off the side of the parent, producing a miniature, genetically identical copy. Regeneration occurs when animals such as earthworms, planarians, or starfish are broken apart and each piece then grows into a separate

organism. Parthenogenesis occurs when an animal’s egg cell begins to divide mitotically without being fertilized by a sperm. The embryo that develops from this unfertilized egg will be

genetically identical to the parent. Populations of animals that reproduce parthenogenically are usually entirely female. Animals ranging from rotifers to some amphibians reproduce through

parthenogenesis.

Sexual reproduction is when two haploid gametes, one from each parent, fuse to form a zygote, which develops into an offspring genetically different from the parents. This fertilization can take place externally, as is the case for many aquatic organisms that release their unfertilized gametes into the water, or internally. As animals have evolved, they have developed special structures for the production of gametes, for the fertilization process, and, in the case of viviparous animals that give birth to live young, for the support and nourishment of the developing young. Collectively, these structures are known as the reproductive system. In most species, including humans, the anatomy of the male and the female reproductive system is significantly different.

The Male Reproductive System

The male reproductive system has two major functions:

• It produces sperm cells, the male gametes, through the process of spermatogenesis. (Spermatogenesis is covered in the chapter on genetics.)

• It produces semen, a fluid that acts as a vehicle and nourishment for sperm as they make their way through the female reproductive system on their way to fertilize the egg.

The testes are the male gonads: they produce the sperm, which is the male gamete. More specifically, sperm cells are produced in the seminiferous tubules of the testes. (In addition to producing sperm, the testes also produce the hormone testosterone.) Since sperm can only develop at a temperature slightly lower than the normal mammalian body temperature, evolution has provided the needed lower temperature by placing the testes outside the body in a sac called the scrotum.

The seminiferous tubules empty into a long tube called the vas deferens, which joins the urethra just below the bladder and thereby provides a means of exit from the body through the penis. The penis is a spongy organ that can become erect during periods of sexual excitement. During erection, arteries in the penis dilate, engorging the erectile tissue with blood. This simultaneously compresses the veins that drain blood from the penis, trapping blood in the spongy tissue, causing it to become rigid.

The Female Reproductive System

While the male reproductive system is designed to produce and deposit sperm in the female, the female reproductive system has the more formidable task of receiving the male gametes, producing the female gametes, and, in the event of fertilization, maintaining and supporting a pregnancy.

The female reproductive system consists of the external genitalia, known as the vulva and vagina, the uterus, which supports the developing fetus, the Fallopian tubes, which connect the uterus with the two ovaries, and the ovaries, which produce the ova, or egg cells, in addition to the female sex hormones. Also included are the mammary glands, which produce milk to nourish the young.