Reproduction In The Sea Urchin (1965)

[Music] [Applause] [Music] the science of marine life is among the most fascinating of the branches of biology knowledge of the life patterns of certain marine animals can often be applied to the study of higher forms of life in Marine environments of Seashore and Tide poool live the purple sea urchins sea urchins have long been used as experimental animals in studies of reproduction and genetics these invertebrates are classified as echinoderms a name taken from two Greek words meaning spiny and skin the many spin-like structures are responsible for this descriptive name the purple seurin is a widely distributed species adults generally measure between 1 and 2 in in diameter and are radially symmetrical the Sexes are separate though difficult to distinguish even after dissection a male specimen is on the right female on the left five reproductive organs gonads radiate from the oral cavity the female gonads ovaries produce OVA or egg cells by removing a portion of an exposed ovary we can obtain some of the OVA under the microscope the OVA or eggs are easily distinguished this time we remove a portion of a male gonad the test soon the fluid secreted by the test appears in this fluid are literally millions of sperm cells within the test the sperm cells are relatively immobile for a more detailed view of the male sex cell will use a higher magnification of the electron microscope as shown by these three specimens the sperm cell consists of a Slender tail and a head which measures about 3 microns in diameter now a rarely seen cross-section through the head to reveal its delicate anatomy in structures at the base are concentrations of sugars the oxidation of sugars provides energy for the vigorous movement of the sperm most of the head is nucleus and contains the hereditary materials the DNA of the cell in the tip is an enzyme that acts to perforate the female egg cell and provides entrance for the sperm resulting in fertilization the tiny sperm cells upon contact with seawater begin their characteristic rapid swimming motion through the beating action of their fella or tails the role played by the sperm and fertilization can be clearly demonstrated in this experiment we place mature female sea urchins into a tank of seawater now we introduce a freshly dissected sea urchin test the whitish liquid spermatic fluid containing numerous sperm cells disperses over the females the presence of sperm appears to act as a stimulus to the females which begin to ovulate by passing fluid containing masses of OVA into the water this stimulation also takes place in reverse that is the presence of OVA in the seawater will stimulate the excretion of spermatic fluid this same phenomenon occurs in the natural habitat of the seurin the breeding season usually begins in August at this time the milky white secretion of the gonads in which the eggs are suspended is passed from the female's body this process ovulation releases anywhere from 150 to 250,000 eggs an hour which are dispersed through the water aided by currents and wave motion numerous eggs can be seen in this photomicrographic view as the eggs come in contact with mature male SE urgin the testes are stimulated spermatic fluid is released in quantity into the sea these are the first motion pictures of this fascinating phenomenon as it occurs in nature during the breeding season fluids containing sperm cells and eggs Cloud the tide pool where the sea urchins live but to observe the actual process of fertilization we must return to the laboratory from our living specimens we select a mature female part of the test or protective covering of the animal may be removed to expose the ovaries we then place the animal over the mouth of a beaker filled with sea water a few drops of potassium chloride are sufficient to stimulate the activity of the ovaries to produce ripe eggs some of the eggs can be rinsed from the white fluid and placed under the microscope in this closeup of a single egg the protective membrane surrounding the ovam is seen as a light colored ring ring while the nucleus is the small light area now we've placed some spermatic fluid on the ovom within seconds the sperm cells gather densely around the membrane of the egg cell the underlying cause of this attraction is a chemical stimulus to demonstrate this attraction chemotaxis will begin with fluid containing live egg cells next spermatic fluid containing live sperm is dropped in the watch glass what follows is dramatically shown in time-lapse photography the sperm cells move toward and concentrate around the mass of egg cells in the center of the watch glass this time instead of live egg cells we'll use crystals of gyog gamon a chemical extracted from purple sea urchin egg cells gyog gamon crystals are made up of complex protein molecules when the crystals are dissolved in seawater the sperm cells are attracted just as they were to the live egg cells such experiments have indicated that gyog gamon is the substance that attracts sperm to the ovom actual fertilization is about to take place along the right edge of the ovom as the sperm perforates the egg membrane what happens during this process is best shown in electron photo micrographs here is a sperm at the moment of contact with the ovam the pointed structure extending from the head at the right is believed to contain Lipson an enzyme which can dissolve the 's protective membrane allowing the head of the sperm to enter the ovam as this occurs the female sex cell begins to develop a protruding structure seen at the upper left called the receptive cone on the surface of the cone a tiny sperm is visible the sperm cell is more or less being pulled through this cone here the head of the sperm cell is emerging from the inner surface of the receptive cone it will proceed toward the nucleus of the ovam where fertilization will take place once fertilization of the ovom by a single sperm occurs there develops around the egg cell another membrane called the fertilization membrane the formation and thickening of the membrane may be easily observed development of the fertilization membrane is rapid usually taking about 20 seconds the thick fertilization membrane does not react to the enzyme lipsin so as we see in this closer view it cannot be penetrated by other sperm cells the streaming cytoplasm of the fertilized egg or zygote now undergo a series of chemical and physical changes as It prepares for cleavage or division of the egg into an increasing number of cells after about half an hour the first cleavage occurs as the chromosomes in the nucleus are duplicated and the cytoplasm is divided through the furrowing of the cell membrane the resulting cells are called blastomeres the process of cleavage is repeated to produce four blastomeres in the third division of cells in the developing sea urch and embryo eight cells are produced subsequent cleavage produces a blastula or Hol ball within which is a structure called the blasto seal or segmentation cavity the blastula is not evident in these pictures because we very gently flattened the developing embryo onto the slide so as to observe the process of cleavage more clearly as cleavage goes on more and more cells are produced by simple division mitosis as cleavage continues and the cells are doubled 16 32 64 and so on gastrulation takes place gastrulation is a complex process whereby layers of cells are moved into the interior of the blastula after gastrulation cells of various body tissues and structures are formed through differentiation this rapid development of the embryo continues for several days by the fifth day the tiny sea urchin is a microscopic free swimming larvae it is bilaterally symmetrical both halves of the body structure are identical at 10 days the larvae has grown to about 4 fths of a millimeter in length it will soon undergo metamorphosis during which it will become radially symmetrical a body form typical of several invertebrate classes at 20 days the test or outer shell appears as as well as spines and tube feet characteristic of the adult during metamorphosis the sea urchin has changed from a swimming bilaterally symmetrical larva to a radially symmetrical adult adapted to life in shallow ocean waters or Tide Pools after maturation the sea urchin embeds itself into a rock which then becomes the urchin's permanent home because of its high rate of reproduction and the relative ease with which its fertilization and developmental processes may be studied the sea urchin has become a highly valued experimental animal in studies of genetics and embryonic [Music] development [Music] a