Biology Lecture Notes PDF

Lecture 11 Senses Humans have 5 senses Smell , taste, vision , hearing and skin CNS Interprets ner ve impulses based on origin and frequency • nerve impulses are transmitted from receptors to specific portions of brain (seeing music?) • Stronger stimuli 1. A greater number of receptors. 2. Trigger...

Lecture 11 Senses Humans have 5 senses Smell , taste, vision , hearing and skin CNS Interprets ner ve impulses based on origin and frequency • nerve impulses are transmitted from receptors to specific portions of brain (seeing music?) • Stronger stimuli 1. A greater number of receptors. 2. Trigger a greater frequency of action potentials in sensory neurons. Receptors receive and convert smell • stimulus: sensory input that causes some change within or outside the body - Pressure, sound waves, light, chemical • receptor: structure that detects stimuli and converts its energy into another form (impulse) • different Kind of receptors - Detect one specific stimulus - Free nerve ending respond to several different stimuli, specifically Sensory transduction • reception - stimuli arrives at receptors, receptor translate, the stimuli into potential • transduction - the stimuli is strong over the dress, transmit action potential through afferent axons to CNS • perception - Brain interpret the signals arrive - Type of stimulus - The location of the stimulus - The duration of the stimulus - Relative intensity of the stimulus Mechanoreceptors Detect limb position , muscle length and tension • mechanoreceptors 1. In joints : detect joint position. 2. In skeletal muscles : muscle spindles • specialized mechanoreceptors for monitoring muscle length , which relay information about limb position. 3. In tendons : detect tension • there are more receptors on your fingertips, then on the back of your arm • taste buds is composed of a group of taste cells and supporting cells • Each taste cells are connected to a sensory neuron • smell : chemoreceptors binds with odorants • Olfactory neurons are bipolar neurons • Each neuron has a single dentrite buried in olfactory epithelium • receptors are specialized specific odors • Vision involves detecting and interpreting visual stimuli by Pain receptors signal discomfort converting light energy to ner ve impulses and transmitting • Unenccapsulated nerve endings respond to injury from excessive pressure, heat, light, them to the brain or chemicals • the photoreceptor cells of the eye where transduction of light • Fast pain (acute, sharp) to nervous system occurs are located in the retinal - Occurs very quickly • On the inner surface of the back of the cell, light passes - Forms of stimuli to be avoided through other layers first. The lens bend light to focus the • slow pain image on the retinal -Occurs more slowly • There are t wo types of photoreceptors in the retinal : - Originates in muscles or internal organs - Referred pain: may be perceived as originating in a different area of the body rod : strong photosensitive and located in outer edges of retinal, detects dim light and use for night time vision • pain receptors do not adapt Cone : weakly photosensitive and located near center of retinal, responds to bright light and primary role in daylight and color vision Thermoreceptors detect temperature • themorereceptors surface provide information about external environment - Surface Thermoreceptors adapt quickly • Thermoreceptors in thoracic and abdominal organ monitor core temperature - Core receptors do not adapt quickly Quiz 1) all sensory receptors converts stimuli to action potential 2) what sensory information were carried to the brain? True or false • • • • physical type of stimuli (sound, light, etc) Duration of the stimuli Location of the stimuli relative intensity of the stimuli 3) pacinian corpuscles detect 4) which sensory impulses directly go to the brain, not through thalamus? • • • • • • • • deep pressure Light touch Temperature Pain vision olfactory stimulation Taste touch 5) what is the main function of the mid ear? 6) if a person is colorblind, which part of the eye is mostly responsible? • • • • • • • • Amplify the sound Convert soundwave into vibration Convert stimuli to action potential Channel the soundwave photo receptor- rods Iris Lens Photo receptor- cones Lecture 12 Respiratory system • primary function is to deliver oxygen to the cells and to remove carbon dioxide • gas exchange happen in our alveoli • The trachea can cause expired air to rush upwards from the lungs at great force • The force, exhalation helps expel mucous when we cough • Diaphragm is important for breathing Lung volume and capacities • the lungs are not usually operate at maximum capacity • The volume in the lung can be divided into four units Gas exchange and transport occur • Tidal volume (TV) measures the amount of air that is inspired and expired during a normal breath passively • The expiratory reserved volume (ERV) is the additional amount of air • gas exchange and transport depends that can be exhaled after a normal exhalation on the pressure gradient , similar to • The IRV is the additional amount of air that can be inhaled after a diffusion from higher concentration normal exhalation to lower concentration • The residual volume is the amount of air that is left after expiratory External respiration : the exchange of reser ve volume is exhaled gases bet ween air and blood • The lungs are never completely empty ; always some air left in lung to • at the site of alveoli , o2 in the air is keep the tissue from sticking together. Very important from higher than o2 in the blood preventing large fluctuations in O2 and CO2 • In the air is 104 and in blood is 40 • The viral capacity measures the maximum amount of air that can be • O2 diffuses from alveoli into blood inhaled or exhaled during a respiratory cycle • CO2 diffuses from blood into alveoli • The inspiratory capacity is the amount of air that can be inhaled • In the tissue capillaries , the pressure after the end of a normal expiration is reversed , CO2 leaves the tissues to • The functional residual capacity measures the amount of additional the blood , O2 leaves blood to tissues air that can be exhale after a normal exhalation • Lung volume are measured by a technique called spirometry which Hemoglobin transport most oxygen measures the force expiratory volume molecules • most oxygen is bond to a protein called Breathing hemoglobin and carried to the tissue • during breathing (ventilation) the contraction and relaxation of • Carbon dioxide levels, blood pH and body muscles acts to change the volume of the thoracic cavity. This changes temperature affects oxygen carrying the volume of the lungs capacity • inspiration is an active phase when air enters the lungs , it is initiated • The increase in carbon dioxide and by contraction of muscles subsequent decreases in pH , reduces the • Muscles elevate the ribs and sternum , extending the anterior and affinity of the hemoglobin for oxygen posterior dimension is the thoracic cavity • Diseases like sickle cell , anemia , and • The actions of the muscles results increase in the volume of the thalassemia which has abnormal thoracic cavity hemoglobin and has much lower • Air moves into the lungs, down the pressure of gradient capacity for carrying oxygen • Expiration is the passive phase when air is expelled from the lung. It is initiated by relaxation of the muscles • Carbon monoxide has a greater affinity • Results in a decrease in the volume of the thoracic cavity decrease in for hemoglobin than oxygen, therefore lung volume increase in pressure when carbon monoxide is present, it binds to hemoglobin preferentially over oxygen , you will have carbon monoxide Bicarbonate buffer poisoning • some of the CO2 will dissolve in the plasma or carried by hemoglobin but about 70% will be converted by enzymes called carbonic anhydrase to bicarbonate • this reaction removes CO2 quickly so carbon dioxide into the blood downs the concentration gradient • Hemoglobin binds to the free hydrogen ions and thus limited shifts in pH • The newly synthesized bicarbonate ion is transported out of the red blood cells into the liquid component of the blood in exchange for a chloride ion ; chloride shift • When the blood reaches the lung, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion • The hydrogen ion dissociated from the hemoglobin and binds to the bicarbonate ion, this produces the carbonic acid intermediate , which is converted back into carbon dioxide through the enzyme • The carbon dioxide produce is expelled through the lungs during exhalation Quiz 1) the amount of air that is inspired and expired during normal breath is tidal volume 2) one cannot exhale all the air . true or false 3) in spirometry, what is the FEV 1/FVC in asthma patients FEV 1/FVC ration is low 4) the direction of O2 and CO2 movement at the Alveoli and tissue capillaries depends on the pressure gradient of the gas. True or false 5) during inhalation , the volume of thoracic cavity increase, lung increase, pressure in the lung decrease 6) the benefit of the bicarbonate buffer system is maintaining pH Lecture 13 The Male reproductive System Delivers sperm • the scrotum houses the testicles or testes, including providing passage for blood vessels, nerves, and the muscles related to testicle function . • The testes is a pair of male reproductive organs that produces sperm and some reproductive hormones. • Sperm are immobile at body temperature, therefore scrotum and the penis are external to the body • Non sperm cells are thereto protect the germ cells and promote their development or produce high levels of testosterone once the male reaches adolescence • When the sperm has developed flagella and nearly mature, they leave the testicle and enter epididymis • The sperm leaves the epididymis and enter the ductus deferens, which carries the sperm through ejaculatory duck than leaves the body • during a vasectomy , a section of the ductus deferens is removed, preventing sperm from being passed out of the body Accessory glands helps sperm survive • sperms are haploid cells, consisting of a flagella as a tail, a neck that contains the cell’s energy producing mitochondria and a head that contains the genetic material • During fertilization , only the materials here are entered into the egg • An acrosome is is formed at the top of the head of the sperm • This structure contains lysosomal enzymes that can digest the protective coverings that surrounds the egg to help the sperm penetrate and fertilize the egg • Semen is a mixture of sperm and the secretions, fluids from accessory glands that contribute to most the semens volume • The seminal vesicles make a solution that is thick , yellowish and alkaline. • As sperm are only motile in an alkaline environment , a basic pH is important to reverse the acidity of the vaginal environment • The solution also contains mucus , fructose , it is a sperm mitochondria nutrient, also enzyme , ascorbic acid and local acting hormones • the prostate gland makes a thin, milky fluid that contains citrate ; a nutrient , enzymes, and prostate specific antigen PSA • PSA is a proteolytic enzyme that helps to liquify the ejaculate several minutes after release from the male • the bulbourethral gland releases its secretion prior to the release of the bulk of the semen. It neutralizes any acid residues in the urethra left over from urine because that part is shared The female reproductive system produces eggs and supports pregnancy • the ovaries releases oocytes , that’s immature eggs and the secrete hormones , estrogen and progesterone • Fallopian tubes ; fertilization occurs in the upper third of the oviduct • Uterus; where fetus grows and develops. Uterus has a lining called endometrium, which supports fetus. Parts of it sloughs off during menstrual flow • Under endometrium , has a muscle layer called myometrium used for labor • The sperm coming through vagina, enters through the cer vix , fertilize the egg in fallopian tube , the fertilized egg comes to develop in uterus and the baby is delivered through cervix passed vagina Oogenesis • egg production occurs in ovary. It begins before birth, is arrested until puberty • one egg is produced for each menstrual cycle • One cell does divide into four cells Like sperm but only one become egg . The other 3 degenerate and reabsorbed by the body Human sexual response, intercourse, and fertilization • The sexual response in humans is both a psychological and physiological • Both sexes experience sexual arousal through psychological and physiological stimulation • There are four phases of the sexual response with increase blood flow and muscle contractions • The female hormone controls involves hypothalamus and pituitary gland , even with the same t wo hormone LH and MSH • The first half of the ovarian cycle , you have a slow rise of FSH and LH , causes the growth of follicles , this process prepares the egg for ovulation Quiz • as women approach their mid 40s to mid 50s, their ovaries begin to loose their sensitivity to FSH and LH 1) why scrotum is outside the body? Because higher temperature to • They might still have eggs, but without the stimulation maintain sperm mobility. of FSH and LH, they will not produce a liable egg to be released 2) prostate specific antigen PSA is a proteolytic enzyme • The outcome of this inability to have children without inter vention 3) ovary secrete estrogen only Pregnancy and birth • when fertilized zygote travels zygote the oviduct to the uterus , the developing embryos must implant into the wall of the uterus once or it will die • Once implantation is successful , placenta start to form. It begins to release a hormone called HCG. This ensures adequate level of progesterone that will maintain the endometrium of the uterus for the support of the developing embryos • Pregnancy test determines level HCG in urine • If hormone is present, then the test is positive • 3 stages to labor, during stage one , the cercus stings and then dilates , stage 2, the baby is expelled from uterus, the uterus contracts , rheumatoid arthritis last state is the passage of the placenta after the baby have been born and the organ has completely disengage from the uterine wall 4) the human eggs and sperm are haploid 5) FSH and LH are secreted by pituitary gland 6) pregnancy, kit, detect hCG, secreted by placenta Lecture 14 Fertilization-fusion of one sperm and one egg • human development starts from fertilization • fertilization is a process in which gametes, an egg and sperm fused to form a zygote • The egg is protected by a layer of extra cellular matrix consisting mainly of glycoprotein called the zona pellucida • only one sperm can fuse into one egg to ensure the offspring has only one complete set of chromosomes • When a sperm binds to zona Pellucida , the sperm’s acrosomes at the tip of the head contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg • The sperm plasma membrane starts to fuse with the egg plasma membrane . The fusion creates an opening through which the sperm nucleus is transferred into the egg • The organelles at the neck such as mitochondria are not transferred in • You inherited mitochondria from your mother • The nuclear membrane of the egg and sperm breaks down , the egg abs sperm each contains one set of chromosomes , now the t wo haploid genomes condense to form a diploid genome, so each of us have t wo sets of; one from father and one from mother • To ensure that no more than one sperm fertilize the egg, once the acrosomal reaction takes place at one location of the egg, as soon as the reaction takes place, the egg releases proteins in other locations to prevent other sperms from fusing with the egg • If this mechanism fails, multiple sperms confused with the egg , resulting in polyspermy . • The resulting embryo is not genetically viable and dies within days Formation of the neural system • organs form from the germ layers through the process of differentiation • During differentiation the embryonic stem cells express specific sets of genes which will determine the cell type • in vertebrate, one of the primary steps is the formation of the neural system • During the formation of the neural system, special signal molecules called growth factors signals some cells at the edge is ectoderm to become epidermis cells. The remaining cells in the center form the neural plate • If the signal by growth factors were distrusted then the entire ectoderm will differentiate into neural tissue • The neural plate role up and forms a tube and this is called neural tube • In the future development, neural tube will give rise to the brain and spinal cord. • Having adequate vitamins helps to make sure the neural tube forms properly, if you are deficient in one of the vitamins, you might have a problem closing the tubes Cleavage- rapid cell division • the zygote undergoes rapid multi round of cell division called cleavage • During the cleavage, the zygote rapidly divided into multiple cells, without increasing the size of the cell, so it gets smaller and smaller • After the cleavage has produced over 100 cells, the cells rearrange themselves to form a hollow ball with a fluid filled or yolk filled cavity called blastula Blastula • • • • • • • • • The blastula is usually a layer of cells, that is called blastoderm The cavity is blastocoel Each cells within the blastula is called blastomere The blastula forms the blastocyst The cells in the blastula arranged themselves in t wo layers; the in inner cell mass and the outer layer The outer layer called trophoblast The inner cell mass known as embryo blast The inner cell mass consists of embryonic stem cells that will differentiate into the cell types needed by the organisms The trophoblast will contribute to the placenta and nourish the embryo Gastrulation • the cells in the blastula rearrange themselves spatially to form 3 layers of cells . This process is called gastrulation • During gastrulation process , the blastula folds upon themselves ti form 3 layers of cells. Each of those layers is called a germ layer • Each germ layer differentiates into different organ systems • The three germ layers are ectoderm, mesoderm, and endodermis • The ectoderm gave rise to the nervous system and epidermis. • The mesoderm gave rise to the muscle cells and the connective tissue • The endodermis gave rise to the digestive system and many internal organs Mesoderm development • the mesoderm that lies on either sides of the vertebrates neural tube will develop into various connective tissue of the animal body. • The spatial pattern of gene expression recognizes the mesoderm into groups of cells called somites • Somites will further develop into the ribs, longs, and segmental muscle . Those are spine muscle • The mesoderm also forms a structure called the notochord which is a rod shape and forms the central axis of the the animal body • Formation of vertebrae axis is another important development stage Forming symmetry • animal bodies have externally invisible symmetry • the internal organs are not symmetrical • For example, your heart is on left side and liver is on right side. The establishment of symmetry and internal asymmetry are through gene expression, expression of many gene Lecture 15 Selective Permeability of Lipid Bilayers • our cells need to achieve balance of molecules on either side of the membrane , this process is all small regulation • Our cell membrane allows gas , water to pass, but not ions or large molecules . It’s called selective membrane • When there’s an imbalance of molecules on 2 sides, there is osmotic pressure, it will drive to reach the same concentration on both sides • Since ions cannot pass the membrane, water will be drawn to the higher concentration side to dilute it • The endgame is to achieve the same concentration, this process is osmosis Osmosis • when the cell is in a solution , the outside salt concentration is greater than inside, this is hypertonic • More water will leave the cell , so the cells shrink • If the cell is in physiological saline , where salt concentrations are the same inside and out, this is isotonic. There’s not net flow of water , the cell is normal • If cells are in hypotonic solution such as water or lower salt, outside then salt concentration inside, more water will go into the cell. The cell will expand and may burst • Mammalian systems have evolved to regulate specified concentrations of important electrolytes in the 3 major fluid compartments : blood plasma , extracelluar fluid and intracellular fluid • Osmosis pressure have a direct bearing on blood pressure • sweat glands, lungs, and digestive system could loose some water , the major osmosis regulatory organ is the kidney • The kidney is a pair of bean shaped structures, the adrenal gland sits on the top of each kidney • The kidney has 3 regions; an outer cortex, medulla in the middle and the renal pelvis where blood vessels and the nerves enter and ext the kidney. It is also the point of exit for the ureters • nephrons are the functional unit of the kidney located in the cortex Nephron - function unit of kidney • a nephron consists of three parts ; a renal corpuscle , a renal tubule and the associated capillary net work • the renal corpuscle located in the renal cortex is made up of a net work of capillaries known as the glomerulus and the capsule, a cup shaped chamber that surrounds it called the glomerular or bowman’s capsule Filter blood • the renal tubule is a long structure • the first part is called the proximal convoluted tubule or PCT, due to its proximity to the glomerulus , it stays in the renal cortex • The 2nd part is called the loop of henle , that goes through the renal medulla • The 3rd part of the renal tubule is called the distal convoluted tubule or DCT • The capillary net work supplies the nephron with blood that needs to be filtered • The branch that enters the glomerulus is called the afferent • The branch that exits the glomerulus is called efferent • Within the glomerulus the net work of capillaries is called the glomerular capillary bed Kidneys maintain homeostasis in many ways • kidney contributes to homeostasis by maintenance of water balance • Salt balance , control of blood volume and blood pressure and maintains acid base balance and blood pH • Kidney also makes a protein called erythropoietin or EPO, which stimulates red blood cell production • Too much blood cells could lead to heart attack or stroke Epinephrine and Norepinephrine - flight or fight • the adrenal gland on top of kidney secreted hormones • Epinephrine and Norepinephrine have flight or fight response, where heart rate , metabolic rate goes up . Renin - angiotensin - aldosterone : increase blood volume and pressure • the renin , angiotensin , aldosterone system increases blood pressure and volume • Renin secreted by kidney acts on angiotensin , which is made in the liver and converted to angiotensin 1 • ACE (another enzyme) secreted in the lungs and kidney converted angiotensin 1 to angiotensin 2 • angiotensin 2 raises blood pressure by constricting blood vessels and also triggers the release of aldosterone from the adrenal cortex which in turn stimulates the renal tubules to reabsorb more sodium • Water follows sodium to maintain osmotic balance, therefore aldosterone manages not only sodium levels but also water levels • Aldosterone also stimulates potassium secretion concurrently with sodium reabsorption • If the the daily dietary potassium load is not secreted and the retention of potassium can cause a dangerous increase in plasma potassium concentration • Patients who have Addison’s disease have a failing adrenal cortex and cannot produce aldosterone . They loose sodium in their urine constantly and if the supply is not replenished , the consequences can be fatal • Angiotensin 2 also triggers the release of antidiuretic hormone or ADH, from the hypothalamus helping body to conserve water. It acts directly on the nephrons and decreases glomerular filtration rate • Medically , blood pressure can be controlled by drugs that inhibit ACE called ACE inhibitors • ADH also acts as a vessel constructor and increases blood pressure during hemorrhaging • The hormone ANP, made with cardiac muscle cells has antagonistic effect meaning it lowers blood pressure by acting as a vessel dilutor. It is released by cells in the atrium of the heart in response to high blood pressure Unit 9 study notes from text book (lecture 14) Fertilization • egg and sperm each contain one set of chromosomes • To ensure that the offspring has only one complete diploid set of chromosomes , only 1 sperm must fuse with one egg. • In mammals , the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called zona pellucida. • When a sperm binds to the Zona Pellucida , a series of biochemical events, called the acrosomal reactions take place • in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane . The fusion of these t wo membranes creates an opening through which the sperm nucleus is transferred into the ovum. The nuclear membranes of the egg and sperm break down and the t wo haploid genomes condense to form a diploid genome. • Fertilization is the process in which sperm and egg fuse to form a zygote • Acrosomal reactions helps the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus • to ensure that no more than one sperm, fertilizes the egg, once the acrosomal reaction takes place at one location of the egg membrane, the egg releases, proteins, and other locations to prevent other sperm from fusing with the egg. If this mechanism, fails, multiple sperm, confused with the egg, resulting in polyspermy. The resulting embryo is not genetically viable and dies within few days. Cleavage and blastula Stage • the development of multi cellular organisms begins from a single celled called zygote which undergoes rapid cell division to form the blastula . • The rapid , multiple rounds of cell division are termed cleavage. After the cleavage has produced over 100 cells , the embryo is called a blastula . • The blastula is usually a spherical layer of cells ( the blastoderm) surrounding a fluid filled or yolk filled cavity (the blastocoel) • Mammals at this stage form a structure called the blastocyst , characterized by an inner cell mass that is distinct from the surrounding blastula • During cleavage , the zygote rapidly divides into multiple cells without increasing in size and the cells rearrange themselves to form a hollow ball with a fluid filled or yolk filled cavity called the blastula • cleavage can take place in t wo ways: holoblastic (total) cleavage or meroblastic (partial) cleavage . • The type of cleavage depends on the amount of yolk in the eggs . • In placenta mammals (including humans ) where nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage . Other species, such as birds , with a lot of yolk in the egg to nourish the embryo during development , undergo meroblastic cleavage . • In mammals , the blastula forms the blastocyst in the next stage of development . • The cells in the blastula arrange themselves in t wo layers : the inner cell mass , and an outer layer called the trophoblast • The inner cell mass is known as the embryoblast and this mass of cells will go on to form the embryo • The rearrangement of the cells in the mammalian to t wo layers - the inner cell mass and the trophoblast , results in the formation of the blastocyst Gastrulation • the typical blastula is a ball of cells • The next stage in embryonic is the formation of the body plan. • The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation. • During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, and each germ layer differentiates into different organ systems. • The three germ layers, are the endoderm, ectoderm, mesoderm. • the ectoderm gives rise to the ner vous system and the Epidermis. • The mesoderm gives rise to the muscle cells and connective tissue in the body. • The endoderm gives rise to columnar cells found in the digestive system and internal organs. Organogenesis • gastrulation leads to the formation of the three germ layers that give rise, during further development, to the different organs in the animal body. This process is called organogenesis . Organogenesis is characterized by rapid and precise movements of the cells within the Embryo • Organs form from the germ layers through the process of differentiation. During differentiation, the embryonic stem cells, express specific sets of jeans which will determine their ultimate cell type. • For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. The process of differentiation is regulated by cellular signaling cascades. • scientists study organogenesis extensively in the lab in fruit flies (Drosophila) in the nematode Caenorhabditis elegans. • Drosophila have segments along their bodies , and the patterning associated with the segment formation has allowed scientist to study which genes play important roles in Organogensis along the length of the embryo at different time points • The nematode C. Elegans has roughly 1000 somatic cells and scientists have studied the fate of each of these cells during their development in the nematode life cycle • there is little variation in patterns of cell linage bet ween individuals, unlike in mammals, where cell development from the embryo is dependent on cellular cues • in vertebrates, one of the primary steps during organogenesis is the formation of the neural system . The ectoderm forms epithelial cells and tissues, and neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue. • The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube • Te central region of the ectoderm forms the neural tube which gives rise to the brain and spinal cord • The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. • A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites with spaces bet ween them. The somites will further develop into the cells that form the vertebrae and ribs, the dermis of the dorsal skin, the skeletal muscles of the back, and the skeletal muscles of the body wall and limbs. • The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.