CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.2. Factors that Influence the Force of Muscle Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings


Spermiogenesis in the Seminiferous Tubules.

 Image drawn by BYU-Idaho student Fall 2013

Recall that within the seminiferous tubules two types of cells are found; the Sertoli cells and the germ cells. The Sertoli cells are tightly connected to each other by attachments called tight junctions. This arrangement creates what is known as the blood-testis barrier. This barrier keeps the developing sperm cells "hidden" from the male's immune system (because circulating immune cells cannot cross the Sertoli cell barrier). It also provides the proper environment for the developing sperm. Serious injury or infection can sometimes compromise this barrier and immune cells attack the germ cells. This is a relatively common cause of male infertility.

Prior to puberty the seminiferous tubules are relatively small in diameter and do not have patent lumens. Also, during this time, the germ cells are in a quiescent state. With the onset of puberty, FSH begins to increase. Sertoli cells have FSH receptors and when FSH stimulates these receptors, the Sertoli cells increase in number and size. They also produce fluid that begins to expand the seminiferous tubule lumen, creating a lumen in the center. This is why testicular size increases rapidly during puberty. Also, the increase in FSH and testosterone initiate spermatogenesis. The stem cells that will eventually develop into mature sperm are the spermatogonia. These cells, like other body cells, have 23 pairs of chromosomes for a total of 46 (diploid). At puberty, they begin to divide by mitosis, producing daughter cells that also have 46 chromosomes. When a spermatogonium undergoes mitosis, one of the new daughter cells remains a spermatogonium to divide by mitosis again. The other daughter cell becomes a primary spermatocyte and begins the journey to become a sperm cell. The primary spermatocyte undergoes the first meiotic division, resulting in the production of two secondary spermatocytes. These secondary spermatocytes will then undergo the second meiotic division, resulting in the production of four spermatids. The process of meiosis results in cells that have only one copy of each chromosome or a total of 23 (haploid).

Author: OpenStax Anatomy and Physiology. License: Creative Commons Attribution License 4.0 license. Link: 

At this stage, the spermatids do not look like a mature sperm but are essentially a round cell. To become a mature sperm, the spermatids undergo a complex process, known as spermiogenesis, resulting in the characteristic shape of the sperm cell (also called spermatozoa). Sertoli cells literally surround the developing germ cells and influence sperm cell development along the entire process from a spermatogonium to a fully formed sperm cell. From beginning to end this process takes roughly 74 days. The sperm then move into the epididymis, where they must spend another 10-14 days to become a fully functional sperm and are temporarily stored. The typical 20-year-old male produces around 6.5 million sperm per gram of testicular parenchyma per day. As men age this number goes down and by age 50 is around 3.8 million per gram per day.

Structure of a Spermatocyte

As described above, the spermatozoon or sperm cell (male gamete) is a haploid cell with one copy of each chromosome pair or a total of 23 individual chromosomes. Following spermiogenesis the mature sperm cell has very little in the way of cytoplasm and cell organelles, but it does have a very long flagellum that can propel it quite vigorously. In the first image below, you see that a sperm cell consists of three segments, the headmidpiece and tail.

The head contains the nucleus (very highly condensed DNA of 23 chromosomes). The nucleus is capped by the remnants of a cell organelle called the Golgi body. This remnant is called the acrosome and it is filled with hydrolytic enzymes that will be used to help digest through the protective layers surrounding the egg. The midpiece contains a centriole and many mitochondria that will be used to generate the ATP that powers the long tail (flagellum) for the swimming motion. The tail is a flagellum which consists of an array of microtubules that slide past each other under the influence of ATP. This sliding motion causes waves of curvature or bending in the tail, propelling the sperm forward.

By Mariana Ruiz [Public domain], via Wikimedia Commons Link:

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