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
5.7.6

Female Reproductive System: Oogenesis

By the end of the fourth month of fetal development there are roughly 7 million oogonia in the ovaries. Oogonia are analogous to spermatogonia and are the cells that will develop to become the egg or oocyte (also called an ovum). Unlike spermatogonia that remain quiescent until puberty, the oogonia begin meiosis I even before birth. However, the future ova, at this point, called primary oocytes, arrest in Prophase I where they will remain until puberty. Many of these oocytes degenerate and by birth only about 2 million remain.

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Oogenesis.
Author: OpenStax Anatomy and Physiology. License: Creative Commons Attribution License 4.0 license. Link: https://cnx.org/resources/cb3a51b134cfa417cf88f924fed1d8731ef8754f/Figure_28_02_03.JPG

The process of attrition continues and by puberty there are only around 400,000 primary oocytes remaining, of which only 400 will be ovulated. Beginning at puberty and continuing throughout the reproduction years of the female, each menstrual cycle some oocytes will complete meiosis I to become secondary oocytes. These secondary oocytes begin meiosis II but again stop, this time at Metaphase II. Typically, only one oocyte will be ovulated each month and then, if it is fertilized, it will complete the second meiotic division. If it is not fertilized, it does not complete meiosis II and degenerates within about 24 hours after ovulation.

Recall that the two meiotic divisions of the spermatocytes result in the production of four spermatids. However, when the oocytes undergo meiosis the divisions produce only one daughter cell and one polar body.  It is essential that the ovum, when ovulated, have enough stored energy to keep the developing embryo alive until it implants in the uterine wall. Consequently, the meiotic divisions are unequal. The daughter cell retains all of the stored nutrients and cellular organelles while the polar body contains only the chromosomes from the nuclear division.

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Stages of Folliculogenesis
Image by BYU-I Student Hannah C. Fall 2013

Primordial Follicle

Refer to the images above as you read the explanation of this process. The process of follicular development (folliculogenesis) begins even before the birth of the woman. In the fetal ovary, primary oocytes become surrounded by flat, squamous shaped cells that will later become granulosa cells. Granulosa cells are analogous to the Sertoli cells that surround and support the spermatogonia. Once the primary oocyte becomes surrounded by these pre-granulosa" cells, we call the group of cells, along with the basement membrane that surrounds it, a primordial follicle. Even though there are millions of primordial follicles in the fetus, by the onset of puberty, a female will have only about 400,000 primordial follicles. This is due to the continual process of atresia (apoptosis, or programmed cell death). It is not fully understood what regulates atresia and why some primordial follicles die and others do not. Beginning at puberty, primordial follicles may be recruited to develop further. It is also not known what regulates or determines which primordial follicles will begin further development, but it is known that several hormones are required for the process: the gonadotropins of the anterior pituitary, FSH and LH, as well as the ovarian hormone estrogen. The process is ongoing and during the female reproductive years there are always follicles in various stages of maturation and growth. Approximately 400 of the primary follicles will actually be released during a woman’s reproductively active life.

Primary Follicle

Beginning at puberty small groups (cohorts) of primordial follicles start developing. The pre-granulosa cells that around the oocyte increase in size and become cuboidal in shape. At this point, we refer to these surrounding cells as granulosa cells. At the same time, the primary oocyte inside this layer of cells begins to increase in size and secrete proteins. Also, the granulosa cells nearest the oocyte secrete mucopolysaccharides. Together the proteins from the oocyte and the mucopolysaccharides from the granulosa cells form the zona pellucida, a clear layer between the oocyte and the granulosa cells. It is this barrier that the sperm will eventually have to penetrate in order to fertilize the ovum. The follicle is now a primary follicle.

Secondary Follicle

Next, the surrounding granulosa cells begin to divide by mitosis and form multiple layers. Simultaneously, stromal cells (connective tissue cells) are recruited by the follicle to form a layer of "thecalcells just outside of the basement membrane. Once these processes are complete we call the follicle a secondary follicle.

Graafian follicle: As the follicle continues to develop, the thecal cells multiply forming several layers on the outside of the follicle. Additionally, the granulosa cells continue to increase in number and begin to secrete fluid (follicular fluid) resulting in the formation of fluid filled spaces among the granulosa cells. With the aid of the granulosa cells the primary oocyte continues to enlarge. Eventually all of the fluid filled spaces will coalesce into one large cavity called the antrum.

Graafian Follicle

The follicle is now called a mature Graafian follicle. Note that in the Graafian follicle the oocyte is located on one side of the antrum and is surrounded by several layers of granulosa cells, the cumulus oophorus. The Graafian follicle is now almost ready for ovulation. The final event, occurring several hours prior to ovulation, is completion of meiosis I to produce a secondary oocyte which immediately starts meiosis II. Once again however, the oocyte arrests, this time at the metaphase II stage where it will remain until it is fertilized by a sperm.

The predominant view is that once recruited, it requires three or four monthly cycles before the follicles reach full maturity (185 days). Typically, only one of the follicles will generally reach maturity and all of the others of that cohort will degenerate.

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