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Lecture

The hormonal regulation of reproduction

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Department
Physiology
Course
PSL201Y1
Professor
Christopher Perumalla
Semester
Winter

Description
← The hormonal regulation of reproduction ← ← ← Gametes ← ← Sexual reproduction occurs by the fusion of gametes which in males are the spermatozoa and ova in females. Each gamete contains half of the usual number of chromosomes so are haploid, they contain 23 sets of chromosomes. This is referred to as n chromosomes. All other cells of the body are said to be diploid because they contain 46 sets of chromosomes and are referred to as 2n chromosomes. ← During reproduction, gametes from each parent fuse together to produce a new cell. This new cell, a zygote, is diploid because it has received one set of chromosomes from each parent. ← ← Chromosomes ← ← Spermatozoa and Ova contain 23 chromosomes. 22 of these chromosomes are called autosomes. Autosomes are the chromosomes that determine everything except an individual’s sex. Sematic cells, all cells of the body except gametes contain two sets of 22 autosomes. Each autosome in a pair carries the same complement of genes and is so said to be homologous. The remaining one chromosome of the two sets of 23 chromosomes and diploid cells is the sex chromosome that determines an individual’s sex. There are two types of sex chromosomes: X and Y. The female set of chromosomes is XX and males are XY. Unlike autosomes, the X and Y chromosomes are distinguishable from each other on the basis of their shape. Because the sex chromosomes differ, they are said to be non homologous. The X and Y chromosomes share very few genes. For example the sex determining gene is only present on the Y chromosome. This gene has the responsibility of determining that the individual becomes male. Lack of this gene will mean that the determining organism will become female. ← ← Gametogenesis ← ← The process in which gametes are produced is called gametogenesis. This process differs in many ways from the process in which somatic cells are derived. ← Cells in the body are derived through a process called mitosis which is normal cell division. In a cell undergoing mitosis, the DNA is replicated so for a short time, the cell contains 4 copies. After it divides, it only contains 2 copies of the chromosomes, the parent cell and the new cell each with 2 pairs of chromosomes. ← The production of gametes occurs by a different, more complex process called meiosis. Meiosis differs in two fundamental ways from mitosis. In meiosis, DNA undergoes replication only once and cells undergo division twice. This reduces the number of chromosomes in each cell by half from 2n to n, from diploid to haploid. Eventually, the cells finishing this process become mature ova and spermatozoa. There is reassortment of genes and the production of unique cells with a unique combination of both parent’s genes. This occurs in a process called non homologous recombination or “crossing over”. Genetic reassortment is the basic for variation in DNA between individuals of the species. ← ← Hormonal control of reproduction ← ← Humans acquire the ability to reproduce during puberty. Any years before puberty, the reproductive organs are immature and incapable of generating spermatozoa or ova. At the beginning of puberty, in responses to changes in brain activity that alters the input to the hypothalamus, a collection of endocrine cells of the hypothalamus begin to produce hormone. This hormone called gonadotropin releasing hormone is secreted in the vicinity of the first capillary in the series of two that make up the hypothalamic pituitary portal system. On arriving in a set of second endocrine cells of the anterior pituitary, GnRH triggers the secretion of two other hormones, LH and FSH. These hormones in turn stimulate the onset of gametogenesis at the gonads and the production of sex hormones by the gonads. In this way, the hormones have driven the chromosomal events that make reproduction possible. ← Various forms of stress can lead to a suppression of reproductive function by acting to increase inhibitory drive to endocrine cells that synthesize and secrete gonadotropin releasing hormone. Low energy availability resulting from malnutrition or various intense exercising can delay the onset of gonadotropin releasing hormone secretion. With exposure to stress, reproductive hormone secretion is often suppressed and with chronic stress, reproduction can be completely inhibited. The gonads, the testes in the male and the ovaries in the female are not capable of gametogenesis or hormone production until puberty. There is the secretion of gonadal hormones, androgens in the male and progesterone’s and estrogens in the female. ← ← ← ← Male reproductive system ← ← The production of spermatozoa from the early germ cell called spermatogonia is termed spermatognesis in the male. The term spermatogenesis is a synonym of gametogenesis. Spermatogenesis occurs in the testes within the walls of specialized tubules called seminiferous tubules which are located throughout the body of the testes. From these tubules, spermatozoa will travel through a series of tubules and ducts, eventually emptying out in the urethra and delivered as ejaculate. As spermatozoa is traveling through the series of tubules and ducts, they pass through a series of three different accessory reproductive organs. The accessory reproductive organs include the prostate, the seminal vesicles and the bulbo urethral glands. The function of these organs is the production of a variety of substances which nourish and prolong the life of spermatozoa as well as the promotion of fertilization. The process gland is a dense structure that is located beneath the bladder. It surrounds the ur
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