THE UROGENITAL SYSTEM — EXCRETION AND REPRODUCTION ^J \ 



242. Mammalian Reproduction 



Fertilization. During copulation, the sperm that have been stored 

 in the epididymis and vas deferens are ejaculated by peristaltic contrac- 

 tions of the male ducts, and the accessory sex glands concurrently 

 discharge their secretions. The seminal fluid that is deposited in the 

 upper part of the vagina may contain as many as 400,000,000 sperm. It 

 also contains glucose and fructose from which the sperm derive energy, 

 mucus that serves as a conveyance, and alkaline materials that neu- 

 tralize the acids produced by sperm metabolism and those normally 

 present in the vagina. Sperm are quickly killed in an acid environment. 



Sperm move from the vagina through the uterus and up the Fal- 

 lopian tube in a matter of a few hours or less. How they do this is not 

 entirely understood. They can swim, tadpole fashion, by the beating of 

 the tail, but muscular contractions of the uterus and Fallopian tubes 

 must help considerably. Fertilization occurs in the upper part of the 

 Fallopian tube (Fig. 28.6), but the arrival of an egg and the sperm in 

 this region need not coincide exactly. Sperm retain their fertilizing 

 powers for a day or two, and the egg moves slowly down the oviduct, 

 retaining its ability to be fertilized for about a day. The chance of 

 fertilization is further increased in many species of mammals (but not 

 in human beings) by the female coming into "heat" and receiving the 

 male only near the time of ovulation. Ovulation, heat, and changes in 

 the uterine lining in preparation for the reception of a fertilized egg 

 are controlled by an intricate endocrine mechanism that will be con- 

 sidered in Chapter 30. 



Only one sperm fertilizes each egg, yet unless millions are dis- 

 charged, fertilization does not occur. One reason for this is that only 

 a fraction of the sperm deposited in the vagina reach the upper part of 

 the Fallopian tube. The others are lost or destroyed along the way. 

 Another reason is that when the egg enters the Fallopian tube, it is 

 still surrounded by a few of the follicle cells that encased the egg within 

 the ovary (Fig. 3.16), and a sperm cannot penetrate the egg until these are 

 dispersed. This requires an enzyme, hyaluronidase, which can break down 

 hyaluronic acid, a component of the intercellular cement. Hyaluroni- 

 dase is believed to be produced by the sperm themselves, and large 

 numbers are apparently necessary to produce enough of it. 



Establishment of the Embryo in the Uterine Lining. The fertilized 

 egg passes down the Fallopian tube into the uterus, undergoing cleav- 

 age along the way. Energy for early development is supplied by the 

 small amount of food within the egg (mammalian eggs are isolecithal), 

 and by secretions from glands in the uterine lining. About a week 

 after fertilization the embryo of most mammals penetrates the uterine 

 lining, apparently by secreting digestive enzymes, and the lining folds 

 over it. The extraembryonic membranes that form the placenta de- 

 velop very rapidly in mammalian embryos. A functional placenta is 

 established in the human embryo about three weeks after fertilization, 

 and this provides for the metabolic requirements of the embryo during 

 the rest of embryonic life (Fig. 28.7). The development of the embryo 

 itself is described in Chapter 31. 



