Lect. 9: Introduction to Human Embryology PDF

Lect. 9; Introduction to Human Embryology Assoc. Prof/ Reda Mansour & Dr. Radwa Lotfy  I- Gametogenesis Gametogenesis is the process of gametes production. It includes spermatogenesis (the production of sperm) and oogenesis (the production of oocytes or eggs). II- Spermatogene...

Lect. 9; Introduction to Human Embryology Assoc. Prof/ Reda Mansour & Dr. Radwa Lotfy  I- Gametogenesis Gametogenesis is the process of gametes production. It includes spermatogenesis (the production of sperm) and oogenesis (the production of oocytes or eggs). II- Spermatogenesis  Spermatogenesis (in seminiferous tubules) Spermatogenesis takes around 64 days to produce spermatozoa from germ cells. 1- Spermatocytogenesis (formation of spermatids): Spermatogonia A divide mitotically forming spermatogonia B which divide by mitosis giving primary spermatocytes. The primary spermatocytes enter meiosis I. Recombination of chromosomes occurs in this stage. One primary spermatocyte becomes two secondary spermatocytes. These cells are now haploid. Each secondary spermatocyte may contain an X or a Y sex chromosome. Secondary spermatocytes enter meiosis II and again divide, forming spermatids. 2- Spermiogenesis: It is a process in which spermatid is transformed into spermatozoa (sperm). The rounded spermatid cell changes in shape & become elongated developing head and tail. The cell loses cytoplasm, the nucleus is packed into the head, mitochondria become concentrated in the first part of the tail and an acrosome forms around the tip of the head. The acrosome contains enzymes that will help the sperm penetrate the outer layers of the ovum during fertilization. At the end of spermiogenesis the spermatids have become spermatozoa. 3- Spermatozoa: From the seminiferous tubules of a testis, the sperm pass into a coiled duct called epididymis. In humans, it takes 3 weeks for sperm to travel the 6 mm length of this duct, during which time the sperm complete maturation and become motile. During ejaculation, the sperm are propelled from each epididymis through a muscular duct, the vas deferens. Each vas deferens (one from each epididymis) extends around and behind the urinary bladder, where it joins a duct from the seminal vesicle, forming an ejaculatory duct. The ejaculatory ducts open into the urethra, the outlet tube for both the excretory system and the reproductive system. The urethra runs through the penis and opens to the outside at the tip of the penis.  III- Oogenesis Oogenesis begins before birth in the female embryo with the production of oogonia from primordial germ cells. The oogonia divide by mitosis to form primary oocytes that begin meiosis, but stop or arrested at meiotic prophase I before birth.  IV- Fertilization Fertilisation: The fusion of haploid male gametes and female gametes to form zygote is called fertilisation. 1. Approach of sperm to ovum: A single ejaculate of sperms contains around 200-400 million sperms in approximately 3 to 3.5 ml of semen. However only sperms are able to reach the fallopian tube. 2. Capacitation: Through alteration of acrosomal membrane (removal of glycoprotein layer for exposing the membrane receptors) in female genital tract. 3. Penetration of sperm into the ovum and acrosome reaction: The ovum at the secondary oocyte stage is extruded from the ovary along with corona radiata cells. These follicular cells are glued together by a mucopolysaccharide, the hyaluronic acid. The sperm produces enzymes lysins & hyaluronidase to dissolve follicular cells, hyaluronic acid and plasma membrane of ovum. 4. Activation of ovum: The entry of sperm nucleus induces cortical granules to release their contents to harden the zona pellucida, so prevent the entry of other sperms in the egg cell (prevent dispermy or polyspermy). 5. Fusion of sperm and egg nucleus to form zygote. With the zona pellucida, and the acrosome and cortical reactions fertilisation by multiple sperm (dispermy or polyspermy) is prevented. This is a very important process in mammalian reproduction as hundreds of sperm reach the egg at the same time and dispermy would create an embryo with three haploid sets of chromosomes (triploidy) that would be extremely unlikely to survive. From zygote to blastocyst Time period: days 0–5 Zygote With fertilisation the oocyte and spermatozoon combine to become a zygote. Cleavage Around 24 hours after fertilisation the zygote begins to increase its number of cells by rapid mitosis, but without increasing its size. The cells become smaller with each cell division. The number of cells doubles with each division. This is cleavage. The cells of the zygote are called blastomeres. Morula From around the 16‐cell stage the ball of cells becomes called the morula, derived from the Latin word for mulberry. The cells of the morula will not only give rise to the cells of the embryo, but also to many of its supporting structures, such as part of the placenta. Blastocyst: The morula passes into the uterus around 4 days after fertilisation. The blastomeres in the middle of the morula become the inner cell mass or embryoblast. These cells will form the embryo. The blastomeres on the outside of the morula become the outer cell mass or trophoblast. These cells will form some of the supporting structures for the embryo. Trophoblast cells pull fluid from the uterine cavity into the center of the morula. The fluid‐filled space that forms is called the blastocoel (or blastocyst cavity). The cells of the inner cell mass are pushed to one end of the cavity and become called the embryonic pole. Implantation: Around 5 days after fertilisation the blastocyst loses the zona pellucida. By doing this it becomes able to grow in size and interact with the uterine wall. Embryogenesis: 16-32 stage Inner cell mass = embryoblast Blastocyst cavity = Blastocoel