Conception Lecture 3

Conception Conception is when sperm and egg meet and fertilization occurs Conception occurs in the outer third of the fallopian tube Zygote - fertilized egg http://www.youtube.com/watch?v=SdZW_K9J-nY

Conception The sperm reaches the caudal epididymis approximately 72 days after the initiation of spermatogenesis. Preservation of optimal sperm function during storage requires adequate testosterone levels and maintenance of a normal scrotal temperature.

Stages of Fetal Development Preembryonic stage: fertilization through 2nd week Fertilization; cleavage; morula Blastocyst and trophoblast Implantation Embryonic stage: end of 2nd week through 8th week Basic structures of major body organs and main external features Fetal stage: end of the 8th week until birth

Embryonic Layers Ectoderm: forms the central nervous system, special senses, skin and glands Mesoderm: forms skeletal, urinary, circulatory, and reproductive organs Endoderm: forms respiratory system, liver, pancreas, and digestive system

Site of sperm formation Seminiferous tubules of Testis

Seminiferous tubule under microscope

Conception The alkaline pH of semen provides protection for the sperm from the acid environment of the vagina. Of an average of 200 to 300 million sperm deposited in the vagina, at most only a few hundred achieve proximity to the egg.

Conception Capacitation is characterized by three accomplishments: The ability to undergo the acrosome reaction. The ability to bind to the zona pellucida. The acquisition of hypermotility.

SPERMIOGENESIS Spermatids are rounded cells. They modify to assume specific shape of the sperm. This process is called Spermiogenesis. In it they elongate and reorganize internal structure to acquire the particular shape.

Accomplishing Fertilization The oocyte is viable for 12 to 24 hours Sperm is viable 24 to 72 hours For fertilization to occur, coitus must occur no more than: Three days before ovulation 24 hours after ovulation Fertilization – when a sperm fuses with an egg to form a zygote

Sperm Transport and Capacitation Fates of ejaculated sperm Leak out of the vagina immediately after deposition Destroyed by the acidic vaginal environment Fail to make it through the cervix Dispersed in the uterine cavity or destroyed by phagocytic leukocytes Reach the uterine tubes Sperm must undergo capacitation before they can penetrate the oocyte

Acrosomal Reaction and Sperm Penetration An ovulated oocyte is encapsulated by: The corona radiata and zona pellucida Extracellular matrix Sperm binds to the zona pellucida and undergoes the acrosomal reaction Enzymes are released near the oocyte Hundreds of acrosomes release their enzymes to digest the zona pellucida

Acrosomal Reaction and Sperm Penetration Once a sperm makes contact with the oocyte’s membrane: Beta protein finds and binds to receptors on the oocyte membrane Alpha protein causes it to insert into the membrane

Acrosomal Reaction and Sperm Penetration Figure 28.2a

Blocks to Polyspermy Only one sperm is allowed to penetrate the oocyte Two mechanisms ensure monospermy Fast block to polyspermy – membrane depolarization prevents sperm from fusing with the oocyte membrane Slow block to polyspermy – zonal inhibiting proteins (ZIPs): Destroy sperm receptors Cause sperm already bound to receptors to detach

Implantation Implantation is defined as the process by which an embryo attaches to the uterine wall and penetrates first the epithelium and then the circulatory system of the mother to form the placenta. The human blastocyst remains in the uterine secretions for approximately 1 to 3 days and then hatches from its zona pellucida in preparation for attachment.

Implantation Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblasts hCG prompts the corpus luteum to continue to secrete progesterone and estrogen Between the second and third month, the placenta: Assumes the role of progesterone and estrogen production Is providing nutrients and removing wastes

Implantation The blastocyst loosely adheres to the endometrial epithelium, a process called apposition, which most commonly occurs on the endometrium of the upper posterior wall of the uterus. The implantation occur in fundus of the uterus, which is rich blood supply.

Placental Hormones HUMAN CHORIONIC GONADOTROPIN maintenance of function of the corpus luteum Stimulation of the maternal thyroid gland Promotion of relaxin secretion Promote uterine vascular vasodilatation and myometrial smooth muscle relaxation

Student Practice 2 1. Fates of ejaculated sperm caused by: 2. Explain the process of implantation?

The END