Chapter 8 Fertilization, Development, and Birth

Introduction Childbirth is one of the most amazing processes that a female can experience. The miracle of life cannot be regarded lightly, or without a natural curiosity as to how it all happens. Fertilization, development and birth are where much of the mystery of life takes place. During our disscussion today, we’ll consider the major aspects of fetal development.

Fertilization, the initial step where and sperm unite, leads to the formation of a zygote. Cells of the corona radiata nourished the egg following its release from the follicle. Now they serves as protection. The zona pellucida is analogous to a cell membrane.

Ferilization Only one sperm usually enters the egg during fertilization. Upon entry of the first sperm, changes in the egg’s plasma membrane and zona pellucida prevent additional eggs from entering. If multiple sperm do get in, then a condition of polyspermy occurs. Retracting of the zona pellucida away from the egg surface further prevents fertilization by “rogue sperm.”

Human Development at a Glance Cell development immediately following fertilization consists of multiple cell divisions without an increase in mass, known as cleavage. (Rem: 2, 4, 8, 16, etc.)j Cellular growth occurs during embryonic development. Following cell growth, a period of morphogenesis takes place when the embryo is shaped. Finally, cells begin to take on specific functions during the differentiation stage.

Extraembryonic membranes, outside the embryo also serve various function in embryonic development. Chorion:placenta Yolk sac:blood cell formation Allantois:unbilical chord Amnion: aminiotic fluid

Pre-embyronic Development: Cells within the morula and blastocyst are undifferentiated. Identical twin development is possible during this time.

Embryonic Development Week 2: Implantation occurs Trophoblast secretions digest endometrium to prepare implantation sight. Human chorionic gonadotropin is secreted at this time as well, which ensures that the uterine lining isn’t shed, resulting in a miscarriage. Also, the amiotic cavity (fluid) is formed to protect the developing embyro and later the fetus.

Fig. 08-04

Embryonic Development Week 2: Gastrulation (a type of morphogenesis) occurs and three cellular layers (ectoderm, mesoderm, endoderm) are formed. All organs can be traced back to the development of on of these tissues.

Embryonic Development Week 3: Formation of the nervous system is now taking place. Spinal chord formation from the nerual tube is evident. The heart also begins to form.

Embryonic Development Week 4-5: Right now the embryo isn’t any larger than a small ant. Development of the umbilical chord, limb buds, ears, eyes and a nose begins.

Embryonic Development Week 6-8: Human form is easily recognized during this stage of development. Reflex actions from a maturing nervous system are also evident. Placental development continues.

Placenta

-The placenta is the medium through which nutrients, wastes, and oxygen are transferred to the fetus from the mother. -Maternal blood rarely mixes with fetal blood, since passage of nutrition, etc. takes place by difusion. -Because of this process, the main organs of the fetus do not function. -REM: Anything mom gets, baby gets (more later)!

Fetal Development From the 3rd through the 9th month its all fetus! 3rd & 4th Month (6 oz.): Appearance of fingernails, nipples, eyelashes, eybrows, and hair are all epidermal developments during this stage. Cartilage to bone transformation occurs Fontanels (become skull) allow flexibility of head during birth. We can hear the fetal heartbeat too!

Fetal Development 5th & 7th Month (3lbs.) Football season begins!! (or so my wife thought) Fetal movement is obvious! Formation of the lanugo (downy covering on skin) and vernix caseosa (cheese-like covering) complete. Babies born prematurely at this stage of development have an excellent chance of surviving.

Fetal Development 8th-9th Month (>7lbs.) Fetus rotates in preparation for birth (if not, then breech and caesarean section is necessary.) At the end of 9 months contractions may begin to occur. Braxton Hicks contractions may occur and result in false labor. True labor is marked by contractions lasting 40 seconds in duration, occuring every 10-20 minutes.

1-2 minute contractions

Among the many changes which females experience during pregnancy, development of breastmilk is one of the most important for new born fetuses. Also, Weight gain Constipation Hormonal flux Cravings Morning sickness It’s all good, right?

Fig. 8B

Fig. 8A

Michael Clancy Vanderbilt University Medical Center: Samuel Armas' tiny hand reaches out of the womb to grip Dr. Joseph P. Bruner's finger during a radical spinal bifita procedure at 21 weeks! The photographer (Michael Clancy) became an active pro-life advocate following this event!