Chapter 12 - REPRODUCTION PRODUCES OFFSPRING

insemination

During sexual intercourse, an erect penis releases semen into the vagina through an ejaculation.

oocyte

an immature egg cell in the ovary

Gene

The factor that determines a hereditary characteristic; part of a chromosome

The secondary oocyte that is released at ovulation is at metaphase II. It is surrounded by two layers.

• The outer corona radiata consists of follicle cells held together by cementing materials that

contain acid.

• The inner zona pellucida is a glycoprotein matrix surrounding the plasma membrane of the

oocyte.

implantation

The blastocyst remains free within the cavity of the uterus for two to three days, and then sinks into the soft endometrium (uterine lining) to become firmly attached to the wall of the uterus. This process is called implantation, and enables the blastocyst to gain nourishment for growth and development by absorbing nutrients from the glands and blood vessels of the uterine lining.

stem cells are very different from other cells because:

• they are not specialised for any particular role

• they are capable of repeated division by mitosis – a process called proliferation

• given the right conditions, they can differentiate into specialised cells. All the 200 or more

types of cells that make up a mature human body develop from the stem cells of the inner

cell mass.

zygote life journey

A zygote develops from an unspecialised totipotent cell to a blastocyst, and then to an embryo and its membranes.

The primary germ layers differentiate to form the specialised structures of the embryo.

Structures formed by the three primary germ layers

what is an embryo

When a fertilized egg has implanted in the uterus, the group of cells that will become a baby

Amnion

The amnion is the first membrane to develop. By the eighth day after fertilisation, it surrounds the embryo, enclosing a cavity into which it secretes amniotic fluid. This fluid serves to protect the embryo against physical injury by acting as a shock absorber. It also helps to maintain a constant temperature and allows the developing embryo, and later the foetus, to move freely. The amnion expands as growth takes place. It usually ruptures just before childbirth, releasing the amniotic fluid, an event commonly referred to as ‘breaking of the waters’.

Chorion

Another embryonic membrane is the chorion. It is formed from the outer cells of the blastocyst together with a layer of mesodermal cells. The chorion surrounds the embryo and the other three embryonic membranes. As the amnion enlarges, it fuses with the inner layer of the chorion. Eventually, the chorion becomes the main part of the foetal portion of the placenta.

Yolk sac and allantois

In addition to the chorion and the amnion, there are two other membranes – the yolk sac and the allantois. These are not as important in humans as they are in the development of many other animals; however, they do form the outer structure of the umbilical cord.

Functions of the placenta

Stages of foetal development

Pregnancy also affects the mother in less obvious ways.

• There is an increase in the size of the heart and in blood volume. This is to cater for the extra blood that is flowing through the placenta.

• The greater blood volume results in an increased blood flow to the kidneys and, therefore, increased urine production. Additionally, during the first three months of pregnancy, the expanding uterus presses on the bladder so that it feels as if it is filled with urine. As the uterus grows, it moves up the pelvic cavity, releasing this pressure. Then, during the last stages of pregnancy, the foetus presses on the bladder again.

• The emotional state of the mother may be affected due to the changes in hormonal balance and as a result of natural fears accompanying pregnancy. The mother may be concerned about her child’s development, the problems that may occur at the time of birth, and the effect the newborn child will have on the rest of the family. Many of these factors are beyond the control of the pregnant woman and so support and reassurance from family and friends are very important in maintaining a positive outlook.

During the first stage of labour,

During the first stage of labour, uterine contractions gradually increase in strength and the cervix dilates until it forms the birth canal with the uterus, cervix and vagina

During the second stage

During the second stage of labour, the membrane bursts and contractions push the foetus through the birth canal.

In the final stage

In the final stage, the placenta, other membranes and the remains of the umbilical cord are expelled.