Prenatal:◦ Pre-Implantation◦ Embryonic◦ Fetal

An oocyte is normally fertilized by a single sperm 12-24 hours after it’s released.

When a sperm penetrates an oocyte conception has occurred (now 46 chromosomes)

This initial cell is called a zygote

About 36 hours after fertilization occurs, the oocyte divides into 2 cells (cell division process called cleavage).

By day 4, it has multiplied to 12-16 cells◦ At this stage it is known as a morula◦ The morula travels down the oviduct, and on

day 7 it enters the cavity of the uterus

Each cell of the morula is capable of forming a complete individual

When the morula fails to reach the uterine cavity, but continues to develop, it may implant on in the wall of the fallopian tube (ectopic pregnancy)

The morula continues to develop. Some fluid (the blastocoel) appears inside it. ◦ It is now called a blastocyst (~day 5)◦ Blastulation: the sprouting of a hollow sphere of cells

formed during the early embryonic development

The blastocyst implants into the endometrium◦ Progesterone and estrogen secreted from the corpus

luteum have influenced the glands and blood vessels in the endometrium to develop and be ready for the blastocyst

When the blastocyst attaches, it has an inner mass of cells which develops into the embryo and an outer mass of cells that are called trophoblasts.

The trophoblast cells begin to devlop their own blood supply and this outer cell mass becomes known as the chorion (which later develops into the placenta). ◦ Also produced the hormone hCG (human

chorionic gonadotrophin) which is the hormone detected by pregnancy tests.

The shaping of an organism by embryological processes of differentiation of cells, tissues, and organs and the development of organ systems.

The development of the body form following the 3rd week of development

The inner cell mass becomes a platelike structure called the embryonic disk

By day 14:◦ Trophoblast is completely embedded and

implantation site in the endometrium heals over

◦ The inner cells begin to line up as 2 layers of different cells

◦ Pregnancy now suspected due to missed period

Through the process of gastrulation, the single layered blastula is reorganized into a structure known as a gastrula. It now possess the 3 germ layers◦ All tissues and organs develop from these 3 layers

During this period the embryo is called a gastrula.

Cell differentiation continues, and (as early as 5 weeks from last period), the inner mass of cells begins to change into tissues and organs)

ECTODERM Top cell layer of the embryonic disc. Will later form skin,

hair, lenses of the eyes, lining of the internal and external ear, nose, sinues, mouth, anus, tooth enamel, pituitary and mammary glands, and all parts of the nervous system.

MEDODERM Middle cell layer of the embryonic disc. The precursor to

the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, reproductive and excretory systems.

ENDODERM Inner cell layer of the embryonic disc. Will later form the

lining of the lungs, the tongue, tonsils, urethra and associated glands, bladder and digestive tract.

Endoderm Mesoderm Ectoderm

Respiratory Tract

Dermis and Muscles

Epidermis, Hair, and Nails

Digestive Tract, Bladder and Thyroid

Bone, connective and lymph tissue

Lens of Eye and Skin Glands

Liver and Pancreas

Reproductive & Cardiovascular System

Central and Peripheral Nervous System

Using prgnancy ultrasound, the blastocyst is seen as a small circle measuring 2-4mm within the uterus.

The first pulse of the heartbeat can been seen as early as 36 days after the last period.

Yolk sac is seen close to the embryo, and measures ~10mm

Gestational period from week 5 – 9 is called the embryonic period.

This is a vital time for organs (most abnormalities or defects that are found after birth have happened during this time frame)

Beginning at ~10 weeks gestation, the outline of the baby becomes recognisable◦ Now called a fetus

Organ formation almost complete, fetus now starts to mature and prepare for delivery (at ~40 weeks from last period)

Skin initially appears red and transparent, but progressively thicken in the last few weeks

The biological sex is recognisable on external appearance by 14 weeks ◦ Ultrasound scans at this time are rather early to be

certain of sec, but usually at the time of the 20-week scan it is possible depending on fetus position

Fetal weight and length changes

Weeks of pregnancy Fetal weight (grams) Crown–rump length (cm)

10 weeks 10 g 3 cm

22 weeks 500 g 20 cm

28 weeks 1,000 g 25 cm

40 weeks 3,500 g 35 cm

The fetus becomes viable at 24 weeks (meaning it is possible for it to survive if delivered after this time)

However the organs and tissues are still very immature, and survival rate is low

Most babies survive after 30 weeks

Human embryo, aged 2 weeks seen in uterus. Embryo measures ~0.2mm and has no organs or nervous system

Human embryo, aged 3 weeks seen in uterus. Embryo measures ~3mm in length

Amnion Yolk Sac Allantois Chorion

Lines the amniotic cavity◦ Provides diffusion area

Amniotic fluid◦ Allows movement◦ Provides cushion◦ Cells lost in fluid can be collected

Allantois◦ Becomes the blood vessels inside the umbilical

cord Yolk Sac

◦ Becomes part of the digestive and respiratory tracts

◦ Produces fetal blood cells until bone marrow takes over

Chorion◦ Outermost membrane◦ Exchange portion of the placenta◦ Produces hCG