Oogenesis and Follicle Development

At birth, females have approximately 1 million primary oocytes.
Only 40,000 primary oocytes survive to puberty.
During each ovarian cycle, 20-100 primary oocytes begin the process of ova formation.
Only one oocyte successfully completes the process.
Oogenesis results in the formation of one ovum from 20-100 oocytes.
Oogenesis stops at menopause.
One ovum is produced each month and lasts for 12-24 hours.

Each primary oocyte is surrounded by a single layer of flat follicular cells.
At puberty, follicular cells become granulosa cells (multilayers of cubical cells).
Cavities appear between granulosa cells.
These cavities enlarge and unite forming an antrum (chamber).
The outer layer of the follicle is the stratum granulosum (granular layer).
The secondary oocyte with its surrounding cells is called the cumulus oophorus and consists of:
- Secondary oocyte
- Zona pellucida (transparent zone)
- Corona radiata (radiating crown)
The ovarian tissue compresses to form a capsule called the theca folliculi.
The theca folliculi is divided into:
- Theca interna: Secretes estrogen
- Theca externa: Formed of fibrous tissue

The enlargement of the follicle and pressure on the ovarian cortex causes the overlying cortex to rupture.
The secondary oocyte with the zona pellucida and corona radiata leaves the ovary and enters the Fallopian tube.
Ectopic pregnancies occur when the fertilized ovum implants outside the uterus.

Cytotrophoblast cells form a new layer called extraembryonic mesoderm (EEM).
Spaces appear inside EEM forming the extraembryonic coelom (EEC).
The EEC divides EEM into an outer layer lining the cytotrophoblast and an inner layer surrounding the amniotic cavity and yolk sac.
Part of the EEM connects the outer and inner layers, forming the connecting stalk (future umbilical cord).
The blastocyst is now called the chorionic vesicle.
The outer wall of the chorionic vesicle is called the chorion.
The chorion gives rise to finger-like processes called chorionic villi that invade the uterine wall.
The spaces between the chorionic villi (intervillous spaces) are filled with maternal blood.

Primary villi consist of syncytiotrophoblast and cytotrophoblast.
Secondary villi consist of syncytiotrophoblast, cytotrophoblast, and extraembryonic mesoderm.
Tertiary villi consist of syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm, and fetal blood vessels.
The tertiary villi are divided into:
- Stem (anchoring) villi: Extend from the base of the chorion towards the uterine wall
- Branching (free absorbing) villi: Small villi branching from the stem villi, responsible for nutrient and gas exchange.

On either side of the notochord, the intraembryonic mesoderm is divided into three parallel craniocaudal masses:
- Paraxial mesoderm
- Intermediate cell mass
- Lateral plate mesoderm

It is divided into cubical masses called somites.
There are 42-44 pairs of somites, formed in a craniocaudal sequence.
Each somite is divided into three parts:
- Sclerotome: Differentiates into vertebral column and ribs
- Myotome: Differentiates into back muscles
- Dermatome: Differentiates into the dermis of the skin covering the back muscles (both the skin and muscles are supplied by the dorsal rami of the spinal nerves)

Cavities appear in the lateral plate and fuse to form the intraembryonic coelom (IEC).
The IEC divides the lateral plate mesoderm into:
- Somatic (parietal) layer: Differentiates into the dermis, bones, joints, muscles, and vessels of the limbs and ventral part of the trunk
- Splanchnic (visceral) layer: Differentiates into connective tissue, smooth muscles, and vessels of the viscera

Occurs during the 4th week of pregnancy.
Caused by rapid growth of the cranial part of the neural tube, expansion of the amniotic cavity, and growth of the embryo.
Results in the formation of head, tail, and two lateral folds.

The endoderm and most of the yolk sac invaginate within the folds.
The endoderm forms a tube inside the folds (gut).
The gut is divided into:
- Foregut: In the head fold
- Midgut: Between the lateral folds
- Hindgut: In the tail fold

A small part of the yolk sac remains outside the fold, connected to the midgut by the vitelline duct.
This part is called the definitive yolk sac.

The cranial part of the neural tube (future brain) becomes the most cranial structure.
The prechordal plate (future mouth) lies caudal to the developing brain.
The cardiogenic area (future heart) becomes caudal to the prechordal plate.
The septum transversum (future diaphragm) becomes caudal to the cardiogenic area.
The connecting stalk (future umbilical cord) becomes ventral near the definitive yolk sac.
The cloacal membrane (future anal canal and urethra) becomes caudal to the connecting stalk.
The amniotic cavity surrounds the whole embryo.

Formed from the chorion frondosum and decidua basalis.
The chorion projects chorionic villi invading the decidua basalis.
The part of the chorion forming the base of chorionic villi is called the chorionic plate.

Separates fetal blood from maternal blood.
Consists of:
- Syncytiotrophoblast
- Cytotrophoblast
- Extraembryonic mesoderm
- Fetal capillary endothelium

Formed from the connecting stalk.
Contains:
- Wharton’s jelly (extraembryonic mesoderm)
- Two umbilical arteries
- One umbilical vein

Abnormally fixed placenta to the uterus, due to deep invasion of chorionic villi reaching the muscular layer of the uterus.

Battledore placenta: Umbilical cord attached to the periphery of the placenta.
Velamentous attachment of umbilical cord: Umbilical cord attached to the fetal membranes away from the placenta.
Diffuse (membranous) placenta: Placenta occupying a wide area of endometrium, due to spread chorion frondosum.