Late embryonic development is the phase where organs and structures continue to develop and mature in the embryo before birth.
Fate of Embryonic layers
There are 3 Germ layers – Ectoderm Mesoderm & Endoderm.
1. Ectoderm:
- (i) Skin: Epidermis, hair follicles, nail, apocrine & eccrine sweat glands, mammary glands, sebaceous gland
- (ii) Epithelial lining of mouth and anus, salivary glands.
- (iii) Cornea of eye, Nasal passage
- (iv) Nervous system [Brain & spinal cord. with all nerve tissue, outer & Inner ear, Eye-lens, retina, cornea, conjustivita, teeth enamel
- (v) Endocrine gland [Posterior pituitary gland, pineal gland. Adrenal medulla.
2. Mesoderm
- (i) Skin – dermis region.
- (ii) Muscle system
- (iii) Skeletal system
- (iv) Eye – Choroid & sclera
- (v) Circulatory system & lymphatic system.
- (vi)Adrenal cortex, endocrine tissue of gonad
- (vii) Reproductive organs, except germ cells.
3. Endoderm:
- (i) Mucous epithelium of digestive canal (except mouth & anus) & respiratory system
- (ii) Exocrine glands, Liver, Pancreas.
- (iii) Middle ear, tympanic cavity,
- (iv) Urethra, urinary bladder,
- (v) Thymus, thyroid, anterior pituitary
- (vi) Germ cells of reproduction system
Extra embryonic membrane:
- The membrane which does not form any part of the embryo but performs in development is called extra embryonic membrane.
- This is only found during the time of embryonic development, so it is a temporary structure.
- This specialised structure helps in gas exchange, waste removal, providing nourishment to the embryo.
There are 4 types of Extra embryonic Membrane: – Amnion, Chorion Allantois & Yolk sac.
1. Yolk Sac:
Macrolecithal egg, Cleidoic egg.
- Yolk sac is formed by a layer of extra embryonic splanchnopleure.
- Chief source of embryonic food.
- The product of digestion reaches the embryos initially by diffusion process.Yolk sacs become gradually small and finally absorbed into the midgut.
Functions
- Yolk sac protects the yolk
- It nourishes the developing embryo.
- Yolk sac secrete a digestive enzyme, which change the folk into digestacure form
2. Amnion:
Amnion is the layer which completely covers the embryo.
- It is filled with fluid, which is called amniotic fluid.
- Formed from somatopleure.
- Two fold of amnion meet the above the embryo this joining is known as sero-amniotic connection.
- Amniotic cavity: The space between embryo and the amnion is called amniotic cavity.
- Amniotic cavity filled with amniotic fluid.
Function:
- Amnion prevents the embryo from desiccation.
- Amniotic fluid prevents amnion from serving as a protective cushion, it absorbs the mechanical shock.
- It helps in maintaining temperature.
3. Chorion:
- The chorion is developed with amnion.
- Outer somatopleure separate from sera amniotic connection to form chorion.
- Sero-amniotic cavity: The space between amnion and chorion is continuous is known as sero amniotic cavity.
Function:
- It helps in respiration.
- It helps to keep away from the shell , so that proper motion of embryo occurs.
4. Allantois:
- The allantois layers are made up of externally splanchnic mesoderm and internally by endoderm.
- Allantois develop from endoderm (amnion & chorion from ectoderm)
- Cavity of allantois is continued with hind gut.
- Part near the gut is called allantoic stalk, and part far away is called allantois vesicles.
Function:
- Allantois serve as urinary bladder.
- When the bird hatches out, urine is left at that time.
- Also help in O2 and Co₂ exchange.
Implantation:
Definition: Attachment of blastocyst (Zygote) in the endometrium of the uterus of a female is called implantation.
Time period: Occurs often on the 7th day of fertilization.
Location: Normally the blastocyst (embryo) implanted on the fundus part or endometrium wall of the uterus.
Process of Implantation.
1. After blastulation, the blastocyst travels from the fallopian tube towards the uterus, at which time the zona pellucida layer disintegrates.
2. At that time, the wall of endometrium was highly developed with blood vessels, glands due to high levels of progesterone & oestrogen.
3. After 7 days of fertilization blastocyst arrived at the endometrium wall and attached with CAM formation.
4. Trophoblast cells of the blastocyst formed two layers. Inner layers- Cytotrophoblast and outer layer Syncytiotrophoblast.
5. Syncytiotrophoblast cells produce proteolytic enzymes that cause lysis of some cells in the endometrium and lead attachment of blastocyst.
6. After that, blastocyst developed to gastrula by gastrulation process and forms- bilayer germ disk, amniotic cavity, york, chorionic cavity.
7. After the implantation of the embryo, the cells of the endometrium modified into decidua.
8. It takes 3 to 5 days to implant.
Placenta:
- Physiological connections between mother and foetus.
- Placenta is developing organ in the uterus during pregnancy.
→ Placenta has two parts.
- Foetal part– Chorionic part
- Maternal part– desidua basalis
→ Decidua capsularis cover the blastocyst.
→ Rest of the part of the endometrium called decidua parietalis.
Connection between from placenta and the embryo is called Umbilical Cord.
Trophoblast layer forms amnion and chorion layer.
Types: Placenta are of 3 types on the basis involvement of foetal membrane.
(1) Chorionic Placenta: Placenta is formed from the chorion layer.
Example: Human.
(2) Chorio-Allantoic Placenta: Placenta is derived from allantois and chorion layer.
Example: Eutherian mammal.
(3) Yolk sac Placenta: Placenta form from yolk sac.
Example: Kangaroo